Thermal conductivity definition

The measure of a material’s ability to conduct heat is called thermal conductivity of a material. It is denoted by the following symbols k, λ, or κ. Thermal conductivity can be measured in several ways. The two major categories of measuring the techniques are: 

Steady-state – On reaching a steady-state temperature, it infers the thermal conductivity by measuring the state of the material.
Transient – It operates during an instantaneous state of a system while approaching a steady state.

Thermal conductivity formula

The equation for thermal conductivity is expressed as below:

q = – k∇T

where,

q = heat flux

K = thermal conductivity

∇T = Temperature gradient

Thermal conductivity unit

In the International System of Units (SI), thermal conductivity is measured in watts per meter-kelvin (W/(m⋅K)). Some papers report in watts per centimetre-kelvin (W/(cm⋅K)).

Sl. no.	Thermal conductivity unit	Measured
1.	International System of Units	watts per meter-kelvin (W/(m⋅K)) or
2.	SI unit	watts per centimetre-kelvin (W/(cm⋅K)
3.	Imperial units	BTU/(h⋅ft⋅°F)

The dimension of thermal conductivity is given as M1L1T−3Θ−1

where,

M is the mass

L is the length

T is the time

Θ is the temperature.

Factors influencing Thermal conductivity

• Chemical phase – When a substance undergoes a phase change thermal conductivity changes abruptly. For example, when the ice melts and forms liquid water at a temperature of 0 °C, the thermal conductivity also changes from 2.18 W/(m⋅K) to 0.56 W/(m⋅K).
• Magnetic field – Righi–Leduc effect or thermal Hall effect is the impact of the magnetic field on thermal conductivity.
• Isotopic purity – The thermal conductivity of any crystal mainly depends on isotopic purity, considering other lattice defects as negligible. One best example is diamond because, at T = 100 K, there is an increase in thermal conductivity from 10,000 W·m−1·K−1 to 41,000.

Various other factors are thermal anisotropy, temperature, electrical conductivity, gaseous phases, etc.

Thermal conductivity chart

The thermal conductivity values of some important materials are given in the table below:

Sl. no.	Material	Thermal Conductivity (W/(m K))
1.	Copper	385.0
2.	Aluminum	205.0
3.	Air at 0°C	24.36
4.	Water at 20°C	0.6
5.	Steel	50.2
6.	Glass (ordinary)	0.8
7.	Plastic (PVC)	0.19

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