polymer compositesolid synthesis

1
Solid State Synthesis
• Solid State Reactions
• Film deposition
• Sol-gel method
• Crystal Growth
2
• 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.RoyJ. Solid State Chem. 111, 11-17 (1994).
"Nonhydrolytic sol-gel routes to oxides"

A. Vioux, Chem. Mater. 9, 2292-2299 (1997).
•
3
• Molten Salt Fluxes & Hydrothermal Synthesis
• "Turning down the heat: Design and mechanism in solid state synthesis"
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).
4
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.
5
(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 cm
3
volume of a reactant
• Edge Length = 1 cm
# of Crystallites = 1
Surface Area = 6 cm
2
• Edge Length = 10 μm
# of Crystallites = 10
9
Surface Area = 6 x 10
3
cm
2
• Edge Length = 100Å

# of Crystallites = 10
18
Surface Area = 6 x 10
6
cm
2
- Pelletize to encourage intimate contact between crystallites.
Solid State Reactions
6
Time (h)
7
Different parts of the crystal have different
structure and different reactivities
8
(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.
9
10
(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.
11
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:
2MO
n
(s) à 2MO
n-1
(s) + O
2
(g)
Due to the release of a gaseous product (O
2
), the products
are favored by entropy, and the entropy contribution to the
free energy become increasingly important as the
temperature increases.

12
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)
Al
2
O
3
1950 °C $30/(20 ml) ZrO
2
/Y
2
O
3
2000 °C $94/(10 ml)
b) Precious Metals (crucibles, boats and tubes)
Pt 1770 °C $500/(10 ml) Au 1063 °C $340/(10 ml)
c) Sealed Tubes
SiO
2

- Quartz, Au, Ag, Pt
13
6)Heat
a) Factors influencing choice of temperature for
volatilization
b) Initial heating cycle to lower temperature can help to
prevent spillage and volatilization
c) Atmosphere is also critical
Oxides (Oxidizing Conditions) – Air, O
2
, Low Temps
Oxides (Reducing Conditions) – H
2
/Ar, CO/CO
2
, High T
Nitrides – NH
3
or Inert (N
2
, Ar, etc.)
Sulfides – H
2
S
Sealed tube reactions, Vacuum furnaces
7) Grind product and analyze (x-ray powder diffraction)
8) If reaction incomplete, return to step 4 and repeat.
14
Example: the synthesis of Sr
2

CrTaO
6
1) Possible starting reagents
Sr Metal – Hard to handle, prone to oxidation
SrO - Picks up CO
2
& water, mp = 2430 °C
Sr(NO
3
)
2
– mp = 570 °C, may pick up some water
SrCO
3
– decomposes to SrO at 1370 °C
Ta Metal – mp = 2996 °C
Ta
2
O
5
– mp = 1800 °C
Cr Metal – Hard to handle, prone to oxidation
Cr
2
O
3
– mp = 2435 °C
Cr(NO
3
)

3
*nH
2
O – mp = 60 °C, composition inaccurate
15
• To make 5.04 g of Sr
2
CrTaO
6
(FW = 504.2
g/mol; 0.01 mol) to complete the reaction:
• 4SrCO
3
+ Ta
2
O
5
+ Cr
2
O
3
à 2Sr
2
CrTaO
6
+
4CO
2
• you need:
SrCO

3
2.9526 g (0.02 mol)
Ta
2
O
5
2.2095 g (0.005 mol)
Cr
2
O
3
0.7600 g (0.005 mol)
16
• Applying Tamman’s rule to each of the
reagents:
• SrCO
3
? SrO 1370 °C (1643 K)
• SrO mp = 2700 K ® 2/3 mp = 1527 °C
• Ta
2
O
5
mp = 2070 K ® 2/3 mp = 1107 °C
• Cr
2
O
3
mp = 2710 K ® 2/3 mp = 1532 °C
• Although you may get a complete reaction

by heating to 1150 °C, in practice there will
still be a fair amount of unreacted Cr
2
O
3
.
Therefore, to obtain a complete reaction it is
best to heat to 1500-1600 °C.
17
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.

18
Coprecipitation Synthesis of ZnFe
2
O
4
• 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 H
2
O decreases, a mixed
Zn/Fe oxalate (probably hydrated) precipitates out.
Fe
2
((COO)
2
)
3
+ Zn(COO)
2
àFe
2
Zn((COO)
2
)
5
*xH
2
O
• After most of the water is gone, the precipitate is filtered and calcined
at 1000 °C.
Fe

2
Zn((COO)
2
)
5
à ZnFe
2
O
4
+ 4CO + 4CO
2
• 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.
19
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 Sr
2
AlTaO
6
in a SrCl
2
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 PbZrO
3
in a B
2
O
3
flux).
20
Example 1
• 4SrCO
3
+ Al
2
O
3
+ Ta
2

O
5
àSr
2
AlTaO
6
(SrCl
2
flux, 900°C)
• Powder sample, wash away SrCl
2
with weakly acidic H
2
O
• Direct synthesis requires T > 1400°C
and Sr
2
Ta
2
O
7
impurities persist even
at 1600°C
21
22
23
24
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.
MoCl
5
+ 5/2 Na
2
S àMoS
2
+ 5NaCl + ½ S
The enthalpy of this reaction is ? H = -213 kcal/mol
25
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 + 6SiO
2
à Ca
6
Si
6
O
17
(OH)

2
(150-350 °C)