Chapter 7 - Methods to produce functional polycrystalline and monocrystalline materials Flashcards
What are the main steps of solid state reactions? Which steps are typically rate determining?
The general reaction steps of the reactions:
- Diffusion of reactant to reactive interface
- Adsorption/desorption/decomposition at the interface
- Reaction on atomic scale at the interface
- Nucleation of product and growth
- Diffusion of products away from the reactive interface
The two common rate determining steps are the nucleation of the product and growth and diffusion to and away from the reactive interface.
What are the main approaches to improve the kinetics of solid state reactions?
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.
One can maximize the rate of nucleation by using reactants with crystal structures similar to that of the product.
What are basic ideas of the hydrothermal and microwave assisted syntheses?
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.
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.
What are the key parameters to control in order to obtain dense ceramic materials after sintering?
The key parameters:
- Compaction (e.g. isostatic pressing)
- High temperatures
- Long sintering times
- Small particle size.
We can also use microwave assisted sintering
Describe main methods for single crystal growth. Analyze pros and cons for applications of these methods.
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.
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.
What is the origin of the Kirkendall effect in solid phase synthesis? Draw an example.
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.
Describe the process of solid state synthesis.
The solid state synthesis procedure:
Powder preparation -> thermal treatment -> Milling -> Thermal treatment -> repeat -> product
Draw schematically the Czochralski method
See slide 36.
Draw schematically the Bridgeman method.
See slide 38.
Draw schematically the Zone melting (float zone) method.
See slide 39.
Draw schematically the Verneuil process.
See slide 40.
Why is it important to be able to control the solid state synthesis?
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.
Name five methods we can use to synthesize non-molecular inorganic solids.
- Solid state reactions
- Hydrothermal methods
- Sol-gel methods
- Precipitation/hydrolysis techniques
- Microwave synthesis
How can one measure the extent of reaction?
We can do successive XRD measurements at various temperatures. We can then plot isotherms of the extent of reaction as a function of time.
Give two examples of ceramics that can be synthesized with solid state synthesis.
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
