Materials ch2

Simple Cubic

one atom in each of the eight corners of the cube

BCC

contains one atom in each corner of the unit cell a well as one atom in the center of the unit cell

doping

The deliberate introduction of substitutional defects is called

FCC

has one atom in each corner of the unit cell and one atom on each face of the unit cell

Atomic Packing Factor

The amount of the unit cell occupied by atoms as opposed to void space (.68 for BCC and .74 for FCC)

Limitation of theoretical density

assumes a perfect lattice

Determining crystallographic dimensions and planes

1. Using a right-handed coordinate system, determine the coordinates of two points that lie on a line in the direction of interest.

2. Subtract the coordinates of the tail point from the coordinates of the head point to determine the number of lattice parameters traveled in the direction of each axis.

3. Clear fractions and reduce the result to the nearest integer (so that 1.25 would become 1).

4. Enclose the numbers in brackets with a line above negative numbers

(e.g., [1 2 0] would correspond to 1 in the x-direction, 2 in the

y-direction, and 0 in the z-direction)

Determining Miller Indices

1. Identify where the plane intercepts the x-, y-, and z-coordinate lines in terms of number of lattice parameters.

2. Take the reciprocal of these three points.

3. Clear fractions but do not reduce the results.

4. Enclose the results in parentheses

Point Defect

A flaw in the structure of a material that occurs at a single site in the lattice, such as vacancies, substitutions, and interstitial defects

Vacancies

Point defects that result from the absence of an atom at a particular lattice site.

Substitutional Defects

Point defects that result when an atom in the lattice is replaced with an atom of a different element.

Interstitial Defects

Point defects that occur when an atom occupies a space that is normally vacant.

Schottky defect

the presence of a cation and anion
vacancy in the lattice

Frenkel defect

diffusion of a cation into an interstitial
space in the lattice

Dislocations

Large-scale lattice defects that occur from alterations to the structure of the lattice itself.

Edge Dislocations

Lattice defects caused by the addition of a partial plane into an existing lattice structure

screw dislocation

results from the lattice being cut and shifted by one atomic spacing

Mixed dislocations

result when a lattice contains both edge and screw dislocations with a discernible transition region in between

Slip

The movement of dislocations through a crystal, caused when the material is placed under shear stress.

Slip factors

• Slip occurs more readily when the atoms are close together. The stress required to induce slip increases exponentially with increasing interplanar spacing.

• Slip requires the breaking of bonds, so materials with strong covalent bonds (such as polymers) are resistant.

• Materials with ionic bonds (such as metal oxides) are resistant because of larger interplanar distances and the repulsions caused when particles with like charge are forced to pass close to each other.

Homogeneous nucleation

occurs when a pure material cools enough to support the formation of stable nuclei

Heterogeneous nucleation

utilized impurities to provide a surface for the nuclei to form.