Thermal conductivity and extinction coefficient of opacified expanded perlite for vacuum super insulation up to 1073 K

Abstract A vacuum super insulation for temperatures up to 1073 K has been developed using an opacified powder mixture with high-density ( ρ = 180 kg m−3) expanded perlite as base material. To analyze radiative transfer, the mass-specific extinction coefficient e of expanded perlite and various opacifiers (SiC, B4C, FeTiO3, TiO2) has been determined by FTIR spectrometry. For reduction of radiative transfer, a SiC powder with mean grain diameter of 2 μm–4 μm and bulk density of 870 kg m−3 has been identified as most suitable opacifier, also under economic aspects. Subsequently, six homogeneous powder mixtures with weight fraction w of SiC between 0% and 60% have been prepared. In order to determine the optimum w , guarded hot plate (GHP) measurements under high vacuum conditions ( p λ e = 13∙10−3 W m−1 K−1 for w = 40% and ρ = 263 kg m−3. This mean temperature corresponds to an insulation application with 950 K hot-side and ambient temperature cold-side. For the optimum mixture with w = 40%, e has likewise been determined with FTIR spectrometry. Furthermore, λ e has been measured with the GHP method also at mean temperatures between 373 K and 873 K, where excellent values between 4∙10−3 W m−1 K−1 and 20∙10−3 W m−1 K−1 have been obtained. For the evaluation of these measurements, two different methods have been applied in order to separate radiative thermal conductivity λ r and solid thermal conductivity λ s : first the common plot of λ e versus T 3 , and second an improved method, which includes the temperature dependencies of e and λ s . Expanded perlite, opacified with SiC, has proven as a highly efficient and economic vacuum super insulation material for industrial applications up to 1073 K.