Effect of grain thermal expansion mismatch on thermal conductivity of porous ceramics

Many ceramics contain microcracks, which are often situated between sintere d grains. These microcracks constitute thermal resistances, which may affec t heat transfer through the material and its effective thermophysical prope rties, The thicknesses and the contact areas of the microcracks change with temperature as a result of the thermal expansion mismatch between the grai ns on opposite sides of the microcracks, This physical mechanism affects ch anges of the material's thermal conductivity, k, with temperature. The abov e mechanism usually plays a minor role at atmospheric pressure, where heat may flow via the gas filling the cracks. Hence, the temperature-induced cha nges of the crack geometry have little effect on heat transfer. However, at low gas pressures, where the heat flow between the grains occurs mainly vi a the contact areas, the grains' thermal expansion mismatch causes unusual temperature behavior of the material's thermal conductivity observed for se veral industrial refractories, In this paper, the influence of the above ph ysical mechanism is discussed relative to other heat transfer mechanisms de scribed in the literature. A simple physical model of the thermal expansion of grains bonded by an agent, having different thermal expansion coefficie nts, is developed. This model allows calculation of the contact area and th e average microcrack opening between the grains as functions of the tempera ture, the characteristic grains sizes and their thermal expansion coefficie nts, and the permanent crack area. These parameters are evaluated and used to calculate the effective thermal conductivity of ceramic materials contai ning microcracks that appear as a result of thermal contraction of grains, The calculated thermal conductivity satisfactorily correlates with the expe rimental data collected for several chrome-magnesite refractories over a Ri de range of temperatures and gas pressures.