Thermal Conductivity Research Paper

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THERMAL CONDUCTIVITY

“Thermal conductivity is the property of material to conduct heat”.
It is due to result of temperature gradient. If the gradient is uniform then the amount of the thermal energy passing through unit area in unit time is directly proportional to the temperature gradient.
Q∝dT/dx
Q= K dT/dx
K is the proportionality constant & is called thermal conductivity. Heat is transfer by free electrons and phonons in metallic conductor and insulators respectively. So it is clear from these two points that heat can transfer both by electrons & phonons. Total thermal conductivity can be written as K_(total=K_(electron+K_phonon ) )
To find the value of K. suppose conductor in …show more content…

Hot electrons from high energy states carry more energy than cold electrons. While electrical conductivity to energy distribution carriers bcz amount of charge that electron carry does not depend on their energy. This physical reason for greater sensitivity of electronic thermal conductivity to energy dependence of density of state & relaxation time respectively.
We make following assumptions to find an expression for thermal conductivity of metals In metals there’s a sea of electrons & positive ions are fixed. Electrons transport thermal energy from the hotter to colder region & they behave as a perfect gas. In a mean free time τ , electrons travels a distance⋋. In the transport process only those electrons are that lie within the range KT of the Fermi level.
From these assumptions we obtain the electronic thermal conductivity
K_e = ((π^2 NK^2 τ)/3m)T

The mean free time varies as T^(-1) above the Debye …show more content…

foamed polystyrene is used for drinking cups). Thermal conductivity of polymers depends on the degree of crystalline – highly crystalline polymer has higher k

Thermal stresses
• can be generated due to restrained thermal expansion/contraction or temperature gradients that lead to differential dimensional changes in different part of the solid body.
• can result in plastic deformation or fracture.
In a rod with restrained axial deformation: σ = E αl ΔT where E is the elastic modulus, αl is the linear coefficient of thermal expansion and ΔTis the temperature change.
Stresses from temperature gradient Rapid heating can result in strong temperature gradients Æ confinement of expansion by colder parts of the sample. The same for cooling –tensile stresses can be introduced in a surface region of rapidly cooled piece of material.
Thermal stresses can cause plastic deformation (in ductile materials) or fracture (in brittle materials). The ability of material to withstand thermal stresses due to the rapid cooling/heating is called thermal shock resistance. Shock resistance parameter for brittle materials

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