Ultrasonic study of the temperature and pressure dependences of the elastic properties of β-silicon nitride ceramic

Ultrasonic wave velocity measurements have been used to determine the elastic stiffness moduli and related elastic properties of high-purity, dense -Si₃N₄ ceramic samples as functions of temperature in the range 150–295 K and hydrostatic pressure up to 0.2 GPa at room temperature. Due to its covalently bonded, rigid structural framework -Si₃N₄ is an elastically stiff material; the elastic stiffness moduli of the ceramic at 295 K are: C _L = 396 GPa, = 119 GPa, B ^S = 238 GPa, E = 306 GPa, Poisson's ratio = 0.285. The longitudinal elastic stiffness C _L increases with decreasing temperature and shows a knee at about 235 K; the decrease in slope below the knee indicates mode softening. The shear elastic stiffness shows mode softening which results in a plateau centred at about 235 K and an anomalous decrease with further reduction in temperature. The hydrostatic-pressure derivatives of elastic stiffnesses at 295 K are (C _L/P) _P=0 = 4.5 ± 0.1, (B ^S/P) _P=0 = 4.3 ± 0.1 and (/P) _P=0 = 0.17 ± 0.02 (pressure < 0.12 GPa). An interesting feature of the nonlinear acoustic behaviour of this ceramic is that, in the pressure range above 0.12 GPa, the values obtained for (/P) _P=0 and the shear mode Grüneisen parameter (_S) are small and negative, indicating acoustic-mode softening under these higher pressures. Both the anomalous temperature and pressure dependences of the shear elastic stiffness indicate incipient lattice shear instability. The shear _S(=0.005) is much smaller than the longitudinal _L(=1.18) accounting for the thermal Grüneisen parameter ^th(=1.09): since the acoustic Debye temperature _D(=923 ± 5 K) is so high, the shear modes play an important role in acoustic phonon population at room temperature. Hence knowledge of the elastic and nonlinear acoustic properties sheds light on the thermal properties of ceramic -Si₃N₄.     

Thirty years with the GÉT 43–73 standard of the unit of pressure

The main results of the use of the GÉT 43–73 State Standard of the unit of pressure over the range of reproducible pressures of 100–1500 MPa, connected with ensuring correctness of high-pressure measurements in this country, by comparing the high-pressure scales of different countries, by improving the theoretical basis of the standard, its material part, and the experimental research techniques and checking work, are presented. Basic results, which, in the opinion of the authors, may be of interest to specialists, are given.Translated from Izmeritelnaya Tekhnika, No. 1, pp. 42–45, January, 2005.     

Influence of the deposition pressure on the preparation of μc-Si:H thin films in hot-wire-assisted MWECR-CVD system

Considering the important influence of the deposition pressure on the growth of thin films, such as deposition rate, crystalline volume fraction and density, etc., and based on the analysis of the advantages and disadvantages on the mono-pressure method, we proposed a new method of high- and low-pressure combination to prepare hydrogenated microcrystalline silicon (μc-Si:H) films, i.e. at first we used high pressure to deposit film in 2 min in order to minish the thickness of incubation layer from the amorphous phase transition to crystalline phase, and then used low pressure to deposit film in 18 min to improve the density and decrease the oxidation of the film. The experimental results showed that using this new method the thin film with high crystalline volume fraction of 61% and low light-induced degradation ratio of 5.6% at 210 min was obtained, and meanwhile, it also possessed higher density and better photoelectronic properties than mono-pressure method.     

Ultrasonic study of the temperature and pressure dependences of the elastic properties of ceramic dimolybdenum carbide (α-Mo2C)

Pulse-echo overlap measurements of ultrasonic wave velocity have been used to determine the elastic stiffness moduli and related elastic properties of ceramic samples of dimolybdenum carbide (-Mo₂C) as functions of temperature in the range 130–295 K and hydrostatic pressure up to 0.2 GPa at room temperature. The temperature dependences of the shear elastic stiffness () and Young's modulus (E) show normal behaviour and can be approximated by a conventional model for vibrational anharmonicity. The longitudinal elastic stiffness (C _L) increases with decreasing temperature and shows a knee at about 200 K; the decrease in slope below the knee indicates longitudinal acoustic-mode softening. The adiabatic bulk modulus (B ^S) is also affected by the mode softening below 200 K. The values obtained for the acoustic Debye temperature (_D) for ceramic -Mo₂C agree well with the thermal Debye temperature determined previously from heat capacity measurements. The velocities of both the longitudinal and shear ultrasonic waves in ceramic -Mo₂C increase approximately linearly with pressure: both the long-wavelength longitudinal and shear acoustic modes stiffen under pressure. The values determined at room temperature for the hydrostatic-pressure derivative (/P) _P=0 of the shear stiffness is similar to those found for ceramic TiC and TaC; while (C _L/P) _P=0 and (B ^S/P) _P=0 have large values, possibly due to the defect microstructure of ceramic -Mo₂C.     

Effects of pressure sensitivity on the η factor and the J integral estimation for compact tension specimens

In this paper, the effects of pressure-sensitive yielding on the factor and the J integral estimation for compact tension specimens are investigated. The analytical expressions for and J for pressure-insensitive von Mises materials are generalized to pressure-sensitive Drucker-Prager materials using a lower bound approach. The factor as a function of the pressure sensitivity and the normalized crack depth for compact tension specimens is derived under plane stress and plane strain conditions. The numerical results indicate that the factor decreases as the pressure sensitivity increases. The effects are more pronounced under plane strain conditions than under plane stress conditions. However, the effects of the pressure sensitivity on are found to be mild in general. For rigid perfectly-plastic materials, the J estimation for pressure-sensitive materials is also reduced to a simple expression of the tensile yield stress times the crack tip opening displacement as for the von Mises materials.     

Gas pressure sintering of β-silicon nitride

The sintering behaviour of -Si₃N₄ powder was investigated in 980 kPa (10 atm) nitrogen at 1800–2000 °C. It is shown that -Si₃N₄ has a higher sinterability than the finer -Si₃N₄. The solution of small grains and reprecipitation on large grains occurred during sintering at >1600 °C. The rate-determining step in the liquid-phase sintering is believed to be the diffusion of material through the liquid phase at grain boundaries. There was no abnormal grain growth during gas pressure sintering of -Si₃N₄. The microstructures of gas pressure sintered materials from -Si₃N₄ were more uniform than those from -Si₃N₄. The densification mechanism of -Si₃N₄ is discussed in relation to that of -Si₃N₄.     

Can the use of pulsed direct current induce oscillation in the applied pressure during spark plasma sintering?

Abstract

The spark plasma sintering (SPS) process is known for its rapid densification of metals and ceramics. The mechanism behind this rapid densification has been discussed during the last few decades and is yet uncertain. During our SPS experiments we noticed oscillations in the applied pressure, related to a change in electric current. In this study, we investigated the effect of pulsed electrical current on the applied mechanical pressure and related changes in temperature. We eliminated the effect of sample shrinkage in the SPS setup and used a transparent quartz die allowing direct observation of the sample. We found that the use of pulsed direct electric current in our apparatus induces pressure oscillations with the amplitude depending on the current density. While sintering Ti samples we observed temperature oscillations resulting from pressure oscillations, which we attribute to magnetic forces generated within the SPS apparatus. The described current–pressure–temperature relations might increase understanding of the SPS process.     

Simulation of the pressure drop across granulated mixtures using a coupled DEM – CFD model

The sinter process converts mixtures of iron ore, iron ore fines and fluxes into a fused aggregate (sinter) that is used as burden material in the blast furnace. The rate of this process is predicted by measuring the pressure drop across the green granulated mixture before ignition. A lower pressure drop corresponds with a higher permeability resulting in a higher sinter rate. The addition of fine material, such as concentrate or concentrate agglomerated into micropellets, to the sinter mixture affects the pressure drop. This study numerically predicts the pressure drop over several granulated mixtures in order to reduce the number of experimental measurements. The pressure drop was studied both experimentally using a pot grate and by coupled DEM (Discrete Element Method) – CFD (Computational Fluid Dynamics) simulations. The validation of the model was performed by comparing the measured and numerical values of the pressure drop across glass beads 3 and 6?mm in diameter respectively. The simulation of the pressure drop was extended to granulated mixtures that contain 0–40% concentrate or micropellets. DEM was also used to numerically simulate iron ore granules and relate their mechanical behaviour to particle size distribution, shape, friction coefficient, Young’s modulus and adhesion force.     

Studies on the mechanical properties of steel–TiB2 composites obtained by high pressure sintering

The currently used composites produced by classical sintering methods are characterised by numerous limitations due to the difficulties in combining different materials with extreme properties. One of the ways to overcome these limitations is in the use of modern sintering methods, including the high pressure-high temperature process. This study describes the composite materials based on 316L austenitic steel reinforced with titanium diboride and examines the effect of sintering conditions on the mechanical properties and microstructure of sintered composites. It has been found that the key parameter in the manufacture of composites with optimal properties is the sintering time and temperature, while martensitic transformation taking place in the composite matrix can be controlled by the properly selected pressure applied during the sintering process.     

Diaphragm forming of continuous fibre reinforced thermoplastics: influence of temperature,pressure and forming velocity on the forming of Upilex-R® diaphragms

In the diaphragm forming process, the thermoplastic composite sheet is clamped between two high temperature thermoplastic diaphragms. In the present study, the influence of temperature, pressure and forming rate on the deformation of high temperature PI diaphragms (Upilex-R^®, Ube Industries) is described. At temperatures below 275 °C the upper diaphragm slides over the bottom diaphragm and shows a more global deformation, above 305 °C, the upper diaphragm cannot slide over the bottom diaphragm and deforms in the same manner. The region 275–305 °C is a kind of transition region between the previous two temperature ranges. A hydrostatic pressure of 1 bar turned out to be sufficient to deform the diaphragms, therefore, no influence of pressure was observed. The deformation of the bottom diaphragm is independent of forming rate, while the upper diaphragm showed some dependence.     

Thermal pressure coefficients of ethanenitrile,propanenitrile, and butanenitrile in the region 295–395 K

The thermal pressure coefficient (p/T) _v has been measured for ethanenitrile from 299 to 364 K, for propanenitrile from 295 to 377 K, and for butanenitrile from 297 to 398 K. The results are discussed in terms of the diminishing role of polarity in the alkanenitrile series and of a corresponding-states approach using gas-liquid critical properties as reduction factors. Although (p/T) _v varies unevenly with chain length, the reduced quantity shows a more regular behavior similar to that of the related quantity the cohesive energy density.     

Investigation of the gapless state induced by pressure in Hg1−x Cd x Te alloys

We report the results of an experimental study of the gapless state (GS) induced inp-type semimetallic alloys Hg _1–x Cd _x Te (0<x<0.15) by pressure. Galvanomagnetic effects in weak magnetic fields (2T<300 K) and the Shubnikov-de Haas effect have been investigated in the pressure interval 1p<15 kbar. Direct evidence for the existence inp-type semimetallic alloys of an impurity hole band overlapping with the conduction band by 3–4 me V is obtained. At liquid helium temperatures the Fermi level is located in the impurity band, so that two groups of carriers take part in transport effects: light electrons in the conduction band and heavy holes in the impurity band. The electron Fermi energyE _F is proved to be essentially constant during the transition to the GS. A linear dependence of the electron effective mass at the band edgem* (0) upon the gapE _g is obtained. A significant role of scattering of electrons into the impurity band at liquid helium temperatures is revealed.     

An analytic solution to the coupled pressure–temperature equations for modeling of photoacoustic trace gas sensors

Trace gas sensors have a wide range of applications including air quality monitoring, industrial process control, and medical diagnosis via breath biomarkers. Quartz-enhanced photoacoustic spectroscopy and resonant optothermoacoustic detection are two techniques with several promising advantages. Both methods use a quartz tuning fork and modulated laser source to detect trace gases. To date, these complementary methods have been modeled independently and have not accounted for the damping of the tuning fork in a principled manner. In this paper, we discuss a coupled system of equations derived by Morse and Ingard for the pressure, temperature, and velocity of a fluid, which accounts for both thermal effects and viscous damping, and which can be used to model both types of trace gas sensors simultaneously. As a first step toward the development of a more realistic model of these trace gas sensors, we derive an analytic solution to a pressure–temperature subsystem of the Morse–Ingard equations in the special case of cylindrical symmetry. We solve for the pressure and temperature in an infinitely long cylindrical fluid domain with a source function given by a constant-width Gaussian beam that is aligned with the axis of the cylinder. In addition, we surround this cylinder with an infinitely long annular solid domain, and we couple the pressure and temperature in the fluid domain to the temperature in the solid. We show that the temperature in the solid near the fluid–solid interface can be at least an order of magnitude larger than that computed using a simpler model in which the temperature in the fluid is governed by the heat equation rather than by the Morse–Ingard equations. In addition, we verify that the temperature solution of the coupled system exhibits a thermal boundary layer. These results strongly suggest that for computational modeling of resonant optothermoacoustic detection sensors, the temperature in the fluid should be computed by solving the Morse–Ingard equations rather than the heat equation.     

The effective oxidation pressure of indium–oxygen system

A theoretical model on oxygen transport at the surface of liquid metals has been validated by dynamic surface tension measurements performed on liquid Indium as test metal. The oxygen contamination conditions have been obtained at different oxygen partial pressures under both low total pressure (Knudsen regime) and inert atmospheric pressure (Fick regime) conditions, confirming that an oxide removal regime occurs under an oxygen partial pressure much higher than the equilibrium one (the “Effective Oxidation Pressure”). Experimental results are reported which give a further insight on the relative importance of the various processes due to the oxygen mass transport between the liquid metal and the gas phase. The critical aspects involved in surface tension measurements of liquid metals, related to the problem of liquid metal–oxygen interactions, are also underlined.     

On the electrical properties of Si-doped InGaP layers grown by low pressure‐metalorganic vapor phase epitaxy

The electrical properties of undoped and silicon doped InGaP layers grown lattice matched on GaAs by low pressure metal-organic vapor phase epitaxy were investigated under different growth conditions. The possible presence of superlattice ordering was excluded by photoluminescence analysis. Undoped layers exhibited a background p-type contamination of the order of 10¹⁶ cm^− 3; the role of possible carbon contamination is discussed. Capacitance-voltage and Hall investigation of Si-doped n-type layers evidenced a room temperature free electron density linearly increasing from 3.6 × 10¹⁶ to 6 × 10¹⁸ cm^− 3 as a function of the Si precursor flow. The corresponding electron mobilities decreased from 1800 to 483 cm²/V s. At lower temperatures, the conductivity and mobility of the n-doped samples showed a metallic like behavior, in some cases with values not consistent with a simple electronic transport into the conduction band, suggesting the presence of an additional parallel transport channel. Five main electron traps were identified by deep level transient spectroscopy and among them, two traps resulted to be dominant, one turned out to play a major role in the bulk, probably associated to the Si doping, and the other was active near the InGaP surface, ascribable to P-related defects.     

On the Correction of the Melting Curve of the Eutectic Co–C for the Effect of the Thermal Inertia of the Furnace

This paper considers the influence of the thermal inertia of the furnace on the shape of the melting curve of the eutectic Co–C. To this end, melting experiments have been performed in a uniform three-zone furnace, with an inherent substantial thermal inertia. The thermal inertia has been quantified by measuring the step-response of the furnace with the sample in its solid state, just below its melting temperature. From the analysis of the effect of the thermal inertia of the furnace, it turned out that during melting the temperature distribution within the furnace, surrounding the crucible, is bound to be in a non-stationary state. This provided the key to properly finalizing the correction to be applied. The shape of the corrected curve differs considerably from that of the curve, as measured, in that the former shows a flatter melting plateau, and a larger curvature on the way down to the solidus point. As regards the liquidus temperature \(T_{\mathrm{liq}}\)—of major interest in the characterization of the transition temperature of high-temperature fixed points—it is demonstrated that the thermal inertia of the furnace shows a kind of self-compensating mechanism. But the effects of the thermal inertia of the furnace on the parameters defining the Scheil fit, involved in the correction procedure, were considerable.     

Gas pressure sintering of silicon nitride — current status

Gas pressure sintering of silicon nitride is now common for obtaining a dense as well as tough product. A review of earlier works has revealed that there are still controversies in prediction of mechanism of the sintering process. Analysis of the kinetics of the process obtained by integration of a dilatometer inside the furnace has been presented. The mechanism of sintering silicon nitride powder compacts in presence of hyperbaric nitrogen atmosphere has been discussed. It has been observed that intermediate stage densification kinetics is very much susceptible to sintering atmosphere and time of pressurisation. The microstructure of the final product has been found to be dependent primarily on the method of incorporation of additive in the starting silicon nitride powders.