Measurement and calculation of cryogenic thermal conductivity of HGMs

Abstract Hollow glass microspheres (HGMs) have been introduced to cryogenic thermal insulation system as an efficient and low maintenance thermal insulation materials. The physical properties of HGMs influences their thermal insulation performance, which has been ignored in the past few decades. In this work, a new apparatus based on steady-state liquid nitrogen evaporation rate method was designed and fabricated to characterize thermal insulation properties for four types of HGMs (T18 (0.18 g/cm 3 ), T19 (0.19 g/cm 3 ), T25 (0.25 g/cm 3 ) and T30 (0.30 g/cm 3 )) at boundary temperature of 77 K–293 K and different cold vacuum pressures. The obtained thermal conductivities of T18, T19, T25 and T30 were 5.0497E−04 W/m·K, 5.2730E−04 W/m·K, 6.5608E−04 W/m·K and 8.0750E−04 W/m·K at high vacuum (1 × 10 −3  Pa) respectively. In order to analyze effect of physical properties on its thermal insulation properties, thermal conductivities of HGMs were also calculated through each mode of heat transfer including solid conduction, gas conduction and radiation. Solid conduction increased as true density of HGMs increasing and was independent of cold vacuum pressure. Gas conduction increased as cold vacuum pressure increasing. Besides, with solid fraction of HGMs remain the same, radiation heat transfer reduced as average particle size reducing. In summary, HGMs are superior powder thermal insulation material in soft vacuum for liquid nitrogen tank and have potential to further reduce the thermal conductivity.