Diffusion Of Nitrogen Atoms Research Articles

Nitrogen atom diffusion into TiO2 anatase bulk via surfaces

Nitrogen atom diffusion from anatase surfaces to the bulk via the interstitial sites is studied by first principle calculations and nudged elastic band method. Anatase surfaces (101), (001), (100), and (103) are chosen for the surface diffusion calculations. It shows that the diffusion from the outermost surface to the sub-surface is the most difficult step for a N atom diffusing into the bulk and it is easier for a N atom diffusing out of the bulk and staying on the surface or at the sub-surface sites. According to the diffusion barriers, N diffusion into the bulk via a (101) surface is relatively easier than the diffusion via other three surfaces for the N doping process.

Study on the trap site in Ni–Cr alloy by first principle approach

First-principle approach for nickel chromium binary alloy with nitrogen was proposed to determine the trapping position of nitrogen in austenitic alloys. The energy of the alloy matrix with the nitrogen atoms at different positions relative to chromium atoms were calculated by density function theory. The calculated results show that the nitrogen atom diffusion along pathways through tetrahedron site consumed less energy than direct diffusion. Among the pathways through tetrahedron site, nitrogen atoms in octahedron sites without nearest neighboring chromium atom need least energy, while those with two chromium atoms need the most energy. Therefore, the trapping site for the nitrogen atom in the Cr containing alloy is the octahedron site with nearest neighboring two Cr atoms.

Detailed surface analyses and improved mechanical and tribological properties of niobium treated by high temperature nitrogen plasma based ion implantation

Transition metal nitrides, as is the case of NbN, exhibit an attractive mixture of physical, chemical and mechanical properties and can be used to overcome a severe constraint of pure niobium, that is the high oxidation rate for temperature above 400°C. In this work nitrogen ions were implanted into niobium by means of plasma based ion implantation (PBII) and high temperature PBII (HTPBII) in order to produce NbN on the surface of the material. In the case of HTPBII the treatment was performed in the temperature of 1000°C and 1250°C. In the process, negative pulses of 10kV/20μs/500Hz were applied to Nb samples for 1h. The depth of the modified layer reached up to 4.5μm due to the diffusion of nitrogen atoms implanted into the material. X-ray diffraction spectra showed the presence of ɛ-NbN, β-Nb2N and γ-Nb4N3 phases. Wear rate was reduced from 1.5×10−2mm3/Nm up to 2.6×10−6mm3/Nm for treated samples at high temperature in comparison with pristine samples, while friction coefficient was reduced from 0.8 to 0.25. Hardness was significantly increased. Surface topography was measured by optical profilometry. Surface roughness increases with the sample temperature but it remains lower than the one obtained by conventional PBII, very probably due to the heating method, which was performed by electron bombardment.

Formation of wear resistant coatings on Ti–6Al–4V by calciothermic reduction

A nitriding method defined as, “calciothermic assisted immersion nitriding (CAIN)” has been developed to modify the surface of titanium alloys and improve their tribological properties. The CAIN process described here involves the heat treatment of titanium alloy specimens (Ti–6Al–4V) in an air environment within a sealed vessel containing a molten flux of calcium and calcium chloride, resulting in the preferential diffusion of nitrogen atoms into the samples. The nitriding process was found to produce a consistent TiCxNyOz surface layer, where the exact chemical composition was dependent on the processing conditions. The tribological properties of the surface layers were tested using a high frequency reciprocating rig under dry lubrication conditions and demonstrated that the CAIN process can be used to produce layers with good adhesion to the Ti–6Al–4V substrate and wear resistance comparable to industrially produced physical vapour deposition (PVD) coatings.

TiN(001)表面上3N1Ti1Si岛构型及其演变的第一性原理研究

In order to study the interface formation in the growth process of Ti-Si-N films,a series of calculations have been carried out with the first principle method to investigate the total energies and adsorption energies of some 3N1Ti1Si island configurations on the TiN(001) surface,and also the activation energies of two kinds of transformations from the Si-in-3N1Ti configuration to the Ti-in-3N1Si configuration.The calculations present some interesting results:(1) According to the energies of all 3N1Ti1Si configurations,the Ti-in-3N1Si configuration is a relative stable structure.It implies that silicon atom outside of TiN island could lead to the structure stable.(2) In the island evolution from the Si-in-3N1Ti configuration to the Ti-in-3N1Si configuration,the diffusion of silicon and titanium atoms need less activation energy than the diffusion of nitrogen atoms.(3) Compared with the evolution of 2Ti2N1Si island,the phase separation of SiN and TiN could be easily performed in the evolution of 3N1Ti-1Si island.This means that properly increasing the partial pressure of nitrogen in the deposition is beneficial to the interface formation in Ti-Si-N film growth process.

Structure and mechanical properties of iron after surface severe plastic deformation under friction with simultaneous nitrogen saturation: I. structure formation

The structure of armco iron after severe plastic deformation under friction with simultaneous nitrogen saturation has been studied by microstructure analysis, X-ray diffraction, and transmission and scanning electron microscopy. A gradient surface layer, in which the grain size varies from micro- to submicro- and nanometer-scale values, is found to form. A mutual intensifying effect of the deformation-induced refinement of a grain structure and the diffusion of nitrogen atoms in iron is shown. The formation of a nanocrystalline state is discussed as a result of dynamic recrystallization.

Effects of sintering temperature on microstructure, nitrogen deficiency and densification of spark plasma sintered Mn 3Cu 0.5Ge 0.5N

Single-phase Mn 3Cu 0.5Ge 0.5N bulks were prepared by spark plasma sintering (SPS) in nitrogen atmosphere at different sintering temperatures, and the effects of sintering temperature on microstructure, nitrogen deficiency and densification were studied. It was found that the densification is accompanied by the thermal decomposition of Mn 3Cu 0.5Ge 0.5N. Nitrogen deficiency and porosity due to the thermal decomposition of the compound do not increase with increase of the sintering temperature; the nitrogen deficiency from decomposition is compensated by the diffusion of nitrogen atoms from the N 2 atmosphere. When the two processes reach the balance, minimum nitrogen deficiency of Mn 3Cu 0.5Ge 0.5N is achieved together with the highest densification.

Oxidation Resistance of CrN and CrAlN Coating Tools

CrN and CrAlN coating tools were heated and heat-preserved in electric stove to make oxidation experiment. The results were obtained through SEM, EDX and XRD. The results are as follows: The oxidation of CrN and CrAlN coating is induced by the micro-holes which were formed by the diffusion of nitrogen atoms and chromium ions towards the coating surface under sufficient thermal energy. With the incorporation of Al, CrAlN coating showed stronger oxidation resistance as compared to CrN. The dense mixture of Cr2O3and Al2O3makes oxygen diffusion through the film difficult and then oxidation and decomposing are alleviated effectively, therefore CrAlN coating can remain high oxidation resistance and high hardness after high temperature which make this coating tool be competent for high speed machining of the hardworking materials. In addition, the failure of the coating is primary induced by oxidation and the different expansion coefficients between coating and substrate.

Nitriding of high speed steel by bipolar PBII for improvement in adhesion strength of DLC films

In the present study, bipolar plasma based ion implantation and deposition (bipolar PBII) was used for plasma nitriding of high speed steel (SKH2), and the effects of the treatment parameters (positive pulse voltage, negative pulse voltage, treatment pressure, treatment time, and precursor gases) on the nitriding process were investigated. The hardness, roughness, and depth of nitride layer were also measured. The adhesion strength of diamond-like carbon (DLC) films coated on the nitride substrate was evaluated by carrying out Rockwell indentation and microscratch tests. Nitriding by bipolar PBII was achieved in the combining of two effects: nitrogen ion implantation by applying a high negative pulse voltage and thermal diffusion of nitrogen atoms under the application of a high positive pulse voltage. However, a very high voltage negative pulse caused surface roughening of the nitride layer. Application of a high positive pulse voltage during nitriding was found to be effective in promoting the thermal diffusion of the implanted nitrogen atoms. Effective nitriding could be achieved under the following conditions: high positive pulse voltage, low negative pulse voltage, high nitrogen gas pressure, and addition of hydrogen to the precursor gas. The adhesion strength of the DLC films on the SKH2 substrate was well improved after nitriding.

Effect of nitrogen bonding states on dipole at the HfSiO/SiON interface studied by photoemission spectroscopy

We have investigated effect of nitrogen bonding states on dipole at the HfSiO/SiON interface using photoemission spectroscopy with synchrotron radiation. Significant increase in the valence-band discontinuity between HfSiO films on a Si substrate upon annealing is observed, which can be related to changes in the interface dipole. Chemical states and in-depth profiles analyses suggest diffusion of nitrogen atoms from the HfSiO/SiON interface to the substrate during annealing processes. It is found that the formation of the interface dipole strongly depends on the nitrogen bonding states and their distributions at the HfSiO/SiON interface.

Effect of Thermal Annealing or Plasma Treatment on Analog Characteristics for High-k Material Capacitors

For TaN/HfO2/TaN capacitors, thermal annealing or plasma treatment is conducted after HfO2 deposition. Rapid thermal annealing or furnace thermal annealing at 400 °C can improve the leakage current but worsens the voltage linearity of capacitance. A capacitor with adequate NH3 plasma treatment can have a lower leakage current and a better voltage linearity property compared with the control sample. Atomic force microscopy (AFM) data indicate that the roughness of thermal-annealed or plasma-treated HfO2 film is related to the voltage linearity property for the capacitor. In addition to thermally induced stress, the oxygen atoms diffuse through the HfO2 film and then oxidize the bottom TaN electrode, leading to a rougher interface during O2 thermal annealing. Because of nitrogen atom diffusion into HfO2 film and a less damaged surface, the quality of the TaN/HfO2 interface and HfO2 film can be improved, which leads to better analog characteristics.

Synthesis of nanocrystalline CrN by arc discharge

Nanocrystalline chromium nitride (CrN) with the cubic rock-salt structure was synthesized by the arc discharge method in nitrogen gas (N 2). The product was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that the nitrogen gas pressure is a crucial factor for the synthesis of cubic CrN. At relatively low N 2 pressure, cubic CrN was formed. With the increase of N 2 pressure, hexagonal Cr 2N and metal Cr were gradually formed. It indicated that the formation of CrN apparently favors a low nitrogen pressure environment, and the diffusion of nitrogen atoms into the Cr was lowered with the increase of N 2 pressure. We explain this experimental observation in terms of the evaporation rate of anode Cr and the ionization of nitrogen.

Improvements in the understanding and application of duplex coating systems using arc plasma technology

There has been interest for some years now in the development of duplex treatments involving plasma nitriding coupled with the deposition of a hard coating. Although this coating system has in service use for various applications, there are still however many underlying uncertainties with regard to how treatment parameters affect coating performance, for example the deposition temperature and the related substrate softening. Different duplex treatments are applied to SKH 51 and SKH 2 steel by changing the nitriding atmosphere and TiN deposition temperature. The nitride layers with the ε-Fe 3N phase are more prone to denitriding than those consisting of either an α or γ′-Fe 4N phase during deposition of the hard coating. The black layer formed through denitriding of the SKH 51 alloy is thinner than that of the SKH 2 alloy. This is considered to be a result of the presence of nitride forming elements. Regardless of the nitride phases that are formed, the diffusion of nitrogen atoms is enhanced at increased substrate temperatures, induced by ion bombardment characteristics during coating growth, causing a significant decrease in the nitrogen content of the original prenitrided layer. The composite surface hardness of the treated specimens is found to be most dependent on substrate softening effects, as opposed to factors such as black layer formation. The adhesion of the TiN coating to the underlying nitrided layer again appears to be influenced more by softening of the substrate during higher temperature deposition as opposed to the formation of a black layer. This is indicated by the fact that the adhesion to the samples with an initial ε-Fe 3N nitrided surface is greater than those with an α phase, even though the black layer is only present for the former. These studies reveal that avoiding substrate softening is essential in producing high performance hard coating/nitride duplex coatings.

The influence of a small amount of active sites on the adsorption kinetics of nitrogen on ruthenium

Recently, Dahl et al. [S. Dahl, A. Logadottir, R.C. Egeberg, J.H. Larsen, I. Chorkendorff, E. Törnqvist, J.K. Nørskov, Phys. Rev. Lett. 83 (1999) 1814; S. Dahl, E. Törnqvist, I. Chorkendorff, J. Catal. 192 (2000) 381] have proposed very interesting hypothesis that the rate of dissociation and adsorption of nitrogen on Ru(0 0 0 1) facet is totally dominated by the presence of a small amount of step sites on Ru(0 0 0 1) terraces. Following this idea, a kinetic model, based on applying the Statistical Rate Theory approach, was developed in order to explain if such mechanism is able to explain the observed features of the system N 2/Ru(0 0 0 1). As a result, it was stated that the activation barrier for adsorption on the active (step) sites is equal to 36 kJ/mol; in turn, the adsorption energy of nitrogen atoms on the active sites is 43 kJ/mol. It implies that the rate of adsorption via the active sites is much faster than direct adsorption on the three-fold hollow sites; moreover, the occupation of the active sites is always close to zero at the investigated temperatures, so they are not blocked and may act as an indirect channel for adsorption. Thus, the rate of nitrogen adsorption on Ru(0 0 0 1) surface is governed by the rate of diffusion of nitrogen atoms from the active sites into the three-fold hollow sites. The analysis of thermodesorption spectra revealed an important role of repulsive interactions between the N atoms adsorbed on the hollow sites, the associated interaction parameter between nearest neighbors was estimated to be 5 kJ/mol. The presence of small amount of gold on Ru(0 0 0 1), apart of blocking the active sites, seems to remove the repulsion between nitrogen atoms.

Diffusion kinetics of explosively treated and plasma nitrided Ti–6Al–4V alloy

Ti–6Al–4V alloys, which were exposed to an explosive shock process, were nitrided in nitrogen plasma in the temperature range of 700–900°C for 3–12 h. During the plasma nitriding, the surface layer consisted of TiN ( δ), Ti 2N ( ε) and nitrogen solid solution layers ( α-Ti). The growth rate of nitride and solid solution layers were found to be controlled by the diffusion of nitrogen. An effective nitriding was achieved due to high dislocation density and vacancy concentration. Based on the present layer growth data, an analytical model for multiphase diffusion was used to estimate the effective nitrogen atom diffusion coefficient in the nitride layers. The interface velocity equations were derived from Fick's law and a numerical method has been used to compute the diffusion coefficients of nitrogen in a binary multiphase Ti–TiN system. Depending on temperature and layer thickness, the activation energies of nitrogen in TiN and Ti 2N phases were found to be 18,950 (±2116) and 27,925 (±1105) cal/mole, respectively.

Characteristics of the Compound Layer on the SCM 435 Steel Produced by Plasma Nitrocarburizing

Plasma nitrocarburizing treatment was performed for SCM 435 steel by using a pulsed plasma ion nitriding system. The effects of the variation of nitrogen and methan contents upon the hardened layer were investigated. Both the thickness of the compound layer and the amount of ε phase in the compound layer increased with nitrogen content. For an atmosphere containing 90% N2 gas content in the gas mixture, the thickness of the compound layer decreased due to the reduction of the number of nitrogen ion and slow diffusion of nitrogen atoms. The relative amount of ε phase in the compound layer increased with CH4 gas content up to 2%. For CH4 gas content over 2% in the gas mixture, the thickness of the compound layer decreased with CH4 content due to the formation of θ-Fe3C phase.

Kinetics of In-situ formation of AlN in Al alloy melts by bubbling ammonia gas

In-situ processing of AlN-Al alloy composites by the gas bubbling method was investigated using ammonia as the gaseous precursor in the temperature range of 1373 to 1523 K. The products were characterized using X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray microanalysis. It was found that in-situ formation of AlN reinforcing particles was feasible by bubbling ammonia through aluminum and aluminum alloy melts. The AlN particles formed in situ were small in size and enriched in the top part of the product formed in the crucible. In comparison with the purified nitrogen bubbling gas, using ammonia as the nitrogen precursor enhanced the formation of AlN. Hydrogen gas generated from the dissociation of ammonia is an effective oxygen getter in the system, which can offset the deleterious effect of oxygen impurities and enhance the formation of AlN. The rate of formation of AlN was controlled by the diffusion of nitrogen atoms in the liquid boundary layer. A kinetic model was developed to describe the rate of formation of AlN, which was in excellent agreement with the experimental results. The influence of various processing variables on the rate of formation of AlN were also investigated. The rate of formation of AlN changed little with the content of silicon in the matrix melt, increased slightly with increase in temperature and decrease in nozzle size, and increased significantly with the increasing gas flow rate.

The effects of nitridation on the permeability-frequency spectra in nanocrystalline Fe88Zr7B4Cu alloys

The effects of nitridation on the permeability-frequency spectra in Nanoperm alloys were studied. In the initial stage of nitridation, the nitrogen atom diffusion into Nanoperm alloys may make the extent of increase of the relaxation frequency f0 exceed the extent of decrease of initial permeability μi. However, as the materials are nitrided for 200 min in a nitrogen atmosphere of 0.03 MPa at 400°C, μif0 shows a tendency to be constant and the increase of the strength of domain wall pinning with nitriding time becomes the main factor affecting the dynamic magnetization process. Nitridation can improve the soft magnetic properties of nanocrystalline soft magnetic materials in the higher frequency range.

Mechanism and kinetics of early growth stages of a GaN film

The growth of GaN islands on the substrate surface covered with an AlN buffer layer is theoretically investigated at the stages of nucleation and Ostwald ripening in the temperature range 480–1000°C. The following inferences are made from analyzing the results obtained. (1) At temperatures T>650°C, the growth of islands is controlled by the chemical reaction of formation of GaN molecules around the periphery of the island surface. Islands nucleated at these temperatures are characterized by a large spread in their sizes. (2) At temperatures T<600°C, the island growth is governed by the surface diffusion of nitrogen atoms. Islands nucleated at these temperatures are virtually identical in size. (3) In the temperature range 600–650°C, the mechanism of island growth gradually changes over from the mechanism associated with the surface diffusion of nitrogen atoms with a large mean free path to the mechanism determined by the diffusion of gallium atoms with a smaller mean free path. The supersaturation, flux, and size distribution functions of GaN nuclei are calculated at different substrate temperatures. The phase diagrams describing the evolution in the phase composition of GaN islands with variations in temperature are constructed.

Diffusion of Nitrogen Atoms Implanted into Synthetic Diamonds

The effect of isothermal annealing on the depth profiles of nitrogen atoms implanted into synthetic diamonds is studied by secondary-ion mass spectrometry. The annealing is performed under vacuum at 1400°C for 1, 5, or 20 h. It is found that the depth profile broadens on a macroscopic scale with annealing time. The broadening is successfully represented by a mathematical model of diffusion. The diffusion coefficient is roughly estimated at 2.3 × 10–15, 8.5 × 10–16, and 3.7 × 10–16 cm2/s for annealing times of 1, 5, and 20 h, respectively.