The melting and pyrolysis of teflon and the melting of silver chloride and iodine under high pressure.

The melting temperatures have been determined for a wide range of substances as a function of pressure. In the present work, Teflon (polytetrafluoroethylene) was investigated up to 30 kb and the decomposition temperatures, and the melting temperatures of silver chloride and iodine were determined up to 30 kb and 20 kb, respectively. General Technique. -The pressure was generated with a piston-cylinder type press, the details of which can be found elsewhere.'-3 A typical high-pressure furnace is shown in Figure 1. The temperature of the sample was varied by means of an electrical resistance graphite heater placed in the high-pressure furnace. Electricity was applied to the heater at a constant rate. Two Alumel-Chromel thermocouples were set in the furnace, one for measuring the temperature of the sample and the other to be used in making a differential thermal analysis (DTA). The slight change of electromotive force of the thermocouples due to pressure4 was not corrected for in the present work. The high-pressure furnace used in the present work consisted of materials such as talc, pyrophyllite, graphite, and Teflon, which served as solid pressure transmitting media. The calculated pressure at the bottom of the moving piston is not transmitted unchanged through such highly viscous pressure transmitters as are used in our high-pressure furnace. There are at least three major factors to be considered. The first factor is the pressure loss due to an interfacial friction between the cylinder wall and the piston surface including the influence of the packing material leaking into the clearance between the piston and the cylinder. The second factor is the pressure loss due to an iiiterfacial friction between the cylinder and the medium used for the outside of the high-pressure furnace. The third factor is the pressure loss due to the shearing or flow properties of the media used. These pressure losses, however, have been carefully studied. ' The total pressure losses were determined as 3.7, 5.5, and 6.6 kb at the gauge pressures 5.0, 10.0, and more than 15.0 kb, respectively, in the high-pressure furnace shown in Figure 1. These values were applied to determine the phase diagram of bismuth and were found to be valid. Therefore, the above values of pressure losses were applied in the following experiments to calibrate the gauge pressures, which were read from a 16-in. diameter Heise gauge set in the primary hydraulic oil line. Samples. -(a) Polytetrafluoroethylene: DuPont's Teflon rod. (b) Silver chloride: 99.992 per cent minimum purity, reagent. (c) Iodine: 99.85 per cent minimum purity, resublimed crystals, analytical reagent.