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Construction Materials > Crystal structure and defects > Flashcards

Crystal structure and defects Flashcards

1
Q

What are the primary interatomic bonds? (3)

A

1) Ionic
2) Covalent
3) Metallic Bonds

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2
Q

What are the secondary interatomic bonds? (2)

A

1) Van der Waals

2) Hydrogen bonding

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3
Q

What is ionic bonding?

A

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

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4
Q

What is covalent bonding?

A

Sharing of valence electrons between adjacent atoms.

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5
Q

What are metallic bonds?

A

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

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6
Q

What are Van der Waal bonds?

A

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

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7
Q

What is hydrogen bonding?

A

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

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8
Q

What are atoms and ions separated by?

A

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

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9
Q

What is the net force at equilibrium distance?

A

Zero (attractive=repulsive).

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10
Q

Define bonding energy.

A

The energy required to separate two atoms to infinite separation.

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11
Q

What do materials which high bonding energies have?

A

High melting temperature and therefore high strengths.

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12
Q

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

A

1) Modulus of elasticity or mechanical stiffness.

2) Linear coefficient of thermal expansion

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13
Q

Define Linear coefficient of thermal expansion.

A

How much materials expand upon heating and contract upon cooling.

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14
Q

How do atoms form a solid structure?

A

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

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15
Q

What type of materials form crystalline structures? (3)

A

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

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16
Q

What is a non-crystalline or amorphous material?

A

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

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17
Q

What does a crystal structure describe?

A

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

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18
Q

What is lattice?

A

A 3D array of points coinciding with atom positions.

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19
Q

What are unit cells?

A

Smallest repetitive entities (building units).

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20
Q

What builds a crystal?

A

Repeated translation of unit cells.

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21
Q

What is the hard-sphere model?

A

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

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22
Q

How many crystal systems are there?

A

7

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23
Q

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

A

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

24
Q

What are the lattice parameters?

A

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

25
Q

What are the interaxial angles?

A

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

26
Q

How are crystalline structures formed?

A

From solidification from a liquid.

27
Q

What forms first when a crystalline structure forms?

A

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

28
Q

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

A

All different.

29
Q

How are grains formed?

A

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

30
Q

What are the unit cells of grains?

A

Misaligned at their interfaces calling them “grain boundaries”

31
Q

Define a polycrystalline structure?

A

Structure made of multiple crystalline grains.

32
Q

What are vacancies?

A

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

33
Q

What are the 3 defects in crystal lattices?

A

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

34
Q

What is a point defect also known as?

A

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

35
Q

What do point defects do?

A

Tune the properties of a material.

36
Q

What is an interstitial atom defect?

A

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

37
Q

What are the properties of interstitial atoms?

A

Smaller than the ones located in the lattice sites.

38
Q

What is the substitutional defect?

A

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

39
Q

What are the properties of the substitutional atoms?

A

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

40
Q

What do substitutional atoms do?

A

Increase the strength of metallic materials.

41
Q

What is the Frankel defect?

A

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

42
Q

What is the Schottky defect?

A

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

43
Q

What are line defects also called?

A

One dimensional defects or dislocations.

44
Q

What are the two types of line defects?

A

1) Edge dislocation

1) Screw dislocation

45
Q

What is the typical movement of an edge dislocation?

A

The sliding of dislocation on straight planes called lattice planes.

46
Q

What do dislocations contribute to?

A

Strain hardening a macroscopic phenomenon.

47
Q

What is the material surface?

A

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

48
Q

Define Grain boundaries.

A

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

49
Q

What happens if you reduce the grain size?

A

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

50
Q

What is the Hall-Petch equation?

A 
σy = σ0 + kd^(-1/2)
where:
σy = yield strength
σ0 and k = material constant
d = average grain diameter
51
Q

What are stacking faults?

A

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

52
Q

What are twin boundaries?

A

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

53
Q

Define strain hardening?

A

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

54
Q

How does grain size strengthening occur?

A

Increasing the number of grains or reducing grain size.

55
Q

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

A

Dopants

56
Q

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

A

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

57
Q

What effect do point defects have?

A

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

Construction Materials (9 decks)

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