Thermal rectification enhancement based on porous structure in bulk materials

Thermal rectification effect refers to an asymmetric heat transfer phenomenon (namely, the amount of heat flux depends on the direction of temperature gradient). A two-segment bar made of two materials that have thermal conductivities with different temperature-dependence, can realize the thermal rectification effect. In the present paper, we propose to use porous structure on the bulk material to modify the thermal conductivity of bulk material. It is found that the thermal rectification effect can be enhanced by the porous structure. The finite element method and effective medium approximation are used to analyze the influence of porosity on the thermal rectification ratio of the two-segment system. The calculation results are consistent with each other. Under low temperature bias, the effect of the porosity is weak, while its influence becomes very significant when the temperature difference is high. Usually, thermal rectification ratio decreases if the porous structure is made on the segment whose thermal conductivity increases with temperature increasing. If the porous structure is made on the segment with negative temperature-dependent thermal conductivity, an optimal porosity can be found. For low porosity, the forward heat flux keeps almost unchanged while the reverse heat flux decreases by more than half, and the thermal rectification ratio can be increased to twice or more than thrice that in the case of no porous structure. For a fixed temperature difference, the influence of porosity on the thermal rectification ratio increases with the augment of the power exponent value.