Schematic diagram of ML measurement devices a using a materials test machine for the compressive test and b using a friction test ma- chine for the friction shear stress test.. Dependenc
Trang 2O −Na+ P O
O −Na+
O −Na+
CH2 CH2 O
O O
O
CH2OH
NH 3+ OH
CH2OH
OH NH 3+
δ − n
n +
Tripolyphosphate (TPP)
Stirring
Figure 6 Scheme for preparing CS nanoparticles.
porosity of the matrix of the freeze-dried hydrogel
com-pared to that of the air-dried hydrogel (23)
The ionic interaction between the positively charged
amino groups of chitosan and negatively charged
coun-terion of tripolyphosphate (TPP, MW 4000) results in
polyelectrolyte complex formation and thus allows the
formation of beads in very mild conditions So, chitosan
nanoparticles can be prepared by ionic gelation with the
counterion TPP (24)
The particle size depends on both the chitosan and
TPP concentrations The minimum size (260 nm) has
been obtained The PEO or PEO–PPO block polymer
Synperonic®(1C1 Iberica, Spain) can be incorporated into
chitosan nanoparticles by dissolving these copolymers in
the chitosan solution either before or after adding the ionic
cross-linker TPP (Fig 6) TEM observation reveals that the
chitosan nanoparticles have a solid and consistent
struc-ture, whereas chitosan/PEO–PPO nanoparticles have a
compact core, which is surrounded by a thin but fluffy coat
presumably composed of amphophilic PEO–PPO
copoly-mer (25) Here, the presence of PEO–PPO within the
tosan nanoparticles can mask the ammonium group of
chi-tosan by a steric effect, thus hindering the attachment of
the BSA (isoelectric point pH = 4.8) These hydrophilic
chitosan/ethylene oxide–propylene oxide block copolymer
nanoparticles are very promising matrices for
administer-ing therapeutic proteins and other macromolecules that
are susceptible to interaction with chitosan (i.e., genes or
oligonucleotides) (26)
Mucoadhesive drug delivery systems can easily be
com-bined with auxiliary agents, such as enzyme inhibitors
Chitosan and EDAC [1-ethyl-3-(3-dimethylamino-propyl)
carbodimine hydrochloride] form chitosan–EDAC
conju-gates (10 mL of 1% chitosan HCl, 0.1M EDAC, and an
amount of EDAC that ranges from 0.454 to 7.26 g) at
pH 3.0 The chitosan–EDAC conjugates offers several
advantages as vesicles for peroral administration of tide and protein drugs: excellent mucoadhesive propertiesand strong inhibition of the proteolytic activity of zinc pro-teases, carboxypeptidase A, and aminopeptidase N Theconjugate that has the lowest amount of remaining freeamino groups seems to be a useful carrier in overcomingthe enzymatic barrier for perorally administered therapeu-tic peptides (27)
pep-The progress in biotechnology, coupled with an creased understanding of molecular mechanism underly-ing the pathogenesis of a variety of diseases of the genelevel, has effected dramatic changes in therapeutic moda-lities Recombinant DNA itself has been used like a “drug”
in-in gene therapy, where genes are applied to produce peutic proteins in the patient Oligonucleotides, relativelysmall synthetic DNA designed to hybridize specific mRNAsequences, are used to block gene expression To achievethese goals, the gene drugs must be administered via anappropriate route and be delivered into the intracellularsite of the target cells where gene expression occurs.Gene drugs have substantial problems as polyanionicnucleic acids, including susceptibility to degradation by nu-cleases and low permeability So, a suitable carrier system
thera-is the key to successful in vivo gene therapy Considering
that viral vectors have a number of potential limitationsinvolving safety, cationic lipid polymers developed as DNA
carriers can improve in vivo transfection efficiency.
Chitosan as a natural amino polysaccharide can form apolyelectrolyte complex with DNA For site-specific DNAdelivery, a quaternary ammonium derivative of trimethyl-chitosan with antenna galactose was synthesized It wasindicated that the galactose-carrying chitosan derivative
as a ligand provides cell-specific delivery of DNA to G2 cells The chitosan derivative binds to DNA via elec-trostatic interaction The resulting complexes retain theirability to interact specifically with the conjugate receptor
Hep-on the target cells and lead to receptor-mediated tosis of the complex into the cell (28) (Fig 7)
endocy-Separation Membranes
Chemically modified chitosan membranes have been used
in various fields, for example, metal-ion separation, gasseparation, reverse osmosis, pervaporation separation
of alcohol–water mixtures, ultrafiltration of biologicalmacromolecular products, and affinity precipitation ofprotein isolates
Pervaporation is a very useful membrane separationtechnique for separating organic liquid mixtures, such asazeotropic mixtures and mixtures of materials that haveclose boiling points In pervaporation, the characteristics
of permeation and separation are significantly governed bythe solubility and diffusion of the permeates
The separation mechanism of pervaporation is based
on the solution–diffusion theory, the adsorption–diffusion–desorption process of the components in the feed comingacross the membrane from one side to the other Therefore,pervaporation properties can be improved by enhancingthe adsorption of one component in the feed to the mem-brane and/or accelerating the diffusion of one component
in the feed through the membrane
Trang 3P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH
DNA complex
ReceptorOsmatic pressure
sink Receptor mediatedendocytosis
DNA
Figure 7 Scheme of gene delivery system via receptor-mediated
endocytosis.
Chitosan has been used to form pervaporation
mem-branes for separating water/alcohol mixtures and shows
good performance in dehydrating alcohol solutions To
en-hance fluxes with more free volume, chitosan membranes
cross-linked with hydrophilic sulfosuccinic acid (SSA) were
developed where two carboxylic acid groups and one
sul-fonic acid group offer ionic cross-linking with amine side
groups of the chitosan molecules Because the SSA
cross-linked membrane bears more binding sites for the target
compound to be separated than the cross-linked chitosan
membrane, the former membrane is better than the latter
membranes reported earlier (29)
Due to high permeability and good mechanical
proper-ties comparable to those of commercial cellulose acetate
membranes, the membranes have potential application in
pervaporation separation of aqueous organic mixtures (30)
The pervaporation properties of isopropanol–water
mix-ture via a chitosan–silk fibroin complex membrane are
shown in Fig 8 The data imply that the flux through the
complex membrane expresses ion (Al3 +) sensitivity, so the
pervaporative flux of the isopropanol–water mixture can
be modulated; the high selectivity was maintained by
op-timizing the AlCl3concentrations in the feed (31)
A benzoylchitosan membrane was designed for
sepa-rating a benzene/cyclohexane mixture, which is very
important in the petrochemical industry The benzene
permeation selectivity of the membrane is attributed to the
smaller molecular size and higher affinity of the benzene
molecules compared to those of cyclohexane (32)
Anionic surfactants for example, sodium lauryl ether
sulfate (SLES), can form complexes with the chitosan chain
through interaction of their opposite ionic charges The
complexes can survive as a skin layer (ca 15 µm) on a
polyethersulfone ultrafiltration membrane Here, the ionic
property of the surfactant–chitosan complex membrane
preferentially promotes the permeation of polar methanol
−6 −5
1020304050607080
20040060080010001200
2h)
Figure 8 Separation properties of an isopropanol–water mixture
through a chitosan–silk fibroin complex membrane by tion [silk fibroin content in membrane: 30% (w/w), isopropanol in feed: 85% (w/w), CAlCl3denotes the AlCl3concentration in the wa- ter part of isopropanol–water mixture].
prevapora-through the membrane, compared with the less polar and
relative bulky methyl-t-butylether (MTBE) (33).
A formed-in-place (FIP) membrane is dynamically ated by depositing polymers on the surface or at the en-trance of the pores of macroporous substrates ChitosanFIP ultrafiltration membranes can be formed on a macro-porous titanium dioxide substrate The chitosan gel mem-brane formed on the substrate is cross-linked enough by asupramolecular interaction, for example, hydrogen bond-ing The estimated mean pore size for the membrane nearneutral conditions (pH 6.0 and 8.2) is about 17 nm, and forthe membrane at pH 3.6, it is 55 nm The contraction andswelling of the chitosan membrane are reversible There-fore, it is possible to control the pore size of the membrane
cre-by simply adjusting the pH of the system according to theseparation requirement (34)
Smart polymers are the basis for a new protein lation technique—affinity precipitation The precipitationapplies a ligand coupled to a water-soluble polymer known
iso-as an affinity macroligand, which forms a complex with thetarget The phase separation of the complex, triggered bysmall changes in the environment, for example, pH, tem-perature, ionic strength, or addition of reagents, makes thepolymer backbone insoluble; afterward, the target proteincan be recovered via elution or dissolution Chitosan itselfhas been successfully used as a macroligand for affinityprecipitation of isolated wheat germ agglutinin from its ex-tract and glycosides from cellulose preparation by changes
in the pH of the media (35)
A partially sulfonated poly(ether sulfone) microporoushollow fiber membrane was coated with chitosan by electro-static attraction After cross-linking by reacting the fiberwith ethylene glycol diglycidyl ether (EGDGE), the hy-droxyl and amino glucose units of chitosan are then modi-fied to bind recombinant protein A (rPrA) as an affinity lig-and at 4.77–6.43 mg rPrA/mL fiber The immobilized rPrAhollow fiber membrane serves as a support for affinity sep-aration of immunoglobulin (IgG) (36)
Trang 4Immobilization Supports
Immobilization is an effective measure for using enzymes
or microbial cells as recoverable, stable, and specific
in-dustrial biocatalysts Immobilization must be carried out
under mild conditions
An ideal support for enzyme immobilization should
be chosen to achieve essential properties such as
chemi-cal stability, hydrophilicity, rigidity, mechanichemi-cal stability,
larger surface area, and microbial attack resistance
Sev-eral methods are used to modify the supports to improve
stability and mechanical strength, and to modify different
functional groups that may be superior for enzyme
immo-bilization One of the ways to improve these properties is
grafting of monomers onto the matrix The graft polymeric
support, for example, chitosan-poly(glycidyl methacrylate)
(PGMA), is used for glucosidase to immobilize urease
(37,38)
Immobilized urease can be used in biomedical
applica-tions for the removing urea from blood in artificial kidneys,
blood detoxification, or in the dialysate regeneration
sys-tem of artificial kidneys In the food industry, it may be
used to remove traces of urea from beverages and foods
and in analytical applications as a urea sensor
In addition to urease, other enzymes, for example,
glu-tamate dehydrogenase, penicillin acylase,β-galactosidase,
and glucosidase, have been immobilized via chitosan gels
Immobilization improves the stability of the enzymes
Extracellular Matrixes For Tissue Engineering
Various synthetic and naturally derived hydrogels have
recently been used as artificial extracellular matrices
(ECMs) for cell immobilization, cell transplantation, and
tissue engineering Native ECMs are complex chemically
and physically cross-linked networks of proteins and
gly-cosaminoglycans (GAGs) Artificial ECMs replace many
functions of the native ECM, such as organizing cells into
a 3-D architecture, providing mechanical integrity to new
tissue, and providing a hydrated space for the diffusion of
nutrients and metabolites to and from cells Chitosan is
similar to GAG Therefore, it is promising for application
as a biomaterial in addition to use as a controlled delivery
matrix
Chitosan is a basic polysaccharide, so it is possible
to evaluate the percentage of amino functions, which
re-main charged at the pH of cell cultivation (7.2–7.4) These
cationic charges have a definite influence on cell
attach-ment by their possible interaction with negative charges
located at the cell surface Chitosan materials give the
best results in cell attachment and cell proliferation for
chondrocytes and keratinocytes of young rabbits compared
with its polyelectrolyte complex with glycosaminoglycans,
such as chondroitin 4 and/or 6 sulfate and hyaluronic
acid (39)
Field Responsive Materials
Chitosan-based gels consist of a positive charged network
and a fluid (e.g., water) that fills the interstitial space of the
network The gels exhibit a variety of unique field
respon-sive behaviors, such as electromechanical phenomena
Figure 9 The EMC behaviors of chitosan/PEG crosslinked with
different concentrations of ECH.
Electromechanochemical (EMC) behavior deals withcontraction of polymers in an electric field The effects of
an electric field on polyelectrolyte hydrogels relate to theprotonation of its alkaline amino groups and redistribu-tion of mobile counterions when chitosan/PEG compositefibers is cross-linked with epichlorohydrin (ECH) and glu-taraldehyde (GA), respectively The EMC behavior of fibers
in a 0.1% aqueous HCl solution in a 25-V dc electric field
is shown in Fig 9
The bending direction of the fiber specimen inverts at
a critical concentration of the cross-linking agents Whenthe ECH concentration is more than 9.0× 103M or the GAconcentration is larger than 5.64× 104M, the fiber speci-mens bend toward the cathode If the ECH or GA concen-tration is less than the critical values, they bend towardthe anode The reason may be attributed to variation inthe mobile ions within the network (40)
Thin films of chitosan and chitosan doped with earth metal ions can be used as wave-guiding materials.They are transparent across the wavelength range of 300–
rare-3000 nm and exhibit low optical loss (less than 0.5 db/cm2)(41,42)
Chitosan/acetic anhydride and acrylate/chitosan gels have an excellent laser-damage threshold (LDT) up
hydro-to 35 times higher than commercial poly(methyl acrylate) (PMMA) bulk materials, and their LDT increases
meth-as water content incremeth-ases As we know, absorbed lmeth-aser ergy can lead to rapid local heating of a laser “hot spot.” Ahydrogel can be considered a composite of statistically dis-tributed microchannels and/or fluctuating pores created bythe movements of polymer segments within the network
en-in the presence of water When a hydrogel is irradiated,the energy generated by laser light can be absorbed anddispersed by the water and the polymer frame These hy-drogels have potential applications as new materials forhigh-power laser-damage usage (43)
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PB091-C1-Drv January 11, 2002 22:18
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T Furusona, H Saito, C Leng, G Chen, X Wu, J.E Matsuvra, and W.R Gombotz, Graft copolymers of stimuli-responsive polymers on biomolecule backbone: Synthesis and biomedical
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COATINGS
CHAO-NANXU
National Institute of Advanced
Industrial Science and Technology (AIST)
Tosu, Saga, Japan
INTRODUCTION
Many materials emit light during the application of a
mechanical energy This phenomenon is usually referred
to as mechanoluminescence (ML) or triboluminescence (1)
The more historical term is “triboluminescence.” It stands
for tribo-induced luminescence, and this was the term used
for more than a century to refer to light emission induced
by any type of mechanical energy (2) The term
“mechano-luminescence” was not proposed until 1978 (3) The
pre-fix “mechano” is correlated to the general mechanical way
used for exciting luminescence, including concepts such as
deformation, piezo, tribo, stress, cutting, grinding, rubbing,
and fracto In recent years mechanoluminescence (ML)
has become the preferred nomenclature (4) Although the
transfer of mechanical stress into light radiation is very
complex, successes in experimental applications suggest
possible uses of the ML phenomena in stress sensors,
me-chanical displays, and various smart systems
In general, ML can be divided into
fractolumines-cence (destructive ML) and deformation luminesfractolumines-cence
(nondestructive ML); these correspond to the
lumines-cence induced by fracture and mechanical deformation of
solid, respectively Roughly 50% of solid materials gives
fractoluminescence by fracture (5): the well-known
ma-terials include sugar (6), molecular crystals (7,8), alkali
halides (9,10), quartz (11), silica glass (12–14), phosphors
(15), piezoelectric complex (16), metals (17), various
min-erals (18,19), and biomaterials (20) Recently, the
frac-toluminescence of rare-earth complexes was investigated
in order to build smart damage sensors capable of simplereal-time detection of the magnitude and location ofstructural damage within materials (21) Deformation lu-minescence can be induced by mechanical deformationwithout fracture, and this is of interest in nondestructiveevaluation Deformation luminescence can be further di-vided into plasticoluminescence and elasticoluminescence.The former is produced during plastic deformation ofsolids, where fracture is not required, and the later is pro-duced during the elastic deformation of solids where nei-ther plastic deformation nor fracture is required Nonde-structive ML due to plastic deformation has been observed
in several materials such as colored alkali halides (22,23),II–VI semiconductors (24), and rubbers (25) However, ML
in the elastic region has been observed only for the ated alkali halides (4,26), and some piezoelectric materials(27) So far nondestructive luminescence intensities of ma-terials have been reported to be too weak and difficult torepeat, and this has deferred any practical application ofthe phenomenon For application of ML in developing newmaterials, repetitive ML must occur with undiminishedintensity
irradi-Devices for ML Measurement
Both mechanical and optical devices are being used to sure ML The objective is to apply the measured mech-anical energy to the ML sample, and then to detect thelight induced by the mechanical energy The various tech-niques already investigated include compression, bending,stretching, loading, piston impact, needle impact, cleavingand cutting, laser, shaking, air-blast, scratching, grindingand milling, and tribo- and rubbing (4) Figure 1 givespopular measurement devices for nondestructive ML;these devices measure compression, tension, bending, andshearing Figure 1(a) shows a schematic diagram of an
mea-ML measurement device capable of measuring mea-ML strain–stress relations simultaneously Stress is applied on eachsample by a materials test machine The ML intensity ismeasured by a photon-counting system that consists of aphoto multiplier tube (R464S, Hamamatsu Photonics) and
a photon counter (C5410-51, Hamamatsu Photonics) trolled by a computer The ML emission light is guided tothe photo multiplier through a quartz glass fiber The MLspectrum is obtained with a photon multichannel analyzersystem (PMA 100, Hamamatsu Photonics) The ML im-ages are recorded with an image intensified charge coupleddevice (ICCD) controlled by a computer system (C6394,Hamamatsu Photonics Corp.) Simultaneously, the stressand strain of the sample are measured by an in-situ sen-sor In addition to compressive test, the materials test ma-chine shown in Fig 1(a) is able to apply tensile and bendingstresses by exchanging the sample holder
con-Figure 1(b) shows a schematic diagram of an ML surement device for applying friction (shear stress); thesame equipment as shown in Fig 1(a) is used to measurethe ML intensity and spectrum The mechanical friction
mea-is applied by a friction rod under a load The friction rodmaterial, as well as the load, can be changed for differentlevels of mechanical stress applied to the test material As
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PB091-C1-Drv January 11, 2002 22:18
COATINGS 191
Computer Image
(a)
ICCDcamera
Loadcell
Sample
Glassfiber
channelanalyzer
Strainamp
Recorder
MotorSpeed
controller
VSample
hυ
(b)
Figure 1 Schematic diagram of ML measurement
devices (a) using a materials test machine for the compressive test and (b) using a friction test ma- chine for the friction (shear stress) test.
the test sample is rotated at a controlled speed, as shown
in Fig 1(b), the friction rod draws a concentric circle on the
test material The ML emission light induced by friction is
guided to the PM through a quartz glass fiber that is 3-mm
in diameter; the distance between the glass fiber and the
friction tip is set at 40 mm The mechanical impact is
ap-plied by using a free-falling ball through a steel guide pipe
The impact velocity is adjusted by the height of the falling
ball, and the impact energy can also be adjusted by both
the weight of the ball and the falling height
NONDESTRUCTIVE ML FROM ALKALINE ALUMINATES
DOPED WITH RARE-EARTH IONS
As previously mentioned, development of materials with
strong nondestructive ML intensity is an important goal
in exploring applications of ML Recently, systematic
terials research has resulted in producing a variety of
ma-terials that emit an intensive and repeatable ML during
elastic deformation without destruction: among these are
ZnS: Mn, MAl2O4:Re (M= alkaline metals, Re = rare-earth
metals), and SrMgAl10O17:Eu (28–32) So far the most
promising ML materials are the rare-earth ions doped
al-kaline aluminates and the transition metal ions doped zinc
sulfide Remarkable upgrading in ML intensity has been
achieved in the SrAl2O4doped with europium by
control-ling the lattice defects in the material
Preparation of Fine Particles of ML Materials and Their Smart Coatings
The luminescence powders are normally produced by asolid reaction process using a flux In the solid reac-tion process, the starting materials of ultrafine powder ofSrCO3, Al2O3, and Eu(NO3)3.2H2O, H3BO3(flux) are tho-roughly mixed The mixture is calcined at 900◦C for 1 h andthen sintered at 1300◦C for 4 h in a reducing atmosphere(H2+ N2) However, this process has the limitation thatsmall particles cannot be obtained because of the growth
of grains that occurs during the calcinations at high ratures To address this problem, a modified sol-gel methodhas been developed for preparing fine powders of SAO-E(33) In this modified sol-gel process, the starting mate-rials of Sr(NO3)2, Al(O-i-C3H7)3, and Eu(NO3)3.2H2O aredissolved in H2O and thoroughly mixed with NH.3H2O Thesol solution is then dispersed by HCON(CH3)2, followed bydrying, calcining, and finally sintering in a reducing at-mosphere at 1300◦C for 4 h In comparison with SAO-Epowders synthesized by the solid reaction process, finerparticles are obtained by the modified sol-gel process, theirmean size being about 1µm; the particles obtained by the
tempe-solid reaction process are about 15 µm The finer
parti-cles exhibit high ML, and as a result smart coating can beapplied in uniform layers on the surfaces of the target ob-jects by mixing it with binders and polymers For example,standard coating techniques such as spin coating and spray
Trang 80 5 10 15 200
50100150200250300350
coating could then be used to create uniform layers (films)
of SAO-E/epoxy on the surfaces of plastics, rubbers, glass,
ceramics, and metals The thicknesses of the resultant
coating could be controlled from micrometer to millimeter
Smart coatings made by mixing SAO-E fine powder with
an optical epoxy can transfer mechanical energy into the
photo energy of light emission, which in turn is capable
of sensing the dynamic stress of substrate materials To
obtain the ML intensity and stress distribution, the
com-posite samples of the SAO-E/epoxy have been included in
the ML measurement together with the coated samples
and the ceramics of SAO-E
ML Response of SAO-E to Various Stresses
In upgrading the ML intensity of SAO-E for the smart
coating application, the composition, the PH value, and
the calcination conditions must be controlled Significant
Figure 3 (a) Typical time history of the
luminescence intensity recorded during a
compressive test for a SAO-E/epoxy
com-posite with a dimension of 55 × 29 × 25
mm3, (b) Diminishment in ML peak
inten-sity during application of repetitive cycles
of loading and its recoverability with UV
4080120160
10005000
1500200025003000
ML intensity is obtained by Sr0.975Al2O3.985:Eu0.01 with alattice defect of 1.5 at% Sr vacancy Such a nonstoich-iometry system is found to produce one order of magni-tude higher ML than a stoichiometry system This is fourorders of magnitude higher in intensity than that of thereported strong ML material of a quartz crystal The lumi-nescence of the defect-controlled SAO-E material gives ahigh enough ML to monitor the stress of the object it coats.The influences of the stress and strain rates on the emit-ted light intensity are measured using the same strain ratebut at different peak stresses, and then the same peakstress but at different strain rates Figure 3(a) shows atime history of a luminescent object recorded during theapplication of a compressive stress with a constant strainrate for a SAO-E/epoxy composite with a dimension of
55× 29 × 25 mm3 As can be seen, over time a linearlyincreased load system results in linearly increased MLintensity That is, the ML intensity emitted is linearlyproportional to the magnitude of the applied stress Fig-ure 3(b) shows the ML intensity diminished during repet-itive cycles of loading The ML intensity decreased withthe repetitive cycles of stress, reaching a stable level atabout 20% of its initial strength The ML intensity ofSAO-E recovered completely after UV irradiation (365 nm)using a handy lamp Such recoverability distinguishedSAO-E from other nondestructive ML materials reported,for example, alkali halides which need γ -ray irradiation
or very high energy irradiation to recover their ity (34) The linear relationship between ML intensitivityand stress has been further demonstrated by the tests inwhich each test had the same strain rate but a differentpeak stress, as shown in Fig 4 The ML intensity increasedlinearly with the increase of strain rate Such a linear re-lation was also reported forγ -ray irradiated single crys-
intensitiv-tals of alkali halides during the elastic deformation (35)
It is evident that the ML of SAO-E is an cence In a comparison test, the defect controlled SAO-Ewas found to give the most intense elasticoluminescenceamong the materials examined to date (36)
elasticolumines-SAO-E was found also to exhibit ML during tic deformation and fracture In the region of plastic
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02.0 × 1044.0 × 1046.0 × 1048.0 × 1041.0 × 1051.2 × 1051.4 × 1051.6 × 105
Mechanical stress (MPa)
Figure 4 Influence of applied stress on ML intensity (strain rate
is 3.0 × 10 −4l/s).
deformation, ML became intensified because of the stress
that was concentrated in this region Upon further load
increases the ML intensity exhibited a sharp rise as the
SAO-E material began to crack, revealing the maximum
value in ML intensity at fracture Similar results were
ob-tained for ceramic samples of SAO-E The results for
al-kali halide crystals confirmed the presence of intense ML
in plastic deformation and that it sharply increases during
fracture of the crystals (4,5) Clearly, the linear relation
between strain and stress is an important factor in the
ap-plication of ML stress sensors
Figure 5 shows the relationship between the strain
rate and ML intensity The relationship is almost linear,
as a higher strain rate produces greater ML intensity
0102030405060
90
7080
Figure 6 Dependence of ML intensity on tensile stress for a
SAO-E/epoxy composite sample with a dimension of 100 × 25 × 5 mm 3 The ML intensity induced by compressive stress is also plotted for comparison.
Expressed mathematically, the ML intensity in terms ofthe stress and strain rate is
I − I0= K σ (t)˙ε(t), (1)
whereσ is the stress and ˙ε is the strain rate Equation (1)
indicates that the linear intensity of ML is consistent withthe concept of an elastic (linear) region
The dependence of ML on tensile stress is shown inFig 6 for a SAO-E/epoxy composite sample with mea-surements of 100× 25 × 5 mm3 For comparison, the MLintensity induced by compressive stress is also plotted.Note that the SAO-E exhibits the same ML intensitywhether or not the stress is compressive or tensile Figure 7shows the dependence of ML on torsion measured by ashearing test machine for a SAO-E composite sample withdimensions ofφ10 mm × 110 mm Note that the ML inten-
sity increases linearly with the increase of torsion Clearly,the nondestructive ML of SAO-E can detect shear stressand torsion changes without any volume changes This isvery different from the thermography technique that candetect stress based on the thermo-elastic effect when stress
is accompanied by a volume change
As previously mentioned, SAO-E ceramics and posites exhibite distinct ML behavior Smart coating withSAO-E/epoxy can be applied to objects to sense changes ofstress Figure 8 shows the dependence of ML intensity onthe stress for a plastic coated with an SAO-E/epoxy layer.The results are similar to those of composites and cera-mics The ML intensity almost linearly increases with the
Trang 10Torsion (kgcm)
Figure 7 Dependence of ML intensity on torsion measured by a
shear test machine for a SAO-E composite sample with dimensions
increase of stress when the strain rate is kept constant
Additionally, the ML intensity increases with the increase
of the thickness in the ML layer A thick layer is generally
not suitable for stress detection because it produces strain
or stress in the ML layer that differs from that of the
ob-ject beneath it As these results indicate, a uniform layer is
necessary for an accurate display of the stress distribution
of the object
050100150200250
Figure 8 Dependence of ML intensity on the stress for a plastic
block coated with SAO-E/epoxy layers with different thickness.
Contrary to the behavior of the destructive ML (37,38),the width of the ML emission band at the peak wavelength
is independent of the stress level during elastic tion However, the ML intensity at peak emission increaseslinearly with the increase of stress level Furthermore, theintegrated intensity under the band also increases linearlywith stress As noted earlier, the ML of SAO-E can be in-duced by compressive, tensile, or shear stress Smart coat-ing using ML materials provides a simple way to detectthese stresses dynamically and remotely
deforma-ML Mechanism of SAO-E
The ML spectrum is measured by a photon multichannelanalyzer The broadband emission peaks at a wavelength ofabout 520 nm, which is the same as the photoluminescence(PL) spectrum measured by a fluorescence spectrometer
As shown in Fig 9, the PL and ML spectra from SAO-Eare characterized by emissions that peak near 520 nm Noother emission bands are found in the ML spectrum at 300
to 700 nm This implies that ML is emitted from the sameemission center of Eu ions as PL; both are produced bythe transition of Eu2 +ions between 4f7and 4f65d1(39,40).Emissions due to N2 discharge have not been observed,which generally occur in destructive ML (fractolumines-cence) (41) These results confirm that the ML of SAO-Edescribed here is produced by a nondestructive deforma-tion of SAO-E Moreover, the recovery of ML intensity by
UV irradiation suggests that the traps in SAO-E samplecan be filled by UV irradiation Measurements of the Halleffect of UV-activated SAO-E reveal traps of holes filled by
UV, and this is consistent with other reports (39,44) Thedepths of these hole traps can be evaluated by the Hoogen-straaten method (42) This technique calls first for thermo-luminescence glow curves to be measured at different rates
of heating (β) to obtain the glow peak temperature (Tm) foreach heating rateβ, and then for the depth of trap (Et) to be
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Figure 10 (a) Glow curves of thermoluminescence from SAO-E at different heating rate of 0.091,
0.187, and 0.259 K/s for A, B, and C, respectively; (b) resultant Hoogenstraate plot, whereβ is a heating rate and Tm is a glow peak temperature.
calculated from the slope of the Hoogenstraaten plots using
the equation
Et= −klog e
β/T2 m1/Tm
where k is the Boltzmann constant Figure 10(a) and (b)
shows the glow curves of SAO-E and the resultant
Hoogen-straaten plots, respectively Two glow peaks are found for
SAO-E as shown in Fig 10 (a), implying that there are
dif-ferent kinds of traps in the material The depth of the trap
associated with lower Tm is about 0.2± 0.1 eV, which is
much higher than the thermal energy of 350 K (0.03 eV)
Consequently these hole traps can not be thermally
acti-vated at room temperature On the other hand, it is
evi-dent that traps at levels near 0.1 eV can be activated by
deformation to release electrons to the conduction band in
the case of KCl (41) The ML kinetic model for SAO-E
pro-posed in Fig 11 takes these results into account During
deformation the strain energy excites the filled traps (T+)
to release holes to the valence band (process 1) The holes
then excite Eu+to produce Eu2 +∗(process 2), and return
to ground state by emitting a green light of about 520 nm
(process 3)
The hole traps in the model are attributable to the
lat-tice defects of Sr2 + (44), which can greatly affect the ML
intensity as was indicated in Fig 12 For this reason the
SAO-E is classified as a defect-controlled ML material
whose intensity is substantially altered by the existence
of the trap (defect) The well-known alkali halides also
be-long to this outgoing However, ZnS:Mn is a
piezoelectric-induced material as will be described in the next
1
5d
4f
Eu2 + (Eu+ 1)
Figure 11 ML kinetic model for SAO-E.
REPEATABLE ML OF TRANSITION METAL IONS DOPED ZINC SULFIDE
As was previously noted, the nondestructive ML of SAO-Eshowed diminished intensity during the application of arepetitive stress cycle, similar to that of alkali halides (45).ZnS-doped Mn has been found to give undiminished MLintensity during the elastic deformation, so it is the repre-sentative material to achieve the most repetitive ML
Preparation of Highly Oriented ZnS:Mn Films
Thin films can be prepared from a ZnS pellet on varioussubstrate materials, including quartz glass, stainless, car-bon, and ceramics (Al2O3, SiC, Si3N4), by physical vapordepositions of the ion plating method (46) The substrates
Trang 12Figure 12 (a) ML response of ZnS:Mn to a friction (shear stress); (b) dependence of ML intensity
on the friction load.
are kept at 160◦C, and the deposition is carried out in a
vacuum of 0.2 Pa in Ar atmosphere The source pellet is
prepared from Zn0.985Mn0.015S powder by a cold isotropic
press method followed by sintering in a vacuum-sealed
quartz glass tube at 1000◦C for 10 h Such a
pretreat-ment is applied to fabricate highly crystallized pellets
whose deposition rate is very stable and easily controlled
A vacuum-sealed technique is used because ZnS begins
to sublimate at temperatures above 700◦C The as-grown
films are annealed at 500 to 1000◦C for 1 h in a
vacuum-sealed quartz glass tube Their chemical composition is
determined by fluorescent X-ray spectrometric analysis
The Mn amount in the film showed the same as that in
the source material, namely 1.5 at% The XRD pattern
ex-hibited a strong diffraction peak at 28.49◦in the 2 range
of 10 to 90◦, which was attributed to the (111) plane of the
ZnS film was highly oriented
ML Characteristics of ZnS:Mn Films
The luminescence intensity depends on the
microstruc-ture of film Post-heat treatment in vacuum is effective
for obtaining high ML in that it increases the crystallinity
of ZnS:Mn and decreases the defects and residual stress
in the film Moreover, the connection between the film
and substrate is strengthened by annealing, even for film
thicker than 1µm, and thus prevents deprivation due to
moisture or mechanical attack The ML intensity is
en-hanced by two magnitudes in order after annealing at
700◦C, and the mechanical strength of the ZnS:Mn film
is also remarkably strengthened (46) Surface observation
shows that the film consists of ZnS grains with a mean size
of several nano-meters
Figure 12(a) shows the ML response of ZnS:Mn to
friction (shear stress) measured by the device shown in
Fig 1(b) The ML intensity increases steeply when the tion on the ZnS:Mn film is turned on, and the frequency
fric-of the oscillating change equals the rotating speed fric-of thetest sample This oscillating change is also found in thefriction force by a strain gauge attached to the frictionrod The oscillation may be caused by nonuniformity ofthe test material, as is similarly the case in bulk ceramics(47) The response curve is reproducible, and this indicatesthat the film is combined strongly onto the substrate, as isconfirmed by the SEM image and adhesive strength test.This reproducibility distinguishes the ML of ZnS:Mn fromthe other destructive ML It has been found that ZnS:Mnshows a repetitive ML response not only to friction but also
to other types of stresses such as compressive stress (30).These results are substantially different from the otherelasticoluminescent materials reported so far like gammacolored alkali halide crystals and the SAO-E, where theintensity decreased in a great deal during the application
of repetitive stress Apparently the reproducibility is sentially important for self-diagnosis materials and appli-cations in various novel smart systems including stresssensors
es-The ML intensity of ZnS:Mn increases with increasingthe mechanical stress Figure 12(b) shows that the inten-sity increases linearly with the increasing applied load.Correspondingly, the mechanical friction can be monitoredwithout any mechanical contacts The linear relation bet-ween ML and stress of the ZnS:Mn has also been found inthe case of compressive stress
Figure 13 shows the ML response to a mechanical pact Note that the ML response transits of ZnS:Mn aresimilar to those of piezoelectric voltage responses, as re-ported previously (48) The energy conversion efficiency forconverting mechanical energy to photon energy, roughlyestimated from the experimental data, is on the order of
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5
010
10050
Figure 13 (a) ML response to a mechanical impact; (b) dependence of ML on the falling height.
10−2and 10−6for a slowly applied stress and impact cases,
respectively; this is also on the same order as was
re-ported previously for the piezoelectrics The ML intensity
increases linearly with the increase of the falling height
of the free ball; that is, ML intensity increases with the
increasing impact energy Correspondingly, the
mechani-cal impact can be remotely monitored using an ML film
More important, the ML intensity emitted by such a
high-oriented films is much higher than that of the bulk material
by more than one magnitude The significant improvement
in the ML intensity is attributed to the high orientation of
the created film and the nano-sized grains of which it is
composed
Mechanism of ZnS:Mn
ZnS is both a piezoelectric and electroluminescent
mate-rial Figure 14 gives the spectrum of the ML for ZnS:Mn
thin films, along with the photoluminescence (PL) and
elec-troluminescence (EL) The ML exhibits a maximum
emis-sion band at 585 nm, which is consistent with the
spec-tra for PL and EL of ZnS:Mn No additional emissions
due to the discharge of N2are found in the ML spectrum
of ZnS:Mn This indicates that the ML is introduced by
stress and the emission arises from the emitting center of
Mn2 +ions, due to the transition of4T1→6A1 The
stress-activated mechanism is supported by other experiments;
for example, when covering the ZnS:Mn film with a
trans-parent film of AlN, similar emission is seen Figure 15
shows the ML and PL intensities for ZnS:Mn films
de-posited on various substrates It is seen that the ML
in-tensities for conductor substrates like stainless and carbon
are much lower than those for dielectric substrates such as
quartz, alumna, silicon nitride, and silicon carbide When
a shear stress is applied on the ZnS:Mn film, a tric voltage based on the piezoelectric coefficient of d14will
piezoelec-be generated piezoelec-between the opposite sides of the thin filmsurfaces If the film is deposited on a conducting substrate,then electrical leakage may occur The presence of suchleakage is indicated by low ML intensity for ZnS:Mn onsteel and carbon substrates as shown in Fig 15
PLELML
Wavelength (nm)
Figure 14 Spectrum of the ML for ZnS:Mn thin films, along
with the photoluminescence (PL) and electroluminescence (EL) spectra.
Trang 14Luminescence intensity (a.u.)
Figure 15 ML and PL intensities for ZnS:Mn films coated on
EvStress applied
Figure 16 ML model of ZnS:Mn film, where the ML of ZnS:Mn
is proposed to be based on the superposition of piezoelectric effect
and electroluminescence.
Taking these results into account, the ML phenomenon
of ZnS:Mn can be interpreted by a piezoelectric-induced
electroluminescence model as shown in Fig 16 In the
figure the ML of ZnS:Mn is considered to be the
super-position of piezoelectric effect and electroluminescence,
and this can also be considered as the inverse effect of
photostriction (49) The proposed ML model can well
ex-plain the distinguished behavior of ML It is evident that
the high orientation and crystallinity of the ZnS:Mn film
give higher piezoelectric performance The higher
piezo-electric voltage produced on the opposite sides of the
nano-sized grains of ZnS leads to a higher intensity of
electroluminescence Meanwhile, the EL efficiency is
also improved with the increase in crystallinity (50–52)
Therefore the total effect is higher ML intensity The
re-peatable ML with an undiminished intensity of ZnS:Mn
is attributed to the reproducibility of the piezoelectric
effect
APPLICATION OF SMART COATING WITH ML
MATERIALS FOR NOVEL STRESS DISPLAY
Since ML intensity increases linearly with the increase of
stress and strain rate in the elastic region, the ML layer is
believed to be able to display a stress distribution of anyobject that it covers
Stress distribution is measured in solids to improvetheir reliability and extend their applications The distri-bution of stress in a solid is conventionally evaluated us-ing several techniques (53,54) Electric resistance straingauges and piezoelectric sensors are typical techniquesusing electrical signals The limitation to these methodsbecomes evident in analyzing the distribution on the mi-cro scale because of their size The sensors must maintainelectrical contact to the target objects, so it is difficult tomeasure the stress distribution of a dynamic moving partsuch as cutting tool or gas turbine Remote detection haspermitted the use of optical signals in experimental stressanalyses utilizing photoelastic and photoplastic effects,X-ray diffraction, optical fiber networks embedded in com-posites, and thermography based on thermoelastic ana-lysis, for example However, until recently there were nosimple techniques for the direct visualization of the stressdistribution in real time Current studies to solve this prob-lem have focused on building self-diagnosis systems usingsmart coatings of piezoelectric (55–57) and the ML mate-rials (30,58,59)
Visualizing Stress Distribution
To view the stress distribution of an object, a smart coating
of ML material is applied on the surface of the object The
ML images are recorded during the application of stress
on the ML-layer/object Figure 17 shows an example of aplastic disk coated with a SAO-E/epoxy film (50µm) The
intense green light emitted into the atmosphere from thetwo ends of the stressed sample can be clearly observed
by the naked eye Also various stress images can be ulated using the finite element method (60) It has beenfound that the strain energy distribution is in agreementwith the ML image, as compared in Fig 17(b) and (c) This
sim-is supported by the argument in the previous sections Inaddition, since the strain rate is a constant in the measure-ment, this ML image is consistent with the stress distribu-tion Figure 18 illustrates the stressed sample and the linedistribution of ML intensity and stress along CCaxis The
ML intensity distribution along COC, which was obtainedexperimentally by the ICCD camera, is given by a solidline, and the stress distribution of the sample, which wassimulated theoretically based on elastics (61), is shown by adotted line As compared in Fig 18(b), the simulated com-pressive stress along COC increases exponentially with
the increasing of r/R This is consistent with the line
pro-file of the ML image, indicating that the ML image reflectswell the stress distribution (stress image) under the ex-perimental conditions Therefore, smart coating with MLmaterials can directly display the stress distribution of theobject beneath the layer
Monitoring in Dynamic Stress and Impact
Dynamic ML images have been successfully recorded ing the application of different stresses, including bend-ing, tension, compression, and impact (62) The ML images
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O
.341E−15.512 E−15.682 E−15.853 E−15.102 E−14.119 E−14.136 E−14.154 E−14
Figure 17 Stress distribution images for plastic disc (φ25 mm × 10t mm) coated with SAO-E/epoxy
layer under compressive test: Stressed sample (a); ML image at a load of 500 N (b); simulated image
of strain energy distribution using the finite element method (c).
Trang 16Oa
Compression stress (a.u.) Emission intensity (a.u.)
Figure 18 Stressed sample (a) and the line distribution profiles
of ML intensity and stress along CC axis (b).
dynamically changed with the loading, and were found
to be in good agreement with the stress
concentra-tion results obtained by computer simulaconcentra-tion and other
experimental stress analyses This imaging method gives
the dynamic stress distribution in real time It is
distin-guished from thermography, which requires repetitive
cy-cles of stresses and thus is limited in evaluating stress
dur-ing the fatigue process Moreover, the present ML images
are strong enough to be seen by the naked eye in a darkened
room
Photographs in Fig 19(a) and (b) show the dynamic
vi-sualization of impact and friction, respectively, for a quartz
substrate coated with the (111)-plane-oriented ZnS:Mn
film After applying mechanical impact caused by a
free-falling ball, the yellow emission shown in Fig 19(a) was
recorded Mechanical friction caused the strong ML
(a)
(b)
Figure 19 Photographs of the dynamic visualization of impact
(a) and friction (b) for a quartz substrate coated with the plane-oriented ZnS:Mn film.
(111)-recorded in real time The ML emission from the ZnS:Mnfilm was strong enough to be clearly seen by the naked eye.The same method can be applied in an aqueous environ-ment Real-time ML images of stress distributions wereobtained in water, ethanol, acetone, and 0.1 M HCl, forexample, although the ML intensity values were depen-dent on the environment due to the different refraction andadsorption values These results show the practicability ofthe present method in environment uses
In particular, the ML smart coating technique holdsmuch promise for observing the stress distribution withhigh spatial resolution using ML materials with nano-scale particles and the optical microscopy with high reso-lution Although more experiments are needed, in the nearfuture stress distributions in a scale smaller than mi-crometers should become observable as image techniquesare combined with microscopy Already the smart coatingtechnique has enabled us to view stress distributions onboth macro and micro scales
The application of smart coating with an ML layer toanalyze dynamic stress not only provides a new methodfor nondestructive evaluation of materials In addition
to the mechanical display, ML smart coating has opened
a window on developing new smart systems and mechanical devices
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Trang 18COLOSSAL MAGNETORESISTIVE MATERIALS
A MAIGNAN
Laboratoire CRISMAT, ISMRA
CAEN Cedex, FRANCE
INTRODUCTION
Magnetoresistance (MR) is defined as the relative change
in the electrical resistivity of a material upon the
applica-tion of a magnetic field and is generally given by %MR=
100× {[ρ(H) − ρ(0)]/ρ(0)}, where ρ(H) and ρ(0) are the
re-sistivities at a given temperature in and in the absence
of a magnetic field, respectively MR is positive for most
nonmagnetic metals, and its magnitude is limited to a few
percent, whereas MR can be negative in magnetic
mate-rials because the magnetic field tends to reduce the spin
disorder For instance, the %MR of Co and Fe is∼−15%
MR is of considerable technological interest IBM is
us-ing the Permalloy (composition: 80% Ni and 20% Fe) MR of
about 3% in a small magnetic field at room temperature for
the magnetic storage of information More recently, larger
magnetoresistance also called giant MR (GMR) was
ob-served in thin films of magnetic superlattices (for instance,
Fe, Cr) for which metallic layers of a ferromagnet and a
nonmagnetic material (or an antiferromagnet) are
alter-nately deposited on a substrate (1,2) By doing so, the MR
magnitude is increased by an order of magnitude Small
ferromagnetic particles deposited on a paramagnetic thin
film also provide an alternative way to obtain GMR devices
(3) For both material classes, small magnetic field
appli-cations (a few oersteds) are sufficient to align the
magneti-zations ferromagnetically and thus to induce a resistivity
decrease originating in decreased scattering
In hole-doped perovskite manganites Ln1 −xAExMnO3
where x∼ 0.3, magnetoresistance values of ∼−100% in
large magnetic fields (several teslas) have been
discov-ered These effects are called CMR to distinguish them
from GMR (4–11) CMR has motivated a large number
of experimental studies of these oxides in bulk
(ceram-ics and crystals) and in thin films and also of
theoret-ical work to understand the origin of the phenomenon
In the 1950s, the double-exchange model (DE) was
pro-posed to explain the simultaneous appearance of
ferromag-netism and metallicity when Mn3 +/Mn4 +valency is created
in La1 −xSrxMnO3(12–14) However, after the CMR
disco-very, several theoretical studies have shown that double
exchange alone cannot explain the magnitude of the
resis-tivity drop upon the application of a magnetic field (15)
The distorted Jahn–Teller Mn3 +O6octahedron introduces
an interaction between the charge carriers and the crystal
lattice so that the bound-state charge and a lattice called a
“polaron” has been proposed and experimentally evidenced
(15–19) Consequently, the Jahn–Teller distortion, static or
dynamic, must be incorporated in any model, built to
de-scribe CMR This time-dependent increasing complexity
has been more recently confirmed by the relevancy of the
phase-separation scenario for manganese oxides (20–25)
Roughly, recent computational studies in which
ex-tended coulombic interactions are included have revealed
that, as the Mn3 +/Mn4 +mixed valency is created by ing x in Ln1 −xAExMnO3, the transition from the antifer-romagnetic insulating state for x = 0 toward ferromag-netism for hole-doped compositions, where x 0, does notoccur via intermediate phases (canted phases) but ratherthrough a mixed-phase process (20) An inhomogeneouselectronic state should be stabilized, and several experi-mental studies have confirmed this phase-separation sce-nario (21–25) Consequently, the large droping resistivity
vary-at the origin of the qualifying “colossal” magnetoresistance
is interpreted in a percolation framework: a magnetic fieldincreases the ferromagnetic metallic regions at the ex-pense of the insulating antiferromagnetic areas, so thatthe macroscopic insulating state becomes metallic beyondthe percolation threshold (26)
In this article, several representative examples of ovskite manganites are given to illustrate the richness oftheir phase diagrams More particularly, the chemical keyfactor governing the CMR of hole-doped manganites thatcontain 30% Mn4 +and have the Ln0.7AE0.3MnO3formulaare reviewed The existence of Mn3 +/Mn4 +charge ordering
per-in the Mn lattice for half-doped manganites (Mn3 +: Mn4 +=
50 : 50, i e., x= 0.5) and also for Mn4 +-rich compositions(electron-doped, x> 0.5) are discussed Finally, the possi-
bility of obtaining CMR properties in Mn4 +-rich nites is shown
manga-CMR IN HOLE-DOPED Ln0.7AE0.3MnO3 PEROVSKITES
Among the first compositions that were investigated, thosewhere x= 0.3 in Ln1 −xAExMnO3, have the best CMR prop-erties (4–10) This is the case for Ln = Pr3 + and AE=
Ca2 +/Sr2 + that have the formula Pr0.7Ca0.3−xSrxMnO3(27,28) Some of the latter compositions show resistivity(ρ) drops in a magnetic field (µ0H = 5T) when the ra-tio ρ(0)/ρ(5T) is from 104 to 1011, as shown in Fig 1 for
Pr0.7Ca0.25Sr0.05MnO3(x= 0.05) and Pr0.7Ca0.26Sr0.04MnO3(x = 0.04), respectively By cooling the x = 0.05 samplefrom 300 K to 5 K in the absence of a field, the activatedcharacter ofρ observed till 90 K evolves to a metallic char-
acter below that temperature;ρ decreases by about four
orders of magnitude at 20 K (Fig 1a) Then, by registeringtheρ data using the same process but in a 5-T magnetic
field applied at 300 K before cooling, one can clearly see
in Fig 1a the dramatic ρ decrease induced by the field
in the temperature vicinity of the insulator–metal (I–M)transition Five orders of magnitude are obtained fromthe isothermal ρ(0)/ρ(5T) at 88 K and consequently, the
pre-∼30 K Again, the field application seems to quench aquasi-T independent metallic state and theρ(0)/ρ(5T) ra-
tio reaches∼1012 This behavior has also been confirmed
by measuring isothermal field dependent ρ(H) curves (T = 50 K, Fig 2) At 50 K, the curve shows that a ρ drop
of 107is reached beyond the critical field of 0.6T
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Figure 1 T dependence of the resistivity ρ upon cooling in (5 T) and in the absence (0 T) of a
magnetic field for (a) Pr 0.7Ca0.25Sr0.05MnO3 and (b) Pr 0.7Ca0.26Sr0.04MnO3
These CMR properties are connected with the
mag-netic properties that show the great interplay between
the carriers and the spins Clear transitions from
para-magnetic (PM) to ferropara-magnetic (FM) are observed from
the T-dependent magnetization (M) curves of the x= 0.05
and x = 0.04 compositions (Fig 3) The corresponding
Curie temperatures (TC) taken at the inflection point of the
transition coincide with the I–M transition temperatures
Thus, for Pr0.7Ca0.25Sr0.05MnO3, the metallicity appears as
the sample becomes ferromagnetic For the other
compo-sition, Pr0.7Ca0.26Sr0.04MnO3, theρ(T) curve (Fig 1b) does
not show an I–M transition in the absence of a magnetic
field, although this ceramic exhibits a ferromagnetic state
(Fig 3) However, the M(T) curve has been obtained by
measuring in an applied field of 1.45T and aρ(T) curve
registered in the same field shows the I–M transition
This first set of data allows two important conclusions:
high magnetic values are required to obtain CMR effects,
Figure 2 Field-dependentρ curve for Pr0 7Ca .26Sr.04MnO
and small chemical changes are responsible for dramaticmodifications in physical properties
ORIGIN OF THE CMR EFFECT: MANGANESE MIXED VALENCY AND DOUBLE EXCHANGE
Manganese oxides Ln1 −xAExMnO3 crystallize in a ovskite structure (Fig 4), but their structures differ fromthat of the ideal cubic perovskite ABO3(29,30) The struc-ture can be described as a tridimensional network of MnO6octahedra linked by their apexes, so that cages are formedand are filled by the Ln3 +and AE2 +cations (A site of theperovskite) The distortion of the structure in manganites
per-is a consequence of the small size of the A-site cations
Figure 3 T-dependent magnetization curves found upon
warm-ing in 1.45 T after a zero-field-coolwarm-ing process (ZFC) for
Pr 0.7Ca0.26Sr0.04MnO3 (Ca 0.26) and Pr0.7Ca0.25Sr0.05MnO3 (Ca .25).
Trang 20which gives rise to the tilting of the MnO6octahedra This
distortion is quantified by the Goldsmith tolerance
fac-tor t = d A −O /[√2(dMn −O)], where dA −O and dMn −O are the
A cation–oxygen and Mn–oxygen bond lengths,
respec-tively Usually, for manganites t is ∼1 or t < 1 and,
con-sequently, because the tilting mode depends on t, several
kinds of crystallographic space groups can be evidenced
as the A-site average cationic size rA changes or as the
manganese valency (which controls the Mn–O distance)
varies
To understand ferromagnetic metallic properties (31),
the electronic configurations of the Mn3 + (3d4) and Mn4 +
(3d3) magnetic species must be considered Full rotational
invariance is broken in the octahedral environment, and
this creates the splitting of 3d orbitals in two egand three
t2g Due to the strong Hund coupling (JH) for this system,
the spins are aligned in the 3d shell (high-spin
configu-ration): three localized electrons populate the t2gorbitals
(t2g 3), whereas one electron (eg 1) or no electron (eg 0)
popu-lates the egorbital for Mn3 +and Mn4 +, respectively
More-over, the eg filling for Mn3 +creates a Jahn–Teller
distor-tion that degenerates the egorbitals in two levels, dz2 and
dx 2 −y 2 ; only the former is occupied (Fig 5) The eg
elec-trons of Mn3 + are mobile and they use the bridging
or-bitals of the oxygens to reach an empty egorbital of a Mn4 +
nearest neighbor This leads to the double-exchange model
proposed by Zener (12): the eg electron delocalization
be-tween nearest neighbor manganese ions that have t2g
para-llel spins (Fig 6) allows paying the energy JH and gains
some kinetic energy for the mobile carriers by minimizing
Figure 5 Electronic configuration of Mn4 + (3d3 ) and
Figure 6 Double-exchange mechanism according to Zener.
the hole–spin scattering Consequently, the FM dropletsaround the holes start to overlap as holes (Mn4 +) are in-jected in the Mn3 +matrix, and a fully FM metallic statecan be reached
From this model, one can understand that the CMR
ef-fect in the TC vicinity results from the field-induced romagnetic alignment, which creates delocalization andthus the resistivity decrease But, several experimentalresults exist, for instance, coexistence of FM and chargeordering (21–26) that have suggested that more complexideas are needed to explain CMR properties At present,the phase-separation scenario (20) seems to be relevantfor manganese oxides Several examples that support thismodel are described in the following
fer-CHEMICAL FACTORS GOVERNING CMR PROPERTIES
Two important factors have to be considered to control themagnetism in these systems: the hole concentration andthe overlap of the Mn and O orbitals (11,32,33) The firstcorresponds to the content of Mn4 +in the Mn3 +matrix andcan be tuned by varying the A-site cationic Ln3 +/AE2 +ra-
tio The best concentration for obtaining the highest TCcorresponds to about 30–40% Mn4 +; far below this con-tent, the FM regions do not percolate (FM insulating “FMI”state), and beyond, other complications arise from thecloseness to the “half-doped” Ln0.5AE0.5MnO3compositionsthat are highly favorable for antiferromagnetism (AFM).This is clearly seen in Fig 7 where the Pr1 −xSrxMnO3phase diagram (34) is given; if one concentrates on thehole region (x < 0.5), a clear TC optimum of ∼280 K is
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COLOSSAL MAGNETORESISTIVE MATERIALS 205
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Figure 7 Magnetic and electronic
phase diagram of Pr 1 −x Sr x MnO 3 that shows the great complexity of these systems as the Mn valency varies The magnetic transition temperatures
N´eel (TN) and Curie (TC ) are zed by black triangles and circles, re- spectively The gray curve (gray cir- cles) corresponds to the magnetization values at 4.2 K in 1.45 T (ZFC) The
symboli-highest TC of 280 K is reached for
Pr0.6Sr0.4MnO3 (x = 0.4).
observed for x∼ 0.4 For the same Mn valency, the TC
max-imum of La1 −xSrxMnO3reaches 370 K (35), and the TCof
La1 −xCaxMnO3is 280 K (36)
The overlap of the 3d orbitals of the Mn species and
of the oxygen p orbitals are controlled by varying the
Mn–O–Mn angle, which can be done by changing rA
The effect of rA on CMR properties was shown
simulta-neously by several groups (11,32,33) If we return to the
Pr0.7Ca0.3−xSrxMnO3series and more especially to theρ(T)
and M (T) curves where x varies by 0.01 increments from
0.04 to 0.10 (Fig 8), the following remarks can be made:
(1) the resistivity drop at the I–M transitionρTI–M/ρ5 K
in-creases as the strontium content dein-creases, from 170 for
0.070.06
0.04
0.050.060.070.101.45 T
T(K)
(b)
Figure 8 (a)ρ(T) and (b) M(T) curves of Pr0 7Ca 0.3−xSr x MnO 3 x values are labeled on the graphs.
x= 0.10 up to 3×105for x= 0.05 (Fig 8a); (2) both TI–Mand
the Curie temperature TC(Fig 8b) increase as x increases.These are very important results because they demon-strate that the physical properties are highly sensitive to
rA The ionic radius of Sr2 + is larger than that of Ca2 +[1.31 ˚A versus 1.18 ˚A (37)], and thus as x increases, rA increases; the Mn–O–Mn angle increases as x increases
so that the bandwidth (W) increases Consequently, TCcreases as rA increases The largest TCof∼370 K is thusobserved for the larger rA such as for La0.7Sr0.3MnO3(35).Finally, a third important parameter exists that gov-
in-erns the T C of these perovskites: the local disorder tends
to weaken the DE process Particularly, the same rA
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0.040.080.12
(b)
Figure 9 T dependence of the real part of the AC susceptibility ( χ) for several Th 0.35Ae 0.65MnO 3 samples characterized by a fixed rA ... data-page= "15 ">
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O
. 341 E? ?15 . 512 E? ?15 .682 E? ?15 .853 E? ?15 .10 2 E? ? 14 .11 9 E? ? 14 .13 6 E? ? 14 .15 4 E? ? 14
Figure 17 Stress... class="text_page_counter">Trang 24< /span>
1. 120 1. 13 1. 14 1. 15 1. 16 1. 17 1. 18 1. 19 1. 2050
10 015 0200250300350
A - Site size <rA>... samples to spin-glass insulators (SGI), as shown inthe electronic and magnetic diagram proposed in Fig 11
2 040 608 010 012 0 14 016 018 0
Figure 11 Electronic and magnetic diagram