Gas Mixture Of NH3 Research Articles

Effect of Prior Heat Treatment on Hardness Profile after Nitrocarburization in Medium Carbon Steel

Gaseous ferritic nitrocarburization is one of the major surface hardening methods to improve the fatigue strength of machinery parts made of medium carbon steels. The fatigue strength of nitrocarburized steel parts depends on the hardness profile below the surface; however, the mechanism of its evolution during nitrocarburization has not been fully understood. Recently, as-forged steels, in which thermal refining process like normalizing is omitted from the viewpoints of environmental considerations, energy savings and manufacturing cost reductions, have been widely used in the machinery parts industry. Therefore, it is important to understand the mechanism of hardness increase below the surface of the nitrocarburized steels with respect to the effect of prior refining heat treatment. In the present study, the hardness increase at the subsurface region of nitrocarburized steels containing 0.4mass%C was characterized, and the influence of prior normalizing treatment was investigated. Microstructure of both the as-forged and the normalized specimens was the ferrite/perlite mixture, while the ferrite volume fraction in the as-forged steel was smaller than the latter. These as-forged and normalized steels were gaseous nitrocarburized at 853K for 2 hours under the atmosphere of RX gas and NH3 gas mixture, and then they were oil-quenched to 373K. Overall hardness below the surface after nitrocarburization was higher for the specimen without prior normalizing treatment, although both specimens had the similar nitrogen concentration profiles and precipitation behaviors of the nitrides. However, it was found that the individual ferrite grains in the as-forged steel were more hardened than those in the normalized steel. These indicate that the most likely cause of the hardness increase near the surface after nitrocarburization is the solid solution hardening by dissolved nitrogen and that the ferrite grains of the as-forged steel were likely to soak up more nitrogen and were hardened to the higher degree since the similar amount of nitrogen were incorporated mainly within ferrite grains. Thus, the prior heat treatment strongly affects the amount of hardening through the ferrite fraction.

Effects of Anode Temperature on Working Characteristics and Performance of a Low Power Arcjet Thruster

An arc-heated thruster of 130–800 W input power is tested in a vacuum chamber at pressures lower than 20Pa with argon or H2-N2 gas mixture as propellant. The time-dependent arc voltage-current curve, outside-surface temperature of the anode nozzle and the produced thrust of the firing arcjet thruster are measured in situ simultaneously, in order to analyze and evaluate the dependence of thruster working characteristics and output properties, such as specific impulse and thrust efficiency, on nozzle temperature.

The role of monomeric iron during the selective catalytic reduction of NOx by NH3 over Fe-BEA zeolite catalysts

A series of highly active Fe-BEA iron zeolite catalysts for selective catalytic reduction (SCR) of NOx with ammonia have been prepared by incipient wetness impregnation and aqueous ion exchange with Fe(NO3)3 solutions. While the catalysts were stable during short term hydrothermal ageing, extended periods of ageing did reduce activity. The catalysts were investigated by diffuse reflectance UV–vis spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, ammonia temperature programmed desorption (NH3-TPD), X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) both ex-situ and in-situ with online gas analysis to measure catalytic activity.UV–vis, EPR and EXAFS showed that low iron loadings (≤1.2wt.% Fe) resulted in mostly iron monomers, especially for the ion exchanged samples. In contrast a mixture of monomers, oligomers and hematite particles was formed at intermediate to high loadings (2.5–5.1wt.% Fe). Linear combination analysis of UV–vis spectra, with real spectra and one Gaussian as basis, was used for quantifying the three iron species. A good correlation between iron monomer content and NO conversion was observed. In-situ XANES and EXAFS showed that heating in oxygen atmosphere removes water from the iron coordination sphere and the resultant vacant positions are occupied by NO, NH3 or H2O upon exposing the catalyst to a gas mixture of NO, NH3 and O2 in He. Heating in this gas mixture gave a distinct correlation between the catalytic performance and the oxidation state of iron, which is more pronounced in the catalysts where mostly iron monomers are present. Iron was more partially reduced when the SCR activity increased. These results support a mechanism where the oxidation of iron or NO is the rate-determining step.

Investigation of Hydrogenated Amorphous SiNx:H Films Grown by Rapid Thermal Chemical Vapor Deposition

Hydrogenated silicon nitride films were deposited with NH3, SiH4 and N2 gas mixture at 700oC by rapid thermal chemical vapor deposition (RTCVD) system. Wafer curvature measurements were carried out at room temperature to study the SiNx:H film stress with various deposition parameters. The NH3/N2 flow ratio and deposition pressure are found to influence the film stress. The stress of SiNx:H films deposited by RTCVD is tensile, which can reach ~ 1.5 GPa in our study. The influence of the reactive gas mixture and working pressure on the film stress, in connection with its material properties, is demonstrated.

Plasma nitriding to selective laser sintering parts made of SCM430 powder

Selective Laser Sintering (SLS) is an attractive Rapid Prototyping process for rapidly manufacturing dies and mechanical parts. Some recent studies have applied the SLS process to molding dies but few have applied it to stamping dies because some of the mechanical properties of SLS parts, such as strength and hardness, are inadequate for stamping dies. Plasma nitriding is widely used for hardening machine components and stamping dies to give them better wear resistance.In this study, SLS parts that are strong enough to be used as stamping dies were produced by plasma nitriding parts that had been sintered from an alloyed steel powder, JIS-SCM430 (corresponding to AISI 4130). The specimens were nitrided in N2–H2 gas mixtures at various temperatures, which hardened the surface layer. The properties of this hardened layer were then investigated.The specimens sintered from SCM430 powder had very low porosity and very high density, much like wrought materials. The porosity of the SLS specimens was 1.2%. After plasma nitriding, the surfaces of the specimens had a compound layer, the so-called “white layer,” composed of ε-Fe2-3N and γ'-Fe4N. The surface hardness of the specimens plasma nitrided at 773 K and 823 K were about 680 HV and 600 HV, respectively and were much higher than that of the original SLS parts — about 330 HV. Moreover, the parts sintered from SCM430 powder and then plasma nitrided showed better wear resistance than the original SLS parts. Therefore, plasma nitriding improved the wear resistance of the SLS parts.

The Characteristics of Surface Layers Produced on SKD 61 Steel by Plasma Radical Nitriding Compared with Conventional Plasma Ion Nitriding

Plasma radical nitriding has been performed to harden the surface of SKD 61 steel for 1- 10 hours at temperature range of 450-550°C. A NH radical, which has played a key role to produce a nitrogen diffusion layer without the formation of the brittle compound layer, has been generated in a gas mixture of NH3 and H2 . One of the main advantages of the plasma radical nitriding is to improve the surface hardness by maintaining the roughness of the initial polished surface. The microstructures and material properties of the radical nitrided layer have been characterized in order to investigate the effects of various radical nitriding processing parameters. The hardness and surface roughness of the hardened layer were compared between two processes. In addition, PVD CrN coating has been deposited on both the radical nitrided substrates and conventional nitrided substrates by an arc ion plating (AIP) technique. The effect of two different of plasma nitriding treatments on the adhesive strength of the coating layer on the substrates was also investigated.

Hydrogenated silicon carbon nitride films obtained by HWCVD, PA-HWCVD and PECVD techniques

Hydrogenated silicon carbon nitride (SiCN:H) thin film alloys were produced by hot wire (HWCVD), plasma assisted hot wire (PA-HWCVD) and plasma enhanced chemical vapor (PECVD) deposition techniques using a Ni buffer layer as catalyst for inducing crystallization. The silicon carbon nitride films were grown using C2H4, SiH4 and NH3 gas mixtures and a deposition temperature of 300°C. Prior to the deposition of the SiCN:H film a hydrogen etching of 10min was performed in order to etch the catalyst material and to facilitate the crystallization. We report the influence of each deposition process on compositional, structural and morphological properties of the films. Scanning Electron Microscope-SEM and Atomic Force Measurement-AFM images show their morphology; the chemical composition was obtained by Rutherford Backscattering Spectrometry-RBS, Elastic Recoil Detection-ERD and the structure by Infrared-IR analysis. The thickness of the catalyst material determines the growth process and whether or not islands form. The production of micro-structured SiCN:H films is also dependent on the gas pressure, gas mixture and deposition process used.

Effects of heat treatments on near surface elemental profiles of Fe–15Cr polycrystalline alloy

For the development of optimised surface modification of Fe–Cr alloys, the surface chemistry of an Fe–15 at.-%Cr alloy specimen during the initial stage of oxidation was investigated as well as the effects of hydrogen during annealing and following oxidation. Samples were exposed to various heat treatment atmospheres at 800°C: annealing in N2–H2 gas mixtures, oxidation in air and annealing then oxidation. After the heat treatments, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterise the morphology, elemental depth profiles and the chemical states of the elements. In the initial stage of oxidation, the chemistry of the oxide layer on the surface transformed in the following sequence: Fe rich oxide → Duplex oxide layer (outer Fe rich and inner Cr rich) → Cr rich mono-phase layer (M2O3). In the transformation, the reaction rate for the formation of the Cr rich oxide layer was controlled by the diffusion of cations. Annealing in H2-containing atmospheres increased the Cr content at the surface. An increase in hydrogen content in the atmosphere further increased both the Cr to Fe ratio in the near surface region, and the thickness of the layer affected by the heat treatment. Selective oxidation of chromium occurred as internal Cr2O3 formation, as a function of the Cr content rather than the oxygen partial pressure. Hydrogen facilitated the diffusion of chromium, probably by cleaning of fast diffusion paths. The addition of hydrogen to the annealing atmosphere before the oxidation was beneficial to obtain a Cr rich oxide layer on the surface.

Mass Spectroscopic Approach to Amino Acids Formation Processes by UV Irradiation to Simple Organic Molecules in Aqueous Solution

We have studied amino acid formation by UV (193 nm) irradiation to organic molecules (amines, alcohols and amides) in aqueous solution. Among several types of detected amino acids, small aliphatic amino acids (Gly and alpha-, beta-Ala and alpha-, beta-, gamma-ABA) were quantitatively identified. Among these small aliphatic amino acids, certain amino acids were formed in its free form, even before hydrolysis, contrary to the results of UV irradiation to a gas mixture of CO, NH3, and H2O, where amino acids were hardly detected before hydrolysis. The species distribution of identified amino acids showed a dependence on the starting organic molecules, and also on the presence of ammonia. The formation processes of the identified small aliphatic amino acids were investigated with the aid of electrospray ionization (ESI) MS and MS/MS measurements of photoproducts. Possible formation processes of these amino acid precursors from each starting molecules are proposed. By identifying the amino acid precursor, which has a chiral carbon atom, a new possibility is suggested for asymmetric photosynthesis of amino acid from achiral organic molecules.

Nitrogen-incorporated multiwalled carbon nanotubes grown by direct current plasma-enhanced chemical vapor deposition

The nitrogen-incorporated multiwalled carbon nanotubes (N-MWCNTs) were synthesized by dc plasma-enhanced chemical vapor deposition with a gas mixture of C2H2, NH3, and N2. Nitrogens in the N-MWCNTs were pyridinic nitrogen and graphitic nitrogen. With increase in the flow rate of N2 gas during the synthesis of MWCNTs, the pyridinic nitrogen increased much more than graphitic nitrogen. The near-edge x-ray absorption fine structure spectra revealed that the density of states such as π*, σ*, and π*+σ* bands of the N-MWCNTs decreased with increase of concentration of pyridinic nitrogen incorporated in the MWCNTs. The intensity ratio of the D band to the G band of Raman spectrum increased with the incorporation of nitrogen into MWCNTs.

On the materials properties of thin film plasma-nitrided austenitic stainless steel

A series of experiments were performed to study the composition and mechanical properties of the surface layers formed on the austenitic stainless steel after plasma nitriding in the temperature range of 400–500 °C with different N2–H2 gas mixtures. The thin layer at the nitrided surface was examined by the glancing-angle XRD and the differential load penetration from the microhardness measurements. The formation of the expanded austenite phase was detected at temperatures below 450 °C and/or 10% N2 in the treatment gas. The distortion of equivalent lattice constant after plasma nitriding was as high as 10%. The modulus of elasticity in the nitrided surface was increased by 33% after the plasma nitriding. The coating-only hardness was measured and it was equivalent to the hardness of coating containing CrN. Thus, it is possible to obtain thin coatings with superior resistance to corrosion and high hardness on the austenitic stainless steel.

Electrostatic probe measurements in an expanding microwave discharge sustained in an Ar–N2–H2 gas mixture: Investigation on plasma parameters

This work is devoted to the study of plasma parameters in the expansion of a microwave discharge sustained within an Ar–N2–H2 gas mixture. The plasma expansion in the reactor depends on the wave propagation conditions.We show that in a pure argon discharge the electron energy distribution function (EEDF) is identical to a Maxwell energy distribution function whatever the distance between the discharge exit and the substrate holder is and whatever the incident power is, ranging between 100 and 400 W. However, results show that the electron density is larger close to the discharge exit than to the substrate holder.When N2 or H2 is mixed with Ar, the discharge shrinks. The EEDF is displaced towards a low electron energy, and this behaviour increases with increasing N2 or H2 percentage. The EEDF cannot be compared with a Maxwell energy distribution function for a gas mixture containing more than 10% N2. Beyond this value, the plasma is unstable 3 cm below the discharge exit, and for larger N2 concentrations, the wave propagation is not observed in these regions.When the three gases H2, N2 and Ar are all mixed together in the gas mixture, the plasma is more stable and the expansion is detected even for gas mixtures containing a large concentration of (N2, H2) mixed with argon, in a 80% Ar–20% (N2, H2) or 48% Ar–52% (N2, H2) gas mixture. However, when the concentration of H2 is too low (49% Ar–50% N2–1% H2), the plasma shrinks and wave propagation is not observed. This behaviour could be explained assuming that these gas mixtures containing Ar–H2–N2 are producing NHx species in plasma that are low ionization energy species. However, numerical models are necessary in order to study such a complex plasma and to confirm the accuracy of this last assumption.

Effects of N2 on the Growth of Multiwalled Carbon Nanotubes Synthesized by Plasma-Enhanced Chemical Vapor Deposition

We have investigated the role of N2 in the synthesis of carbon nanotubes (CNTs) by dc hot-filament plasma-enhanced chemical vapor deposition. An NH3 and C2H2 gas mixture with a ratio of 4:1 was used as a precursor for the synthesis of CNTs on nickel-coated TiN/Si (100) substrates. N2 gas was introduced at a flow rate of 30–120 sccm with the precursor. The structure and composition of CNTs synthesized with/without N2 were investigated by scanning electron microscopy, transmission electron microscopy, X-ray photoemission spectroscopy (XPS), and Raman spectroscopy. The multiwalled carbon nanotubes (MWCNTs) synthesized with N2 were 2–5 times longer than those synthesized without N2, and the morphology of the MWCNTs was improved. An XPS core level spectrum shows that nitrogen is in the CNX and C–NH2 forms.

Properties of thin film silicon nitride deposited by hot wire chemical vapor deposition using silane, ammonia, and hydrogen gas mixtures

The structure of thin film SiN, deposited by the hot wire chemical vapor deposition (HWCVD) technique using SiH4 and NH3 gas mixtures, is examined as H2 dilution is added to the gas flow mixture. For NH3/SiH4 gas flow ratios greater than 1/2, all films are a-SiN:H for H2/SiH4 gas dilution ratios as high as 20/1. While H2 dilution does not change the basic film structure, it does increase the efficiency of NH3 dissociation in the gas phase, and causes a further reduction in the already small amount of N-H bonding in a-SiN:H films deposited by HWCVD. Differences in local N bonding sites are observed when the nitrogen source gas is changed from NH3 to N2. For NH3/SiH4 gas ratios less than 1/2 and with high H2 dilution, deposition of μc-SiN by HWCVD is demonstrated. X-ray diffraction measurements show that the structure of these films consists of silicon crystallites embedded in an a-SiN:H matrix. An upper limit for N incorporation with the preservation of microcrystallinity is found, beyond which the films again become amorphous. The existence of this limit is explained in terms of structural disorder in the a-SiN:H tissue brought about by N incorporation.

Emission properties of carbon nanotubes grown on various catalytic layers coated glass using plasma-enhanced chemical-vapor deposition with CO gas

Using plasma-enhanced chemical-vapor deposition with a gas mixture of CO and NH3, carbon nanotubes (CNTs) were vertically grown on a glass substrate with various catalyst metals and buffer layers. The effects of catalyst metals and buffer layers on the growth and emission characteristics of CNTs have been investigated. The difference in the field-emission characteristics between CNTs with various catalyst metals and buffer layers was mainly attributed to the crystallinity of CNTs, i.e., sp2-binding states of CNTs. These states could be the most effective electron-emission sites.

Synthesis and sintering properties of aluminium nitride nanopowder prepared bythe gas-reduction–nitridation method

Aluminium nitride (AlN) nanopowder was successfully synthesizedfrom transition alumina nanopowder via a new single-stepnitridation process. A gas mixture of NH3–C3 H8was employed as a reduction–nitridation agent, which enables direct nitridation oftransition alumina at processing temperatures of 1200–1400 °C. AlNpowder prepared under typical reaction conditions was found to bephase-pure and nanocrystalline, with a specific surface area of36.4 m2 g−1 and a mean particle size around50 nm. The AlN nanopowders produced could be densified to a pore-freestate in conventional processing, without resort to the use ofsintering aids. SEM observation revealed markedly small grain sizesin the sintered specimens compared with the commonly processed AlN.

The Mechanisms of Formation and Decay of Atoms in N2-H2 Plasmas

The formation rate and number densities of H(2S) and N(4S) atoms, rate constants of their heterogeneous decay, and some electrophysical characteristics of plasma were measured in the positive column of a dc discharge in an H2–N2 gas mixture at a pressure of 266 Pa and a discharge current of 50 mA. From these data, by the combined solution of the Boltzmann equation, vibrational kinetics equations for Н2(X1Σg) and N2(X1Σg), chemical kinetics equations, and plasma energy and heat balance equations, it was found that vibrationally excited nitrogen molecules must play a substantial role in the formation of both hydrogen and nitrogen atoms.

Cell attachment and biocompatibility of polytetrafluoroethylene (PTFE) treated with glow-discharge plasma of mixed ammonia and oxygen

The plasma generated from a gas mixture of NH3 plus O2 (NH3 + O2) has been used to impart unique chemical and biological characteristics to polytetrafluoroethylene (PTFE). PTFE treated with NH3 + O2 plasma was physiochemically distinct from surfaces treated with plasma of either NH3 or O2 alone, as determined by electron spectroscopy for chemical analysis (ESCA). The contact angle analysis revealed that the PTFE surfaces became less hydrophobic after plasma treatments. ESCA results indicate the presence of oxygen-containing groups and nitrogen-containing groups at the plasma-treated surfaces. PTFE treated with NH3 + O2 plasma resisted the attachment of platelets and leukocytes in a manner similar to untreated PTFE; however, the attachment of bovine aorta endothelial cells was substantially increased. Once attached, these cells grew to confluency. The increased endothelial cell attachment was higher than that observed following plasma treatment with each gas used separately, which could be attributed to the considerable amount of CF(OR)2-CF2 formed on the NH3 + O2 plasma-treated PTFE surface. At 14 days after subcutaneous implantation in rats, the PTFE wafers treated with NH3 + O2 plasma demonstrated less encapsulation and lower levels of inflammatory cells compared to controls. Collectively, the results suggest that NH3 + O2 plasma treatment imparts a unique character to PTFE and could be useful in certain in vivo applications.

Optical properties and electrical characterization of p-type ZnO thin films prepared by thermally oxiding Zn3N2 thin films

In this paper, we report a simple method for preparing p-type ZnO thin films by thermal oxidization of Zn3N2 thin films. The Zn3N2 films were grown on fused silica substrates by using plasma-enhanced chemical vapor deposition from a Zn(C2H5)2 and NH3 gas mixture. The Zn3N2 film with a cubic antibixbyite structure transformed to ZnO:N with a hexagonal structure as the annealing temperature reached 500 °C. When the annealing temperature reached 700 °C, a high-quality p-type ZnO film with a carrier density of 4.16 × 1017 cm−3 was obtained, for which the film showed a strong near-band-edge emission at 3.30 eV without deep-level emission, and the full width at half-maximum of the photoluminescence spectrum was 120 meV at room temperature. The origin of the ultraviolet band was the overlap of free exciton and the bound exciton. The N concentration was as high as 1021 cm−3, which could be controlled by adjusting the parameters of the annealing processes.

Systematic analysis and control of low-temperature GaN buffer layers on sapphire substrates

The growth of low-temperature (LT) GaN buffer layers on sapphire substrates was systematically studied using x-ray photoelectron spectroscopy with regards to processes such as substrate treatment and deposition conditions, along with annealing treatments of the GaN buffer layer during two-step metalorganic chemical vapor deposition. Variations observed in the LT-buffer layer depended strongly on both the chemical state of the sapphire surface as a result of the substrate treatment and the subsequent annealing conditions. A 20 nm buffer layer on non-nitrided sapphire evaporated after the formation of islands during the conventional annealing process (N2, H2, and NH3 gas mixture). Adding H2 gas to the annealing ambient enhanced the evaporation and reduced the surface coverage. It was found that AlxGa1−xN was formed at the interface, which has a low evaporation coefficient. In contrast, a buffer layer deposited onto a nitrided sapphire substrate evaporated completely in a layer-by-layer mode. The buffer layer contained domains with N face (−c) polarity that were almost covered with a Ga face (+c) layer. It was found that using Ga-rich conditions (a lower V/III ratio) for the deposition suppressed the formation of the −c domains, even on the nitrided sapphire. High temperature (HT) GaN layers were deposited on these well-defined LT-buffer layers. The influence of the various conditions used in preparing the LT-buffer layer on the HT-GaN layers are discussed in terms of the crystalline quality and the polarity of the HT-GaN layers.