Measurement Of The Isentropic Compression Curve And W-J Parameter Via A Pressure-Jump Method

The W-J EOS, which was proposed by Q. Wu and F. Q. Jing, is a volume equation based on two independent variables, i.e., pressure and temperature, that complements the Gruneisen EOS. Establishment of the W-J EOS will help develop and enrich general EOS theory. The W-J parameter R is important in the W-J EOS, as it is involved in combining macroscopic and microcosmic characteristics. Therefore, determination of W-J parameters is of great significance in understanding and applying thermodynamic properties and the EOS under high pressures and temperatures.In this thesis, previous theoretical and experimental investigations on isentropic compression were first reviewed. The W-J EOS and W-J parameters were then summarized and evaluated. Experimental results from conventional methods showed inherited uncertainties of some necessary input parameters. To avoid influences from these uncertainties, this study mainly focused on establishing a practicable experimental method to measure the isentropic compression curve T (P) and W-J parameters directly. Several types of pressure-jump instruments, including their characteristics and deficiencies, were reviewed. Furthermore, a unique high pressure-jump apparatus was developed by ourselves, and the structure, working principles, advantages, and some preliminary achievements of this novel apparatus are discussed in this work.This thesis describes the following:(1) Using known phase transitions of bismuth and barium, the relationships between oil pressure and actual pressure in the pure pyrophyllite plates and in the samples of NaCl, Ta, Mo, C60, and graphite around the pyrophyllite gaskets were calibrated in Bridgman anvils respectively. The pressure rising slope was found to increase with increasing oil pressure.(2) An experimental method for measuring W-J parameters was proposed; in this method, the large pressure-jump and mean value theorem in differential calculus were applied. Using the proposed method, the W-J parameters of NaCl, Ta, Mo, and graphite at mean pressure during compression were obtained. The W-J parameters of Cu, Fe, and Pb were also determined using the same principle on the basis of previous experimental data. For comparison, the W-J parameters and their relationship with the pressure values of NaCl, Ta, Mo, graphite, Cu, Fe, and Pb were further calculated using the related equation of state, the empirical formula, and some known parameters. Results from this experiment agreed well with the calculations.(3) An experimental method for determining the isentropic compression curve T (P) s was established. It was recognized that maximum pressure and maximum temperature in a rapid compression correspond to a point of an isentropic compression curve, and then the isentropic compression curve T(P)S can be determined by a series of points obtained from the separated experiments with same initial conditions. The isentropic compression curves T(P)S of Mo, Ta, and graphite were measured in this way within 12.8 and 5.0 GPa. The results of this method can be reasonably extended to include larger ranges of pressure.(4) Considering the relationship between W-J parameters and pressure and temperature during isentropic compression, the W-J parameters and their relationship with the pressure of Ta, Mo, and graphite were obtained in the corresponding pressure ranges. In principle, compared with the mean value theorem, this method is more rigorous and reasonable, can acquire more data, and enlarges the measuring range. Comparison experiments using two Mo samples with different relative densities and substitution of the W/Re25%-W/Re3% thermocouple for the NiCr-NiSi thermocouple were further performed to investigate the effect of sample density and thermocouple hardness on the experimental results. No observable effects resulting from sample density differences or thermocouple hardness alteration were observed. (5) A new method to derive Gruneisen parameters from W-J parameters was explored. The Gruneisen parameters of NaCl, Ta, Mo, and graphite samples were obtained from the measured W-J parameters. Compared with previous methods, the proposed approach can gain a series of continuous values and is appropriate for use even in wider pressure ranges.(6) The thermodynamic behavior of a compacted powder material of C6o and graphite was investigated using a series of pressure-jump processes, respectively. The temperature of graphite nearly simultaneously increased with the pressure jump before slowly decreasing because of heat conduction as the pressure remained constant. By contrast, the temperature of C6o rapidly increased with the pressure jump and then continued to increase as the pressure was held constant. The maximum temperature was reached approximately 1 s after the pressure jump before the temperature gradually decreases. The temperature increase in C6o was much larger than that in graphite. Furthermore, both the duration and extent of temperature increase in C60 decreased with repeated compression. Finally, the thermal behavior of this sample resembled that of graphite. Combining these results with TEM characterizations of the initial and recovered C6o samples, the novel thermal behavior of C60 was attributed to pressure-induced polymerization, which releases a large amount of heat. This method is also applicable in thermodynamics research, specifically in pressure-induced phase transition.(7) As an exploration, the critical thickness of pyrophyllite plates was measured under average surface pressures of 0.5,1.0,1.5, and 2 GPa by using an improved anvil with a strip-shaped top surface. The relationship between oil pressure and actual pressure at the centerline of the 0.8 mm-thick pyrophyllite was calibrated. Pressure distribution in the strip shape gasket was discussed in principle, thought that the pressure should be uniform along the center line of the strip shape anvil, and suggested that the improved anvil is more convenient for measuring the properties of linear sample.