Lecture 5 - Order in Solids

Question 1

What is amorphous structure?

Answer 1

Non-dense, random packing of atoms/molecules within a solid.

Question 2

What is crystalline structure?

Answer 2

Dense, regular packing of atoms/molecules within a solid.

Question 3

Which is a lower energy structure and why: Crystalline or amorphous?

Answer 3

Crystaline structures are typically more stable/lower energy due to their dense, regular-packed structure.

Question 4

What is a single crystal?

Answer 4

entire solid is one long crystal

Question 5

What is a polycrystal?

Answer 5

Several crystals packed together in one solid

Question 6

What is the best 2-dimension packing structure?

Answer 6

“Closed packed” structures (hexagonal) because it minimizes the amount of empty space within a crystal

Question 7

What are properties of metallic crystals?

Answer 7

Densely packed crystals (extra), simplest structure

Question 8

Why are metallic crystals so densely packed?

Answer 8

• Typically only one element present (all atomic radii are equal)
• Metallic bonding is non-directional
• small intramolecular or atomic distance lowers bond energy
• Electron cloud shields positive cores from repulsive forces

Question 9

What is a unit cell?

Answer 9

smallest repetitive volume which
contains the complete pattern of a crystal.

Question 10

What is the coordination number (CN)?

Answer 10

Number of neighbouring particles that come in direct contact with one given particle in a hard sphere model.

Question 11

For the simple cubic structure: CN, atoms/unit cell, a/R relationship?

Answer 11

CN: 6
atoms/unit cell = 1
a = 2R

Question 12

What is the formula for the atomic packing factor?

Answer 12

APF = (# of atoms/unit cell)(volume of atom with R in terms of a)/(volume of unit cell which is a^3)

Question 13

For the body centred cubic structure: CN, atoms/unit cell, a/R relationship?

Answer 13

CN: 8
atoms/unit cell = 2
a = 4R/sqrt(3)

Question 14

For the face centred cubic structure: CN, atoms/unit cell, a/R relationship?

Answer 14

CN: 12
atoms/unit cell: 4
a = 2Rsqrt(2) –> a^2+a^2=(4R)^2

Question 15

For the hexagonal close packed structure: CN, atoms/unit cell?

Answer 15

CN: 12
atoms/unit cell: 6

Question 16

Why do compounds have more complex crystal structures?

Answer 16

Differences in atomic radii yields more possible structures.

Question 17

How are compound structures typically described?

Answer 17

describing the lattice of one particle type and describing how the voids in this structure are filled by other types of particles.
ex: CsCl: simple cubic atom cell with central voids occupied by the other atom.

Question 18

What are the two types of voids within FCC structures of compounds?

Answer 18

1. Octahedral (CN = 6)
2. Tetrahedral (CN = 4)

Question 19

How many types of each void appear in FCC structure per unit cell?

Answer 19

Octahedral = 4 voids

Tetrahedral = 8 voids

Question 20

What is the “Rock Salt” structure?

Answer 20

FCC structure with all octahedral voids occupied by the other atom.

ex: LiF, NaF, KCl… all ionic salts

Question 21

What is the “Zincblende” structure?

Answer 21

FCC structure with half of the tetrahedral voids occupied by the other atom.

ex: diamonds (except voids are filled with other carbons)

Question 22

What is the theoretical density formula?

Answer 22

density = (atoms/unit cell)(atomic weight)/(volume of unit cell)(Avogadros number)

Question 23

What is polymorphism?

Answer 23

When metals or non-metals can have different types of crystal structures depending on pressure/temperature conditions.

Question 24

What is allotropy?

Answer 24

When elemental solids have different types of crystal structures depending on pressure/temperature conditions

Question 25

What is a crystal system?

Answer 25

The overall geometry of the unit cell.

Question 26

What is a crystal lattice?

Answer 26

The actual position of the atoms within the geometry of the unit cell.

Question 27

What is Bragg's law? (definition ish)

Answer 27

The law explains the relationship between an x-ray light shooting into and its reflection off from crystal surface.

Question 28

When does diffraction occur?

Answer 28

BCC: h+k+l = even # FCC: h,k,l are either all even or all odd HCP: when (h+2k)/=3n (n is any integer)

Question 29

What is anisotropy?

Answer 29

When properties vary with direction ex: wood shows different strength or conductivity properties along different axes due to its grain structure.

Question 30

When are single crystals anisotropic?

Answer 30

Always!!

Question 31

When are polycrystals anisotropic?

Answer 31

If grains are randomly oriented: isotropic If grains are properly oriented: anisotropic

Question 32

What are examples of amorphous solids?

Answer 32

glasses and most polymers

Question 33

How does an amorphous structure form?

Answer 33

When fast solidification does not allow time for atoms/molecules to reorganize into lower energy states (crystals), the solid is left in an amorphous state.

Question 34

How does the transition between solid crystals and liquid occur in crystal structures?

Answer 34

Before and after melting: atomic vibrations increase with temperature causing slower volume expansion At melting: crystals begin to form/unform which causes sudden volume change

Question 35

How does the transition between solid crystals and liquid occur in amorphous structures?

Answer 35

At Tm, the liquid turns into a supercooled liquid (basically solid but not in a crystalline structure so that atoms/molecules can technically still move around) At Tg, the supercooled liquid becomes a glass where the solid becomes hard and brittle.

Question 36

What are the three types of defects in crystals?

Answer 36

- Vacancies (point defects) - Dislocations (line defects) - Grain boundaries (surface defects)

Question 37

What is a vacancy defect?

Answer 37

When an atom within a crystal structure is missing

Question 38

What is an interstitial impurity?

Answer 38

When there is an extra atom (typically of another kind) where there should be a vacancy.

Question 39

what is a self interstitial defect?

Answer 39

When there is an extra atom in the crystal structure where there should be a vacancy according to the crystal structure

Question 40

What is a substitutional impurity defect?

Answer 40

When a wrong atom replaces a right atom in the correct spot

Question 41

How do vacancy defects promote diffusion?

Answer 41

extra space allows atoms increased mobility thus facilitating movement of atoms

Question 42

what are the consequences of point defects?

Answer 42

Induce local strain by distorting the plane of atoms

Question 43

How do vacancies affect thermal expansion?

Answer 43

Vacancies provide extra space within the lattice structure, allowing more freedom for neighboring atoms to move. This increased freedom can lead to a more pronounced expansion response to changes in temperature --> increase in the thermal expansion.

Question 44

What is a dislocation defect?

Answer 44

A type of line defect around which atoms are misaligned --> if a crystal is a stack of planes of atoms, then a dislocation defect would be like rotating or pushing the top half of the stack slightly over

Question 45

What are the two types of dislocation defects + explanations?

Answer 45

1. edge dislocation: extra half plane of atoms inserted in a crystal structure 2 screw dislocation: spiral planar ramp resulting from shear deformation.

Question 46

What is a grain boundary?

Answer 46

interface or boundary separating two crystalline grains within a material

Question 47

How do grain boundaries form?

Answer 47

Solidification: When a material solidifies from a molten state, it forms grains. During this process, atoms or molecules arrange themselves into ordered structures and form nuclei which grow into crystals. The boundaries between these growing grains become the grain boundaries.

Question 48

What are some properties of grain boundaries?

Answer 48

Low density: - high mobility - high diffusivity - high chemical reactivity

Question 49

How do grain boundaries affect the hardness of a material?

Answer 49

Dislocations allow for plastic deformation. Grain boundaries impede the movement of these dislocations, making the material harder. Small grains result in a stronger material

Question 50

Why do large grain materials have less energy than fine grain materials?

Answer 50

The boundary area/unit volume is smaller in large grains (same volume of material but bigger grains)

Question 51

what is an interstitial solid solution?

Answer 51

When a solute (substitutional interstitial) ion is within the solvent (normal atom) lattice

Question 52

What is a substitutional solid solution?

Answer 52

when one substitutional atom replaces an original atom in a crystal.

Question 53

What are the conditions for a substitutional solid solution?

Answer 53

1. atomic radius of substitution atom must be within 15% of normal atom 2. they must have proximity on the periodic table (similar properties) 3. they must have the same crystal structure when in pure crystals 4. substitutional atom must have a high valency

Question 54

What does the lever rule show?

Answer 54

the relative amounts of a material in the liquid and solid phase at equilibrium.

Question 55

How does cooling speed affect equilibrium of a binary system?

Answer 55

Slow cooling allows an equilibrium structure in which the entire solid has a uniform concentration of the two species Faster cooling causes cored structures in which certain layers contain varying concentrations.

Question 56

What causes cored structures in solids?

Answer 56

Diffusion is a difficult (long) process in solids, leading to non homogeneities if the cooling time is too fast and insufficient time is given to allow the solid to gain an equilibrium structure.