Nitrogen Concentration Profiles Research Articles

Deuterium retention and ammonia production from D-implanted 316 L stainless steel: insights for future fusion reactors

We present a systematic study that quantifies deuterium (D) retention and ammonia (ND3) production from 316 L stainless steel (SS316L) following the implantation of D ions in conditions similar to the ones expected in the ITER tokamak, i.e. with kinetic energy below 300 eV. Using Temperature Programmed Desorption (TPD) after deuterium ion implantation at 250 eV/D, we show that deuterium retention increases linearly with the D fluence up to 1021 D+m−2, with a retention probability of 18%. For higher D fluence, deuterium retention increases sub-linearly. Analysis of the TPD spectra evolution with varying storage time in vacuum after D implantation, shows that D retention is influenced by D diffusion into the bulk of SS316L. Subsequent to D ion implantation, we evidence the efficient production of ND3 molecules during TPD, between 400 K and 750 K, from the nitrogen present naturally in SS316L. Up to 21% of the D release during TPD can be found in ND3 molecules, indeed. The fraction of ND3 in the total D release depends both on the D ion fluence and the nitrogen concentration profile in the bulk. At least 7% of the D release is found in the form of ND3 molecules, even at a fluence of 2 × 1021 D+m−2 and for a natural N concentration bulk profile. Both N diffusion and D diffusion into the bulk appear to dictate the kinetics of ND3 production. Our findings of efficient production of ND3 in D-implanted austenitic 316 L stainless steel underline the need for similar studies on reduced-activation ferritic/martensitic (RAFM) steels that contain similar content of nitrogen and will be used in fusion reactor prototypes.

Comprehensive performance and numerical analysis of pressure swing adsorption process-based medical oxygen concentrators under various operating conditions

ABSTRACT One common application of pressure swing adsorption technology is medical oxygen concentrator (MOC) which directly produces ~ 94% O2 from air. The operating condition determines the separation efficiency of MOC and is varied with practical requirements. A better understanding of performance, mass and heat transfer inside adsorption bed at wide operating conditions is of great importance. The impacts of key operating parameters on the performances, the concentration and temperature distributions of MOC have been numerically investigated. The numerical results demonstrate that high comprehensive performances are achieved at an optimal condition with combination of adsorption pressure, product and feed flowrate and feed temperature. As adsorption pressure increases, the front of gas concentration and temperature becomes sharp, which effectively benefits for improving the performance. It is nearly identical shapes of nitrogen concentration and gas temperature profiles after adsorption and the profiles are pushed forward near production end with increasing of product flowrates and decreasing of feed flowrates. The increasing of feed temperature is beneficial to improve the performance. However, the adverse mass transfer and thermal effects are dominant at very high pressures, flowrates and temperatures conditions and this variation increases the unit power of MOC.

Mass and Heat Transfer of Pressure Swing Adsorption Oxygen Production Process with Small Adsorbent Particles

Rapid-cycle pressure swing adsorption (PSA) with small adsorbents particles is intended to improve mass transfer rate and productivity. However, the mass transfer mechanisms are changed with reduction of particle size during rapid-cycle adsorption process. A heat and mass transfer model of rapid-cycle PSA air separation process employing small LiLSX zeolite particles is developed and experimentally validated to numerically analyze the effects of mass transfer resistances on the characteristics of cyclic adsorption process. Multicomponent Langmuir model and linear driving force model are employed for characterizing the adsorption equilibrium and kinetic. The results of numerical analysis demonstrate that the dominant mass transfer resistance of small adsorbents particles is a combination of film resistance, axial dispersion effect and macropore diffusion resistance. The oxygen purity, recovery and productivity of the product are overestimated by ~2–4% when the effect of axial dispersion on mass transfer is ignored. As particle size decreases, the front of nitrogen-adsorbed concentration and gas temperature become sharp, which effectively improves the performance. However, the adverse effect of axial dispersion on the mass transfer becomes significant at very small particles conditions. It is nearly identical shapes of nitrogen concentration and gas temperature profiles after adsorption and desorption steps. The profiles are pushed forward near the production end with an increase in bed porosities. The optimal oxygen recovery and productivity are achieved with a particle diameter of 0.45 mm and bed porosity of 0.39 during the PSA process.

Influence of Plasma Power and Oxygen-Containing Process Gases in Active Screen Plasma Nitrocarburizing with Carbon Solid Source*

Abstract Plasma nitrocarburizing by means of active screen technology using an active screen made of carbon fiber-reinforced carbon was carried out by varying the power at the active screen and using oxygen-containing fresh gas components (O2, CO2) in the N2:H2 plasma using the example of the quenched and tempered steel AISI 4140 (42CrMo4). The investigations focused on the analysis of the process gas by means of laser absorption spectroscopy, the evaluation of the produced compound layers with regard to structure and phase composition, as well as the resulting properties. It was shown that by varying the process gas atmosphere, the structural composition of the compound layer and the concentration profiles of nitrogen and carbon can be specifically influenced. The high concentrations of carbon-containing compounds in the process gas resulted in complete suppression of γ’-Fe4N formation, but cementite was detected in the lower part of the compound layer. The addition of oxygen-containing fresh gases and the resulting change in process gas composition suppressed cementite formation. The results suggest that, in particular, high powers at the carbon active screen and the simultaneous addition of oxygen-containing gases results in the generation of nitrogen-rich, single-phase ε-compound layers.

Nitrate transport in a fracture-skin-matrix system under non-isothermal conditions.

Subsurface leaching of agricultural runoff has been identified to pose a serious hazard to the soil-water ecosystem and human health, mostly due to the associated contamination with nitrate. Our understanding of the nature of contaminant spread in the vadose and aquifer zones has been improved from recent mechanistic models on the flow and transport of contaminants through fractured porous media. The present study aims to explore the impacts of skin formation in a fracture-matrix aquifer system onto the nitrogen species transport under non-isothermal settings using numerical modeling. A finite-difference scheme was employed to capture the nitrogen concentration profile and kinetics of transformation by solving the derived partial differential equations. The results show evidence of an additional mass transfer from fracture to skin so as to reduce the migration of nitrogen species (NO3-N and N2) at the fracture-matrix interface thereby reducing the peak concentration of N2 by nearly 1.5 times in fracture after denitrification. Although the thermal conductivity of the rock matrix has a direct impact on the temperature distribution in fracture-skin-matrix profiles, the presence of skin has a cooling effect for a high-temperature influent (45°C), which also deteriorates the propagation of organic N2 and NO3-N, within the fracture. An increase in the temperature coefficient of skin has resulted in an apparent reduction in nitrogen species migration, indicating the thermo-chemical feasibility of an intermediate skin favoring the mass transfer processes. The findings of this study can be extended toward realistic estimation of groundwater contamination risks and for the design of biological filters for in situ remediation.

Evolution of Microstructure and Hardness of the Nitrided Zone during Plasma Nitriding of High-Alloy Tool Steel

Plasma nitriding is widely used in various industrial applications to improve surface hardness and wear properties. Especially for tool steels, it is also used to improve the support and adhesion of diamond-like carbon (DLC) coatings. The properties of the nitrided zone produced by plasma nitriding are influenced by the applied process parameter, in particular temperature and time. However, for high-alloy tool steels, a deeper understanding of the underlying diffusion processes of the nitrogen and the interaction with the existing microstructure, as well as the effects on the case depth is still lacking. Therefore, in this study, specimens of high-alloy tool steel X153CrMoV12 were plasma nitrided at varying temperatures (480 °C, 520 °C, 560 °C) and treatment times (2 h, 4 h, 16 h). The resulting nitrided zones were investigated by optical and scanning electron microscopy (OM and SEM), depth-dependent glow discharge optical emission spectroscopy (GDOES), X-ray diffraction (XRD), and hardness measurements to characterize their microstructure, chemical composition, and hardness depending on the process parameters. The distribution of carbides (M7C3), e.g., chromium carbides, affects the diffusion of the nitrogen and the layer growth. An increase of temperature and duration leads to an increased layer thickness. The composition of the compound layer is, e.g., influenced by the process parameters: ε nitrides (Fe2–3N) occurred preferentially at lower temperatures, while γ′ nitrides (Fe4N) appeared mostly at higher temperatures. In order to investigate the influence of the carbides of the high-alloy tool steel on the nitriding process, a new methodology was developed by means of finite element analysis (FE), which makes it possible to analyze this influence on the development of the nitrogen concentration profile. This methodology makes it possible for the first time to map the heterogeneous nitrogen evolution and distribution.

Effect of Nitrogen Concentration Profile on the Wear Rate of Expanded Austenite Phase

Effect of Nitrogen Concentration Profile on the Wear Rate of Expanded Austenite Phase

THERMODIFFUSION SATURATION OF THE SURFACE LAYERS OF α-TITANIUM BY OXYGEN, NITROGEN, CARBON

The purpose of the study is to analytically assess the depth of the gas-saturated zone in the case of a single-component diffusion saturation of alpha-titanium with nitrogen, oxygen and carbon from a rarefied controlled gas environment. Results. Based on the analysis of the literature data, the work schematically shows the interaction of alpha titanium with the elements of implementation and presents the processes with the corresponding parameters that characterize. It is shown that the surface impurity concentration is equal to the equilibrium concentration and is established instantly and does not depend on time. Consequently, with the proposed generalized nonstationary boundary condition in the absence of diffusion of impurities into the volume of the metal, the time dependence of its surface concentration is given, determined by the intensity of surface processes. The dependence of the relative change in the microhardness in the diffusion zone of titanium due to dissolved nitrogen (without taking into account the contribution of nitride inclusions) is presented. Analytically calculated concentration profiles of nitrogen generally correlate well with the distribution of the corresponding relative changes in microhardness in the surface layer. Analytical calculations of the concentration profiles of oxygen, nitrogen and carbon in titanium at a saturation temperature of 700 °C are presented. Practical value. The results obtained will make it possible to preliminarily estimate the size of the fortified near-surface layer depending on the parameters of chemical-thermal treatment and select the optimal parameters of thermal diffusion treatment to ensure the formation of reinforced layers on products made of alpha-titanium based on elements of interstitial in order to increase the functional properties.

Numerical modeling of the diffusional transport of nitrogen in multi-phase solid titanium and its application to determine diffusion coefficients

As part of a program to understand the dissolution of nitrogen-rich titanium solids in liquid titanium, this manuscript presents a numerical study of nitrogen transport in titanium at elevated temperatures. A Landau transformation was applied to the equations governing mass transport which were used as the basis for the numerical model. The numerical model was used initially to simulate nitrogen transport in a planar geometry and the predicted nitrogen concentration profiles and displacement of Ti–N phase boundaries showed good agreement with analytically derived solutions. The numerical model was then used to simulate nitrogen transport in commercially pure titanium cylinders. The model results were shown to be sensitive to the diffusion coefficients of Ti–N phases present in the system. Based on the sensitivity analysis, diffusion coefficients at 1650 °C of 4.3 × 10 − 11 m 2 s − 1 , 1.6 × 10 − 11 m 2 s − 1 and 1.7 × 10 − 12 m 2 s − 1 for β-Ti, α-Ti and TiN phases, respectively, were back calculated using the model. The model predictions, using the new diffusion coefficients, showed good agreement with previously published data in terms of both the nitrogen concentration profiles and displacements of Ti–N phase boundaries under the conditions examined in the study. The comparison indicates the presented model is capable of accurately approximating the transport of nitrogen in titanium at elevated temperatures.

Simulation of Nitrogen Indiffusion and Its Impact on Silicon Wafer Strength

Herein, the concentration profiles of nitrogen after nitriding rapid thermal annealing (RTA) of Si wafer surfaces are simulated. A model describing the fundamental partial differential equations for diffusion and interaction of nitrogen, vacancies, interstitials, and nitrogen vacancy (NV) complexes is used for this thermal process and the equations are solved simultaneously. In addition, thermal stress tests via intentionally induced stress due to the temperature gradient in an RTA process are conducted. By these wafer strength tests, which apply a controlled stress level, the relative slip robustness of wafers with nitrogen‐indiffused RTA and polished wafers are compared quantitatively.

Diffusion of nitrogen in solid titanium at elevated temperature and the influence on the microstructure

Nitrogen introduction to solid commercially pure titanium has been carried out at 1650 °C in an electric induction furnace using two different methods. An effective way to avoid the formation of the hard and brittle nitride layer (TiN and Ti2N) is reported. Microstructure and microhardness were examined on the cross-section of the nitrided samples. Multiple phase layers can be observed, and the phases in each layer were identified using X-ray Diffraction. The effects of the experimental conditions such as temperature and nitriding time on the kinetics of nitrogen diffusion were investigated. The nitrogen content within the samples was increased with increasing temperature and nitriding time. Correlations between microhardness and the nitrogen concentration have been developed for the phase(s) present in the core and the outer layers. The diffusion of nitrogen in solid titanium was simulated numerically, and the predicted nitrogen concentration profile in the rods and displacement of Ti–N phase interfaces show good agreement with the experimental observations. Energy-dispersive X-ray spectroscopy and the numerical simulation results suggest that β phase boundary composition is 2.8 wt. % N, the α phase exists within the compositional range of 5.5–6.5 wt. % and the Ti–N phase within the compositional range 7.0–15.0 wt. %, which differs from data extracted from a published phase diagram.

Enhanced Performance of Automotive and Industry Precision Components by Advanced Carbonitriding Technology

Due to their enhanced mechanical properties, gas carbonitrided steel components containing martensite and nitrogen stabilized austenite are currently widely used for highly loaded and severe application conditions. Carbon and nitrogen (C–N) concentration profiles developed during the gas carbonitriding have significant effect on the final properties of the steel. To achieve consistent properties and increase the reliability of processes, simulation of the C–N profile and evolving precipitates during the carbonitriding is essential. Generally, it is observed that the surface nitrogen content developed in the low-alloyed bearing-grade steels is much higher compared to the nitrogen potential in the furnace atmosphere during gas carbonitriding. The formation of nitrides/carbonitrides is one of main reasons for this difference. Thus, diffusion equations cannot be directly applied to calculate the C–N profile, as they do not include precipitation. To solve this problem, a mathematical approach is developed in this work. Thermodynamic data from MatCalc and experimental data are used to formulate equations to calculate the precipitated fraction of C–N during carbonitriding. Furthermore, these equations are integrated with diffusion equations to predict C–N profile that includes precipitates and the developed model is validated with experiments.

The Effect of Nitrogen Concentration at Oxynitride Gate Insulators Formed by 28N2 + Implantation into Silicon with Additional Conventional or Rapid Thermal Oxidation

Silicon oxynitride (SiOxNy) insulators have been obtained by nitrogen ion implantation into Si substrates prior to conventional or rapid thermal oxidation. These films have been used as gate insulators in nMOSFETs and MOS capacitors. nMOSFET electrical characteristics, such as field effect mobility between 390 cm2/Vs and 530 cm2/Vs, and sub-threshold slope between 70 mV/decade and 150 mV/decade, were obtained. MOS capacitors were used to obtain capacitance-voltage (C-V) and current-voltage (I-V) measurements. The Equivalent Oxide Thickness (EOT) of the films were obtained from C-V curves, resulting in values between 2.9 nm and 12 nm. SiOxNy gate insulators with EOT between 2.9 nm and 4.3 nm have presented gate leakage current densities between 3 mA/cm2 and 50 nA/cm2. The electrical characteristics were compared and correlated with the nitrogen concentration profiles at SiOxNy/Si of the structures, obtained by Secondary Ion Mass Spectrometry (SIMS).

Effects of shallow plasma nitriding on the surface topography of gray cast iron specimens

A concept of plasma treatment for enhancing pre-textured surfaces of components made in pearlitic gray cast iron is proposed. It consists of a plasma nitriding at low temperatures and short times, to increase the hardness of the surface employing a shallow depth nitride formation in the surface of the iron. As a result, the roughness pattern of a previously textured surface is preserved after the treatment. A series of plasma nitriding experiments was run on specimens produced from gray cast iron. Flat specimens, obtained from blanks extracted from the cylinder wall of an engine cylinder block, were textured in laboratory to emulate a typical roughness distribution of an ordinary cylinder bore plateau honing process. ε‑iron nitride (Fe3N) formation was identified via X-ray diffraction analysis. Also, the nitrogen concentration profile was quantitatively evaluated using wavelength-dispersive (X-ray) spectroscopy microanalysis, substantiating the nitride formation at shallow depth. Microhardness measurements indicated that ε‑iron nitride and solid solution hardening at shallow depth increased the hardness of the textured surface. 3-D interferometry roughness measurements were performed before and after the plasma treatment to assess the topographic stability of the textured specimens. The results demonstrated that the roughness texture pattern was satisfactorily preserved after the treatment, which was supported by direct observation of the surfaces via scanning electron microscopy.

Finite Element (FE) Modeling of the Nitrogen Concentration Profile in 31CrMoV9 Steel After Gas Nitriding

This paper presents a possibility for the numerical calculation of the profile of nitrogen (N) concentration in 31CrMoV9 steel after gas nitriding. Samples subjected to gas nitriding at three different temperatures were investigated with different experimental methods. The concentration profile of nitrogen was determined using a Jeol JXA-8900 RL electron probe microanalyzer (Epma). The distance between two points was 5 μm, while the acceleration voltage was 20 kV and the sample current was 40 nA. The nitrogen concentration profile was then modeled using the finite-element method. The program was written in Ansys Parametric Design Language, and the basic element was the three-dimensional element Solid70, while simulation of the model was done in Ansys. Comparison of the results obtained from Epma and modeling confirmed the accuracy of the theoretical approach.

Analysis of boron profiles as function of nitrogen content in silicon

In this paper, the Secondary Ion Mass Spectroscopy (SIMS) technique is used to study the experimental depth concentration profiles of boron, nitrogen, and other expected atoms in amorphous nitrogen doped silicon (NiDoS) layers deposited at Td = 460 °C by low-pressure chemical vapor deposition (LPCVD) process from disilane. It has been observed that the reduction of implanted boron penetration is important when the film deposition temperature is low and the nitrogen in-situ doping step is initially improved. And, the simulation results with the experientially measured projected range values using SIMS are in good agreement.

A steady-state analytical profile method for determining methane oxidation in landfill cover

Gas concentration profiles of carbon dioxide (CO2), oxygen (O2), methane (CH4) and nitrogen (N2) are usually measured during tests investigating microbial aerobic methane oxidation in landfill cover. However, only qualitative/limited information can be obtained from gas concentration profiles by existing methods. A new method is proposed to determine methane oxidation in soil quantitatively and comprehensively, including methane oxidation efficiency, stoichiometry, gas transfer mechanism, methane generation rate and gas reaction rate distributions. Governing equations are established based on mass balance for O2, CO2, CH4 and N2 at one-dimensional and steady-state condition. Gas transfer mechanisms considered include gas diffusion, advection and gas reaction. The method utilizes gas concentration profiles to determine gas diffusion for each gas component according to Fick's law. Then gas advections and reactions can be determined by mass balance. The method is validated by (i) published soil column tests investigating methane oxidation and (ii) a calibrated numerical model based on a selected soil column test. The new method is capable of determining methane oxidation efficiency, stoichiometry, gas transfer mechanism, methane generation rate and gas reaction rate distributions for CH4, CO2 and O2.

Effect of Nitriding on Antibacterial Performance of 8Cr17 and 4Cr13 Martensitic Stainless Steel

Abstract Martensitic stainless steel is widely used for medical apparatus, but it can carry a large number of bacteria and cause a potential risk of infection because it has no antimicrobial ability. In this paper, nitriding of 4Cr13 and 8Cr17 martensitic stainless steel was conducted to improve the antibacterial activity. The surface morphology and phase constituents of nitrided 4Cr13 and 8Cr17 specimens were characterized using scanning electron microscopy and X-ray diffraction. Nitrogen concentration profiles in the cross sections of the nitrided specimens were obtained by glow discharge spectroscopy. The antibacterial properties of this nitrided layer were evaluated by a plate-counting method. The antimicrobial ratios of nitrided 4Cr13 and 8Cr17 martensite stainless steel against Staphylococcus aureus were 98.13 % and 99.22 %, respectively, after 12 h. The results showed that the formations of chromium nitride (CrN) and two iron nitrides (Fe3N and Fe4N) are of vital importance to the antibacterial properties of martensitic stainless steel.

Tribological behaviour of plasma nitrided cast iron D6510 and cast steel S0050A under the inclined-impact sliding condition with extremely high contact pressure

Plasma nitriding as a surface modification was applied on two substrate materials: cast iron D6510 and cast steel S0050A. After measurement of the friction coefficients of the treated samples using a pin-on-disc tribotester, an inclined impact-sliding wear tester was utilized to investigate their tribological behaviour under tilting contact with extremely high contact pressure. While numerous surface fatigue cracks, severe chipping, and peeling of the compound layer were observed for the treated cast steel sample, the treated cast iron sample had far fewer surface fatigue cracks without chipping or peeling of the compound at the same test condition. The governing mechanisms of the treated cast iron sample’s superior resistance to surface fatigue failure were revealed by studying the cross-sectional hardness and nitrogen concentration profile. Energy-dispersive X-ray spectroscopy (EDS) analysis indicated that the treated cast iron sample had a smaller nitrogen concentration gradient, which led to a smaller hardness gradient as measured. The results suggest that a smaller hardness gradient between the compound layer and the diffusion zone and a thicker hardened case was able to improve the wear resistance and surface fatigue cracking resistance against high contact loads. Moreover, the smaller friction coefficient of the treated cast iron sample could also be beneficial for improving the wear resistance.

Numerical and Experimental Study on the Residual Stresses in the Nitrided Steel

In the present work, residual stresses distribution in the gas nitrided AISI 4140 sample has been studied using finite element (FE) simulation. The nitrogen concentration profile is obtained from the diffusion-controlled compound layer growth model, and nitrogen concentration controls the material volume change through phase transformation and lattice interstitials which results in residual stresses. Such model is validated through residual stress measurement technique—micro-ring-core method, which is applied to the nitriding process to obtain the residual stresses profiles in both the compound and diffusion layer. The numerical and experimental results are in good agreement with each other; they both indicate significant stress variation in the compound layer, which was not captured in previous research works due to the resolution limit of the traditional methods.