Crystal structure and defects

Question 1

What are the primary interatomic bonds? (3)

Answer 1

1) Ionic
2) Covalent
3) Metallic Bonds

Question 2

What are the secondary interatomic bonds? (2)

Answer 2

1) Van der Waals

2) Hydrogen bonding

Question 3

What is ionic bonding?

Answer 3

Transfer of electrons creates electrically charged ions which are attracted together.

Question 4

What is covalent bonding?

Answer 4

Sharing of valence electrons between adjacent atoms.

Question 5

What are metallic bonds?

Answer 5

Valence electrons not bound to any atom in the solid, firing a sea of electrons or an electron cloud.

Question 6

What are Van der Waal bonds?

Answer 6

Considerably weak compared to primary ones; arise from atomic or molecular dipoles.

Question 7

What is hydrogen bonding?

Answer 7

A special type of bonding where hydrogen is involved as one of the constituents.

Question 8

What are atoms and ions separated by?

Answer 8

An equilibrium distance (r0) where the interatomic bonding energy is minimum (E0).

Question 9

What is the net force at equilibrium distance?

Answer 9

Zero (attractive=repulsive).

Question 10

Define bonding energy.

Answer 10

The energy required to separate two atoms to infinite separation.

Question 11

What do materials which high bonding energies have?

Answer 11

High melting temperature and therefore high strengths.

Question 12

What dictates the shape of the force or energy vs interatomic separation curve?

Answer 12

1) Modulus of elasticity or mechanical stiffness.

2) Linear coefficient of thermal expansion

Question 13

Define Linear coefficient of thermal expansion.

Answer 13

How much materials expand upon heating and contract upon cooling.

Question 14

How do atoms form a solid structure?

Answer 14

1) Through Crystalline materials- periodic arrangement of atoms over long distances, therefore, long-range order.
2) Through non-crystalline or amorphous materials.

Question 15

What type of materials form crystalline structures? (3)

Answer 15

1) All metals
2) Many ceramics
3) Some polymers

Question 16

What is a non-crystalline or amorphous material?

Answer 16

Short-range order, sd long-range order do not exist. They are complex structures that rapidly cool.

Question 17

What does a crystal structure describe?

Answer 17

Describe the manner of how atoms, ions and molecules are spatially arranged.

Question 18

What is lattice?

Answer 18

A 3D array of points coinciding with atom positions.

Question 19

What are unit cells?

Answer 19

Smallest repetitive entities (building units).

Question 20

What builds a crystal?

Answer 20

Repeated translation of unit cells.

Question 21

What is the hard-sphere model?

Answer 21

Atoms or ions are thought of as being solid spheres ave well-defined diameters.

Question 22

How many crystal systems are there?

Answer 22

7

Question 23

What happens when you combine one of the crystal systems with and entering?

Answer 23

Creates 14 bravos lattices such as BCC (Body-centered-cubic).

Question 24

What are the lattice parameters?

Answer 24

a,b & c in x,y & z.

Question 25

What are the interaxial angles?

Answer 25

Angles between the two edges: angles between a and b is gamma, b and c is alpha and a and c is beta.

Question 26

How are crystalline structures formed?

Answer 26

From solidification from a liquid.

Question 27

What forms first when a crystalline structure forms?

Answer 27

The nucleus, consisting of only a few unit cells. The nucleus grows by the addition of unit cells forming a larger crystal. (crystallite).

Question 28

When multiple nuclei form at the same time what is the orientations of these?

Answer 28

All different.

Question 29

How are grains formed?

Answer 29

When different crystallites impinge on each other, they stop growing forming a structure consisting of multiple crystallites called grains.

Question 30

What are the unit cells of grains?

Answer 30

Misaligned at their interfaces calling them "grain boundaries"

Question 31

Define a polycrystalline structure?

Answer 31

Structure made of multiple crystalline grains.

Question 32

What are vacancies?

Answer 32

Missing atoms leaving holes in the lattice reducing mechanical strength of the material.

Question 33

What are the 3 defects in crystal lattices?

Answer 33

1) Point defects 2) Line defects 3) Surface defects

Question 34

What is a point defect also known as?

Answer 34

zero-dimensional defect concerning the position of one or, pre isolated atoms in a crystal lattice.

Question 35

What do point defects do?

Answer 35

Tune the properties of a material.

Question 36

What is an interstitial atom defect?

Answer 36

An extra atoms/ion inserted at an interstitial site, small void space not usually occupied.

Question 37

What are the properties of interstitial atoms?

Answer 37

Smaller than the ones located in the lattice sites.

Question 38

What is the substitutional defect?

Answer 38

A lattice atom or ion is replaced by a different atom/ion.

Question 39

What are the properties of the substitutional atoms?

Answer 39

Can be larger or smaller than the host atoms in the structure.

Question 40

What do substitutional atoms do?

Answer 40

Increase the strength of metallic materials.

Question 41

What is the Frankel defect?

Answer 41

A vacancy interstitial pair, formed when a lattice atom jumps to an interstitial site, leaving behind a vacancy.

Question 42

What is the Schottky defect?

Answer 42

Removal of a stoichiometric number of anions and cations to preserve the electrical neutrality.

Question 43

What are line defects also called?

Answer 43

One dimensional defects or dislocations.

Question 44

What are the two types of line defects?

Answer 44

1) Edge dislocation | 1) Screw dislocation

Question 45

What is the typical movement of an edge dislocation?

Answer 45

The sliding of dislocation on straight planes called lattice planes.

Question 46

What do dislocations contribute to?

Answer 46

Strain hardening a macroscopic phenomenon.

Question 47

What is the material surface?

Answer 47

A crystal abruptly ends, the atomic bonding is disrupted and each atom at the surface has no longer got a proper coordination number.

Question 48

Define Grain boundaries.

Answer 48

Narrow zones where the atoms are not properly spaced, separating the individual grains.

Question 49

What happens if you reduce the grain size?

Answer 49

Increase the grain boundaries leads to an increase in materials strength.

Question 50

What is the Hall-Petch equation?

Answer 50

``` σy = σ0 + kd^(-1/2) where: σy = yield strength σ0 and k = material constant d = average grain diameter ```

Question 51

What are stacking faults?

Answer 51

An error in the stacking sequence of close-packed planes.

Question 52

What are twin boundaries?

Answer 52

A plane across which there is a mirror image disorientation of the crystal structure.

Question 53

Define strain hardening?

Answer 53

Process of strengthening a material by deforming it increases dislocation density.

Question 54

How does grain size strengthening occur?

Answer 54

Increasing the number of grains or reducing grain size.

Question 55

What are important in semi-conductors to achieving the desired performance?

Answer 55

Dopants

Question 56

How can we produce dramatic colour changes in the original material?

Answer 56

Adding small amounts of foreign atoms. e.g. coloured lass.

Question 57

What effect do point defects have?

Answer 57

Dramatic effects on optical, magnetic and electrical properties of materials.