Factors leading to variation of thermal conductivity of different materials
This article gives the reason why the thermal conductivity of different materials/phases varies with different factors like material structure, density, phase, and composition as well as temperature. Read the following article about the variation of thermal conductivity.
Introduction to heat transfer
In the real world, we normally feel the heat by virtue of its temperature. If the temperature of a body is high, we can say a body is having high heat energy content compared to that at a low temperature. Now, this heat always flows from high temperature to low-temperature region or towards a positive temperature gradient according to the second law of thermodynamics. This heat transfer can take place in three different modes viz. Heat conduction, Heat convection, and Thermal Radiation. Heat convection can occur due to the bulk motion of fluid and contact with the solid surface. Thermal radiation occurs due to the temperature of any body which is at a temperature more than zero Kelvin. All the bodies having a temperature of more than zero Kelvin transmits thermal radiation. But the heat conduction or heat diffusion is proportional to the negative temperature gradient and proportionality constant called 'Thermal Conductivity. Heat transfer by conduction = -(thermal conductivity)*(High temperature-Lower temperature)/Width of the solid.Introduction to thermal conductivity
In simple words, Thermal conductivity is the ability of any material to be heated or to be cooled or the capacity to pass heat. Thermal conductivity differs from one material to others and also under different conditions. Thermal conductivity is a property of the material which depends mainly on the structure of the material in terms of chemical composition, the phase of the material, and its texture of it. Thermal conductivity also depends on the content of moisture present in the material as well as how closely atoms are packed in the lattice, also with operating conditions like pressure and temperature. The unit of thermal conductivity is W/mK or W/mC.Factors influencing thermal conductivity
Free electrons
Metals are having more free electrons in their outer shell compared to that liquids and gases, so metals are good conductors of heat due to the migration of free electrons. Metals are having closely packed lattices or crystal structures compared to liquids and gases.Purity of material
Thermal conductivity of the pure material is higher than that of alloy materials. Alloying of metals and the presence of impurities cause a decrease in thermal conductivity. E.g. thermal conductivity of pure copper is 385 W/mK but copper has a content of arsenic, and its thermal conductivity is 142 W/mK.Effect of forming
Treatment of metals like heat treatment and metal forming like bending, drawing, and forging decreases the thermal conductivity of material compared to the material before treatment.High temperature
Solid: At elevated temperature lattice vibration increases and free electrons movement decreases, thus the thermal conductivity of metal decreases when the temperature is increased.Gases: But for gases, thermal conductivity increases. The reason behind it at a higher temperature, the mean travel velocity of gas molecules and specific heat increases, because the thermal conductivity of gas is = (The mean travel velocity)X (specific heat)X (mean free path times density). While, Liquid: Thermal conductivity of the liquid is also proportional to the density and at a higher temperature, the density of liquid decreases thus, thermal conductivity also.
Pressure
Thermal conductivity is weakly dependent on pressure of substance. This means a change in pressure does not affect much in thermal conductivity.Density
Thermal conductivity is highly dependent on density of material. The increase in density increases thermal conductivity.Crystalline structure
Material having a regular crystalline structure has higher value of thermal conductivity compared to that of amorphous(irregular) form.Different materials have different thermal conductivity. The descending order of thermal conductivity for different forms of materials is as under-
- Pure metals
- Alloys (Combination of different metals)
- Non-metallic crystalline structures
- Liquids
- Gases
Also, thermal conductivity plays important role in the selection of a conductor or insulator. The material having higher thermal conductivity can be used as a thermal conductor and the material having lower thermal conductivity can be used as a thermal insulator. Diamond has the highest thermal conductivity among all the materials available naturally on the earth.
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A nice article regarding the thermal conductivities of different materials. Heat transfer through different material depends on their thermal conductivities only. I have some experience in the Geothermal area in Geophysics and Geology and I want to supplement the author's narration with it. The Earth crust is composed of solidified rocks practically floating on the mantle and subsequently the lava beneath it. The thickness of the Earth crust varies 5 to 30 km depending upon the oceans and landmasses. When we drill a hole in the Earth then the temperature gradually increases downwards as the temperature of inner Earth is very high. This temperature change is known as the geothermal gradient and is a characteristic of the rocks. On an average, the geothermal gradient is 2.5–3 °C /100 m. Different rocks have different geothermal gradient. By knowing the geothermal gradient we can predict the temperature expected at a particular depth so that the pipes or other tools lowered in oil exploration or geothermal water exploration can be designed suitably to handle the work. So the thermal conductivities of rocks are the important parameters which result in this gradient.