Effect of surface segregation kinetics on the effective thermal conductivity of porous ceramics

Thermophysical properties of some porous refractory materials depend upon the thermal history of the samples tested. Sometimes repetitive measurements of thermal conductivity, performed at constant thermodynamic conditions, yield results that continue to change during tens of hours after a thermal equilibrium seems to be established. This phenomenon is shown to be attributable to a redistribution of impurities (normally present in all industrial ceramics) in the vicinity of pores existing in the grain boundary region, separating poorly sintered granules, This mass redistribution process is induced by changes of the material temperature. Its rate is shown to be governed by diffusion of impurities to and from the pores. This process in refractory ceramics is very slow (i.e. occurs on the time scale of tens of hours), which incurs the concomitant slow changes of the effective thermal conductivity. A physico-mathematical model of !he surface segregation kinetics is developed. Relative influences of the pore sizes and the segregation kinetic parameters on the mass redistribution rare and surface concentration of impurities are analyzed. These results are further used to calculate the temporal variation of the effective thermal conductivity, resulting from an instantaneous change of the material temperature. The results are shown to agree with experimental data collected for porous yttrium bride ceramic material. The model developed here can be used to explain and rationalize the dependence of thermal conductivity on the material thermal history and the measurement method.