Density - The Science

MEE1004: Mechanics of Materials - Lecture 4

What determines density

Atomic weight, atom size, atomic packing.

Crystalline Materials

Have a regularly repeating pattern of structural units called unit cells.

Amorphous / non-crystalline materials

Materials that lack an ordered internal structure.

Unit cell

Characterising unit of a crystal structure.

Atomic packing factor

The fraction of volume in a crystal structure that is occupied by the atoms.

Atomic packing factor equations

Types of structures

Simple cubic

Body-centred cubic (BCC)

Face-centred cubic (FCC)

Close packing hexagonal (CPH)

Simple cubic

Face centred cubic (FCC) structure

An atom at all 8 corner positions and at the centre of all six faces.

4R = √2a

APF = 0.74

Body centred cubic (BCC)

Close packed hexagonal (CPH)

Each atom is surrounded by six nearest neighbours arranged in a hexagonal ring, and twelve next nearest neighbours arranged in a slightly distorted octahedral shape.

Close packed atomic structures

Packing fraction for CPH and FCC is 0.74 - meaning spheres occupy 74% of all available space.

Non close packed atomic structures

BCC APF = 0.68

Simple cubic APF = 0.52

Amorphous material: APF less than and equal to 0.64

Atomic packing in Ceramics

Generally compounds made of two or more elements (A and B).

A pair of atoms (A + B) is associated with each lattice point in the structure.

Atomic packing in Glasses

Made of natural materials such as sand.

Amorphous silica (SiO2) forms the basis of almost all glasses.

APF for amorphous material

≤0.64

Technique used for producing amorphous structured materials

Quenching (rapid cooling).

Thermoplastics

Any plastic polymer material that becomes pliable or mouldable at a certain elevated temperature and solidifies upon cooling,

Thermoplastic properties

Weak hydrogen bonds between molecule chains easily broken / rearrange

Melt easily

Easily shaped / mould / recycled

Thermoset

A polymer that is obtained by irreversibly hardening (“curing”) a soft solid or viscous liquid prepolymer (resin). T

Thermoset properties

Stiffer and stronger inter-molecular bonding

Crosslinks cannot be broken easily

Cannot be shaped / mould / recycled