Title: ETH_Sem4 / ACII

Q: How would you expect the formation of color centers to affect the density of the crystal?
A: A crystal containing F-centers contains anion vacancies. We would expect, therefore, that the density would be lower than that of the colorless crystal

Q: Primitve is [...] symmetric than face
centered.
A: less

Q: [...] ist die Fähigkeit von Kristallen, [...]
A: Elektrochromie; ihre optischen Eigenschaften durch ein äußeres elektrisches Feld oder einen Stromfluss zu ändern.

Q: mobility in electronic conductivity decreases with increasing temperature because of [...]
A: more friction with atoms

Q: mobility in electronic conductivity decreases with increasing the amount of dopants/impurities because [...]
A: of scattering at dopant site

Q: mobility in electronic conductivity decreases with increasing the presence of defects because of [...]
A: scattering at grain boundaries, point defects,...

Q: In the Kröger Vink Notation, an effective positive charge is shown by [...]
A: ^.

Q: In the Kröger Vink Notation, an effective negative charge is shown by [...]
A: ^/

Q: In the Kröger Vink Notation, an interstitial metal A at the metal M site is shown by [...]
A: A_M

Q: ist nur einmal beschriftet ( zB als alpha ) aber mit Linien unterteilt. Diese Linien sind [...]
A: phase transitions.

Q: face centered is [...] symmetric than primitve.
A: more

Q: CdI2 and CdCl2 are usually adopted by [...]
A: heavy halides

Q: For stronger bonding, the atomic pair potential is more harmonic and thermal expansion is [...]
A: smaller

Q: More covalent nature will result in the electrons being [...], which in turn means that electrons will be [...]
A: more localized to the orbitals; less mobile

Sketch a unit cell of a cubic halide perovskite ABX3

Q: The degree of ionicity (polarity of bonding) increases from I to Cl, so does the [...]
A: bandgap.

interplanar spacing of cubic lattice

One can derive selection rules for the Miller indices for different cubic Bravais lattices; here, selection rules for several will be given as is.

Combination of bragg equation and cubic lattice equation gives

Q: How does the electrical conductivity change with increasing temperature? What is the reason for this change?
A: - Conductivity first increases steeply with increasing temperature as the energy barrier for ionic conduction is overcome.
- It later stagnates when ion diffusion can lead to defect clusters but the thermal energy is yet too small for the formation of new defects.
- In a third region, when the thermal energy is sufficient to overcome the free energy of defect formation, the conductivity increases more steeply again.

equation for electron mobility

Compute the intrinsic carrier concentration

Q: Why conductive polymers can be used in gas sensing? Which gases you could imaging can be detected?
A: - The chemical sensing behavior of PPy is based on the interaction between the polymer layer and the ab- or adsorbed gas molecules.
- The interaction leads to a heterogeneous charge transfer reaction and hence to a chemical modulation of the polymer doping level, which is directly related to the Fermi level of the organic semiconductor.
- This type of interaction is often described as a secondary doping of the sensitive polymer layer.
- The gas molecule behaves as an electron acceptor and donor, respectively.
- Essentially any reactive molecule should be detectable such as ammonia (donor) or oxygens/halogens (acceptors).

Q: The [...] band is the one [...] the Fermi Level
A: valence; below

Q: The [...] band is the one [...] the fermi level.
A: conduction; above

Q: Please compare thermal conductivities of crystalline and amorphous ceramics
A: Thermal conductivities are higher for crystalline than for non-crystalline ceramics because, for non-crystalline materials phonon scattering occurs more efficiently at the structural imperfections, and thus resistance to heat transport is much more effective due to the highly disordered and irregular atomic structure (no long-range phonon transport).

Q: Please compare thermal conductivities of metals and ceramics
A: Metals are typically better thermal conductors because most of the heat is transported by free electrons (of which there is a very large number).
In ceramic materials, the primary mode of thermal conduction is via phonons, and phonons are more easily scattered than free electrons.

Q: Porosity [...] the thermal conductivity of ceramic and polymeric materials
A: decreases
Rückseite Extra: because the thermal conductivity of a gas phase that occupies pore space is extremely small relative to that of the solid material. Furthermore, contributions from gaseous convection are generally insignificant.

Q: Why does the thermal conductivity first decrease and then increase with rising temperature in certain ceramics?
A: For some ceramic materials, the thermal conductivity first decreases with rising temperature because the scattering of lattice vibrations increases with temperature. At higher temperatures, the thermal conductivity will increase for some ceramics that are porous because radiant heat transfer across pores may become important, a process increasing with rising temperature.

Q: The thermal conductivity of a single crystal is [...] than a polycrystalline specimen of the same material.
A: greater
Rückseite Extra: This is because both phonons and free electrons are scattered at grain boundaries, thus decreasing the efficiency of thermal transport.

Q: The band gradient or the steepness of the conduction band is correlated with the [...].
A: effective mass of the electron
Rückseite Extra: - Steep bands correlate to a low effective mass higher conductivity
- flat bands correlate to high effective mass and therefore lower conductivity.

Q: The Motif is the [...]
A: smallest multiple of each type in the unit cell
Rückseite Extra: => Summenformel

The density D of a crystal structure is calculated by using

sketching directions in unit cells

Perovskite Structure

Derive Structure types for given compounds

Functionality of solar cells

Functionality of Photo diode

What happens if you connect a p- type with an n - type semiconductor ?

Functionality of a Laser

Q: How to differentiate between amorphous and crystalline solid?
A: → Method 1: X- ray Diffractian : amorphous solids only scatter, crystals can also diffract
→ Method 2: heating: glasses soften gradually , crystals will melt suddenly
→ Method 3: Tensile Stress: Crystal will deform by slipping on parallel planes , glasses will just fracture

DSC

TG/TGA

Q: how would you detect the residual water?
A: DTA & TGA analysis: Dehydration can be observed by a decrease in weight and an endothermic behaviour.
Alternatives: Proton NMR

Q: Which analysis methods can be used to analyze alloy microstructures?
A: Scanning Electron Microscopy

Q: it is p-type doping, if the Fermi level [...]
A: is lowered, since less electrons

kröger vink notation examples