Low-temperature specific heat measurements on bulk technical polymer samples (polytetrafluoroethylene and polychlorotrifluoroethylene) were carried out between 2,5 and 20 K. In these experimental investigations a heat pulse method and an adiabatic vacuum calorimeter were used.The low-temperature specific heats of polytetrafluoroethylene (T4) and polychlorotrifluoroethylene (T3) have not been studied in sufficient detail [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The extrapolation of the experimental values at 20 K down to 4 K by means of the Debye-model is inadmissible [5]. Thus, investigations of the thermal properties giving the necessary physical quantities and constants for cryogenic design of these highly crystalline polymers are quite important.In the present work, calorimetric measurements on the bulk polymer samples were made in the range 2.5-20 K. The length and diameter of the samples did not surpass 50 mm and 12 ram, respectively. By using a shield in a vacuum calorimeter [16], entirely adiabatic conditions of operation were ensured. The cooling of the calorimeter, as well as that of the sample and the shield, was effected with a mechanical heat switch [17]. The heat capacity was derived from a well-known relation between the increase of the sample temperature (A T ~ 10-2 T) and the amount of heat supplied to the sample by a 650 f2 heater, consisting of 0.050 mm diameter constantan wire. This heater gives heat impulses lasting 40 seconds. The temperatures of the samples and the shield were measured by 300~ Allen-Bradley resistors. The calibration of these resistors was performed using a germanium (NIG) substandard. The time for establishment of the heat equilibrium was a few seconds at 2.5 K, and about 15 minutes at 20 K.
ExperimentalThe temperature-dependence of the specific heat is presented in Fig. 1 for polychlorotrifluoroethylene, and in Fig. 2
A STUDY OF THE Y-Ba-Cu-Ag-O SYSTEM: The specimens studied were produced using the standard ceramic technique. High purity source materials Y2O3, BaCO3CuO, and Ag2O were used. Solid state synthesis was carried out at 900 C for six hours in oxygen ambient. Pressed samples of 16 mm diameter and 2 mm thick were processed in an atmosphere of pure oxygen at 950 C for 12 hours. The transition temperature to the superconducting state was measured by the four point probe resistance technique. The ac magnetic susceptibility was determined by the method outlined in [1].
The thermal conductivities of bulk technical samples of polytetrafluoroethylene and polychlorotrifluoroethylene were measured in the temperature range 4.2--20 K by application of the steady-state method. An interpretation of the thermal conductivity data in terms of the available theoretical and empirical models is presented.
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