Nanoporous Refractory Insulating: Solution or Illusion?

Abstract

The physical factors affecting the thermal efficiency of ceramic insulating refractories were evaluated in order to find out the effects of nanopores containing microstructures. Mathematical analysis and simulations were carried out to quantify the influence of the pore size distribution on the thermal conductivity, which is the main physical property associated to the microstructure’s insulating capacity. Based on those analyses it was found out an ideal pore size range (between 1 and 3 μm) that optimize the reduction of thermal energy transmission through the material. Therefore, the presence of nanopores ( 1000°C). Such argument is supported by experimental data for aerogels, which contain nanopores and show significant thermal conductivity increase with the temperature. Additionally, such nanopores would hardly present dimensional stability during continuous use at high temperature. INTRODUCTION High temperature insulating ceramics are used to decrease the thermal losses in industrial processes by reducing the energy transfer rate between two distinct temperature regions. Therefore, these materials are considered fundamentals to increase the energy efficiency in most industrial processes. Concern about thermal efficiency of materials is not a recent matter, however it has achieved great importance recently due to the huge costs associated to energy losses. Furthermore, the literature has shown that the higher the process operating temperature, the higher the efficiency gain associated to improvements in thermal insulation [1] . Thermal insulation capacity depends on a well-designed microstructure in order to reduce the thermal energy transmission from the hot face to the cold one. The literature has suggested that the pore size greatly influences thermal conductivity values at high temperatures [2,3] . In addition, these publications have shown that pore size reduction could lead to lower thermal conductivity values, which should be extremely beneficial for the insulators performance. However, the literature does not indicate any minimum threshold for the pore size, implying that they should be as small as possible. Moreover, it suggests that materials containing nanopores would be the best thermal insulation choice. These materials have been reported in the literature for a long time and they are known as aerogel, which present high porosity (>90%) and pore sizes in the nanometer range (10 to 100nm) [4] . In this paper, fundamentals of the thermal-optic properties of ceramic materials are presented in order to point out an optimized range of pore sizes as a function of temperature, where the thermal insulation efficiency can be maximized. THERMAL ENERGY TRANSMISSION FUNDAMENTAL CONCEPTS Thermal conductivity (k) is the main physical property that defines thermal insulation efficiency of material [5] . That is clearly shown by the Fourier equation [2] :