NANOPRECIPITATES Flashcards
Describe the Hume Rothery criteria for when you may expect to have a complete solid solubility between two compounds. Give two examples; one metallic system and one oxide system for which you will argue that there is full solid miscibility.
Requirements:
- Moderate differences in size between solvent and solute atoms; Hume Rothery criterion of 15 %.
- Same type of crystal structure. Same type of coordination polyhedra for the atoms.
- Same oxidation state for the involved anions/cations.
- No major difference electronegativity
Ex: fcc metals: Pt - Rh
Rock salt oxides: MgO - NiO
Perovskites: LaAlO3 - LaCoO3
Consider the system MgO (rock salt structure) – ZnO (wurtzite structure), and describe solid solubility in the system.
Different crystal structures, NOT possible with full solid solubility.
May have limited solubility.
What is heterovalent (aliovalent) substitution? Which types of defects are connected with such substitution?
Heterovalent substitution: the solute and solvent atoms do not have the same oxidation state. Can take place at the anion part of the structure, or the cation part. E.g. Zn(II) is exchanged (to a small extent) by
Al(III). Then one will have a surplus of positive charge in the structure. In this case charge compensation mechanisms become activated; either by formation of vacancies, or by formation of interstitials
Imagine that you substitute Fe(II) into ZnO at high temperature. Draw a figure showing how the solid solubility depends on temperature. Describe what will happen if you rapidly or slowly cool a sample which at high temperature contains a substantial amount of Fe(III), see lecture slides for relevant phase diagrams.
On slow cooling one will follow the equilibrium phase boundary. A two-phase will be formed. The second phase will the be spinel compound – which is ferromagnetic.
Slow cooling: large particles will grow of the second phase.
Rapid cooling; nanoprecipitates are likely to form at grain boundaries or inside mother particles.
Can you generalize this (question 4) to what can be expected on rapid and on slow cooling of supersaturated solid solutions?
Yes, similar behavior as described is expected.
Imagine that you have incorporated some Fe(II) into an oxide matrix by ion implantation, and that you subsequently heat treat the sample at specific (different) temperature and oxygen partial pressure. How will the magnetic properties of the sample depend on the conditions for the treatment? Which methods may you use for identifying the oxidation state of iron in these samples?
As long as one is doing the annealing in a T, pO2 window where Fe(II) or Fe(III) is stable, it is expected that one will have well dissolved iron cations in the ZnO matrix. These cations will show paramagnetism. If one on the other hand is at reducing conditions, then Fe(s) NPs may form. These will be superparamagnetic. If these nanoparticles grow into larger sizes, then they will show ferromagnetic behavior.
How are nanoprecipitates formed in Al-based light metal alloys, and how to the presence of these influence the properties of the alloys?
By careful heating (annealing) of a quenched supersaturated solid solution. Then NPs will precipitate at grain boundaries, and/or inside primary particles. They prevent mobility of dislocations and are mechanisms for hardening of the alloys.
