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The ability of a material to conduct/transfer heat is referred to as thermal conductivity. The letter ‘k’ is commonly used to represent it. Thermal resistivity is the reciprocal of this parameter. High thermal conductivity materials are employed as heat sinks, while low thermal conductivity materials are used as thermal insulators. If two molecules collide, an energy transfer from the hot to the cold molecule occurs. The cumulative effect from all collisions results in a net flux of heat from the hot body to the colder body. We call this transfer of heat between two objects in contact is thermal conduction.
Thermal conduction formula
Thermal conductivity is defined on Fourier’s Law of heat conduction. The rate at which heat is transported through a material is proportional to the negative of the temperature gradient and also proportional to the area through which the heat flows, according to Fourier’s law of heat conduction.
Mathematically,
q = -k∇T
Where
q = heat flux or the thermal flux
∇T = temperature gradient
k = thermal conductivity
Now, the thermal conductivity of any material is described by the following equation:
k = (QL)/(AΔT)
Where
Q = the rate at which heat is transported through a substance
L = distance between two isothermal planes
A = area of the surface
ΔT = temperature difference
k = thermal conductivity
Unit of thermal conductivity
Thermal conductivity is expressed in terms of the following dimensions: Temperature, Length, Mass, and Time.
The SI unit of thermal conductivity is in ‘watt per meter kelvin’.
The CGI unit of thermal conductivity is in ‘cal/cm/sec/K’.
The dimensional formula of thermal conductivity is [M¹L¹T⁻³K⁻¹]
Measurement of thermal conductivity
There are mainly two techniques for the measurement of thermal conductivity:
Steady-state technique
Transient state technique
Steady-state technique
Steady-state approaches entail measurements in which the temperature of the material in question remains constant across time. Because the temperature is constant, these approaches have the advantage of being relatively simple to analyze.
One of the major drawbacks of steady-state approaches is that they typically necessitate a very well-engineered set up to do the tests.
Transient state technique
During the heating-up phase, transient approaches take measurements. The benefit is that measurements may be completed rapidly.
The drawback is that data mathematical analysis is more complicated in general.
Factor affects the thermal conductivity
Temperature: According to these findings, the value of thermal conductivity normally increases as the temperature rises.
Chemical phase: A sudden shift in a material’s heat conductivity can occur when its phase changes.
Moisture content: All three stages of moisture (solid, liquid, and gas) can be detrimental to building materials in normal ambient circumstances around buildings. Moisture can reduce the effective thermal characteristics of building envelopes, insulated walls, and roofs.
Density: For the same types of materials, the thermal conductivity decreases with increasing density. Furthermore, specimens with lower densities had higher heat conductivity than those with higher densities.
Thermal anisotropy: When thermal anisotropy exists, the direction in which heat flows may differ from the direction in which temperature gradients exist.
Electrical conductivity: Only metals are subject to the Wiedemann-Franz law, which establishes a relationship between electrical and thermal conductivity. Non-metals heat conductivity is mostly influenced by their electrical conductivities.
Magnetic field: When magnetic fields are applied, the creation of an orthogonal temperature gradient is noticed, which describes the change in the thermal conductivity of a conductor when it is placed in a magnetic field.
Purity of metal: Pure materials have a higher thermal conductivity than alloy materials. Thermal conductivity is reduced when metals are alloyed and impurities are present.
Isotopic purity: The effect of isotopic purity on thermal conductivity is explained as the concentration of isotopes increases, the thermal conductivity also increases.
Aging time: The mechanical properties and thermal performances of insulating materials are well known to change significantly over time. The dispersion of highly insulating blowing agents and the infusion of air from the environment, which may absorb moisture into the material, are two of the most significant aging effects on thermal conductivity.
Conclusion
One of the thermophysical qualities is thermal conductivity. It will define the ease with which heat can be transferred from one location to another. Thermal conductivity determines how we keep warm or cool, as well as how we insulate ourselves from heat or cold. Metal, for example, is an excellent heat conductor. We utilize it in the kitchen to allow heat to circulate quickly through it and around the food we’re cooking. Rubber, for example, is a poor heat conductor. We then use it to defend ourselves by wrapping it around hot or cold objects.
