Title: ETH_Sem4 / ACII / AC 2 stuff

Q: Bardeen-Cooper-Schrieffer (BCS) mechanism:
A: -Resistivity in metals is due to electron-lattice interactions.
− An electron passing through a lattice attracts the cores
− At low T: atoms do not move right back when the electron leaves
− a local positive charge remains that can attract a second electron (forming a cooper pair)
− They can have reasonably long distances (some nm) but they are spin paired
− The energy lost by the first electron is used to attract the second electron
− no net loss in energy and therefore no resistivity.

Q: inorganic transparent conductive oxides:
A: - similar to transparent conductive polymers
- Large bandgap
- High conductivity through heavy doping
- e.g ITO (In2O3 doped by 8% SnO4)

Q: Transparent conductive polymers
A: − Organic polymers with extended pi-system have a band gap
− Heavy doping induces almost metallic conductivity
− Applications: LASERs, LEDs (OLED), Solar cells, ...

Q: electrical conductivity
A: electronic conductivity + ionic conductivity

Q: Charge-transfer complexes
A: − Stacked aromatic systems
− Conductivity through charge transfer
− Some feature almost metallic conductivity

Q: doping semi-conductors strategies
A: - introduce higher/lower valency elements during synthesis
- through red/ox reactions
- intercalation

Q: How does electrical conductivity change with increasing temperature?
A: - T ^ conduct. increases , more energy to overcome barrier
- T ^^ stagnates due to clusters of voids
- T ^^^ generate more voids => vacancy mechanism => conductivity increases

Q: Doping semi conductors
A: -small amounts of dopant ( usually less than 1%)
- band structure is not affected!!!
- only carrier concentrations change

Q: Space Groups
A: a space group is the symmetry group of an object in space, usually in three dimensions

space group notation

Q: X_3_ oder X_3_ mit strich oben,
 X=P, F, I
A: Cubic

Q: X4_ _, X4/m, X422, X4mm etc.
X=P, I
A: Tetragonal

Q: Xmm2, Xmmm
X= P, F, I, A, B, C
A: Orthorhombic

Q: P6_ _
A: Hexagonal

Q: R3_ _
A: Trigonal

Q: X_2 oder X_m
X = P,C
A: Monoclinic

Q: P1_ _
A: Triclinic

Kröger-Vink Notation

Q: ·    Gibbs phase rule
A: o    P+F = C+2

Q: Liquidus
A: o    Above liquidus is all liquid
o    Linie zwischen liquid und liquid-solid mischphase

Q: Solidus
A: o    Below solidus is all solid
o    Linie zwischen solid und liquid-solid mischphase

Lever Rule

Q: Tie line
A: - Connects phases in equilibrium with each other
- Also known as isotherm

Q: Melting points |
A: Punkt zwischen liquid und puren Phasen

Q: eutectic point
A: L ⇄S_A + S_B
Hinreaktion ist kühlen, Rückreaktion ist erhitzen

Q: eutectoid point
A: S_C ⇄ S_A + S_B
Hinreaktion ist kühlen, Rückreaktion ist erhitzen

Q: peritectic point |
A: L_ ⇄ L + S_A
Hinreaktion ist kühlen, Rückreaktion ist erhitzen

Q: peritectoid point |
A: S_A + S_B _ ⇄ S_C
Hinreaktion ist kühlen, Rückreaktion ist erhitzen

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

Q: monotectic | point
A: a monotectic phase diagram is that in which their is no solid/liquid solubility for the given number of phases

Q: Critical point |
A: The point on a phase diagram at which the substance is indistinguishable between liquid and gaseous states

Q: Triple point |
A: The point on a phase diagram at which the three states of matter: gas, liquid, and solid coexist

Q: Fermi level
A: Bei den Band diagrams eine horizontale Linie bei eV=0

Q: Metal bei band diagram
A: o    Linien gehen durch Fermi level
o    Kein bandgap

Q: Semimetal bei band diagram
A: -Linien berühren Fermi level, gehen aber nicht durch
-Bandgap is null

Q: Semiconductor bei band diagram
A: o    Linien berühren Fermi level nicht
o    Bandgap ist kleiner als 4 eV

Q: Insulator bei band diagram
A: o    Linien berühren Fermi level nicht
o    Bandgap ist grösser als 4 eV

Q: Indirect bandgap
A: Minima und maxima stehen nicht genau übereinander

Q: Direct bandgap
A: Minima und maxima stehen genau übereinander

Q: Transform wavelength to bandgap
A: Lambda = 1240/E (in eV) bzw h*c/E (in Joule)

Q: Application for Semiconductor
A: o    indirect bandgap semiconductors can be used in electronics and photovoltaics
o    Many digital consumer products in everyday life such as mobile phones / smartphones, digital cameras, televisions, washing machines, refrigerators and LED bulbs also use semiconductors.

Q: Application for Metals
A: Depends on the metal, various applications in all kinds of fields

Q: Application for insulator
A: used in high-voltage systems, circuit boards & also in electric wire coating & cables

Q: Application for semi-metal
A: often used incomputers and other electronic devices, as well as ceramics, polymers and batteries

Q: Characterizing techniques
Element composition: ??
Crystal structure: ??
Local structure: ??
Distribution of oxidation numbers: ??
Surface morphology: ??
Magnetic properties: ??
A: Element composition: ICP-AAS
Crystal structure: XRD
Local structure: EPR
Distribution of oxidation numbers: XPS
Surface morphology: SEM
Magnetic properties: Squid

Q: DTA measurement
A: ·    DTA = Differential thermal analysis
·    measure the temperature rate change due to consumption or liberation of heat by chemical or physical processes taking place when heating a sample within a certain temperature range.
·    The experiment can also be conducted by comparing the temperature difference between the sample and a reference under identical experimental conditions.
o    This relative mode provides more sensitive results than the absolute one.
·    Glass transition temperatures are rate dependent.
o    The faster the cooling rate the higher the glass transition temperature.
o    the faster the heating rate the lower the melting points. Consequently a DTA will expand with slower rates.
·    An exothermic chemical reaction will show a broad, positive peak because it takes place after overcoming the activation energy up to the point until the reaction is complete.
·    In some cases, the activation energy can even be seen as an endothermic peak before the exothermic one.

Q: First order phase transition in DTA
A: o    will ideally happen at a fix temperature
o     the peak will be narrow.
o    The peak width is due to the experimental impossibility to heat infinitely slowly.

Q: Second-order phase transition in DTA
A: show broad peaks since the
heat capacity of the material will change gradually during the transition and
not abruptly, like a step function

electronic vs ionic conduction

Q: Electronic conductivity
A: ·    electronic conduction is the movement of valence electrons from one place to another
·    electrons have to be free electrons in order to move and there has to be an electric field present.
·       electronic conduction is a very effective way of heat transport in solids.

Q: Ionic conductivity
A: ·    ionic conduction is the movement of ions from one place to another
·    ionic conduction refers to the movement of ions from one defect to another in the crystal lattice

Q: Good vs Bad thermal insulators?
A: ·    Electronic conductors are bad thermal insulators.
·    good thermal insulators: amorphous, porous, electrical insulators such as polystyrene foam

Q: Drude Model
A: ·    The Drude model attempts to explain the resistivity of a conductor in terms of the scattering of electrons (the carriers of electricity) by the relatively immobile ions in the metal that act like obstructions to the flow of electrons.
·    Assumptions made
o    Electrons are delocalized
o    No coulomb interactions
o   E_kin=E_therm=(3/2)*k_B
_

Q: The Piezoelectric (effect) is
A: a physical phenomenon, whereby electric charge is accumulated within the solid upon application of mechanical pressure or strain.

Q: BCS-type superconductor
A: ·       Bardeen-Cooper-Schrieffer
·       explains the phenomenon in which a current of electron pairs flows without resistance in certain materials at low temperatures.
·       electrons can combine into pairs (called Cooper pairs) at low temperatures to travel through superconductors without resistance
·       However, electrons normally repel each other, and some intermediary is required to induce them to pair.

Q: Bloch equations
A: Hat mit NMR zu tun
·       Sie ermöglichen eine physikalische Deutung des paramagnetischen Resonanzeffekts in der magnetischen Kernresonanz und in der paramagnetischen Elektronenresonanz.

Q: [h k l]
A: direction
miller indices

Q: ⟨h k l ⟩
A: family of directions
miller indices

Q: (h k l)
A: plane
miller indices

Q: {h k l}
A: family of planes
miller indices

Q: [h k l] ⊥ (h k l) gilt in..
A: cubic systems

Distance between adjacent planes

Q: Doping
A: In semiconductor production, doping is the intentional introduction of impurities into an intrinsic semiconductor.
 Purpose: modulating its electrical, optical and structural properties.
The doped material is referred to as an extrinsic semiconductor.

Q: P-type doping
A: -P-type semiconductors
o    are created by doping an intrinsic semiconductor with an electron acceptor element
o    have a larger hole concentration than electron concentration.
- P-type refers to the positive charge of a hole
-The Fermi level is below the intrinsic semiconductor(= vor doping) and lies closer to the valence band than the conduction band.
-Holes are the majority carriers and electrons are the minority carriers.
-A common p-type dopant for silicon is boron or gallium (links davon im Periodensystem).
-Examples: boron, aluminium, gallium, etc.

Q: N-type doping
A: - N-type semiconductors
o    are created by doping an intrinsic semiconductor with an electron donor element.
o    electrons are the majority carriers and holes are the minority carriers
o    The term n-type comes from the negative charge of the electron.
o    A common dopant for n-type silicon is phosphorus or arsenic.
- The Fermi level is greater than that of the intrinsic semiconductor(= before doping) and lies closer to the conduction band than the valence band.
- Examples: phosphorus, arsenic, antimony, etc

Q: P-N-Junction
A: - When p-type and n-type materials are placed in contact with each other, the junction behaves very differently than either type of material alone.
- current will flow readily in one direction (forward biased) but not in the other (reverse biased), creating the basic diode.
- This non-reversing behavior arises from the nature of the charge transport process in the two types of materials.
- The open circles on the left side of the junction represent "holes" or deficiencies of electrons in the lattice which can act like positive charge carriers. The solid circles on the right of the junction represent the available electrons from the n-type dopant. Near the junction, electrons diffuse across to combine with holes, creating a "depletion region"

Q: Characterizing techniques
Element composition: ??
A: ICP-AAS

Q: Characterizing techniques
Crystal structure: ??
A: XRD

Q: Characterizing techniques
Local structure: ??
A: EPR

Q: Characterizing techniques
Distribution of oxidation numbers: ??
A: XPS

Q: Characterizing techniques
Surface morphology: ??
A: Characterizing techniques  SEM

Q: Characterizing techniques
Magnetic properties: ??
A: Squid

Q: mobility in electronic conductivity decreases
A: o    with increasing temperature ( more friction with atoms )
o    with increasing the amount of dopants/impurities (scattering at dopant site)
o    with increasing the presence of defects (scattering at grain boundaries, point defects,..)

Draw (111), (110) and (100) diffraction planes in TiFe structure. What are the corresponding 2 values and how did you calculate them?

Q: Metallic glasses vs. silicate glasses
A: - Metallic glasses are electronic conductors
- Silicate glasses are insulators.

Both pure Au and Al adopt the same crystal structure - FCC. Please draw it and identify all voids.

Q: How is resistance related to conductivity ?
A: Specific resistance is inversely proportional to conduction, higher resistance means lower conductivity

Q: Propose how to distinguish between two materials
A: X has higher conductivity than Y
X has higher density as it is a metal and has a more densely packed structure
materials will give different powder diffraction patterns

Q: How is the color of the material related to the band gap value ?
A: The band gap is corresponding to the absorption edge: all the energy above band gap value is absorbed by the material. E.g. FeS2 has a band gap of 1 eV which corresponds to the wavelength 1240 nm. All the spectrum above 1240 nm is absorbed (which is all visible spectrum), which means FeS2 should be black.

Q: A reduction reaction means the Fermi level shifts [...] into the conduction band.
A: up

Q: Something can be used in Photovoltaic solar panels because of the suitable bandgap of about [...]
A: 1 eV

Q: What is the structural prerequisite for elecrochromicity?
A: That there is some direct corner sharing between octahedral, so that small domains of the hexagonal  structure can be formed

Q: piezoelectric materials have [...] in their crystal structure
A: no inversion center

Q: crystal structures with Stoichiometry AB_2
A: CaF_2, TiO_2, CdI_2, CdCl_2 or MoS_2

Q: What does it mean when the bond is smaller than the sum of Wan-der-Waals radii ?
A: there is an orbital overlap and a proper intermetallic bond and it was not just the packing density that caused the structure to form

CdCl2 (rhombohedral) , TiO2 (tetragonal) and MoS2(hexagonal)  and CdI2 are not cubic .

Q: What does SEM stand for?
A: Scanning electron microscopy

Q: Please explain why some regions are bright and other dark in the SEM images
A: SEM is measured in reflection geometry.
Material with higher electron density scatters more electrons and such also scatters more of them back into the direction of the detector.

Q: congruent melting
A: -directly from solid AB to liquid
- represents a maximum in melting or liquidus curves.

Q: Which is the most probable point defect you would expect for AgCl? Describe and draw it, specifically for AgCl
A: Frenkel defect, is a point defect where an Ag-atom is torn out from its lattice position and located on an interstitial cite .
The favored interstitials are Td-voids

indicate one distorted tetrahedral, one trigonal and one linear void

Q: How many carriers can be excited across the bandgap in the dark, at room temperature ?
A: E_g ist band gap energie

Q: How would you produce highly crystalline InSb on an industrial scale? Name and describe a method.
A: By the Czochralski method. In the Czochralski process a large crystal will be drawn from a melt starting to grow epitaxially from a small single crystal. The impurities will stay in the melt because most compounds are better miscible in the melt than in the solid. Turning the growing crystal seed helps to get a uniform cylindrical crystal.

Q: it is n-type doping,iif the Fermi level [...]
A: rises, since more electrons

Q: For a ferroelectric structure [...] is a prerequisite, i.e. the structure should have no inversion center
A: a permanent dipole moment

Q: What type of distortion do you need to introduce in the ABO_3 structure in order to obtain the structure of ferroelectric ABO_3?
A: Destroy inversion center, create a permanent dipole by moving atom from the center along two axes to a different extent -> orthorhombic . This is a Jahn-Teller effect 2nd order.

Q: Someone claims that since electrons in a metal get scattered at the crystal-lattice and this causes resistivity, glasses have to be way more conductive since they don’t have a crystal lattice. Does this make sense?
A: This does not make sense. The electrons get scattered at the atoms which move around their lattice position because of thermal energy at normal Temperatures (not 0K). This happens irrespective of a material being ordered or not.

Q: Which are the requirements needed for electronic conductivity? Which classes of materials fulfill these requirements?
A: Mobile electrons or holes.
=> Metals (no bandgap) or doped semiconductors (mobile electrons or holes induced by the dopant).

FCC
R :
Oh void:
Td void:

BCC
R :
Oh void:

Q: bonding orbitals are occupied and therefore [...] than the fermi energy.
A: lower

Q: The Bandgap decreases with [...] [...]
A: heavier elements; due to smaller electronegativity difference between the two atoms.

𝑠𝑖𝑛^2𝜃 =

Q: Wenn steht dass man "infinitely slow" runterkühlt dann, [...]
A: geht man einfach gerade runter bis zur gewünschten temperatur und wendet lever rule an

Q: wenn steht dass man schnell runterkühlt, dann [...]
A: wird kein GGW angenommen und man kann nicht einfach "gerade nach unten" sondern geht der Linie entlang bis zu peritectic resp. eutectic points. Erst wenn keine points mehr gerade nach unten. bei jedem schritt einzeln lever rule anwenden

Q: XRD shows [...] means it is [...]
A: broad peak; amorphous.

Q: XRD shows [...] means it is [...]
A: characteristic, sharp, narrow peaks; a crystal

Q: The only AB structures that are formed in intermetallic structures are [...]
A: CsCl and NiAs structure

Q: [...] is characterized by [...]
A: Metallic conductivity; mobile electron cloud

Q: I means [...]
A: body-centered

Schottky Defect

Frenkel Defect

Schottky vs. Frenkel Defect

DOS Darstellung von Metal, Semiconductor und Insulator

p-n-junction

Glass vs. Crystal

Q: Knock-on-process
A: an equivalent description: whereby Ag+ cations of a Td-interstitials knocks out a Ag+ from a Oh-lattice position into another Td-void (interstitial) and takes over its lattice position at the Oh-void of the fcc Cl lattice

Q: The lattice parameter will [...] for heavier/ larger elements, therefore the diffraction peaks will shift to [...] angles for the heavier elements.
A: increase; smaller

Q: More grain boundaries (also between different structures) will [...]. Increased mechanical properties are hardness, tensile strength and shear strength.
A: stop the progression of slip plains i.e. 2D defects

Q: Bonding orbitals are occupied and therefore [...] than the fermi energy.
A: lower

Q: When dopants are close with their atomic radii we can only have [...] doping.
A: substitutional doping and no interstitial

Q: DTA: An exothermic chemical reaction will show a [...] peak because [...] In some cases, the activation energy can even be seen [...]
A: broad, positive; it takes place after overcoming the activation energy up to the point until the reaction is complete.; as an endothermic peak before the exothermic one.

Q: Principal difference between congruent and incongruent phase transformations
A: For congruent no compositional changes occur with any of the phases that are involved in the transformation.
For incongruent there will be compositional alterations of the phases.

Q: Thermal conductivity of amorphous metal is
A: - lower than that of crystalline metal
- higher than that of silicate glass.

Q: [...] are [...] than oxide glasses and ceramics. Because of [...]
A: Amorphous metals, while technically glasses,; much tougher and less brittle; the nature of bonding there is more tolerance for bond deformation, which leads to a less brittle product.