Diffusion Of Nickel Research Articles

A molecular dynamics study of helium diffusion and clustering in fcc nickel

Helium diffusion and clustering in nickel are studied by molecular dynamics simulations. Extensive atomistic simulations are performed using a modified analytic embedded-atom model. We find that the rate of helium clustering increases with increasing temperature, and interstitial helium clusters can create nickel interstitials when they reach sufficient size and temperature is higher than 600K. Finally, the mobility of helium clusters is also investigated using trajectory time decomposition method. The diffusion properties of He clusters with 1–3 helium atoms are found to obey the Arrhenius relationship over the whole temperature range considered while He clusters with 4 helium atoms show an non-Arrhenius relationship at higher temperatures.

Structure, texture, and properties of superconductive electrolytic niobium coatings on nickel and copper substrates

Structure, texture, chemical purity, and other characteristics of superconductive niobium coatings prepared via electrochemical deposition onto nickel and copper substrates in (LiF + NaF + KF)eut + 5 wt % K2NbF7 molten salt are studied. It is shown that, when depositing the coatings onto nickel, the intermediate layer of Ni3Nb and NiNb intermetallides is formed at the substrate-coating interface, which inhibits the diffusion of nickel in niobium. The nickel concentration in niobium coatings above this layer is (2–7) × 10−2 wt %. In coatings deposited onto the copper substrate the copper content does not exceed 1.5 × 10−4 wt %, and from the viewpoint of the amount of impurities, niobium is considered as a high purity metal. The dynamics of structure modifications in niobium coatings and changes in microdeformations and dislocation density in the deposited metal are explored depending on the current density at the cathode.

Characterization of Copper Diffusion in Silicon Solar Cells

Abstract In this work we investigate the degradation behavior of solar cells with a copper front side metallization based on a fine-line screen-printed silver seed-layer, a plated nickel diffusion barrier, a plated copper conductive layer and a silver capping. The results reveal that the cell degradation depends, besides the effectiveness of the nickel diffusion barrier, on the used seed-layer and the firing temperature of the seed-layer. The degradation behavior of cells, produced with different Ag seed-layer pastes resulting in different contact finger geometries after screen-printing and firing, was evaluated. With similar nickel diffusion barrier masses the seed-layer that generates the biggest metallized area at the cell surface shows the fastest degradation. Apart from that, the composition of the seed layer also has an impact on the degradation. The analysis of different firing temperatures shows that higher set-peak temperatures result in faster cell degradation due to copper migration.

TEM analysis of the hafnium-doped aluminide coating deposited on Inconel 100 superalloy

Hafnium-doped aluminide coating was deposited on Inconel 100 superalloy by the CVD method. The coating consists of two layers: the additive and the interdiffusion one. The coating was formed by the predominant outward diffusion of nickel from the substrate. The additive layer consists of the β-NiAl phase and the interdiffusion layer consists of the β-NiAl phase with M6C and M23C6 carbides. No hafnium rich particles were observed. It is the evidence, that hafnium has dissolved in the coating. The oxidation resistance of hafnium-doped and undoped coatings was tested. Hafnium significantly improved oxidation resistance of coated Inconel 100 superalloy.

Improving the sintering resistance of Ni/Al2O3 steam-reforming catalysts by promotion with noble metals

The role of Rh, Ir and Ru promoters on the structural properties and catalytic behavior of Ni/α-Al2O3 catalyst during steam methane reforming (SMR) was investigated. The catalysts were subjected to aging treatments at 800°C and PH2O=22 bar g in order to induce sintering of the metal nanoparticles and alloying between Ni and the promoters. The crystallite sizes and the extent of alloying were determined in the reduced and aged catalysts by combination of in situ extended X-ray absorption fine structure (EXAFS), X-ray powder diffraction (XRPD), transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy (STEM-XEDS) mapping. The results reveal that the degree of alloying between Ni and Rh or Ir promoters increases during the aging process, as a result of the high mobility of the nickel species and diffusion into the FCC structures of Rh and Ir. Moreover, the formation of Ni–Rh and Ni–Ir alloy particles has a positive effect in mitigating the sintering of the nickel particles. In contrast, sintering inhibition effect of Ru was poorer due to its lower miscibility in nickel as well as its lower sintering resistance under the aging conditions employed. Based on density functional theory (DFT) calculations the diffusion of the Ni atoms into the lattice structure of Ru is energetically not favorable, whereas Ni atoms are equally stable in the first and second surface layers of Ir and Rh. Hence, a synergistic effect between Ni and Rh or Ir is achieved due to the alloy formation leading to conservation of larger active metal surface area and consequently to superior SMR activity with respect to the monometallic systems.

Hydrogen influence on diffusion in nickel from first-principles calculations

We propose a method to evaluate the diffusion coefficient of vacancy-hydrogen clusters (${\mathrm{VH}}_{n})$ in metals. The key is a good separation of time scales between H diffusion and the metal-vacancy exchange. The Ni-H system is investigated in details, using ab initio calculations, but the arguments can be transposed to other systems. It is shown that cluster diffusion can be treated as an uncorrelated random walk and that H is always in equilibrium before the vacancy-metal exchange. Then, the diffusion coefficient is a sum over jump paths of the equilibrium probability of being in a specific ${\mathrm{VH}}_{n}$ configuration times the corresponding activation terms. The influence of H on the energy barrier is well reproduced by effective pair interactions between the jumping Ni and the H atoms inside the vacancy. This model is motivated by an analysis of the electronic charge redistribution in key saddle configurations. The interaction is repulsive and decreases with distance. The model is used to find easy jump path, reduce the number of saddle searches, and provide an estimate of the error expected from this reduction. The application to the Ni-H system shows that vacancies are drastically slowed down by H. The effects of temperature and bulk H concentration are explored and the origin of the non-Arrhenius behavior is explained. At equilibrium, ${\mathrm{VH}}_{n}$ clusters always induce a speedup of metal diffusion. The implications concerning H induced damage, in particular in regards to Ni-Cr oxidation, are discussed.

In situ Raman and optical characterization of H2S reaction with Ni-based anodes for SOFCs

In this paper, we re-assess the use of Raman spectroscopy as an in situ method to monitor the sulfidation of SOFC (solid oxide fuel cell) anode-based materials. The technique allows studying in situ kinetics of the sulfidation of nickel at low temperature (up to 500°C). A severe poisoning happens at 500°C with a layer of Ni3S2 covering the entire pellet surface. At higher temperatures, optical microscopy helps to observe in situ the formation and growth of nickel sulfide crystals for the first time. The crystals' growths observed in situ strongly suggest the diffusion of nickel toward sulfide species. A reaction between H2S and CGO was clearly observed at 750–790°C.

Long term stability of copper front side contacts for crystalline silicon solar cells

In this work the long term stability of silicon solar cells with a copper front side metallization based on a fine-line screen-printed silver seed-layer, a plated nickel diffusion barrier, a plated copper conductive layer and a silver capping is investigated in detail. Silicon nitride layers deposited by PECVD or sputtering effectively hinder copper diffusion, which might occur e.g. at contact edges. To investigate the plated nickel diffusion barrier, fast degradation of full size 156×156mm² cells at elevated temperatures on hotplates and module degradation in the climate chamber executing a damp heat test were performed. Plated nickel thickness influences the degradation speed on cell level. On module level, only the cell without diffusion barrier shows a degradation of 1.2% rel. in efficiency (IEC criteria passed) after 1500h damp heat test (85°C, 85% r.h.). The cells with diffusion barrier do not show any degradation. Comparison of cell and module results indicate that fast degradation on hotplates at cell level gives a reasonable first estimate regarding cell degradation due to copper diffusion.

Electrical Resistance Measurements and Microstructural Characterization of the Anode/Interconnect Contact in Simulated Anode-Side SOFC Conditions

Metallic interconnects in solid oxide fuel cell (SOFC) stacks are often in direct contact with a nickel/yttria stabilized zirconia (Ni/YSZ) cermet anode. Interdiffusion between the two components may occur at the operating temperature of 700–850°C. The alteration of chemical composition can result in phase transformation of the steel and in formation of oxides with a poor electrical conductivity in the anode. In this study, the area specific resistance (ASR) of the steel Crofer 22 APU, in contact with a Ni/YSZ anode with and without a tape casted CeO2 barrier layer was measured in simulated SOFC anode conditions at 800°C. The microstructure in the contact area was characterized using scanning electron microscopy techniques. The ASR was low for the steel in direct contact with the Ni/YSZ anode. Nickel diffusion into the steel resulted in a fine grained zone, which was identified as ferrite. The zone is austenitic at the exposure temperature but transforms to ferrite during cooling. When a CeO2 nickel diffusion barrier layer was used The ASR was considerably higher. These results imply that nickel diffusion is not only detrimental: It leads to microstructural instability but also results in a low electrical resistance of the anode/interconnect contact.

Effect of La2O3/CeO2 particle size on high-temperature oxidation resistance of electrodeposited Ni–La2O3/CeO2 composites

Ni–La2O3/CeO2 composite films were prepared by electrodeposition from a nickel sulfate bath containing certain content of micrometer and nanometer La2O3/CeO2 particles. The effect of La2O3 or CeO2 particle size on the oxidation resistance of the electrodeposited Ni–La2O3/CeO2 composites in air at 1000 °C was studied. The results indicate that, compared with the electrodeposited Ni-film, Ni–La2O3/CeO2 composites exhibit a superior oxidation resistance due to the codeposited La2O3 or CeO2 particles blocking the outward diffusion of nickel. Moreover, compared with nanoparticles, La2O3 or CeO2 microparticles have stronger effect because La2O3 or CeO2 microparticles also act as a diffusion barrier layer at the onset of oxidation.

Molecular dynamics simulations of the effects of vacancies on nickel self-diffusion, oxygen diffusion and oxidation initiation in nickel, using the ReaxFF reactive force field

A ReaxFF reactive force field was developed for the nickel–oxygen system. The quantum mechanical (QM) data used to derive the force field parameters included the equations of state of various phases of nickel and that of nickel oxide (NiO), the vacancy formation energy and the vacancy-mediated self-diffusion barrier in the face-centered cubic nickel. Furthermore, in order to study the interstitial diffusion of oxygen atoms in the nickel matrix, the oxygen insertion energies and the diffusion barriers were included in the training set. The force field was validated by performing molecular dynamics (MD) simulations of self-diffusion of nickel and the interstitial diffusion of oxygen. The predicted diffusivity and the activation energy achieved quantitative agreement with their respective published values. Furthermore, this force field enables study of the effects of vacancies on the diffusion of dissolved oxygen and the successive initiation of internal oxidation. A new oxygen diffusion mechanism is proposed in which the oxygen atom diffuses via the movement of the oxygen–vacancy pair. In addition, the MD simulation results suggest that the voids at the grain boundaries can induce local oxygen segregation due to the strong oxygen–vacancy binding effect, which is responsible for the formation of a nickel oxide particle in the void. These results demonstrate that the ReaxFF MD study can contribute to bridging the gap between the QM calculations and the experimental observations in the study of metal oxidation.

Nanoparticles in zirconium-doped aluminide coatings

The microstructure of the zirconium doped aluminide coating deposited on the nickel substrate by the CVD method is presented. TEM analysis proved that the ß-NiAl and the γ′-Ni3Al are the main phases of the deposited coating. Some dislocations in the nickel substrate and in the coating were observed. The cross section chemical composition examination revealed, that the coating was formed via the inward aluminum diffusion and the outward nickel diffusion. SEM analysis revealed small zirconium content (0.2–0.4at%) on the cross-section of the aluminide coating. Such zirconium content leads to zirconium dissolution in the aluminide coating. Long time of zirconizing (4h) caused the appearance of zirconium nano particles at the surface of the coating. TEM investigations confirmed that zirconium formed inclusions on the coating’s surface.

The effects of applied current on one-dimensional interdiffusion between copper and nickel in spark plasma sintering

Spark plasma sintering (SPS) is a powder metallurgy technique that employs the use of fast sintering kinetics to produce final consolidated components in a matter of minutes. In order to use blended powders in SPS to obtain fully alloyed parts, diffusion during sintering must be understood. An investigation into the effects of current on the diffusion of copper and nickel was performed using SPS. Bulk specimens were used to generate diffusion couples in SPS in alternating orientations with respect to the direction of the current. Control samples were produced using a horizontal insertion vacuum furnace. Experiments were performed at temperatures between 850 °C and 1000 °C for 3 h. Concentration profiles were obtained by the use of both energy-dispersive spectroscopy and a Monte Carlo simulated correction curve. Diffusion coefficients and activation energies were calculated for samples produced by SPS and annealing without current. It was shown that, at temperatures near 0.9 Tm, the application of current in SPS inhibits diffusion between copper and nickel due to the re-orientation of electrons caused by the loss of ferromagnetism in nickel. Activation energy for diffusion is, however, decreased due to the temperature gradients arising from the difference in resistivity between the two species.

Formation and Evolution of Nickel Silicide in Silicon Nanowires

Thermally activated axial intrusion of nickel silicides in silicon nanowires (SiNWs) is utilized to form nickel silicide/silicon contacts in SiNW field effect transistors. The growth of different nickel silicides is often accompanied by local thickening and tapering of the NW, up to its full disintegration. In this paper, this process was investigated in SiNWs of 30-60 nm in diameters with prepatterned Ni electrodes after annealing cycles at different temperatures of 300 °C-440 °C and times up to 120 s. From the temperature dependence of the intrusion lengths, activation energy of 1.45 eV for the surface diffusion of nickel was extracted. In several cases, periodic thickening of the nickel-rich part is accompanied by tapering of the monosilicide part up to its full dissolution. The kinetics of the nickel silicides growth was described by phenomenological model. For a certain set of parameters, tapering and dissolution of the monosilicide part of the intrusion were predicted, similar to the experimental results.

Diffusion Coefficient of Nickel in Ni<sub>2</sub>Al<sub>3</sub> Intermetallic Compound

Nickel alloys have been widely used for high temperature applications such as gas turbine engine, turbine blade and many high temperature resistance components. Aluminizing is one of effective to increase oxidation resistant of nickel alloys by forming nickel aluminide compounds on nickel surface. Nickel aluminide is formed by the diffusion mechanism. This research aims to study the diffusion behavior of nickel in Ni2Al3 compound. The diffusion coefficient is determined using Ni/Al diffusion couple forming Ni2Al3 and NiAl3 phases. The temperatures under study are 873, 973, and 1073 K, which are at above and below melting point of aluminum. Determination of diffusion coefficient of Ni in Ni2Al3 is from mass balance concept: flux difference at interphase causes accumulation of atoms in compound layer, which as be derived as: dxαβ/dt = [1/(nβ-nα)] * [Dα *(dnα/dx) - Dβ *(dnβ/dx)] From this equation, diffusion coefficient of Ni in Ni2Al3 at 873 and 1073 K are calculated as 6.243×10-11 and 6.82×10-9 m2/s, respectively. From Arrhenius equation of diffusivity, D = Doexp (-Q/RT), the activation energy for diffusion of nickel in Ni2Al3 is found to be 183 kJ/mol. The result obtained in this research is of great use in controlling aluminizing process.

Elevated temperature, strain rate jump microcompression of nanocrystalline nickel

Nanocrystalline and ultrafine-grained materials show enhanced strain rate sensitivity (SRS) in comparison to their coarse grained counterparts. Majority of SRS measurements on nanocrystalline thin films reported in literature have focused on nanoindentation-based approaches. In this paper, micropillar strain rate jump tests were demonstrated on an electrodeposited nanocrystalline nickel film from 25 to 100 °C. SRS exponent, m, and activation volume, V, values were determined as a function of temperature. The measured values were found to be in good agreement with previously reported literature on bulk and nanoindentation measurements. Apparent activation energy for deformation was found to be about 100 kJ/mol, which is close to that for grain boundary diffusion in nickel. Grain boundary sliding was observed in the deformed pillars from scanning electron microscopy images.

Experimental and Finite Element Simulation Study of Thermal Relaxation of Residual Stresses in Laser Shock Peened IN718 SPF Superalloy

An integrated experimental and modeling/simulation approach was developed to investigate and secure a quantified knowledge of the impact of high temperature exposures on the stability of residual stresses in a laser shock peened (LSP) high temperature aero-engine alloy, IN718 SPF (super-plastically formed). Single dimple LSP and overlap LSP treatments were carried out utilizing a Nd:Glass laser (λ = 1.052 μm), and subsequent heat treatments on the LSP-treated coupons were conducted at different temperatures between 550 and 700 °C. A 3-D nonlinear finite element (FE) computational model and the rate-dependent Johnson-Cook material model were calibrated using the experimental results of residual stress from the single dimple LSP and thermal relaxation treatments, and were further extended to the overlap LSP treatment case. Both experimental and FE simulations show that: a) a high level of compressive residual stress (~700 MPa at surface) and residual stress depth (~0.4–0.6 mm) were achieved following LSP, and b) the overlap LSP treatment gave higher residual stress and greater depth. The magnitudes of the initial residual stress (and plastic strain), heating temperature and exposure time were identified as the key parameters controlling the thermal relaxation behavior. The stress relaxation mainly occurs initially before 20 min exposure and the extent of relaxation increases with an increase in temperature and a higher magnitude of the initial as-peened residual stress. In addition, in regions deeper than ~300 μm or after initial thermal exposure where the residual stress was lower than ~300 MPa, stress relaxation was found to be negligible. Kinetic analysis of the experimental thermal relaxation data based on Zener-Wert-Avrami model gave an activation enthalpy of 2.87 to 3.77 eV, which is near that reported in the literatures for volume and/or substitutional solute diffusion in Nickel. These results suggest that thermal relaxation of the LSP-induced residual stress occurs by a creep-like mechanism involving recovery, rearrangement and annihilation of dislocations by climb.

A Computational Study of Nickel Migration in SOFCs Anode Due to Phosphine Induced Degradation and Applied Electrical Current

The poisoning of Solid Oxide Fuel Cells (SOFCs) anode materials by impurities in coal syngas is a significant problem for utilization coal symgas in SOFCs. One such impurity, phosphine is known to cause catastrophic failure of SOFC anode at ppm level concentrations (below 10 ppm). A significant phenomenon observed in SOFC anodes, made of Ni-YSZ cermets, exposed to phosphine is migration of the nickel from porous matrix to the surface, which is believed to be one of the reasons for performance degradation. The mechanisms responsible for the experimentally observed Nickel migration are not well understood. A possible mechanism, which considers the Nickel diffusion and the formation of secondary phases (Ni-P compounds) as responsible phenomenon of Nickel migration, was proposed by our group. However this consideration does not allow us to predict the experimentally observed effect of the applied current. In this study, a reasonable mechanism is proposed to reveal the effect of electrical current on Nickel migration in SOFCs. The postulation of this mechanism is that the applied electrical current could possibly cause the transport of Nickel by generating an electrical force in the current flow direction. In SOFCs, the electrical current flows from anode/electrolyte interface to anode/fuel interface. Therefore the direction of the Nickel transport is from anode active layer to the anode surface. A transport model for nickel migration is formulated based on these postulations and it is integrated into an existing one dimensional in house code for predicting SOFC anode degradation due to fuel impurities. Simulations show that the proposed mechanism of Ni diffusion driven by secondary phase formation and the electrical force can reveal the experimentally observed accumulation of Ni and secondary phases on the SOFC anode surface when driven by a current and exposed to phosphine.

Effect of high temperature deposition on the diffusion of nickel in amorphous silicon thin films

The diffusion of nickel into amorphous Si (a-Si) thin films and the growth of different nickel silicides is reported. a-Si and Ni films were deposited on Borosilicate Glass substrates and annealed in vacuum at temperatures from 200 to 500°C. The average grain size of the films increases from 150 nm to 360 nm with deposition temperature. Cross-sectional scanning electron microscopy and energy dispersive x-ray spectroscopy have been used in conjunction to reveal the process of diffusion. It is inferred from x-ray diffraction and Raman spectroscopy analysis on these stacks that Ni3Si is formed at 200°C, NiSi at 300°C and NiSi2 at 400°C. This study reveals that Ni-Si bilayer films deposited at an elevated temperature and subsequently annealed at the deposition temperature formed monophase Ni silicides.

First-principles study of 4d solute diffusion in nickel

Diffusion of the 4d transition elements in Ni has been investigated within the five-frequency model framework using migration energy barriers calculated from the first principles. Agreement with counterintuitive experimental/calculated data is observed; atoms in the middle of 4d row have the smallest atomic radii while exhibiting the lowest diffusivity as compared to larger atoms at the beginning and the end of 4d row. We show that 4d solute diffusion is controlled mainly by the size misfit. The larger atoms have higher solute–vacancy binding energies and lower migration barriers. Both were shown to correlate with a displacement of the equilibrium solute position toward the adjacent vacancy. The difference in mechanisms controlling sp- and transition elements diffusion rates in Ni is discussed.