Chapter 3: Structure of Crystalline Solids Flashcards
2 Types of Packing
- Non Dense, Random Packing
2. Dense, Ordered Packing
This structure tends to have lower energies.
Dense, Ordered Packing
Crystalling Materials
Packed in periodic, 3D arrays.
Metals, ceramics, Polymers…
Noncrystalline / Amorphous Materials
No periodic packing.
Complex structures, rapid cooling.
Metallic Crystal Structures
Densely Packed
Simplest Crystal Structures
3 Metal Crystal Structures
Simple Cubic Structure (SC)
Body Centered Cubic Structure (BCC)
Face Centered Structure (FCC)
Simple Cubic Structure
Rare due to low packing density.
Close-packed directions are cube edges
1 atom / unit cell
APF: 0.52
Atomic Packing Factor (APF) Formula
Volume of atoms in unit cell / Volume of unit cell
Body Centered Cubic Structure (BCC)
Atoms touch each other along cube diagonals
2 atoms / unit cell
APF: 0.68
Face Centered Cubic Structure (FCC)
Atoms touch each other along face diagonals
4 atoms / unit cell
APF : 0.74
FCC Stacking Sequence
ABC ABC
2D Projection
Hexagonal Close-Packed Structure (HCP)
ABAB Stacking Sequence
6 atoms / unit cell
APF: 0.74
A Sites: Hexagon
B Sites: Triangle
Theoretical Density (p)
Mass of Atoms in Unit Cell / Total Volume of Unit Cell
(n * A) / (V * N)
n: Number of atoms / unit cell
A: Atomic weight
V: Volume of Unit Cell
N: Avogadro’s Number
Density of Material Classes
Metals > Ceramics > Polymers
Density of Metals
Close-Packing (Metallic Bonding)
Large atomic masses
Density of Ceramics
Less Dense Packing
Lighter elements
Density of Polymers
Low-packing density (amorphous)
Lighter elements
Density of Composites
Intermediate values
Polycrystals
Most engineering materials fall into this category
Each “grain” is a single crystal
Grain size: 1 nm - 2 cm
Single Vs Polycrystals
Single Crystals:
Properties vary with direction (ansiotropic)
Polycrystals:
Properties may or may not vary with direction
If grains randomly oriented : Isotropic
Else textured: Ansiotropic
Poymorphism
Consists of two or more distinct crystal structures for the same material
Unit Cell
Smallest repetitive volume which contains the complete lattice pattern of a crysta
Linear Density (LD)
Number of Atoms / Unit length of directional vector
X-Ray Diffraction
Used for crystal structure and interplanar spacing determinations
