Pure Gas Flow Research Articles

Numerical Study on Pressure Oscillations in a Solid Rocket Motor with Backward Step Configuration Under Two-Phase Flow Interactions

The pressure oscillation caused by vortex–acoustic coupling is one of the main gain factors that results in the combustion instability of motors. Focusing on a solid rocket motor with a backward step configuration that can generate a corner vortex, this study aims to investigate the pressure oscillation characteristics in a combustion chamber under two-phase flow interactions through numerical simulations. The two-phase flow discrete phase model (DPM) was chosen to study particle motion and two-phase interactions. The numerical methodology was hence established by coupling the DPM with the large eddy simulation (LES) method. Taking the Clx motor as a reference and introducing aluminum oxide particles, two important particle parameters (diameter and concentration) and the key geometric parameters of the backward step were numerically studied. The numerical results show that both increased particle diameter and concentration can decrease the frequency and amplitude of pressure oscillations; additionally, the effects of geometric parameters on the pressure oscillations of the backward step, such as the downstream aspect ratio, the expansion ratio, and the step position, are basically consistent under both pure gas and two-phase flows. The influences of those geometric parameters are mainly reflected in defining the space for the development of upstream flow instability and the motion of downstream vortices. Compared with the pure-gas flow, the presence of aluminum oxide particles in two-phase flow globally decreases the vortex shedding frequency, the primary frequency, and the amplitude of pressure oscillations. It can also weaken the effects of vortex–acoustic coupling due to increased turbulent viscosity, which hinders the orderly development of vortices.

Gas extraction generation of gas mixtures of polar organic compounds at trace concentration levels

The patterns of generating gas mixtures of polar organic compounds at the level of their MAC (several µg/m³) were studied using the method of continuous gas extraction from aqueous solutions with a known concentration in combination with dilution by a stream of diluent gas. The necessary distribution coefficients of phenol, isomeric cresols, nitrobenzene, and alkanols С4–С6 between the aqueous and gas phases (nitrogen) were determined. A two-stage generation scheme is proposed, based on saturating the sorbent (activated carbon) with a flow of extraction gas containing the target components at a specified concentration, followed by desorption by a flow of pure extraction gas.

Novel Gas Phase Route Toward Patterned Deposition of Sputter‐Free Pt/Al Nanofoils

AbstractThis article reports a new approach toward fabrication and directed assembly of nanoparticulate reactive system (Nanofoils) on patterned substrates. Different from current state‐of‐the‐art, gas phase electrodeposition uses nanoparticles instead of atoms to form densely packed multilayered thin films at room temperature‐pressure. On ignition, the multilayer system undergoes an exothermic self‐propagating reaction. The numerous contact points between two metallic nanoparticulate layers aid in high heat release. Sub‐10‐nm Platinum (Pt) and Aluminum (Al) particles are synthesized through cathode erosion of metal electrodes in a flow of pure nitrogen gas (spark ablation). Pt/Al bilayer stacks with total thickness of 3–8 µm undergo self‐propagating reaction with a 10.3 mm s−1 wavefront velocity on local ignition. The reaction wavefront is captured using high speed videography. Calorimetry studies reveal two exothermic peaks suggesting Pt/Al alloy formation. The peak at 135 °C has a higher calorific value of 150 mW g−1 while the peak at 400 °C has a 12 mW g−1 exothermic peak. X‐ray diffraction study shows reaction‐products are cubic Al2Pt with small quantities of orthorhombic Al6Pt and orthorhombic AlPt2. Electron microscopy studies help draw a correlation between film morphology, bimetallic interface, nanoparticle oxidation, and self‐propagating reaction kinetics that is significant in broadening our understanding towards nanoparticulate reactive systems.

COMPUTATIONAL ANALYSIS OF THE IMPACT OF BOUNDARY CONDITIONS ON A PARTICLE-LADEN FLOW: A CASE STUDY IN A PRESSURIZED OXY-COAL COMBUSTOR

Designing an effective burner is vital for the development of coal combustion technologies. Because of high pressure, the volumetric fraction of the coal particles in the injected fuel in a pressurized oxy-combustion (POC) burner approaches or even exceeds the limitations allowed by the commercial computational fluid dynamics codes (e.g., Ansys Fluent). Consequently, for such high particle volumetric fractions, the interplay between the particles, the fluid flow, and the burner wall needs to be re-evaluated. The present computational work is a first step in a systematic analysis of the roles of various characteristics involved in the POC process, such as the method of particle release, its location, and the particle size. Specifically, pulverized coal is burned under an elevated pressure of 15 bar in an O<sub>2</sub>/CO<sub>2</sub> environment. A 100 kW, a POC combustor, is modeled with Ansys Fluent using the Reynolds-averaged Navier-Stokes approach. It is revealed that for this pilot-scale, pressurized burner, the gas phase flow velocity in the near-wall region exhibits anomalies. With the major focus on POC, this work aims to eliminate/reduce the impact of high particle loading on the gas-phase flow. To scrutinize the role of particle loading in the near-wall region and eliminate the impact of this velocity on POC downstream, the particle-gas interplay in the boundary layer is investigated by means of the computational simulations incorporating the coupling between the turbulent flow and the particles. It is found that the tuning of the particle release location makes the gas-phase flow velocity in the presence of particles consistent with the pure gas flow velocity profile. The particles size is also found to have a significant impact on the particle trajectory.

A computational analysis of the impact of boundary conditions on a particle-laden flow: A case study in a pressurized oxy-coal combustor

Designing an effective burner is vital for the development of coal combustion technologies. Because of high pressure, the volumetric fraction of the coal particles at the fuel inlet of the POC burner approaches or even exceeds the limitations allowed by the commercial computational fluid dynamics (CFD) codes (e.g., Ansys FLUENT). Consequently, for such high particle volumetric fractions, the interplay between the particles, the fluid flow, and the wall need to be re-evaluated. The present computational work is a first step in a systematic analysis of the roles of various characteristics, specifically, the method of particle release, its location, and the particle size. In the POC process, pulverized coal is burned under elevated pressure in an O2/CO2 environment. Specifically, a 15-bar, 100 kWth, POC combustor is modeled with Ansys FLUENT using the Reynolds-averaged Navier-Stokes (RANS) approach. It is revealed that for this pilot-scale, pressurized burner, the gas phase flow velocity in the near-wall region exhibits some anomalies. In order to scrutinize the role of particle loading in the near-wall region, the particle release location is investigated. The numerical simulations incorporate coupling between the turbulent flow and the particles. It is found that the tuning of the particle release location makes the gas phase flow velocity consistent with the pure gas flow velocity profile. Moreover, the particle releasing location also influences the flame stability. The particle size is also found to have a significant impact on the particle trajectory, flame stability, and temperature.

Dynamic surface-coverage alteration based on microkinetic analysis for enhanced ammonia synthesis over ruthenium catalysts at low temperatures

The ammonia (NH3) synthesis reaction has been well established via Harbor-Bosch process, but recent demands of NH3 as a hydrogen carrier necessitates the synthesis operations at much milder conditions for renewable energy utilization. Catalytic ammonia synthesis is among the most investigated heterogeneous catalytic reactions but the detailed kinetic studies especially at low temperatures are still missing. This study investigated thorough microkinetic analysis at a wide range of reaction temperatures from 200 to 400 °C for NH3 synthesis. Although the united kinetic expression for the NH3 synthesis was established at all temperature range, the rate at low temperatures was found to be strongly reduced not only by hydrogen but also NH3 (NHx) surface intermediates that become more significant as the conversion increases. Knowing these strong inhibitions by both the reactant (H2) and product (NH3), transient perturbation of the surface intermediates by alternating between pure H2 and N2 gas flow was tested, which may lead to rate jumps by controlling the most abundant surface intermediates. The results show dramatic enhancement of the overall NH3 synthesis rate of as high as > 1 mmol g−1 h−1 over 5 wt% Ru/CeO2 catalysts at 200 °C and 0.1 MPa. The results not only give insight into improving existing catalysts and catalytic reaction operations, but also cast doubts on the consistency of how to report the rates of ammonia synthesis due to the strong dependence of the catalysts on ammonia yield, especially at low temperatures.

Fluid Flow and Heat Transfer of a Gas Stream Containing Dust Particles in a Parallel-Plates Duct

Abstract Fluid flow and heat transfer of a gas stream in various ducts have been studied thoroughly before. However, in real applications, a gas stream usually contains dust particles, whose effects have typically been neglected. In this study, the effects of the dust particles on the flow and heat transfer characteristics in a parallel-plates duct were numerically investigated in detail. A lattice Boltzmann method combined with a modified immersed boundary approach was employed to calculate the velocity and temperature distribution in the duct. The effects of the particles on the development of the hydrodynamic and thermal boundary layers in the duct were predicted. The product of friction factor and Reynolds number (fRe) and local Nusselt number (NuL) along the flow direction were obtained for a particle-laden flow and compared with those for a pure gas flow. The results indicated that for particle-laden flows, the “fully-developed” flow was just an approximation. Both the flow and thermal boundary layers were disrupted by the accompanying particles. The particles would form a stable and dense particulate fouling layer at the walls; this could increase the local (fRe) and reduce the NuL in “fully developed” regions. Moreover, ducts with superhydrophobic properties would be less influenced by the particles due to decreased particle deposition because of the anti-dust property of the surface.

Numerical Optimization of Drilling Parameters for Gas Predrainage and Excavating-Drainage Collaboration on Roadway Head

In order to improve efficiency of gas drainage, the reasonable layout parameters for borehole on roadway head are investigated. Assuming that the coal mass has a dual pore structure with fractures and pores, a multiphysical field coupling mathematical model is proposed for gas drainage. The governing equations of gas adsorption, seepage, and diffusion are considered. The process of gas preextraction and excavating-extraction collaboration on roadway heading face in 2606 haulage roadway of Zhangcun mine is modeled by finite element (FE) method. The effects of gas drainage under different drilling arrangements are analyzed. The results show that the fastest decreasing rate of gas content occurs at the beginning of drainage, and decreasing rate of gas content tends to be stable in the later stage. After removing the predraining on roadway head, the gas content at the center of coal wall on the roadway heading face will rebound. The included angle between the borehole and the roadway middle line determines the spatial area of gas drainage. Too small included angle will reduce the area of gas content reduction, while too large included angle will result in a blind area on both sides of the roadway. The change in borehole spacing affects the decreasing rate of gas content in the rock and coal around the roadway. Large spacing may cause an inadequate decreasing rate in the middle of the roadway, while small spacing may cause an inadequate decreasing rate in both sides of the roadway. In the field application, the pure gas flow and gas concentration of drainage show a downward trend in whole. The gas concentration decreased from 13.2% to 10.8%, and the pure gas flow decreased from 10.6 m3/min to 8.15 m3/min. Research achievements can provide a basis for gas drainage in underground roadway.

Adsorption of CO2 on ZSM-5 and Cu-MOF at room temperature and low pressure conditions for Carbon Capture and Storage (CCS) application

The application of Carbon Capture and Storage (CCS) is viewed as a key strategy to achieve CO2 reduction targets. In capturing CO2 process, both adsorption and absorption are promising techniques for CO2 capture, but adsorption processes using solid adsorbents are the prevailing technique nowadays. In this study, the performance of the prepared adsorbents on the CO2 capture capability is investigated and compared. Zeolite (ZSM-5) and copper metal–organic framework (Cu-MOF) are selected as adsorbents because of high surface area and the excellent performance in the adsorption process. The performance of both adsorbents in CO2 adsorption is evaluated in a fabricated setup consists of a metal cylindrical vessel equipped with flow of pure CO2 gas from a cylindrical tank. The efficiency of the adsorption system was explored as a function of the experimental parameters: effect of adsorbent dosage (0–500 mg) and inlet flow pressure (0–1 bar). Based on the findings from FTIR, EDX and XRD analyses, the results suggested successful formation of ZSM-5 and Cu-MOF. The highest particle size distribution and area for ZSM-5 were in the range of 25–66 µm and 1.7–442 µm2, while 102–149 µm and 1.7–1400 µm2 for Cu-MOF, respectively. SEM images show an agglomeration and flaky particles. From the TGA analysis, ZSM-5 tends to remain stable compared to Cu-MOF where lacks the ability to retain its framework at temperature above 400 °C. The optimum conditions obtained in the adsorption process for both adsorbents were 200 mg adsorbent dosage and 1 bar inlet pressure. Based on the results, ZSM-5 shows the best performance with high CO2 adsorption capacity as compared to Cu-MOF at the low pressure system. Under the optimum condition, 800 and 1000 mg/g adsorption capacity of Cu-MOF and ZSM-5 were achieved within 50 min of reaction to adsorb CO2.

Synergy of Mediator and LiNO3 in Electrolyte Solution on Li-Air Battery Performance

Li-air batteries (LABs) have a theoretical energy density (3500Wh kg-1) and are attracting worldwide attention for longer cruising range of electric vehicles. However, the discharge product Li2O2 is difficult to be decomposed during charging with a large overvoltage, and repeated discharge/charge cycles gradually cause clogging of the air electrode, decomposition of the electrolyte, and air electrode corrosion, which shorten the cycle life. Recently, redox mediator (RM) to reduce the charging overvoltage is widely studied by adding it to the electrolyte solution, while RM+ generated at the air electrode during charge process diffuses and reacts with Li negative electrode, causing shuttle effect [1], [2]. We have found that coating RM on the air electrode suppressed the shuttle effect and stabilized the cell performance. In this study, we used both RM to reduce the discharge/charge overvoltages and LiNO3 dissolved in tetraglyme (G4)-based electrolyte solution to suppress the shuttle effect by forming Li2O layer on the Li negative electrode. [3] We introduced two kinds of RM, LiBr and 10-methylphenothiazine (MPT), to electrolyte solution or air electrodes coated on a carbon paper substrate with Ketjen black (KB)-PVdF/NMP-based slurry. The amount of RM dosed on the air electrode was the same as that in the electrolyte solution in the cell. The cells consisted of the air electrode of 2 cm2, a separator, a Li negative electrode and an electrolyte solution, which were tested under pure O2 gas flow at 1.0 mL s-1 at 50 oC. Discharge/charge cycle tests were conducted at constant current of 200 mA gKB - 1 and constant capacity of 500 mAh gKB -1 between 2.0 V and 4.5 V.Figure 1 shows discharge/charge curves of the LAB cells using a LiBr-containing air electrode in 1.0 M LiTFSI/G4 and LiNO3/G4 electrolyte solutions. The LiTFSI/G4 cell shows capacity fading before 10 cycles, while the LiNO3/G4 one shows limited fading even after 10 cycles. The difference should be due to the suppressed shuttle effect by Li2O protective layer formed on the Li electrode and the prolonged RM function as demonstrated by a plateau at around 3.5 V. Figure 2 shows comparison of cycle performance among four LAB cells, which includes RM-free, LiBr on the air electrode (AE), MPT in EL, and MPT on the air electrode (AE) cells. MPT-containing cells revealed better performance in the less overvoltage and the longer cycle life than the others. SEM-EDS and XPS analyses of the electrode surface after cycle tests showed a good consistency with the above results. Details will be presented on the day.This study was supported by JST Project ALCA-SPRING (JPMLAL1301) and NIMS Joint Research Hub Program, Japan.[1] S. Ha et al., J. Mater. Chem. A, 5, 10609 (2017).[2] Y. Hayashi, et al., J. Electrochem. Soc., 167, 020542 (2020).[3] M. Saito et al., J. Electrochem. Soc., 168, 010520 (2021). Figure 1

Reactive surface coating of metallic lithium and its role in rechargeable lithium metal batteries

Realization of an outermost layer on the surface of metallic lithium is performed via reactive exposure of a thin lithium foil to a constant flow of pure O2 gas in a controlled, airtight environment to form spontaneously a ceramic-type, Li-ion conductive film for use in rechargeable lithium metal batteries with a liquid electrolyte. This layer acts as possible intermediate ‘buffer’ between the underlying lithium and solid electrolyte interphase (SEI) formed in contact with the electrolyte. The impact of this oxygen-containing layer on the cycle performances of thin lithium anodes and associated surface evolution are studied here in cells having LiFePO4 as stable cathode. The influence of this protective layer on 30 μm-thick lithium metal anodes and related effects on the electrochemical behaviour of corresponding cells are investigated with both standard LiPF6 electrolyte and lithium bis-oxalato-borate (LiBOB) as F-free alternative. Combining lithium oxide coating and LiBOB appears to play a key role in extending cell cycling and hindering dendrite formation, although a clear surface roughening of the cycled lithium is observed too. This protected lithium cycled against LiFePO4 with LiBOB provides good capacity retention at different C-rates, displaying adequate Coulombic and round-trip efficiencies and simultaneously enhancing the number of charge-discharge cycles.

Impact of particle loading and phase coupling on gas–solid flow dynamics: A case study of a two-phase, gas–solid flow in an annular pipe

The present study is devoted to a two-phase, gas–solid flow in an annular pipe (hollow cylinder) at an elevated pressure of 15 bars and moderate Reynolds number of circa 6000. The influence of the particle loading, the interaction between the phases, and turbulence dispersion on the flow dynamics is systematically studied by means of computational fluid dynamics simulations, employing the Ansys FLUENT commercial package. The cases with a particle volumetric fraction of 1.2% are referred to as “high particle loading,” and those with 0.13% are denoted as “low particle loading.” The following cases are investigated: (1) pure gas flow; (2) low particle loading two-phase flow with one-way coupling and with turbulence dispersion; (3) low particle loading two-phase flow with two-way coupling but without turbulence dispersion; (4) low particle loading two-phase flow with two-way coupling and with turbulence dispersion; (5) high particle loading two-phase flow with one-way coupling and with turbulence dispersion; (6) high particle loading two-phase flow with two-way coupling but without turbulence dispersion; and (7) high particle loading two-phase flow with two-way coupling and with turbulence dispersion. The boundary layer is found to grow without fluctuations of the turbulent kinetic energy (TKE) for cases 1, 2, and 5. For case 4, the TKE fluctuations have been identified, although they appear to be less substantial than those in cases 6 and 7. The authors attribute the semi-chaotic nature of the TKE fluctuations to the particle loading and two-way coupling. In addition, the onset and development of the flow instability have been observed at a random axial distance in cases 4, 6, and 7. Such instability is also attributed to the two-way coupling with turbulence dispersion in the flow. It is concluded that the particle loading, one-way, or two-way coupling between the phases, and the turbulence dispersion models significantly influence the development of the flow dynamics with the same inlet and boundary conditions. Consequently, it is not a trivial question, which result a user should trust. The present computational results inspire to perform verification as well as experimental validation of the simulations, so the simulation results can subsequently be used with confidence for design analysis.

Experimental Study on Gas Extraction Effect of Conventional Drilling in Coal Mining Face

In order to study the effect of conventional drilling gas extraction technology, combined with the observation results of upward drilling and downlink drilling field test in 81309 working face of Baode Mine. The analysis of field test data shows that under the same gas control effect, the average gas concentration and gas purity of uplink drilling are higher than that of downlink boreholes. In the extraction range, the average pumping concentration of uplink boreholes is 1.134 times higher than that of downlink boreholes, while the average pure gas flow rate of uplink boreholes is 1.296 times higher than that of downlink boreholes. The test results show that the uplink conventional drill Hole has better pumping concentration stability and extraction effect than downlink conventional drilling.

An approach based on the porous media model for numerical simulation of 3D finned-tubes heat exchanger

Developing efficient and compact heat exchangers is of important significance to improve the energy utilization efficiency of entire society. 3D finned-tubes, as a kind of passive enhanced heat exchange technology, are widely used for improving the thermal-hydraulic performance of the heat exchanger. In the numerical study of 3D finned-tube heat exchanger, on account of the limit by computing capacity, it is very hard to carry out the full-size numerical simulation, so the method based on porous media model becomes a common approach for numerical simulation of the heat exchanger. In the present study with the discrete 3D finned-tube heat exchanger as the study object, a kind of approach based on the porous media model under the cylindrical coordinate system is proposed. In this approach, only the fin region on the surface of base tube is simplified to the annular porous media zone while the region beyond the fin tips is still regarded as the pure gas flow region. The resistance coefficients of annular porous media, which are used to represent the retardation effects of the fin on the fluid motion, are given under the cylindrical coordinate system. Based on serialized numerical simulations of the fin configuration model seen by the air when it flows in the circumferential, axial and radial direction, the viscous resistance coefficients and inertial resistance coefficients of the annular porous media zone in each direction are obtained through fitting. Through comparison of simulated values between the porous media model and 3D finned-tube practical physical model, it is found that the pressure drop calculated by the method based on porous media model proposed in this study agree well with that based on the real physical model. The pressure drop predicted by the porous media model also agree well with the experimental results. In the approach based on porous media model, the grid number and calculation cost are smaller than those in practical physical model. Especially in case of tube bundles, number of grids applied in the simulation based on porous media model is far less than the grid quantity in the practical physical model simulation, but the pressure drop obtained by the porous media model is almost equal to the result obtained from the practical physical model. Besides, in the present method, only the fin zone is deemed as the porous media, so the flow field in the region beyond the fin tips will still be solved as same as that in the full-size simulation. Thus more flow details within the heat exchanger can be captured. The approach based on porous media model under the cylindrical coordinate system proposed can be used in the simulation study of the internal flow field of 3D finned-tube heat exchangers in actual scale. On this basis, this approach can contribute to optimization design of the 3D finned-tube heat exchanger.

Long-time drift induced changes in electrical characteristics of graphene–metal contacts

Chemical vapour deposition (CVD) graphene is commonly recognized as promising 2D material for development of electronic devices. However, the long-term drift of electrical parameters still requires deeper understanding before the technological means can be selected for an individual type of the devices. In this work, the changes in the electrical resistance were investigated over long time in the planar samples based on the CVD graphene with Au and Ni contacts. The samples were arranged as arrays of the resistors on a silicon substrate covered with a 250 nm layer of thermally grown silicon dioxide. The annealing in pure argon gas flow at 573 K was used to return the electrical properties of samples to the initial state. The effects of drift and annealing were compared for the three parts of structures, namely the electrical contact, the graphene sheet and the edge of the metal film with a hanging graphene sheet. For these parts, the resistance changes were related to the strain and doping of supported and hanged parts of the graphene sheet. Raman spectroscopy and Kelvin force probe microscopy were used to characterize charge doping, strain and work function in the graphene. The drift was explained in terms of the most prominent changes in the doping, strain and work function detected within the edge zone of the contact. It was proved that the annealing significantly changed the p-type doping and work function in the graphene layer in this edge zone. The properties were almost independent of test conditions in the SiO2 supported graphene. The changes in the contact parameters produced by drift mechanisms were proved being reversible under proper annealing conditions.

Isothermal reduction of V2O5 powder using H2 as oxygen carrier: Thermodynamic evaluation, reaction sequence, and kinetic analysis

V2O3 was prepared by reducing V2O5 powder using H2 as the oxygen carrier. To initially determine reasonable experimental for the H2-based reduction, oxygen potential and equilibrium calculations were performed. The reaction sequence of the V2O5 powder reduction was predicted by VO phase diagram assessments and was then verified by XRD and XPS measurements under various experimental condition. It was observed that the reduction followed the path of V2O5-VnO2n+1(n = 3 and 6)-VO2-VnO2n-1(n = 7, 6, 5, 4, and 3)-V2O3. Isothermal reduction experiments and kinetic analysis were performed using thermogravimetric analysis (TGA) at 813 K, 823 K, and 833 K, under a pure H2 gas flow. The entire reduction process, from V2O5 to V2O3, obeyed the unreacted shrinking core model (with random instant nucleation and three-dimensional growth of nuclei) and was controlled by chemical reactions. Mathematical fitting showed that the reduction process could be described as G(α) = [−ln(1-α)]1/3, and the apparent activation energy was determined to be 47.11 kJ·mol−1.

A clean process of preparing VO as LIBs anode materials via the reduction of V2O3 powder in a H2 atmosphere: Thermodynamic assessment, isothermal kinetic analysis, and electrochemistry performance evaluation

VO is one of the most promising anode materials for lithium ion batteries (LIBs). Herein, we present the clean preparation of VO powder via reduction of V2O3 powder under high-purity H2. Thermodynamic calculation and V–O phase diagram analysis were carried out to ensure reasonable oxygen potential conditions. Isothermal reduction experiments and kinetic analysis using thermogravimetric analysis (TGA) were then performed at 1623 K, 1648 K, and 1673 K under a pure H2 gas flow. The reduction of V2O3 powder appears to obey an unreacted shrinking core model and can be divided into two steps. The first step was controlled by a chemical reaction, the kinetics of which can be described as G(α) = [-ln(1-α)]1/3 with an apparent activation energy of 107.3 kJ·mol−1. The second stage was controlled by gas diffusion, the kinetics of which can be described as G(α) = [1-(1-α)1/3]1/2 with an apparent activation energy of 45.5 kJ·mol−1. Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) also were employed for characterizing the morphology of the prepared VO powder. As a result, when the VO was served as the LIBs anode material, the resulting electrodes exhibited a high specific capacity (843 mAh·g−1 after 30 cycles) and remarkable cyclic stability.

High-dielectric-constant silicon nitride thin films fabricated by radio frequency sputtering in Ar and Ar/N2 gas mixture

In this work, dielectric behaviour, photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS) analyses of silicon nitride (Si-nitride) dielectric films sputtered with radio frequency and Si3N4 sputtering target under pure Ar and Ar/N2 (50/50) mixed gas flow sputtering ambient. The dielectric constant of sputtered Si-nitride dielectric film with Ar/N2 (50/50) mixed gas flow sputtering ambient can be up to 17, which is higher than that with pure Ar gas flow sputtering ambient. In addition, the dielectric constant of 17 is higher than all the reported dielectric constants of sputtered Si-nitride dielectric films and matches the dielectric constant of the Si-nitride dielectric films fabricated by the chemical vapour deposition process. XPS analysis shows that N-deficient SiNx is the major phase (approximately 54.7%) in the Si-nitride dielectric film sputtered with Ar/N2 mixed gas flow. The N species (neutral and ionized N) generated in the sputtering ambient enable the formation of the N-deficient SiNx phase in the Si-nitride dielectric film. N vacancy defects with negative charges in the Si-nitride film prepared with Ar/N2 (50/50) mixed gas flow are responsible for the enhancement of the spontaneous polarization under the electric field and the dielectric constant. PL analysis also confirms that the Si-nitride film prepared with Ar/N2 (50/50) mixed gas flow contains more N vacancies than the Si-nitride film prepared with pure Ar gas flow.

Effect of annealing atmosphere (CO and N2 gas flow) on surface morphology and crystal quality of AlN buffer layer

In this study, the effect of the annealing atmosphere (N2–CO and pure N2 atmosphere) on the crystal quality, surface morphology, and surface O content of the annealed AlN buffer layers was studied. The FWHM of the (0002) plane of an XRD rocking curve showed that the screw dislocation density of the as-grown AlN film can be reduced by annealing in both N2–CO and pure N2 atmosphere. The AlN film showed an improvement in crystal quality under pure N2 annealing atmosphere. As annealing the AlN buffer layer improves the crystal quality, surface roughness, and surface O content of the AlN film, it should be well controlled for the subsequent growth of III-nitride layers. The surface oxidation that occurred on the AlN films during annealing in N2–CO atmosphere resulted in an increase in surface O content and surface roughness of the AlN films, which is undesirable for the subsequent growth of III-nitride layers. We observed that the surface O content (in the depth of 2-nm) of all annealed AlN films was reduced in the N2 annealing atmosphere, which could be due to the O outgassing from the AlN surface to the pure N2 annealing atmosphere. The O outgassing of the surface O content in the AlN surface layer was related to the volume of space in the annealing setups. However, a large volume of annealing space under pure N2 gas flow resulted in higher thermal decomposition on the annealing AlN films, leading to pits forming on the annealed AlN surface and a smearing of the original step-terrace structure. Therefore, we found that there is an optimal annealing atmosphere space in maintaining the original surface morphology and achieving low surface O content in the annealed AlN film.

Liquid carbon dioxide fracturing application and its effect on gas drainage in low permeability coal seams of underground coal mine

ABSTRACT Outbursts comprise the predominant natural hazards in Chinese underground coal mines. Extracting high content gas within coal seams is one of the most effective ways to reduce the risk of triggering outbursts. However, with the depth increase of coal mining production, conventional gas drainage might be failed due to the low permeability of coal seams. Additional permeability enhancement methods should be adopted to improve the gas drainage efficiency. In this study, we proposed the application of liquid carbon dioxide fracturing for enhancing gas drainage in a underground coal mine in China with low permeability coal seams. By drilling cross-seam boreholes, liquid carbon dioxide fracturing was simultaneously performed in the two drilled fracturing boreholes. The effective damage radius of liquid carbon dioxide fracturing was further identified by continuously monitoring pure gas flow rates in different observation boreholes. After liquid carbon dioxide fracturing, there was no significant decrease in pure gas flow rate of observation boreholes for a period of 2 weeks. A conceptual model of liquid carbon dioxide fracturing was proposed for illustrating the mechanisms of improving gas drainage efficiency. Some methods for further optimizing gas drainage with the operation of liquid carbon dioxide fracturing under various thickness of coal seams and geological conditions were discussed. These results suggest that the application of liquid carbon dioxide fracturing are effective for gas control and hazards prevention in underground coal mines.