Effective thermophysical properties of ceramic materials (mainly insulating materials) with porosity (II) >30% are reviewed. Nonmonotonic pressure and temperature dependences of the effective thermal conductivity (X) are analyzed, based on the ceramic microstructure (pores, cracks, and grain boundaries present in many industrial refractories) and several heat‐transfer mechanisms in composite multiphase materials. These mechanisms include heat conduction in solid and gas phases, thermal radiation, gas convection, and the mechanism originating from intrapore chemical conversion processes accompanied by gas emission. For high temperatures, λ of porous insulations is governed by thermal radiation. Contact‐heat‐barrier resistances play a less‐important role in highly porous ceramics than in their dense counterparts. This underlies a weaker pressure dependence at low temperatures (<500°C) of λ of the majority of industrial insulating materials than in dense materials possessing microcracks and small pores in the grain‐boundary region. For high gas pressure, λ of porous insulating materials is governed by free convective‐gas motion. For low gas pressures (normally <1 kPa), where heat transfer in pores occurs in the free‐molecular regime, X is controlled by the pressure‐dependent mean free path of gas molecules in pores. A classification of the porous material structure and thermophysical properties is proposed, based on the geometric model described in Part 1 of this series.

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Thermal conductivity of MgO (magnesia) foam thermal insulation with porosity 0.49–0.81 have been measured by the non-steady plane flow method in the temperature range of 500–2000 K at atmospheric pressure. We have demonstrated a significant influence of porosity on the apparent thermal conductivity of MgO insulating materials in the temperature range 500–1500 K. Materials with porosities exceeding 0.75, have relatively low radiation attenuation coefficients. This results in a relatively large contribution to the radiative component of the apparent thermal conductivity. For such materials this property measured at temperatures above 1700 K weakly depends on porosity. The measured apparent thermal conductivities are analyzed on the basis of a theoretical model, accounting for total material porosity and particle size distribution. We discuss the suitability of the data on particle and pore size distributions, measurable by various experimental methods, for calculation of the apparent thermal conductivity.

Experimental data on thermal conductivity of packed beds composed from various refractory particles (corundum, silica, magnesia, baddeleyite, yttrium oxide, spinel) obtained in the temperature range 400-2000 K in various gases are presented. It is found that thermal conductivity of a bed composed from crushed refractory particles may change after the first and subsequent heatings. This occurs as a result of smoothing of particle surfaces and decreasing of contact heat barrier resistances between the granules. The influence of smoothing is most significant for beds composed from particles with sizes below 2 mm. In polydisperse beds, containing micrometer-size particles, sintering processes were found to occur at temperatures above 1600 K. This led to a sharp increase of the bed thermal conductivity. In regimes where sintering did not take place, decreasing of particle size resulted in a decrease of the effective thermal conductivity. This is attributed to the increased number of contacts between the particles and the scattering of thermal radiation.