Thermal Insulating Materials

Abstract

In this section, some classifications of porous thermal insulation materials are given and different techniques for porosity investigation are described. Special attention is focused on the standard contact porosimetry, which provides no destruction of the samples and gives a possibility to determine pores in a wide diapason of sizes. Owing to these advantages, the technique allows us to research evolution of porous structure at different stages of the product preparation and identify the synthesis phase, when functional properties of the material are transformed to diametrically opposite ones. Effect of porosity on such properties as thermal conductivity and compression strength is estimated, the appropriated correlations are represented. The information dealt to research of thermal conductivity is given, the heat transfer through porous media is considered. It is noted, that the main way to reduce thermal conductivity is to increase porosity of the material, the contribution of solid phase can be diminished by this manner. This principle is used for manufacture of most of thermal insulators such as widespread polymer and inorganic foams. They are characterized by extremely low thermal conductivity, the order of magnitude of which is 10⁻² and 10⁻¹ W m⁻¹ K⁻¹, respectively. Modern approaches to development of new thermal insulating materials and structures are considered. These approaches are based on a decrease in heat conductivity of gaseous phase. In order to minimize the fluid contribution, inert gases, which are characterized by lower conductivity in a comparison with air, can be encapsulated in closed pores. Other ways are degassing of thermal insulating materials and decrease of their pore sizes, simultaneously high porosity has to be provided.