Inhomogeneity in pore size appreciably lowering thermal conductivity for porous thermal insulators

It has been years since the concept that inhomogeneity in pore size has an adverse effect on the thermal transport came into view. Typically, although some porous materials possess the identical porosity, they could show a strong inhomogeneity in pore size, making the physical parameters change greatly. However, one major and often overlooked challenge in understanding the underlying mechanism behind the above observation involves quantifying the effect of inhomogeneity. In this paper, the inhomogeneity in pore size is quantitatively evaluated to explain the thermal conductivity diminishment in the porous material system. By means of self-developed adaptable interfacial thermo-sensor technology, the thermal conductivity of a series of micro-porous foams with homogeneous pores are accurately characterized, and its evolution trend versus porosity agrees well with the typical homogeneous model. To compare with homogeneous materials, the thermal conductivity of the inhomogeneous porous materials is calculated by coupling 3D tomographic modeling and finite element method. An appreciable thermal conductivity reduction up to 13.5% is found as a result of the constructed inhomogeneity for pore size distribution. Furthermore, the distinction between the homogeneous and inhomogeneous models would remarkably diminish as the porosity approaches a very high value, probably owing to the increment of the content of the solid-gas interface. Our work opens up fresh opportunities for research of super thermal insulation materials. In contrast to harnessing high porosity, developing inhomogeneity in pore size distribution could play a critical role in further lowering the thermal conductivity of porous thermal insulators.