Phase behavior of mixtures at very high pressures

The invention of the diamond anvil cell (DAC) has been an enormous stimulus to high-pressure research. This technique has only recently been used to investigate the behavior of binary systems, probably because of the extra experimental problems which arise in the study of mixtures. It will be shown that a variety of aspects of the behavior of the mixture can, nevertheless, be studied under extreme conditions. Although the first investigations were carried out only recently, some very interesting results have already been obtained. A variety of two-components systems has been studied, e.g., He-Kr, Ne-Xe, He-H₂, NH₃-H₂O, and N₂-He₂. Some of these results are discussed. Finally, a comparison is made between experimental results and theoretical calculations.Invited paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Dielectric relaxation in sulfur at very high pressures

The ac electrical properties of sulfur were measured at pressures from 20 to 50 GPa. The loss tangent vs. frequency data indicate dielectric anomalies in the pressure range of structural transformations: tanS rises by about one order of magnitude as the frequency decreases from 0.8 to 0.4 kHz and by a factor of more than 2 in the range 3–10 kHz.     

Sorption kinetics of oxygen at very low pressures by zirconium

The sorption rate of oxygen by zirconium at very low pressures of 4×10^?6 to 4×10^?4 torr has been investigated in the temperature range 500–850°C using a microbalance. The rate law of sorption was separable into two types, i.e., the linear rate law in the initial period and the parabolic rate law in the latter period. In the initial linear sorption, the rate was exactly proportional to the oxygen pressure at each temperature.The data of sorption kinetics of zirconium were rearranged from the viewpoint of vacuum technology, i.e., sticking probability and pumping speed in bulk gettering.The transition in the rate law of sorption is discussed on the bases of Wagner's model which involves simultaneous scale formation and oxygen dissolution into the metal bulk.     

Conductivity measurements on ceria at high oxygen pressures

The a.c. conductivity of high purity ceria has been measured at oxygen pressures and temperatures in the ranges 0.01 to 400 atm and 700 to 1100° C respectively. Similar measurements have been made on impure ceria over the same temperature range for oxygen pressures up to 100 atm. The conductivity isotherms obtained exhibit flattish minima where the ionic contribution to the conductivity is a maximum, a region at low oxygen pressures, where n-type conduction occurs and increases with diminishing pressure, and a high pressure region where p-type conduction occurs and increases with pressure. Activation energies derived for the conductivity minima are 1.55 and 0.85 eV for the high purity and impure ceria respectively; the higher value for the high purity material implies that a significant electronic contribution is present.     

Piezoelectric composites with high sensitivity and high capacitance for use at high pressures

A type of piezoelectric composite has been developed for oceanographic applications. The composites have a large figure of merit (d(h)xg(h) or d(h )xg(h)/tan delta, where d(h ) is the hydrostatic piezoelectric voltage coefficient), a large dielectric constant (K) and low dielectric loss, and great mechanical strength. A shallow cavity between the PZT ceramics and thick metallic electrode is designed to convert a portion of the z-direction stress into a large radial and tangential stress of opposite sign. thereby causing the d(33) and d(31) contributions to d(h) to add rather than subtract, and raising the figure of merit. Theoretical stress analysis was carried out using an axisymmetric finite element method. Experimental results show that the d(h)x g(h), K, and withstandable pressure are extremely high.     

Corrosion of alumina ceramics in acidic aqueous solutions at high temperatures and pressures

Alumina is resistant against corrosive aqueous solutions and could be used as a reactor material in the Supercritical Water Oxidation (SCWO) process. For this reason, the corrosion resistance of alumina and zirconia toughened alumina (ZTA) ceramics was investigated in aqueous solutions containing 0.1 mol/kg H₂SO₄, H₃PO₄ or HCl at T = 240°C–500°C at p = 27 MPa. In sulfuric acid, the solubility of alumina and its corrosion products was high at temperatures of 240°C–290°C. The corrosion rate was still high at higher temperatures (340°C–500°C), but the corrosion products were less soluble and formed a non-protecting scale on the samples. Phosphoric acid was less corrosive due to the formation of berlinite (AlPO₄) on the surface of the specimens. In hydrochloric acid, the dissolution of the alumina grains was the predominant corrosion phenomenon at temperatures of 240°C–290°C. At higher temperatures, intergranular corrosion was observed, but a dissolution of the grains did not occur.     

Fracture and deformation behaviour of melt growth composites at very high temperatures

Unidirectionally solidified Al₂O₃/Y₃Al₅O₁₂ (YAG) or Al₂O₃/Er₃Al₅O₁₂ (EAG) eutectic composites, which are named as Melt Growth Composites (MGCs) has recently been fabricated by unidirectional solidification. The MGCs have a new microstructure, in which continuous networks of single-crystal Al₂O₃ phases and single-crystal oxide compounds (YAG or EAG) interpenetrate without grain boundaries. The MGCs fabricated are thermally stable and has the following properties: 1) the flexural strength at room temperature can be maintained up to 2073 K (just below its melting point), 2) a fracture manner from room temperature to 2073 K is an intergranular fracture different from a transgranular fracture of sintered composite with the same composition, 3) the compressive creep strength at 1873 K and a strain rate of 10^–4/sec is 7–13 times higher than that of sintered composites, 4) the mechanism of creep deformation is based on the dislocation creep models completely different from the Nabarro-Herring or Coble creep models of the sintered composites, and 5) it shows neither weight gain nor grain growth, even upon heating at 1973 K in an air atmosphere for 1000 hours. The above superior high-temperature characteristics are caused by such factor as the MGCs having a single-crystal Al₂O₃/single-cryatal oxide compounds without grain boundaries and colonies, and the formation of the thermodynamically stable and compatible interface without amorphous phase.     

Processing and oxygenation of YBaCuO melted textured ceramics at high and enhanced pressures and temperatures

It has been established that the oxygenation of the structure of YBa₂Cu₃O_7??? (Y123) textured ceramic materials under elevated pressure (up to 10?C16 MPa) and high temperatures (700?C900°C) for a relatively short time (72 h) makes it possible to attain the record superconducting (at 77 K in the ab plane of Y123 phase j _c ?? 100 kA/cm² (0 T), 55 kA/cm² (2 T) and H _irr 9.5 T) and mechanical (microhardness up to H _V = 7.6 GPa and fracture toughness up to K _Ic = 4.4 MPa·m^0.5 under 4.9 N) properties. The high material characteristics have been explained by the formation of a nanostructure from twins acting as pinning centers (the density of twinning being 20?C35 m^?1) and a considerably smaller amount of microcracks (< 0.2 m^?1) as well as by prevention of macrocracking.     

Friction and wear properties of monolithic silicon-based ceramics

Friction and wear properties of two forms of silicon nitride (reaction bonded and hot pressed) and two of silicon carbide (reaction sintered and sintered) are reported. The materials were slid against themselves under unlubricated conditions. Tests were run using a simulated inertial sample dynamometer. Sliding speeds ranged from 0.5 to 5.5 m sec^–1 with applied loads of 225 or 450 N. Friction coefficients were found to be in the range of 0.15 to 0.8 for both types of material. Friction response was qualitatively correlated with changes in surface chemistry at the sliding interface. Wear rates were of the order of 10^–14 to 10^–13m³ (Nm)^–1, in order-of-magnitude agreement with previous pin-on-disc results reported in the literature. Wear surfaces exhibit plastic deformation, ploughing, and oxide film formation and removal.     

Interaction of metals and alloys with silicon-based ceramics

Silicon-based ceramics have been found to interact chemically with a number of metals and alloys when the materials were placed in physical contact in air at temperatures in the vicinity of 1000 C. Among the reaction products found after 100 h were silicides, silicates and carbides and the severity of the interaction depended on the temperature and the ease of migration of free silicon from the ceramic to the metallic phase. The occurrence of these reactions may be deleterious in applications in which silicon based ceramics and alloys are in contact for extended periods at high temperatures.     

Superconducting and mechanical properties of YBCO/Ag composites fabricated at high pressures up to 5.4 GPa

The YBCO/Ag superconducting composites were fabricated by compressing powders of YBCO and silver mixed at various volume proportions 0 to 100% under pressures of 2.0 and 5.4 GPa and subsequently sintering at 900 °C for 10 h in a flowing oxygen atmosphere. The superconducting transition temperature, particularlyT _c (R=0), was found to retain the value above 85 K up to 50 vol % Ag while the normal resistivity at 300 K to reduce below 10 cm. The highestJ _c of 400 Acm^–2 was attained for the sample containing 20 vol % Ag pressed at 5.4 GPa. The mechanical properties were also improved by the addition of silver: the compressive strength is increased by two to three times and the maximum strain, which is a measure of ductility, reached a value almost four times as large as that in the pure YBCO prepared under the ordinary pressure of 0.5 GPa. The persistence of the superconducting characteristics up to 70 vol % Ag is discussed in terms of the microstructure taken with the scanning electron microscope.     

Viscoelasticity of ceramics at high temperatures

The dependence of the fracture toughness, K _IC, on the loading rate has been calculated. On the basis of linear elastic fracture mechanics (LEFM) a strong dependence of the fracture toughness on the loading rate is obtained if subcritical crack growth is taken into account. If the subcritical crack growth parameters n and B are sufficiently small, which correspond to a high velocity of crack extension, the fracture toughness should decrease at lower loading rates. This behaviour is similar to the well-known decrease of bending strength. The experimental results for alumina containing glassy phase as a model material, however, show a maximum in a certain regime of loading rates. A model is established, which combines LEFM and the viscoelasticity, and leads to a maximum of K _IC at a certain loading rate dependent on the viscosity of the glassy phase.     

Thermal conductivity of polymethylphenylsiloxanes at high pressures

Results are presented of an experimental investigation of the thermal conductivity of polymethylphenylsiloxanes in the 20–200°C temperature range and up to 200-MN/m² pressure range. An equation is proposed to compute the heat conduction in the temperature and pressure ranges investigated.     

Thermal conductivity of nitrogen at high pressures

The results of the measurements of the thermal conductivity coefficients of nitrogen at 298.15 K from atmospheric pressure up to 1 GPa are reported. The experimental values are used to test the Modified Enskog Theory and the corresponding state principle. The experimental values are also compared with the results of computer simulation of the thermal conductivity of a Lennard Jones fluid.     

Strength behavior of materials at high pressures

Strength is an important aspect of material behavior for armor performance, planetary science, and accurate property measurement under quasi-isentropic loading and static high pressure. The advent of time-resolved diagnostics allowed the elastic–plastic behavior of solids under shock loading to be detected for the first time. These early experiments revealed a two-wave structure of elastic and plastic shock waves for stresses above the elastic limit. However, the full stress state in the shocked state remained unknown because the conservation equations provide only the longitudinal stress. Since then, a variety of techniques has been used to determine strength in the shocked state. One of the most successful has been using shock/release and shock/reload to find the stress state and strength in the shocked condition. This technique has been applied to a variety of metals and ceramics at stresses of 100 GPa or higher. These experiments have revealed a rich set of behaviors, many of which are still not fully understood. Recently, there has been significant interest in isentropic loading where the material can be significantly cooler and strength even more important. In this paper, we present a current perspective on strength under high pressures, particularly in the dynamic regime, with emphasis on the techniques used to measure strength and their advantages and disadvantages. Results of strength measurements on several materials will be discussed, and interesting aspects of behavior will be highlighted. In addition, shock results will be contrasted with those under isentropic loading. Finally, some new directions in the study of strength will be outlined.     

Viscosities of some liquids at high pressures

Viscosity and solidification pressure are measured for spindle and castor oils, liquids PES-5, PES-S-2, PFMS-4, and polymethyltrifluoropropylsiloxane FS-169.     

Thermophysical properties of liquids at high pressures

Most of the thermophysical properties of fluids are greatly altered at high pressures, and the studies of these changes are of much scientific and technological importance. In this paper, the effects of temperature and pressure on the density, viscosity, and thermal conductivity of various liquids are described briefly, based on recent experimental results from the author's laboratory. The objectives of this investigation, methods of measurements, and some of the experimental results are reviewed, as well as the present aspects in this field. Several important problems to be interpreted are also pointed out from the present measurements.Presented at the Japan-United States Joint Seminar on Thermophysical Properties, October 24–26, 1983, Tokyo, Japan.     

Thermal characterization of materials at high pressures

This review deals with the development of some novel techniques for the measurement of thermal properties of materials at ambient and high pressures. Thermal characterization techniques like Absolute thermoelectric—power, AC electrical Resistivity, Differential thermal analysis and AC Calorimetry with specific reference to their usage at high pressures will be discussed in some detail. Some typical data on materials like Chromium alloys, Chalcogenide and metallic glasses will be presented.     

The measurement of humidity at high pressures

A humidity calibrator for measurements at pressures up to 15 MPa is designed. The effect of pressure on the characteristics of capacitive humidity sensors is demonstrated. A method of taking this effect into account is proposed. A procedure for calibrating humidity transducers with capacitive sensors at high pressure is described. __________ Translated from Izmeritel’naya Tekhnika, No. 2, pp. 61–64, February, 2008.