Physical properties of metals

Dr. Dmitri Kopeliovich

Optical properties

Metals reflect equally nearly all visible electro-magnetic waves. Therefore the color of the most of the metals is white or silvery-white (except copper and gold).
Metals are lustrous due to the metallic bonding, contributing free electrons to the metal crystal structure and providing an ability of metals to reflect light when polished.

Physical state

Metals are solid at normal temperatures (except mercury).
Metals transform to liquid from solid and to gas from liquid at definite temperatures(melting and boiling points), which are high for most of metals (except mercury, sodium and potassium).
Most of metals have relatively high densities (except sodium and potassium with densities lower, than density of water).

Electrical properties

Metals have high electrical conductivity, provided by free electrons available in the metal crystal structure.

• Peltier effect:

When there is an electric current, passing through a junction of two different metals, one of them evolves heat and another absorbs heat.

• Thomson effect:

A current is produced in a metal conductor when there is a temperature gradient along its length.
The Peltier and Thomson effects are widely used in thermocouples.

Thermal properties

Thermal Conductivity

Thermal Conductivity (λ) is amount of heat passing in unit time through unit surface in a direction normal to this surface when this transfer is driven by unite temperature gradient under steady state conditions.
Thermal conductivity may be expressed and calculated from the Fourier’s law:
ΔQ/ Δt = λ*S *ΔT/ Δx
Where
Q -heat, passing through the surface S;
Δt - change in time;
λ - thermal conductivity;
S - surface area, normal to the heat transfer direction;
ΔT/Δx-temperature gradient along x – direction of the heat transfer.
Fourier’s law is analogue of the First Fick’s law, describing diffusion in steady state.
Metals have high thermal conductivity. Heat is transferred through the metal crystal by free electrons. Compare:
λ of alumina = 47 BTU/(lb*ºF) (6.3 W/(m*K)).
λ of Al = 1600 BTU/(lb*ºF) (231 W/(m*K)).

Coefficient of Thermal Expansion

Thermal Expansion (Coefficient of Thermal Expansion) is relative increase in length per unite temperature rise:
α= ΔL/ (LoΔT)
Where
α -coefficient of thermal expansion (CTE);
ΔL – length increase;
Lo – initial length;
ΔT – temperature rise.
Thermal expansion of metals is generally higher, than that of ceramics.
Compare:
CTE of SiC = 2.3 ºFˉ¹ (4.0 ºCˉ¹).
CTE of Al = 13 ºFˉ¹ (23 ºCˉ¹).

Specific Heat Capacity

Heat Capacity is amount of heat required to raise material temperature by one unit.
Specific Heat Capacity is amount of heat required to raise temperature of unit mass of material by one unit:
c= ΔQ/(mΔT)
Where
c -specific heat capacity;
ΔQ – amount of heat;
m – material mass;
ΔT – temperature rise.
Specific Heat Capacity of metals is lower, than that of ceramics.
Compare:
“c” of alumina = 0.203 BTU/(lb*ºF) (850 J/(kg*K)).
“c” of steel = 0.115 BTU/(lb*ºF) (481 J/(kg*K)).

Magnetic properties

Most of metals are slightly magnetic, but only few of them (iron, nickel, cobalt and their alloys) display pronounced magnetic properties, called ferromagnetism.

• Magnetically soft metals – metals, which are demagnetized after the magnetic field is removed. Magnetically soft metals are used in electric motors and transformers.

• Magnetically hard metals – metals, retaining their magnetization after the magnetic field is removed.Magnetically hard metals are used for permanent magnets.

• Magnetostriction – effect of changing dimensions of a ferromagnetic metal when its magnetization is changed.