Theoretical study of the thermodynamic properties of nitrogen at high temperatures and pressures

A theoretical analytical equation of state is derived and used to compute new thermodynamic data for nitrogen at high tempratures and pressures.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 53, No. 6, pp. 975–983, December, 1987.     

Destruction of silicon nitride whiskers by reaction with metals at high temperatures

A preliminary study has been made of the reaction, at elevated temperatures, between whiskers of silicon nitride and pure metals (aluminium and nickel). The subject is important in the context of composite materials using refractory whiskers as a reinforcing component. The method involves reacting the whiskers with a thin, evaporated layer of the metal concerned and observing the results under the electron microscope. The effects of reaction are described and may be explained in terms of either crystallographic etching or recrystallisation of the whisker, though the former appears the more likely. The results do not necessarily reflect the behaviour of bulk composites made from the same constituents.     

Deformation of silicon at low temperatures

The dislocation arrangements produced around microhardness indentations made in silicon at room temperature have been studied by transmission electron microscopy. Loops consisting of 30°- and 60°-dislocations are produced and move on the {111} planes. It is suggested that, during indentation, the theoretical shear strength is exceeded locally and that the observed dislocations arise as a result of the accommodation of the displacements due to block slip. On annealing up to 1030° C the loops do not appear to be mobile, rather new loops consisting of edge and screw components are formed which can move large distances.     

Stabilization of luminous properties of porous silicon by vacuum annealing at high temperatures

The degradation properties of porous silicon caused by vacuum annealing at high-temperatures have been measured. Vacuum annealing within the temperature range of 150–300°C provided the complete removal of water molecules from the pore surface and partial destruction of hydrogen complexes. Our studies showed that vacuum annealing at 150°C considerably stabilizes the luminous properties of porous silicon: the subsequent UV-irradiation of silicon specimens changed the photoluminescence intensity only by 8–10% of its initial value.     

Chemical techniques for measuring and controlling the thermodynamic properties of aqueous fluids at high pressures and temperatures

Several chemical techniques have recently been developed for measuring and controlling the fugacitiesf _{O 2},f _{H 2},f _{H 2 O}, andf _HCl in supercritical aqueous fluids. Experimental samples consisting of fluid components and solid chemical sensors or buffers are sealed in H₂-permeable noble metal (Pt or Ag Pd alloy) capsules and reacted at high pressure and temperature, Hydrogen diffusion through the walls of the capsule(s) allowsf _{H 2} to be controlled or measured. After quenching, the fluids and solids are analyzed to quantify one or more thermodynamic properties of interest. Methods for determining the activity Composition relations of H₂O in aqueous fluid mixtures are discussed.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder. Colorado. U.S.A.     

Theory of superconductivity at high temperatures

Superconductivity is generally explained by an electron-lattice interaction which results in the pairing of electrons and the condensation of these pairs into a state of lower entropy. In high-temperature superconductivity the pairs consist of hybrids in which the top of the oxygen band of the crowded perovskite layer is mixed with the bottom of unoccupiedd- orf-bands from monoxide layers in the crystal. Only electrons and phonons with low quasi-momentum (k) values participate. This makes it possible to localize the lattice perturbation into broad regions in which the van der Waals forces are reduced and the perovskite planes are contracted. The low entropy state associated with superconductivity manifests itself as the formation of a superlattice of lattice distortions which is in actual motion in the current-carrying states. The observability of this superlattice is discussed.     

Sulphidation of cobalt at high temperatures

The kinetics and mechanism of cobalt sulphidation have been studied as a function of temperature (773–1023 K) and sulphur partial pressure (1–104 Pa) by means of thermogravimetric, SEM and X-ray techniques, and also using inert-marker and ratio-tracer methods. It has been shown that the sulphidation process is diffusion controlled, the rate-determining step being the outward volume diffusion of cations. According to the phase diagram of the Co–S system, the sulphide scale on cobalt is heterogeneous. At sulphur pressures higher than the dissociation pressure of the CoS2 phase, the sulphidation rate is pressure independent, and at lower pressures it increases with rising pressure, in agreement with theoretical predictions. The apparent activation energy of sulphidation is considerably higher for multilayer than for double-layer scale formation, because the main part of multilayer scale is growing at the dissociation pressure of the CoS2 phase, which increases with increasing temperature. Over the whole temperature and pressure range studied, the rate of cobalt sulphidation is more than three orders of magnitude higher than the oxidation rate of this metal. Rapid degradation of cobalt in a sulphur atmosphere results mainly from a very high defect concentration in Co1-yS and Co9S8 sulphides, participating in comparable amounts in the scale formation on this metal at T>900 K. The only sulphide of cobalt in which the defect concentration may be very low is CoS2, the growth rate of this sulphide layer being more than two orders of magnitude lower than that of other cobalt sulphides. © 1998 Chapman & Hall     

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.     

Cyclic fatigue of reaction-bonded silicon nitride at elevated temperatures

Cyclic and static loading tests were performed on reaction-bonded silicon nitride from 1000–1400 °C in air. This porous, fine-grained material contained no glassy grain-boundary phase and exhibited no slow crack growth at room temperature. Under cyclic loading, the crack-growth behaviour at 1000 °C was similar to room-temperature results; however, at 1200 and 1400 °C crack-growth rates increased significantly. Under static loading, significant crack growth was detected at 1000 °C and increased with temperature. Most of the crack growth under cyclic loading was attributed to slow crack-growth mechanisms, but evidence of cyclic crack-growth mechanisms were also observed. Oxidation played a major role in crack-growth velocity at high temperature. This revised version was published online in November 2006 with corrections to the Cover Date.     

Friction and wear of single-crystal silicon at elevated temperatures

Single-crystal silicon wafers ((1 1 1) and (1 0 0)p-type) were abraded at room temperature 300 °C, and 600 °C by a polycrystalline partially stabilized zirconia ball in a ball-on reciprocating flat geometry. The sliding direction was 1 1 0. The friction coefficient was recorded as a function of reciprocating strokes and the deformation mode of the silicon. The friction coefficient at room temperature decreased with the number of strokes, and this variation was less affected by the number of strokes at the higher temperatures. The wear track width and depth were measured at the three temperatures. Wear increases as the temperature is raised to 300 and 600 °C. Optical and scanning electron microscopy of the subsurface damage reveals that cracks are generated at RT and 300 °C and dislocations are produced at 600 °C. The change in deformation mode with temperature from brittle fracture to plastic deformation accounts for the differences in wear.