Physics_Everything You Should Know About Thermal Conductivity

In simple terms, thermal conductivity is the ability of any material to transfer or conduct heat. It is either denoted by λ,k, or even κ. Among the three methods of heat transfer, it is one of them, while the other two are radiation and convection. Thermal conductivity comprises the rate equation based on Fourier’s law of heat conduction. In this blog, we will discuss thermal conductivity, its formula, the different thermal conductivity of materials, the influence of temperature, and variations in thermal conductivity. In addition to that, we will also discuss the different thermal conductivities of metals.

What is thermal conductivity?

Thermal conductivity is the property that connects the heat loss per unit area that a material experiences to the rate of temperature change. The value is the change that any material undergoes to conduct heat. The SI Unit of thermal conductivity is signified by watts per metre kelvin. In addition, thermal conductivity is denoted by BTU per hour per foot Fahrenheit in imperial units. If materials have higher thermal conductivity, they are considered good thermal energy conductors.

Thermal conductivity of materials

Thermal conductivity depends significantly on the structure of the material and varies accordingly. Depending on the direction of the heat travel, different materials will have different transfers of heat through thermal conductance. The materials whose thermal conductivity depends on the direction of the heat travel are known as anisotropic materials. Due to the arrangement of the material, heat tends to travel effortlessly in a specific direction. 

There are three classifications of materials when discussing thermal conductivity, metallic solids, non-metallic solids, and gases:

1. Gases have low thermal conductivity due to the molecular packaging. Thus, one can imply that thermal conductance depends significantly on the molecular velocity and the free movement of the molecules.
2. Since molecules in non-metallic solids are bound in a lattice network, thermal conductivity primarily occurs through vibrations.
3. The dense packaging of the non-metallic solids ensures that they are better conductors of heat than gases.

Thus, materials with large air pockets are better insulators than densely packed molecules. 

Temperature and thermal conductivity

The lack of motion of the material, while there is a heat transfer taking place through conduction, ensures that the rate of heat transfer depends on the thermal conductivity of the material and the difference in temperature between the two locations.

The formula of thermal conductivity

Since molecular movement depends on thermal conductance, thermal conductivity is influenced severely by the temperature of the material. Molecules move quickly at a higher temperature and vice versa. The rapid movement of molecules due to a high temperature compels the transfer of heat to take place faster. This implies that the same material will perform thermal conductivity at a different rate with higher temperatures. With the decrease in temperature, the thermal conductivity will be slower than the material’s thermal conductivity power if the temperature increases. 

Understanding the effect of temperature on thermal conductivity is essential to ensure that the materials behave as they are supposed to when they experience thermal stress. 

Variations in the thermal conductivity of materials

Several factors determine the thermal conductivity of different materials. The factors include temperature gradient, path length, and even the properties of the materials. Low thermal conducting materials or substances such as air have a thermal conductivity of 0.024 W/m.k at 0° celsius. While materials like copper have a high thermal conductivity rate of 385 W/m.k. A significant amount of thermal conductivity of the various materials depends on the utilisation. For example, materials with low thermal conductivity have more significant potential to become effective insulators. On the other hand, materials with higher thermal conductivity have more significant potential where there is a requirement for the heat to travel faster from different locations. For example, the cooling systems in electronic devices or appliances. The key lies in selecting the material with the suitable thermal conductivity for the application to utilise optimal performance.

Thermal conductivity of metals

Except for graphene, metals have different thermal conductivity and the highest thermal conductivity. Metals have the unique ability to become both electrical and thermal conductors. Wiedemann-Franz Law explains the relation between the two attributes. The law states that at a specific temperature, the thermal and the electrical conductivity of metal will be the same. Still, as the temperature rises, the thermal conductivity will increase while the electrical conductivity decreases. 

Conclusion

Thermal conductivity is the property that denotes the rate of loss of heat per unit area of any material concerning the temperature change. Therefore, it is the value of any property of a material that changes while it conducts heat. Since there is a transfer of heat without the physical movement of the material, the rate of heat transfer would depend on the temperature change between the two locations along with the thermal conductance value of the material. Thermal conductivity depends on the temperature gradient of the material, the molecular packaging, properties, and even the pathway following which the heat will supposedly flow. Metals are great conductors of heat, while gases are bad conductors of heat. Nonmetallic solids, on the other hand, are average conductors of heat.