Heat transfer in microsphere thermal insulations

An extensive analytical and experimental study on heat transport in aluminized and clear hollow glass microspheres is presented. These microspheres are first investigated for their particle size distribution, shell thickness, surface and volume texture, and composition. The size distribution, based on a statistical sampling using a Zeiss Counter, is described by logarithmic normal probability distribution functions. Values of the effective conductivity of the fully aluminized and also the clear glass microspheres were obtained from 80 to 300 K from the steady-state experiments. The effective thermal diffusivity of the coated microspheres is also found based on the transient data. The conductivity data are analyzed for the effects of the sample size, the microsphere surface texture, and the thermal strain loading. These parameters are found to have considerable influence on the lattice conductivity values for both types of microspheres. Comparison between the experimental results and the theoretical predictions is also presented.