Ni Lattice Research Articles

Nucleation and growth mechanisms of an electrodeposited Ni–Se–Cu coating on nickel foam

Electrocatalysts for water splitting have been widely explored among recent years. In this study, nickel–selenium–copper (Ni–Se–Cu) coating was synthesized on nickel foam through potentiostatic electrodeposition. The electrochemical kinetics and nucleation mechanisms of the deposition were investigated, and the diffusion coefficient D from different deposition potentials and temperatures was calculated. Results reveal that the electrodeposition of Ni–Se–Cu follows an instantaneous nucleation and diffusion-controlled three-dimensional (3D) growth mechanism. Deposition potential and bath temperature slightly effect the nucleation mechanism of electrodeposition. The apparent activation energy Ea of the hydrogen evolution reaction (HER) in 1.0 M KOH electrolyte of Ni–Se–Cu is 21.1 kJ·mol−1, which is lower than that of Ni–Se (37.7 kJ·mol−1). The majority phase formed by nickel and selenium is Ni3Se2, and a Ni(Cu) solid solution forms after the incorporation of Cu atoms into a Ni lattice.

NiMo Nanoparticles Anchored on N-Doped Carbon Rods for High-Efficiency Hydrogen Electrooxidation in Alkaline Media.

Anion-exchange membrane fuel cells (AEMFCs) are promising clean energy devices of high efficiency; however, their large-scale applications suffer from high cost and/or poor activity of electrocatalysts, especially at the anode. Herein, an electrocatalyst composed of NiMo nanoparticles (≈50 nm) in situ anchored on N-doped carbon rods (denoted as NMNC) has been synthesized by a facile calcination method. The as-prepared NMNC catalyst shows an especially high hydrogen oxidation reaction (HOR) activity (3.23 mA cm-2 at 0.10 V vs reversible hydrogen electrode (RHE)), excellent stability during 10,000 cycles within -0.10 to 0.50 V vs RHE, and extraordinarily high CO tolerance in a 0.1 M KOH electrolyte. Particularly, the doping of Mo atoms into Ni lattices of the electrocatalyst results in Ni lattice distortion and well-regulated graphitization degree of the carbon matrix, contributing to the enhanced reducibility and accelerated HOR reaction kinetics. This facilely synthesized NMNC electrocatalyst of enhanced HOR activity is expected to serve as a promising anodic electrocatalyst for AEMFCs.

Hierarchically-porous metallic scaffolds via 3D extrusion and reduction of oxide particle inks with salt space-holders

3D ink-extrusion of powders followed by sintering is an emerging additive manufacturing method capable of creating metallic microlattices. Here, we study the creation of hierarchically porous Fe or Ni scaffolds by 3D extrusion of 0/90° lattices from inks consisting of fine oxide powders (Fe 2 O 3 or NiO, < 3 µm), coarse space-holder particles (CuSO 4, < 45 µm) and a polymer binder within a solvent. After space-holder leaching and debinding of the lattices, a sintering step densifies the metallic Fe or Ni powders created by oxide reduction with H 2 , while maintaining the larger pores templated by the space-holder particles within the printed struts. The resulting Fe or Ni lattices exhibit a hierarchical porosity: (i) open submillimeter channels within the lattice between the printed struts; (ii) open micropores (5–50 µm) within the struts, from leaching of space-holder particles; (iii) closed submicron pores within the micropore walls, from incomplete sintering of the metal microparticles created by oxide reduction. While the channel porosity is defined by the printed geometry, strut porosity is controlled by the sintering temperature, with the finest porosity removed first. Sintering at 873–1373 K for 1 h leads to strut porosities of 38–19% in Fe and 59–29% in Ni. In situ synchrotron X-ray tomography reveals the sintering kinetics and porosity evolution upon debinding, oxide reduction by H 2 and sintering of the porous metal, which, combined, lead to ~50% shrinkage of the scaffolds as compared to the printed state. The space-holder templated pores are not eliminated by sintering and remain in the struts, but they shrink upon reduction and sintering of the surrounding metal matrix (e.g., by 40% in Fe and by 30% in Ni upon sintering at 1173 K for 1 h).

Heat Transport without Heating?—An Ultrafast X‐Ray Perspective into a Metal Heterostructure

AbstractWhen the spatial dimensions of metallic heterostructures shrink below the mean free path of its conduction electrons, the transport of electrons and hence the transport of thermal energy by electrons continuously changes from diffusive to ballistic. Electron–phonon coupling sets the mean free path to the nanoscale and the time for equilibration of electron and lattice temperatures to the picosecond range. A particularly intriguing situation occurs in trilayer heterostructures combining metals with very different electron–phonon coupling strength: Heat energy deposited in few atomic layers of Pt is transported into a nanometric Ni film, which is heated more than the Cu film through which the heat is released. Femtosecond pump‐probe experiments with hard X‐ray pulses provide a layer‐specific probe of the heat energy. A purely diffusive two‐temperature model with increased thermal conductivity of hot electrons excellently reproduces the observed signals from all three layers. At the time when the Ni lattice is maximally heated, no significant heat has entered the Cu lattice. This phenomenon would be enhanced in thinner layers where ballistic transport dominates. In this context it is shown that purely diffusive transport can lead to a linear time‐to‐length dependence that must not be misinterpreted as ballistic transport.

The Obstruction Effect of Ni Layer on the Interdiffusion of Cu Substrate and Sn Solder: A Theoretical Investigation

The protective effect of a Ni layer on Cu-Sn solder joints was investigated from the perspective of interface diffusion. The results indicate that temperature is the primary external factor affecting interfacial diffusion, and the effect of external load also has an influence. Atom diffusion is promoted by the increasing temperature and applied compressive load perpendicular to the interface. In contrast, the tensile load perpendicular to the interface inhibited the diffusion behavior. Ni atoms at both the Ni/Cu and Ni/Sn interfaces act as the main diffusion factor and play a vital role in the growth and evolution of the diffusion layer. Because of the high stability of the Ni lattice and the large radii of Cu and Sn atoms, it is difficult for Cu and Sn atoms to migrate to the Ni atoms, which effectively delays the contact and reaction between Cu and Sn. To reveal the micro-mechanism of the diffusion, the diffusion coefficient and activation energy were studied. Ni atoms need more energy than Cu and Sn atoms to drive the diffusion, which delays the consumption of Ni and helps to maintain the Ni layer. The single vacancy formation energy of the Ni lattice is the largest, which means that it is difficult for Cu and Sn atoms to migrate into the lattice. By comparing the atomic diffusion rate, the final formation of Cu-Sn intermetallic compounds in the Ni layer on the side of tin-based solder was predicted at a macroscopic time scale. Using density functional calculation, the slight lattice distortion of the interfaces was measured. The analysis of charge transfer and electronic properties show that a strong electron transfer and energy level overlap occurs between the Ni and Cu atoms, implying that the Ni/Cu interface has a more considerable electronic thermal conductivity and should be more sensitive to the increasing temperature.

Implanting Ni-O-VOx sites into Cu-doped Ni for low-overpotential alkaline hydrogen evolution

Nickel-based catalysts are most commonly used in industrial alkaline water electrolysis. However, it remains a great challenge to address the sluggish reaction kinetics and severe deactivation problems of hydrogen evolution reaction (HER). Here, we show a Cu-doped Ni catalyst implanted with Ni-O-VOx sites (Ni(Cu)VOx) for alkaline HER. The optimal Ni(Cu)VOx electrode exhibits a near-zero onset overpotential and low overpotential of 21 mV to deliver –10 mA cm−2, which is comparable to benchmark Pt/C catalyst. Evidence for the formation of Ni-O-VOx sites in Ni(Cu)VOx is established by systematic X-ray absorption spectroscopy studies. The VOx can cause a substantial dampening of Ni lattice and create an enlarged electrochemically active surface area. First-principles calculations support that the Ni-O-VOx sites are superactive and can promote the charge redistribution from Ni to VOx, which greatly weakens the H-adsorption and H2 release free energy over Ni. This endows the Ni(Cu)VOx electrode high HER activity and long-term durability.

Density functional theory and kinetic Monte Carlo simulation study the strong metal–support interaction of dry reforming of methane reaction over Ni based catalysts

Abstract Oxide supports modify electronic structures of supported metal nanoparticles, and then affect the catalytic activity associated with the so-called strong metal–support interaction (SMSI). We herein report the strong influence of SMSI employing Ni4/α-MoC(111) and defective Ni4/MgO(100) catalysts used for dry reforming of methane (DRM, CO2 + CH4 → 2CO + 2H2) by using density functional theory (DFT) and kinetic Monte Carlo simulation (KMC). The results show that α-MoC(111) and MgO(100) surface have converse electron and structural effect for Ni4 cluster. The electrons transfer from α-MoC(111) surface to Ni atoms, but electrons transfer from Ni atoms to MgO(100) surface; an extensive tensile strain is greatly released in the Ni lattice by MgO, but the extensive tensile strain is introduced in the Ni lattice by α-MoC. As a result, although both catalysts show good stability, H2/CO ratio on Ni4/α-MoC(111) is obviously larger than that on Ni4/MgO(100). The result shows that Ni/α-MoC is a good catalyst for DRM reaction comparing with Ni/MgO catalyst.

Asymmetric Strain in the Oxidized and Reduced States of Heterogeneous Electrocatalysts

The growing number of functional examples of “strain engineering” is drawing attention to the influence that mechanical or geometric considerations can have on the electronic structure and electrochemical behavior of electrocatalysts. It will be shown that the concept of lattice strain, and the changes in electronic structure that accompany it, can explain the experimentally observed trends in electrochemical behavior of common mixed-metal oxide catalysts for the oxygen evolution reaction. Structural analysis of a series of iron-nickel hydroxide materials reveal a significant change in O-Ni-O bond angles upon electrochemical oxidation.1 Subsequent analysis shows that insertion of ions with varied ionic radii into a Ni(OH)x host lattice introduces localized geometric distortions in the nickel environments that can be observed by tracking Ni-based d-d transitions in the near-infrared.2 The type (tensile or compressive) and magnitude of strain is correlated to the oxidation state of nickel ions and the ionic radius of the secondary ion. Experimentally observed trends in electrochemical behavior track the ionic radius of the secondary ion, indicating that the degree of strain directly correlates to electrochemical behavior. Density functional theory calculations indicate that inequivalence of this internalized strain in the oxidized and reduced states of the catalyst material introduces asymmetry into the potential energy surface landscape. This asymmetry effectively decreases the activation energy for electron transfer and may simultaneously decrease the electrochemical transfer coefficient. Evidence suggests that the synthetic protocol employed exerts significant influence over the placement of secondary ions and ability to observe this effect.3 (1) Smith, R. D. L.; Pasquini, C.; Loos, S.; Chernev, P.; Klingan, K.; Kubella, P.; Mohammadi, M. R.; González-Flores, D.; Dau, H. Energy Environ. Sci 2018, 11, 2476–2485.(2) Alsac, E. P.; Whittingham, A.; Liu, Y.; Smith, R. D. L. Chem. Mater. 2019, 31, 7522–7530.(3) Rong, W.; Stepan, S.; Smith, R. D. L. MRS Adv. 2019, 4, 1843–1850.

Facile fabrication of ultrafine nickel-iridium alloy nanoparticles/graphene hybrid with enhanced mass activity and stability for overall water splitting

• A facile and general method was developed for fabricating Ni-Ir alloy nanoparticles/graphene electrocatalysts. • The incorporation of Ir into metallic Ni lattice greatly improves the intrinsic activity for HER and OER. • The bifunctional catalyst shows much higher electrochemical performance than commercial 20 wt% Ir/C. Developing active and durable electrocatalysts for overall water splitting is desirable but challenging to realize sustainable hydrogen production. Here, we report a facile and general method to prepare ultrafine nickel (Ni)-iridium (Ir) alloy nanoparticles/graphene hybrids for overall water splitting. The optimized hybrid with 4.9 wt% Ir exhibits much higher catalytic activity and durability than commercial 20 wt% Ir/C for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Theoretical simulations reveal that the incorporation of Ir in metallic Ni lattice regulates hydrogen adsorption free energy to the optimum level, thus improving HER activity, while in situ generated amorphous Ir-Ni hydr(oxy)oxides around metallic Ni-Ir core have been demonstrated to be the active species under OER conditions, which switches OER rate-determining step to energy-favorable pathway. The overall water splitting electrolyzer assembled by the optimized electrocatalyst shows a low cell voltage of only 1.52 V and excellent stability to deliver a current density of 10 mA cm −2 . This work provides a powerful strategy toward general synthesis of ultrafine alloy nanoparticles for high-performance overall water splitting. Ultrafine nickel-iridium alloy nanoparticles/graphene hybrids serve as highly efficient bifunctional electrocatalysts for overall water splitting. The incorporation of Ir greatly improves the reaction kinetics for hydrogen evolution reaction and oxygen evolution reaction.

The partitioning behavior of dual solutes at the antiphase domain boundary in the B2 intermetallic: A microscopic phase-field study

The partitioning behavior of dual solutes at the antiphase domain boundary (APDB) in B2 intermetallic is studied by establishing a ternary BCC phase-field model of atomic-resolution. The influence factors doping concentration of the third alloying solute, temperature and lattice misfit strain are considered. The elemental co-distribution caused by the partitioning behavior is characterized by the topographic map of solute and micro deviation of concentration. Theoretical results reveal that the co-distribution is Ni segregation coupled with Al depletion and Fe segregation. At the microscopic level, the micro deviation level is confirmed to be strongly dependent on the magnitudes of Ni concentration, temperature and lattice misfit. At the atomic level, the micro deviation is heterogeneous. The mechanism, regular dependence, and heterogeneity of the elemental partitioning under three factors are revealed from the viewpoint of energy. This work provides a new perspective on understanding the origin of the partitioning behavior of constituent elements at the APDB in B2 intermetallics and will be beneficial to evaluate elemental distribution-APDB-properties in future FeAl-base superalloys design with multi alloying additions and the interface engineering.

Atomic-scale characterization of the site occupancies in the B2 FeAl phase: Effects of Ni addition, temperature and lattice misfit

A microscopic phase-field model is constructed to study the influences of Ni addition, temperature and lattice misfit on the site occupancies in complex B2 FeAl–Ni intermetallic phase. The tendencies of six site occupancies are explored through tracing the atomic site occupation probability. The results reveal that six site occupancies are monotone functions of Ni concentration, temperature and the lattice misfit, respectively. For the antisite behavior of Fe (AlFe), Al (FeAl) and the substitution behavior of Ni (AlNi, FeNi), the ranks of four defect occupancies are identical under three factors. The transformation rate and ordering degree of the complex FeAl–Ni are dependent of three factors. The mechanism of configuration of six site occupancies is revealed from the energy analysis. The results are in accordance with experimental and theoretical data and provide theoretical support to the questioned validity of the lattice misfit effect on the ordering behavior in intermetallics.

Rapid solidification kinetics and mechanical property characteristics of Ni–Zr eutectic alloys processed under electromagnetic levitation state

The rapid solidification mechanism of Ni–Zr alloys was investigated by electromagnetic levitation (EML) technique assisted with high speed videography. A systematic analysis of the competitive growth mode between the primary phase and eutectic structure was conducted for three types of alloys (hypoeutectic Ni-5 at.%Zr, eutectic Ni-8.8 at.%Zr and hypereutectic Ni-13 at.%Zr alloys), whose maximum undercoolings reached up to 260, 192 and 270 K, respectively. With the increase of undercooling, the primary (Ni) phase of hypoeutectic Ni-5 at.% Zr alloy transferred from well-defined dendrites to dendrite fragments. Zr atom was difficult to diffuse into the Ni lattice because the radius of Zr atom was larger than Ni atom. For eutectic Ni-8.8 at.% Zr alloy, the competitive nucleation and growth between primary (Ni) phase and eutectics could happen when the undercooling exceeded 18 K. The refined dendrites of primary (Ni) phase and various eutectic morphologies were observed at the undercooling of 192 K. For the hypereutectic Ni-13 at.% Zr alloy, the growth morphology transition of the primary Ni5Zr intermetallic compound from faceted to non-faceted crystals occurred if the liquid alloy achieved a high undercooling. Furthermore, the atomic scale structure of the primary phase was explored by the transmission electron microscopy (TEM), which revealed that the solid solubility and the lattice constant were consistent at various undercoolings. Moreover, the compressive performance of these three type alloys were determined for EML solidified samples. Due to the effects of undercooling and composition on microstructures, Ni-5 at.%Zr and Ni-8.8 at.%Zr alloys exhibited plastic deformation at the large strain condition, while Ni-13 at.%Zr alloy showed brittle fracture feature. The compressive strength and yield strength displayed a non-linear relationship with the rise of undercooling.

Ni-Doped ZnO Thin Films Deposited by Pneumatic Spray Pyrolysis

The objective of the present work is the study the Ni concentration effect on the optical, structural and electrical properties of ZnO:Ni thin films deposited by pneumatic spray pyrolysis method. Un-doped and Ni-doped ZnO thin films were prepared from zinc acetate dihydrate and nickel chloride hexahydrate dissolved in distilled water onto glass substrates by pneumatic spray pyrolysis method. Solution concentration, substrate temperature and nozzle-substrate distance were kept constant during all deposition processes. Effect of nickel content on the optical, structural and electrical properties of as-prepared films obtained was investigated by UV–Vis-NIR spectrophotometry, X-ray diffraction and four-point probe technique. The results indicate that the deposited films are well adherent to the substrates, present surface roughness and have a preferential growth in the (002) direction. The observed transmittance in the visible region was situated between 60% and 70%. For Ni-doped ZnO, there are two separate phases. The Urbach energy, grain size, and electrical conductivity increase with Ni content. The same behavior for both the optical energy gap and the ZnO:Ni lattice parameters.

Ni-based transition metal trichalcogenide monolayer: A strongly correlated quadruple-layer graphene

We investigate the electronic physics of layered Ni-based trichalcogenide NiPX$_3$ (X=S, Se), a member of transition-metal trichalcogenides (TMTs) with the chemical formula, ABX$_3$. These Ni-based TMTs distinguish themselves from other TMTs as their low energy electronic physics can be effectively described by the two $e_g$ d-orbitals. The major band kinematics is characterized by the unusal long-range effective hopping between two third nearest-neighbor (TNN) Ni sites in the two-dimensional Ni honeycomb lattice so that the Ni lattice can be equivalently viewed as four weakly coupled honeycomb sublattices. Within each sublattice, the electronic physics is described by a strongly correlated two-orbital graphene-type model that results in an antiferromagnetic (AFM) ground state near half filling. We show that the low energy physics in a paramagnetic state is determined by the eight Dirac cones which locate at $K$, $K'$, $\frac{K}{2}$ and $\frac{K'}{2}$ points in the first Brillouin zone with a strong AFM fluctuation between two $K (K')$ and $\frac{K'}{2} (\frac{K}{2})$ Dirac cones and carrier doping can sufficiently suppress the long-range AFM order and allow other competing orders, such as superconductivity, to emerge. The material can be an ideal system to study many exotic phenomena emerged from strong electron-electron correlation, including a potential $d\pm id$ superconducting state at high temperature.

On Improving the Cycling Stability of P2-Type Na0.67Ni0.33Mn0.67O2 Cathode Material By Ti-Substitution for Na-Ion Batteries

The extensive studies over the last decade have established Na-ion batteries (NIBs) as one of the cheaper alternatives to Li-ion batteries (LIBs) [1-2]. P2-type Na0.67Ni0.33Mn0.67O2 has stood out among layered oxide based electrode materials providing the best over-all electrochemical performance. The electrodes can exert up to 92.5% of its theoretical capacity (160 mAhg-1) at a voltage higher than 3 V accounted for the Ni2+/Ni4+ redox. However, at higher voltages, electrodes suffer irreversibility due to P2-O2 structural transition [3]. Recent studies in suppressing this transition by partial substitution with various metals on either Ni or Mn lattice site have suggested enhancing cycling stability [4-7]. In this study, a novel cathode material with Ti-substitution on Ni site, P2-type Na0.67Ni0.25Ti0.08Mn0.67O2 has been synthesized via solid-state synthesis method and characterized electrochemically. Na0.67Ni0.25Ti0.08Mn0.67O2 electrodes have been observed to be highly reversible at higher voltage ranges. The electrodes have an initial discharge capacity of 125 mAhg-1 and can retain around 84% of this capacity (105 mAhg-1) even after 50 cycles at 0.1C when cycled at an upper cut-off voltage of 4.3 V (Figure 1). Na0.67Ni0.25Ti0.08Mn0.67O2 electrodes are believed to suppress the irreversible P2-O2 transformation by diverting the charging reaction through a more reversible P2-OP4 transition.

Interatomic interactions and structural formations in WFeNi(Ti) and MoFeNi(Ti) layers under intense plastic deformation induced by ball collisions

Abstract WFeNi(Ti) and MoFeNi(Ti) alloy layers were fabricated onto Ti surfaces from W, Mo, and Ni targets through ball collisions initiated by a mechanically vibrated vial. Steel balls were used for processing as the Fe source. The as-fabricated alloy structures could be represented as a heterogeneous multi-modal nanocrystalline structure with elemental flakes embedded within a matrix composed of nanocrystalline grains encapsulated within an amorphous phase. The deformation-induced chemical interactions between dissimilar atoms in the WFeNi(Ti) and MoFeNi(Ti) systems affected the structural formation of the alloy layers. Fe atoms in the WFeNi(Ti) alloy system tended to react with Ni, forming fcc (Fe,Ni) phase, while W atoms dissolved into the fcc (Fe,Ni) lattice. In the MoFeNi(Ti) alloy system, Fe tended to react with Mo over Ni, resulting in the formation of a bcc Fe(NiMo) solid solution. Formation of an amorphous phase was mainly related to compositional changes and distortion of the bcc Fe atomic rows with refractory metal atoms. The W and Mo nanocrystalline grains reduced their size continuously during processing via amorphization and wear effects. The Ti particles absorbed oxygen atoms. Chemical interactions between Ti and oxygen atoms made the Ti and oxygen components passive with regard to other present metallic elements. The formation of atomic clusters was related to the attractive bonding forces between dissimilar atoms that bonded the atoms together. The yield stress value of the as-fabricated layers evaluated via nanoindentation was (2.5 ± 0.4) GPa, which was much higher than the yield stress for tungsten heavy alloys.

First principles study of graphene on metals with the SCAN and SCAN+rVV10 functionals.

Integrating graphene into electronic devices requires support by a substrate and contact with metal electrodes. Ab initio calculations at the level of density functional theory are performed on graphene-fcc-metal(111) [Gr/M(111)] (M = Ni, Cu, Au) systems. The strongly constrained and appropriately normed (SCAN) and SCAN with the revised Vydrov-van Voorhis (SCAN+rVV10) functionals are relatively new approximations to the exchange-correlation (xc) energy shown to account for van der Waals (vdW) interactions which many non-empirical semi-local functionals fail to include. Binding energies and distances as well as electronic band structures are calculated with SCAN, SCAN+rVV10, Perdew-Burke-Ernzerhof (PBE), and PBE-D3 with and without Becke-Johnson damping, Bayesian error estimation functional with van der Waals correlation (BEEF-vdW), and optB86b-vdW. SCAN and SCAN+rVV10 succeed in describing chemisorption and physisorption in the Gr/Ni(111) system and physisorption in the Gr/Cu(111) and Gr/Au(111) systems. Incorrectly, the physisorption is found to be more favorable than chemisorption in the Gr/Ni(111) system with SCAN, but the result is reversed when the experimental bulk Ni lattice parameter is used as opposed to the SCAN calculated lattice parameter. The SCAN+rVV10 functional produces binding energies and distances comparable to those calculated using the random phase approximation as well as the experiment. The SCAN based functionals produce the highest spin magnetic moments in the bulk Ni and Gr/Ni(111) systems compared to the rest of the functionals investigated, overestimating the experiment by at least ∼0.18 μB. Also, in contrast to the rest of the functionals, the induced spin magnetic moment in graphene is found to be larger in magnitude in the physisorption region than the chemisorption region. The pristine graphene band structure is preserved in the physisorbed systems but with a shift in the Dirac point away from the Fermi energy causing graphene to become n-doped in the Gr/Cu(111) system and p-doped in the Gr/Au(111) system. Chemisorption occurs in the Gr/Ni(111) system where carbon pz states mix with the nickel d states causing a gap to form at the K point, destroying the Dirac point and conical dispersion.

Formation of Metallic and Carbide Phases via Codecomposition of [NiEn3]WO4 and Lithium Hydride in the Range 410–1060°C

The formation of metallic and carbide phases during the thermal decomposition of a mixture of the coordination compound [NiEn3]WO4 (En = ethylenediamine) and LiH in a He atmosphere has been studied by X-ray diffraction. The results demonstrate that heat treatment of the mixture at 410°C for 10 min leads to the formation of a Ni(Cx) interstitial solid solution based on the HCP Ni lattice and a W1 –xNix substitutional solid solution based on the BCC W lattice. Starting at 510°C, a Ni1 –xWx solid solution based on the FCC Ni lattice is also formed. The double carbide Ni2W4Cx appears at 660°C, and Ni6W6Cx forms at 860°C. Under similar conditions (700°C, He), the thermal decomposition of a mixture of NiWO4 and LiH leads to the formation of W and an FCC Ni1 –xWx (x ~ 0.12) solid solution.

First-principles study of diffusion and interactions of hydrogen with silicon, phosphorus, and sulfur impurities in nickel

Using density functional theory (DFT), we systematically study the effect of Si, P, and S impurities on the diffusion and binding of an H atom in a face-centered-cubic (FCC) Ni lattice. First, we quantify binding energies of an H atom to a vacancy, an impurity atom, and a vacancy-impurity atom defect pair. The energetics of H interactions show that a vacancy-impurity atom defect pair with larger binding energy traps the H atom more strongly and correlates with electronic bonding. Next, we study how the impurities influence diffusion of an H atom by using the Climbing Image Nudged Elastic band method to evaluate the Minimum Energy Path and the energy barrier for diffusion. The H atom preferentially diffuses between tetrahedral to octahedral (T-O) interstitial positions in pure Ni and when impurities are present. However, the activation energy significantly decreases from 0.95 eV in pure Ni to 0.47 eV, 0.52 eV, and 0.46 eV, respectively, in the presence of Si, P, and S impurities, which speeds up H diffusion. We rationalize this by comparing the bonding character of the saddle point configuration and changes in the electronic structure of Ni for each system. Notably, these analyses correlate the lower values of the activation energies to a local atomic strain in a Ni lattice. Our DFT study also validates the hypothesis of Berkowitz and Kane that P increases the H diffusion and, thereby, significantly increases H embrittlement susceptibility of Ni. We report a similar effect for Si and S impurities in Ni.

Effect of nitrogen doping on surface morphology, microstructure, chemical composition and intrinsic stress of nickel thin films deposited by reactive sputtering

N-doped nickel thin films were fabricated by direct current reactive magnetron sputtering, with varying nitrogen contents (0%–20%) added to the sputtering gas (Ar). To investigate the effect of nitrogen contents on morphology, microstructure, chemical composition and intrinsic stress of nickel thin films, characterizations were carried out using grazing incidence X-ray reflectivity (GIXRR), X-ray scattering (XRS), atomic force microscopy (AFM), X-ray diffraction (XRD), cross-sectional transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and intrinsic stress measurements. GIXRR and AFM results showed that the roughness achieves minimum with 8% nitrogen, corresponding to minimum average grain size obtained by XRS and XRD measurements. XRD patterns simultaneously indicated that the position and intensity of the Ni(111) and Ni(200) peaks shifted. This is because Ni lattice structure changed after the introduction of N atoms at interstitial sites, which were octahedrally surrounded by Ni atoms. Cross-sectional TEM images confirmed that Ar + 8%N2 as sputtering gas can suppress the crystallization and effectively smoothen the surface of Ni film. Moreover, no nickel nitride was found in the N-doped Ni films by XPS analysis and the intrinsic stress remained unchanged after the addition of 8% nitrogen.