Volume Fraction Of Nitrogen Research Articles

Distribution of the electrical resistivity of a n-type 4H-SiC crystal

Nitrogen is commonly doped to obtain n-type 4H-SiC crystals, which have been commercialized for the development of power electronics in recent years. Now the uniformity of electrical resistivity becomes an important issue for the growth of n-type 4H-SiC crystals. In this work, the effects of the radial thermal gradient and growth facet on the distribution of the electrical resistivity were investigated for a n-type 4H-SiC crystal with a diameter of 150 mm during its physical-vapor-transport growth. It is found that the resistivity at the center of the crystal is low in the early growth stage. As the crystal grows, the growth facet expands, accompanied by a reduction in the resistivity of this facet. The change in the distribution of the resistivity in the crystal is initially governed by the radial thermal gradient and then influenced by the spiral growth mode of the growth facet. In the non-facet region of the crystal step-flow growth occurs, where the doping of nitrogen is primarily affected by the temperature and C/Si ratio. In the facet region, the volume fraction of nitrogen in the mixture of argon and nitrogen input into the growth system mainly governs the doping of nitrogen during the spiral growth. The insights gained in the current work may help the fabrication of n-type 4H-SiC crystals with uniform resistivity.

Simulation and Control Strategy Study of the Hydrogen Supply System of a Fuel Cell Engine

The hydrogen supply system is one of the important components of a hydrogen fuel cell engine, and its performance has an important impact on the economy and power of the engine system. In this paper, a hydrogen supply system based on cyclic mode is designed for a hydrogen fuel cell stack with a full load power of 150 kW, and the corresponding hydrogen fuel cell engine simulation model is built and validated. The control strategy of the fuel cell hydrogen supply system is developed, and its effect is verified through bench tests. The results show that the developed control strategy can keep the volume fraction of nitrogen below 6%, the hydrogen excess ratio does not exceed 1.5 under medium and high operating conditions, the anode pressure is relatively stable, and the stack can operate efficiently and reliably.

Influence of Mixture Composition on Electrophysical Parameters and Emission Spectra of Tetrafluoromethane and Trifluoromethane Plasma with Nitrogen

The influence of the mixture composition on the electrophysical parameters and plasma emission spectra of mixtures of tetrafluoromethane and trifluoromethane with nitrogen under dc glow discharge conditions has been analyzed. It has been established that the reduced electric field strength changes nonlinearly with an increase in the proportion of the second gas in both mixtures. The reduced intensity of atomic fluorine emission in the CF4/N2 mixture is shown to pass through a maximum at a nitrogen volume fraction of 0.2, whereas a monotonic change in this dependence is observed for the CHF3/N2 mixture. The reduced emission intensities of CF2 radicals decrease monotonically with an increase in the nitrogen fraction in both mixtures.

Research on the Mixing Mechanism and Parameter Optimization of Liquid Nitrogen Foam Generator

Nitrogen foam is expected to be an effective method to extinguish large-scale fires and suppress explosions. The key to its foaming process is that gas nitrogen (N2) and a foam solution are mixed uniformly in a foam generator. Moreover, liquid nitrogen (LN2) has been proposed as a source of gas nitrogen to generate nitrogen foam in previous experimental works. In this paper, the flow and heat transfer characteristics between liquid nitrogen and the foam solution are explored by numerical methods, which are then utilized to optimize the parameters of the foam generator. It is found that the flow pattern of the foam generator with a cone spoiler is a stratified flow by establishing the mixture and Lee’s evaporation-condensation model in ANSYS Fluent. Moreover, the spoiler in the foam generator plays a crucial role in breaking LN2 into droplets and increasing the contact area between phases, and a distance of 10 mm from the inlets to the spoiler is recommended. From previous results, an unreasonable flow rate ratio of LN2 to a foam solution may lead to icing or a low volume fraction of nitrogen, thus a flow rate ratio of 1:50 is determined in the foam generator. As for the shape of the spoiler, the spiral spoiler shows the best foaming performance compared to cone and semisphere spoilers, due to its fluid rotation instead of translation, which effectively increases the N2 volume fraction of foam from 0.616 to 0.717. Therefore, the mixing characteristics of the foam generator in this work lays a foundation for devising a practical nitrogen foam generator.

Improvement on pitting wear resistance of gears by controlled forging and plasma nitriding

In the present investigation, the use of a bainitic steel for forged gears applications and the suitability of different plasma nitriding treatments is discussed. The gears were forged, machined, ground, polished, and nitrided at 500 °C with three sets of N2–H2 gas mixtures, containing 5, 24, and 76 vol.% N2, to develop a nitrided case of approximately 300 μm depth. Gears were characterized before and after the nitriding concerning the phase composition, microstructure, microhardness, fracture toughness, and residual stresses states. Pitting wear tests were performed on a standard Forschungsstelle für Zahnräder und Getriebebau (FZG) test rig. The nitrided gears with 24 and 76 vol.% N2 formed a biphasic compound layer of ε-Fe2-3(C)N and γ′-Fe4N. As the volume fraction of nitrogen in the gas mixture was decreased, the detected content of γ′-Fe4N in the compound layer increased, but a monophasic compound layer was only reached with 5 vol.% N2. The nitrogen rich gas composition increased the surface hardness and decreased the fracture toughness of the compound layer, as they have more ε-Fe2-3(C)N. The diffusion zone of the different nitrided surfaces showed residual compressive stresses. The best performance was obtained in the nitrided gears with 24 vol.% N2, due to the better combination between the surface hardness, fracture toughness, residual stresses, and compound layer thickness. The nitrided gears with 24 vol.% N2 have ten times improvement over the non-nitrided gears, while the nitrided gears with 5 and 76 vol.% N2 have an improvement of three point seven and five point four times.

Transparency Effect in Zr−B−N Coatings Obtained by Magnetron Sputtering of ZrB2 Target

Thin fims in the Zr−B−N system possessing optical transparency have been studied. It is established that increase in the volume fraction of nitrogen in the gas mixture and, hence, in the concentration of nitrogen in the coatings leads to an increase in their optical transmission due to the formation of a BN phase.

In Situ Doping of Nitrogen in -Oriented Bulk 3C-SiC by Halide Laser Chemical Vapour Deposition.

Doping of nitrogen is a promising approach to improve the electrical conductivity of 3C-SiC and allow its application in various fields. N-doped, <110>-oriented 3C-SiC bulks with different doping concentrations were prepared via halide laser chemical vapour deposition (HLCVD) using tetrachlorosilane (SiCl4) and methane (CH4) as precursors, along with nitrogen (N2) as a dopant. We investigated the effect of the volume fraction of nitrogen (ϕN2) on the preferred orientation, microstructure, electrical conductivity (σ), deposition rate (Rdep), and optical transmittance. The preference of 3C-SiC for the <110> orientation increased with increasing ϕN2. The σ value of the N-doped 3C-SiC bulk substrates first increased and then decreased with increasing ϕN2, reaching a maximum value of 7.4 × 102 S/m at ϕN2 = 20%. Rdep showed its highest value (3000 μm/h) for the undoped sample and decreased with increasing ϕN2, reaching 1437 μm/h at ϕN2 = 30%. The transmittance of the N-doped 3C-SiC bulks decreased with ϕN2 and showed a declining trend at wavelengths longer than 1000 nm. Compared with the previously prepared <111>-oriented N-doped 3C-SiC, the high-speed preparation of <110>-oriented N-doped 3C-SiC bulks further broadens its application field.

The purification of hydrogen isotopes expelled from nuclear fusion reactors by a combined process

Hydrogen isotope gases expelled from nuclear fusion reactors can be purified by a process combining catalytic deoxygenation and cryogenic adsorption (77.4 K). In the present work, a novel purification process design is proposed. The experimental result shows that the oxygen volume fraction of the purified hydrogen isotopes is below 0.1×10−6 with the nitrogen volume fraction less than 1×10−6. And it was discovered that the waste amount of hydrogen isotopes after purification is only 100–300 ppm which occupies just 10–30% of the amount of nitrogen impurity through the theory calculation. Therefore, the impurity content of hydrogen isotopes is removed sufficiently by the combined process and the proposed process for purification of the exhaust gases expelled from nuclear fusion reactors can be used as an alternative to the traditional method.

Optimization schemes extraction of inert and acid gases from a trade flow

To increase the efficiency of gas-lift method of gas condensate and oil recovery nitrogen canbe used as a natural gas substituting agent. It should be noted at problem actuality because at“mature” gas-condensate and oil fields high-pressure wells drop in pressure with duration of fieldlife or there is no sources of high-pressure natural and associated gases at all (eg, oil fields).When using nitrogen gas as an operating agent for gas-lift operation the possibility ofincreasing the volume fraction of nitrogen in natural gas composition grows to maximum at theproduction wells wellhead. In these cases it should further pursue the collection and systematizationof natural gas physical and chemical parameters data at the outlet of the gas producing facilities asthe results of necessity of complying with the qualitative indicators for commercial gas transmittedto pipelines. Operator needs to predict the maximum concentration of nitrogen in the gascomposition after the gas-lift operation for individual wells and define the optimal nitrogen contentand net calorific value of marketable gas feeding to customers. For gathering systems with maintransported product: natural gas artificially enriched with nitrogen for technological operations of&nbsp;&nbsp; improved condensate or oil recovery, the authors propose to apply a centralized scheme for gasgathering during production instead of gas gathering at group gas-oil separation plant.In the article the basic aspects of such schemes, construction of production and processingequipment for nitrogen removing are represented. The algorithm of nitrogen content calculation,optimum capacity of nitrogen units are created and prospects for their further use are shown.The authors argue that the optimization of gas gathering schemes in the fields ofcentralization of equipment will save investment and operating costs.

Surface Potential Behavior of Direct-Fluorinated PPLP Under Quench Condition

With the emergence of superconductor technology, high-temperature superconducting (HTS) cable is receiving considerable attention for application in power systems. Polypropylene laminated paper (PPLP) and liquid nitrogen (LN 2 ) have been widely used as insulation and coolant, respectively, in HTS cable systems, where the insulation is threatened by surface charge and quench. Direct fluorination is an effective method to improve the electrical properties on the material surface. To restrain the surface charge accumulation and prolong the insulation life, the surface potential behaviors of fluorinated PPLP under the quench condition has been studied to clarify the effects of fluorination time and low temperature. PPLP specimens were fluorinated in a reaction kettle using a F 2 /N 2 blend with an 80% volume fraction of nitrogen and pressure of 0.1 MPa for respectively 5, 10, and 15 min. Corona charging was performed with dc voltage at a relative humidity of ~15% in atmospheric air on room temperature specimens and after the specimens had been cooled by LN 2 for 20 s before the corona charging. Then, the surface potential was measured with an electrostatic voltmeter. The results indicate that the accumulation and decay behaviors of the surface potential for PPLP are affected by the charge polarity, specimen temperature, and fluorination time.

Mechanism of pressure management by injecting nitrogen in casing annulus of deepwater wells

The influence factors of annular pressure buildup are analyzed, and the temperature and pressure characteristics of annulus trap medium are simulated for deepwater wells. The method of pressure management by injecting nitrogen in casing annulus is proposed based the experiment result. Limited by the well structure and subsea system, long casing annulus exists between technical casing and production casing and it is filled with water-based, synthetic based or oil-based drilling fluid. In the process of oil/gas test and production, the temperature of the trapped fluid rises significantly under the influence of the well-bore fluid and the trap pressure buildup appears because of liquid heat expansion. Experiments show that isothermal compressibility coefficient and thermal expansion coefficient are the key influencing factors for annulus pressure buildup. Trap pressure is very sensitive to the type of trapped medium (liquid, gas). Injecting 5%−20% volume fraction of nitrogen into the annulus can effectively control the annulus pressure build-up, and avoid casing collapse. Field practice shows that the method, convenient and highly reliable, can ensure the borehole safety during testing and production of deepwater oil and gas.

Numerical Investigation of Gas Mixture Length of Nitrogen Replacement in Large-Diameter Natural Gas Pipeline without Isolator

Nitrogen replacement is a key process for natural gas pipeline before it is put into operation. A computational fluid dynamic model coupled to a species-transportation model has been used to investigate the gas mixture length of nitrogen replacement in large-diameter pipeline without isolator. A series of numerical simulations are performed over a range of conditions, including pipe length and diameter, inlet rate, and inclination angle of pipe. These affecting factors are analyzed in detail in terms of volume fraction of nitrogen, the maximum gas mixture length, and gas mixture length varied with time. Gas mixture length increases over time, and the maximum gas mixture length is present at outlet of pipe. Long and large-diameter pipe and fast speed of nitrogen lead to long length of mixed gas, while large inclination angle of pipe brings about short length. Several fitting formulas have been obtained, which can predict the maximum gas mixture length in gas pipelines. The used method of fitting formula is shown in the paper by examples. The results provide effective guidance for practical operation of nitrogen replacement.

Measuring the laminar burning velocity and Markstein length of premixed methane/nitrogen/air mixtures with the consideration of nonlinear stretch effects

There exists a vast amount of methane-based combustible gases in the earth, such as natural gas, coalbed gas and shale gas. Usually there are non-combustible gases such as nitrogen in them and their combustion properties vary with composition. The knowledge on burning velocity and flame stability of those gaseous fuels is in great demand in order to achieve their efficient usage in industry and transportation. When measuring flame speed of an outwardly propagating spherical flame, the conventional analysis method assumes a linear relationship between flame speed and stretch rate. However, recent studies showed that the linear assumption could bring in errors when calculating unstretched flame speeds and Markstein lengths. In this study, we investigate laminar flame of methane/nitrogen and air mixtures by using a constant volume combustion chamber together with high-speed schlieren photography system. The experimental data have been analyzed by using both conventional method and nonlinear method that considers the effect of the stretch nonlinearly. Comparison shows that different methods do not affect flame speed results greatly, but have significant impact on Markstein length. Therefore nonlinear stretch effect should be considered. In this paper, unstretched laminar burning velocity and Markstein length of nitrogen diluted methane obtained by using nonlinear method are reported. The data covers a wide range of nitrogen diluted methane fuels whose nitrogen volume fraction varying from 0% to 50% at an interval of 10%. The effects of nitrogen concentration on laminar burning velocity and flame stability are then studied at different equivalence ratios (0.6–1.2). A correlation has been summarized for laminar burning velocity of methane/nitrogen and air mixtures obtained by using nonlinear analysis in this study.

Investigation of combustion characteristics and emissions in SI engine fuelled with LHV gas

Abstract In this study, an experimental research on the combustion and emission characteristics of a spark ignition engine fuelled with a blended gas was performed at different engine load operations. The blended gas, which was composed by the compressed natural gas and nitrogen, was lower heating value gas. In the experiments, the parameters of combustion characteristics were analysed under 0-35% nitrogen volume fraction in the blends. The results show that the maximum cylinder pressure, the maximum rate of pressure rise and the maximum rate of heat release decrease with increasing nitrogen fraction. The flame development duration and the main combustion duration increase with the higher level of the nitrogen fraction. Meanwhile, the HC, CO emission increase and the NOx decreases remarkably with the increase in the nitrogen volume fraction.

Investigation into secondary phases in steels microalloyed with vanadium and nitrogen

In the present work two nitrogen and vanadium microalloyed steels were compared in terms of the nitride or carbonitride phases present. The thermal evolved gas analysis clearly showed that one of the steels contains a higher volume fraction of nitrogen bound as vanadium carbonitride than the other steel in a natural relation to the total nitrogen content in steels. For the promotion of this statement, the content of free nitrogen with the use of the hot hydrogen extraction method was analysed and the thermodynamic prediction of the equilibrium composition of precipitating phases in microalloyed steels by the CALPHAD method with the use of the Thermo-Calc software was carried out.

Effect of buoyancy on the radiative extinction limit of low-strain-rate nonpremixed methane–air flames

The structure and extinction of nonpremixed flames were investigated through comparison of experiments and calculations using a counterflow configuration. Experiments were conducted at the NASA Glenn Research Center's 2.2-s drop tower to attain suppression and temperature measurements in low-strain nonpremixed methane–air microgravity flames. Suppression measurements using nitrogen added to the fuel stream were performed for global strain rates from 7 to 50 s −1. Judicious hardware selection and an optimized experimental procedure facilitated rapid, controllable, and repeatable flame extinction measurements. The minimum nitrogen volume fraction in the fuel stream needed to ensure suppression for all strain rates in microgravity was measured to be 0.855 ± 0.016 , associated with the turning point, which occurred at a global strain rate of 15 s −1. This value was higher than the analogous value in normal gravity. Flame temperature measurements were attained in the high-temperature region of the flame ( T > 1200 K ) using visible emission from a SiC filament positioned axially along the burner centerline. The suppression and temperature measurements were used to validate a two-dimensional flame simulation developed here, which included buoyancy effects and finite-rate kinetics. The simulations yielded insight into the differences between microgravity and normal gravity suppression results and also explained the inadequacy of the one-dimensional model results to explain the microgravity suppression results.

Grey theory applied to evaluate the tribological performances of the a-C:H(N) coating films prepared by differing the nitrogen content and the film thickness

The purpose of this paper is to study the tribology performances of the aC:H(N) films by using a nanotester under different scratch loads and velocities. From the measurements of the friction coefficient and wear volume, the tribological performances including wear resistance and friction coefficients were evaluated for the hydrogenated amorphous carbon films prepared by differing film thickness and nitrogen volume friction in the gas mixture of (C2H2+N2). Taguchi experimental design and the grey relational analysis were used to investigate the influence of specimen parameters (film’s thickness, nitrogen content in the film), and operating conditions in tribological tests (scratch load and scratch velocity) on the friction coefficients and the wear volume arising in the specimens with different coating films. It is found that the wear volume of thin film is increased by increasing either the nitrogen volume fraction or film thickness. Moreover, the optimal combination of the testing parameters was also determined in the use of the present model.

Influence of welding parameters on nitrogen content in welding metal of 32Mn-7Cr-1Mo-0.3N austenitic steel

The transfer behavior of nitrogen into the welding metal during gas tungsten arc welding process of 32Mn-7Cr-1Mo-0.3N steel was investigated. The effects of gas tungsten arc welding process variables, such as the volume fraction of nitrogen in shielding gas, arc holding time and arc current on the nitrogen content in the welding metal were also evaluated. The results show that the volume fraction of nitrogen in gas mixture plays a major role in controlling the nitrogen content in the welding metal. It seems that there exhibits a maximum nitrogen content depending on the arc current and arc holding time. The optimum volume fraction of nitrogen in shielding gas is 4% or so. The role of gas tungsten arc welding processing parameters in controlling the transfer of nitrogen is further confirmed by the experimental results of gas tungsten arc welding process with feeding metal.

The effects of hydrogenated carbon films with different film thickness and nitrogen content on specimen mechanical properties, scratch critical load, adhesion work and tribological behavior

The contact mechanics developed for a sphere in contact with a flat plane is employed in this study to obtain the stress functions distributing in the elastic and plastic-deformation regions formed in a specimen with a hydrogenated carbon film. These stress functions are then used in the evaluation of the adhesion work of the specimen. All coating films using a silicon wafer as the substrate were prepared by differing the film thickness and the volume fraction of nitrogen in the gas mixture of (N 2+C 2H 2) during the deposition process. The critical load of a specimen with a coating film can be determined from a scratch test on a nanotester. The proportional relationship of the adhesion work and the critical load is found to be valid only for the specimens having the same film thickness and the same nitrogen content in the coating film. Nanoindentation tests have been carried out on a nanotester to obtain the Young's modulus of all specimens. The wear volumes of all specimens were also obtained from the nanoscratch tests under various operating conditions. The wear volume results are then tried to establish their connection with the parameters including adhesion work, Young's modulus and operating conditions. The behavior analysis indicates that the adhesion work of a specimen is not the unique factor to the wear volume; the mechanical properties of a specimen and the operating conditions are also involved as dominant factors. The elevation in either the adhesion work or the Young's modulus is helpful for the reduction of the wear volume.

Effect of nitrogen content on mechanical properties and tribological behaviors of hydrogenated amorphous carbon films prepared by ion beam assisted chemical vapor deposition

In the present study, hydrogenated amorphous carbon (a-C:H) films with different atomic concentrations of nitrogen were prepared by differing the volume fraction of the nitrogen in the (N 2+C 2H 2) gas mixture during the deposition process. The characteristics of the coating films exhibited in terms of the chemical bond and bonding style, the surface topography, the material mechanical properties and the tribological behaviors were evaluated by a variety of experiments in order to examine the effect of the nitrogen content in the coating films. The experimental results indicate that the increase on the volume fraction of nitrogen in the gas mixture of (N 2+C 2H 2) would decrease the index of refraction but increase the extinction coefficient of the coating film. We found that the addition of nitrogen to the coating film lowers the nanohardness, irrespective of the content. A film with a relatively small internal stress can be produced by increasing the volume fraction of nitrogen in the gas mixture. The critical load for the coating layer to spall from the substrate is lower in a specimen having a high average internal stress. The endurance time in the friction and wear test is increased in the film that has a high nitrogen content.