Factors leading to variation of thermal conductivity of different materials
Posted Date: 28-Oct-2012
This article gives reason why thermal conductivity of different material/phase varies with different factors like material structure, density, phase and composition as well as temperature. Read the following article about variation of thermal conductivity.
Introduction to heat transfer
In real world we normally feel the heat by virtue of its temperature. If temperature of body is high, we can say 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 occurs due to bulk motion of fluid and in contact with solid surface. Thermal radiation occurs due to temperature of any body which at temperature more than zero Kelvin. All the body having temperature more than zero Kelvin transmits the thermal radiation. But the heat conduction or
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
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 structure of material in terms of chemical composition, phase of material and texture of it. Thermal conductivity also depends on content of moisture present in material as well as how closely atoms are packed in lattice, also with operating conditions like pressure and temperature.
Factors influencing thermal conductivity
Metals are having more free electrons compared to that of liquid and gases, so metal are good conductors of heat due to migration of free electrons. Metals are having closely packed lattice compared to liquids and gases.
Purity of material
Thermal conductivity of pure material is higher than that of alloy materials. Alloying of metals and presence of impurities cause 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 material before treatment.
At elevated temperature lattice vibration increases and free electrons movement decreases, thus thermal conductivity of metal decreases when temperature is increased.
But for gases, thermal conductivity increases. The reason behind it at 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,
Thermal conductivity of liquid is also proportional to the density and at higher temperature, density of liquid decreases thus, thermal conductivity also.
Thermal conductivity is weakly dependent on pressure of substance. Means change in pressure does not affect much in thermal conductivity.
Thermal conductivity is highly dependent on density of material. Increase in density increases thermal conductivity.
Material having a regular
has higher value of thermal conductivity compared to that of amorphous(irregular) form.
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
and the material having lower thermal conductivity can be used as
An introduction to thermodynamics laws
Thermodynamics - First Law of Thermodynamics
Basics of Thermodynamics (Part-1)
Brief introduction to thermodynamic heat & thermodynamic work
How It Is Possible For A Thermos Flask To Retain Heat For A Longer Time
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