Determination of the Critical Point of Gold

Wire-shaped gold specimens are placed in a new, improved high-pressure vessel, which is part of a fast capacitor-discharge circuit and in which static pressures above 600 MPa can be reached with distilled water as the pressure-transmitting medium. The specimens are self-heated resistively by a current pulse. The current through the specimen, the voltage drop across it, and its temperature are recorded as a function of time with submicrosecond resolution. The radial expansion of the specimen is determined with a CCD camera, Experiments are performed at different pressures. When the critical pressure is exceeded, there is no liquid–gas phase transition; hence, no sudden change in the thermal expansion rate is observed. The results for temperature, pressure, and specific volume at the critical point of gold are as follows: T _c =7400±1100 K, p _c=530±20 MPa, and v _c=0.13±0.03 × 10^–3m³·kg^–1.     

Thermophysical properties of liquid iron

Wire-shaped iron samples are resistively volume heated as part of a fast capacitor discharge apparatus. Measurements of current through the specimen, voltage across the specimen, radiance temperature, and thermal expansion of the specimen as functions of time allow the determination of specific heat and various dependencies among enthalpy, electrical resistivity, temperature, and density for liquid iron up to 5000 K. High pressures. up to 3800 bar, are used to obtain the liquid state far above the normal boiling point. An estimate of critical-point data for iron is given by using experimental data of the vapor pressure of liquid iron.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

固化压力对一种纸基摩擦材料摩擦性能的影响

基于本实验室制备的一种炭纤维增强纸基摩擦材料, 研究了固化压力对材料孔隙率, 动摩擦系数, 静动摩擦系数之比, 摩擦力矩曲线的影响。研究结果表明: 孔隙率随固化压力增加而下降; 动摩擦系数随制动压力的增加均呈下降趋势; 静动摩擦系数比值随制动压力的增加分布趋势较为复杂: 较低的固化压力条件下, 静动摩擦系数之比略有上升, 而较高的固化压力条件下静动摩擦系数之比则略有下降趋势; 摩擦力矩曲线随固化压力升高有轻微翘曲出现。综合考虑固化压力对动摩擦系数、 静动摩擦系数比值以及摩擦力矩曲线的影响, 在这种纸基摩擦材料成分不变的条件下, 其对应较为合适的固化压力约为3.4MPa。      

Thermodynamic properties of 2,2,4-trimethylpentane

p, V, T data for 2,2,4-trimethylpentane (TMP) have been obtained in the form of volume ratios for six temperatures in the range 278.15 to 338.15 K for pressures up to 280 MPa. Isothermal compressibilities, isobaric expansivities, and internal pressures have been evaluated from the volumetric data. There are strong indications that the combination of the present results with literature data at 348 and 373 K enable accurate extrapolations in the liquid range up to 473 K, and possibly to as low as 170 K, for pressures up to 980 MPa; use of only the present results with the requirement that the B coefficient of the Tait equation should become equal to the negative of the critical pressure at the critical temperature provides interpolations and extrapolations of comparable accuracy. It is suggested that 2,2,4-trimethylpentane is a suitable secondary reference material (because of its large liquid range at atmospheric pressure and the similarity of its volumetric properties to a wide range of fluids) for calibration of measuring cells used for determining volumes of fluids under pressure.     

Thermodynamic properties of 2,2,2-trifluoroethanol

(p, V, T) data for 2,2,2-trifiuoroethanol (TFE) have been obtained in the form of volume ratios for six temperatures in the range 278.15 to 338.15 K for pressures up to 280 MPa. Isothermal compressibilities, isobaric expansivities, and internal pressures have been evaluated from the volumetric data. The compressibilities and internal pressures indicate that the behavior of TFE is closer to that of methanol than of ethanol for most of the pressure range. The use of only the present volumetric results together with the requirement that the B coefficient of the Tait equation should become equal to the negative of the critical pressure at the critical temperature provides interpolations and extrapolations up to 413 K of comparable accuracy.     

Measurement of thermophysical properties of lead by a submicrosecond pulse-heating method in the range 2000–5000 K

A submicrosecond ohmic pulse-heating technique with heating rates of more than 10⁹K· s^–1 allows the determination of such thermophysical properties as heat capacity and the mutual dependences among enthalpy, electrical resistivity, temperature, and volume up to superheated liquid states for lead. Also, an estimation of the critical point data is given from investigations at elevated static pressures.Paper presented at the First Workshop on Subsecond Thermophysics, June 20–21, 1988, Gaithersburg, Maryland, U.S.A.     

Vapor pressure and liquid and gas densities of 2,2,2-trifluoroethanol

Forty-three vapor pressures were measured for temperatures from the normal boiling point to the critical point. These data were obtained with a phase-equilibrium cell designed for precise static measurements at pressures up to 20 MPa. The cell is located within an oil-operated thermostatic bath which provides a homogeneous temperature field with variations less than ±1 mK. The vapor pressure data were fitted to a Wagner-type equation. Sixty-two liquid densities were measured on seven isotherms between 20 and 140°C for pressures up to 16 MPa. These measurements were carried out with a precision density meter operating on a vibrational technique. Sixty-nine gas-PVT triples were determined from Burnett expansion series on five isotherms between 140 and 200°C for pressures up to the saturation line. In all experiments, temperature measurements were made with platinum resistance thermometers. Precise pressure measurements were performed using a mercury column of 6-m height and a standard deadweight gauge for the higher pressures.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Critical data of type II superconductors under hydrostatic pressure

Thin films of Nb₃Sn, Nb₃Ga, and NbN deposited on sapphire substrates by different preparation techniques and diffusion wires were measured under hydrostatic pressures up to 15 kbar. The results on critical current and upper critical induction under pressure can be described by scaling laws similar to those already known for temperature as parameter. The anisotropies of the critical currents and fields depend on the microstructure of the samples. The pinning mechanisms are not changed under hydrostatic pressure, so that the pinning forces in these superconductors can be compared to existing pinning theories.Supported by the Deutsche Forschungsgemeinschaft.     

Thermal conductivity and heat capacity of solid nabr under pressure

Using the transient hot-wire method, measurements were made for solid NaBr of both the thermal conductivity and the heat capacity per unit volume. The measurements were performed in the temperature range 100 to 400 K and at pressures up to 2 GPa. An adiabatic compression technique allowed the determination of the thermal expansivity as a function of pressure at room temperature. The heat capacity did not vary with pressure. Analysis of the thermal conductivity data showed that it can be described adequately by the Leibfried-Schlömann formula. For temperatures up to 400 K only acoustic modes needed to be taken into account. A small contribution of optic modes to the heat transport might be apparent at the highest temperatures.     

Volumetric properties of mixtures of 1,4-dioxane and water at high pressures

Densities of aqueous of 1,4-dioxane have been measured at temperatures from 298 to 348 K and at pressures up to 40 MPa by a vibrating-tube method. Molar volumes obtained with an estimated uncertainty of ±0.2°,% are correlated with pressure by the Tait equation within the experimental uncertainty. Pressure and composition dependences of the excess molar volume, partial molar volume, and isothermal compressibility are determined and they are compared with those of other aqueous solutions.     

Thermophysical properties of quinoline as a function of temperature (303–503 K) and pressure (0.1–400 MPa)

Measured and derived thermophysical properties of quinoline are reported for pressures up to 400 MPa at temperatures from 303 to 503 K. The specific volume at 353 K was determined from the specific volume at atmospheric pressure measured b) pycnometry and from isothermal compressibilities measured as a function of pressure up to 400 M Pa. Specific volumes, isothermal compressibilities, thermal coellicients of pressure, and isobaric and isochoric heat capacities at pressures up to 4011 MPa are derived at several temperatures. The effects of pressure on the isobaric heat capacity of quinoline, a weakly self-associated liquid. are discussed and compared with the pressure effects on heat capacities ofn-hexane andm-cresol.     

An optical technique forin situ density determination in electrolyte solutions under hydrothermal conditions

We report on a technique which determines the density of hydrothermal solutions through the relationship between density and refractive index given by the Lorentz Lorenz equation. An optical cell is described which allows the refractive index of fluids to be measured at temperatures up to 823 K and pressures up to 1200 bar. The validity of the Lorentz-Lorenz equation is demonstrated for pure water and water/NaCl solutions. New experimental results are presented for the density of water/NaNO₃ solutions at high temperature and pressure. The use of density data to derive other properties such as compressibility, thermal expansion coefficient, and apparent molar volume is demonstrated.     

The thermal conductivity of liquid 1,1,1,2-tetrafluoroethane (HFC 134a)

The thermal conductivity of HFC 134a was measured in the liquid phase with the polarized transient hot-wire technique. The experiments were performed at temperatures from 213 to 293 K at pressures up to 20 MPa. The data were analyzed to obtain correlations in terms of density and pressure. This study is part of an international project coordinated by the Subcommittee on Transport Properties of Commission 1.2 of IUPAC, conducted to investigate the large discrepancies between the results reported by various authors for the transport properties of HFC 134a, using samples of different origin. Two samples of HFC 134a from different sources have been used. The thermal conductivity of the first sample was measured along the saturation line as a function of temperature and the data were presented earlier. The thermal conductivity of the second one, the round-robin sample was measured as a function of pressure and temperature. These data were extrapolated to the saturation line and compared with the data obtained, previously in order to demonstrate the importance of the sample origin and their real purity. The accuracy of the measurements is estimated to be 0.5%. Finally, the results are compared with the existing literature data.     

Comparative study of electron beam-gas interaction in an SEM operating at pressures up to 300 Pa

The effects of three different gas environments formed by pure argon, helium and nitrogen with variable pressures up to 300 Pa inside the specimen chamber of a scanning electron microscope on the electron beam parameters were investigated and compared using Monte Carlo calculations. The calculations show that the skirt of the electron beam probe is nearly independent of pressure in the case of helium, which is also supported by experimental evidence. Both theoretical simulation and experimental facts indicate helium as the environment of choice in specimen chambers for minimizing the effect of beam-gas interactions.     

Density and viscosity of linear perfluoropolyethers under high pressures

New experimental data on the density and viscosity of linear, unbranched perfluoropolyethers are presented at temperatures from 273 to 333 K and pressures up to 180 MPa. The measurements were carried out by a high-pressure burrette apparatus and a falling-cylinder viscometer. The uncertainties of the measurements are estimated to be less than 0.09% for the specific volume and 2.5% for the viscosity. The P-V data at each temperature are correlated satisfactorily by the Tait equation. The viscosity data are also analyzed and correlated with pressure or molar volume by several empirical and theoretical equations.     

Thermophysical properties ofm-cresol as a function of temperture (303 to 503 K) and pressure (0.1 to 400 MPa)

Measured and derived thermophysical properties ofm-cresol are reported for pressures up to 400 MPa at temperatures from 303 to 503 K. Isobaric thermal expansivities were measured by pressure-scanning calorimetry from 303 to 503 K and 0.1 to 400 MPa. The specific volume at 353 K was determined by pycnometry at atmospheric pressure and calculated from isothermal compressibilities measured as a funtion of pressure up to 400 MPa. Specific volumes at other temperatures and pressures are calculated from isothermal compressibilities measured as a function of pressure up to 400 MPa. Specific volumes, isothermal compressibilities, thermal coefficients of pressure, and isobaric and isochoric heat capacities at pressures up to 400 MPa are derived at several temperatures. The effects of pressure on the isobaric heat capacities ofm-cresol,n-hexane, and water are compared. The effects of self-association ofm-cresol are apparent in both the thermal expansivity and the heat capacity data.     

Comments on “size and temperature dependences of the resistivity of thin alkali metal wires at liquid nitrogen,hydrogen and helium temperatures

The conductivity of PbF₂ thin films was studied as a function of the hydrostatic pressure of a neutral gas at different temperatures. This conductivity varies with pressure according to an exponential law up to a value of about 4 kbar; the measured activation volume is 2.3 cm³ mol^-1. The activation energy of the conductivity at these pressures remains unchanged in comparison with that found at atmospheric pressure, showing that the conduction mechanisms remain unchanged. For pressures above 4 kbar a fast rise in conductivity is attributed to a phase change accompanied by the creation of a large number of defects.     

Volumetric properties of mixtures of fluoroalcohols and water at high pressures

Densities of aqueous solutions of 2,2,2-trifluoroethanol, 2,2,3,3-tetranuoropropanol, and 2,2,3,3,3-pentanuoropropanol have been measured with a vibrating-tube densitometer. Measurements were performed at temperatures of 298 and 323 K and at pressures up to 80 MPa with an estimated uncertainty of ±0.2 %. Molar volumes obtained for these mixtures are correlated with pressure by the Tait equation within the experimental uncertainty. Excess molar volume, isothermal compressibility, and partial molar volume of these mixtures are determined in terms of this correlation equation and compared with those of the aqueous solutions of hydrocarbon alcohols. Composition dependence of partial molar volume is discussed in comparison with that of Raman spectroscopic data.     

Volumetric behavior of pure alcohols and their water mixtures under high pressure

The specific volumes of C₁-C₄ alcohols and binary mixtures of water with methanol, ethanol, 1-propanol, 2-propanol, and 2-methyl-2-propanol are presented as functions of temperature, pressure, and composition. The measurements were carried out using a modified Adams piezometer and a high-pressure burette method in a temperature range from 283.15 to 348.15 K at pressures up to 350 MPa. The uncertainties in the specific volume obtained are estimated to be less than 0.09%. The specific volumes of the pure alcohols and their mixtures with water are found to decrease monotonously with increasing pressure. The numerical P-V relations at each temperature and composition are correlated satisfactorily as a function of pressure by the Tait equation. Definite inflections appear on the isobars of isothermal compressibility or partial molar volume versus composition of alcohol + water mixtures.     

Combustion-resistance of silicon-based ceramics and composites at very high oxygen pressures

The resistance of ceramic matrix composites to combust at high oxygen pressures is expected to provide information in developing oxygen-rich propulsion systems. In contrast to metals, silicon based ceramics, SiC, Si₃N₄ and SiC-composites, have prevalently covalent bonding characteristics and their promoted combustion characteristics were different. These materials dissociate at very high temperatures rather than going through discrete solid—liquid phase transformation (melting). The silicon based ceramics and composites burn heterogeneously at very high oxygen pressures and the critical threshold pressure at which no test specimen sustains combustion correlated with composition, the nature of the bonding and the solubility of oxygen.