Gas Flow Geometry Research Articles

Microcrystalline thin-film solar cell deposition on moving substrates using a linear VHF-PECVD reactor and a cross-flow geometry

A concept for high-rate plasma deposition (PECVD) of hydrogenated microcrystalline silicon on moving substrates (dynamic deposition) is developed and evaluated. The chamber allows for substrates up to a size of 40 × 40 cm2. The deposition plasma is sustained between linear VHF electrodes (60 MHz) and a moving substrate. Due to the gas flow geometry and the high degree of source gas depletion, from the carrier's point of view the silane concentration varies when passing the electrodes. This is known to lead to different growth conditions which can induce transitions from microcrystalline to amorphous growth. The effect of different silane concentrations is simulated at a standard RF showerhead electrode by intentionally varying the silane concentration during deposition in static mode. This variation may decrease the layer quality of microcrystalline silicon, due to a shift of the crystallinity away from the optimum. However, adapting the input silane concentration, state-of-the-art solar cells are obtained. Microcrystalline cells (ZnO : Al/Ag back contacts) produced by the linear VHF plasma sources show an efficiency of 7.9% and 6.6% for depositions in static and dynamic mode, respectively.

Effect of the gas flow geometry and turbulence on the hydrodynamic drag of a body in the flow

The effect of the incoming flow geometry on the hydrodynamic drag of a body is investigated in a numerical experiment simulating a free gas flow past a sphere as well as flows in cylindrical tubes of various radii, in a confuser, and a diffuser. The results of calculations lead to the conclusion that the confinement of the flow by the tube walls, its contraction and expansion may change the hydrodynamic force and the drag acting on the body insignificantly (not more than by 30%). This cannot explain the early drag crisis, in which the values of these quantities decrease by 4–7 times for Reynolds numbers on the order of 100. This phenomenon is explained theoretically by the effect of strong turbulence of the incoming flow to the body.

Structured catalytic substrates with radial configurations for the intensification of the WGS stage in H 2 production

Weight and size reduction of fuel processing reactors is one of the key issues in the development of catalytic H 2 production and purification systems for distributed power plants. The search for innovative solutions is focused on the development of both catalyst formulations and geometries which could help to achieve an effective process intensification. Precious metals based structured catalysts in form of monolith or foam prepared from ceramic or metallic substrates could be a viable option to overcome heat transfer limitations, which typically characterize these systems. They allow high efficiency of fuel conversion even at high GHSV, and better stability and durability, a critical issue for industrial WGS catalysts. In this work we report on the development of an integrated kW-scale system, CH 4 fuelled, for H 2 production arranged with an autothermal reforming (ATR) and a WGS stage. Both reactors operated with noble metals based structured catalysts. Since in an adiabatic WGS reactor the reaction rates are not optimized throughout the reactor we employ noble metals foam structured catalysts assembled in such a way to perform inside the reactor axial or radial gas flow geometry. The aim of the work is to verify the influence of flow geometry on the reformer performance.

Transparent and semitransparent conducting film deposition by reactive-environment, hollow cathode sputtering

Highly transparent and conductive In2O3 and ZnO films containing different doping elements such as Ti, Mo, Zr, Nb, Ta, W (for In2O3), and B (for ZnO) have been prepared by reactive-environment, hollow cathode sputtering (RE-HCS). The use of Nb and W as effective dopants is reported for the first time. Metallic targets were used exclusively, and the dopant concentration was easily controlled using a second sputtering power supply. As a result of the cathode and gas flow geometry, the sputtering is conducted in metal mode, and the target and doping materials are free from oxidation during the deposition process. Film resistivities achieved with the various dopants are reported. For In2O3:Mo (IMO), a resistivity of 1.6×10−4Ωcm and a mobility of 80cm2∕Vs were achieved for Mo concentrations in the range 0.5–5.0% as measured by inductively coupled plasma (ICP). X-ray photoelectron spectroscopy (XPS) analysis indicates Mo with a +6 valence state and that the film is stoichiometric. For In2O3:Ti (ITiO), a superior optical transmission is achieved relative to IMO, while carrier mobility and conductivity were similar. Remarkably, semitransparent films of InN:O having sheet resistances of 9.5Ω∕square have also been prepared. ZnO:B films deposited by RE-HCS exhibit superior optical properties relative to ZnO:Al, and when applied as a window layer to CIGS solar cells yield higher quantum efficiencies.

Numerical analyses of the internal conditions of a molten carbonate fuel cell stack: comparison of stack performances for various gas flow types

A three-dimensional numerical analysis model of cell voltage, temperature and current profile in a molten carbonate fuel cell (MCFC) stack has been developed. `The Formula for MCFC Performance' that was derived from the test results of a single cell having the same active components as the stack has been applied to the calculation of electrochemical performance. Calculation result of cell voltages and temperature profile showed good agreement with 10–100 kW class stack experimental data. Using this model, cell voltages, temperature profile and net output powers (MCFC output power minus required cathode recycling power) of five gas flow geometries have been compared. The calculation results showed that the net output power is highest in the co-flow geometry. From the viewpoint of a stack performance, a cooling power, and a control of maximum temperature, the co-flow type stack has advantages over the external reforming type MCFC power plant.

Heat flow modeling of chemical vapor deposition of tungsten for improved film thickness uniformity

The boundary element method is used to model the heat flow in a tungsten chemical vapor deposition (CVD W) reactor to determine the origin of the tungsten film thickness variation across a 125 mmp-type silicon wafer. The typical thickness nonuniformity is 7-10%. The model predicts temperature variations in the graphite susceptor which heats the wafers. The resulting temperature difference between the center and edge of a wafer can be up to 20 °C. Deposition rate experiments in a CVD W reactor confirm this. Experiments involving changes in the gas flow geometry have been conducted. It is concluded that the temperature gradient across the wafer, not the gas flow geometry, is the dominant factor controlling film thickness uniformity for this CVD W reactor.

Problems in Large Size Amorphous Silicon Plate Manufacturing

ABSTRACTThe growing aSi:H based component industry needs large PECVD machines for high throughput coating of flat substrates up to 1 meter in size. Deposition uniformity can be affected by gas depletion in diffusion and laminar gas flow geometries. Deposition rate is shown to be in conflict with large size uniformity. Main uniformity problems are found to originate from uneven RF voltage distribution, the effect is shown to be amplified near plasma equilibrium transitions. Contamination, a key factor for production quality, is controlled by the hot pressurized plasma box by both reducing unwanted background gas near UHV level and by allowing systematic fast plasma cleaning without machine corrosion or post contamination. Powder formation and dynamic is discussed based on several novel experiments and powder formation threshold is shown to be pushed away if the plasma is made intermittent.

The Stellar X-Ray Polarimeter for the Spectrum-X-Gamma Mission

AbstractWe describe an X-ray polarimeter which will be flown on the SPECTRUM-X-Gamma mission. The instrument exploits three distinct physical processes to measure polarization: Bragg reflection from a graphite crystal, Thomson scattering from a metallic lithium target, and photoemission from a Cesium Iodide photocathode. These three processes allow polarization measurements over an energy band of 0.3 keV to 12 keV. The polarimeter will make possible sensitive measurements of several hundred known X-ray sources. X-ray polarization measurements will allow us to constrain the geometry of gas flow in X-ray binaries, identify nonthermal emission in supernova remnants, test current models for X-ray emission in radio pulsars, determine the radiation mechanisms in active galactic nuclei, and search for inertial frame dragging (Lense-Thirring effect) around the putative black hole in Cygnus X-1.

Stellar X-Ray Polarimeter: a focal plane polarimeter for the Spectrum X-Gamma mission

This paper describes an X-ray polarimeter that will be flown on the Spectrum X-Gamma mission. The instrument exploits three distinct physical processes to measure polarization: Bragg reflection from a graphite crystal, Thomson scattering from a metallic lithium target, and photoemission from a cesium iodide photocathode. These three methods allow polarization measurements over an energy band from 0.3 to 12 keV. The polarimeter will make possible sensitive measurements of several hundred known X-ray sources. X-ray polarization measurements will make it possible to constrain the geometry of gas flow in X-ray binaries, identify nonthermal emission in supernova remnants, test current models for X-ray emission in radio pulsars, determine the radiation mechanisms in active galactic nuclei, and search for inertial frame dragging (Lense-Thirring effect) around the putative black hole in Cygnus X-1.

Technical note: Comparison of film density, stoichiometry, optical and electrical properties of thin metal oxide films produced by reactive d.c. magnetron sputtering and electron beam evaporation techniques

Multilayer thin metal oxide films are used in the fabrication of some electro-optic devices. Many of these devices are produced by reactively sputtering metals or by evaporating the metal oxide. To design and engineer such devices, a thorough knowledge of the physical properties of these films is essential. One can then relate these properties to the conditions under which the films were made. Critical parameters in the fabrication process are then identified and the process is not only better controlled but can also be optimized for the end product required. In this study, film thicknesses varied from 20 nm to 360 nm and were determined by a surface profiling technique. The film density and stoichiometry were then determined by Rutherford backscattering measurements. The reactively sputtered metal oxide films were made between base argon pressures of 2.5 and 20 mTorr, oxygen partial pressures of 1% and 65% of the total pressure and direct current magnetron power levels of 2 and 6 kW. Significant enhancements to deposition rates and variation in physical properties were accomplished with non-standard gas flow geometries. Comparable metal oxide films were made with electron beam evaporation. Optical properties of the films were determined from transmission and reflection measurements from 380 nm to 780 nm. The index of refraction and absorption as a function of wavelength were deduced from these data. The index of refraction exhibits a surprising, significant and changing structure with deposition parameters and film thickness. The resistivities of the sputtered films vary greatly with process parameters. The values of film resistivity and index of refraction can be altered in a controlled manner by subsequent in-line substrate heating in an enriched O2 atmosphere. TaxOy films will be used as typical examples to illustrate the above phenomena.

Strong pressure waves in air-breathing engines

Fuel ingestion, damage from foreign objects, or a simple high-pressure compressor stall can cause strong pressure waves in air-breathing engines. These waves travel at sonic and supersonic velocities throughout all engine compartments, where they may cause structural damage. A computer program for unsteady compressible gas flow has been used to analyze and evaluate the transient flow phenomena caused by these waves. Starting with a steady-state gas flow input defining nominal pressures, velocities, and temperatures, a pressure pulse simulating an explosion is placed at some specified location. The program then computes the variations of all the flow parameters as a function of time at every point throughout the engine. The program is modularly constructed and can simulate any engine gas flow geometry by means of judicious numbering and connection of the various modules. The program has been successfully used to assess the severity of high-pressure waves in numerous cases and has correlated very well with experimental data.

Effect of carrier gas flow geometry on the response of sulfur flame photometric detectors for gas chromatography

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTEffect of carrier gas flow geometry on the response of sulfur flame photometric detectors for gas chromatographySten Aake. Fredriksson and Anders. CedergrenCite this: Anal. Chem. 1981, 53, 4, 614–618Publication Date (Print):April 1, 1981Publication History Published online1 May 2002Published inissue 1 April 1981https://doi.org/10.1021/ac00227a013RIGHTS & PERMISSIONSArticle Views56Altmetric-Citations19LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (666 KB) Get e-Alerts Get e-Alerts