Thermal Contact and Insulation below 1°K

Experiments on rapid thermal contact with a single crystal of ferric ammonium alum are discussed and the heat flow rates fitted to the empirical formula Q̇=½aA(THn−TCn). Thermal links of crystalline quartz and 99.999% copper and bonding agents of GE 7031-Toluene and Apiezon J oil were used and gave substantially the same results with a≅104 erg/(sec cm2 K°n) and n≅2.5. The heat flow rates have also been measured for a strand of No. 40 copper wire 55 mm long, and for a bundle of ten glass tubes, each 50 mm long, 2.0 mm diameter, and 0.19-mm wall thickness. These data were fitted to the formula Q̇=BTHn. For the copper and for a single glass tube one finds, respectively, B=28 and 0.95 erg/(sec K°n) and n=1.7 and 2.6. Solid and gaseous conduction, radiation, and vibration, the four main problems in thermal insulation below 1°K, have also been studied. Guiding principles have been established for the design of sample holders yielding reproducibly low heat leaks. A sample holder is discussed in which an ultimate heat leak of 1.6 erg/min was obtained. The techniques developed have been applied to the design and construction of a simple two-stage adiabatic demagnetization apparatus for use with single crystals.