Showing papers on "Total pressure published in 1991"

A Mathematical Model of the Hydrodynamics and Gas‐Phase Reactions in Silicon LPCVD in a Single‐Wafer Reactor

Abstract: A mathematical model has been developed for the fluid dynamics and the chemical reactions in silicon low pressure chemical vapor deposition (LPCVD) from silane in a cold-wall single-wafer reactor. The fluid dynamics model includes the mass, momentum, heat, and species balance equations and equations for multicomponent (thermal) diffusion. The chemical model includes five reversible homogeneous reactions and five heterogeneous deposition reactions. The equations are solved numerically using a control volume based finite difference method. The mathematical model has been used to simulate silicon growth on large (0.24 m) wafers, at varying pressures, wafer temperatures, and silane concentrations in different carrier gases. It was found that both the fluid dynamics and the chemical kinetics have a large influence on the predicted growth rates. At conventional LPCVD process conditions gas-phase reactions are negligible, and deposition is very uniform, but the growth rate is low. An increase in the wafer temperature leads to an increased growth rate, whereas uniformity is still very good. Increasing the total pressure or the silane partial pressure leads to an increased importance of gas-phase reactions and highly deteriorated uniformity. The model predictions were found to be relatively insensitive to most of the model parameters.

Mass-volume vortex flowmeter

Abstract: A flowmeter has a vortex generating bluff body (102) disposed across the flow passage (106), a planar member (103) disposed immediate usptream of the bluff body (102) including a plurality of total pressure ports emerging through the leading edge of the planar member (103) and a plurality of static pressure ports emerging through the side faces of the planar member (103), and a vortex sensing device including a planar member (104) and one or two transducers (111 and 112), which flowmeter determines the mass and volume flow rates of the fluid and the density of the fluid from the dynamic pressure determined from the difference between the total and static pressures and the fluid velocity determined from the vortex shedding frequency.

A comparison of ULF fluctuations in the solar wind, magnetosheath, and dayside magnetosphere: 2. Field and plasma conditions in the magnetosheath

Abstract: Data from three spacecraft (AMPTE IRM, AMPTE CCE, and ISEE 1 or 2) are used to study the correlation among the field and plasma conditions in the subsolar magnetosheath region, ULF wave activity in the magnetosphere, and the cone angle of the IMF. A disturbance parameter, R, which is the magnitude of the normalized resultant of unit vectors (calculated from measurements of the magnetic field or the plasma bulk velocity in a time interval), is used to describe the disturbance of the magnetosheath region. A “quiet” state has R values close to unity. We have studied five time intervals and found that when the R values of the magnetosheath magnetic field were below 0.8, indicative of a disturbed magnetosheath near local noon, transverse harmonic oscillations of magnetic field lines in the Pc 3, 4 range were observed in the magnetosphere and small cone angles were observed in the solar wind. We have also investigated the variation of other magnetosheath parameters (such as the magnetic pressure, the thermal pressure, the dynamic pressure, and the perturbation energy, etc.) under various magnetosheath conditions by comparing them with the disturbance parameter. It is found that the thermal beta (the ratio of the thermal pressure to the magnetic pressure) and the perturbation energy increase greatly as R decreases (i.e., as the magnetosheath region becomes more disturbed). The total pressure, which is the sum of the magnetic, thermal, and dynamic pressure of the subsolar magnetosheath region, decreases as the region becomes more disturbed. The dynamic pressure and the dynamic beta (the ratio of the dynamic pressure to the magnetic pressure) measured in the magnetosheath are poorly correlated with R, indicating that the changes in magnitude of the plasma bulk velocity in the subsolar magnetosheath have little effect on the occurrence of Pc 3-4 waves in the outer magnetosphere. All magnetosheath parameters we examined became more variable during disturbed periods than during quiet ones. The implication of the above results to the transport of wave energy from the solar wind to the magnetosphere is discussed.

High-Temperature Wetting of Sapphire by Aluminum

Abstract: Sessile drop studies of molten aluminum on single-crystal sapphire substrates were conducted to investigate the effects of atmosphere on contact angle, substrate reactions, and interfacial crystal growth. Unlike previous investigations performed briefly in a vacuum environment in a temperature range within 600°C of the aluminum melting point, these experiments were conducted at higher temperatures (1200° to 1600°C) and at 1-atm total pressure over longer experimental times to more closely approach equilibrium conditions. A continuously flowing buffered gas system utilizing high-purity metered mixtures of hydrogen and helium in combination with a thoria ceramic electrolyte sensor were employed to achieve variations of the oxygen partial pressure from 10−19 to 10−15 atm while continuously maintaining the total pressure at 1 atm. At constant temperature, it was found that neither the oxygen partial pressure nor the crystallographic orientation of the sapphire substrate had a significant effect on the observed contact angles. A continuous decrease of acute contact angles and a single reaction ring characterized the 8-h experiments without the alternating spreading and contracting behavior repeatedly reported in the literature. This phenomenon can be attributed to the lower rate of metal evaporation and interfacial reaction at the higher total gas pressure and yet extremely low oxygen partial pressure of these experiments. Profilometric analysis of sapphire substrates subsequent to the removal of the quenched sessile drops indicates a reduction in metal–solid interaction due to the closer approach to equilibrium than in previous studies. An epitaxial orientation with respect to the substrate was observed in α-alumina crystallite formation at the metal–ceramic interface. Experimental evidence suggests that it was formed by a nucleation and growth process during the cooling period.

Dynamic modeling of chemical vapor infiltration

Abstract: A mathematical model is developed to describe the dynamic and pseudosteadystate behavior of chemical vapor infiltration, a process used to fabricate fiber-reinforced ceramic matrix composites. The three-parameter dusty-gas model is used to describe the interaction of mass transport fluxes, partial pressures, and partial pressure gradients in the interior of the porous medium, with the viscous flux related to the total pressure gradient through Darcy's law. The model is applied to study the deposition of SiC via decomposition of methyltrichlorosilane in a porous medium whose structure can be represented by a population of uniformly-sized, randomly-overlapping pores. The results show that use of simplified mass transport flux models can lead to significantly different results, even if the concentration of reactants is low and the effects of the products of the reaction on the deposition rate are ignored. It is also shown that operation of chemical vapor infiltration reactors under pressure pulsing can lead to conversion gradients in the densifying structure by a few orders of magnitude smaller than those seen at the same reaction conditions under constant pressure.

Formation of silver clusters in nozzle expansions

Abstract: The first successful experiments to generate continuum silver cluster beams from nozzle expansions are described. A mixture Ar/Ag expands out of a conical nozzle (0.35 mm dia., 10° cone angle, length 17 mm). At 2150 K, total pressure 300 kPa, silver partial pressure 8 kPa the silver intensity measured with a rate meter 479mm away from the source is 1.8 nm/s, or 0.02 g m−2s−1. The data for the onset of clustering confirm the predictions of the scaling laws developed to compare condensation in nozzle expansions of metal vapors with that of rare gases.

Measuring chlorinated hydrocarbons in combustion by use of Fourier-transform infrared spectroscopy

Abstract: The quantitative measurement of products of incineration of chlorinated hydrocarbons using Fourier transform infrared (FTIR) spectrometry was evaluated during a study of the fundamental processes of toxic waste incineration. Combustion products were sampled from a turbulent flow reactor into which chlorinated hydrocarbons were injected. Chlorinated species were detected mainly in the C-Cl stretching region, away from H{sub 2}O and CO{sub 2} interferences. Absorbances of species having rotational lines narrower than the instrument resolution, such as CO, CO{sub 2}, and HCl, were nonlinear under most conditions. The measured absorbance of these species was very sensitive to total pressure, increasing with an increase in total pressure; at very low optical densities, however, absorbance became independent of total pressure. Thus, while FTIR spectroscopy has many features that make it attractive for measuring combustion species, care must be taken to ensure the accuracy of quantitative measurements.

Kinetic and Thermodynamic Analyses of Chemical Vapor Deposition of Aluminum Nitride

Abstract: AIN coatings were prepared by chemical vapor deposition from the AlCl3—NH3—Ar reagent system using an impinging jet reactor in the temperature range of 700° to 1100°C. A mass transfer model and thermodynamic calculations were used to analyze the deposition data. The AIN-CVD process could be approximated by calculating mass transfer—thermodynamic limits at low AlCl3 concentrations. The AIN deposition rate decreased drastically with increasing temperature above 1000°C in agreement with thermodynamic predictions. At high AlCl3 concentrations, a surface kinetic mechanism involving AlCl3 adsorbed on the deposition surface appeared to be the rate-limiting step. The AIN deposition rate decreased on increasing the AlCl3 concentration or total pressure. The crystalline structure of AIN was strongly influenced by the processing parameters. The AIN coatings became highly crystalline and preferentially oriented with an increase in the AlCl3 concentration or pressure.

How a limited mass transfer in the gas phase may affect the steady-state response of a Pd-MOS hydrogen sensor

Abstract: The hydrogen sensitivity of palladium MOS sensors in an oxygen-containing atmosphere is shown to depend on the area of the Pd film, the rate of the gas flow, the total pressure and the type of carrier gas. All observations can be explained by the influence of a limited mass transfer in the gas phase. This model is confirmed by mass spectrometric studies using a local gas-sampling technique. Requirements upon the sensor size and the H 2 supply to minimize mass-transfer influence are discussed.

Effects of Strain Rate and Confining Pressure on the Deformation and Failure of Shale

Abstract: Previous work on shale mechanical properties has focused on the slow deformation rates appropriate to wellbore deformation. Deformation of shale under a drill bit occurs at a very high rate, and the failure properties of the rock under these conditions are crucial in determining bit performance and in extracting lithology and pore-pressure information from drilling parameters. Triaxial tests were performed on two nonswelling shales under a wide range of strain rates and confining and pore pressures. At low strain rates, when fluid is relatively free to move within the shale, shale deformation and failure are governed by effective stress or pressure (i.e., total confining pressure minus pore pressure), as is the case for ordinary rock. If the pore pressure in the shale is high, increasing the strain rate beyond about 0.1%/sec causes large increases in the strength and ductility of the shale. Total pressure begins to influence the strength. At high stain rates, the influence of effective pressure decreases, except when it is very low (i.e., when pore pressure is very high); ductility then rises rapidly. This behavior is opposite that expected in ordinary rocks. This paper briefly discusses the reasons for these phenomena and their impact on wellbore andmore » drilling problems.« less

Pressure drag on the dinaric Alps during the ALPEX SOP

Abstract: During the ALPEX SOP (March–April, 1982), microbarographic measurements were conducted on the Northern Adriatic as a part of research on the Bora. In this paper the measured pressure field around the Dinaric Alps is used to compute the total pressure drag vectors using Archimedes law.

Total pressure measurements for n-pentane-methanol-2-butanol at 303.15 K

Abstract: In this paper, the total pressure is reported as a function of liquid-phase composition for n-pentane-methanol-2-butanol and each of its constituent binary systems at 303.15 K. The data were reduced by using Baker's method. The modified Margules equation was found to fit the binary measurements adequately. The ternary measurements were modeled with an expression for the excess Gibbs free energy which includes contributions from the binary systems plus additional terms containing ternary parameters.

Synthesis of high quality thallium superconductors at elevated oxygen pressure

Abstract: Preparation of thallium superconductors at pressures up to 200 bar is reported. We find that thallium loss can be completely suppressed, so that samples can be heated for long durations to ensure complete reaction and densification without loss of thallium. The relative phase fractions of Tl2Ba2CuO6 (2201), Tl2Ba2Ca1Cu2O8 (2212) and Tl2Ba2Ca2Cu3O10 (2223) are found to depend upon ( a ) starting composition ( b ) temperature of reaction/sinter( c ) total pressure and ( d ) oxygen partial pressure. Phase fractions were determined by X-ray powder diffraction. SQUID magnetization data reveal high superconducting volume fraction and sharp transitions. These dense, textured, polycrystalline high temperature superconductors show high critical current values (determined magnetically) which approach those of single crystal and oriented thin films.

Experimental Study of Thermodiffusiophoresis By Use of a Thermal Diffusion Cloud Chamber

Abstract: Thermodiffusiophoresis and growth of dioctylphthalate droplets formed by nucleation in a thermal diffusion cloud chamber were studied with helium, argon, hydrogen, nitrogen, and methane as inert carrier gases. The experiments were carried out in a total pressure range of 1–50 kPa. Distributions of droplets in the chamber, as obtained from photographs of their tracks in a flat vertical laser beam, were compared with predictions by the theory developed by Annis and Mason in 1975. At higher pressures of inert gases the agreement with the theory is as expected. At lower pressures the theory predicts higher thermodiffusiophoretic effects than observed in experiments.

Inviscid-Viscous Interaction Analysis of Compressor Cascade Performance

Abstract: An inviscid-viscous interaction technique for the analysis of quasi-three-dimensional turbomachinery cascades has been developed. The inviscid flow is calculated using a time-marching, multiple-grid Euler analysis. An inverse, finite-difference viscous-layer analysis, which includes the wake, is employed so that boundary layer separation can be modeled. This analysis has been used to predict the performance of a transonic compressor cascade over the entire incidence range. The results of the numerical investigation in the form of cascade total pressure loss, exit gas angle, and blade pressure distributions are compared with existing experimental data and Navier–Stokes solutions for this cascade, and show that this inviscid-viscous interaction procedure is able to predict cascade loss and airfoil pressure distributions accurately. Several other aspects of the present interaction analysis are examined, including transition and wake modeling, through comparisons with data.

The low-temperature synthesis of YBa2Cu3O7−δ under reduced oxygen pressure

Abstract: Thermogravimetric data on the synthesis of YBa 2 Cu 3 O 7−δ below 800°C under reduced pressure are presented. It is demonstrated that the reaction pathway and rate are determined by the CO 2 , O 2 , and total pressure in the reaction vessel. The importance of these variables to the impurities generated during the synthesis is discussed.

Search for pressure dependence in the sensitivity of several common types of hot-cathode ionization gauges for total pressures down to 10-7 Pa

Abstract: Dependence of sensitivity on pressure in He, N2, and H2 has been investigated for a group of 16 hot‐cathode ionization gauges, representing both extractor and Bayard–Alpert types, for total pressure as low as 5×10−8 Pa. Absolute sensitivities were determined using a primary high vacuum standard. An independent method, that of measuring the sensitivity ratio of one gauge to another, was also employed. Within a scatter of about ±3%, ±4%, and ±4% respectively, the N2, He, and H2 sensitivity ratio data showed no clearly discernible pressure dependence down to total pressures as small as 5×10−8 Pa. The absolute sensitivity measurements in He and N2 over the range 10−7 to 10−3 Pa in total pressure, also were constant within a scatter of about ±4% and ±3%, respectively. As a consequence of drift in the background component of the total pressure, all the absolute sensitivity measurements at total pressures below 10−7 Pa exhibited an apparent pressure dependence not evident in the sensitivity ratio results. In the case of H2, drift in the pressure persisted at total pressures orders of magnitude larger than the background pressure, and for all the gauges led to an apparent difference of approximately 10% between the H2 sensitivities at 10−8 and 10−5 Pa. Results of further investigation suggest that the apparent pressure dependence in the sensitivities is an artifact produced by the well‐known phenomenon of thermal dissociation of H2 at hot filaments and associated processes of H2 pumping and production of other species such as CO and C2H4.

Low‐pressure chemical vapor deposition of low in situ phosphorus doped silicon thin films

Abstract: In situ low phosphorus doped silicon films are deposited onto glass substrates by low‐pressure chemical vapor deposition method. The deposition parameters, temperature, total pressure, and pure silane gas flow are, respectively, fixed at 550 °C, 0.08 Torr, and 50 sccm. The varying deposition parameter is phosphine/silane mole ratio; when this ratio varies from 2×10−6 to 4×10−4, the phosphorus concentration and the resistivity after annealing, respectively, vary from 2×1018 to 3×1020 atoms cm−3 and from 1.5 Ω cm to 2.5×10−3 Ω cm.

A thermodynamic and kinetic study of chemical vapor deposition of tungsten from WF6 and GeH4

Abstract: In this paper a thermodynamic and kinetic study of the deposition of tungsten on silicon (100) from tungsten hexafluoride (WF6) and germane (GeH4) is presented Thermodynamic calculations, as well as experiments with a closed reactor, indicate that the reaction occurring during deposition is WF6+3GeH4→W+3GeF2+6H2 The growth rate as a function of process parameters is obtained for depositions in the temperature range from 600 to 800 K and a total pressure range from 150 to 1000 mTorr Experiments show that the germane reduction of tungsten hexafluoride is of 09 order in WF6, −02 order in GeH4, and zero order in H2 The activation energy is 34 kJ/mol The deposition rate does not change when SiH4 is added to the GeH4/WF6 mixture, while, on the contrary, a small amount of GeH4 reduces the growth rate from a SiH4/WF6 mixture considerably The kinetic data indicate that the formation of GeF2 might be the rate‐limiting step

Phosphorus Doping of Silicon Using a Solid Planar Diffusion Source at Reduced Pressures

Abstract: A silicon pyrophosphate-containing (SiP 2 O 7 ) solid planar diffusion source has been evaluated for phosphorus doping of silicon at reduced pressures. A reduction in pressure generally results in improvements in the uniformity of doping. At the edges of 3 in. silicon wafers diffused at 1000°C, maximum deviations in sheet resistance are reduced from approximately 15 to approximately 5%, if total pressure is reduced from 1 atm to 5 torr. This improvement is even more significant for diffusion at 900°C, where the maximum deviation is reduced from approximately 100% to approximately 10% by the same reduction in total pressure

Probe shapes for streamwise momentum and cross-stream turbulence intensity

Abstract: When the highly turbulent flowfields at the edges of jets, in augmentors, and in other jet-mixing devices are surveyed with conventional pitot probes, the values indicated by the instruments may contain a significant increment brought about by the dynamics of the eddies. Although the influence of turbulence on the measurements is usually negligible in streams where the turbulence level is 1 percent or less, the effect of turbulence on static and total pressure measurements can be around 20 percent when the turbulence level exceeds 40 percent. This paper describes a theoretical study that develops probe shapes that directly measure the time-averaged total pressure based on the streamwise component of the velocity vector to obtain a direct measurement of the streamwise momentum. The difference between the time-averaged pressure indicated by such a probe and one that measures the total head based on the entire velocity vector yields the cross-stream turbulence intensity.

The Effect of Total Pressure on the Oxidative Coupling of Methane Reaction under Cofeed Conditions

Abstract: The effect of total pressure on the catalytic properties of an oxidative coupling of methane (OCM) catalyst, strontium doped lanthanum oxycarbonate, has been studied using nondiluted methane-oxygen mixtures at pressures of up to 7.5 bar. The results indicate that increased pressure has a negative effect on both methane conversion and C 2+ selectivity. The C 2+ yield falls from 9.9 % at atmospheric pressure to 3.4 % at 7.5 bar when operating at constant flow rate and using a charge containing 9 % oxygen. Increasing both the pressure and the flow rate by a factor of 7.5 does not significantly improve the results. However, the negative pressure influence on the OCM reaction can be minimized by appreciably increasing the linear space velocity for pressures up to 3 bar. Under these conditions, the yield to C 2+ products increased to 10.3 %. The possibility of operating under pressures of up to 3 bar represents a significant advantage in terms of reactor size and down stream treatment of product gases.

Application of three-dimensional viscous analysis to turbofan forcedmixers

Abstract: A combined analytical and experimental study was conducted to establish an analytical procedure for optimizing the design of forced mixers for application to high bypass ratio turbofan engines. This two part study involved the application of a fully three-dimensional viscous procedure to a series of mixer configurations, followed by an experimental program to validate the analytical procedure. The viscous analysis used for this study involved combining two robust and versatile codes: PARC-3D, a flow solver, with INGRID-3D grid generation code. Comparison between the Navier-Stokes solution and detailed experimental measurements were made for five different mixer configuration. Both global performance parameters and detailed temperature distributions were compared. Predicted results were found to be in excellent agreement with test data. Gross thrust coefficient, defined in Section 4 Specific heat at constant pressure Thrust coefficient, defined in Section4 Diameter of mixing duct at lobe exit plane Gross thrust Length of mixing duct b s s flow rate Pressure Total pressure Radius of shroud Total temperature Velocity Radial distance measured from center body surf ace