The specific heat of Cry-Con grease, a copper loaded electrically insulating grease, was measured between 0.5 and 20 K. The low specific heat and high thermal conductivity of Cry-Con grease render it a useful thermal bonding material for calorimetric measurements on samples with low thermal conductivity. Low temperature specific heat of Cry-Con grease M.S. Torikachvili, K.N. Yang, R. Calvo, O.R. Nascimento and M.B. Maple Key words: cryogenics, insulation,grease, thermal conductivity Frequently, measurement of the specific heat (69 at low temperatures with the conventional heat pulse technique is cumbersome because of the poor thermal conductivity of the sample. The heat pulse is generally produced with a resistance heater in good thermal contact with the addenda of the calorimeter. The change in temperature (T) is measured with a thermometer that is also in good thermal contact with the calorimeter addenda. Our experiments are performed with a semiadiabatic calor- meter, ~ one in which the temperature, in the equilibrium situation, drifts slowly and approximately linearly with time. The temperature is continuously recorded on a strip- chart recorder; the change in temperoture due to the heat pulse is inferred from linear extrapolations of T vs time, both before and after the pulse, to the time at which half of the pulse has elapsed. The duration of a heat pulse lasts from 10 to 30 s. After the heat pulse is applied, the tempera- tures of the sample and the addenda relax to the same value approximately exponentially with time; ie, as ~ exp (-t/r), where ~" is the relaxation time constant. This relaxation time r is an inverse function of the thermal diffusivity of the sample, and the accuracy of the method depends on how fast thermal equilibrium is reached, since the semi- adiabatic condition does not prevail indefinitely. An alternative way of making specific heat measurements on a sample with poor thermal conductivity is to grind the sample and mix it with a medium of good thermal diffusi- vity and then subtract the contribution of the medium from the results. Two greases, Apiezon T 2 and Apiezon N 3 , have been successfully used for this purpose. Cry-Con 4 is a copper bearing electrically insulating grease, and its low temperature thermal conductivity was found to be substan- tially higher than that of silicone-based vacuum greases, s which makes it an attractive candidate for a thermal con- tact agent in calorimetry. We frequently measure the specific heat of magnetic materials at very low temperatures (0.5 - 20 K range) and sometimes encounter a sample whose thermal conductivity at these temperatures is very small, especially if it orders The authors are with the Institute for Pure and Applied Physical Sciences and the Department of Physics at the University of California, San Diego, La Jolla, CA 92093, USA. The research was supported by the US Department of Energy under contract No. DE-AT03-76ERT0227. MST is on leave from UNICAMP (Brazil) and is also supported by FAPESP. RC is on sabbatical leave from IVIC, Caracas (Venezuela). ORN is on leave from USP, Sao Carlos (Brazil) and is also supported by CNPq. Paper received 10 May 1982. magnetically. In such cases, the elapsed time for reaching thermal equilibrium, after a heat pulse is applied, may be very long. In a recent experiment, we attempted to measure the specific heat below 1.5 K of a powdered specimen of Ni(NH2CO2CH2):2HzO that had been pressed into a copper sample holder and observed relaxation times of over 1 h, which considerably limited the accuracy of the results and made data acquisition an extremely long and tedious process. Subsequently, we ground the sample into a rme powder and mixed it with Cry-Con grease (sample: Cry-Con mass ratio of approximately 40:60) after which it was again pressed into the copper sample holder. This procedure reduced the relaxation times to less than 1 min and greatly facilitated the specific heat measurement .6 The purpose of this paper is to present an accurate charac- terization of the specific heat of Cry-Con grease between 0.5 and 20 K, so that its contribution can be estimated when it is used as a thermal bonding material in calorimetry. We also compare the specific heat and thermal diffusivity of Cry-Con grease to Apiezon T 1 and Apiezon N 2 greases. The experiment was performed in a He3 refrigerated semi- adiabatic calorimeter, in which the temperature sensor was a germanium resistor. A 0.430 g amount of Cry-Con grease was placed in a 4.536 g copper sample holder whose specific heat has been measured previously. At 0.6 K, the heat capacity due to the Cry-Con grease was ~ 3.6% of the total (Cry-Con + copper sample holder + addenda of the calori. 30 '7 v ? i- .- Cry - con // 0'06 t ./~ /2 0"041 /'/ /'/" ~""¢P': /° 0 Or5 I0 r5 20 ..."/'" ,,d- . j-*'- ,o.,, I ~ J ~ n ~ in4 i 8 [ 0 2 4 6 8 I0 12 16 I Temperature, K Fig. 1 Specific heat vs temperature for Cry-Con grease between 0.5 and 20 K. The solid line is a least squares fit of the data with a 5th-order polynomial 20 52 0011-2275/83/001052-03 $03.00 © 1983 Butterworth & Co (Publishers) Ltd. CRYOGENICS. JANUARY 1983