Three-dimensional structure of high-performance heat insulator produced with micro and nano particle alumina

Abstract

The internal structure of heat insulator materials, which were built by mixing micrometer-size α-alumina with nanometer-size fumed alumina though changing their mixture ratios, was inspected using transmission electron microscopy (TEM) tomography and synchrotron radiation nano-tomography. The heat insulator samples formed by two kinds of alumina phase have high porosity that consist of hierarchical structures of nanometer and micrometer size. From TEM observation, it is found that the porosity was 90.7% in the aggregate of fumed alumina with an average particle diameter of 35 nm whilst the average pore size was 70–80 nm. From observation of synchrotron radiation nano-tomography, it is found that α-alumina particles distribute homogeneously in samples that have fumed alumina phase of more than 50 mass%. The effect of internal microstructure on heat insulator properties were considered by performing a simple heat transfer simulation using image based models prepared from synchrotron radiation nano-tomography images. Similar changes in heat transfer with the experiment were obtained by the simulation. Therefore, it is suggested that the change of heat transfer depending on a mixture ratio of α-alumina and fumed alumina is mainly caused by the internal microstructure, that is, the distribution and connectivity of α-alumina phase within fumed alumina phase.