polymer compositesolid synthesis

• The following references discuss various aspects or methods in solid state synthesis in greater detail.. • Low Temperature & Precursor Techniques • "Crystallization of Solid State Mate

Solid State Synthesis

• Solid State Reactions

• Film deposition

• Sol-gel method

• Crystal Growth

• Synthesis References

• The material we discussed in class was drawn primarily from the following sources:

• A.R West

"Solid State Chemistry and its Applications"

Chapter 2 – Preparative Methods

• "Solid-State Chemistry – Techniques"

Chapter 1 – Synthesis of Solid-State Materials

J.D Corbett – book edited by A.K Cheetham and P Day

More detailed treatment, including practical details such as what sort of containers to use, how to avoid introducing impurities, what reactants to choose, etc., than above references Corbett’s treatment is less oriented toward oxides, and more focussed on materials such as chalcogenides, halides and metal rich compounds No discussion of thin films or growth of large crystals.

• "Preparation of Thin Films"

Joy George

This book has a nice succinct treatment of the various thin film deposition methods.

• The following references discuss various aspects or methods in solid state synthesis in greater detail I have listed them according to synthesis method.

• Low Temperature & Precursor Techniques

• "Crystallization of Solid State Materials via Decomplexation of Soluble Complexes"

K.M Doxsee, Chem Mater 10, 2610-2618 (1998).

"Accelerating the kinetics of low-temperature inorganic syntheses"

R.Roy J Solid State Chem 111, 11-17 (1994).

"Nonhydrolytic sol-gel routes to oxides"

A Vioux, Chem Mater 9, 2292-2299 (1997)

•

• Molten Salt Fluxes & Hydrothermal Synthesis

• "Turning down the heat: Design and mechanism in solid state synt hesis"

A Stein, S W Keller, T.E Mallouk, Science 259, 1558-1563 (1993).

• "Synthesis and characterization of a series of quaternary chalcogenides BaLnMQ3 (Ln = rare earth, M = coinage metal, Q = Se or Te)"

Y.T Yang, J.A Ibers, J Solid State Chem 147, 366-371 (1999).

• "Hydrothermal Synthesis of Transition metal oxides under mild conditions"

M.S Whittingham, Current opinion in Solid State & Materials Science 1, 227-232

• Chimie Douce & Low Temperature Synthesis

"Chimie Douce Approaches to the Synthesis of Metastable Oxide Materials"

J Gopalakrishnan, Chem Mater 7, 1265-1275 (1995).

•

• High Pressure Synthesis

"High pressure synthesis of solids"

P.F McMillan, Current Opinion in Solid State & Materials Science 4, 171-178 (1999)

"High-Pressure Synthesis of Homologous Series of High Cricitcal Temperature (Tc)

Superconductors"

E Takayama-Muromachi, Chem Mater 10, 2686-2698 (1998).

"Preparative Methods in Solid State Chemistry"

J.B Goodenough, J.A Kafalas, J.M Longo, (edited by P Hagenmuller) Academic Press, New York (1972).

Classification of Solids

There are several forms solid state materials can adapt

Single Crystal

Preferred for characterization of structure and properties

Polycrystalline Powder (Highly crystalline)

Used for characterization when single crystal cannot be easily obtained, preferred for industrial production and certain

applications

Polycrystalline Powder (Large Surface Area)

Desirable for further reactivity and certain applications such

as catalysis and electrode materials

Amorphous (Glass)

No long range translational order

Thin Film

Widespread use in microelectronics, telecommunications,

optical applications, coatings, etc

(1) Area of contact between reacting solids

- We want to use starting reagents with large surface area to

maximize the contact between reactants

Consider the numbers for a 1 cm3 volume of a reactant

- Pelletize to encourage intimate contact between crystallites.

Solid State Reactions

Time (h)

Different parts of the crystal have different

structure and different reactivities

(2) The rate of diffusion

Two ways to increase the rate of diffusion

are to

• Increase temperature

• Introduce defects by starting with reagents that decompose prior to or during reaction, such as carbonates or nitrates

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(3) The rate of nucleation of the product phase

• We can maximize the rate of nucleation by

using reactants with crystal structures

similar to that of the product (topotactic and epitactic reactions).

a topotactic transformation is characterized by internal

atomic displacements, which may include loss or gain of

material so that the initial and final lattices are in coherence

epitaxy - The growth of the crystals of one mineral on the crystal

face of another mineral, such that the crystalline substrates of both minerals have the same structural orientation

What are the consequences of high

reaction temperatures?

• It can be difficult to incorporate ions that readily form

volatile species (i.e Ag+)

• It is not possible to access low temperature, metastable

(kinetically stabilized) products

• High (cation) oxidation states are often unstable at high

temperature, due to the thermodynamics of the following

reaction:

2MOn (s) à 2MOn-1(s) + O2(g)

Due to the release of a gaseous product (O2), the products

are favored by entropy, and the entropy contribution to the

free energy become increasingly important as the

temperature increases

Steps in Conventional Solid State Synthesis

1) Select appropriate starting materials

a) Fine grain powders to maximize surface area

b) Reactive starting reagents are better than inert

c) Well defined compositions

2) Weigh out starting materials

3) Mix starting materials together

a) Agate mortar and pestle (organic solvent optional)

b) Ball Mill (Especially for large preps > 20g)

4) Pelletize

5) Select sample container

Reactivity, strength, cost, ductility all important

a) Ceramic refractories (crucibles and boats)

c) Atmosphere is also critical

Oxides (Oxidizing Conditions) – Air, O2, Low Temps

Oxides (Reducing Conditions) – H2/Ar, CO/CO2, High T

Nitrides – NH3 or Inert (N2, Ar, etc.)

Sulfides – H2S

Sealed tube reactions, Vacuum furnaces

7) Grind product and analyze (x-ray powder diffraction)

8) If reaction incomplete, return to step 4 and repeat

1) Possible starting reagents

Sr Metal – Hard to handle, prone to oxidation

• Although you may get a complete reaction

by heating to 1150 ° C, in practice there will still be a fair amount of unreacted Cr2O3

Therefore, to obtain a complete reaction it is best to heat to 1500-1600 ° C

Precursor Routes

• Approach : Decrease diffusion distances through intimate mixing of

cations.

• Advantages : Lower reaction temps, possibly stabilize metastable

phases, eliminate intermediate impurity phases, produce products

with small crystallites/high surface area.

• Disadvantages : Reagents are more difficult to work with, can be

hard to control exact stoichiometry in certain cases, sometimes it is

not possible to find compatible reagents (for example ions such as

Ta5+ and Nb5+ immediately hydrolyze and precipitate in aqueous

solution)

• Methods : With the exception of using mixed cation reactants, all

precursor routes involve the following steps:

1 Mixing the starting reagents together in solution

2 Removal of the solvent, leaving behind an amorphous or

nano-crystalline mixture of cations and one or more of the following anions: acetate, citrate, hydroxide, oxalate, alkoxide, etc

3 Heat the resulting gel or powder to induce reaction to the desired

product

• The following case studies illustrate some examples of actual

syntheses carried out using precursor routes.

• Mix the oxalates of zinc and iron together in water in a 1:1 ratio Heat

to evaporate off the water As the amount of H2O decreases, a mixed

Zn/Fe oxalate (probably hydrated) precipitates out.

Fe2 ((COO) 2) 3 + Zn(COO) 2àFe2Zn((COO) 2) 5*xH2O

• After most of the water is gone, the precipitate is filtered and calcined

at 1000 ° C.

Fe2Zn((COO) 2) 5à ZnFe2O4 + 4CO + 4CO2

• This method is easy and effective when it works It is not suitable

when

Reactants of comparable water solubility cannot be found The

precipitation rates of the reactants is markedly different

These limitations make this route unpractical for many combinations of ions Furthermore, accurate stoichiometric ratios may not always be

maintained.

Molten Salt Fluxes

• Solubilize reactants → Enhance diffusion → Reduce reaction

temperature

• Synthesis in a solvent is the common approach to synthesis of organic

and organometallic compounds This approach is not extensively used

in solid state syntheses, because many inorganic solids are not soluble

in water or organic solvents However, molten salts turn out to be good solvents for many ionic-covalent extended solids

• Often slow cooling of the melt is done to grow crystals, however if the flux is water soluble and the product is not then powders can also be

made in this way and separated from the excess flux by washing with

water.

• Synthesis needs to be carried out at a temperature where the flux is a

liquid Purity problems can arise, due to incorporation of the molten

salt ions in product This can be overcome either by using a salt

containing cations and/or anions which are also present in the desired

product (i.e synthesis of Sr2AlTaO6 in a SrCl2 flux) , or by using salts

where the ions are of a much different size than the ions in the desired

product (i.e synthesis of PbZrO3 in a B2O3 flux).

Example 1

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Solid State Metathesis Reactions

A metathesis reaction between two salts merely

involves an exchange of anions, although in the

context we will use there can also be a redox

component If the appropriate starting materials are chosen, a highly exothermic reaction can be devised.

The enthalpy of this reaction is ? H = -213 kcal/mol

Hydrothermal Synthesis

• Reaction takes place in superheated water, in a

closed reaction vessel called a hydrothermal bomb (150 < T < 500 ° C; 100 < P < 3000 kbar).

• Seed crystals and a temperature gradient can be

used for growing crystals

• Particularly common approach to synthesis of

zeolites

• Example :

6CaO + 6SiO2 à Ca6Si6O17(OH)2 (150-350 ° C)

26

Sol-gel process

29

• Involves inserting ions into an existing

structure, this leads to a reduction (cations

inserted) or an oxidation (anions inserted) of the host.

• Typically carried out on layered materials

(strong covalent bonding within layers, weak van der Waals type bonding between layers, i.e graphite, clays, dicalchogenides,).

• Performed via electrochemistry or via

chemical reagents as in the n-butyl Li

technique.

• Examples :

TiS2 + nBu-Li à LiTiS2

b-ZrNCl + Naph-Li à b-LixZrNCl

34

• By removing water and/or hydroxide groups from a compound, you can often perform

redox chemistry and maintain a structural

framework not accessible using

conventional synthesis approaches

• Examples :

Ti4O7(OH)2*nH2O à TiO2 (B) (500° C)

2KTi4O8(OH)*nH2O à K2Ti8O17 (500° C)

Ion Exchange

• Exchange charge compensating, ionically

bonded cations (easiest for monovalent

cations)

• Examples :

LiNbWO6 + H3O + à HNbWO6 + Li+

KSbO3 + Na + à NaSbO3 + K +

Chemical vapor deposition (CVD) is a chemical process for

depositing thin films of various materials In a typical CVD

process, the substrate is exposed to one or more volatile

precursors, which react and/or decompose on the substrate surface

to produce the desired deposit Frequently, volatile by-products

are also produced, which are removed by gas flow through the

reaction chamber

Types of CVD Processes

• Atmospheric Pressure Chemical Vapour Deposition (APCVD)

• Low Pressure Chemical Vapour Deposition (LPCVD)

• Metal-Organic Chemical Vapour Deposition (MOCVD)

• Plasma Assisted Chemical Vapour Deposition (PACVD)

or Plasma Enhanced Chemical Vapour Deposition (PECVD)

• Laser Chemical Vapour Deposition (LCVD)

• Photochemical Vapour Deposition (PCVD)

• Chemical Vapour Infiltration (CVI)

• Chemical Beam Epitaxy (CBE)

41

(a) (b)

Film formation

MOCVD to prepare METAL, METAL OXIDE,

NITRIDE and SULFIDE FILMS

Metal-organic CVD (MOCVD) - CVD processes based on

metal-organic precursors, such as Tantalum Ethoxide,

Ta(OC2H5)5, to create TaO, Tetra Dimethyl amino

Titanium (or TDMAT) to create TiN.

The philosophy behind this work is the discovery of:

• volatile organometallic precursors

• sometimes single source containing more than one of the

required elements

• that are pure enough

• and cleanly produce the required elements on a desired

substrate

• at as low a temperature as possible

• often epitaxially to minimize interfacial defects

MOCVD PRECURSORS

A favorite ligand is the bulky

2,2',6,6'-tetramethyl-3,5-heptanedionate, basically a bulky acac ligand (TMHD)

Y(TMHD) 3 T sub = 160 o C, Ba(TMHD) 2 T sub = 70-190 o C,

and Cu(TMHD) 2 Tsub = 125 o C are very useful

MOCVD precursors

MOCVD to prepare METAL, METAL OXIDE,

NITRIDE and SULFIDE FILMS

• Best precursors for copper films used in microelectronics

are Cu(hfacac)2 (VP 0.25 Torr at 60oC) at 250-350oC, and

Cu(hfacac)PR3 (VP 0.1 Torr at 60oC) at 120-350oC

hfacac = hexafluoroacetylacetonate

• Rare earth doped semiconductor films make use of the

sterically crowded encapsulated (C5H4Me)3Nd and

(C5H4CMe3)3Nd can sublime at 110oC and 10-3 Torr

allowing them to be doped into III-V semiconductors, the

idea is to excite the sharp 4f-4f intra-shell luminescence of

the rare earth center optically and electrically via the host

semiconductor crystals, which is of interest in fiber optical

communication:

GaMe3 + AsH3 + (C5H4Me)3Nd ? Nd:GaAs

• Nitride films are important as they display unique properties

including metallic behavior, extreme hardness, very high meltingpoints, high chemical resistance

This has generated considerable interest in MOCVD precursors

to nitride films

Homoleptic dialkyamides and ammonia react at temperatures as low as 200oC to afford excellent quality TiN films:

Ti(NMe2)4 + NH3 (200-450oC) ? TiN + organics

• Sulfide films possess a wide range of fascinating solid state

properties and have been the focus of much MOCVD research

Most prominent application is in the area of cathodes for thin film lithium batteries

Promising materials are TiS2 and MoS2

TiCl4(HSC6H11)2 (VP 1-2 Torr, 25oC, single source precursor,

~200oC) ? TiS2 + 2HCl + 2C6H11Cl

CVD to produce diamond at low pressure and around

1000 o C

Single crystal synthetic diamonds (3000oC and 130 kbar from

graphite) make excellent heat sinks for semiconductors in

device applications

Example of high thermal conductivity of diamond laser diode

heat drain (yellow diamond n-doped with N, blue diamond

p-doped with B), conductivity of diamond 4x greater than copper

or silver at RT (10-20 watts/cmoC)

By 1996, it was estimated that semiconductor applications could take 60% of worldwide diamond thin film market, other

contenders for use of diamond film made by CVD are coated

tools (abrasion resistance), optical disk coatings (protective

coatings), lens and window coatings, loudspeakers (sound

distortion control), UV laser coatings (reduces laser heating)

Synthetic methods for making diamond films all employ low

pressure deposition of 1%CH 4 /H 2 onto 1000 o C substrate

Heated filament method uses a hot wire to decompose the methane,

2200oC, produces atomic C/H, 50 Torr pressure silica bell jar,

diamond film deposited on 1000oC substrate

Direct current plasma jet arc discharge focuses coating on a small area of substrate and can be scanned across a substrate

Microwave plasma discharge is used for commercial production of diamond films