Performance Evaluation of the Nanopore Heat Exchanger for Dilution Refrigerator

A crucial problem in the design of heat exchangers (HEx) for dilution refrigerators is thermal boundary resistances between liquid helium and substances rapidly increasing at low temperatures. In addition, thermal resistivity of liquid helium in restricted geometries also increases with decreasing temperature. To solve these problems, recently we developed the nanopore heat exchanger (NHEx), where nanoporous silicate and 75000-nm silver powder are used. Nanoporous materials have huge specific area about of 1000 m \(^{2}\) ${}^2$ /g, which is 3 orders larger compared with conventional fine silver powder sinters widely used for HExs. We have measured the Kapitza resistance of NHEx down to 22 mK and found that it is nearly 10 times smaller than that of conventional sinters with the same size. Use of 75000-nm silver powder instead of 70 nm also allows to reduce thermal resistivity of the liquid. In order to examine performance of the NHEx, quantitative evaluation is performed using numerical simulations. The result shows 3–10 times better performance of NHEx, compared with conventional HExs.