7. Methods To Produce Functional Polycrystalline And Monocrystalline Materials

Question 1

Define Solid state synthesis/ reaction?

Answer 1

Reactions in which the reactants and target products are in solid state.

Question 2

What are the main steps of solid state reactions? Which steps are typically rate determining?

Answer 2

The general reaction steps of the reactions:

• Diffusion of reactant to reactive interface(x)
• Adsorption/desorption/decomposition at the interface
• Reaction on atomic scale at the interface
• Nucleation of product and growth(x)
• Diffusion of products away from the reactive interface(x)

The most important part is the diffusion rate of reactants to the reactive interface. The diffusion og Mg2+ and Al3+ in this example. Not the concentration like in liquid phase reactions.

The two common rate determining steps are the nucleation of the product and growth and diffusion to and away from the reactive interface.

Question 3

What are the main approaches to improve the kinetics of solid state reactions?

Answer 3

There are mainly two ways how to increase the rate of diffusion:

• Increase the temperature
• Introduce defects by e.g. starting with reagents that decompose prior to or during the reaction. Example use carbonates (Litium carbonate), material that has CO3, that will decompose into CO2 gas. Then this creates space, more degrees of freedom for the ions to diffuse and travel. But the product is rather porous in the end.
• One can maximize the rate of nucleation by using reactants with crystal structures similar to that of the product.

Question 4

What is the origin of the Kirkendall effect in solid phase synthesis?

Answer 4

The Kirkendall effect is the fact that one reaction interface will move more quickly than the other. The reason for this is the number of ions needed to diffuse to each interface can be different. Consider the system MgO and Al2O3 as reactants.

The reaction on MgO/MgAl2O4-interface:
2Al3+ - 3Mg2+ + 4MgO -> MgAl2O4

The reaction on MgAl2O4/Al2O3-interface:
3Mg+ - 2Al3+ + 4Al2O3 -> 3MgAl2O4

Here we see that for every three Mg2+ ions that diffuse to the right-hand interface, two Al3+ ions must diffuse to the left-hand interface. From this we can see that the right-hand interface should move three times as quickly as the left-hand interface.

Question 5

What are basic ideas of the hydrothermal and microwave assisted syntheses?

Answer 5

Hydrothermal synthesis involves heating reactants in water/steam at high pressures and temperatures. Here the water has two function: 1) as a pressure-transmitting medium and as a solvent, in which the solubility of the reactants is P,T-dependent. (making Quartz crystals)

Microwave assisted synthesis uses the fact that microwaves generally heat any material containing mobile electric charges, such as polar molecules in a solvent or conducting ions in a solid, to selective heat heterogenous systems. This leads to reaction times that are orders of magnitude less than required for regular solid state reactions, and side reactions are less problematic. This heating is different than conventional heating, as conventional heating requires in-diffusion of heat.

Question 6

What are the key parameters to control in order to obtain dense ceramic materials after sintering?

Answer 6

The key parameters:

• Compaction (e.g. isostatic pressing) - shorter diffusion length
• High temperatures
• Long sintering times
• Small particle size (smaller empthy space between the particles that has to be sintered)
• relative uniform particlesize

We can also use microwave assisted sintering

Question 7

What are the driving force for sintering?

Answer 7

To reduce the surface free energy

Question 8

Describe main methods for single crystal growth. Analyze pros and cons for applications of these methods.

Answer 8

Czochralski process:
Grown from a nucleation seed from a melt of the same composition. The seed is then gradually pulled out of the crucible to form a boule.

Pros:
• Good quality single crystal, can vary the orientation of the since crystal by using a different seed crystal (111 or 100 orientation).
• Have high purity, because impurities will want to stay in the melted phase, diffuse there.
• The crystal doesn’t come in contact with the cruicible material
• Easy to control the diameter of the crystall

Cons:
• Take a long time (has to move the seed crystal very slow)
• Difficult to maintain chemical homogeneity of the crystall along the

Bridgeman and Stockbarger methods:
Here the material is passed through a temperature gradient, where the crystallization occurs at the cooler end.

Zone melting (Float zone):
An inductor heat very selectively an area of the material, keeping only this part molten.

Verneuil process (flame fusion):
Powder is passed through a flame from combustion of O2 and H2. When passing through the flame, it melts into small droplets. They will then fall to a crystal support where the crystal growth occurs.

Question 9

Describe the process of solid state synthesis.

Answer 9

The solid state synthesis procedure:

Powder preparation -> thermal treatment -> Milling -> Thermal treatment -> repeat -> product

Question 10

Why is it important to be able to control the solid state synthesis?

Answer 10

Because we want to replace liquids with solid materials in energy provision devices, for example as electrolytes in fuel cells, electrolysers and batteries. Solid state solar cells are more practical than dye-sensitised ones.

A lot of other important energy materials are polycrystalline ceramic materials, such as the superconductors and a lot of piezoelectrics.

We need porous structures for metal electrodes, to increase the surface area, but for ion-conducting membranes we want it normally to be as dense as possible.

Question 11

Name five methods we can use to synthesize non-molecular inorganic solids.

Answer 11

• Solid state reactions
• Hydrothermal methods
• Sol-gel methods
• Precipitation/hydrolysis techniques
• Microwave synthesis

Question 12

How can one measure the extent of reaction?

Answer 12

We can do successive XRD measurements at various temperatures. We can then plot isotherms of the extent of reaction as a function of time.

Question 13

Give two examples of ceramics that can be synthesized with solid state synthesis.

Answer 13

Li4SiO4: parent phase for a family of Li+ ion conductors.
2Li2CO3 + SiO2 —(~800 C, 24h) —> Li4SiO4 + 2CO2

YBa2Cu3O7 (YBCO), classic high-temperature superconductor.
Y2O3 + 4BaCO3 + 6CuO + 1/2 O2 –(950 C) –> 2YBa2Cu3O7 + 4CO2