Lecture 9: Crystalline Defects Flashcards
What is a crystalline defect?
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.
What are point defects?
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.
Define a vacancy defect.
A vacancy defect occurs when a position in the crystal lattice that should be occupied by an atom or ion is empty.
What is an interstitial defect?
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.
Describe a substitutional defect.
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.
What is a Frenkel defect?
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.
What is a Schottky defect?
A Schottky defect occurs when equal numbers of cations and anions are missing from an ionic crystal, maintaining overall electrical neutrality.
How does temperature affect the formation of defects in crystals?
As temperature increases, the entropy of the crystal increases, which can lead to the formation of more defects.
Explain intrinsic defects.
Intrinsic defects are those that occur naturally during the formation of the crystal and are not due to external impurities or conditions.
What are extrinsic defects?
Extrinsic defects are introduced into the crystal structure through doping or during the manufacturing process.
Describe how doping influences semiconductor properties.
Doping introduces extrinsic defects by adding impurity atoms to a semiconductor, which can alter its electrical properties, such as its conductivity.
What is the Kröger-Vink notation?
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.
Define an edge dislocation.
An edge dislocation occurs when an extra half-plane of atoms is inserted into a crystal, creating a misalignment in the lattice.
What is a screw dislocation?
A screw dislocation occurs when the crystal layers are misaligned along a spiral ramp around a cylindrical axis.
How do dislocations affect material properties?
Dislocations can weaken materials but also allow for plastic deformation, which is crucial for ductility in metals.
Explain the role of dislocations in the mechanical strength of materials.
The presence and movement of dislocations determine the mechanical strength of a material; more dislocations can lead to stronger, but more brittle, materials.
What is a stacking fault?
A stacking fault is a planar defect where the normal stacking sequence of crystal planes is disrupted.
How do vacancies affect material properties?
Vacancies can lead to a decrease in density and may affect the electrical and thermal properties of materials.
What is the effect of grain boundaries on the properties of a material?
Grain boundaries can block the movement of dislocations and impede electron flow, which can strengthen materials but may also decrease electrical conductivity.
Describe the twin boundary in crystallography.
A twin boundary is a type of grain boundary where the crystal lattice is mirrored across the boundary, creating a symmetrical orientation.
What are color centers in crystals?
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.
How does irradiation create defects in crystals?
Irradiation can knock atoms out of their lattice positions, creating vacancies and interstitials, leading to radiation damage.
What is an alloy? How do defects play a role in alloys?
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.
Explain how defects influence the diffusion process in materials.
Defects like vacancies and interstitials provide pathways that can enhance the diffusion of atoms or ions in a material.
What is the role of defects in the electrical properties of semiconductors?
Defects can act as electron donors or acceptors in semiconductors, significantly affecting their conductivity and other electrical properties.
How are defects studied and analyzed in materials science?
Defects are studied using techniques such as X-ray diffraction, electron microscopy, and electrical conductivity measurements.
What is the effect of thermal expansion on defect formation?
Thermal expansion can lead to the formation of thermal vacancies as the lattice expands and more interstitial spaces become available.
Discuss the concept of defect chemistry.
Defect chemistry studies the formation, distribution, and impact of defects in materials, particularly ionic and semiconductor materials.
How do point defects influence chemical reactivity?
Point defects can act as active sites for chemical reactions, particularly in catalysis, where they can enhance the reactivity of a material.
What is a Burger’s vector?
The Burger’s vector is a vector that describes the magnitude and direction of a dislocation in a crystal lattice.
Explain the significance of defect density in materials.
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.
What are coherent and incoherent twin boundaries?
Coherent twin boundaries have atoms that match across the boundary with good lattice matching, whereas incoherent boundaries do not match and often contain defects.
