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
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)|
The dimension of thermal conductivity is given as M1L1T−3Θ−1
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))|
|3.||Air at 0°C||24.36|
|4.||Water at 20°C||0.6|
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