Silica-Based Materials for Thermoelectric-Legs Embedding

Sol-gel chemistry was used to synthesize low-density SiO2 aerogel for matrix embedding of silicide-based (Mg2Si and MnSi(2−x)) thermoelectric legs. In thermoelectric (TE) modules, the heat conduction in air and the convective and radiative contribution to the heat transfer play an important role in the reduction of the efficiency of the module. Silica aerogels are known for the lowest thermal conductivity of any non-evacuated solid. With this in mind, silica-based aerogel materials were employed to fill the void spaces between the thermoelectric legs of a module. In order to do this, different synthesis procedures were taken into account to produce suitable silica materials. It is important that the silica can be easily cast into place, avoiding mechanical cracks of the matrix. Silica aerogel typically requires a supercritical drying step to remove the pore fluid from the SiO2 gel, avoiding the collapse of the pores. This procedure is not practical for TE-legs embedding and it is dangerous, expensive, and time-consuming. It is known that replacing the –OH groups with organic hydrophobic substituents in the SiO2 pores prevents the pore-shrinkage and the sintering of the matrix during solvent evaporation step. This allows synthesizing relatively light materials at low temperature and ambient pressure, with no need of supercritical drying of the gel. The obtained aerogels were characterized by thermogravimetric analysis and differential scanning calorimetry to evaluate the stability of the material and the chemical modification with increasing temperature. The thermal expansion of the silica was evaluated by means of dilatometry. Finally, the thermal diffusivity was measured with the laser flash method.