Heat exchange performance of sintered fine silver powders in ultra-low temperature cooling of superfluid 4He

The aim of this study is to explore the heat exchange performance of sintered fine silver powders by measuring thermal boundary resistances of them to superfluid 4He below 700 mK. The sinters used in this study were made of pure fine silver powders having nominal particle sizes of 0.07 mu m and 0.13 mu m. Thermal boundary resistances of the sinters increased with decreasing temperature with a temperature dependence close to T-3 down to about 40 mK. The thermal boundary resistance of the 0.13 mu m sinter was smaller than that of the 0.07 mu m sinter below about 100 mK. Although the sinters have inhomogeneous and rough surfaces, no anomalous Kapitza resistance was observed at high temperatures. The thermal boundary resistances of the sinters were found to be in the same order of magnitude as that of a bulk silver. It appears that the microscopic structure of the sinter, with its large surface area, does not function efficiently to exchange heat with superfluid 4He at ultra-low temperatures. It is concluded that efficient cooling of superfluid 4He down to ultra-low temperatures requires heat exchangers with structural scales much larger than 0.13 mu m.