Ammonia Flow Rate Research Articles

Removal of Boron from Molten Si and Si-Cu Using Ammonia Containing Gas

All the reported thermodynamics analysis implied that ammonia is a promising reagent for removing boron from silicon, but efforts to employ it in silicon refining have failed in practice. As such, there are few reports detailing this process. In this study, this process was analyzed experimentally. Various concentrations of ammonia were introduced to remove boron from silicon in the temperature range of 1200–1550 ∘C with a total pressure of 1 atm. Boron containing nitrides precipitates were detected in the furnace tube. The effect of ammonia content in the feeding gas was explored. It implied that higher ammonia partial pressures promote the boron removal. The experimental results have suggested that ammonia could remove boron from silicon in the form of volatiles, such as BHx (x = 1, 2, 3) and B3 H 6 N 3, in practice. The reaction-rate constant was limited to 10−6– 10−7 m/s in pure ammonia at 1450 ∘C. Moreover, a higher ammonia flow rate resulted in lower boron removal ratio. It was indicated that the rate determining steps of boron removal and silicon loss in this process were the chemical reaction at the surface of the melt and the transport of ammonia from gas phase to the surface, respectively. The relationship of the boron-removal rate with temperature followed a “V”-shaped curve, which implied the limit of thermodynamic factors at high temperature and the limit of kinetic factors at temperatures lower than 1300 ∘C. Based on the analysis results, temperature-programed reaction was designed to promote the boron-removal efficiency doubled. Cu was used to decrease the liquidus temperature of Si based alloy in the process. As a result, more than 80% boron in Si-Cu alloy could be removed.

Numerical model to analyze Nox reduction by ammonia injection in diesel-hydrogen engines

Nowadays, the even increasing stringent environmental legislations have promoted interest in alternative fuels for internal combustion engines. Particularly, hydrogen is becoming a promising fuel due to its high specific energy and low emissions production. Environmentally, the main disadvantage of hydrogen is the high level of nitrogen oxides (NOx) which produces. In this regard, this work proposes a NOx reduction method which consists on direct injection of ammonia (NH3) into the combustion chamber. A numerical model validated with experimental measurements was carried out to analyze emissions and brake specific consumption in a commercial engine operating with diesel-hydrogen blends. Comparing to diesel operation, a 10% hydrogen content increases a 5.3% the peak pressure and 5.7% the maximum temperature. The CO2, CO and HC emissions are reduced but NOx emissions increase up to 18.3%. Several injection instants and ammonia flow rates were analyzed, obtaining more than 70% NOx reductions with a negligible effect on other emissions and brake specific consumption. It was found that the start of ammonia injection is too critical since the maximum NOx reduction takes place when the temperature is around 1200 K. The NOx reduction increases with the ammonia flow rate but an excessive quantity of ammonia can lead to un-reacted ammonia slip to the exhaust.

The study of nitrogen inclusion in carbon nanotubes obtained by catalytic laser-induced chemical vapour deposition (C-LCVD)

Nitrogen doped carbon nanotubes were grown on Fe2O3 nanoparticles deposited on silicon substrates, by laser-induced chemical vapour deposition of acetylene/ammonia mixtures. The concentration of the nitrogen has been controlled in the range 1–6 atomic% by adjusting the flow rate of ammonia, pressure and laser power. XPS and Raman spectroscopy were used to quantitatively assess the compositional and structural properties of the nitrogen-doped carbon nanotubes (NCNTs). First order Raman spectra were deconvoluted assuming five vibrational modes and the integrated peak intensity ratio ID/IG and I2D/IG of all samples are displayed. We demonstrate that the relative amount of sp2 CC carbon has the same trend as ID4/IG and the pyrrolic relative amount exhibits the same trend as I2D4/IG. The high resolution TEM images are consistent with the Raman and XPS results, revealing that the surface of the NCNTs outer walls becomes more distorted at the highest content of N while the inner walls of the nanotube preserve a high crystallinity, corresponding to the lowest ID/IG ratio.

Effects of hydrogen etching on stress control in AlN interlayer inserted GaN MOVPE on Si

Clarification and control of the hydrogen etching of GaN is essential to achieving high-quality GaN-on-Si virtual substrates and devices based on it produced by metalorganic vapor phase epitaxy. This phenomenon and its effects on GaN-on-Si stress control were studied in this work. Without deliberate protection, voids with lateral sizes on the micrometre level underneath AlN interlayers emerged in GaN. Such voids made stress balancing in GaN-on-Si systems by inserting AlN interlayers less efficient. By flowing a protective large flow rate of ammonia, voids were eliminated while GaN decomposition still happened, which led to AlGaN alloy in the interlayers. Interfaces of Ga incorporated AlN interlayers grown under large-scale varied conditions were characterised by scanning transmission electron microscopy and energy dispersive spectrometry. Higher growth temperatures caused more Ga in the interlayers and weakened their capacity to induce compressive stress in the overlying GaN.

Design and operation of an ammonia-fueled microchannel reactor for autothermal hydrogen production

The present work describes the design and operation of an autothermal microchannel reactor for portable and distributed hydrogen production via ammonia decomposition. The microreactor consists of an array of alternating catalytic plate channels where the heat for the endothermic ammonia decomposition reaction is supplied in adjacent microchannels via exothermic oxy-fuel combustion of ammonia. The reactor performance was investigated under various operating conditions, such as ammonia flow rate in the decomposition channel, combustible feed flow rate, and fuel equivalence ratios. The best operating conditions were obtained at ammonia flow rate of 0.4 NLPM, combustible feed flow rate of 0.8 NLPM, and fuel-rich operation corresponding to fuel equivalence ratio of 1.2. At these conditions, >99% NH3 conversion was obtained, and the reactor could generate enough hydrogen for a 45We fuel cell system at an equivalent reactor power density of 1.5kWe/L. Overall, the work reported here successfully demonstrated the feasibility of ammonia decomposition for distributed hydrogen generation and delineated the attainable operating region, results of which could be used further to advance and scale-up autothermal ammonia-fueled microchannel reactors in the kW-range.

Correlation between mobility collapse and carbon impurities in Si-doped GaN grown by low pressure metalorganic chemical vapor deposition

In the low doping range below 1 × 1017 cm−3, carbon was identified as the main defect attributing to the sudden reduction of the electron mobility, the electron mobility collapse, in n-type GaN grown by low pressure metalorganic chemical vapor deposition. Secondary ion mass spectroscopy has been performed in conjunction with C concentration and the thermodynamic Ga supersaturation model. By controlling the ammonia flow rate, the input partial pressure of Ga precursor, and the diluent gas within the Ga supersaturation model, the C concentration in Si-doped GaN was controllable from 6 × 1019 cm−3 to values as low as 2 × 1015 cm−3. It was found that the electron mobility collapsed as a function of free carrier concentration, once the Si concentration closely approached the C concentration. Lowering the C concentration to the order of 1015 cm−3 by optimizing Ga supersaturation achieved controllable free carrier concentrations down to 5 × 1015 cm−3 with a peak electron mobility of 820 cm2/V s without observing the mobility collapse. The highest electron mobility of 1170 cm2/V s was obtained even in metalorganic vapor deposition-grown GaN on sapphire substrates by optimizing growth parameters in terms of Ga supersaturation to reduce the C concentration.

Review—Hexagonal Boron Nitride Epilayers: Growth, Optical Properties and Device Applications

This paper provides a brief overview on recent advances made in authors’ laboratory in epitaxial growth and optical studies of hexagonal boron nitride (h-BN) epilayers and heterostructures. Photoluminescence spectroscopy has been employed to probe the optical properties of h-BN. It was observed that the near band edge emission of h-BN is unusually high and is more than two orders of magnitude higher than that of high quality AlN epilayers. It was shown that the unique quasi-2D nature induced by the layered structure of h-BN results in high optical absorption and emission. The impurity related and near band-edge transitions in h-BN epilayers were probed for materials synthesized under varying ammonia flow rates. Our results have identified that the most dominant impurities and deep level defects in h-BN epilayers are related to nitrogen vacancies. By growing h-BN under high ammonia flow rates, nitrogen vacancy related defects can be eliminated and epilayers exhibiting pure free exciton emission have been obtained. Deep UV and thermal neutron detectors based on h-BN epilayers were shown to possess unique features. It is our belief that h-BN will lead to many potential applications from deep UV emitters and detectors, radiation detectors, to novel 2D photonic and electronic devices.

Structural and electronic properties of InN epitaxial layer grown on c-plane sapphire by chemical vapor deposition technique

Growth of InN epilayers on c-plane sapphire substrate by chemical vapor deposition technique using pure indium metal and ammonia as precursors has been systematically explored. It has been found that [0001] oriented indium nitride epitaxial layers with smooth surface morphology can be grown on c-plane sapphire substrates by optimizing the growth conditions. Bandgap of the film is observed to be Burstein–Moss shifted likely to be due to high background electron concentration. It has been found that the concentration of this unintentional doping decreases with the increase in the growth temperature and the ammonia flux. Epitaxial quality on the other hand deteriorates as the growth temperature increases. Moreover, the morphology of the deposited layer has been found to change from flat top islands to faceted mounds as the flow rate of ammonia increases. This phenomenon is expected to be related to the difference in surface termination character at low and high ammonia flow rates.

Growth of silicon-doped Al0.6Ga0.4N with low carbon concentration at high growth rate using high-flow-rate metal organic vapor phase epitaxy reactor

The relationship between the carbon concentration and electrical characteristics of silicon-doped AlGaN (Al > 0.5) was investigated using a high-flow-rate metal organic vapor phase epitaxy (MOVPE) reactor. The carbon concentration and electrical properties of AlGaN (Al > 0.5) were measured as a function of the growth rate, V/III ratio, and growth temperature. The growth rate of Al0.6Ga0.4N was linearly controlled up to 7.2 µm/h under a constant ammonia (NH3) flow rate. However, a decrease in V/III ratio resulted in an increase in carbon concentration to 8 × 1017 cm−3. With increased growth temperature, the carbon concentration decreased to less than 2 × 1017 cm−3 without showing any reduction in growth rate. As a result, n-type Al0.6Ga0.4N with a carrier concentration of 5.4 × 1018 cm−3 and a resistivity of 2.2 × 10−2 Ω·cm was obtained.

CO2 Absorption Characteristics in Ammonia Solution inside the Structured Packed Column

Ammonia has proved an alternative and potential solvent for postcombustion CO2 capture. It is of benefit to the reduction of energy consumption for CO2 capture to clarify the CO2 absorption characteristics in ammonia solution. On the basis of the representative elementary volume method and pseudo-single-liquid model, the computational model of CO2 absorption into ammonia solution in the structured packed column of a pilot CCS plant was developed, in which the gas-phase flow was also taken into account. The modeling and computational methods were validated by comparing with the experimental data from two pilot CO2 absorption towers. The CO2 removal efficiency and volumetric overall mass transfer coefficient were obtained at various inlet CO2 volume fractions, gas flow rates, aqueous ammonia solution flow rates and temperatures, and the ammonia mass fractions. The results show that the absorption process is predominated by the liquid film, so the flow rate and ammonia concentration of the aqueous ammonia so...

Effect of NH3 flow on incorporation efficiency of Al composition and reduction of surface donor states in AlGaN grown by MOVPE

AbstractWe have studied the effect of various ammonia (NH3) flow rates on AlGaN film grown on GaN using c‐plane sapphire as substrate by metal organic chemical vapour deposition. The influences of NH3 flow on the species diffusion anisotropy of thick AlGaN films were investigated by scanning electron microscopy, atomic force microscopy and high‐resolution X‐ray diffraction measurements. It is demonstrated that, in addition to the larger NH3 flow, growth of the AlGaN epilayer under the same trimethylaluminum (TMAl) flow condition as others is a critical factor for less micro‐cracks. The results of X‐ray double crystal diffraction showed that the AlGaN films with larger ammonia flow exhibited lower Al content and smaller (0002) rocking curve width, and while NH3 flow rate reached 23 liter per minute, a (0002) rocking curve width of 295 arcsecond was reached. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The origins of near band-edge transitions in hexagonal boron nitride epilayers

Photoluminescence spectroscopy has been employed to probe the near band-edge transitions in hexagonal BN (h-BN) epilayers synthesized under varying ammonia flow rates. The results suggest that the quasi-donor-acceptor pair emission line at 5.3 eV is due to the transition between the nitrogen vacancy and a deep acceptor, whereas the 5.5 eV emission line is due to the recombination of an exciton bound to a deep acceptor formed by carbon impurity occupying the nitrogen site. By growing h-BN under high ammonia flow rates, nitrogen vacancy related peaks can be eliminated and epilayers exhibiting pure free exciton emission have been obtained.

Hydrogen production from ammonia decomposition over a commercial Ru/Al2O3 catalyst in a microchannel reactor: Experimental validation and CFD simulation

In this work, an integrated experimental and CFD modelling technique was used to evaluate a microchannel reactor producing hydrogen from ammonia decomposition using a commercial Ru/Al2O3 catalyst. The microchannel reactor performance was first assessed in a series of experiments varying the reaction temperature (723–873 K) and ammonia flow rates (100–500 Nml min−1) at atmospheric pressure. A global rate expression based on Temkin-Pyzhev kinetics that accurately predicts the entire experimental operating space was established using a model-based technique with parameter refinement and estimation. The kinetic model provided the reaction source term for subsequent CFD simulations aiming to obtain a more fundamental understanding of the reaction-coupled transport phenomena within the microchannel reactor. The transport processes and reactor performance were discussed in detail using velocity, temperature, and species concentration profiles. Finally, the influence of mass transport limitations within the various regions of the microchannel reactor was evaluated and discussed by means of dimensionless numbers vis-à-vis Damköhler and Fourier numbers. Overall, results presented in this paper provide valuable data for the efficient design of ammonia-fuelled microchannel reactors for hydrogen generation aimed at portable and distributed fuel cell applications.

Correlation between the photoluminescence and chemical bonding in silicon nitride nanowires deposited by chemical vapour deposition

We have fabricated needle-shaped silicon nitride (Si3N4) nanowires (NWs) on p-type Si substrate by chemical vapour deposition reactor using Si powder and (H2 and NH3) as precursor gases. The NWs were characterized by fourier transform infrared (FTIR) spectroscopy, photoluminescence (PL) spectroscopy and X-ray photoelectron spectroscopy (XPS) in order to investigate structural and electronic properties. The vibrational signature at 467, 799 and 1085 cm−1 confirmed the Si–N stretching vibration bands. The qualitative of Si3N4-NWs network were analysed by curve fitting of FTIR signature. The room temperature photoluminescence spectra of Si3N4-NWs reveals several emission bands centred at 3.219, 3.265, 3.319, 3.373, 3.432, 3.504, 3.577 and 3.631 eV, which are associated with electronic transitions between the conduction band and defect states. The valence band of appeared p, sp, sp2, sp3 and 2s orbitals at 3.36, 6.42, 8.11, 9.73 and 12.36 eV respectively and shifted to higher binding energy with increasing of ammonia flow rate.

Effects of Carbon and Nitrogen on the Microstructure and Mechanical Properties of Carbonitrided Low-Carbon Steel

Various carbonitriding processes have been applied to low-carbon steel. The carbon and nitrogen contents in the case of carbonitrided low-carbon steels have been evaluated, and their influences on microstructure, distortion, hardness, and strength have been investigated. The factor limiting the ferrite and pearlite formations with increasing nitrogen content was a mechanism that resulted in more retained austenite in the case of low-carbon steel treated using a high flow rate of ammonia. The high-nitrogen content and high amount of retained austenite in the case of carbonitrided low-carbon steel slightly reduced the hardness, case depth, and strength. However, the elongation was improved, and the distortion was reduced. In the case zones, fractures occurred along the grain boundaries, i.e., intergranular fractures, whereas the fracture surfaces in the core regions were typical ductile fractures, i.e., dimples were observed.

Growth of bulk AlN crystals by vapor-phase epitaxy from atomic Al and NH3

A new approach to obtaining bulk AlN single crystals by vapor-phase epitaxy has been tested. NH3 and Al vapor were used as growth reagents. The following ranges of growth parameters were admissible for the laboratory equipment (experimental growth installation): temperatures of 1050–1500°C at ammonia flow rates of up to 50 sccm and pressures on the order of 10–5–10–4 bar, growth rates of up to 200 μm h–1. At a temperature of 1450°C, samples of strained bulk block AlN crystals with thicknesses of up to 200 μm were obtained in the wurtzite phase in the [0001] direction on MBE templates based on sapphire substrates with a diameter of 2″.

Growth behaviors of platinum-assisted GaN nanostructures in vapor–liquid–solid mechanism

This article reports the investigation of the growth behaviors of platinum-catalyzed GaN nanostructures grown via chemical vapor deposition method by manipulating growth temperature and ammonia flow rate. Morphological observation revealed the tapering behavior of GaN nanostructures at high growth temperatures and high ammonia flow rates in which the GaN nanowires were observed to be grown from the surface of GaN microcrystal structures. Growth mechanism of the tapering effect was discussed. X-ray diffraction showed the synthesized GaN nanostructures are of wurzite structure with no existence of impurities and found that GaN nanostructures grown at 1050°C under ammonia flow rate of 250 sccm possessed the highest degree of crystallinity. Raman measurement exhibited no peak shift in E2(high) while a redshift in A1(LO) mode with rising growth temperatures, indicating the decreased of carrier concentration in the GaN nanostructures.

The effect of cordierite/Pt catalyst on the NOx reduction in a diesel and Jatropha bio-diesel operated single cylinder engine

Bio-diesel is an alternative energy resource, which can be successfully used in diesel engines. One major disadvantage of using bio-diesel is the higher emission of NOx. Hence some after treatment devices have to be adopted to reduce NOx in order to meet the stringent limits on emission standards. In the current research work honeycomb structured circular Cordierite/Pt SCR catalyst is used as an aftertreatment device. Studies revealed that SCR (Selective catalytic reduction) catalyst exhibits better performance at a higher temperature range (>500°C), which can be successfully used in Trucks and Buses. But many catalysts failed to exhibit better performance at a lower temperature range varies from 180 to 400°C, which makes these catalysts unsuitable to use with light and medium duty vehicles. The main objective of this study is to assess the suitability of Cordierite/Pt catalyst for light and medium duty vehicles and study the effect of SCR catalyst on bio-diesel blends in order to reduce NOx in a single cylinder diesel engine. The experimental work has been carried out using diesel and bio-diesel as a fuel and the NOx reduction is tabulated. The results are validated with CFD code AVL FIRE. The ammonia flow rate has been varied from 0.2 to 0.8kg/h. It has been found that among all the blends B15 has shown maximum NOx conversion of 60% and NH3 conversion of 34%.

Fabrication, structure and hydrogen-gas-sensing properties of multiple networked GaN nanostructure gas sensors

The controlled synthesis of nanostructures with different shapes and morphologies has attracted considerable interest because the sensing properties of nanostructures depend on their structure, shape, phase, size, and size distribution, as well as their composition. This paper compares the responses of GaN nanostructures with different morphologies to hydrogen. The underlying mechanism for the hydrogen gas sensing of the multiple networked GaN nanowire sensor can be explained based on the well-established surface depletion model. On the other hand, the difference between two different samples, the GaN nanostructure sample synthesized at 1,100 °C and that synthesized at 1,000 °C, could be explained as follows: The major process behind the interaction between the nanostructures and hydrogen is the chemisorption of the dissociated hydrogen on the GaN surface. The chemisorption creates an electron accumulation layer on the GaN surface that enhances its electrical conductance. The GaN nanostructure sample synthesized at 1,100 °C with a higher ammonia flow rate showed a higher response to H2 gas than that synthesized at 1,000 °C with a lower ammonia flow rate, which might be attributed to the higher surface-to-volume ratio of the former.

The origin of deep-level impurity transitions in hexagonal boron nitride

Deep ultraviolet photoluminescence (PL) emission spectroscopy has been employed to investigate the origin of the widely observed deep level impurity related donor-acceptor pair (DAP) transition with an emission peak near 4.1 eV in hexagonal boron nitride (h-BN). A set of h-BN epilayers were grown by metal-organic chemical vapor deposition (MOCVD) under different ammonia (NH3) flow rates to explore the role of nitrogen vacancies (VN) in the deep-level transitions. The emission intensity of the DAP transition near 4.1 eV was found to decrease exponentially with an increase of the NH3 flow rate employed during the MOCVD growth, implying that impurities involved are VN. The temperature-dependent PL spectra were measured from 10 K up to 800 K, which provided activation energies of ∼0.1 eV for the shallow impurity. Based on the measured energy level of the shallow impurity (∼0.1 eV) and previously estimated bandgap value of about 6.5 eV for h-BN, we deduce a value of ∼2.3 eV for the deep impurity involved in this DAP transition. The measured energy levels together with calculation results and formation energies of the impurities and defects in h-BN suggest that VN and carbon impurities occupying the nitrogen sites, respectively, are the most probable shallow donor and deep acceptor impurities involved in this DAP transition.