Lecture - 4
Solid State Synthesis
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 can not 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.
Controlling Parameters
(1) The area of contact between reacting solids
To maximize the contact between reactants, we want to use starting reagents with
large surface area. Consider the numbers for a 1 cm3 volume of a reactant
Edge Length = 1 cm Edge Length = 10 μm Edge Length = 100Å
No. of Crystallites = 1 No. of Crystallites = 109 No. of Crystallites = 1018
Surface Area = 6 cm2 Surface Area = 6 x103 cm2 Surface Area = 6 x106 cm2
(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.
(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.
Relation between diffusion distance and temperature
The number of atoms diffusing in unit time across unit area through a unit
concentration gradient is known as the diffusivity or diffusion coefficient (D)
Diffusion Coefficient depends on the temperature and concentration of the reactants
Do = frequency factor (m2s-1), Q = activation energy (J mol-1), R = Gas Constant (J mol-1
K-1).
Under isothermal condition the characteristic diffusion distance d(m), assumed to be
predominantly controlled by volume diffusion
d2 = 2Dt
For a thermal cycle T(t) the characteristic diffusion distance is given by
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) 2MOn-1(s) + O2(g)
Due to the presence 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 batch > 20g)
4. Select sample container
Reactivity, strength, cost, ductility all important
a) Ceramic refractories (crucibles and boats)
Al2O3 1950° C $30/(20 ml), ZrO2/Y2O3 2000° C $94/(10 ml)
b) Precious Metals (crucibles, boats and tubes)
Pt 1770° C $500/(10 ml) , Au 1063° C $340/(10 ml)
5) Heat c) Sealed Tubes: SiO2 - Quartz, Au, Ag, Pt
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, 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 (if necessary) and analyze (x-ray powder diffraction)
8) If reaction incomplete return to step 3 and repeat.
Example: BaTiO3 Nanopowders for MLCC
D. Sarkar, J American Ceramic Society, 1–5 (2010)
DOI: 10.1111/j.1551-2916.2010.04049.x
1100oC / 5Min
Dielectric and Ferroelectric Properties of BaTiO3
