Correlation of pore size distribution with thermal conductivity of precipitated silica and experimental determination of the coupling effect

Abstract Vacuum insulation panels are high-performance insulating materials with considerably better thermal properties than conventional thermal insulation. Unfortunately, their use is limited to applications where their high price does not matter. To extend the application range of vacuum insulation, the authors try to replace the high-priced core material fumed silica with the cheaper precipitated silica. For this purpose, five commercially available precipitated silica samples were tested for their suitability as core material for vacuum insulations. They were pre-pressed with 5 bar and 30 bar each. To compare their thermal performances, a guarded hot plate apparatus for measuring thermal conductivities under vacuum was developed. The pore size distributions of the samples were measured by mercury intrusion porosimetry and used to calculate the gas thermal conductivity as a function of the residual pressure. For this purpose, a correction factor for the measured pore size distribution is introduced. Additionally, the coupling effect between gaseous and solid thermal conductivity could be determined through comparison with measured data. A model is presented to predict the thermal conductivity curve, even of unknown silica samples, solely using mercury intrusion porosimetry data.