Platinum black sinters for heat exchangers at very low temperatures

We have investigated a series of Pt black sinters for use as heat exchangers to cool liquid or solid helium to temperatures below 1 mK. They had thicknesses between 60 and 80 μm and specific surface areas up to 39.7 m²/g and 304 m²/cm³. The sintering was on highly polished Cu surfaces of 12.7×15.2 mm², a geometry close to one we plan to use in neutron scattering experiments from solid ³He. The electrical resistivity of one broken piece of sinter was about 60 μΩ cm which should be sufficient to give good thermal contact internally. Since the sinters were bonded very well to the copper surface, good thermal contact between the helium and the bulk metal is expected even at very low temperatures.