Interstitial Nitrogen Atoms Research Articles

The Effect of La on the Surface Properties of Plasma Nitrided CoCrCuFeNi High-Entropy Alloys at 440 Degrees Celsius

This paper investigates the effect of the element La on plasma nitriding of the CoCrCuFeNi high-entropy alloy (HEA) at 440 °C for 8, 16, and 24 h. The phase composition, morphology, and hardness distribution of the nitrided layer are characterized using XRD, optical microscopy, and a microhardness tester. Furthermore, the corrosion resistance is tested using an electrochemical workstation. The study evaluated the friction and wear performance using a testing machine and scanning electron microscope. The thickness of the effective hardening layer after 16 h of treatment with La was similar to that after 24 h of treatment without La. The addition of La significantly increased the growth rate constant of the effective hardening layer from 0.53 × 10−14 m2/s to 0.72 × 10−14 m2/s. In addition, an expanded FCC phase with greater interplanar spacing can be formed on the surface of the sample by introducing La into the plasma nitriding process. This indicates that the expanded FCC phase, with a higher concentration of interstitial nitrogen atoms, can effectively improve the corrosion resistance of the specimen surface. The corrosion rate of the specimen surface was reduced by 27.5% and the wear rate was reduced by 41.7% after 16 h of treatment with the addition of La compared to 24 h of nitriding without the addition of La. It has been shown that the addition of La to the plasma nitriding process results in a higher quality nitrided layer in a shorter time and also demonstrates that La has the potential to optimize the surface properties of plasma nitrided HEAs.

Electrochemical properties and interfacial contact resistance of plasma ion nitrided titanium as PEMFC bipolar plate

Plasma ion nitriding was applied to grade 1 titanium, which was used as a bipolar plate a for proton exchange membrane fuel cell (PEMFC). Electrochemical experiments were performed in a solution of pH3 (Sulfuric acid + 0.1 ppm Hydrofluoric acid, 80 °C) required by the Department of Energy (DoE). Air and hydrogen gas was bubbled to simulate cathode and anode environments, respectively. Several experiments (potentiodynamic polarization and potentiostatic), analyses (XRD, EDS, FE-SEM, and EIS), and interfacial contact resistance measurements were performed to evaluate plasma ion nitrided titanium used as a PEMFC bipolar plate. Plasma ion nitrided titanium showed improved corrosion resistance and interfacial contact resistance compared to the titanium substrate. The nitrided layer was approximately 2.0–2.5 μm, with dense formation of the layer on the titanium substrate. The results of the EDS analysis indicated that a nitrogen diffusion layer of approximately 2.2 μm formed due to interstitial nitrogen atoms.

Investigation of dilute ferromagnetism induced at room temperature in undoped crystalline AlN thin film after isothermal annealing in a controlled oxygen environment

The crystalline aluminium nitride (AlN) thin films were isothermally annealed at 850 °C in a controlled oxygen environment. The oxidation process has led to significant enhancement in lattice parameters and degradation in the crystallinity of as-grown AlN film. The AlN bonding at the film’s surface was systematically reduced and finally vanished when the film was annealed at 20% oxygen mixed with an argon environment. During annealing, nitrogen in AlN is replaced by oxygen, and a fraction of replaced nitrogen is trapped at the interstitial sites in AlN, which systematically enhances with an increment in oxygen content. Magnetic measurements exhibit dilute ferromagnetism induced in as-grown AlN film at 300 K after annealing. Density functional theory (DFT) calculation reveals interaction between N 2p orbitals of interstitial nitrogen and first neighbouring nitrogen atoms contributing to observed magnetic properties.

Enhanced strength-ductility synergy via high dislocation density-induced strain hardening in nitrogen interstitial CrMnFeCoNi high-entropy alloy

• Nitrogen addition induced a remarkable increase in strength but only a negligible decrease in uniform elongation of CrMnFeCoNi HEA. • First-principles calculations were carried out to analyze the effect of nitrogen on generalized stacking fault energy ( γ usf , γ isf , and γ utf ) and twinning. • The pinning effect of nitrogen on dislocations contributed to the increased dislocation density and thus the higher strain-hardening rate. The present work demonstrates that nitrogen doping inhibits the formation of deformation twins in a CrMnFeCoNi high entropy alloy, while significantly increasing the strength without sacrificing much ductility at 77 K. Microstructural characterization and first-principles calculations were employed to unveil the role of interstitial nitrogen atoms in obtaining such an excellent combination of strength and ductility at 77 K. It is found that nitrogen addition increases generalized stacking fault energy (GSFE) and reduces twinning. However, the pinning of dislocations by nitrogen atoms effectively suppresses dislocation cross-slip and dynamic recovery and in turn, promotes the accumulation of dislocations. The high dislocation density induces a high strain hardening capacity and improves uniform elongation, which compensates for the ductility loss accompanied by solid solution strengthening. The effect of nitrogen doping enriches the design concept of high- and medium-entropy alloys, providing an economical and effective strategy to develop ultra-high-performance alloys that are suitable for cryogenic applications.

Effect of nitrogen content on the mechanical properties and deformation behaviors of ferritic-pearlitic steels

The effect of nitrogen content (0.001 wt%, 0.009 wt%, 0.017 wt%) on the microstructure, mechanical properties and deformation behaviors of a medium-carbon ferritic-pearlitic steel is investigated in this study. The addition of nitrogen is shown to have little effect on the ferrite-pearlite microstructure and only marginally coarsens it because austenite grain growth is promoted. Although nitrogen is a strong element for solid solution strengthening, nitrogen addition only marginally improves the strength of the steel and has little effect on the ductility because fine Fe3N phases are precipitated during the tensile process. The toughness of the steel is markedly decreased by the addition of nitrogen because the dislocation locking effect caused by interstitial nitrogen atoms is enhanced. When the nitrogen content increases from 0.001 wt% to 0.017 wt%, the yield-point phenomenon in the tensile process is first strengthened and then weakened. The yield-point phenomenon is closely related to the dislocation behaviors in proeutectoid ferrite, and the different yield-point phenomena under different nitrogen contents are related to the dislocation locking effect caused by interstitial nitrogen atoms and to the dislocation pinning effect caused by Fe3N phases.

Influence of Nitrogen Incorporation Sites on Structural and Optical Properties of Sputtered TiO2-N Thin Films with Improved Visible Light Activity

The influence of deposition and annealing parameters on optical, structural and photocatalytic properties of N-doped titanium-dioxide (TiO2) thin films have been studied. Anatase and anatase-rutile crystalline phase mixture TiO2 thin films were obtained depending on the reactive direct current (DC) sputtering and annealing conditions. Deposition in the nitrogen atmosphere produced N-doped TiO2 thin films with energy gap values shifted towards visible region in comparison with thin TiO2 films deposited without nitrogen in the atmosphere. The role of substitutional and interstitial nitrogen atoms incorporated in obtained crystalline structures during the deposition was shown important and was reflected in the efficiency of photocatalysis. It was found that thin TiO2 films with more substitutional incorporated nitrogen exhibits better photocatalytic properties. These results could be of importance towards achieving controlled synthesis of N-doped TiO2 thin films with desired structural and optical properties, especially with improved photo-activity in the visible part of the solar spectrum.

Pressure-induced disordering of site occupation in iron–nickel nitrides

Controlled disordering of substitutional and interstitial site occupation at high pressure can lead to important changes in the structural and physical properties of iron–nickel nitrides. Despite important progress that has been achieved, structural characterization of ternary Fe–Ni–N compounds remains an open problem owing to the considerable technical challenges faced by current synthetic and structural approaches for fabrication of bulk ternary nitrides. Here, iron–nickel nitride samples are synthesized as spherical-like bulk materials through a novel high-pressure solid-state metathesis reaction. By employing a wide array of techniques, namely, neutron powder diffraction, Rietveld refinement methods combined with synchrotron radiation angle-dispersive x-ray diffraction, scanning electron microscopy/energy dispersive x-ray spectroscopy, and transmission electron microscopy, we demonstrate that high-temperature and high-pressure confinement conditions favor substitutional and interstitial site disordering in ternary iron–nickel nitrides. In addition, the effects of interstitial nitrogen atoms and disorderly substituted nickel atoms on the elastic properties of the materials are discussed.

High entropy nitride (HEN) thin films of AlCoCrCu0.5FeNi deposited by reactive magnetron sputtering

Thin films of high entropy alloys (HEAs) are of great interest for surface engineering applications due to their exceptional properties including superior hardness, resistance to oxidation and corrosion, high temperature stability and high hydrophobicity. The microstructural and the physical properties of high entropy nitride (HEN) thin films, that are chemically inert and possess superior properties, can be effectively tunned using magnetron sputtering processes in the reactive mode. In this study, high entropy nitride thin films of AlCoCrCu0.5FeNi were deposited using RF reactive magnetron sputtering with three different nitrogen gas flow fractions (RN) of 6.25, 12.5, and 25%. X-ray diffraction technique and transmission electron microscopy revealed that the crystallinity of the HEN films changed from a mixed cubic FCC and BCC to a partially amorphous phase with increasing RN due to the accumulation of more interstitial nitrogen atoms in the film. Modifications of film morphology, such as decreases in grain size and surface roughness, were observed from scanning electron microscopy and atomic force microscopy, respectively. Energy dispersive spectroscopy recorded the highest nitrogen concentration of 10.6% for the film deposited at an intermediate RN of 12.5%. X-ray photoelectron spectroscopy detected significant levels of surface protective oxides (such as Al2O3 and Cr2O3) and nitrides (such as AlN and CrN) for the HEN thin films deposited at higher gas fractions of 12.5 and 25%. According to nanoindentation results, the film deposited at the RN of 25% attained a hardness of 9.8 GPa. The highest hardness, significant oxidation resistance and the highest hydrophobicity with a WCA of 112° were achieved for the HEN film deposited using the highest RN of 25%. Such high entropy nitride films deposited by reactive magnetron sputtering have important implications as protective coatings for a wide range of applications including turbines, engine blades and aircraft bodies used in aerospace industries.

Early recrystallization of Ni/Ti multilayer due to disorder in the Ni layer

Disorders present at the interfaces drive solid state amorphization (SSA) and recrystallization in Ni/Ti multilayers. In this communication, the effect of disorders present in the bulk Ni layers is studied on SSA and recrystallization. The disorder in the Ni layers is created by sputtering Ni layers of the Ni/Ti multilayers under the mixed ambience of argon and nitrogen. It has been observed recently that the neutron optical performances of Ni/Ti graded multilayers or supermirrors can be improved by sputtering of Ni layers under the mixed ambience of argon and nitrogen. In this work, Ni/Ti periodic multilayers deposited under pure argon ambience and under mixed ambience of argon and nitrogen have been annealed under vacuum at 200 °C, 400 °C, and 600 °C for 2 h. All the annealed multilayer samples along with as-deposited multilayers are characterized by x-ray diffraction and grazing incidence x-ray reflectivity measurements in specular and non-specular modes. Atomic diffusion within the layers and across interfaces has been studied as a function of annealing temperature for both sets of multilayers. It is found that the interstitial nitrogen atoms present in the Ni layers play a crucial role in the diffusivity at room temperature as well as high temperature and thus influence the SSA and recrystallization phenomena.

Structural, electronic and magnetic properties of Fe20Cr6Mn6 austenitic alloys with interstitial carbon and nitrogen atoms from density functional theory

Based on a 32-atom supercell system, the structural, electronic and magnetic properties of Fe20Cr6Mn6 alloys are calculated and analyzed using first-principles calculations. The effects of interstitial atoms (N and C) on the properties of Fe20Cr6Mn6 alloys are also discussed. In this 32-atom supercell system with an octahedral interstice in the center, there are four different atomic sites occupied by metal atoms (Fe, Cr and Mn). According to the differences of metal atoms at the different atomic sites, the Fe20Cr6Mn6 alloys have been defined as two structural models. Interstitial atoms (N or C) change the phase stability of Fe20Cr6Mn6 alloys. The supercell volumes increase linearly with the content of N (C) in the supercells for each alloy system. The bond lengths of Mn-C/N or Cr-C/N in the octahedral interstice, not as the supercell volume, do not increase linearly with the content of N (C). The spin moments of the Fe20Cr6Mn6 alloys are very small and the magnetic moments of I-Mn-Cx/Nx alloys and I-Cr-Cx/Nx alloys increase slightly. In Fe20Cr6Mn6 alloys, the electrons transfer from Cr atoms to Fe and Mn atoms, and the ability to gain and lose electrons of metal atoms is changed by the addition of C or N atoms. There is an atomic attraction between metal atoms and C or N atoms from bond population analyses.

Experimental and computational study of visible light-induced photocatalytic ability of nitrogen ions-implanted TiO2 nanotubes**Project supported by the National Natural Science Foundation for Joint Fund Key Project of China (Grant No. U1865206), the National Science and Technology Major Project of China (Grant No. 2017-VII-0012-0107), the National Defense Science and Technology Key Laboratory Fund of China (Grant No. 614220207011802), and the Key

Nitrogen-doped TiO2 nanotubes (TNTs) were prepared by ion implantation and anodic oxidation. The prepared samples were applied in photocatalytic (PC) oxidation of methyl blue, rhodamine B, and bisphenol A under light irradiation. To explore the influence of doped ions on the band and electronic structure of TiO2, computer simulations were performed using the VASP code implementing spin-polarized density functional theory (DFT). Both substitutional and interstitial nitrogen atoms were considered. The experimental and computational results propose that the electronic structure of TiO2 was modified because of the emergence of impurity states in the band gap by introducing nitrogen into the lattice, leading to the absorption of visible light. The synergy effects of tubular structures and doped nitrogen ions were responsible for highly efficient and stable PC activities induced by visible and ultraviolet (UV) light.

Density-Functional-Theory Calculations of Formation Energy of the Nitrogen-Doped Diamond

The geometry optimization of the nitrogen-doped diamond has been carried out by the density functional theory (DFT) calculations. We model the defective diamond of substitutional and interstitial nitrogen atoms by using a simple-cubic supercell. Atoms in the supercell are relaxed by allowing them to move so that the atomic forces are less than 5.0 × 10-3 eV/Å. We calculate the formation energy for substitutional and interstitial sites. We find that the formation energy for the substitutional defect is10.89 eV. We check the convergence of the calculation with respect to the k×k×k - Monkhorst-Pack grids. We show that the energy difference between k = 4 and 6 is very small (7.0 meV). We also check the calculations by using a 216-sites supercell and find that the energy difference is 0.10 eV. Thus, the calculations of the formation energy converge well. As for the interstitial defect, we model some possible configurations and find that the smallest formation energy is 21.88 eV. Therefore, the most stable configuration of the nitrogen-doped diamond belongs to the substitutional site.

Diffusion Paths for Interstitial Impurities in Different Polymorphic Modifications of Niobium Silicide Nb5Si3

Structural models of the α, β, and γ modifications of Nb5Si3 silicides, which are used as a reinforcing phase in composites obtained in situ based on the Nb‒Si system, have been constructed by computer simulation methods. A geometric analysis of unit cells is performed using the H-poisk program to estimate the voids existing in the structures. The results of measuring the number of voids and their sizes are reported. A conclusion about possible diffusion paths of interstitial boron, carbon, nitrogen and oxygen atoms is drawn based on the calculation results, and the solubility of these impurities in the structure of each Nb5Si3 modification is estimated.

Strengthening effects of interstitial nitrogen on rhenium

Introducing interstitial nitrogen atoms into the rhenium lattices is accepted for enhancing the mechanical and thermal properties of the metal. Here, we present an experimental study on the strengthening effects of interstitial nitrogen in rhenium on bulk modulus B0 and elastic shear constant C44 by comparing the difference in the compression and vibration behaviors between Re3N and Re in the in situ high pressure angle dispersive X-ray diffraction and high-pressure Raman scattering. The incorporation of interstitial N atoms introduces the covalent bonding and the special Z-type structure along the c-axis, which caused the bulk modulus B0 and elastic shear constant C44 increase by 14% and 15%, and the incompressibility of the c-axis is obviously enhanced. In addition, this work well explains the strengthening effects of interstitial N atoms on mechanical and vibrational properties.

Interstitial nitrogen atoms in diamond. A quantum mechanical investigation of its electronic and vibrational properties.

The electronic and vibrational features of the single- (I1N) and double- (I2N) nitrogen interstitial defects in diamond are investigated at the quantum mechanical level using a periodic supercell approach based on hybrid functionals constructed from all electron Gaussian basis sets within the Crystal code. The results are compared with those of the well characterized 100 split self-interstitial defect (I2C). The effect of defect concentration has been investigated using supercells with different size, containing 64 and 216 atoms. Band structure, formation energy, charge and spin density distributions of each defect are analyzed. Irrespective of the defect concentration, these defects show important features for both IR and Raman spectroscopies. Stretching modes of the two atoms involved in the defect are calculated to be around 1837, 1761 and 1897 cm-1 for the I1N, I2N and I2C case, respectively. Since they are well removed from the one-phonon mode of pristine diamond (1332 cm-1), they are, in principle, detectable from the experimental point of view.

Microstructure evolution and enhanced mechanical properties in SUS316LN steel processed by high pressure torsion at room temperature

Nowadays, it is of interest to enhance the strength of metals for light-weight design of nuclear reactor, so the objective of the present study is to improve the mechanical properties of SUS316LN steel through grain refinement. The nanostructured SUS316LN steels were produced by high pressure torsion (HPT) at room temperature. The variations of microstructure and strain-induced martensite transformation during HPT were investigated. The homogeneous nanostructured SUS316LN steel with an average grain size of approximately 40nm was achieved by HPT for 5 revolutions under a pressure of 5GPa, which demonstrated extremely high tensile strength (1828MPa) and high hardness (530 HV). The interstitial nitrogen atoms can reduce the stacking fault energy of the material and also play an important role in interaction with mobile dislocations during HPT. Therefore, compared to SUS316L steel, the formation of the nanograined structure was much faster and the strain-induced martensite transformation was restrained in the HPT-produced SUS316LN steel.

Effects of Interstitial Nitrogen Atoms on Atomic Oxygen Adsorption on Fe (001) Surface from <i>Ab Initio</i> Calculations

The effects of interstitial nitrogen atoms on the adsorption of atomic oxygen on fcc Fe (001) surface have been studied using ab initio density functional theory calculations to understand the initial stage of oxidation on nitrogenous austenitic stainless steel. It has been found out that the N atoms can improve the adsorption ability of the O atom at the hollow site on the surface, and thus promote the rapid passivation of nitrogenous austenitic stainless steel. This improvement is possibly because the Coulombic interactions between the O atom and the neighboring Fe and Cr atoms are enhanced due to the electron transfer from the Fe and Cr atoms to the N atoms.

AuNx stabilization with interstitial nitrogen atoms: A Density Functional Theory Study

Researchers have been studying 4d and 5d Series Transition Metal Nitrides lately as a result of the experimental production of AuN, PtN, CuN. In this paper, we used the Density Functional Theory (DFT) implementing a pseudopotential plane-wave method to study the incorporation of nitrogen atoms in the face-centered cube (fcc) lattice of gold (Au). First, we took the fcc structure of gold, and gradually located the nitrogen atoms in tetrahedral (TH) and octahedral (OH) interstitial sites. AuN stabilized in: 2OH (30%), 4OH and 4TH (50%), 4OH – 2TH (close to the wurtzite structure) and 6TH (60%). This leads us to think that AuN behaves like a Transition Metal Nitride since the nitrogen atoms look for tetrahedral sites.

Modification of the yellow luminescence in gamma-ray irradiated GaN bulk single crystal

We report the variation of the yellow luminescence (YL) in GaN bulk single crystals by gamma-ray irradiation. The crystals are irradiated at room temperature with gamma-rays of 1.17 and 1.33 MeV from a cobalt-60 source. Gamma-ray dose is 160 kGy. The resistivity varies from 30 Ωcm for an un-irradiated sample to 104 Ωcm for gamma-ray irradiated one. The nitrogen displacement in gamma-ray irradiated samples is observed by Rutherford backscattering channeling experiments using proton beam, suggesting the existence of the deep energy level relating to interstitial nitrogen atoms. The YL from the un-irradiated GaN with a peak at 557 nm (2.22 eV) is observed at around 440 nm to 800 nm, whereas that of the gamma-ray irradiated GaN shows a peak at 532 nm (2.33 eV) although the YL spectrum is almost overlapped with un-irradiated ones. Compton electrons emitted by the gamma-ray irradiation induce the shallow donor located at about 50 meV bellow the conduction band. This energy level is close to that of nitrogen vacancy. The modification of YL is attributed to a transition from the shallow donor induced by the gamma-ray irradiation to the native gallium vacancy.

Highly Visible Light Activity of Nitrogen Doped TiO2 Prepared by Sol–Gel Approach

A simple approach was explored to prepare N-doped anatase TiO2 nanoparticles (N-TiO2 NPs) from titanium chloride (TiCl4) and ammonia (NH3) via sol–gel method. The effects of important process parameters such as calcination temperatures, NH3/TiCl4 molar ratio (R N) on crystallite size, structure, phase transformation, and photocatalytic activity of titanium dioxide (TiO2) were thoroughly investigated. The as-prepared samples were characterized by ultraviolet–visible spectroscopy, x-ray diffraction, transmission electron microscopy, energy dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy. The photocatalytic activity of the samples was evaluated upon the degradation of methylene blue aqueous solution under visible-light irradiation. The results demonstrated that both calcination temperatures and NH3/TiCl4 molar ratios had significant impacts on the formation of crystallite nanostructures, physicochemical, as well as catalytic properties of the obtained TiO2. Under the studied conditions, calcination temperature of 600°C and NH3/TiCl4 molar ratio of 4.2 produced N-TiO2 with the best crystallinity and photocatalytic activity. The high visible light activity of the N-TiO2 nanomaterials was ascribed to the interstitial nitrogen atoms within TiO2 lattice units. These findings could provide a practical pathway capable of large-scale production of a visible light-active N-TiO2 photocatalyst.