Measurements of the thermal conductivity of bcc 3He at very low temperatures

The thermal conductivity of bcc 3He has been measured in samples grown at constant volume using the blocked capillary technique. Molar volumes between 24.4 cm3/mole and 22.4 cm3/mole were investigated to temperatures in the vicinity of 30 mK using a dilution refrigerator. The 4He impurity levels in these samples was 10 ppM. In most cases the thermal conductivity was observed to vary with temperature very nearly as T3 in the low temperature limit, characteristic of diffuse phonon scattering from crystal boundaries in a Debye solid. In a few cases, samples were grown for which the temperature dependence of the thermal conductivity was weaker than T3. It is suggested that this departure from T3 behaviour represents additional resistive phonon scattering from structural imperfections within sample crystals and does not indicate any anomalous thermal properties of bulk bcc 3He. Samples of bcc 3He with 4He impurities of 500 ppM and molar volumes between 24.4 cm3/mole and 21.5 cm3/mole were investigated briefly. These samples also displayed T3 behaviour in the limit of low temperatures. In addition, however, a thermal hysteresis was observed in the conductivity, particularly in the higher density samples. This is attributed to the onset of the isotopic phase separation known to occur in solid helium mixtures at these temperatures