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 of at low temperature. Now this heat always flows from high temperature to low-temperature region or towards 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. The heat convection can occur due to bulk motion of fluid and in contact with the solid surface. Thermal radiation occurs due to a temperature of any body which at a temperature more than zero Kelvin. All the body having the temperature more than zero Kelvin transmits the 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 capacity to pass heat. Thermal conductivity differs from one material to other and also with 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 material and texture of it. Thermal conductivity also depends on the content of moisture present in material as well as how closely atoms are packed in the lattice, also with operating conditions like pressure and temperature.
Factors influencing thermal conductivity
Free electrons Metals are having more free electrons compared to that of liquid and gases, so metal are good conductors of heat due to the migration of free electrons. Metals are having closely packed lattice 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 presence of impurities cause a decrease in thermal conductivity. E.g. thermal conductivity of pure copper is 385 W/mK but copper having content of arsenic, 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.
But for gases, thermal conductivity increases. The reason behind it at a higher temperature, 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. Means 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.
Conclusion Thus, we can conclude that different materials have different thermal conductivity. The descending order of thermal conductivity for different forms of materials are as under
1. Pure metals
2. Alloys (Combination of different metals)
3. Non-metallic crystalline structures.
Also, thermal conductivity plays important role in selection of conductor or insulator. The material having higher thermal conductivity can be used as thermal conductor and the material having lower thermal conductivity can be used as thermal insulator.
FYI: Diamond has highest thermal conductivity
More articles: Thermodynamics