Ni Crystals Research Articles

Численное моделирование трёхмерных дискретных бризеров в ГЦК решётке Ni

Molecular dynamics study of discrete breathers in fcc Ni crystal is undertaken. The initial conditions for the excitation of breathers are constructed by imposing a spherically symmetric function, exponentially decreasing with the distance from the center of the sphere, on the delocalized vibrational normal mode, the frequency of which lies above the phonon spectrum of the crystal. This method allows to obtain a three-dimensional discrete breather of a new type, in the core of which the atoms oscillate like in the normal mode, and the amplitude of these oscillations decreases exponentially with distance from the center of the breather. A detailed discussion of the properties of the normal mode on which the breather is built is presented. It is shown that in the two extreme cases, constant volume and zero pressure, the mode shows hard type of non-linearity in a wide range of amplitudes, which ensures the hard type of the nonlinearity of the breather and its frequency lying above the phonon spectrum of the crystal. The spatial localization parameter of the breather is determined as the function of breather amplitude so that the breather life time is maximal. Found discrete breathers have relatively long lifetime of the order of 10 ps, which substantially exceeds the lifetime of thermal fluctuations.The proof of the existence of three-dimensional breathers in fcc Ni, presented in this paper, is an interesting and important result in the theory of discrete breathers.

Synthesis of YSZ@Ni Nanoparticle by Modified Electroless Plating Process

Ni-YSZ (Y2O3-stabilized ZrO2) composites with core-shell structure (YSZ@Ni) were produced by modified electroless plating process. It was found that YSZ nanoparticles were well encapsulated by nickel powders at 65 degrees C with pH = 12. The spherical nanopowders had core-shell structure and the shell layer was less than 20 nm. The X-ray diffraction (XRD) analysis inferred the production was composed of YSZ and Ni crystals. In the end, the formation mechanism was discussed.

First-principles investigation of quantum mechanical effects on the diffusion of hydrogen in iron and nickel

The diffusion coefficients of interstitial hydrogen in bulk Fe and Ni crystals have been calculated over a wide range of temperatures employing first-principles methods based on density functional theory. Quantum mechanical effects have been included by means of the semiclassical transition state theory and the small-polaron model of Flynn and Stoneham. Our results show that to include such effects is crucial for a quantitative simulation of H diffusion in bcc Fe even at room temperature, while in the case of fcc Ni this is less important. The comparison with other theoretical approaches as well as with experimental studies emphasizes the main advantages of the present approach: it is quantitatively accurate and computationally efficient.

Adequacy of damped dynamics to represent the electron-phonon interaction in solids

In time-dependent density functional theory and Ehrenfest dynamics are used to calculate the electronic excitations produced by a moving Ni ion in a Ni crystal in the case of energetic MeV range (electronic stopping power regime), as well as thermal energy meV range (electron-phonon interaction regime). Results at high energy compare well to experimental databases of stopping power, and at low energy the electron-phonon interaction strength determined in this way is very similar to the linear response calculation and experimental measurements. Our approach to electron-phonon interaction as an electronic stopping process provides the basis for a unified framework to perform classical molecular dynamics of ion-solid interaction with ab initio type nonadiabatic terms in a wide range of energies.

Carbon Tolerant Electrodes for SOFC and Reversible SOFC (RSOFC) Cells Operating on Carbon Containing Fuels

Solid Oxide Fuel Cells (SOFC) and Electrolyzers (SOEC) hold great potential for the security of energy sector, providing solutions for efficient power generation and production of hydrogen and synthesis gas. A challenging barrier for the successful implementation of SOFC and SOEC technologies, remains the long term stability under realistic operating conditions by the effective control and minimization of degradation due to carbon built up. The problem arise from the fact that the commonly used anode cermets, e.g. Ni-YSZ and Ni-GDC, are prone to carbon deposition onto nickel, as a result of the Boudouard reaction (CH4cracking, disproportionation of CO), leading to low activity and fast degradation. In this study, a class of Au-modified commercially available NiO/GDC and NiO/YSZ cermet powders has been developed and studied for their performance and tolerance to carbon deposition, operating as SOFC anodes for CH4 steam reforming, as well as bi-functional electrodes in a Regenerative SOFC operating on the CO2 cycle (SOEC mode: electrolysis of CO2, SOFC mode: electrochemical reaction of CO and oxygen for power generation). In- situ HT-XRD tests revealed that the presence of Au, in an optimum nominal loading of 3wt%, affects the Ni crystal phase and has a significant positive effect in inhibiting carbon deposition. The results from cell testing in the temperature range of 800-1000°C, showed promising, stable performance of electrodes under carbon forming conditions (e.g. H2O/CH4=0.25, CO or CO/CO2mixtures of 0.7/0.3). The work is supported by the FCHJU project T-CELL (298300).

Ultrasound-assisted electrodeposition of thin nickel-based composite coatings with lubricant particles

Thin Ni composite coatings with hBN and WS2 particles were ultrasonically-electrodeposited on Cu with no need of a surfactant. Although the combination of mechanical agitation and ultrasound yielded the best dispersions, ultrasound on its own during plating yielded coatings with a more uniform distribution of particles. Ni/hBN and Ni/WS2 composite coatings electrodeposited under ultrasound were characterised by different methods: GD-OES to estimate particle content, XRD to analyse the preferred orientation, FIB-SEM to analyse the surface morphology and microstructure, and microhardness tests to measure the hardness. Whereas Ni/hBN composite coatings showed little difference compared to pure Ni deposits, the incorporation of WS2 into Ni had a significant effect on the preferred orientation, the surface morphology and the grain size of the coatings, refining the Ni crystal down to the nano-scale. The latter had a significant effect in the hardness of the coatings, despite the ‘soft’ nature of the WS2 particles.

Carbon Tolerant Electrodes for SOFC and Reversible SOFC (RSOFC) Cells Operating on Carbon Containing Fuels

A challenging barrier for the broad, successful implementation of SOFC/SOEC technologies, remains the long term stability by the effective control and minimization of degradation resulting from carbon built up. A class of Au-modified commercially available NiO/GDC and NiO/YSZ cermet powders has been developed and studied for their performance and tolerance to carbon deposition, employed as SOFC anodes for CH4 steam reforming, as well as bi-functional electrodes in a Regenerative SOFC operating on the CO2 cycle (SOEC: CO2 electrolysis, SOFC: power generation through the electrochemical reaction of CO and oxygen). In-situ HT-XRD tests revealed that the presence of Au, in an optimum nominal loading of 3wt%, affects the Ni crystal phase and has a significant positive effect in inhibiting carbon deposition. Cell testing in the temperature range of 800-1000° under carbon forming conditions, showed promising stable performance of the modified electrodes.

Keeping argon under a graphene lid—Argon intercalation between graphene and nickel(111)

We report on the intercalation of graphene grown on a Ni(111) crystal with argon. Argon is implanted in the Ni(111) crystal by ion bombardment before graphene growth, and diffuses to the surface during the growth of graphene at elevated temperatures. Graphene acts as an atomically thin barrier and keeps the argon underneath. We investigated this system with high resolution X-ray photoelectron spectroscopy. From our experiments we determined the mean quantities of argon under graphene. From our analysis, a simple model to determine the pressure under the graphene layer is presented. In our measurements, we find an increased thermal stability of the intercalated graphene as compared to non-intercalated graphene on Ni(111).

Structural, electronic and optical properties of Ilmenite ATiO3(A=Fe, Co, Ni)

Ilmenite-type ATiO3 (A=Fe, Co, Ni) crystals have been investigated via Generalized Gradient Approximation (GGA) in the scheme of Revised Perdew-Burke-Ernzerhof (RPBE) using the first-principles method. The band structures, densities of states, bond orders and charge populations, optical properties including the dielectric function ε(ω), absorption coefficient I(ω), refractive index n(ω), extinction coefficient k(ω), electron energy loss function L(ω) and reflectivity function R(ω), are calculated. The results show that the GGA-optimized geometries agree well with the experimental data. FeTiO3 has a direct band gap, but both CoTiO3 and NiTiO3 exhibit indirect band gap. The analysis for densities of states and atomic charge populations exhibits that TiO bonds possess the stronger covalent bonding strength than AO bonds. The calculated optical properties along [100], [010] and [001] as well as polycrystalline directions demonstrate the significant optical anisotropy parallel and perpendicular to c-axis for ATiO3. Finally, the origins of main peaks for optical spectra are presented based on electron transitions. Theoretical insights into the microscopic intrinsic properties of ATiO3 should provide fundamental investigations for further understanding the Ilmenite ATiO3 materials and improving their practical applications.

Structure and Morphology of Electrodeposited Nickel Nanowires at an Electrode Distance of 20mm

The objective of this work is to study the effect of two key factors - external magnetic field and applied current density during template-based electrodeposition of nickel nanowires using an electrode distance of 20 mm. Morphology, length, crystallite size and crystallographic characterization of the grown nickel nanowires at an electrode distance of 20mm are presented. For this electrode distance of 20 mm, these two key electrodeposition factors when coupled was found to reduce crystallite size with a higher growth length and preferred orientation of Ni crystals. These observed changes can be inferred to be due to coupled interaction forces induced by the intensity of applied electric field (current density) and external magnetic field known as magnetohydrodynamic (MHD) effect during the electrodeposition process.

Enhanced saturation magnetization in buckypaper-films of thin walled carbon nanostructures filled with Fe3C, FeCo, FeNi, CoNi, Co and Ni crystals: the key role of Cl.

We report an advanced chemical vapour deposition approach which allows the direct in situ synthesis of cm-length ultrathin buckypapers comprising carbon nanostructures filled with Fe3C, FeCo, FeNi, CoNi, Co and Ni by sublimation and pyrolysis of single or combined metallocenes with very low quantities of dichlorobenzene. As a result, extremely high saturation magnetizations of 117 emu g(-1), 90 emu g(-1) and 80 emu g(-1) are obtained for the specific cases of Fe3C, FeCo and FeNi, respectively, while variable saturation magnetizations of 70 emu g(-1), 58 emu g(-1) and 6.7 emu g(-1) are obtained for Co, CoNi and Ni respectively.

Thick Nickel Coating on Surface of Quartz Optical Fiber by Electrochemical Deposition Method

Electrochemical deppsition methods (electroless plating and electroplating) were used to deposit a thick nickel on surface of quartz optical fibers. A thin Ni--P film was firstly to be deposited on the fiber by electroless plating. Then a thick Ni coating with thickness of about 1.7 mm was deposited on the Ni-P film,as a substrate, by electroplating. The effects of the concentrations of NiSO4·6H2O, NaH2PO2·H2O , C3H6O2 , H3BO3 , pH value and temperature on the quality of Ni-P coatings were investigated and their optimal values were determined by orthogonal test method..The effects of concentrations of NiSO4·6H2O and C12H25―OSO3Na and current density on Ni deposition were studied and the optimum technical conditions were discussed. Under the optimal conditions, the thin Ni-P film possessed good smoothness and strong adhesive force and the Ni-coating had good quality with fine Ni crystals and uniform surface.

Field-regulated switching of the magnetization of Co-porphyrin on graphene

Different magnetic coupling mechanisms have been identified for a few monolayers of Co-porphyrin molecules deposited on a graphene-covered Ni(111) single crystal. A relatively strong antiferromagnetic coupling of the first molecular layer via graphene to the Ni crystal in comparison to a weaker intermolecular coupling gives rise to a complex field-dependent response of this hybrid system. By continuously increasing the magnetic field strength, the net magnetization of the molecular system switches from antiparallel to parallel to the field direction at 2.5 T. Utilizing x-ray absorption spectroscopy and x-ray magnetic circular dichroism, the element-specific magnetization and field dependence was probed. The nature of the magnetic couplings is identified by means of density functional theory and orbital-dependent susceptibilities.

Morphology and Laser-Induced Photochemistry of Silicon and Nickel Nanoparticles

The structural and photoinduced properties of silicon nanoparticles obtained by plasmachemical and electrolytic techniques and the nickel particles deposited on aluminum oxide film in ultra-high vacuum are investigated by Auger electron spectroscopy, transmission electron microscopy, Fourier-transform infrared spectroscopy and time-of-flight spectroscopy. It is found that substantial increase of silicon nanoparticle photoinduced luminescence can be attributed to particle specific structure, as well as to the SiO2 thin film which is formed on the nanocrystalline silicon surface. In case of Ni particles deposited on aluminum oxide film at low mean coverage of about 0.04 monolayers, when the film can be viewed as consisting of separated single adsorbed atoms or very small clusters, the photon irradiation by nanosecond pulsed laser leads to NO desorption. At monolayer Ni coverage formed at a substrate temperature of 80 K laser irradiation causes dissociation of NO molecules. Efficiency of this process at the initial stage is notably enhanced compared to that of NO on the bulk Ni (111) crystal. This enhancement can be attributed to the effect of underlying aluminum oxide support.

TiO2-supported catalysts for the steam reforming of ethanol

TiO2 was used as support for Ni, Co and Cu to prepare catalysts for the steam reforming of ethanol, due to its known tendency to form strong metal–support interaction (SMSI) with some metals. The samples were prepared following different procedures, tuning the reducibility of the active phase and its interaction with the support. The latter parameter showed pivotal to impart suitable catalytic activity and most of all stability towards coking. Indeed, the insurgence of SMSI may allow to keep the active phase dispersed, improving activity and inhibiting the formation of carbon filaments over the active phase.The comparison between different active phases (Ni, Co, Cu, 10wt%) confirmed Ni as very active, although it has a higher tendency to form carbon filaments. This drawback may be at least partially controlled by favouring high Ni dispersion through the formation of a mixed oxide with the support. The calcination temperature, and in general the preparation procedure for the catalyst, showed of pivotal importance to establish SMSI. In particular, calcination at relatively low temperature (i.e. 500°C) induced initially a higher dispersion of the active phase (mean Ni crystal size 7nm), however not accompanied by sufficient stabilisation during activity testing (mean Ni crystal size increased to 44nm). By contrast, calcination at higher temperature (i.e. 800°C) favoured the instauration of a SMSI and the formation of a mixed oxide (NiTiO3), which, after activation, allowed the coexistence of smaller particles, more active and resistant to deactivation and sintering, together with the more sintered ones (mean Ni crystal size 27nm before and after activity testing). Different characterisation data (XRD, FT-IR, TPR, TEM) allowed to conclude the need of calcination at high temperature to achieve sufficient activity and resistance of the catalyst for this application.Co and Cu proved more promising as for C balance, although their activity at low temperature was unsatisfactory, mainly due to poor activity for CC bond cleavage.The characterisation of the spent catalysts by XRD, TEM and Raman allowed to evidence the different types of C deposed and to check for active phase stability against sintering.

The effects of Si substitution on the glass forming ability of Ni–Pd–P system, a DFT study on crystalline related clusters

In order to investigate the influence of Si in the glass forming ability of Pd–Ni–P based bulk metallic glasses, the topological and electronic structure of stoichiometry related crystals is explored using density functional theory. The results indicate that Si based clusters based on a rich Pd environment with a symmetric configuration of the transition metal are the most stable, while P prefers a Ni rich environment with an asymmetric arrangement of the transition metals. Si tends to be at the center of the clusters, while in general the P atoms present a displacement toward some of the Ni atoms. The coordination of Si in rich Ni crystals is always higher than the one observed for P atoms. Also, the inclusion of Si induces a local pressure field in a radius of 14Å leading to a disordered displacement of P from the equilibrium positions. The favoring of different competing crystals by adding Si, as well as the local pressure effect, suggests a mechanism on how devitrification is suppressed.

CO2 reforming of methane over MCM-41-supported nickel catalysts: altering support acidity by one-pot synthesis at room temperature

Highly structured neat MCM-41, Na-form, and Al-incorporated MCM-41 mesoporous materials were synthesized using a simple, one-pot procedure at room temperature. The neat MCM-41 material possessed a surface area comparable with other MCM-41 materials prepared using more conventional higher temperature methods. Incorporating Al into the MCM-41 framework increased support acidity while entrapping Na ions in the MCM-41 pores lowered acidity. The neat MCM-41, Na-form, and Al-incorporated MCM-41 were impregnated with 2.5, 5 and 10% Ni and catalytic performance was assessed for CO2 reforming of methane (CH4). XRD, TEM and H2-TPR indicated at low loadings the Ni was highly dispersed on the support, whereby increasing the Ni loading encouraged the formation of larger Ni crystals on the external MCM-41 surface. 2.5% Ni catalysts supported on neat MCM-41 exhibited excellent catalytic activity invoking up to 98% CH4 conversion at 800°C and 26% at 500°C. Carbon accumulation on the catalyst was greater at higher Ni loadings although this was not to the detriment of activity over the reaction time-frame. This result coupled with variations in the H2/CO product ratio and CH4/CO2 conversion difference suggested increasing the Ni loading promoted the Boudouard, CH4 cracking and reverse water gas shift side reactions. Incorporating the MCM-41 with either Al or Na lowered catalyst performance with the promoters found to enhance or suppress the side reactions to varying degrees.

Charge Conduction Properties at the Contact Interface between (Phthalocyaninato)nickel(II) and Electron Acceptor Single Crystals

Single-component crystals of both (phthalocyaninato)nickel(II) (Ni(Pc)) and 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane (F2TCNQ) are typical band insulators. However, the contact interface between them demonstrates metal-like transport properties. Although Ni(Pc) and F2TCNQ are an electron donor and an acceptor, respectively, the combination of these two components does not yield any charge transfer (CT) complex crystals. Infrared spectra show that the highly conductive feature originates from charge injection at the contact interface. The thermoelectric power of the mixed powder reveals that the transport at the contact interface is dominated by the holes in the Ni(Pc) crystal.

Ab initiostudy of thermodynamic, electronic, magnetic, structural, and elastic properties of Ni4N allotropes

We have employed parameter-free density functional theory calculations to study the thermodynamic stability and structural parameters as well as elastic and electronic properties of Ni${}_{4}$N in eight selected crystallographic phases. In agreement with the experimental findings, the cubic structure with Pearson symbol cP5, space group $Pm\overline{3}m$ (221) is found to be the most stable and it is also the only thermodynamically stable structure at $T=0$ K with respect to decomposition to the elemental Ni crystal and N${}_{2}$ gas phase. We determine structural parameters, bulk moduli, and their pressure derivatives for all eight allotropes. The thermodynamic stability and bulk modulus is shown to be anticorrelated. Comparing ferromagnetic and nonmagnetic states, we find common features between the magnetism of elemental Ni and studied ferromagnetic Ni${}_{4}$N structures. For the ground-state Ni${}_{4}$N structure and other two Ni${}_{4}$N cubic allotropes, we predict a complete set of single-crystalline elastic constants (in the equilibrium and under hydrostatic pressure), the Young and area moduli, as well as homogenized polycrystalline elastic moduli obtained by different homogenization methods. We demonstrate that the elastic anisotropy of the ground-state Ni${}_{4}$N is qualitatively opposite to that in the elemental Ni, i.e., these materials have hard and soft crystallographic directions interchanged. Moreover, one of the studied metastable cubic phases is found auxetic, i.e., exhibiting negative Poisson ratio.

Contribution of the inverse flexoelectric effect to counterpropagating two-wave mixing of light beams in photorefractive crystals

This paper presents the results of a theoretical analysis of the contribution of the inverse flexoelectric and photoelastic effects to the photorefractive response that accompanies counterpropagating mixing of a steady-state reference wave with a phase-modulated signal wave on reflective holograms in samples of X-cut crystals of symmetry classes 23, 4¯3m, 4¯2m, 422, 622, 222, and 3m. Experimental studies of such mixing of waves with circular polarization of opposite signs in a Ba12TiO20:Ni crystal made it possible to estimate its flexoelectric coefficient.