On the anisotropy of thermal conductivity in ceramic bricks

This study presents results of investigation on anisotropy of thermal conductivity of masonry bricks. Few results of anisotropic thermal properties were presented in literature. Most of them were focused on the thermal conductivity measurement across the sample thickness only or additionally in one direction. In this work, thermal conductivities of three types of bricks were determined with an indirect method which involved measurements of thermal diffusivity, specific heat and density. The thermal diffusivity of ceramic bricks has been measured using the flash technique while differential scanning calorimetry was applied for the specific heat measurement. Apparent densities were determined geometrically. Measurements taken in three directions normal to the main planes of the brick revealed that thermal diffusivity of the bricks is anisotropic. Investigations were repeated on several bricks coming from different local manufacturers. Differences of the thermal conductivities determined for samples cut in various directions were up to 36%. The connection between principal directions of thermal diffusivity tensor and microstructure of the material was also investigated using the scanning electron microscopy and infrared thermography. It was found that silicate bricks were more isotropic than fired red bricks. The study confirmed earlier reports about the relation of microstructural alignment with thermal conductivity anisotropy. Interesting difference in the degree of anisotropy at two different depths was revealed. The precisely evaluated thermal conductivity tensor might be of relevance in the modeling of heat and moisture transport phenomena in building materials.