Reactive Gas Flow Research Articles

Electrical Characterization of Hydrogenated Boron Carbon Nitride Thin Films at Cryogenic Temperatures

Compounds in boron-carbon-nitrogen ternary triangle such as diamond (C), cubic boron nitride (c-BN), and boron carbide (B4C) have gained attention due to their covalent bonding, short bond lengths, low atomic mass[1]. These materials exhibit low dielectric values along with excellent mechanical and thermal strength. Boron carbon nitride (BCN) compounds are known to combine unique properties of boron carbide and boron nitride, with the properties being tunable with varying elemental composition and structure. These outstanding properties have made BCN attractive and finds multi-functional applications in anti-wear and protective coatings, supercapacitors, UV detectors, nanobiotechnology and nanomedicine[2]. BCN thin films also find opto-electronic applications in the harsh environments. While addition of small amounts of hydrogen has been proven beneficial for electrical and optical properties in BCN films, the electrical behavior of hydrogenated BCN films at low temperatures is not evaluated. Such evaluations will further expand the use of of hydrogenated BCN for extreme temperature applications.In this study, BCN thin films are deposited using RF magnetron sputtering with varying hydrogen/nitrogen gas flows. Substrate temperature is kept constant at room temperature. Dual target sputtering from B4C and BN targets is performed to deposit BCN thin films. I-V characterization is performed from room temperature to 70 K. The variation in resistivity with reactive gas flow rate at low temperature is reported.

3D Printed Scaffolds for Monolithic Aerogel Photocatalysts with Complex Geometries.

Monolithic aerogels composed of crystalline nanoparticles enable photocatalysis in three dimensions, but they suffer from low mechanical stability and it is difficult to produce them with complex geometries. Here, an approach to control the geometry of the photocatalysts to optimize their photocatalytic performance by introducing carefully designed 3D printed polymeric scaffolds into the aerogel monoliths is reported. This allows to systematically study and improve fundamental parameters in gas phase photocatalysis, such as the gas flow through and the ultraviolet light penetration into the aerogel and to customize its geometric shape to a continuous gas flow reactor. Using photocatalytic methanol reforming as a model reaction, it is shown that the optimization of these parameters leads to an increase of the hydrogen production rate by a factor of three from 400to 1200µmol g-1 h-1 . The rigid scaffolds also enhance the mechanical stability of the aerogels, lowering the number of rejects during synthesis and facilitating handling during operation. The combination of nanoparticle-based aerogels with 3D printed polymeric scaffolds opens up new opportunities to tailor the geometry of the photocatalysts for the photocatalytic reaction and for the reactor to maximize overall performance without necessarily changing the material composition.

Numerical modeling of subsonic axisymmetric reacting gas flows

A numerical algorithm is developed and implemented for modelling axisymmetric subsonic reacting gas flows based on a previously created program for plane flows. The system of Navier-Stokes equations in the low Mach number limit is used as a mathematical model. Calculations of ethane pyrolysis for axisymmetric and plane flow of mixture at heat supply from the reactor’s walls are carried out. Through the interplay of the developed code and the code for plane flows it becomes possible to identify the geometric factor role at the presence of a large number of nonlinear physicochemical processes. We found that diffusion of synthesized molecular hydrogen mainly influences heat supply from the reactor’s walls to gas and pyrolysis products distribution along its length.

Chemistry of the Gasification of Carbonaceous Raw Material

The paper represents the studies of the process of carbonaceous raw material gasification. The initial material is represented by bituminous coal of grade H with the carbon (C) content of 79.2-85.3 %. Experimental studies have been used to substantiate the parameters of combustible generator gases (СО, Н2, СН4) output depending on the temperature of a reduction zone of the reaction channel and gas flow velocity along its length. It has been identified that the volume of the raw material input to be used for gasification process changes in direct proportion depending on the amount of burnt-out carbon and blow velocity. The gasification is intensified in terms of equal concentration of oxygen and carbon in the reaction channel of an underground gas generator. The gasification rate is stipulated by the intensity of chemical reactions, which depend immediately on the modes of blow mixture supply. Moreover, they depend directly on the intensity of oxygen supply to the coal mass and removal of the gasification products.

Hydrogenation of Boron Carbon Nitride Thin Films for Low-k Dielectric Applications

The influence of hydrogenation on boron carbon nitride (BCN) thin films was investigated for low-k dielectric applications. The BCN thin films were deposited using radio-frequency magnetron sputtering in hydrogen, nitrogen, and argon ambiance. The hydrogen/nitrogen reactive gas flow was varied from 0/10 to 10/10 to achieve a varying range of hydrogen doping. Elemental composition and chemical bonding studies of the films were analyzed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). XPS results confirmed the formation of B-C-N atomic hybridization, and FTIR confirmed the hydrogen doping by evidence of C–H bonds. Metal insulator metal structures were fabricated using Al and BCN thin films to measure electrical properties such as dielectric constant and resistivity. Hydrogenation of BCN caused a 68% decrease in the k value from 6.2 to 2 due to the formation of non-polar bonds. The k value of 2 reported in this study is the lowest value achieved for hydrogenated BCN films deposited by the RF magnetron sputtering technique.

Formation of Mobile Hydridic Defects in Zirconium Nitride Films with n-Type Semiconductor Properties

The incorporation of mobile hydridic defects was demonstrated for low-crystalline zirconium nitride (Zr3N4−δ) films deposited by radio frequency reactive sputtering under a flow of nitrogen-rich reactive gases. Extended X-ray adsorption fine structure analysis confirmed that the local coordination environment around the Zr atoms was very close to that of the orthorhombic Eu3O4 phase. Pristine Zr3N4−δ films exhibited n-type semiconductor behavior due to the presence of nitrogen vacancy donors with a carrier concentration of 1.4 × 1020 cm–3 and an electron mobility of 2.2 × 10–3 cm2 V–1 s–1 at room temperature. Electrochemical and spectroscopic measurements confirmed that Zr3N4−δ was readily hydrogenated upon exposure to H2 gas at 300 °C to form hydridic defects via electron donation to hydrogen adatoms. Hence, the hydrogenated film exhibited H– ion/electron mixed conductor behavior in a H2 atmosphere. These findings open the possibility for exploring new hydride ion conductors based on thermodynamically stable transition metal nitrides.

Numerical Investigation of Mass Transfer in PEM Fuel Cell with Straight Gas Flow Channels using CFD

Interest in PEM fuel cells has grown rapidly in recent years because of its possible applications. The performance of PEM fuel cells is strongly affected by various physical factors, such as the flow of reactant gas, thermal management and water management. The performance and characteristics of a PEM fuel cell have been analysed through the development of a 3D model and numerical simulation. The result obtained from the computational model shows details of species movement, charge Transport and mass transfer phenomena. This paper also investigates the influence of input parameters on the output of the PEM fuel cell model. The result from the analytical study is compared with experimental results to check the accuracy of the model.

A Numerical Study of the Influence of Process Parameters on the Efficiency of Staged Coal Gasification Using Mixtures of Oxygen and Carbon Dioxide

The paper considers a staged conversion process of pulverized coal fuel in the MHPS-type gasifier, which uses mixtures of oxygen and carbon dioxide as a gasifying agent instead of air. Similar conversion processes can be applied in the process diagrams with the capture and disposal of carbon dioxide. The research tool is a reduced-order mathematical model of coal particles' conversion in a reacting gas flow. Replacement of nitrogen with carbon dioxide leads to significant changes in the gasification process characteristics: the average reaction temperature decreases, but this decrease is partially compensated by an increase in the concentration of gaseous reactants. Thus, the gasification process efficiency and the fuel conversion degree increase. Calculations make it possible to identify a range of parameters with the highest cold gas efficiency values. The influence of oxygen concentration is estimated, the dependence of the fuel conversion degree on the reaction temperature is analyzed.

Characterization Method for Gas Flow Reactor Experiments—NH3 Adsorption on Vanadium-Based SCR Catalysts

In this study, NH3 adsorption isotherms for a commercial vanadium-based SCR catalyst coated on a monolith substrate were obtained using a gas flow reactor over a wide range of parameters which have not been performed before in a single study. The isotherms were obtained under different conditions, where adsorption temperature, NH3 concentration, water concentration, washcoat loading, and catalyst oxidation state were varied. For this purpose, a systematic data processing method was developed, which characterizes the dispersion and delay effects in the experimental setup using a residence time distribution model, and artifacts such as NH3 adsorbed in the experimental setup and uncertainties in the washcoat loading were removed. As a result, data from different catalyst samples were integrated, and adsorption isotherms with low data spread and well-defined regions were obtained. This allows the identification of the complex nature of the catalyst and dynamics, where multiple types of adsorption sites are present. For instance, the oxidized catalyst has 50% higher NH3 storage capacity compared to the reduced state of the sample. Moreover, water reduces the NH3 storage capacity at high concentrations (5.0%), whereas at low concentration (0.5%), water increases the NH3 adsorption capacity for an oxidized catalyst. The proposed data processing method can be extended for the analysis of further phenomena in catalysts studied using gas flow reactors, complementing current methods and providing information for models with extended validity and lower parameter correlations.

Design of a TiAlON multilayer coating: Oxidation stability and deformation behavior

During the cutting of high-speed steel, thermal and mechanical loads occur. Thin hard coatings like TiAlN deposited by physical vapour deposition (PVD) are applied on the tool surface to prevent wear and oxidation. In the scope of this article, a multilayer coating concept was developed to improve the cutting performance of the tools. It consists of a Ti bond coat, a TiN/AlN nanolayer as an interlayer and an oxynitride TiAlON toplayer. The manuscript presents results of the investigation on the influence of the oxygen content in the reactive gas flow during an industrial direct current/high power pulsed magnetron sputtering hybrid process on the coating properties and on the oxidation stability. The reaction layer forming after the PVD process under ambient atmosphere on the surface of hard coatings has a thickness of a few nanometers. The chemical composition of the coatings and of the reaction layer were measured using electron probe micro analysis (EPMA) and X-ray photoelectron spectroscopy (XPS), respectively. At a high oxygen content of y(O / (O + N) = 85% the oxynitride TiAlON shows the presence of both, crystal and amorphous phases, as proven by transmission electron microscopy. As investigated by nanoindentation, indentation hardness, indentation modulus and resistance against plastic deformation decrease with increasing oxygen content in the coatings. When the oxygen content increases from y = 49% to y = 85%, the oxidation stability decreases. This might be caused by a decreased aluminum ratio of the reaction layer of x(Al / (Al + Ti) = 73% compared to x = 49%.

Numerical study of polyoxymethylene burning in a combustion reactor

The paper presents numerical results of polyoxymethylene burning behavior in a reactor. To estimate the effect of reactor geometry on polymer burning several configurations of a combustion reactor are introduced, which includes size variation of air inlet, reactor width (polymer sample width) and reactor height. A reactor is designed to allow air supply by natural convection. A mathematical model developed resolves primary features of combustion process such as multicomponent reacting gas flow, heat and mass transfer, radiative heat transfer, gas phase combustion and polymer pyrolysis. A set of air-to-fuel ratios is obtained to provide input for cases arranged for studying the efficiency of various municipal solid waste (especially polymers) incineration techniques. The paper presents hydrodynamic and thermal parameters distribution in a combustion chamber, which displays distinct two-dimensional nature of combustion process. Combustion of polymer in a reactor is shown to proceed in diffusion regime. The global balance shifts towards air excess.

TRIGGERING OF DETONATION PROCESSES IN PROPULSION CHAMBER

The Navier-Stokes equations have been used for numerical modeling of chemically reacting gas flow in the propulsion chamber. The chamber represents an axially symmetrical plane disk. Fuel and oxidant were fed into the chamber separately at some angle to the inflow surface and not parallel one to another to ensure better mixing of species. The model is based on conservation laws of mass, momentum, and energy for nonsteady two-dimensional compressible gas flow in the case of axial symmetry. The processes of viscosity, thermal conductivity, turbulence, and diffusion of species have been taken into account. The possibility of detonation mode of combustion of the mixture in the chamber was numerically demonstrated. The detonation triggering depends on the values of angles between fuel and oxidizer jets. This type of the propulsion chamber is effective because of the absence of stagnation zones and good mixing of species before burning.

Graded solar selective absorbers deposited by non-equilibrium RF magnetron sputtering

A new approach to deposit graded solar selective absorbers (SSA) is proposed. High entropy composite SSAs with antireflection layers have been prepared by non-equilibrium reactive RF magnetron sputtering. In order to obtain the gradient composition a relaxation process of target poisoning in the reactive O 2 /Ar atmosphere is employed. After introduction of oxygen the target gradually poisoned, resulting in a smooth change of the absorber composition from pure metals to their oxides. The poisoning relaxation time controls the thickness of SSA and the profile distribution of elements, and can be adjusted by selecting the reactive gas flow rate and RF power. Cintered Al:Cr:Nb:Si:Ta:Ti:Y mixture of powders is used as a target material. The graded SSAs demonstrate encouraging results: a coating with maximal selectivity of 13.4 has the solar absorptivity of 0.884 and emissivity of 0.066. In order to test the deposition method, we also try another target materials (AlTi and AlSc), which show better optical properties with the absorptivity of 0.949 (0.978 for textured surface) and selectivity of 18.6. The non-equilibrium reactive sputtering method proposed in this study has the advantages of simplicity and high production yield compared with the existing methods to deposit effective SSAs including double absorbing layer and multilayer coatings. • A new approach to fabricate graded solar selective absorbers (SSA) is developed. • High entropy composite SSAs was synthesized using nonequilibrium RF sputtering. • Graded composition is obtained using a relaxation process of target poisoning. • The relaxation time of the poisoning defines thickness and profile distribution. • Absorbers demonstrate selectivity of 13.4 and absorptivity of 0.909

Improved Electrical Properties of AlGaN/GaN High-Electron-Mobility Transistors by In Situ Tailoring the SiNx Passivation Layer.

In situ metal-organic chemical vapor deposition growth of SiNx passivation layers is reported on AlGaN/GaN high-electron-mobility transistors (HEMTs) without surface damage. A higher SiNx growth rate, when produced by higher SiH4 reactant gas flow, enables faster lateral coverage and coalescence of the initial SiNx islands, thereby suppressing SiH4-induced III-nitride etching. The effect of in situ SiNx passivation on the structural properties of AlGaN/GaN HEMTs has been evaluated using high-resolution X-ray diffraction. Electrical properties of the passivated HEMTs were evaluated by clover-leaf van der Pauw Hall measurements. The key findings include (a) a correlation of constituent gas chemistry with SiNx stoichiometry, (b) the degree of suppression of strain relaxation in the barrier layer that can be optimized through the SiNx stoichiometry, and (c) optimum strain relaxation by tailoring the SiNx passivation layer stoichiometry that can result in near-ideal AlGaN/AlN/GaN interfaces. The latter is expected to reduce the carrier scatterings and improve electron mobility. Under optimized conditions, low sheet resistance and high electron mobility are obtained. At 10 K, a sheet resistance of 33 Ω/sq and a mobility of 16,500 cm2/V-s are achieved. At 300 K, the sheet resistance is 336 Ω/sq and mobility is 2020 cm2/V-s with a sheet charge density of 0.78 × 1013 cm-2.

On supersonic combustion and hypersonic propulsion

The advanced engine has been the core technology of the aviation industry for several decades. The air-breathing hypersonic propulsion is the top problem for future aerospace flight. The engine's performance depends on its energy conversion method and combustion mode, and its relevant theory is of fundamental and revealing significance. In this paper, the supersonic combustion is discussed first since it is the theoretical basis for the research and development of scramjet engines. Then, by reviewing related research progresses, three criteria of the air-breathing hypersonic ramjet propulsion are established. The first one can be used to determine the local subsonic or supersonic flow states of combustion products in supersonic reacting gas flows, revealing the mechanism of the upstream-traveling shock wave. The second one defines the critical Mach number for hypersonic ramjet operation for all the combustion modes, and is a necessary condition that needs to be considered in the engine design under the equivalent ratio combustion. The last one gives a critical wedge angle corresponding to the CJ oblique detonation, and its physical basis is the critical initiation state of detonation. Finally, the experimental research progress on the stationary oblique detonation ramjet (Sodramjet) engine is summarized, and its feasibility as a hypersonic engine for future aerospace flight is demonstrated.

Grid-Assisted Co-Sputtering Method: Background, Advancement, and Prospect

In this paper, the incorporation of auxiliary grids/electrodes into the design of the sputtering method is scrutinized by studying the impact of factors such as grid size, grid position, grid bias voltage, electrode sheath design, substrate bias voltage, reactive gas partial pressure, and magnetic trap configuration on the discharge condition and thin-film growth. In this regard, utilizing the updated Berg model for reactive sputtering, the authors obtained a formulation for the reactive grid-assisted sputtering. By the newly-developed formulation, the sputtering rate and the reactive gas partial pressure were simulated as a function of the reactive gas flow rate for various grid-to-target distances, argon partial pressures (0.67, 1.6, and 3.6 Pa), and discharge current densities (0.01, 0.015, and 0.02 A/cm2). According to the computation results, with a grid being utilized, the width of the hysteresis loops can be modified in a broader range by changing either the grid-to-target distance, the argon partial pressure, or the discharge current, resulting in better control over the deposition process. Moreover, the novel configuration of anode-spot-assisted sputtering was proposed to significantly ionize sputtered/neutral atoms near the substrate and to simultaneously deposit sputtered species and products produced as the direct result of introducing dust into the dense plasma. Finally, diverse configurations of the grid-assisted co-sputtering method were introduced for two prime purposes: deposition of composite films in any desired ratios of constituents without significantly changing the other co-sputtering parameters, and favorable alternation of a hysteresis loop for one of the guns at shared gas flow rates.

Selective reduction of lead sulfate containing slag using the fluidized bed reactor

Lead sulfate containing residue is a hazardous waste that should be treated cleanly and efficiently for environment protection and resource sustainability. In this work, a novel reduction-sulfurization process using a fluidized bed reactor was investigated for treating lead sulfate containing residue. The effects of temperature, hydrogen ratio, reaction time and gas flow rate on the conversion of PbSO4 and selectivity of PbS were studied. The results showed that reducing the reaction temperature and hydrogen ratio improved the PbS selectivity, while increasing the reaction time and gas flow rate had no significant effect. In the optimal process conditions, i.e., the reaction temperature 650 °C, hydrogen ratio 20%, reaction time 5 min, and gas flow rate 550 mL min−1, the conversion and sulfurization ratios of PbSO4 were 96.2% and 84.8%, which were 2.0% and 40.1% higher than those obtained in a fixed bed reactor, suggesting that enhanced mass transfer in the fluidized bed reactor benefited the phase transformation of PbSO4 to PbS. This work offers a highly potential approach for processing lead sulfate containing wastes in a clean and economical manner.

Control-oriented plasma modeling and controller design for reactive sputtering

The modeling and control of reactive sputtering processes to deposit aluminum oxide thin films by an RF driven magnetically-enhanced low-pressure plasma is investigated. The new model describes the nonlinear deposition process with respect to the reactive gas flow as the input and the plasma density as the output. The process behavior is characterized by an unstable and ambiguous behavior, where different plasma states refer to the same input value, whereas different input values can lead to the same plasma state. Based on the identified model the process can be classified by two process modes, which qualitatively determine the parametrization of a stabilizing pseudo-derivative feedback controller. A new controller design to achieve set-point following for the control loop with a specific transient behavior is proposed. Experiments are presented to validate the model and the control system.

Four-Dimensional Identical-Location X-ray Imaging of Fuel Cell Degradation during Start-Up/Shut-Down Cycling

In this work, local electrode degradation effects from start-up/shut-down cycling of polymer electrolyte fuel cells are visualized using X-ray tomographic imaging of specialized, miniature fuel cell hardware. This combination enables non-invasive in situ tracking of the same cathode catalyst layer domain throughout various degradation stages in four dimensions. Critical, localized regions are identified within the cathode catalyst layer where progressive structural deterioration occurs from carbon support corrosion leading to thinning and collapse of the material. A greater structural change is observed under the landing area than under the channels due to delayed resident gas purge. This finding differs from the results of voltage cycling accelerated stress test, where more structural change was observed under the channel area than the landing area. However, overall similarities in degradation and performance loss supports the use of voltage cycling for accelerated degradation studies. A direct correlation between the structural deterioration and the electrochemical performance reduction of the fuel cell is found. In addition, reduced reactant gas flow in a restricted anode flow channel enhances the local cathode degradation due to delayed gas purge. However, no influence on the degradation is observed in a cell with intentional anode/cathode channel misalignment, compared to generic test cells.

Heat and mass transfer analysis of a high-pressure TGA with defined gas flow for single-particle studies

The test reactor HITECOM was designed to study in situ the conversion of an isolated particle in a well-defined gas flow at a gas temperature up to 1400°C and a pressure up to 40bar. As is typical of high-temperature, high-pressure systems, the temperature field inside the reactor is not perfectly constant and causes natural convection, which interferes with the gas flow in the test rig. To gain a deeper insight into the complex flow field and temperature and species distribution, a detailed CFD study was carried out and the results validated against experimental data. The numerical model comprised reactive gas flow, heat and mass exchange with a reactive char particle, convective and radiative heat transfer in the reactor and heat conduction in the solid parts of the reactor. The present study revealed that, depending on the operating conditions, the flow field shifts from an idealized parabolic profile to a complex, buoyancy-driven flow. It could be shown that comprehensive CFD modeling is necessary to identify critical operating conditions and estimate the real species and temperature distribution around the reacting particle, facilitating an improved evaluation of the experimental data.