Lecture 9: Crystalline Defects

Question 1

What is a crystalline defect?

Answer 1

A crystalline defect is an imperfection in the regular geometrical arrangement of the atoms in a crystal structure, which can vary in size from a single atom to a large area of the lattice.

Question 2

What are point defects?

Answer 2

Point defects are imperfections that occur at or around a single lattice point within the crystalline structure and are not extended to neighboring lattice points.

Question 3

Define a vacancy defect.

Answer 3

A vacancy defect occurs when a position in the crystal lattice that should be occupied by an atom or ion is empty.

Question 4

What is an interstitial defect?

Answer 4

An interstitial defect happens when an extra atom or ion is added to a crystal lattice at a position not usually occupied in the perfect lattice.

Question 5

Describe a substitutional defect.

Answer 5

A substitutional defect occurs when an atom or ion in the lattice is replaced by a different type of atom or ion of similar size.

Question 6

What is a Frenkel defect?

Answer 6

A Frenkel defect is a type of point defect that involves an atom moving from its lattice site to an interstitial site, creating a vacancy and an interstitial defect simultaneously.

Question 7

What is a Schottky defect?

Answer 7

A Schottky defect occurs when equal numbers of cations and anions are missing from an ionic crystal, maintaining overall electrical neutrality.

Question 8

How does temperature affect the formation of defects in crystals?

Answer 8

As temperature increases, the entropy of the crystal increases, which can lead to the formation of more defects.

Question 9

Explain intrinsic defects.

Answer 9

Intrinsic defects are those that occur naturally during the formation of the crystal and are not due to external impurities or conditions.

Question 10

What are extrinsic defects?

Answer 10

Extrinsic defects are introduced into the crystal structure through doping or during the manufacturing process.

Question 11

Describe how doping influences semiconductor properties.

Answer 11

Doping introduces extrinsic defects by adding impurity atoms to a semiconductor, which can alter its electrical properties, such as its conductivity.

Question 12

What is the Kröger-Vink notation?

Answer 12

Kröger-Vink notation is a way to describe point defects in crystals by indicating the type of defect, its charge, and its position relative to the lattice.

Question 13

Define an edge dislocation.

Answer 13

An edge dislocation occurs when an extra half-plane of atoms is inserted into a crystal, creating a misalignment in the lattice.

Question 14

What is a screw dislocation?

Answer 14

A screw dislocation occurs when the crystal layers are misaligned along a spiral ramp around a cylindrical axis.

Question 15

How do dislocations affect material properties?

Answer 15

Dislocations can weaken materials but also allow for plastic deformation, which is crucial for ductility in metals.

Question 16

Explain the role of dislocations in the mechanical strength of materials.

Answer 16

The presence and movement of dislocations determine the mechanical strength of a material; more dislocations can lead to stronger, but more brittle, materials.

Question 17

What is a stacking fault?

Answer 17

A stacking fault is a planar defect where the normal stacking sequence of crystal planes is disrupted.

Question 18

How do vacancies affect material properties?

Answer 18

Vacancies can lead to a decrease in density and may affect the electrical and thermal properties of materials.

Question 19

What is the effect of grain boundaries on the properties of a material?

Answer 19

Grain boundaries can block the movement of dislocations and impede electron flow, which can strengthen materials but may also decrease electrical conductivity.

Question 20

Describe the twin boundary in crystallography.

Answer 20

A twin boundary is a type of grain boundary where the crystal lattice is mirrored across the boundary, creating a symmetrical orientation.

Question 21

What are color centers in crystals?

Answer 21

Color centers are defects that involve vacancies occupied by an electron or an impurity, which can absorb specific wavelengths of light and give color to crystals.

Question 22

How does irradiation create defects in crystals?

Answer 22

Irradiation can knock atoms out of their lattice positions, creating vacancies and interstitials, leading to radiation damage.

Question 23

What is an alloy? How do defects play a role in alloys?

Answer 23

An alloy is a mixture of two or more elements where at least one is a metal. Defects in alloys can affect their mechanical properties, such as strength and hardness.

Question 24

Explain how defects influence the diffusion process in materials.

Answer 24

Defects like vacancies and interstitials provide pathways that can enhance the diffusion of atoms or ions in a material.

Question 25

What is the role of defects in the electrical properties of semiconductors?

Answer 25

Defects can act as electron donors or acceptors in semiconductors, significantly affecting their conductivity and other electrical properties.

Question 26

How are defects studied and analyzed in materials science?

Answer 26

Defects are studied using techniques such as X-ray diffraction, electron microscopy, and electrical conductivity measurements.

Question 27

What is the effect of thermal expansion on defect formation?

Answer 27

Thermal expansion can lead to the formation of thermal vacancies as the lattice expands and more interstitial spaces become available.

Question 28

Discuss the concept of defect chemistry.

Answer 28

Defect chemistry studies the formation, distribution, and impact of defects in materials, particularly ionic and semiconductor materials.

Question 29

How do point defects influence chemical reactivity?

Answer 29

Point defects can act as active sites for chemical reactions, particularly in catalysis, where they can enhance the reactivity of a material.

Question 30

What is a Burger’s vector?

Answer 30

The Burger’s vector is a vector that describes the magnitude and direction of a dislocation in a crystal lattice.

Question 31

Explain the significance of defect density in materials.

Answer 31

Defect density can determine the material’s mechanical strength and electrical properties; high defect density can lead to weaker materials but can enhance certain properties like conductivity.

Question 32

What are coherent and incoherent twin boundaries?

Answer 32

Coherent twin boundaries have atoms that match across the boundary with good lattice matching, whereas incoherent boundaries do not match and often contain defects.