Nitrogen Partial Pressure Research Articles

Evolution of chemical, structural, and mechanical properties of titanium nitride thin films deposited under different nitrogen partial pressure

In this study, to explore the impact of nitrogen on chemical composition, morphology, microstructure, as well as intrinsic stress of titanium films, nitrogen-doped titanium thin films prepared by direct current reactive magnetron sputtering, with different ratio of nitrogen doping into the Argon gas, have been characterized. After the check of grazing incidence X-ray reflectometry (GIXR), thickness of all samples were ensured to be approximately 40 nm. Using the X-ray photoelectron spectroscopy (XPS) combined with X-ray diffraction (XRD), nitrogen atoms were found to transform Ti crystals from the hexagonal close-packed (hcp) α-Ti phase with Ti(100) and Ti(002) orientation to the face-centered cubic (fcc) δ-TiN phase with TiN(111) and TiN(200) orientations, accompanied by the formation of TiO and TiON. XRD diffraction patterns further revealed the expansion of the TiN lattice and the intensity transform of TiN(111) and TiN(200). Atomic force microscopy (AFM) results indicated that the surface roughness declined continuously. A compressive stress was found to increase gradually with increasing nitrogen content. Variations in stress are caused by the expansion of the crystal lattice. It is obvious that the incorporation of nitrogen into titanium films can effectively reduce the roughness, but it will result in an increase in the compressive stress. Therefore, an appropriate nitrogen partial pressure should be chosen to control the intrinsic stress while reducing the roughness effectively.

Robust Model-Free Adaptive Interval Type-2 Fuzzy Sliding Mode Control for PEMFC System Using Disturbance Observer

This paper proposes a novel robust model-free adaptive interval type-2 fuzzy sliding mode control (MF-AIT2FSMC) for the proton exchange membrane fuel cell (PEMFC) system using nonlinear disturbance observer. The main control objective is to adjust the oxygen stoichiometry at its reference trajectory under load disturbances and parameter uncertainties, to avert the oxygen starvation problem and to obtain maximum net power. The referred MF-AIT2FSMC control strategy is composed of three parts: first, a nonlinear disturbance observer is proposed to estimate both oxygen and nitrogen partial pressures which are considered as unmeasurable variables. Second, a nonlinear disturbance observer-based intelligent proportional-integral (NDBO-iPI) control is derived, whereas the NDBO is used to estimate the unmodeled system dynamics via the knowledge of input and output signals. Third, adaptive interval type-2 fuzzy nonsingular fast terminal sliding mode control is inserted to NDBO-iPI control to compensate the NDBO estimation error, enhance the control performance and ensure the global controlled system stability. Furthermore, the entire controlled system stability is verified via the Lyapunov approach. Finally, the corresponding numerical simulation on the PEMFC system is obtained to demonstrate the effectiveness and efficiency of the proposed MF-AIT2FSMC technique by comparing with the NDBO-iPI and standard PID controller.

High-quality Ti(C,N)-based cermets via solid-state nitrogen-pressure sintering: Influence of the sintering atmosphere

Owing to the denitrification behavior of Ti(C,N)-based cermets during the sintering process, nitrogen atmosphere sintering is essential for the fabrication of high-quality products. Disclosing the roles of the sintering atmosphere on the microstructure and properties of cermet is of significance. Herein, Ti(C,N)-based cermets were fabricated under six types of sintering atmosphere. Effects of sintering atmosphere on the microstructure, coercive force (Hc), specific saturation magnetization (Ms (%)), and mechanical properties of cermets were investigated. More rimless Ti(C,N) grains with increased fraction of black cores were formed with increasing the nitrogen partial pressure. Three types of gradient structure of cermets were confirmed caused by the different nitrogen activity at the liquid sintering state. Vacuum sintering induced the formation of thicker rims on smaller Ti(C,N) cores at the surface due to the low nitrogen potential, while fcc solid solutions rich in N was formed in nitrogen atmosphere. Vacuum sintered cermets showed the lowest Hc and Ms. (%) of 3.66 KA/m and 2.13%, owing to the denitrification process. High-quality Ti(C,N)-based cermets were achieved via the solid-state nitrogen atmosphere sintering, benefiting from the optimized core-rim structure, i.e. moderate thickness of rims and certain amount of black Ti(C,N) cores.

Thermodynamics of nitrogen solubility in nickel-based alloys at plasma-arc remelting

Heat-resistant nickel-based alloys are widely used in the domestic aircraft industry, rocket engineering, and instrument making. Increasing the basic mechanical and operational characteristics of the metal is achieved mainly by alloying the base with various elements such as rhenium, ruthenium, hafnium, etc. However, increasing the operational properties can also be achieved by metal nitriding, as a result of which interstitial solid solutions are formed (finely dispersed nitrides), which increase the strength of the alloy. The work is devoted to the study of nitriding process of nickel-based complex alloys. Various options of the interaction of nitrogen with the melt are considered under conditions of open melting and processing with low-temperature nitrogen-containing plasma. The use of plasma-arc remelting allows one to obtain various types of gas in form of atoms, ions and molecules in plasma. The first two forms are much more active than molecular nitrogen, which leads to super-equilibrium concentrations in the alloy. A thermodynamic analysis of nitrogen solubility in a nickel-based melt was performed during open melting and under plasma-arc remelting. The nitrogen solubility in model EP741NP alloy was calculated depending on the partial pressure of nitrogen above the melt surface and in the plasma-forming gas. It was shown that treatment of the melt with nitrogen-containing plasma makes it possible to obtain higher content of nitrogen in the alloy. In this work, temperature of the metal in the zone of contact with plasma arc was estimated using the technique according to which the evaporation of the melt components from the arc spot area occurs when the metal boiling point is reached.

Effect of N2/TMS Gas Ratio on Mechanical and Erosion Performances of Ti-Si-C-N Nanocomposite Coatings

To optimize the tribo-mechanical performance of thick Ti-Si-CN nanocomposite coatings for a wide range of harsh industrial applications, reactive gases of nitrogen and trimethylsilane were employed with specific flow rates of PEMS process. Plasma-enhanced magnetron sputtering (PEMS) was employed for depositing thick Ti-Si-C-N nanocomposite (22-27 µm) on Ti-6Al-4V substrates at relatively high deposition rate up to 4.5 µm/h. Controlling the nitrogen partial pressure ratio PN2/(PN2 + PTMS) from 0.29 to 0.69 resulted in controlling the chemical and physical properties of the coatings. The XRD results demonstrated that the crystallinity of the nanocomposite structure increased with the increase in nitrogen pressure ratio. The coating hardness, erosion resistance, sliding wear resistance and corrosion resistance were augmented with increasing the nitrogen content in the plasma atmosphere. The results displayed that the sliding wear resistance of Ti-Si-C-N coatings increased by approximately three orders of magnitude comparing with the uncoated Ti-6Al-4V substrate. At low nitrogen content, low coefficient of friction (0.13-0.15) was achieved. Furthermore, the coating prepared at high nitrogen content reflected greater values of ratios H/E* and H3/E*2 that correlated well with the coating erosion resistance.

Large enhancement of the photocurrent density in N-doped Cu3N films through bandgap reduction

Copper nitride (Cu3N) has attracted wide attention for solar energy conversion applications owing to suitable Eg of 1.6–1.9 eV, non-toxicity, and possibility to fabricate homojunction solar cells. The Cu3N has a body-centered site of anti-ReO3 structure, which can be easily occupied by transition metals. Hence, many studies on the band gap tuning of Cu3N have been conducted with doping of various elements. However, N-doped Cu3N film has not been reported experimentally yet although it was theoretically predicted to have very stable doping and new partially filled narrow band gap. Herein, via systematically controlling the nitrogen partial pressure (R = N2/N2 + Ar), we successfully fabricated N-doped Cu3N film using reactive RF sputtering. The N-doped Cu3N film (only specific R = 0.5) exhibits significantly reduced optical bandgap (1.1 eV) and improved photocurrent density (1.66 mA/cm2 at 10 V) compared with that of pristine Cu3N film.

Gallium nitride films of high n-type conductivity grown by reactive sputtering

The origin of n-type conductivity in as grown, undoped GaN has been debated over several decades, with contradictory views. This work deals with reactively sputtered, undoped GaN films in which a large decrease of resistivity from ∼105 to ∼2 × 10−3 Ω cm is seen, as the nitrogen percentage in argon–nitrogen sputtering atmosphere is decreased from 100% to 10%, resulting in high electron concentration ∼1020 cm−3 at the lower end of nitrogen pressure. The electrical behavior of these films has been correlated with their composition and chemical state to understand the nature of point defects which influence n-type conductivity. Powder x-ray diffraction and phi scans of the undoped GaN films grown by sputtering of GaAs at 700 °C on c-sapphire reveal epitaxial growth of single phase GaN. SIMS results show the absence of arsenic in the films and uniformly distributed gallium and nitrogen along with oxygen impurity, across their thickness. XPS data reveal the decrease of N/Ga ratio, as the nitrogen partial pressure is decreased along with the increase of uncoordinated Ga, indicating the formation of nitrogen vacancies. In the high resistivity films grown at higher nitrogen percentage in sputtering atmosphere, oxygen impurities appear to be primarily responsible for creation of n-type carriers, which are compensated by Ga vacancies and possibly VGa–ON complexes. The drastic enhancement of n-type conductivity and carrier concentration in the low resistivity films grown at lower nitrogen percentage, which reveal significantly smaller nitrogen content, is attributed dominantly to the increase in nitrogen vacancies and uncoordinated Ga.

Effect of nitrogen concentrations on optical, structural and mechanical properties of self organized a-C:N films

Low friction and surface hardness has become an important aspect to study and understand surface engineering of diamond like coating. Investigation of structural and mechanical properties of nitrogenated amorphous Diamond Like Carbon coating has been done. The cross-sectional microstructures, elemental compositions and various phase constituent of the coated layers under different processing conditions have been characterized. Films are deposited in presence of 5%–20% with the increasing rate of 5% and 40% of N2 partial pressure along with Ar gas. 20% N2 pressure shows a critical behavior in Raman spectroscopy and XPS. In this condition the film shows more uniform coating with vertical growth structures as well as brittle behavior. The micro-structural changes on the surface due to migration of N2 and its related surface properties have been examined. AFM studies clearly show that the percentage change in average roughness decreases to 17% as the film thickness increases at 20% N2 incorporation. The positive changes in its mechanical properties have been observed by Nano-indentation techniques. Significance of DLC coating in this condition is clearly seen with the increasing sp3compositions by XPS analysis. All results have been correlated and hence critical range of nitrogen partial pressure has been observed between 20%-23% to give similar sp3 fraction and hence properties that of pure DLC films.

Densification of a high chromium cold work tool steel powder in different atmospheres by SLPS: Microstructure, heat treatment and micromechanical properties

The degradation of moulds, dies and tools employed in plastic, food and chemical processing industries has necessitated the development of suitable wear and corrosion-resistant materials. As improving the wear and corrosion resistance of iron base alloys tend to have opposing demands regarding chemical composition and heat treatment, optimisation of both parameters has to be kept in mind. One alloying element that is known to improve both corrosion and wear resistance of steels is nitrogen. Hence, an investigation into the densification of high chromium X190CrVMo20-4-1 cold work tool steel in a vacuum and under a nitrogen atmosphere at different pressures via supersolidus liquid-phase sintering (SLPS) process is reported in this paper. The investigation aimed to elucidate the influence of different atmospheres and nitrogen partial pressures employed during densification on the microstructure, optimal heat treatment parameters and micromechanical properties of the steel. Experimental findings were supplemented by computational thermodynamics calculations. The results revealed that increasing nitrogen pressure promoted the diffusion of vanadium from Cr-rich carbides (M7C3) to form V-rich carbonitrides, M(C,N). Optimum quench-hardening temperature was strongly influenced by the matrix chemistry. Upon tempering, the nitrogen-sintered samples had higher secondary hardening potential than the vacuum-sintered at a higher temperature, but a low-temperature tempering is beneficial to the corrosion resistance of the steel. The mechanical properties of the carbides in the densified steels in different atmospheres were influenced by their chemical composition. Experimental observations are in good agreement with computational thermodynamic evaluations.

An experimental study of Ti-6Al-4V powder reactivity in gaseous environment: Application to additive manufacturing

Currently, one of the current issues related to the additive manufacturing is the change in the properties of Ti-6Al-4V powder during this process. This change has been attributed to the interaction of this powder with its gaseous environment that can contain a small quantity of oxygen, nitrogen, carbon dioxide and/or water vapour. Therefore, the aim of this work is to study the effect of oxygen and nitrogen partial pressure as well as the effect of the temperature on the corrosion of the Ti-6Al-4V powder. The results show that the temperature and the oxygen partial pressure have an effect on the corrosion kinetics, which is not the case of nitrogen partial pressure. This study is a first step towards a better understanding of the ageing of Ti-6Al-4V powder during additive manufacturing processes.

Quartz optical cells with alkali-metal vapour for aerospace

This work for the first time analyses the impact of cosmic radiation on quartz cells with rubidium vapour and buffer gas, as well as the ensuing change in stability of atomic frequency standards (AFS) relying on such cells. It is demonstrated that the main effect consists in variation of partial pressure of nitrogen, which is a part of the buffer gas mix. This leads to degraded AFS stability. It was established by modelling that this effect is not significant because material degradation of the AFS stability is only likely to happen over a long period of time, as long as several decades. It can be concluded on the basis of the proposed analysis that a gas cell with rubidium vapour and buffer gas is the most reliable element of a satellite-borne AFS and that it is less affected by exposure to ionising radiation than electronic components of AFS.

Co-doped p-type ZnO:Al-N Thin Films Grown by RF-Magnetron Sputtering at Room Temperature

This study reports the structural properties of zinc oxide thin films co-doped with aluminum and nitrogen (ZnO:Al-N) grown by RF magnetron sputtering from an AZO (ZnO with 2 wt% Al2O3) target under nitrogen (N2) atmosphere at room temperature (RT). Nitrogen partial pressures of 0.00, 0.10, 0.25 and 1.00 mTorr were used. The film thickness was around 270 nm. Ultraviolet-Vis-NIR transmittance (T) spectra of the films revealed T values of 80 to 85% in the 400 to 700 nm wavelength range. XRD results indicated that the films had a hexagonal wurtzite structure and were preferentially oriented in the (002) plane. Analyses by EDS indicated that the N atoms tend to be incorporated into the ZnO matrix at the expense of oxygen atoms. The ideal [N]/[Al] was obtained at a N2 partial pressure of 0.25 mTorr, producing a p-type film. For a [N]/[Al] of 1.53, the film also exhibited p-type conduction with an electrical resistivity of 31.92 Ω cm, mobility of 18.65 cm2/V s and carrier density of 1.22 x 1016 cm-3. The low carrier density is attributed to the energetically favorable formation of inactive nitrogen phases instead of acceptor-receiver-acceptor complexes, even at the ideal [N]/[Al].

Nitrogen Solubility in Liquid Ni-V, Ni-Ta, Ni-Cr-V, and Ni-Cr-Ta Alloys

An investigation has been made concerning the solubility of nitrogen in liquid binary alloys (Ni-V and Ni-Ta) and ternary alloys (Ni-Cr-V and Ni-Cr-Ta) by sampling method. The experiments were carried out in the temperature range from 1773 K to 1873 K and under the nitrogen partial pressure up to 1.0 bar. A technique involving induction melting and a rapid quench device has been used in this study. It has been found that Sieverts’ law successfully describes the nitrogen dissolution in all these liquid alloys up to 1.0 bar of nitrogen gas partial pressure. The additions of vanadium, tantalum and chromium significantly increase the nitrogen solubility, especially vanadium-alloying addition. Furthermore, we have determined the interaction parameters, enthalpies and entropies of nitrogen dissolution in these liquid alloys.

Optical, Mechanical, and Thermal Properties of Free-Standing MoSi2Nx and ZrSi2Ny Nanocomposite Films

The optical, mechanical, and thermal properties of free-standing structures based on nitrided molybdenum and zirconium silicides have been studied. It has been shown that silicide nitriding considerably improves the thermal stability of films. While as-prepared amorphous MoSi2 and ZrSi2 films crystallize in the interval 330–370°C, nitriding of films has made it possible to increase their working temperatures in vacuum to 600–700°C (at least, for x ≥ 0.25, y ≥ 1.3). Tensile tests have shown that the ultimate strength of MoSi2Nx films (0 ≤ x ≤ 0.55) depends on nitrogen content only slightly. Comparison between the properties of MoSi2Nx and ZrSi2Ny films magnetron-sputtered at the same partial pressure of nitrogen has demonstrated that with transmission coefficients of films at a wavelength of 13.5 nm being close to each other, ZrSi2 films are more effective as protection coatings (they are less prone to oxidation and more degradation-resistant).

Experimental Production of Cu–Cr–N Composite Alloys and Thermodynamic Modeling of Their Phase Composition

Composite Cu–Cr–N alloys were obtained in situ under vibration of “copper melt–chromium powder” compositions before their crystallization. Two types of alloys were prepared, where chromium powder was freely dispersed or compacted into a tablet. Atmospheric nitrogen was used as a source of chromium nitrides in the alloys. The microstructure of the alloys is represented by a copper matrix hardened with chromium particles and numerous inclusions of nonstoichiometric chromium nitrides Cr2N1 – x. Thermodynamic modeling showed that the composition and quantities of chromium nitrides in the Cu–Cr–N alloy depend on the partial pressure of nitrogen above the melt.

Plasma Chemical Synthesis of Amorphous Hydrocarbon Films Alloyed by Silicon, Oxygen and Nitrogen

The paper presents the synthesis of amorphous hydrocarbon (a-C:H) films alloyed by silicon, oxygen and nitrogen. The films obtained in polyphenyl methylsiloxane vapor and argon/nitrogen environment are deposited onto crystalline silicon surface using plasma chemical deposition. It is shown that the physical and mechanical properties of the films depend on the nitrogen concentration. The film composition is studied by Xray fluorescence spectrometry and Fourier-transform infrared spectroscopy. Raman spectroscopy is used to explore the film structure. A nanoindenter is used for testing the indentation hardness and other mechanical parameters of the films. It is shown that the chemical composition and properties of a-C:H:SiOx:N film can be maintained by changing the partial nitrogen pressure during the film deposition. The increase in the nitrogen content in a-C:H:SiOx:N film leads to the growth in the root mean square roughness and the contact angle. It also leads to the reduction in the carbon content and the film hardness caused by the lower content of sp3 carbon phase.

Numerical Simulation and Experimental Investigation of Nitrogen Transfer Mechanism from Gas to Liquid Steel During Pressurized Electroslag Remelting Process

Since the gas–slag–metal three-phase transfer mechanism of nitrogen is not suitable to account for the nitrogen pickup during pressurized electroslag remelting process, the laboratory experiments have been carried out to study the transfer mechanism of nitrogen using different nitrogen partial pressures and electrode immersion depths. Then, two possible transfer mechanisms of nitrogen have been proposed, and a 2D transient numerical model has been developed to investigate the behavior of nitrogen based on the new proposed mechanisms. The simulated results are compared with the experiment to validate the feasibility of new proposed mechanism. The results show that the nitrogen content in ingot increases with the increasing nitrogen partial pressure and decreasing electrode immersion depth. At the current experimental condition, the electrode immersion depth seems to play a more important role in the nitrogen pickup than the nitrogen partial pressure. Due to the limits of thermodynamic and kinetic conditions, the nitrogen pickup is negligible via the transfer from gas to liquid metal through molten slag whether nitrogen chemically or physically dissolves into slag. The mechanism that nitrogen directly reacts with the partially exposed liquid metal film under the electrode tip due to the fluctuation of gas/slag interface could reasonably account for the experimental results. Furthermore, based on the mechanism, the predicted variation tendency of nitrogen content in ingot is in agreement with the experiment.

Habitable Underwater Hyperbaric Facilities: Respiratory Balance in the Human Organism During Adapting to Saturation Nitrogen-Oxygen Hyperbaria

Abstract There were evaluated responses of the respiratory system to changes in the variables of the external environment under increased pressure. To the model of professional underwater human activity underwater served the conditions of full saturation in compressed air or nitrogen-oxygen gas mixtures. Technical devices were presented by a number of underwater laboratories, mounted at the bottom (Ikhtiander-66, 67 and 68), hyperbaric chambers, submersible drilling rigs (Bur-1 and 2), and an autonomous diving Ikhtiander-2 for a long stay in the water. Studies of respiratory gases mass transport conditions in man showed than within the pressure range of 0.25-1.1 MPa at density of moderate hyperoxic and nitrogen-helio-oxygen environment up to 14 kg/m3 oxygen and carbon dioxide regimes of the organism come to a new functional level which provides the adaptation to the extremal conditions. It is determined that an increase of physiological dead breathing space, a decrease of the rate of the O2 diffusion through the alveole-capillary barrier, intensification of unevenness of ventilator-perfusional relations in lungs and an increase of blood shunting in lungs are the main respiratory mechanisms which regulate mass transfer of O2 and CO2 in man under hyperbaria. The leading hemodynamic mechanism is the retention of volume blood circulation and cardiac output. It is studied how the compression rate, high partial pressures of oxygen and nitrogen, microclimate parameters in inhabited hypebaric chambers influence changes of functional breathing system. Absence of hypoxic state is proved in man (full saturation of man with nitrogen) under normoxia in nitrogen-oxygen environment with the density 6.34 kg/m3. These are also the data about accelerated rehabilitation of divers using the method of active adaptation o high altitudes. Basic directions in physiological studies of functional breathing system under increased pressure of gas and water environment are described.

Surface Modification of Titanium Implants

The influence of nitriding temperature (900 and 850°C) and partial pressure of nitrogen (105 and 1 Pa) on morphology and phase structural state of modified surface layers of commercially pure titanium (cp-Ti) was determined. It was shown that nitrided titanium provided much better anti-corrosion protection than nitrided Ti-6Al-4V alloy, and its corrosion resistance increased with decreasing nitriding temperature and partial pressure of nitrogen.

High-rate deposition of silicon nitride thin films using plasma-assisted reactive sputter deposition

Functional thin films on organic materials are important in flexible electronics including next-generation flexible displays and photovoltaic cells. Herein, we describe the formation of SiNx films by plasma-assisted reactive sputtering deposition at low temperature, and detail the properties of the formed films. The effect of plasma assistance on the deposition rate and the properties of the SiNx films deposited by this system are reported as functions of relative N2 partial pressure. Plasma assistance led to a deposition rate as high as 1.63 nm/s. The thin film formed at a relative nitrogen partial pressure of 21.4% exhibited a density of 2.73 g/cm3 and was more than 95% transparent over the entire visible region of the spectrum. The properties of SiNx films formed by plasma-assisted reactive sputtering deposition demonstrate the feasibility of forming SiNx films that are suitable for passivation applications in devices such as organic light-emitting diodes (OLEDs) and solar cells.