High-Temperature Thermal Transport in Porous Silica Materials: Direct Observation of a Switch from Conduction to Radiation

Efficient thermal insulation at high temperatures poses stringent requirements on suitable materials. Low density, porous inorganic structures with pore sizes in the sub-micrometer range are of particular interest for such materials to control heat conduction. Simultaneously, thermal radiation has to be suppressed, which depends on the optical properties of the constituents. Here, the authors demonstrate a direct observation of the transition from a conduction dominated to a radiation dominated thermal transport mechanism for the case of particulate silica materials at temperatures reaching up to 925 degrees C. A detailed analysis of the radiative transport through bulk silica as well as solid and hollow silica particles is provided. Optical transparency at high temperatures is the driving force, whereas surface wave modes barely contribute, particularly in case of the insulating particle packings. The existing analytical framework of laser flash analysis is extended to qualitatively describe the radiative and conductive heat transport by two independent diffusive transport models. The analysis provides a better understanding of the challenges to fabricate and analyze efficient thermal insulation materials at high operating temperatures, where multiple heat transport mechanisms need to be controlled.