Optimization of thermal insulation performance of porous geopolymers under the guidance of thermal conductivity calculation

Porous geopolymers are energy saving, environment-friendly and simple in preparation. However, the thermal conductivity (TC) of present porous geopolymers as well as other inorganic thermal insulation materials can be hardly below 0.050 W m(-1) K-1 in the markets. In order to further decrease the TC of porous geopolymers, it is vital to understand the heat transfer mechanism of this type of materials. In this work, we made a comparison among the main heat transfer models for porous two-component system reported so far including series model, parallel model, geometric mean model, Maxwell-Eucken equation, Hashin spherical structure model and novel effective medium theory (NEMT) based on the data reported in references and obtained by our group for porous geopolymers, and found that NEMT could describe the heat transfer mechanism of porous geopolymers better. Then we systematically studied the relationship among the effective TC (k(e)), porosity (epsilon), TC of solid skeleton (k(s)) and TC of the uniform medium reflecting the heat conduction of the solid skeleton to air (k(m)) based on the calculations with NEMT. It was found that it is essential to increase epsilon and decrease k(s) and k(m) for reducing the TC of porous geopolymers. Under the guidance of above calculations, we successfully designed and obtained some porous geopolymers with TC as low as 0.040 W m(-1) K-1. This paper offers not only several porous geopolymers with low TC, but also an idea to design novel thermal insulation materials.