Encyclopedia of Smart Materials (Vols 1 and 2) - M. Schwartz (2002) Episode 12 pps

On the other hand, if some B cations are reduced andtheir valence state is decreased from+3 to +2, the AO33 −layers are required to compensate for the excess negativecharge, resulting in

BaCeO3 can be good conductors of ions and electrons or

protons by simply changing the valence of the B cations to

mixed valency

Superconductivity

Superconductivity is the phenomenon of vanishing

elec-trical resistance below the superconducting transition

emperature (Tc) Superconductivity has important

applica-tions in power transmission, nuclear magnetic resonance,

extremely strong magnets, and high-speed computing

High-temperature superconductors, such as YBa2Cu3O7,

Bi2Sr2CaCu2O8 +d and Tl2Ba2Ca2Cu3Ox, are the focus

of today’s research (22) Most ceramic superconductors

are based on the perovskite structure The structure of

YBa2Cu3O7 will be used as an example to show the

construction of its structure from anion-deficient

per-ovskites (see later section)

A superconductor is characterized by three physical

quantities The first is the critical transition temperature

Tc, below which superconductivity appears The second is

the critical magnetic field Hc, below which a

supercon-ducting body exhibits perfect diamagnetism and excludes

a magnetic field If the applied magnetic field is higher

than Hc, the materials revert to the normal state Hc is

temperature-dependent The third is the critical current

density Jc, above which superconductivity is destroyed and

the superconductor reverts to the normal state

Polycrys-talline superconducting materials are limited mainly by

low Jcdue to the weak link between grain boundaries

Im-proving Jcin the field is the essential task of current

re-search in superconductivity

THE FUNDAMENTAL STRUCTURAL CHARACTERISTICS

OF ABO 3 PEROVSKITE

Perovskite and related structures cover a large portion of

smart materials, and their crystal structures can vary to

a large extent The key questions are do the

perovskite-type structures have smart properties and are there any

intrinsic connections among the structures? The answers

may be found in the following areas: (1) nonstoichiometry

(a)

BO

(b)

ABO

(c)

A

B

O

Figure 16 ABO3perovskite structure formed by corner-sharing octahedron chains The corners

of the Bravais cell are the B cations (a) A postulated structure where the A cation is absent at the center, and (b) a structure that has the A cation The anions shadowed by the octahedra are not shown for clarity (c) ABO3perovskite structure drawn by setting the A cations as the corners of the Bravais cell, showing the BO octahedron located in the A-cation cube.

of the cation and/or the anions; (2) distortion of the cationconfiguration; and (3) the mixed valence and the valencemixture electronic structure From the viewpoint of crystalstructure, each of these features can be introduced by dop-ing a third type of cation into the stoichiometric phase of abase structure It is important to understand how oxygenstoichiometry and lattice distortion are introduced as a re-sult of doping another type of cation that has different va-lence states First, we explore the fundamental perovskitestructure (2)

Vertex Sharing of Oxygen Octahedra

In the ABO3 type structure, the cation B whose valence

is 4+ is usually a transition-metal element that prefers

to form a six-coordinated octahedron with its neighboringoxygen anions, and itself is located at the center The oc-tahedron is the basic unit of these structures The geomet-ric configuration of the arrangement of the octahedra thathas the lowest interactive energy is a linear 180˚ vertex-sharing connection If the octahedra are connected to eachother at every vertex, they form a 3-D network (Fig 16a).Because the oxygen at every vertex is shared by adjacentoctahedra, the composition of this configuration is BO3,and the unit cell is a simple cubic, as given in Fig 16a.This structure, however, cannot exist unless B has a va-lence of 6+ because the valence charges are not balanced.Thus, a cation of valence 2+ must be introduced into thestructure to balance the local excess negative charge Avertex-sharing octahedral network, on the other hand, has

a large cavity in the center of the unit cell A cation of lence 2+ can occupy this cavity Then , the unit cell stillpreserves the simple cubic structure, and the composition

va-is ABO3, simply the basic model of the perovskite structure(Fig 16b) It is clear, therefore, that six-coordinated octa-hedra are essential structural and compositional buildingblocks, and sharing of all of the vertexes is required for thestoichiometry and the structure of the perovskite Based

on this ideal, we can outline some characteristics of theperovskite structure:

1 Any cation, that prefers to have six coordinationcould occupy the B position even if its valence is

1002 SMART PEROVSKITES

different from 4+, but to balance the valence charges,the average valence at the B site usually equals 4+

2 The A cation of valence 2+ is expected to have a

larger radius because, as a general rule, the valenceincreases as the radius of the cation decreases andvice versa The coordination number of the A cation

is 12 Elements that have a coordination number ofmore than six are likely to bond ionically This is pos-sible because the A cation in the perovskite structure

is usually an alkaline-earth metal element From thisviewpoint, we may say that any cation, of higher ioni-city or less polarity could occupy the position of the Acation, although its valence is different from 2+ But

to balance the charge, the average valence at this site

is likely to be 2+

3 From 1 and 2, we can see that B cations have some

degree of covalency, or in other words, they are easierpolarize than A cations, and bonding between the Bcation and the adjacent oxygen is stronger than that

of the A cation The octahedron of oxygen is the basicunit of the perovskite structure although it may bedistorted, as is discussed later Oxygen anions havesignificant influence on the cation valence if it can bechanged For example, a vacancy of oxygen can re-sult in modifying the valence state of the B cation

The A cation may not be easily modified because itbonds ionically to the adjacent oxygen However, ifthe A cation changes valence, the surrounding oxy-gen anions must be affected to balance the valencecharges by creating vacancies Because the oxygenanion cannot have a valence other than−2, an oxy-gen deficiency must form to balance the local charge

The roles of the A and B cations in the structural

evolu-tion of ABO3type perovskite can be understood as follows

The A cation plays a key role in oxygen deficiency because

of its stronger ionic interaction with oxygen anions We will

see later that the A cation is closely packed together with

oxygen The B cation prefers six-coordination (i.e., an

octa-hedron) although its valence can vary The flexibility in the

valences of the B cation makes the oxygen anion deficiency

acceptable, and in a reverse process, the oxygen deficiency

is autocontrolled by adjusting the valence of the B cation

This process makes the perovskite structure the most

fas-cinating structural configuration for smart materials

If the A cation changes its valence, for example, from+2

to+3, the oxygen anion must modify its occupancy to match

the valence variation of the A cation and to balance the

lo-cal charge, resulting in oxygen deficiency But this change

will feed back to the B cation, leading to disproportionation

of the local valence state The disproportionation of the

B cation’s oxidation state usually changes the electronic

band structure of the perovskite compound, resulting in a

transition from an insulator to a semiconductor,

conduc-tor, or even superconductor CaMnO3 is an insulator, but

when the Ca (i.e., A2 +cation) is totally replaced by La(+3),

LaMnO3 could be a conductor if the monovalence Mn4 +

were replaced by mixed valent states of Mn3 +and Mn4 +

From the viewpoint of charge balance, if the valence of the

Mn cations is+3 and +4, the relative content of the oxygen

anions should be more than 3, in other words, its formula

should be LaMnO3 +x Based on the crystallography of theperovskite structure illuminated discussed before, the ex-cess oxygen anions have no place to locate because theexisting oxygens have already been closely-packed in thestructure Therefore, if Mn3 +and Mn4 +must coexist in thesystem, the only choice is to change the A cation’s valencestate from La3 +to a mixture of La3 +and A2+, a divalentcation For example, if a small portion of La3 +is replaced

by Ca2 +, the La3 +stoichiometry will change from 1 to 1−

x Then we can have a perovskite type structure that has+3 and +4 mixed Mn cations at the B site

Unit Cell by Taking the A Cation as the Origin

The perovskite structure has a simple cubic Bravais cell,

in which the octahedra share corners and the origin of theBravais cell is at the B cation, as shown in Fig 16a Alter-natively, we can also take the A cation as the origin, andthe unit cell is transformed into the form given in Fig 16c

In this configuration, the A cations locate at the cubic texes, and the oxygens occupy the face centers of the sixfaces to form an octahedron where the B cation is at the cen-ter This geometric arrangement makes the oxygen formlinear O2 −–B4 +–O2 −triples parallel to the x, y, and z axes,

ver-and the oxygens are located at the centers of the squaresformed by the A2 + cations If an electric field is applied

parallel to the z axis, for example, the O2 −–B4 +–O2 −chain

parallel to the z axis is polarized, but the O2 −–B4 +–O2 −

chains parallel to the x and y axes may not be disturbed, resulting in polarization of the crystal parallel to the z axis

(ferroelectricity) Moreover, the displacement of the oxygenanions could cause a distortion in the configuration of the

A cations, resulting in a change in the shape of the unitcell (the piezoelectric effect)

Oxygen Cubic Close Packing

Now, we reexamine the ABO3 perovskite structure from

a different viewpoint We use the structural model ofperovskite formed by the corner-sharing chains of octa-hedra, for example, the unit cell that has B cations as theorigin, as shown in Figure 16b, in which the octahedron issketched for clarity The relationship between the oxygenanions and A cations in the{111} stacking layers are re-vealed in Fig 17a, where an A cation is surrounded by sixoxygen anions and forms a closely packed structure Notethat three oxygen atoms form a triangular unit Naturally,the A cations also form a hexagonal array, but are sepa-rated by the triangularly packed oxygen anions The chem-ical composition of this layer is AO3 Because the charge

of A is 2+, the valence charge of the layer is (AO3)4 − To ance the local negative charge, a layer that has a charge

bal-of 4+ must be introduced Therefore, a B4 + cation layer(without oxygen anions) should be the next stacking layer(Fig 17b) The A cation has three stacking positions, in-dicated byα, β, and γ , respectively, and the B cation has

similar stacking An A cation hexagon contains six sharing oxygen triangles This characteristic of the fun-damental stacking layer (AO3)4 −determines the features

apex-of the perovskite structure The (AO3)4 −and the B4 + ers are stacked together to form a combined layer, and theperovskite structure is obtained by stacking the new lay-ers following a sequence ofα β γ by translating the cation

αγβ

(AO3)4 − (111) stacking layer

(b)

αγ

β

B4 + (111) stacking layer

Figure 17 (a) The fundamental (AO3 )4−(111) close-packing

layer, and (b) the B4+(111) close-packing layer in the perovskite

structure The shadowed area represents the hexagonal unit

from stacking the two layers into one.α, β, and γ represent the

three cation stacking positions for forming the three-dimensional

The introduction of oxygen vacancies in the unit cell can

create a wide range of perovskite structures If one of

the oxygen sublattices is vacant, the layer composition is

(AO2)2 − (see Fig 18b) If two of the oxygen sublattices

are vacant, the layer composition is AO; thus, the layers

are neutral, provided the valence of cation A is 2+ There

is no electrostatic force attracting the B4 + cation layers,

and thus, the perovskite structure cannot be formed If

oxygen vacancies are partially formed in the (AO3)4 −layer,

the close-packing layer should be (AO3 −x)(4 −2x)−(Fig 18c,d

where x = 0.67 and x = 0.33, respectively) Therefore,

there are two choices to have oxygen vacancies in the

three oxygen sublattices If one oxygen vacancy is formed

in the oxygen sublattice, it can locate in different layers

and/or different sublattices of oxygen The symmetry of

oxygen triangles makes vacancies possible at different

cor-ners of the triangles that belong to alternate stacking

lay-ers This creates the five- and four-coordinated oxygen

polyhedra, shown in Fig 19, where the structure of the

B cations does not change, but their valence states may

change to balance the local charge The A cations in the

(a) (AO3)4− layer (b) (AO2) 2 − layer

2.67 ) 3.33 − layer

Figure 18 Two-dimensional oxygen sublattices in the (AO3 )4− layers (a) no vacancy [(AO 3 )4−], (b) one vacancy [(AO

2 )2−], and(c, d) partial vacancies.

(AO3)4 −layer may change their valence states and/or theirtotal number in the layer (e.g., stoichiometry) Therefore,the substitution of the A cations by an element of differentvalence may be the optimum choice to induce oxygen defi-ciency and/or the mixed valence states of the B cations This

is the tailoring characteristic of the perovskite structure

In the ABO3structure, the A cations are closely packedwith the oxygen anions and are dominated by ionicbonding The B cations may be regarded as the dependentsubordinations that rely on the structure of the (AO3)4 −layer Bonding of B cations with the surrounding oxygensmay be ionic, covalent, or partially covalent If the (AO3)4 −layer has defects, such as vacancies, stacking these layerswill form distorted or deficient oxygen octahedra Thus,the B cation must have the flexibility of modifying its va-lence state to balance the local charge In other words, if

we intend to modify the valence of the B cation, ing the structure surrounding the A cations via doping isrecommended This is an important principle for modifyingthe structure and properties of perovskites

chang-ANION-DEFICIENT PEROVSKITE STRUCTURAL UNITS—THE FUNDAMENTAL BUILDING BLOCKS FOR NEW STRUCTURES

Perovskite types of structures are the basis of many oxides,

in which oxygen-deficient perovskites are a key group ofmaterials that possess functionality Anion-deficiency canchange the coordination number of the B cation octahedraand the size and type of the Bravais unit cell of the com-pounds It is important to elucidate the possible types ofanion-deficient perovskite structures to enhance insightinto fabricating new materials

As shown in Fig 18, there are three equivalent stackingpositions for the layers parallel to the (111) plane Fig-ure 19a shows anαβ stacking of the (AO )4 − layers (the

Figure 19 The local oxygen vacancy

arrange-ments and the stacking of the (AO 3 )4−type layers

(with vacancies) (a) Single and double oxygen

va-cancies in BO6octahedron, where 1, 2, and 3

rep-resent the anions in theα, β, and γ layers,

respec-tively (b, c) Connections of the octahedra without,

with one, and with two oxygen vacancies.

(b)

(c)

23

322223

22

Anions in first, second and third stacking layers, respectively

A cation in the first stacking layer

α, β, and γ positions are indicated in the left part of this

figure) The octahedra are formed by superimposed oxygen

triangles that belong to theα and β layers (this is most

eas-ily seen in Fig 18) If an oxygen vacancy is created in the

β layer, as indicated by V2, a five-coordinated polyhedron

is formed By the same token, oxygen vacancies can be

created in the octahedra formed by the superposition of the

β and γ layers, as shown in Fig 19a,b, where the sequence

numbers 1, 2, and 3 represent the anions in theα, β, and γ

layers, respectively As discussed before, any (AO3)4 −

stack-ing layer can have oxygen vacancies A five-coordinated

polyhedron is formed if only one oxygen vacancy is created

in theβ layer, and the α layer is perfect If two oxygen

va-cancies (V1and V2) are created in the two adjacent layers

within the same octahedron, a four-coordinated square

is formed, as shown in Fig 19a It is impossible to have

more than two vacancies in one octahedron because four

apexes are the minimum to form a 3-D connection for the

perovskite type of frame The geometric assemblies ofthe four-, five-, and six-coordinated A cations are given inFig 19c

Figure 20a,b shows the BO5and BO4structures, tively, in an octahedral sheet The lack of one oxygen at

respec-a vertex forms respec-a five-coordinrespec-ated unit (Fig 20respec-a), respec-and thelack of two oxygens located at two opposite vertexes results

in a four-coordinated square (Fig 20b) The lack of threeoxygens in one octahedron forces the unit to break its con-nections to other octahedra Figure 20c shows a clusterconstructed on the basis of the symmetrical distribution

of two oxygen octahedra, resulting in four 5-coordinatedpolyhedra and two 4-coordinated squares It has the com-position AB8O30, and it demonstrates how the different co-ordinated polyhedra may be connected to each other in 3-D.Figure 20d–n shows eight possible interconnecting con-figurations of the five- and four-coordinated polyhedra(2) Any of these types of anion-deficient perovskite-like

BO5 unitV

OB

Figure 20 A total of 14 perovskite-type structural modules that have oxygen vacancies, where

the oxygen anions are represented by different patterns to distinguish their stacking layers and V stands for vacancy The combination of these modules can reproduce the crystal structures of many compounds (see text).

1006 SMART PEROVSKITES

Figure 20 (Continued )

structural modules may be connected to the ideal

perovskite structure’s unit cells to form a new structure

The compound YBaCuFeO5(23), for example, has a

struc-ture that contains CuO5and FeO5units, corresponding to

configuration (f) shown in Fig 20 LaBa2Cu2TaO8 (24)

and Ba2La2Cu2Sn2O11 (25) belong to the module shown

in Fig 20(l) The idealized structure of this compound

con-tains TaO6or SnO6octahedra and five-coordinated CuO5

The structure of YBa2Cu3O7 (26) is built by the unit in

Fig 20(l), and it contains five-coordinated CuO5and

four-coordinated CuO4 squares The compound La2Ni2O5 is

constructed by the (h) type in Fig 20, and it contains NiO6

octahedra and NiO4four-coordinated squares These types

of anion-deficient perovskite-like units can be combined

with tetrahedra, octahedra, or others including themselves

to form a variety of different structures The compound

Ca2Mn2O5 (27), for example, contains distorted MnO5

units similar to the (m) type unit shown in Figure 20 The

modules shown here are based the squares, tetrahedra,

square-based pyramids (half-octahedra) and octahedra

These are the most fundamental “bricks” for constructing

anion-deficient perovskite structures

It must be pointed out that although the structural

modules proposed in Fig 20 assume that the atom sites are

the same as in perfect perovskite structures, in practice,

lattice relaxation/distortion is possible due to unbalanced

anion coordination and oxygen vacancies Fine-tuning of

the structure must rely on quantitative fitting of X-ray or

neutron diffraction data

STRUCTURAL EVOLUTION IN THE FAMILY

OF PEROVSKITES

High-Temperature Superconductors

In high Tc superconductors, the B cation is Cu that has

+1, +2, and +3 valence states and coordination

num-bers 2, 3, and 4 for Cu(I) and 4 and 6 for Cu(II) and

Cu(III) If the A and B cations in ABO3 −x have valences

+3 and +2, respectively, the (AO3)4 − layers must be

re-placed by(AO3 −x)(3 −2x)− Because the Cu cation (the B

cation) may have valence+1, +2, and +3, the

perovskite-type cell that contains oxygen vacancies gives the Cu

cation the possibility of disproportionating its valence from

+2 to +1 plus +3 Figure 21a gives the module shownpreviously in Fig 20(l) The B cation in this module hastwo types of coordinations: BO5 and BO4 If we flip thismodule over, as shown in Fig 21b, and combine thesetwo by superimposing the BO4 units of the two, we ob-tain the new module given in Fig 21c This new module

is the building block for the Y–Ba–Cu–O system of high

Tcsuperconductors and introduces Ba cations between themodules The structure of YBa2Cu3O7is given in Fig 21d.Combining BO4 units and the YBa2Cu3O7 module(Fig 21d) produces the structure of YBa2Cu4O8(Fig 21f).Stacking YBa2Cu3O7and the YBa2Cu4O8modules createsthe structure of Y2Ba4Cu7O15(Fig 21g)

Anion Deficiency-Induced Brownmillerite Structure

As we discussed before, the ABO3 type perovskite has afundamental stacking layer (AO3)4 −in which the A cationsplay a key role in creating oxygen vacancies In general,the A cations have the valence+2 and an ionic character,which means that their coordination numbers are higherthan six, for instance, 12 If the A cations have the valence+3 and an ionic character, the oxygen in the (AO3)4 −layersmust be modified by creating oxygen vacancies The charge

of the layer is (AO3)3 −; thus, the B cation layers must bemodified to balance the local excess positive charge The

B cations usually have polarization or partial covalence,and they have favorable coordination numbers for reduc-ing energy If the valence of partial A cations is changedfrom +3 to +2, the negative charge of (AO3) layers in-creases from −3 to −(3 + x), where x depends on therelative population of A2 + Simultaneously, part of the Bcations should modify their valence state from +3 to +4

to balance the local charge The percentage of B cationswhose valence is modified is related to the x value, as

La1 −xSrxMnO3 Mn3 +and Mn4 +cations are both favorablefor six-coordinated octahedra, but the situation is differentfor Co cations Both Co3 +and Co4 +can have six and four co-ordinations Four-coordinated Co4 + requires the presence

of oxygen vacancies; thus, the (AO3) layer is replaced by(A31+−xA2+x O3 −y)(3 +x−2y)−

On the other hand, if some B cations are reduced andtheir valence state is decreased from+3 to +2, the (AO3)3 −layers are required to compensate for the excess negativecharge, resulting in oxygen vacancies and the change in

Figure 21 Evolution of the oxygen-deficient perovskite modules into the crystal structures of the

Y–Ba–Cu–O system (a) The module given in Fig 20(l); (b) the module is flipped over vertically and horizontally (c) The modules in (a) and (b) are combined by superimposing the BO4units to form a new module, which is the structural building block for YB2Cu3O7 (d) The structure of YB2Cu3O7 (e) A new module created by combining the two modules in (a) and (b) to share the edges of the

BO4units This new module is the building block of the structure of YBa2Cu4O8 (f) The structure

of YBa2Cu4O8 (g) The structure of Y2Ba4Cu7O15is a combination of the modules in (c) and (e).

the coordination number of the B cations If the percentage

of reduced B cations is small, the perovskite structure still

holds If the percentage of reduced B cations reaches an

upper limit, the perovskite structure has to be changed

to another structure that might be related to perovskite

The structural evolution from perovskite into the

brown-millerite structure is an example

ACoO3(A= La, Pr, Nd, Gd) perovskite is a typical

exam-ple Methane gas can be oxidized by LaCoO3above 1000◦C

This means that LaCoO3can release lattice oxygen (28,29)

Co2 +cations can have coordination numbers of six, five, and

four During the reduction process, Co3 + can be reduced

to Co2 + and Co0 The coordination is changed from

octa-hedral to tetraocta-hedral During the reduction process, the

anion framework should hold, but it can have vacant sites

The perovskite unit and the possible corner-sharing

tetra-hedron chain are shown in Fig 22a When these chains

are connected to neighboring octahedra, the remaining two

corners of each tetrahedron are shared If we randomly

insert these tetrahedra chains into the perovskitestructure (Fig 22b), the compound is ABO3 −x As thenumber of tetrahedron chains increases and reaches anumber at which two octahedron slabs (that have the thick-ness of the perovskite unit cell) and one corner-sharingoctahedron slab are separated by a “slab” of the tetra-hedron chains, as shown in Fig 22c, the ABO2.75 struc-ture is formed Ordered structures of A2 +(B3+0.5 B4+0.5)O2.75

and A2 +(B2+0.5 B5+0.5)O2.75 can be formed If the octahedronslab and the slab of the tetrahedron chains are stacked al-ternately via corner-sharing, the brownmillerite structureABO2.5is constructed (Fig 22d) As the relative number ofthe tetrahedron chains increases, the interaction between

A cations also increases If the number of the tetrahedronslabs is more than that of the octahedron slabs, the systemwill be unstable Then, LaCoO3 −y is likely to be reduced

to two phases: La2O3and CoO In other words, all of the

Co2 + will have tetrahedral coordination, and there is nooctahedral-coordinated Co2 +

Tailoring Perovskite-Related Structures

In general, compounds that have perovskite-like

struc-tures can be represented by the chemical formula

AmBmO3m −x In this formula the (AO3)4 −and the B4 +layers

are the basic stacking layers, and they are stacked

alter-nately following the sequenceαβγ , respectively, as defined

by the three positions shown in Fig 17 If the compound

contains m layers of (AO3)4 −and m layers of B4 +and they

are stacked together in the sequence αβγ , respectively,

alternately, we can have AmBmO3m, such as ABO3 when

m= 1, the perovskite structure If oxygen vacancies are

in-troduced, an (AO3)4 −layer may have one oxygen sublattice

possessing a vacancy, and the layer (AO3)4 −is transformed

into (AO2)2 − If these types of anion-deficient layers are

mixed with the ideal (AO3)4 − layers, the new compounds

should have the formula AmBmO3m −n (where n≤ m), in

which n represents the number of the (AO2)2 −layers that

contain sublattices that have oxygen vacancies Ca2Mn2O5

(m= 2 and n = 1), YBa2Cu3O6or YBa2Cu3O6(m= 3 and

n = 3), YBa2Cu3O7 (m = 3 and n = 2), LaBa2Cu2TaO8

(m= 3, n = 1), and Ba2La2Cu2Sn2O11(m = 4 and n = 1)

are typical examples

Furthermore, if the (AO3)4 − layer lack only a portion

of the oxygens, the composition and the charge of the

layer becomes (AO3 −x)(4 −2x)− If n and m are the numbers

of the (AO3 −x)(4 −2x)−type and the (AO3)4 −type layers,

re-spectively, the compounds formed by stacking these layers

and a total of (m+n) B cation layers alternately, should

be Am +nBm +nO3(m +n)−xn For example, when n= m = 1, it

is A2B2O6 −x (x < 1) These two types of oxygen-deficient

perovskite compounds have been found Compounds that

have (AO3 −x)(4 −2x)−layers may be more stable than those

that have (AO2)2 −layers The high Tcsuperconductor is an

example (21)

Compounds that have some (AO2)2 − layers may have

an integral number of oxygens, but the compounds that

have (AO3 −x)(4 −2x)−layers may have a nonintegral number

of oxygens As discussed before, the A cation can be

sub-stituted by cations that have different valences or by

par-tial cation vacancies to satisfy the local electron orbital

and charge balance requirements These results may help

us to understand the structure of oxide functional

mate-rials Most of the useful compounds that have

functional-ity contain anion-deficient perovskite-like structural units,

especially those that contain (AO3 −x)(4 −2x)−layers The 14

structural configurations shown in Fig 10 are the

fun-damental building blocks for building these structures

The perovskite-related compounds that have A cation

and/or oxygen anion deficiency may have the general

formula

A[(m +n)−yn]B(m +n)O3(m +n)−xn

where 0≤ x < 1, 0 ≤ y < 1, 0 ≤ n < m, and n and m are

integral numbers

Although A or B cations can be partially substituted by

another element, the average valences are usually close

to+2 and +4 (see later), respectively The A cation, for

example, may be replaced by (A+0, A3+) or (V+, A 3+), where

V stands for a positively charged vacancy site, and B cation

Figure 23 High-magnification TEM images of

La 0.5Sr 0.5CoO 2.25 recorded along [100], where the white spots correspond to the projected atom columns.

can be (B3+, B5+) or (B2+, B6+) Substitution of thesecombinations can give a series of compounds that belong tothe perovskite family Cations of different valences usuallyhave different sizes and electron configurations, and theytend to have a strong influence on the oxygen close packing,especially in the (AO3)4 −layers A change in the valence ofthe A cation should have a much stronger effect because it

is in the fundamental stacking layer

Figure 23 shows a cross section of a TEM image of

La0.5Sr0.5CoO2.25viewed along [100] The La and Sr atomsare distributed in different (001) atomic planes and exhibitLa–Co–Sr–Co–La–Co–Sr–Co– (001) layered structure (31).The crystal structure is based on a fundamental perovskitemodule of LaSrCo2O6 (or La0.5Sr0.5CoO3) without aniondeficiency, as shown in Fig 24a, which is a combination

of two perovskite unit cells of LaCoO3 and SrCoO3 Thestructure of La0.5Sr0.5CoO2.25 is composed of eight of thistype module that has ordered anion vacancies in each.Two anion-deficient modules of LaSrCo2O4.5 are derivedfrom this stoichiometric module, denoted by M1 and M2(Fig 24b) A divalent Co is likely to be coordinated by oneoxygen, on average, in the top and bottom layers Thesemodules are the building blocks for constructing the fullunit cell of La0.5Sr0.5CoO2.25(Fig 24c) The unit cell is or-thorhombic The 3-D atomic distribution in the full unitcell is given in Fig 24d The coordination numbers of Laand Sr are nine, respectively, and those of Co are five andfour (Fig 24e); thus, the structure is chemically stable.The three-oxygen-coordinated cation is a tetrahedron, thefour-oxygen-coordinated cation is a square sheet, and thefive-oxygen-coordinated cation forms a half-octahedron(e.g., a square-based pyramid)

QUANTIFICATION OF MIXED VALENCES BY EELS

Transition- and rare-earth metal oxides are the

fundamen-tal ingredients of mixed valences in the structural unit (2).

The valence states of metal cations in such materials can be

chemically determined by using the redox titration, but it

is inapplicable to nanophase or nanostructured materials

such as thin films The wet chemistry approaches usually

do not provide any spatial resolution X-ray photoelectron

spectroscopy (XPS) can provide information on the average

distribution of cation valences for nanostructured rials that have certain spatial resolution, but the spatialresolution is nowhere near the desired nanometer scale,and the information provided is limited to a surface layer2–5 nm thick

mate-Electron energy-loss spectroscopy (EELS), a ful technique for materials characterization at nanome-ter spatial resolution, has been widely used in chemicalmicroanalysis and studies of solid-state effects (32) InEELS, the L ionization edges of transition-metal and

power-rare-earth elements usually display sharp peaks at the

near edge region, which are known as white lines In

transi-tion metals that have unoccupied 3d states, the transitransi-tion

of an electron from the 2p state to the 3d levels leads to

the formation of white lines The L3 and L2lines are the

transitions from 2p3/2to 3d3/23d5/2and from 2p1/2to 3d3/2,

respectively, and their intensities are related to the

unoc-cupied states in the 3d bands (33)

Numerous EELS experiments have shown that a

change in the valence state of cations introduces a dramatic

change in the ratio of the white lines, leading to the

possi-bility of identifying the occupation number of the 3d orbital

using EELS The 3d and 4d occupations of transition-metal

and rare-earth elements have been studied systematically

(34,35) The oxidation states of Ce and Pr have been

de-termined in an orthophosphate, in which the constituents

of Ce and Pr are of the order of 100 ppm (36) It has been

applied to quantifying the valence transition in Mn and

Co oxides (37), determining the concentration of oxygen

vacancies (38), refining the crystal structure of an

anion-deficient perovskite (31), identifying the crystal structure

of nanoparticles (CoO and Co3O4) (39), and determining

magnetic ordering in spinel (40) The principle of this

analysis is illustrated following

Figure 25 shows an EELS spectrum of Co oxide The

EELS data must also be processed first to remove the

gain variation introduced by the detector channels and to

10K 20K 30K 40K 50K 60K 70K 80K

Figure 25 An EELS spectrum acquired from a Co oxide that

shows the O–K and Co-L ionization edges The fine structures

arise from the atomic and solid structure of the specimen (b) The

technique used to extract the intensities of the white lines.

decon-EELS analysis of valence states is carried out by ence to the spectra acquired from standard specimens thathave known cation valence states Because the intensityratio of L3/L2is sensitive to the valence state of the corre-sponding element, if a series of EELS spectra is acquiredfrom several standard specimens that have known valencestates, an empirical plot of these data serves as the refer-ence for determining the valence state of the element in

refer-a new compound The L3/L2 ratios for a few standard Cocompounds are plotted in Fig 26a EELS spectra of Co-L2,3

ionization edges were acquired from CoSi2 (Co4 +), Co3O4(Co2.67+), CoCO3 (Co2 +), and CoSO4 (Co2 +) Figure 26b

is a plot of the experimentally measured intensity ratios ofwhite lines L3/L2for Mn The curves clearly show that theratio L3/L2is very sensitive to the valence state of Co and

Mn This is the basis of our experimental approach for suring the valence states of Co and Mn in a new material

mea-To demonstrate the sensitivity and reliability of ing white line intensity for determining valence states in

us-1012 SMART PEROVSKITES

Co L3/L21/3Co2 + + 2/3Co3 +

In-situ temperature (°C)

300 400 500

Co2 +

Figure 27 An overlapped plot of the white line intensity ratio

of Co L 3 /L 2 and the corresponding chemical composition of n O /n Co

as a function of the in situ temperature of the Co3 O 4 specimen

that shows the abrupt change in valence state and oxygen

com-position at 400 ◦C The error bars are determined from the errors

introduced in background subtraction and data fluctuation among

spectra.

mixed valence compounds (37), the in situ reduction of

Co3O4 is examined first Figure 27 shows the Co L3/L2

ratio and the relative composition of nO/nCofor the same

crystal, as the specimen temperature was increased The

specimen composition was determined from the integrated

intensities of the O–K and Co-L2,3ionization edges by

us-ing ionization cross sections calculated from the SIGMAK

and SIGMAL programs (32) The L3/L2ratios

correspond-ing to Co2 + determined from the EELS spectra of CoSO4

and CoCO3at room temperature and Co2.67+obtained from

Co3O4are marked by shadowed bands, whose widths

rep-resent experimental error and the variation among

dif-ferent compounds The Co L3/L2 ratio and the

composi-tion, nO/nCo, simultaneously experience a sharp change

at T = 400◦C The chemical composition changes from

O:Co= 1.33 ± 0.5 to O:Co = 0.95 ± 0.5 that

accompa-nies the change of the average valence state of Co from

+2.67 to +2 when the temperature is above 400◦C

Elec-tron diffraction has also confirmed the reduction process

observed

HIGH-SPATIAL-RESOLUTION MAPPING

OF VALENCE STATES

Energy-filtered transmission electron microscopy

(EF-TEM) (41) is a rapidly developing field for high

spatial-resolution chemical imaging Images (or diffraction

patterns) formed by electrons that have specific energy

losses can be obtained by using an energy filter If the

energy-selected electron image can be formed using the

white lines described earlier 6, one can map the spatial

dis-tribution of the valence states (42) According to Fig 25, an

energy window 12 eV wide is required to isolate the L3from

the L2white lines A five-window technique is introduced

(see Fig 25a): two images are acquired at the energy lossesbefore the L ionization edges, and they are to be used to sub-tract the background for the characteristic L edge signals;two images are acquired from the L3and L2white lines,respectively, and the fifth image is recorded using the elec-trons right after the L2line that will be used to subtract thecontinuous background underneath the L3 and L2 lines.Then, a L3/L2ratio image will be obtained, which reflectsthe distribution of valence state across the specimen Itmust be pointed out that the thickness effect has been re-moved in the L3/L2image

A partially oxidized CoO specimen that contains a CoOand Co3O4grain structure was chosen for this study (44).The CoO and Co3O4phases are separated by clear bound-aries, and it is an ideal specimen for testing the optimumresolution Figure 28 shows a group of energy-filtered TEMimages from a triple point in the CoO–Co3O4 specimen.The energy-filtered images using the L2 and L3lines andthe post-L2line region (Fig 28b–d) show distinctly differ-ent contrast distributions due to differences in the relativewhite line intensities From these three images, the L3/L2ratio is calculated after subtracting the contribution fromthe continuous energy-loss region that is due to single atomscattering, the image clearly displays the distribution ofcobalt oxides that have different valence states (Fig 28e),where the diffraction contrast disappears The region oflower oxidation state (Co2 +) shows a stronger contrast, andthose that have high oxidation states show darker contrast(see the L3/L2ratio in Fig 26a) The O/Co image (Fig 28f)was calculated from the images recorded from the O–Kedge and the L3 + L2 white lines for an energy windowwidth of = 24 eV Although the energy-filtered O–K edge

image exhibits some diffraction contrast and the thicknesseffect, the O/Co compositional ratio image greatly reducesthe effect The high-intensity region in the O/Co imageindicates a relatively high local concentration of oxygen(e.g., higher Co oxidation states); the low intensity regioncontains relatively less oxygen (e.g., a lower Co valencestate), entirely consistent with the information provided

by the L3/L2 image A line scan across the valence statemap clearly illustrates that a spatial resolution of 2 nmcan be achieved (42) This is remarkable compared to anyexisting techniques

SUMMARY

Perovskite is probably the most important structural type

of smart materials The properties of perovskites dependstrongly on their structures In this article, the character-istics of ABO3perovskites are analyzed to reveal the in-trinsic connection among the A, B, and O elements andthe roles of tetrahedrons and octahedrons in the structure.The (111) alternate stacking of the close-packed (AO3)4 −and B4 + layers is responsible for the cation substitutionand the creation of anion vacancies Careful analysis of thepossibilities of creating oxygen vacancies results in a total

of 14 fundamental structural units, which are the buildingblocks for constructing the unit cells of complex functional

materials, such as high Tcsuperconductors

BF-TEM (a)

Figure 28 A group of images recorded from the same specimen region using signals of (a)

elec-trons without energy loss, (b) the Co-L 2 edge, (c) the Co-L 3 edge, and (d) the post-Co-L 2 line (e) The processed L 3 /L 2 image that displays the distribution of valence states (f) The atomic concentration ratio image of O/Co The O/Co image is normalized with reference to the standard composition of CoO for the low portion of the image to eliminate the strong influence of the white lines on the ionization cross section Each raw image was acquired using an energy window width of = 12 eV

except for O–K at = 24 eV (g) and (h) are line scan profiles from (e) and (f), respectively, proving

the achievement of 2-nm spatial resolution.

2 Z.L Wang and Z.C Kang Functional and Smart Materials—

Structure Analysis and Structural Evolution Plenum Press,

NY, 1998, Chap 3.

3 C.N.R Rao and B Raveau, Transition Metal Oxides, VCH,

1995.

4 F.S Galasso, Perovskites and High T c Superconductors,

Gordon and Breath, 1990.

5 O Auciello, J.F Scott, and R Ramesh, Phys Today, pp 22–27

(July 1998).

6 M Tanaka and G Honjo, J Phys Soc Jpn 19: 954–970 (1964).

7 B Jeffe, W.B Cook, and H Jaffe, Piezoelectric Ceramics.

Academic Press, London, 1971.

8 A.J Moulson and J.M Herbert, Electroceramics—Materials,

Properties, Applications Chapman & Hall, London, 1990.

9 K Uchino, Piezoelectric Actuators and Ultrasonic Motors.

Kluwer Academic, Boston, 1997.

10 S Nomura, M Mizuno, J Kuwata, M Abe, and J.C Burfoot,

Ferroelectrics 23: 183–186 (1980).

11 Z.C Kang, C Caranoni, I Siny, G Nihoul, and C Boulesteix,

J Solid State Chem 87: 308–320 (1990).

12 R.E Newnham, MRS Bull pp 20–34 (July 1997).

13 S Jin, T.H Tiefel, M McCormack, R.A Fastnacht, R Ramech,

and L.H Chen, Science 264: 413–415 (1994).

14 R von Helmolt, J Wecker, B Holzapfel, L Schultz, and K.

Samwer, Phys Rev Lett 71: 2331–2334 (1994).

15 D.B Studebaker, M Todd, T.H Baum, Y Berta, and Z.L Wang, unpublished.

16 G.H Jonker and J.H Van Santen, Physica 16: 337–349 (1950).

17 C Zener, Phys Rev 82: 403–405 (1951).

18 J.B Goodenough, Met., in Progress in Solid State Chemistry,

H Reiss, ed., Pergamon Press, 1971, Vol 5, pp 145–399.

19 P.G de Gennes, Phys Rev 118: 141–154 (1960).

20 Z.L Wang and J Zhang, Phys Rev B 54: 1153–1158 (1996).

21 A.Q Pham, M Puri, J.F Dicarlo, and A.J Jacobson, Solid State

Ionics 72: 309–313 (1994).

22 B Reveua, C Michel, M Hervieu, and D Groult, Crystal

Chemistry of High T c Superconducting Copper Oxides.

26 M.A Beno, D.W Soderholm, D.W Capone, J.D Jorgensen, K.I.

Schuller, C.U Serge, K Zhang, and J.D Grace, Appl Phys.

1014 SOIL-CERAMICS (EARTH), SELF-ADJUSTMENT OF HUMIDITY AND TEMPERATURE

29 A Baiker, P.E Marti, P Keusch, and A Reller, J Catal 146:

268–276 (1994).

30 H.L Yakel, Acta Crystallogr 8: 394–398 (1955).

31 Z.L Wang and J.S Yin, Philos Mag B 77: 49–65 (1998).

32 R.F Egerton, Electron Energy-Loss Spectroscopy in the

Electron Microscope 2e, Plenum Press, NY, 1996.

33 F.M.F de Groot, M Grioni, and J.C Fuggle, Phys Rev B 40:

5715–5723 (1989).

34 H Kurata and C Colliex, Phys Rev B 48: 2102–2108 (1993).

35 D.H Pearson, C.C Ahn, and B Fultz, Phys Rev B 47: 8471–

39 J.S Yin and Z.L Wang, Phys Rev Lett 79: 2570 – 2573 (1997).

40 Z.J Zhang, Z.L Wang, B.C Chakoumakos, and J.S Yin, J Am.

Chem Soc 120: 1800–1804 (1998).

41 L Reime, ed., Energy-Filtering Transmission Electron

Microscopy Springer Series in Optical Sciences, Vol 71.

Springer, Berlin, 1995.

42 Z.L Wang, J Bentley, and N.D Evans, J Phys Chem B 103:

751–753 (1999).

43 J Bentley, S McKernan, C.B Carter, and A Revcolevschi,

Microbeam Anal 2 (Suppl.): S286–287 (1993).

SOIL-CERAMICS (EARTH), SELF-ADJUSTMENT

OF HUMIDITY AND TEMPERATURE

INAX Corporation

Minatomachi, Tokoname, Aichi, Japan

INTRODUCTION

Earth ceramics are hydrothermally solidified soil bodies

that have been developed mainly for use as interior and

exterior materials for housing Generally, manufacture of

industrial materials requires the use of refined raw

ma-terials In the case of earth ceramics, a wide range of soils

can be used for manufacture without extensive refining

Besides, since solidification is carried out at 150 to 200◦C

under saturated steam pressure, the energy required for

solidification is relatively small Therefore, the load

im-posed on the earth by the manufacturing of this material

is kept very low Earth ceramics possess excellent thermal

insulation and self humidity control properties because

the original nano-pores of the soil remain in the bodies

after solidification Because of these properties, earth

ceramics are effective in controlling the interior climate in

dwellings, particularly in monsoon regions like Japan It

has been reported that in dwellings where earth ceramics

were actually used as the floor material, the humidity

control properties of the material allows comfortable and

healthy living with almost no necessity for the use of air

conditioners throughout the year and that the amount of

energy consumed for living is reduced significantly

The necessity of a new concept of manufacturing goodsbased on due consideration for humans and the earth, andthe thinking behind the development of earth ceramicsbased on investigations of this concept, are described below.Examples of actual use of earth ceramics are also outlined

A NEW DEFINITION ON MATERIALS WITH CONSIDERATION FOR HUMANS AND THE EARTH The Inevitability of a Recirculation-Based Society

The global environment has now begun to have great fects on our lives Environmental issues such as globalwarming, desertization, depletion of the ozone layer, andacid rain stem mainly from global scale expansion of theeconomic activities of the industrially advanced nationsand population expansion in the developing countries.These two problems are compelling enough for us to con-sider radical reforms of the global social structure

ef-In their book Beyond the Limits published in February

1992, Meadows and his co-authors (1) warned that less countermeasures are taken, the world economy whichmay still expand until around the year 2020, will cease itsgrowth after that because of various limiting factors, andthat the world’s social structure will have disintegratedcompletely by the year 2100 A number of similar reportssupplementing these warnings were published (2,3) andcreated a great stir among material scientists

un-The United Nations Conference on Environment andDevelopment (Earth Summit) held in Brazil in June of thesame year issued the Rio Declaration on Environment andDevelopment, adopting Agenda 21 as the action plan ofthe summit Out of all the activities carried out by hu-mans, those that impose the largest load on the earth arethe economic and industrial activities These activities de-pend on a flow of materials and energy To counter thedanger of extinction of humankind and build a sustain-able society, it is essential that we institute a recirculation-based society A recirculation-based society is a rejection

of the consumption-based economic structure of the dustrialized countries The idea is to construct a type ofsociety not ever experienced in the past What will themanufacture of goods be like in a recirculation-based soci-ety? Here, we need to redefine the basic notion of manu-facturing

in-Manufacturing of Goods with Awareness

of a Recirculation-Based Society

A recirculation-based society can be expressed by a simplediagram as shown in Fig 1 Since the dawn of civilizationthe human being has demarcated a border (system bound-ary) between the natural ecological system and the humanlife system (human ecological system)

Humans thought that they could not sustain their hood without exploiting nature The amount of intakefrom nature and the resulting amount of release into na-ture he increased rapidly after the Industrial Revolution.These now far exceed what nature can offer without se-rious consequences for the preservation of humankind Arecirculation-based society is one that reduces much of the

Natural resourcesfossil fuels

System border

Output

Waste materialswaste gases

Developmentmanufacturingdistribution

Consumptionmaintenance

Recovery

Human ecosystemNatural ecosystem

Figure 1 Re-circulation based society.

input from nature (fossil energy, raw materials) as well as

its output into nature’s ecosystem (exhaust gases, exhaust

heat, waste materials), and makes efforts to recirculate

and regenerate the input from nature within the human

ecosystem In other words, when manufacturing goods for

a recirculation-based society, it is very important to think

about the total energy balance and the material balance

In practice, it is necessary to reduce the output by reducing

the input, and in addition, synthesize new materials from

materials and energy that have the possibility of

produc-ing fresh output We must, therefore, change our attitude

from recycling to recirculation

Manufacturing of Goods with Consideration for the Earth

Although manufacturing of goods with an awareness of

a recirculation-based society is unavoidable, it is

doubt-ful whether goods manufactured based on such a concept

would be accepted by the world Let us consider an extreme

example An electric refrigerator not only consumes

elec-tricity but also the refrigerating agent, which is Freon gas

and a burden to the earth’s environment In order to lower

the input and output, should we then stop using electric

refrigerators and return to the earlier era of ice-boxes for

refrigeration? Most people would answer no It is not easy

to abandon a convenience once it has been experienced

Furthermore, denial of the existence of electric

refriger-ators means also denial of large businesses that sell

re-frigerated foods like department stores, supermarkets, and

convenience stores The invention of the electric

refrigera-tor brought great changes to the social system As long as

irreversibility of life values exists, we cannot easily return

to the “good old age.” If we reach an ultimate state where

there is no alternative but to return to the past such a

re-versal would probably be at the expense of unprecedented

patience and great pain Conversely, if it were easy to

re-turn to a former way of living, environmental problems

would not occur, there might not be the need to developnew materials Our inability to return to a former, energy-efficient way of living is why we need to address the seriousissue of environmental pollution and set guidelines for newmaterials:

Most important, the manufactures need to be mademore conscious of the earth The goods they developeshould be useful, convenient, or improvements of existinggoods that are necessary to people Of course, as environ-mental problems become more severe, the balance betweenpeople and the earth will shift, and without doubt, moreemphasis will start to be placed on earth friendly mate-rials and manufacturing Materials and goods producedwithout consideration of their value or usefulness to peo-ple would cease to exist Taking this argument from a dif-ferent perspective, no one could argue that the manufac-turing of goods does not exploit something (input) from theearth and then discharge some waste to the earth (output)which is produced as the input from the earth is convertedinto goods with functions that have some value to people(Fig 2) We could express this relation of the development

of goods as,

Value= P

I + O ,

where P is the performance of the material, I + O are

the input and output of the material when manufactured,used, and scrapped

In general, if the value for people is less than one, there

is no merit to developing the material or goods ment should aim to raise this value to five, ten, or eventhe hundreds Attain entirely new approaches, might benecessary, disregarding any current concepts Now, let usthink specifically about materials that could be developed

Develop-using the P /(I + O) valuation concept.

1016 SOIL-CERAMICS (EARTH), SELF-ADJUSTMENT OF HUMIDITY AND TEMPERATURE

Figure 2 A new value for the manufacturing.

SMART MATERIALS FOR THE LIVING ENVIRONMENT

Foremost among materials for maintaining a comfortable

living space, while reducing the burden on the earth’s

environment, is the development of high heat insulation

houses Of course, this is not a consideration in hot and

humid regions with monsoon climates Primarily, there is

the case of Japanese dwellings

Japan, which is located in a monsoon zone, has a

relatively distinct climate compared to other developed

nations In this unique hot and humid climate, much

dam-age is caused by the high humidity Although the

humid-ity itself is lower than that of say, London, Paris, or San

Francisco, Japan is at the top with regard to

humidity-related damage to houses This is due to the fact that fungi

and bacilli that affect human health and cause damage

to houses are able to breed rapidly under the warm and

humid Japanese climate To counter these adverse

influ-ences, elevated houses emerged in Japan as far back as the

nineth century Elevated floors allow for underfloor

ven-tilation The materials used for construction were paper,

wood, and soil Some 50 years ago airtight houses were

in-troduced in Japan following examples in Europe and

Amer-ica A new apartment house building trend then took root

in Japan However, it has turned out that the Western

air-tight dwellings were unsuited to a humid climate, and thus

uncomfortable for living The search for comfort led to the

introduction of various indoor implements—electric fans,

followed by coolers and air conditioners Then the first oil

shock in 1973 forced Japanese to make significant tions in their energy consumption, and the idea of airtightand heat-insulated housing became national policy In theadoption of such dwelling, the technology to control indoortemperature has improved considerably, although the hu-midity has proved to be difficult to control Because therooms in Japanese houses are small, the interiors of theseairtight dwellings tend to be inferior compared to houses inEurope and America (4, 5) In fact the interior environment

reduc-in Japan has degenerated to a clutter of such applianceshumidifiers, dehumidifiers, and air cleaners (Fig 3) Yet,despite these measures, the sick house syndrome and al-lergic diseases are continuing to proliferate Compared to

1973, when the industrial energy consumption in Japan in

1997 was 104%, today dwellings consume energy as high

as 217%, and this figure has been rapidly climbing (6) Theenergy increases have occurred even though much progresshas been made in the development of low-energy consump-tion type of appliances

The technological solution to this problem would be tohave a self-monitoring and self-regulating the indoor cli-mate (humidity, in particular) by materials of the house—

the floor, walls, and ceiling materials—that have high P

value At the same time, it is necessary to examine themethods of synthesizing these materials from the perspec-tive of not using even more energy and natural resources

for the synthesis—that is, to maintain a low (I + O) value.

If such materials could be developed, indoor climate controlcould made effective even in the most airtight and heat-insulated homes Such materials would possess high value

as those to society with due consideration to humans andthe earth

CLIMATE CONTROL BY POROUS BODIES

The humidity range in which a person feels comfortable

is said to be 40% to 70% It has been reported that bymaintaining humidity within this range, allergy sourcessuch as mites, as well as the breeding of wood-eating bac-teria, molds, and the like, that cause degradation of wood

in wooden houses, could be restricted (7) This humidityrange is also thought to be effective in curbing the spread

of viruses and even the accumulation of static electricity.Chemical and physical methods of humidity regulation areavailable, but here, let us think about a safe method, which

is humidity regulation by porous bodies

The target material would be one that does not absorbthe water vapor when the humidity is less than 40%, but

if the humidity rises higher, it should work to lower thehumidity by rapidly absorbing the water vapor from theatmosphere Then, as the humidity starts to fall, the ma-terial should act to increase the humidity rapidly to thepreferred 40% to 70% In other words, the water vapor ab-sorption isotherm of the material should be steep in the40% to 70% humidity range (Fig 4)

So these humidity-regulating porous materials should

be capable of making the water vapor in the atmosphere

to condense within the capillary pores that exist on theirsurface when the humidity is high Conversely, when thehumidity is low, they should function to vaporize the

Advanced materials

Cut of all drafts

Using much electricity

Energy-saving andcomfortable life!!

Leave it to me !!

We hate this heat

Air-conditioner

Electric fanExtractor fan

Summer is coolbut winter

Drier

Figure 3 Earth and people conscious materials for the living environment.

condensed water The relation between the vapor pressure,

P/P0, required for capillary condensation and pore size,

with curvature radii r1, r2, is expressed by Kelvin’s

equa-tion of capillary condensaequa-tion:

Here, r1 and r2are the radii of curvature of the pores in

two perpendicular directions,γ is the surface tension of the

condensate, V is the molecular volume of the condensate,

andθ the contact angle of the condensate within the pore.

Calculations based on this equation, corrected for the

preexistence of a certain thickness of the adsorbed layer

prior to capillary condensation (8), yield pore radii values

of 3.2 nm for 40% relative humidity and 7.4 nm for 70%

humidity High-humidity regulating performance can be

expected from materials synthesized with their pore radiibeing controlled to be within this range

USING THE GREATNESS OF NATURE WISELY Utilizing Soil

There are many possibilities of synthesizing porous terials with humidity-regulating properties For example,taking petroleum as the starting material, the synthesiscould be done by chemical polymerization or biomimeticmethods However, the use of these methods makes itdifficult to lower the input and output of the synthesizedmaterial For this reason, we should select “soil,” as thestarting material Natural soil is a material containing in-cipient micropores that can be effective in imparting hu-midity regulating performance Even after its use in the

ma-1018 SOIL-CERAMICS (EARTH), SELF-ADJUSTMENT OF HUMIDITY AND TEMPERATURE

Figure 4 Property for the humidity-regulating porous

mate-rials.

human ecosystem is over, soil will not inflict a large

bur-den on the natural ecosystem

Soil appeared some 400 million years ago at a time when

plenty of oxygen was supplied by the atmosphere The

com-position of soil is almost the same today The excess

oxy-gen was decomposed in the stratosphere to form the ozone

layer The ozone layer has prevented strong ultraviolet rays

from pouring onto the surface of the earth, and allowed the

movement of animals and plants from the sea to land to

be-gin With the help of this organic matter, the land of stone

and sand turned into a land of green, and soil appeared for

from the weathering and decomposition of rocks (9) Were

it not for soil, the perfect recirculation performance of the

current natural ecosystem would not exist So one cannot

ignore the benefits obtained from soil for the existence of

humankind Humankind is, of course, indebted to soil with

regard to food crops, but more so, it was through the aid of

earthen dwellings that humankind was able to survive the

glacial era without running out of seed

Soil contains numerous pores, both large and small

These pores collect air, water, and many nutrients,

allow-ing soil to carry out its functions For example, a survey of

the virgin forests of the Shiga plateau (Japan) has shown

that in a 1 m by 1 m by 15 cm volume of soil, there are

360 living creatures such as centipedes and earthworms

ranging about 2 cm in size, then some 2 mm in size thread

earthworms, beach fleas amounting to 2.3 million in count,

and finally any number of protozoan, mold, and bacteria

0.010

50

100

1Pore diameter (µm)

100

Figure 5 Examples for the pore size distribution of the general

soil after dry-pressed under 30 MPa (solid line) and 20 MPa ken line).

(bro-of more than 10 trillion in count (10) Figure 5 shows thepore size distribution of common earth (soil) It is clear thatthe 10 nm (0.01µm) pores considered suitable for humidity

regulation are incipient in the material It is also clear thatthe cohesive structure in the neighborhood of 10 nm doesnot collapse easily even under pressure However, evenany kinds of soil if lost for some reason or the other, can-not be regenerated for an extremely long time Further-more, because of its complex structure, soil is a materialthat has not been successfully synthesized artificially up tonow

Humankind has utilized earth very cleverly as a struction material in many ways In its natural state, ithas been used for cave-type and pit-type dwellings Soilhas been processed and used to obtain various types ofconstruction materials such as sun-dried bricks In Japan,

con-it was used as Tataki, or earthen walls The humidcon-ity

reg-ulation and thermal insreg-ulation properties arising from theinnumerable pores in soil were utilized in Japan’s Edoperiod to build earthen storehouses that protected stockfrom wind, fire, and water The technology of soil uti-lization was even raised to an artistic level, as can be

seen from the Nurikabe walls of the Edo era Once these

soil materials have served their purpose in the humanecosystem, they can be returned to the natural ecosys-tem Therefore, soil is an extremely rare material thatcan cross the system boundary zone freely Unfortunately,the use of earth as soil, is usually not the possible in cur-rent construction practice If, for example, soil is used inits natural state for the flooring of today’s airtight andheat-insulated dwellings, the house would become dustyand the health of the inhabitants may be affected adversely

In addition, there are problems regarding strength, bility, and workability

Sun - dried bricksoil wall

< 200 °C

Roomtemp

Figure 6 A culture of soil in human history.

Solidified ceramics such as bricks, blocks, and tiles made

their appearance early in history in order to solve these

problems However, ceramics, which could be considered

to have been developed in the Stone Age, are produced

through high-temperature reactions For this reason, it is

difficult to say that ceramics maintain the inherent

prop-erties and performance of soil In order to sustain the

inherent properties and performance of soil (pore size

distribution), the temperature of manufacture of earthen

products must be lower than 500◦C In the case of

earth/organic material composites, an even lower

temper-ature is desirable

The new technology of solidifying soil by hydrothermal

treatment is a low-temperature process developed to

ob-tain a material with properties and performance between

those of soil and ceramics (6)

Hydrothermal Processing

The largest application of hydrothermal processing is in

the field of building materials manufacture It was

de-veloped in Europe and has over 100 years of history as

represented by sand lime bricks (11,12) Usually, the

pro-cess involves mixing quartz (SiO2) with about 10% lime

(Ca(OH)2) and exposing the mixture to saturated steam

at about 200◦C This results in the formation of acicular

calcium silicate hydrates (Fig 7) (13) It is believed that

strength development is obtained because these hydrates

(tobermorite, xonotolite, etc.) are entwined with each other

in the solidified bodies From the point of view of energy

consumption, this method of synthesis is an extremely

high efficiency process Numerous studies have been made

regarding the reactions involved during synthesis by

hydrothermal treatment and regarding the behavior of

20 µ m

CaO + SiO2 + H2O CaO − SiO2 − H2O (C − S − H)

Hydrothermalprocessing

Calcium silicate hydrates

Figure 7 Calcium silicate hydrates, the most important

bond-ing materials in the hydrothermally solidified buildbond-ing material (photo: tobermorite in autoclaved sand lime brick).

calcium silicate hydrate materials, mainly in the CaO–SiO2–H2O system (14–16) Although many of the reportsdeal with the purity of the starting materials, it is gen-erally accepted that it is desirable to use SiO2with highpurity (12) Almost no studies seem to have been madewith significantly altered SiO2 compositions Mitsuda et

al (17,18) have studied the effects of Al2O3source tions to the CaO–SiO2–H2O system and the replacement

addi-of Si by Al in tobermorite (Ca5(Si6O18H2)· 4H2O) However,the addition of Al2O3in this study was up to Al/(Al+ Si) =

0.16, which corresponds to the solid solution limit of Al in

tobermorite (19,20) An investigation by Kalousek hasreported that when kaolinite is used as the Al source,hydrogarnet forms in the range Al/(Al+ Si) = 0.12 − 0.5

(20) Generally, this hydrogarnet has a cubic structureand contains Ca3Al2Si3O12–Ca3Al2H12O12in solid solution(21,22) In a recent study of metakaolin–quartz–lime se-ries slurries, the effect of amount of metakaolin additionsand of quartz grain size on hydrogarnet formation havebeen clarified (23–25) These reports indicate that with in-creasing Al/(Al+ Si) ratio in the CaO–SiO2–Al2O3–H2Osystem, hydrogarnet forms in addition to calcium silicatehydrates such as C–S–H (calcium silicate hydrate gels)and tobermorite, and that the hydrogarnet becomes themain phase for Al/(Al+ Si) ratios higher than about 0.2.However, in the composition range over which hydrogar-net is the main phase formed, not much study seems tohave been carried out on the strength development of hy-drothermally synthesized bodies Investigations about therelationship between strength development and the mi-crostructural changes that accompany the formation of hy-drogarnet cannot be found either This is probably because

of the fact that it was thought that formation of net causes strength deterioration in calcium silicate mate-rials (26) Because of this belief, avoiding the formation ofhydrogarnet has been an important direction of research

hydrogar-up to now

1020 SOIL-CERAMICS (EARTH), SELF-ADJUSTMENT OF HUMIDITY AND TEMPERATURE

CaO(lime)

Ca5(Si6O18H2)⋅4H2O

(tobermorite)

K/(K + Q)

SiO2(quartz)

Al2Si2O5(OH)4(Kaolinite)

Al2O3

Lime/(K + Q) = 0.20.5

0.1

Al /(Al + Si)<0.16 (molar ratio)

Al2O3/(Al2O3 + SiO2) < 0.14 (mass ratio)

Figure 8 Experimental compositions (mass ratio) in the CaO–

SiO 2 –Al2O 3 system Hatched area was mainly discussed in the

past.

An approach completely different from current

direc-tions of research is required here Soil, which usually

con-sists of minerals such as quartz, feldspar, and kaolinite

(Al2(Si2O5)(OH)4), contains besides SiO2 a considerable

amount of Al2O3 (about 30%) For this reason, it is

un-suitable as the raw material for calcium silicate hydrate

formation In order to produce solidified materials with

soil as the starting material, sufficient strength

develop-ment would be required in solidified bodies that contain

hydrogarnet as the main phase

Hydrothermal Solidification of Kaolinite

An example of a solidification experiment using nearly

pure quartz (Indian quartz), kaolinite (Georgia kaolin—a

clay mineral without micropores), and lime (CaO) is

out-lined here In this experiment, the mass ratio of quartz,

kaolinite, and lime were varied (Fig 8, Table 1) So that

kaolinitequartz+ kaolinite:

K

(Q + K) = 0, 0.1, 0.5, 0.9, 1.0,

and

limequartz+ kaolinite= 0.21.

After weighing the materials, the lime was slaked and

a further 10% of water was added and mixed to allow for

easy forming The specimens were obtained by uniaxial

Table 1 Compositions of the Experimental Mixtures

K /(Q + K) Al/(Si + Al) Ca/Al Ca/(Si + Al)

(mass ratio) (atomic ratio) (atomic ratio) (atomic ratio)

TobermoriteGyroliteHydrogarnet

20

Figure 9 Generation of the flexural strength and the phases

along with the hydrothermal processing.

press forming of this mixture at 30 MPa The press-formedspecimens were cured under saturated steam pressure at

200◦C for 2 to 20 hours (The compositional range of thespecimens was Ca/(Si+ Al) = 0.23 to 0.25, AI/(Si + Al) =0.0 to 0.50

The variation in the phases formed with treatment time

is shown in Fig 9, together with the strength ment characteristics It is recognized that in all specimens,hydrothermal treatment results in increased flexuralstrengths It is also clear that hydrothermal solidification

develop-is possible even in those specimens with Al/(Si + Al) =0.24 to 0.50 where hydrogarnet is the main phase formed

In the case of specimens with AI/(Si+ Al) = 0 and 0.05 thatcorrespond to compositions investigated frequently in thepast, mainly calcium silicate hydrates are formed, and themaximum flexural strength of about 30 MPa is attained

in 2 hours of treatment Longer treatment times lead todecreased strength, particularly for the specimens withAl/(Si+ Al) = 0

In kaolinite-rich specimens with Al/(Si+ Al) = 0.24 −

0.50, flexural strength reaches approximately 15 to 20 MPa

in 2 hours along with the formation of hydrogarnet Longertreatment times lead to only slight strength increases Theflexural strength is maximum for specimens with Al/(Si+Al)= 0.05, and becomes lower for larger Al/(Si + Al) values.

However, the rate of decrease in strength becomes smallwith increasing Al/(Si+ Al) value In the specimens withAl/(Si+ Al) = 0.24 to 0.50, the decrease in maximum flex-

ural strength with increase in Al content is clearly smallerthan in those with Al/(Si+ Al) < 0.24.

These results are extremely significant:

1 By making hydrogarnet as the main phase, sufficientstrength allowing the synthesized bodies to be uti-lized as building materials is obtained, although thestrength may be somewhat lower than those of con-ventional calcium silicate hydrate materials

2 Although the strength decreases with increasingamounts of kaolinite, it is possible to limit the

Figure 10 Influence of the starting materials on the reaction

rates of Ca source with processing time.

strength decrease to acceptable levels by makinghydrogarnet as the main phase As a result, in actualpractice, variations in composition of natural soil willnot significantly affect the final physical properties ofthe synthesized material In other words, this meansthat the range of starting materials that can be used

is wide

Unfortunately, the mechanism of strength development

by hydrogarnet formation still remains unclarified

al-though there is a possibility that this is related to the

re-action scenario of Ca as explained below In the case of

specimens with Al/(Si+ Al) ratios lower than 0.24, almost

100% of the calcium is consumed within the first two hours

of reaction (Fig 10) For specimens with Al/(Si+ Al) = 0.24

to 0.50, calcium is consumed rapidly during the first two

hours, but since little or no further calcium consumption

occurs, after that, the reaction ratio stagnates in the range

50% to 75% This low reaction ratio may be one of the

fac-tors influencing strength development by hydrogarnet

for-mation

Some interesting results are provided by

microstruc-tural evaluation (Fig 11) For Al/(Si+ Al) = 0 and 0.05,

acicular and platelike reaction products (identified by XRD

to be C–S–H, tobermorite) can be observed to fill up the

interparticle spaces after two hours of treatment In

con-trast, for Al/(Si+ Al) = 0.45 and 0.50, only platelike

kaoli-nite particles are recognizable, and hydrogarnet cannot

be observed despite the fact that its existence has been

confirmed by XRD Hydrogarnet crystals in sizes

rang-ing from a few µm to some tens of µm have been

re-ported to have been observed in hydrothermally

synthe-sized bodies using slag as the starting material (27) and

in slurries (23–25) In the current specimens, however,

such hydrogarnet crystals could not be recognized, even

through TEM observation This indicates the possibility

that the hydrogarnet formed may be existing as ultra-fineparticles

There is a clear difference in the pore size distributionbetween the specimens that contain mainly calcium sili-cate hydrate phases and those that contain hydrogarnet

as the main phase formed (Fig 12) The pore size tion of specimens with Al/(Si+ Al) = 0 and 0.05 shows apeak at about 1µm in the press-formed state This peak

distribu-location shifts to smaller values and the total pore volumedecreases as hydrothermal treatment progresses This in-dicates that the 1µm pores in the press-formed state are

filled by the C–S–H and tobermorite formed by mal treatment, and this densification is accompanied bystrength increase (28,29) When the treatment time is in-creased to five hours or more in Al/(Si+ Al) = 0, the peak

hydrother-in the neighborhood of 0.01 µm formed by two hours of

hydrothermal treatment shifts toward coarser sizes Thispore-coarsening phenomenon is due to the formation of gy-rolite (28), and it appears to be connected to the observedstrength reduction

The behavior of specimens with Al/(Si+ Al) = 0.45 and0.50 differs greatly from that of those with Al/(Si+ Al) =

0 and 0.05 For Al/(Si+ Al) = 0.45 and 0.50, the pore sizedistribution in the press-formed state does not shift withhydrothermal treatment time, but the number of pores de-crease as the treatment time increases As a result, poresaround 0.04µm are formed Another characteristic of these

compositions is that the pore volume hardly changes ing this time

dur-From the preceding results, it is clear that the changes

in pore size distribution of specimens with Al/(Si+ Al) =0.45 and 0.50, where hydrogarnet is the main phaseformed, are different from those with Al/(Si+ Al) = 0 and0.05 in which pores are filled up by the calcium silicate hy-drates formed The formation of hydrogarnet and strengthdevelopment without alteration of the pore sizes existing

at the time of press-forming suggests that the solidifiedstructure may be one that corresponds to that shown inFig 13 It is envisaged that ultra-fine hydrogarnet parti-cles grow densely and in situ from the surface of kaolin-ite particles inward (forming 0.04µm pores) Through this

solidification mechanism, bonding of the kaolinite particlesoccurs with almost no alteration to the pore sizes existing

at the time of press forming The reason for the amount ofhydrogarnet formed reaching a near-saturation level aftertwo hours of hydrothermal treatment may be that the re-action has become diffusion controlled because of the kaoli-nite particles being covered by the hydrogarnet layer

In similar experiments using silica sand containing purity clay minerals as the starting material, it has beenfound that although the type of phases formed during hy-drothermal treatment change greatly with the treatmenttemperature and time, the strength of the solidified mate-rial is strongly influenced not by the type but by the amount

low-of formed phases (Fig 14) (30) On the other hand, it hasbeen clearly shown that the strength of bodies formed byextrusion and casting processes, which require the addi-tion of large amounts of water (binder) to the raw mate-rial, is much lower than that of bodies formed by uniax-ial pressing (dry pressing) Although the detailed mecha-nism of strength development by hydrothermal treatment

(a) (b)

Figure 12 Pore size distributions of the specimen with processing time, a= Al/(Al + Si) of 0, b = 0.05, C = 0.45, and d = 0.50.

1024 SOIL-CERAMICS (EARTH), SELF-ADJUSTMENT OF HUMIDITY AND TEMPERATURE

Kaolinite

Hydrogarnet

Pore size ; 0.04 µm

Pore size; 0.1 − 0.2 µm

Figure 13 Estimated schematic figure for the formation of

hy-drogarnet under higher Al/(Al + Si).

of soil needs further investigations, it is believed that the

strength development is attained through ultra-fine

par-ticles becoming uniformly dispersed within the densified

body, that is, through a mechanism similar to the that of

strength development in DSP (densified system particles—

containing homogeneously arranged ultra-fine particles)

(31–33)

As described previously, sufficient strength

develop-ment is obtained through hydrothermal treatdevelop-ment of

bod-ies that have been press-formed from a mixture of lime

and the starting material, kaolinite The strength is

be-lieved to be attained not by the filling up of the pores in the

material but by the formation and dispersion of ultra-fine

hydrogarnet particles within the press-formed, dense body

This solidification mechanism allows strength to be

at-tained without destroying the agglomerated micropore

structure This is important from the point of view of the

humidity regulation performance of the solidified bodies

30

160170180Curing temp (°C)

Figure 14 Examples for the generation of the flexural strength

with amount of formed phases under the various curing time The

phases formed change with curing time and temperature

How-ever, the strength seems to be controlled by the amount of the

formed phases independently.

150°C

Figure 15 Actual processing of the earth ceramics manufacture

PERFORMANCE OF THE HYDROTHERMALLY SOLIDIFIED SOIL BODIES

Earth Ceramics

The industrial method of synthesizing hydrothermally lidified soil bodies is illustrated in Fig 15 A little lime andwater are added to the soil and mixed well Since the treat-ment temperature is low, straw or other organic additivescan be added if necessary in order to obtain higher strength

so-or to enhance the finish The mixture is then dry-pressedinto tiles and then cured for a few hours at about 150◦C insaturated steam pressure to obtain solidified bodies.Among those industrial ceramics that utilize smalleramounts of energy (34) for their manufacture, ceramic tilesare considered to consume relatively little energy The en-ergy required for synthesizing earth ceramics is even lower,being about 2.7 GJ/m3 (35), which is only 1/6th that of theenergy needed for ceramic tiles (Fig 16)

Since there is little limitation with regard to the startingmaterials and the energy required for synthesis is small, itcan be concluded from the point of view of nature’s ecosys-tem that earth ceramics are materials with very small in-put and output

Pore sizes in earth ceramics are concentrated in two gions: at around 0.05µm corresponding to the initial pores

re-at the time of press forming, and re-at around 0.01µm (10 nm)

reflecting the agglomerated structure of soil This is about1/10,000th of the pore diameters usually found in concrete

0.41.09.7

8.9

40.716.0

Actual valueTheoretical value

20Manufacturing energy

Hydrothermally solidified soil

Autoclaved aeratedconcreteSanitary wareCeramic tile

and pottery (Fig 17) The amount of water vapor absorbed

at equilibrium when the relative humidity is varied from

40% to 80% at 25◦C is shown in Fig 18 Although the

re-sponse in the case of earth ceramics is somewhat slower, it

can be seen that they exhibit humidity absorbency

proper-ties as good as, or better than, that of wood because of the

presence of micropores in the starting material (soil)

Living in a House of Soil

Earth ceramic tiles (size: 200× 200 mm) were used as the

flooring material for the living room of a highly airtight

Pore diameter (µm)

Earth ceramics0.118 cm3/gConcrete brick0.102 cm3/gPottery 0.068 cm3/g

Figure 17 Pore size distribution of the earth ceramics.

and heat-insulated apartment (Fig 19), and the changes

in temperature and humidity were measured An ment in the same apartment complex with acrylic carpetflooring was used for comparison (reference apartment),the floor plan and family makeup being the same for bothapartments The measurements during the winter of 1997(December to June) are shown in Fig 20 The room withearth ceramic flooring exhibits very stable temperaturevariation Since there were differences in the heating sys-tems of the two apartments, the measurements were car-ried out after the heaters were switched off for the night(Table 2) The high heat insulation performance of earth ce-ramics was confirmed by the fact that the average rate ofdecrease in temperature (because of the difference betweenindoor and outdoor temperatures) in the earth ceramicfloor apartment was 1.3◦C compared to 5.7◦C for the refer-ence apartment In the former apartment the humidity wasalso unaffected by the outdoor atmosphere and remained

apart-200180160140120

100806040200

Curing time (days)

Earth ceramicsWood (cedar)Plaster board

Figure 18 Variation of the moisture absorption capacity with

time at 25 ◦C when specimens (earth ceramics, wood, and board) were kept under relative humidity from 40% of the equi- librium condition to 80%.

plaster-1026 SOIL-CERAMICS (EARTH), SELF-ADJUSTMENT OF HUMIDITY AND TEMPERATURE

Figure 19 Photo for the application of the earth ceramics flooring

in the room.

30

Earth ceramics floorCarpet floorOpen air25

2015

12:00 18:00 6:00 12:00 18:00100

Earth ceramics floorCarpet floorOpen air80

Figure 20 (a) Example of the variation of the temperature when

the earth ceramics or carpet was used on the floor in winter (b)

Example of the variation of the relative humidity when earth

ce-ramics or carpet was used on the floor in winter.

Table 2 Examples of the Temperature Variation after Turning off the Air Conditioner

Earth Ceramics Carpet

6 am, 12 noon, and at 8 pm during one winter month areshown in Fig 21 The temperature and humidity in theearth ceramic flooring were within the range 15 to 18◦Cand 40% to 50%, respectively This shows that extremelystable and comfortable living environment can be obtained

by the use of earth ceramics The nighttime temperatures

in the earth ceramic floor apartment were about four orfive degrees lower compared to the reference apartment,but there was little recognizable difference in the effec-tive temperature felt by the human body This might bebecause the air temperatures near the ceiling, floor, andintermediate locations were about the same, the tempera-ture difference being only about 0.5 to 1.0◦C

The measurements were continued for one year Itwas found that compared to the reference apartment, the

100

Earth ceramics floorCarpet floor80

Figure 21 Variations of temperature and relative humidity

dur-ing one winter month when the earth ceramics or carpet was used

on the floor.

Table 3 Examples of the Relative Humidity Variation after Turning off the Air Conditioner

Earth Ceramics Carpet

variations of temperature and humidity were small

throughout the year in the earth ceramic flooring

apart-ment In particular, the humidity was within the 40% to

70% RH range, which is the normal range of comfort for

humans Therefore, the use of humidifiers or dehumidifiers

was not necessary, and the period of air-conditioner

oper-ation was short, resulting in low use of fossil energy The

amount of energy utilized for living (electricity, gas,

wa-ter) in the apartment before and after the earth ceramic

floor was installed is shown in Fig 22 converted into an

equivalent amount of CO2 After remodeling, the amount

of electricity thought to have been used for air-conditioning

dropped sharply, and the seasonal fluctuation of energy

consumption was controlled On average, there was a 17%

reduction of energy consumption that year This energy

consumption refers to the entire quantity required for

liv-ing in the apartment (floor area = 72 m2) The energy

Electricity69.1

47.050.444.040.431.535.244.936.041.065.7

39.946.6

43.7Jan

43.8Non measured

(re-modeling)

(kgc)(kgc)

Figure 22 Amount of energy utilized for living (electricity, gas

and water) before (carpet) and after the use of the Earth Ceramics

floor.

consumption reduction effect with respect to the livingroom (floor area= 32 m2) where the earth ceramics floorwas laid was considerably greater

In addition to the above beneficial effects, earth ics proved to be effective in controlling a chronic allergysuffered by the occupants of the apartment Breeding testswith ticks, which are one of source of allergy, showed that at

ceram-25◦C, 90% RH, 200 ticks multiply in number to 990, 1470,and 1895 in earth ceramics, vinyl cloth, and acrylic car-pet, respectively This clearly demonstrates the advantage

of earth ceramics over other materials in restricting thebreeding of allergy sources It remains unclear at present,however, whether the low breeding rate in earth ceramics

is due to the good humidity regulation characteristics ofthe material or whether it is the result of changes in the

pH of the surface by humidity absorption

NEW FUNCTIONAL MATERIALS

It is thought that hydrothermally solidified soil materialsattain strength through the ultra-fine hydrogarnet par-ticles becoming uniformly dispersed within the densifiedpress-formed body, similar to DSP materials This type

of material synthesis holds many possibilities as the cess for producing new functional materials Such possibil-ities need to be investigated through further experiments.For example, another functional material could be tried inplace of the ultra-fine dispersed particles

pro-From the point of view of humidity regulation, theauthor has investigated here only the process of hydro-thermal synthesis There are many other promisingpossibilities of using low-energy processes for producingnew materials One is the use of natural porous materials.Sepiolite (Mg5Si8O20(OH)2· 8H2O) contains micropores ofabout 1 nm and mesopores that are a few nm in size Allo-phane (1–2SiO2· Al2O3· nH2O) is an amorphous substancealumino-silicate formed during the weathering of volcanicglass, which is the major constituent of volcanic ash It iswidely distributed in nature in the form of hollow spheres

of 3–5 nm in diameter (36, 37) As shown in the water vaporabsorption/desorption isotherms of Fig 23, both sepioliteand allophene exhibit high humidity absorption/desorptionability Even at relative humidities less than 40%, theyshow this high ability This is thought to result from thedisordered surface structure and micropores smaller than

1 nm formed by the adsorbates By adding a small quantity

of binder to allophane-rich soil (Kanuma-soil/Japan), forming the material to shape and firing at about 900◦C, it

dry-is possible to obtain a solidified material that has high midity regulation ability (trade name: Eco-carat) The hu-midity absorption/desorption characteristics of Eco-carat

hu-at 40% to 80% RH is shown in Fig 24 Although there

is limitation in the choice of material, the micropore ume of Eco-carat is around three times that of earth ce-ramics, and it shows extremely high humidity regulationperformance However, since these micropores disappear

vol-at about 1000◦C because of phase changes, it is not sible to raise the firing temperature With this restrictedfiring temperature, one cannot expect sufficient strength

pos-to develop by the characteristic sintering mechanism of

1028 SOIL-CERAMICS (EARTH), SELF-ADJUSTMENT OF HUMIDITY AND TEMPERATURE

Plaster boardSepiolite absorption

Sepiolite desorptionAllophane absorption

Figure 23 Water vapor absorption/desorption isotherms of

sepi-olite, allophane, and plasterboard.

ceramics But because of the high humidity regulation

per-formance, sufficient performance is obtained even if thin

material is used In actual practice, the material has been

laid on interior walls to good effect

CONCLUSIONS

The present age is one in which ignoring the global

envi-ronment will have serious consequences for humankind

In maintaining, as far as possible, the inherent highly

ad-vanced properties and abilities of nature, it is important to

develop technologies that convert, using the least amount

of energy possible, these gifts of nature into forms that can

be utilized in the human ecosystem Earth ceramics can

200

Absorption Desorption

Eco - karatEarth ceramicsWood(cedar)150

The development of new values of life and living bychanging today’s synthetic materials into smarter onesthat minimize the load on Mother Earth and creation ofsuch technology and culture should be considered as ourgreatest responsibility toward the next generation

ACKNOWLEDGMENT

The author would like to thank Dr H Maenami and Mr O.Watanabe, INAX Corp Japan and Prof Dr T Mitsuda,Univ of East Asia for the helpful discussions

5 H Komine, Effects from house air quality on health, Annual

of Housing Research Foundation, 23: 5–17 (1997).

6 ’99 Handbook of Energy and Economic Statistics in Japan, The Energy data and Modeling Center, 1999, pp 32–33.

7 T Uemura, J Kohara and S Tokoro, Materials and Structure

of the Wall for Humidity Controll (in Japanese), Shoukokusha, Japan, 1991, pp 18–20.

8 C Arai, T Mizutani, Y Murase, T Hanakawa, and Y Sano, Measurement of Pore Distribution by Water Vapour Adsorp-

tion, Soc Powder Tech Japan, 20(3): 115–121 (1983).

9 V.G Carter and T Dale, Topsoil and Civilization, University

of Oklahoma Press, Norman, OK, 1974, pp 10–31.

10 S Iwata, Ecological Life (in Japanese), Ienohikarikyoukai, Japan, 1991, pp 12–13.

11 G.E Bessey, The History and Present Day Development of the Autoclaved Calcium Silicate Building Products Indus- tries, Society of Chemical Industry, pp 3–6, London, UK 1965.

12 G.E Bessey, Sand-Lime Brick, National Building Studies Special Report No 3, 1–21 (1948).

13 P.D Rademaker, H Hibino, T Mitsuda, Electron Micrographs

of Calcium Hydrates, Annual Report of the Ceramics Research

Lab Nagoya Institute of Technology, 1: 33 (1991).

14 H.F.W Taylor, The Chemistry of Cements, Academic Press, New York, 1964, pp 168–232.

15 S Sohmiya, Handbook for Hydrothermal Science (in Japanese), Gihodoh, Japan, 1997, pp 292–320.

16 F.H Wittmann, Advances in Autoclaved Aerated Concrete, A.A Balkema, Rotterdam, 1992, pp 11–34.

17 T Mitsuda, Synthesis of Tobermorite from Zeolite, Mineral J.,

6(3): 143–58 (1970).

18 T Mitsuda and H.F.W Taylor, Influence of Alumina on the

Con-version of Calcium Silicate Hydrate Gels into 11 ˚ A Tobermorite

at 90 ◦C and 120◦C, Cem Concr Res., 5(3): 203–209 (1975).

19 S.A.S El-Hemaly, T Mitsuda and H.F.W Taylor, Synthesis of

Normal and Anomalous Tobermorites, Cem Concr Res., 7(4):

429–38 (1977).

20 G.L Kalousek, Crystal Chemistry of Hydrous Calcium

Sili-cates: I Substitution of Aluminum in the Lattice of

Tober-morite, J Am Ceram Soc., 40: 74–80 (1957).

21 E Passaglia and R Rinaldi, Katoite, a New Member of the

Ca3Al2(SiO4)3-Ca3Al2(OH)12 Series and a New

Nomencla-ture for the Hydrogrossular Group of Minerals, Bull Mineral,

107: 605–18 (1984).

22 J.L Larosa-Thompson and M.W Grutzeck, C-S-H,

Tober-morite and Coexisting Phases in the System

CaO-Al2O3-SiO2-H2O, World Cem., 27(1): 69–74 (1996).

23 D.S Klimesch and A Ray, Hydrogarnet Formation during

Au-toclaving at 180 ◦C in Unstirred Metakaolin—Lime—Quartz

Slurries, Cem Concr Res., 28(8): 1109–17 (1998).

24 D.S Klimesch and A Ray, Effects of Quartz Particle Size on

Hydrogarnet Formation during Autoclaving at 180 ◦C in the

CaO-Al2O3-SiO2-H2O System, Cem Concr Res., 28(9): 1309–

16 (1998).

25 D.S Klimesch and A Ray, Effects of Quartz Particle Size and

Kaolin on Hydrogarnet Formation durong Autoclaving, Cem.

Concr Res., 28(9): 1317–23 (1998).

26 I Stebnicka-Kalicka, Application of Thermal Analysis to the

Invastigation of Phase Composition of Autoclaved Cement

Pastes and Mortars, Therm Anal 1: 369–74 (1980).

27 S.A Abo-El-Enein, N.A Gabar, and R.Sh Mikhail,

Morpho-logy and Microstructure of Autoclaved Clinker and Slag-Lime

Pastes in Presence and in Absence of Silica Sand, Cem Concr.

Res., 7(3): 231–38 (1977).

28 N Isu, S Teramura, H Ishida, and T Mitsuda, Influence of

Quartz Particle Size on the Chemical and Mechanical

Pro-perties of Autoclaved Aerated Concrete (II) Fracture

Tough-ness, Strength and Micropore, Cem Concr Res., 25(2): 249–54

(1995).

29 T Mitsuda, K Sasaki, and H Ishida, Phase Evolution During

Autoclaving Process of Aerated Concrete, J Am Ceram Soc.,

75(7): 1853–63 (1992).

30 O Watanabe, K Kitamura, H Maenami, and H Ishida,

Hydrotheraml Reaction of Silica Sand Complex with Lime,

J Am Ceram Soc., (2000) in press.

31 L Hjorth, Microsilica in Concrete, Nordic Concr Res., 1: 1–

18 (1982).

32 L Hjorth, Development and Application of High-density

Cement Based Materials, Phil Trans R Lond., A310, 167/73

(1983).

33 S Brunauer, J Skalny, I Odler, and M Yudenfreund, Hardend

Portland Cement Pastes of Low Porosity, Cem Concr Res., 3:

279–93 (1973).

34 M.F Ashby, Materials Selection in Mechanical Design,

Perga-mon Press, New York, 1992, p 245.

35 H Shin and T Kurushima, Thermodynamic Consideration on

Energy Consumption for Processing of Ceramics, Bull Ceram.

Soc Japan, 32(12): 981–84 (1997).

36 G.W Brindley and G Brown, Crystal Structures of Clay

Min-erals and their X-Ray Identification, Mineralogical Soc

INTRODUCTION

In the last decade, there has been an increased interest

in developing methods for the active control of sound diation from vibrating structures (1) In one promisingmethod, termed active structural acoustic control (ASAC),actuators are attached directly to the structure and areused to modify its structural vibration characteristics (spa-tial and temporal) in order to minimize the sound radiation(1) In ASAC, the actuators tend to be compact and thuscover only a very small portion of the structure; their ef-fect is achieved because of the distributed elastic response

ra-of the structure This technique has worked well for a ber of applications, usually where the structure has a rea-sonable mobility and a low modal density of response Insome applications, however, the structure is quite massive

num-or stiff (e.g., an electrical transfnum-ormer casing), and it isextremely difficult to elicit the necessary control field re-sponse with practical control actuators In this article wediscuss a variant of the ASAC approach in which the con-

trol inputs come from a smart or active skin that covers all

or most of the vibrating surface A schematic of the smartskin approach is shown in Fig 1

The objective of the smart skin is to locally change theradiation impedance (the resistive component) of the struc-ture in order to control the total radiated power in contrast

to the conventional ASAC, which alters the dynamic sponse of the host structure The sound radiation levels

re-are directly coupled to the normal displacement wskof thesmart skin Thus modification of the transfer function be-

tween the structural displacement wsand the smart skin

displacement wskwill lead to a change in sound radiation.This modification can occur via a decrease in the amplitude

of wsk, thus decreasing the sound levels, or via a change

in the amplitude distribution of wsk, causing the normalskin surface to be an inefficient sound radiation over anextended area Since it does not drive the host structure,

Structure

Smart skin

Normal structural vibration

Normal skin vibration

1030 SOUND CONTROL WITH SMART SKINS

an active skin is suitable for heavy or stiff structures with

low mobility The negative trade-off, however, of the active

skin approach, is that since it covers the structure

com-pletely, it may require many independent sections if the

structural response is complex and near or above the sound

radiation critical frequency (1) For illustrative purposes,

two different skins are described: one based on a composite

of acoustic foam and the piezoelectric polymer PVDF and

the other on ceramic piezoelectric elements arranged in a

motion amplification configuration

SMART FOAM SKIN

Acoustic foam is commonly employed as a form of passive

liner or skin in order to reduce sound radiation For

exam-ple, in aircraft interiors, a layer of acoustic foam is located

between the inside surface of the fuselage and the

inte-rior trim panels Generally, the foam completely covers the

fuselage surface, and its acoustic purpose is to reduce noise

transmitting through the fuselage to the interior

How-ever, it is well known that passive treatments such as foam

work well at high frequencies (>1000 Hz), and their

per-formance is poor at low frequencies Here we describe the

use of a smart foam skin, which is a hybrid of the

mate-rials of passive foam and active piezoelectric elements (2)

The objective is to develop a flexible active skin that

com-bines good high-frequency passive performance with the

low-frequency performance of an active system

The smart skin has to be of such a form that it can be

extended to cover complex, distributed surfaces similar to

conventional acoustic foam A typical configuration of the

smart foam is shown in diagrammatic form in Fig 2 The

smart foam can be seen to consist of conventional acoustic

foam with embedded layers of the flexible polymer

piezo-electric material, PVDF (1) Two aspects of the embedded

PVDF are significant First, the PVDF is curved in order

to couple the predominantly in-plane piezoelectric effect

++

Figure 2 PVDF actuator configuration: (a) Original and

(b) parallel.

(a)

(b)

Figure 3 (a) Actuator and error sensor configuration (b)

Spheri-cal dome for power measurements.

with the required normal motion (in effect creating a tion amplifier as discussed below), and second, as shown

mo-in Fig 2(b), alternate half-wave sections of the PVDF can

be wired out of phase This second aspect is designed toincrease the radiation efficiency of the active element (andthus its control authority) by causing all the PVDF sections

to move in the same direction when a voltage is appliedacross the PVDF electrodes

Figure 3(a) shows a picture of a smart foam activeskin covering a plate of dimensions 170× 50 × 1.5 mm(3).

The smart foam skin can be seen to be comprised of sixindependently controllable skin cells constructed as de-scribed above In addition, each cell has a lightweightbalsa wood wall used to increase the normal displace-ment by constraining the foam edge Above each smartfoam cell is a microphone located close to the foamsurface and used as an error sensor in a MIMO feedfor-ward Filtered-x LMS control approach (1) (see Fig 11 for

a typical feedforward LMS arrangement) The plate andsmart foam skin is located in a rigid baffle located in ananechoic chamber Figure 3(b) shows a hemispherical ar-ray of ten microphones located over the plate and in theradiated sound field The microphones are used to esti-mate the total radiated sound power from the plate–smartfoam system The disturbance to the plate was provided by

a piezoelectric patch actuator (1), bonded to the back The

800Frequency (Hz)

PlatePassive controlActive/Passive control

4005

101520253035404550

600 Figure 4 Radiated power for a broadband1I1O case using multiple smart foam modules

operating in phase.

reference signal for the LMS control approach was taken

from the internal signal generator used to drive the

distur-bance (termed internal reference)

Figure 4 presents the radiated power with and without

control, when all the smart skin cells are wired together

in phase as a single channel of control The error signal is

provided by a single microphone located close to the smart

foam surface and at the plate-foam center Also shown is

the passive effect of the smart skin when it is located on

the plate but not activated It is apparent that the passive

effect of the skin is good at high frequencies above 1000 Hz

but is limited to resonant frequencies of the base plate

be-low this value Turning on the active control provides

rea-sonable attenuation at low frequencies, though there are

some frequency ranges where the control is negligible, for

instance near 900Hz In this case the smart skin transfer

function is reduced in level We now extend the controller

so that the six smart skin modules can be controlled

in-dependently with a six by six LMS control arrangement

Figure 5 presents these new results for the low frequency

range For the results of Figure 5 three different reference

signals control configuration are also studied: one using

300

−20

−10

010203040

Figure 5 Attenuated SPL for broadband 6I6O

case using multiple-independent smart foam modules.

an internal reference, one using an external referencesignal taken from an accelerometer located on the plate(representing a more realistic arrangement), and an ex-ternal reference signal with feedback (FB) from theactive component of the smart skin removed (1) It is appar-ent that much improved performance is achieved over theSISO case of Fig 4, particularly for the internal referencecase, due to the multicell active skin being able to matchthe complex radiation impedance load near 900 Hz, for ex-ample (3) In this case the smart skin transfer function isalso modified in a distributed manner

Using an external reference signal also provides able attenuation; however, it is reduced from the internalcase implying that the system is acausal (1) Some of thelost performance is recovered when feedback removal isemployed, indicating that the smart foam vibration hassome input to the plate system

reason-Recently the smart foam skin has been used to strate control of interior noise in aircraft (4) Figure 6shows a smart foam skin covering four panels in thecrown section of the fuselage of a Cessna business jet Theapplication is focused toward reducing cockpit noise due

demon-1032 SOUND CONTROL WITH SMART SKINS

Error microphones

M3

C3Top euselage ribs Microphone traverse

C1

M4

Figure 6 Cessna crown panel control arrangement.

to exterior flow separation over the crown of the aircraft

Error microphones were located as shown at the ear

lo-cations of the crew and a microphone traverse was used

to measure the sound pressure levels in a plane at the

crew head height The flow noise disturbance was

simu-lated by an exterior speaker located just over the crown

of the aircraft and driven by band-limited white noise A

four by four feedforward LMS control approach was

im-plemented using a realistic reference signal taken from an

interior mounted accelerometer located on the fuselage at

the aircraft crown (i.e., just under the excitation location)

Figure 7 presents tabular results of the attenuation

achieved at the error sensors (near the crews’ ears) with

an excitation band of 500 to 900 Hz The reference speaker

refers to the use of a reference signal from the signal

driv-ing the disturbance The reference accelerometer refers to

the use of a fuselage-mounted accelerometer as a

refer-ence sensor and in this case attenuation of the order of 2

to 4 dB are achieved The global attenuation measured

us-ing the microphone traverse was found to be 2.5 dB with

the active skin turned on However, the active skin also

provides a passive attenuation of 4 dB when it is installed

over the bare fuselage panels and not turned on Thus the

total global attenuation of the smart foam active skin is

around 6.5 dB, a significant difference It also apparent

from Fig 7 that one of the main limitations on achievable

attenuation is the causality of the controller when using an

accelerometer as a reference signal When the reference

signal is taken from the speaker drive signal the control

path delay is less, and the performance increases markedly

The results do, however, demonstrate the potential of the

smart foam skin in reducing structurally radiated sound

in a realistic application

PIEZOELECTRIC DOUBLE AMPLIFIER SMART SKIN

Piezoelectric transducers tend to be high-force,

low-displacement devices (1) In contrast, active noise control

applications in air require high-displacement actuators,

particularly at very low frequencies Thus much of the

work in developing piezoelectric based actuators for

ac-tive noise control applications has been in designing

de-vices that amplify their displacement This amplification

Error mic 1

0510152025

Error mic 2Reference speakerReference accelerometer with feedback removalReference accelerometer without feedback removal

Error mic 3 Error mic 4

Figure 7 Averaged attenuation microphones for band-limited

500 to 900 Hz excitation.

is usually based on a geometric lever-type principle, andthus results in lower output force More explicitly, the ac-tuators are designed to have the correct source impedancerelative to their load In our application, the load is air with

a relatively low impedance, thus the device needs to have

a low source impedance for maximum power output.Figure 8 shows a schematic diagram of a piezoelectricdouble amplifier actuator, which is the basis of the secondactive skin concept (5) The legs of the element consist

of piezoelectric bimorphs or unimorphs In this case, thepiezoelectric transducers are manufactured from the ce-ramic material PZT (1) These devices are amplifiers inthat due to their asymmetry, small in-plane motions areamplified to larger transverse tip motions at the top of thelegs The tops of the legs are connected to a triangular orcurved stiff, lightweight diaphragm as shown Thus as thelegs move in, the diaphragm is squeezed upward Sincethe diapraghm axis is transverse to the tip motion, verysmall tip motions cause very large diapraghm motions (i.e.,amplify it) in a vertical direction Thus the complete struc-ture comprises a double amplifier actuator and gives am-plification ratios of diaphragm to piezoelectric elementin-plane deflection of the order of 20 : 1 The wholeconfiguration can be built in heights typically ranging from

1.3

234

PZT-Brass-PZTBimorph leg

Speaker paperdiaphragm

0.56

Figure 8 The active skin element (domains in mm)

Vibrating plate surface

PZT Bimorphs

Active-Skindiaphragm

Figure 9 Smart skin constructed from piezoelectric

double-amplifier elements.

3 to 6 cm, leading to a fairly compact device In

construct-ing an active skin of such devices, a number of them are

positioned to completely cover the surface of a structure as

shown in Fig 9 The devices can be either located directly

on the structure as shown or positioned just above it with a

small air gap In addition, the devices can be wired together

as one channel of control or independently controlled,

de-pending on the complexity of the base structural response

Figure 10 shows an actual device designed and

constructed by the Materials Research Laboratory at

Pennsylvania State University The device is 50× 60 mms,

34 mms high, and was found to have a maximum cover

displacement of 300µm at 100 Hz Figure 11 shows six

of the devices arranged to completely cover the surface of

a 170× 150 mm aluminum plate of 1.5 mm thickness In

this test arrangement, the active skin cells are located on a

perforated aluminum sheet which is located 5 mm from the

surface of the radiating plate Thus the active skin has a

small air gap between its bottom surface and the radiating

surface of the structure (5) Small accelerometers located

on each active cell diaphragmn are also apparent in Fig 11

These accelerometers are used to provide time domain

estimates of the radiated pressure in the far-field from the

Figure 10 A single active-skin cell.

Figure 11 The active-skin in a top-mounted SAS configuration.

measured surface vibration data, termed structural tic sensing (SAS) and described in (6) Such approachesallow integration of the sensors into the smart skin itself.The test plate and the active cells were mounted in arigid baffle located in the anechoic chamber at VAL Anoise disturbance to the plate was provided by a smallshaker attached to the back of the plate and drivenwith band-limited random noise The radiated sound fromthe plate-skin structure was measured using an array of

acous-16 microphones located on a hemispherical tube structure

as described above and a microphone traverse that couldmeasure the sound directivity in the horizontal midplane

of the plate The total radiated power from the plate could

be calculated from the 16 pressure levels measured by themicrophone hemispherical array (5)

Figure 12 depicts a schematic of the experimental rigand the control arrangement The control approach usedwas the Filtered -x LMS algorithm (1) implemented on

a TMSC40 DSP The shaker was driven with band ited noise of 175 to 600 Hz The Filtered -x algorithm was

lim-Amplifier

Shaker

Aluminumpanel

Baffle

Microphones

AccelerometersActive-skin

C40 DSPFiltered-x LMScontroller

C30 DSP SASFilter controller

Figure 12 The active-skin experimental setup.

1034 SOUND CONTROL WITH SMART SKINS

75

−90°

0°θ

90°

69 62 56 50Sound pressure level (dB)

Before controlAfter control

56 62 69 75

Figure 13 Total in-plane acoustic directivity (SPL), top-mounted

accelerometer configuration with microphone error sensing.

executed with a 2000 Hz sample rate, and 175 and 96

tap FIR filters were used for the control and system

iden-tification paths, respectively Since six independent cells

were located on the structure to comprise the active skin,

a six by six controller was implemented (5) Two tests were

performed using different error sensors In the first test, six

microphones evenly distributed over the microphone array

were used as conventional pressure error sensors located

in the radiated far field In the second test, the diaphragm

accelerometer signals were used in the structural acoustic

sensing approach, described in (6), to estimate the

pres-sures at the same locations as the previous error

micro-phones These estimates were then used as error signals

for the LMS algorithm

Figure 13 presents experimental results of the

directiv-ity of the total radiated sound power measured using the

far-field microphone traverse before and after the control

using the active skin elements It is apparent that the

active skin provides global sound pressure level

atten-uation of the order of 10 dB, which is impressive since

the excitation band encompasses multiple modes of

vi-bration of the radiating plate (5) Figure 14 shows the

corresponding radiated power versus frequency Good

con-trol is seen over the complete bandwidth of 170 to 600 Hz

except around 350 and 530 Hz, where anti-resonances

occur in the plate-active skin system The overall sound

power reduction for the results of Fig 14 is 10.9 dB

Fur-ther experiments were conducted using the accelerometers

in the SAS approach, and the results are presented in

Fig 15 Good attenuation is evident across the frequency

band, except near the system anti-resonance The

over-all reduction is now 9.5 dB, which is still impressive

Thus the results demonstrate that it is possible to utilize

an active skin that can provide significant attenuation of

sound radiated from a structure vibrating in complex

re-sponse shapes The successful use of the accelerometers

is significant in that it shows that an active skin with

completely integrated actuators and sensors can be

con-structed to provide very significant broadband

attenua-tion of sound radiated from structures under broadband

excitation (5)

200303540455055606570

250 300

Before controlAfter control

350 400Frequency (Hz)

450 500 550 600

Figure 14 Radiated sound power spectra, top-mounted

accelero-meter configuration with microphone error sensing.

SMART SKINS FOR SOUND REFELECTION CONTROL

It should be noted that the above mentioned smart skinapproaches could also be used to absorb sound imping-ing on structures by coating the structure with the smartskin However, in this application, a modified sensing ap-proach is needed in which the reflected or scattered wavecomponents are independently (than the total pressurefield) sensed and minimized by the controller Fuller et al.(7) discuss such approaches using the smart foam notedabove, and a combination of two microphones located nearthe smart foam surface are used to separate out the re-flected wave information from the total pressure field (7).Figure 16 shows a schematic of the experimental testing

in a plane wave acoustic standing wave tube The noise isgenerated by a speaker at the right end of the tube and im-pinges on the smart foam The two microphones are used

to separate out the reflected and incident wave responsesfrom the total pressure field The reflected wave signal isused as error information to the LMS controller The con-troller thus provides a control signal to the smart foam tominimize the reflected signal

Figure 17 presents the measured intensity of the dent and reflected wave intensities versus frequency withthe control off and on With the control off, the incident andreflected intensities are almost equal at low frequencies

inci-20030354045505560657075

Figure 15 Radiated sound power spectra, top-mounted

ac-celerometer configuration with SAS error sensing.

Reflectedwave

Disturbanceinput signal

ComputerwithLMScontroller

Control signal

Signal generator signal

error signal

AcousticsourceLP

filter

Figure 16 Smart skin reflection

con-trol experimental arrangement.

below 300 Hz, implying that the smart foam is acting like

a rigid surface with very little sound absorption Above

300Hz, as is expected, the foam provides increasing

pas-sive sound absorption, and the reflected intensity is less

than the incident When the active control is turned on,

the incident intensity remains the same, but the smart skin

leads to a significant reduction in reflected sound energy

below 300 Hz This reduction in reflected sound due to the

smart skin is apparent over the complete frequency range

of Fig 17 The two microphones can also be used to measure

the acoustic impedance of the smart foam When the

con-trol is turned on at low frequencies, the normal acoustic

impedance of the foam falls from very large values to be

almost identical to the characteristic impedance of the air

Thus the active element in conjunction with the controller

150 200 300 400 500

Frequency (Hz)

Incidentwave

Reflectedwave

Reflected wave undercontrol

600 700 800 900 1000

Figure 17 Reflection control using a smart

skin.

of the smart foam have modified the smart foam dynamics

so that it looks like a perfectly sound absorbing surface

ADVANCED CONTROL APPROACHES FOR SMART SKINS

The conventional control approaches used with a smartskin can be divided into two types; multi-channel feedfor-ward, which is generally used when access to a coherentreference signal is available, and multiple input-multipleoutput state space feedback methods, which are often usedwhen such a convenient reference signal is not available.These approaches are summarized in (1) As discussedabove, the smart skin approach relies on covering a ma-jor part of the structure with independently controllable

1036 SOUND CONTROL WITH SMART SKINS

Performancemetrics

Centralizedprocessor

Local control rules

Multiple independent control signals

Figure 18 Biological control approach.

elements It can thus be seen that when the structure

is large and/or the frequency of interest high (or

wave-length relative to the structure short), many smart skin

elements are required, implying a control approach with

a very high number of control channels In this case, the

conventional approaches are likely to be unsuitable due

mainly to computational limits on the control processor

and stability/performance aspects There are two different

approaches suitable for high sensor/actuator count

sys-tems (8, 9) Both approaches are hierarchical and are

in-spired by biological systems of muscle control They are

thus termed BIO controllers

In the first approach, the smart skin elements are

ar-ranged into groups of “slave” actuators under the

con-trol of a “master” actuator A schematic of the concon-troller

is shown in Fig 18 A top-level centralized controller is

used to send signals to the master actuators Simple local

control laws are used to modify and apply the same

sig-nal to nearby slave actuators For example a very simple

local law discussed in (8) would be take the same control

signal, apply it to an in-phase, out-of-phase, or off-phase

H2

G

H1FIR

filterReference

Figure 19 BIO controller with phase local control law.

Radiated sound

Structure

ActuatorSensor

Controllaw

Localcontroller

Local controllercommandsignal

Top levelcontrollerAveraged

performancemetric

Figure 20 Schematic of a BIO controller arrangement.

slave actuator via simple analog switches and keep thesetting that gives the lowest cost function value Figure 19shows a block diagram realization of such a control systemfor a feedforward approach The process then continues

to the next slave actuator, and so on, in a predeterminedpattern For the system of Fig 19 the top-level controllercould be digital, while the local control changes occur viasimple analog-switching circuits The approach in effecttakes many independent actuators and connects them to-gether via the local controller to create a suboptimal dis-tributed actuator driven by one (or few) channel of controlfrom the top-level centralized controller The net result ofsuch approaches is a large reduction in control channels

to the top-level digital controller, and thus the tional overhead requirements are vastly reduced The BIOapproach in effect takes advantage of some limited knowl-edge of the dynamics of the distributed system to be con-trolled in order to reduce the extensive number crunchingrequired in fully coupled optimal approaches

computa-In the second approach, local analog feedback loops areclosed around individual smart skin elements and associ-ated sensors as shown in Fig 20 The analog local feedbackloops have programmable feedback gains that are adapted

by a higher-level digital controller in order to minimize aglobal cost function (obtained from an array of sensors)such as radiated sound power from the structure covered

by the smart skin (9) Such approaches have been used

to control sound radiation from very large structures Aswith all feedback approaches, stability is an important is-sue Thus work has also been performed to increase thestability margins via using directional feedback sensors topartially de-couple each local feedback loops In addition,specialized distributed actuators are used that rolloff inlevel in the higher-frequency regions where the local openloop transfer function becomes non-minimum phase

CONCLUSION

The results presented have demonstrated the high tial for the implementations of a smart skin approach forreducing sound radiated from vibrating structures whenthe radiating structure is massive, stiff (i.e., low mobility),

poten-or the source vibration pattern is complex The smart skin

has also demonstrated the possibility of combining active

and passive control approaches in order to increase the

control bandwidth and the efficiency of the active portion

A configuration has been demonstrated that further shows

that the error sensors can be integrated directly into the

skin and still result in a far-field sound reduction

BIBLIOGRAPHY

1 C.R Fuller, S.J Elliott, and P.A Nelson, Active Control of

Vibration Academic Press, San Diego, CA, 1996.

2 C.A Gentry, C Guigou, and C.R Fuller JASA 101(4): 1771–

1778 (1997).

3 C.A Gentry, C Guigou, and C.R Fuller Submitted to JASA,

1999.

4 C Guigou and C.R Fuller Proc SPIE Smart Structures and

Materials Conf., San Diego, CA, SPIE Vol 3044, pp 68–78,

1997.

5 B.D Johnson, M.S Thesis VPI& SU Blacksburg, VA, 1997.

6 J.P Maillard and C.R Fuller JASA, 98(5): 2613–2621

(1995).

7 C.R Fuller, M.J Bronzel, C.A Gentry, and D.E Whittington

Proc NOISE-CON 94, pp 429–436, 1994.

8 C.R Fuller and J.P Carneal JASA, 93(6): 3511–3513 (1993).

9 M Kidner and C.R Fuller, Proc 8th Conf on Nonlinear

Vibra-tions, Stability and Dynamics of Structures Blacksburg, VA,

July 2000.

SPIN-CROSSOVER MATERIALS

University of Utah, Chemistry

Salt Lake City, UT

Smart materials respond to their environment as

illus-trated by photochromic eyeglasses, that darken upon

ex-posure to ultraviolet light to attenuate additional

ultra-violet light Hence, materials that have fast reversible

responses to environmental stimuli are sought as

compo-nents of smart systems Similar to photochromic

materi-als, thermochromic materials reversibly respond to heat

and exhibit substantial color changes upon small changes

in temperature Spin-crossover materials (1) are a class

of thermochromic materials that possess fast, reversible

color changes amenable to display and memory devices (2)

These color changes can also be induced by light

(photo-chromic) or pressure (piezo(photo-chromic) as well as heat Due

to the nature of the mechanism of their thermo-, photo-, or

piezochromic responses (i.e., redistribution of the electron

density at a metal ion site within the molecule), they are

extremely fast and reversible As a consequence of the (1)

fast color change, (2) strong contrast between colors, and (3)

the intermolecular interactions within the solid, the

differ-ing colors can be maintained for a long period of time, and

(4) due to the lack of moving parts (i.e., no bond breaking

or forming), these materials are completely recyclable and

amenable to fast, low power-consuming, high-data-density

display (2,3) and storage devices and “smart” materials and

systems of the future

Low spin

1A1g

Spectrochemical seriesincreasing ligand field, ∆

be induced by light or pressure.

Thermochromism results from transition-metal plexes, such as Fe(II), which can be thermally stimulated

com-to change from a colored low-spin electronic state com-to a quently colorless high-spin state (1a) (Fig 1) The high-spin 5T2g ground state for Fe(II) has a t2g–egsplitting,

fre-of <11,000 cm−1, and the low-spin1A1g excited state forFe(II) has a of >21,000 cm−1. ∼16,000 cm−1for Fe(II)surrounded with six unsaturated nitrogen-bound ligands,FeN6, can be induced to switch between the high-spinand low-spin states Upon switching between the high-and low-spin states on cooling, FeN6has a significant de-crease in Fe–N distances by 19± 5 pm, and an increase

in the magnetic susceptibility χ due to a change of four

in the number of unpaired electrons From a namic perspective, the enthalpy H is 10 ± 6 kJ/mol,

thermody-and the entropyS is 52 ± 13 J/Krmol; hence, the

transi-tion is entropy-driven (1a) Additransi-tionally and importantly,the color changes from deeply colored red/purple to color-less upon switching to the high-spin state (see later) Con-comitantly, the unit cell typically changes significantly.The color change of the spin state switch is similar tothat of liquid crystal displays (LCD) prevalent in digitalwatches; however, as a consequence of the mechanism, thethermochromic metal complexes change colors much fasterwithout degradation upon cycling with respect to LCDs(3) Due to the change from low to high spin, this class ofmaterials is referred to as spin-crossover materials Inaddition to the technologically important color changes,spin-crossover materials also exhibit a small, but mea-surable change in magnetic susceptibility This sharptransition in the change in the magnetic properties

is illustrated by the temperature dependence of the

magnetic susceptibility–temperature product for

Fe(o-phenanthroline)2(NCS)2, which undergoes a first-orderphase transition from a low- to a high-spin state at−97◦C(4), (Fig 2)

Materials that can be easily and reversibly stimulated

to change colors for an innumerable number of cycles havebeen exploited for display devices Liquid crystal displays(LCD) found in digital watches, are a common example(3) Materials that have greater switching speeds, sharpercontrast, and enhanced stability enabling more dutycycles may lead to improved display and memory devices

in the future Spin-crossover materials can exhibit sharpcolor changes from small changes in temperature (i.e., theyare dramatically thermochromic) As a consequence of thethermochromic mechanism (redistribution of the electrondensity within the molecule without either bond breaking

Figure 2 Temperature dependence of the magnetic

suscepti-bility–temperature product for Fe(o-phenanthroline)2 (NCS) 2 ,

which undergoes a first-order phase transition from a low- to a

high-spin state at 176 K ( −97 ◦C) (4a).

or forming), they are extremely fast and recyclable and

hence are candidates for high-data-density display and

storage devices of the future

For display/memory devices, it is necessary that the

transition temperature (T) is near room temperature,

∼22◦C This is, however, insufficient because the ambient

temperature fluctuates and hence the transition needs to

be effected over a broad temperature range, 17± 27◦C To

achieve this, the system must exhibit history-dependent

behavior (hysteresis) such that the transition temperature

for color change upon increasing temperature (T↑)

ex-ceeds the transition temperature for color change upon

de-creasing temperature (T↓) ideally by at least 50◦C, that is,

T ↑ − T↓ > 50◦C Molecules cannot exhibit hysteretic

ef-fects, but in a solid or film, interactions between molecules

can lead to hysteretic effects Hysteresis has been reported

for FeL2(NCS)2(L= (N2(CH)2N–)2], where T↑ = −128.7◦C

and T↓ = −149.5◦C (Fig 3) (5) Thus, although the

transi-tion and T↑ − T↓ temperatures are too low to be practical,

the necessary phenomena have been demonstrated, and

new systems that exhibit higher temperatures are needed

Using a mixture of triazole, HN(CH)2N2 (trz), and

aminotriazole, H2NN(CH)2N2 (H2Ntrz) ligands

coordi-nated with Fe(II), a polymer of [Fe(trz)3 −3x(H2Ntrz)3x]

23

Figure 3 Temperature dependence of the magnetic behavior

of FeL2(NCS)2 showing the low-moment (purple) behavior

be-low−149.5◦C (T↑) and high-moment (colorless) behavior above

Figure 4 Temperature dependence of the magnetic behavior

of Fe(trz) 2.85(H2 Ntrz) 0.15](ClO4 ) 2rnH2O showing the low-moment(purple) (Figs 5 and 6) behavior below 39 ◦C (T↑) and high-moment (colorless) behavior above 13 ◦C (T↓) (6).

(ClO4)2 r nH2O composition has been isolated, which for

x= 0.05 exhibits T↑ = 39◦C and T↓ = 13◦C (2,6,7 ) (Fig 4).These values bracket room temperature and demonstratethe feasibility of room temperature applications In ad-dition to the change in magnetic behavior, the color con-comitantly as with hysteresis occurs (Fig 5), from pur-ple to colorless at 21◦C (Fig 6) Solid solutions of triazoleand aminotriazole can be blended to lead to a systematic

change in the transition temperatures: T↑ = 296 − 160x

and T↓ = 313 − 180x in units of Kelvin.

Smart materials for future applications need to respond

to environmental stimuli, and spin-crossover materials (1)are a moderately large class of materials that respond toheat, light, and/or pressure This summary focuses on theuse of heat to change the electronic structure of a material,which in turn leads to substantial and reversible color,magnetic, and structural changes Most of the materialsdiscussed in this context are inorganic coordination com-plexes demonstrating that (1) reversible first-order transi-tions occur, (2) such materials exhibit the technologicallyimportant property of hysteresis, and (3) both the transi-tions and hysteresis can occur at room temperature

320 340 360 380 400 420Temperature, T, K

Figure 5 Temperature dependence of the optical density of

Fe(trz) 2.85(H 2 Ntrz) 0.15](ClO 4 ) 2rnH2O at 520 nm showing sis (purple) (1a).

hystere-Figure 6 Dramatic color change for the dark purple low-spin

state of [Fe(trz) 2.85(H 2 Ntrz) 0.15](ClO 4 ) 2rnH2O below 21◦C to the

colorless high-spin state at 21 ◦C (6).

ACKNOWLEDGMENTS

The author acknowledges continued partial support

by the Department of Energy Division of Materials

Science (Grant Nos FG02-86ER45271.A000,

DE-FG03-93ER45504, and DEFG0296ER12198) and helpful

discussions with Prof O Kahn

BIBLIOGRAPHY

1 (a) P G ¨utlich, A Hauser, and H Spierling, Angew Chem 33:

2024 (1994) (b) E Konig, G Ritter, and S.K Kulshreshtha,

Chem Rev 85: 219 (1985), P G ¨ utlich, Struct Bond 44: 83

(1981).

2 (a) O Kahn, E Codjovi, Y Garia, P.J van Koningsbruggen,

R Lapouyade, and L Sommier, ACS Symp Ser. 644: 298

(1996) (b) O Kahn and C.J Martinez, Science 279: 44 (1998)

O Kahn, J Kr¨ober, and C Jay, Adv Mater 4: 718 (1992).

3 C Esher and R Wingen, Adv Mater 4: 189 (1992) R Bissell,

N Boden, Chem Brit 31: 38 (1995).

4 (a) B Gallois, J-A Real, C Hauw, and J Zarembowitch, Inorg.

Chem 29: 1152 (1990) (b) M Sorai and S Seki, J Phys Chem Sol 35: 555 (1974).

5 W Vreugdenhil, J.H van Dieman, R.A.G de Graaff, J.G Haasnoot, J Reedijk, A.M van der Kraan, O Kahn, and