Thermal conductivity measurements of particulate materials: 5. Effect of bulk density and particle shape

Abstract

Thermal conductivities were measured with a line‐heat source for three particulate materials with different particle shapes under low pressures of a carbon dioxide atmosphere and various bulk densities. Less than 2 μm kaolinite exhibited a general decrease in thermal conductivity with increasing bulk density. For the range of atmospheric pressures appropriate for Mars, a reduction in porosity of 24% decreased the thermal conductivity by 24%. Kaolinite manifests considerable anisotropy with respect to thermal conductivity. As the particles align the bulk thermal conductivity measured increasingly reflects the thermal conductivity of the short axis. When kyanite is crushed, it forms blady particles that will also tend to align with increasing bulk density. Without any intrinsic anisotropy, however, kyanite particles, like other particulates exhibit an increase in thermal conductivity with increasing bulk density. Under Martian atmospheric pressures, a reduction in porosity of 30% produces a 30% increase in thermal conductivity. Diatomaceous earth maintains a very low bulk density due to the highly irregular shape of the individual particles. A decrease in porosity of 17% produces an increase in thermal conductivity of 27%. The trends in thermal conductivity with bulk density, whether increasing or decreasing, are often not smooth. Whether oscillations in the trends presented in this paper and elsewhere have any physical significance or whether they are merely artifacts of the precision error is unclear. Clarification of this question may not be possible without higher‐precision measurements from future laboratories and further development of theoretical modeling.