Nickel Grains Research Articles

Improvement of the redox durability of Ni-gadolinia doped ceria anodes due to the use of the composite particles prepared by spray pyrolysis method

The redox behaviors of Ni-gadolinia doped ceria (GDC) anodes with different compositions and microstructures were investigated. When the NiO-GDC composite particles prepared by spray pyrolysis were used for an anode (SP-cells), the decrease of the cell voltage during the redox cycles was smaller than that of the cell with anode using the mixed powder of NiO and GDC (Mix-cells). Current interruption and AC impedance measurements were examined before redox cycle, after 1st redox cycle, and after 10th redox cycles. The ohmic and polarization losses after 10th cycles for SP-cells were smaller than those for Mix cells. The morphology changes of the Ni-GDC anodes between before-redox cycle and after-10th redox cycles were observed by scanning electron microscopy (SEM). These results indicate that the use of NiO-GDC composite particles suppressed the excessive agglomerations of nickel grains during the redox cycles.

Erosion–corrosion resistance of Ni composite coatings with embedded SiC nanoparticles

This study assessed the erosion–corrosion behavior of pure Ni and Ni–SiC composite coatings evaluated in a corrosive environment using in situ potentiondynamic polarization and electrochemical impedance. Coatings were obtained from a Watts classical solution in which SiC nanoparticles were added. Coatings were galvanostatically deposited on carbon steel on a rotating disk electrode. The hydrodynamic conditions of the bath and the content of SiC in the electrolyte were varied. The morphology, structure and properties of the coatings were evaluated in relation to the electro-synthesis conditions. It was found that incorporation of SiC nanoparticles into the metal matrix produces morphological changes in the deposit. These are responsible for increases in the microhardness of the composite coatings and a better resistance against erosion–corrosion compared with pure nickel coatings. The increases in SiC nanoparticle content in the coating and the hydrodynamic conditions in the electroplating process cause grain refining and changes in the crystal growth of the nickel grains in the Ni deposit. As consequence of structural modification in the crystalline orientation of the Ni deposit, grain refining and strain occurred in the nickel lattice. These structural changes in the nickel deposit led to enhanced performance of the Ni–Sic composite coating in the erosion–corrosion test. Additionally, the presence of inert particles of SiC in the conductive matrix coating may have blocked the passage of the anodic Faradaic current, diminishing the corrosion rate of the coating.

Electrodeposition of Ni–Fe2O3 nanocomposite coating on steel

Aqueous solutions, containing appropriate amounts of sodium dihydrogen phosphate and iron chloride, were transformed into dispersions of the precipitated particles of iron oxide by heating them at 98°C for various periods of time (24–94h) in tightly stoppered Pyrex glass vessels. These particles were employed as reinforcement additive in the electrodeposed Ni–Fe2O3 composite coatings on the steel substrate. The pure Ni and Ni–Fe2O3 composite coatings were subjected to Vickers hardness tests, which indicated that in most of the cases, microhardness of the Ni–Fe2O3 composite coatings was higher than the pure Ni coatings. Furthermore, XRD analysis showed that coating temperature as well as the presence of particles in the formed coatings had obvious effect on the preferred orientation of the crystalline faces of the nickel grains. The value of the texture coefficient was also calculated for pure Ni and Ni–Fe2O3 composite coatings, produced at different temperatures. Significant differences were noted in these values, which were attributed to the changes in the microstructure of the matrix, caused by the embedded Fe2O3 particles. Wear resistance properties and friction coefficient of the pure Ni and Ni–Fe2O3 composite coatings were determined with ball-on-disc tribometer in a unidirectional sliding mode. Results revealed that wear resistance of Ni–Fe2O3 composite coatings was higher than the pure Ni coating. Similarly, the friction coefficient turned out to be smaller for the composite coatings as compared to the pure Ni coating at a given sliding distance. Corrosion resistance of the bare, pure Ni coated, and Ni–Fe2O3 composite coatings was also evaluated. It was noted that corrosion resistance of these specimen increased in the order; Bare substrate<Pure Ni coating<Ni–Fe2O3.

Origin of the Bismuth-Induced Decohesion of Nickel and Copper Grain Boundaries

Ductile metals such as Ni and Cu can become brittle when certain impurities (e.g., Bi) diffuse and segregate into their grain boundaries (GBs). Using first-principles calculations, we investigate the microscopic origin of the Bi-induced loss of cohesion of Ni and Cu GBs. We find that the Bi bilayer interfacial phase is the most stable impurity phase under the Bi-rich condition, while the Bi monolayer phase is a metastable phase regardless of the value of the Bi chemical potential. Our finding is consistent with the recent experimental observation for Ni GBs [Luo et al. Science 333, 1730 (2011)]. The electric polarization effect of the Bi bilayer substantially enhances the strength of the Bi-metal interfacial bonds, stabilizing the bilayer phase over other phases. The Bi-Bi interlayer bonding is significantly weakened in the GBs, leading to a factor of 20 to 50 decrease in the GB cohesion, which has strong implications for the understanding of Bi-induced intergranular fracture of Ni and Cu polycrystals.

Study on Preparation and Tribology Behaviors of Ni-P Electroless Deposited Nanocrystalline Coating

Ni-P nanocrystalline and amorphous coating were fabricated by electroless deposition technique in the same bath except for varying complexing agent. Heat treatment at lower temperature was carried out to the two kinds of coatings above. The phosphorus content and organize structure of the as-deposited coatings was determined by means of inductively coupled plasma atomic emission spectrometry, X-ray diffractometer and transmission electron microscope. Moreover the effects of heat treatment and structure on microhardness and tribology property of coatings were studied by means of microhardness tester, friction and wear tester and scanning electron microscope. Outstanding tribology property of Ni-P nanocrystalline coating compared with the amorphous one owed to the fine nickel grains with a size of several nanometers. While the proper heat treatment process was found to be responsible for improved property of both coatings.

Production and characterization of Ni and Cu composite coatings by electrodeposition reinforced with carbon nanotubes or graphite nanoplatelets

Electrodeposition is well-known as a versatile and economical processing technique to produce metal coatings on conductive substrates. Recently, it has been gaining increasing interest also for the production of tailored composite coatings, containing for instance floropolymers or silicon carbide. A more novel approach concerns the use of carbon nanotubes or even graphene, in the form of graphite nano-platelets. The production of Ni- and Cu-based nanocomposites containing carbon nanoreinforcements was carried out by using standard electrodeposition conditions, but with a particular attention to the dispersion of the nanotubes. The obtained coatings were strong and well adherent to the steel substrate, and presented rather well dispersed carbon nanotubes or graphite nanoplatelets, even if some agglomerates could be present in samples obtained from highly concentrated suspensions. In the case of nickel-based composite coatings, the size of nickel grains was reduced, and pin-on-disc tests demonstrated a significant increase in the life of the coating. In the case of copper-based composite coatings, thermal diffusivity measurements demonstrated that the carbon nanomaterial does not reduce the conductivity of the pure copper coating.

Self cleaning behaviour of Ni/nano-TiO2 metal matrix composites

Titanium dioxide (TiO2) nano-particles were immobilized in nickel matrix coatings using the direct current electrodeposition technique. The influence of nickel surface morphology and crystallographic orientation on the photocatalytic activity and photo-induced hydrophilicity of the composite coatings were investigated under UVA illumination. The overall self-cleaning character of the composites was improved following the titania codeposition, as both hydrophilic and photocatalytic effects are present. Simultaneous optimization of the two functionalities was possible via tuning the titania percentage, indicating that the corresponding properties of the composite coatings are associated also to the induced crystalline nickel orientation and the nickel grain size. In particular, the data indicate that both enhanced photocatalytic activity and photo-induced hydrophilicity are achieved by increasing amounts of titania nano-particles embedded in the Ni matrix in combination with reduced grain size of nickel crystallites that are oriented towards the [110] axis.

Quantitative elemental microscopy on lateral highly inhomogeneous meteorite samples using ion beam analysis

A simple system for lateral dead-time correction was developed at the Leipzig ion nanoprobe LIPSION in order to investigate diverse meteorite samples. The meteorite building crystals can often be found in close proximity to massive iron–nickel grains which provides a challenge for ion beam analysis methods, like Particle Induced X-ray Emission (PIXE) or Rutherford Backscattering Spectrometry (RBS) due to the strongly varying dead-time of the detector electronics on such inhomogeneous samples. This leads to incorrect concentration mapping on a local scale. In order to account for this problem, a noise-triggered setup was developed that generates defined pulses statistically in time which are fed into the preamplifiers of the detector electronics and act as pseudo events in the data processing. A comparison of the number of generated and processed pulses allows the computation of a dead-time map and thereby a correction of the collected spectrum on a pixel per pixel basis. The proper function of this technique was verified by measurements on a simple test specimen and a chondrule of the Acfer 094 meteorite.

Microstructures of Ni–AlN composite coatings prepared by pulse electrodeposition technology

Ni–AlN composite coating was fabricated onto the surface of steel substrates by using pulse electrodeposition (PED) technique in this work. The effect of pulse current on the nucleation and growth of grains was investigated using transmission electronic microscopy (TEM), X-ray diffraction (XRD), scanning electronic microscopy (SEM) and atomic force microscopy (AFM), respectively. The results show that the contents of AlN nanoparticles increase with density of pulse current and on-duty ratio of pulse current increasing. Whereas the size of nickel grains decreases with density of pulse current increasing and on-duty ratio of pulse current decreasing. Ni–AlN composite coating consists of crystalline nickel (∼68nm) and AlN particles (∼38nm). SEM and AFM observations show that the composite coatings obtained by PED showed more compact surfaces and less grain sizes, whereas those obtained by direct current electrodepositing have rougher surfaces and bigger grain sizes.

Transport mechanism in granular Ni deposited on carbon nanotubes fibers

We investigate the transport properties of granular nickel electrodeposited on carbon nanotube fibers by measuring the electrical resistance and the current voltage characteristics as a function of the temperature. The bare fiber is governed by a three-dimensional variable range hopping transport mechanism, however, a semiconducting to metallic transition is observed after the Ni deposition as a consequence of the evolution from weak to strong coupling between the deposited nickel grains. The experimental results indicate that the charge transport in the Ni-coated fiber develops from hopping governed by the Coulomb blockade in the case of small grains dimensions to a metallic electron phonon interaction mechanism for large grains dimensions. Tunneling enhanced by thermal fluctuation is responsible for the transport in the intermediate conductivity range. The role of the fiber and the effects due to the magnetic nature of the nickel grains are also discussed.

Nickel-rich metasomatism of the lithospheric mantle by pre-kimberlitic alkali-S–Cl-rich C–O–H fluids

Metasomatism of the lithospheric mantle sometimes produces unusual assemblages containing native metals and alloys, which provide important insight into metasomatic processes in the mantle. In this study, we describe the metasomatic enrichment of a refractory harzburgite xenolith in Ni, Fe and, to a lesser extent, Cu, Co, As and Sb. The xenolith (XM1/422) derives from the Bultfontein kimberlite (Kimberley, South Africa) and hosts Ni mineralisation that includes native nickel (Ni84.5-98.0), heazlewoodite (Ni3S2) and Ni-rich silicates (e.g. up to 37.5 wt % NiO in olivine, and 22.4 wt % NiO in phlogopite). The presence of several mineral phases enriched in alkali and volatile species (e.g. phlogopite, phosphates, carbonates, chlorides, djerfisherite) indicates that the transition metal cations were likely introduced during metasomatism by alkali-rich C–O–H fluids or alkali-carbonate melts. It is postulated that sulphide breakdown and fluid reaction with refractory mantle rocks contributed to the fluid’s enrichment in Ni and other metallic cations. The Ni-rich assemblages of xenolith XM1/422 show local chemical disequilibrium, and modelling of the Ni diffusion profiles adjacent to olivine-native nickel and olivine-heazlewoodite grain boundaries, suggests a close temporal relationship between Ni-rich metasomatism and subsequent entrainment by the kimberlite magma. However, metal-rich metasomatism has also been observed in other lithospheric mantle domains, including orogenic peridotitic massifs and the suboceanic mantle; regions unaffected by kimberlite magmatsim. As micro-scale occurrences of metallic phases are easily overlooked, it is possible that metal-rich metasomatism is more widespread in the Earth’s mantle than previously recognised.

Self-Assembled Carbon Nanostructures in Nickel Containing Hydrogenated Amorphous Carbon Matrix by One-Step Electrodeposition

Carbon nanostructures incorporated hydrogenated amorphous carbon matrix composite was facilely assembled by the electrolysis of a methanol solution of nickel pentanedionate at ambient atmosphere pressure. Results confirmed that, nickel grains were embedded in amorphous carbon matrix forming nanocrystallite/amorphous composite structure, meanwhile fiber-like carbon were interconnected and dispersed in nickel containing hydrogenated amorphous carbon matrix. The growth mechanism of the as-deposited nanocomposite film mainly involved fiber-like carbon growing onto an enriched nickel template at the un-equilibrium micro-zone around the cathodic substrate during the electrochemical process.

A high density of vertically-oriented graphenes for use in electric double layer capacitors

The growth, capacitance and frequency response of vertically-oriented graphenes grown by radio frequency plasma-enhanced chemical vapor deposition on nickel substrates and used as electrodes in electric double layer capacitors (EDLCs) are presented. The graphenes grown on the grain boundary of substrates show a faster growth rate, but less ordered structure than in the center of the nickel grain. At a few nanometers away from the grain boundaries the graphenes grow vertically at the rate of 70–80nm per minute. The film height increased linearly with growth time from 700nm (10min sample) to 3.1μm (40min sample). Raman spectra show that the intensity ratio of the D band to G band gradually decreased with growth time to a value of 0.5, indicating that the crystalline order of the graphene increases with height. The specific capacitance of symmetric, parallel plate EDLC devices fabricated with these films was found to increase in a linear fashion with growth time up to values greater than 120μF/cm2 at 1kHz. An impedance phase angle of −45° was reached at 30kHz. Specific capacitance normalized to growth height suggests that mechanisms other than double layer charge storage on planar surface area were operative.

A study on the stability of a NiO–CaO/Al2O3 complex catalyst by La2O3 modification for hydrogen production

This paper details the study of La2O3 modifications and their effect on the stability of a NiO–CaO/Al2O3 sorption complex catalyst used in the ReSER (reactive sorption enhanced reforming) process of hydrogen production. The La2O3-modified NiO–CaO/Al2O3 sorption complex catalyst was prepared by isometric impregnation. The microstructure, morphology and reducibility of the La2O3-modified sorption complex catalyst were characterized by means of BET, TEM, XRD and TPR. The stability of the catalyst used in the ReSER process was evaluated on a laboratory-scale fixed-bed reactor. Our results showed that modifying the sorption complex catalyst with La2O3 improved its stability up to 30 cycles of the ReSER process for hydrogen production, while only seven cycles were obtained without La2O3 modification. We showed that the source of the stability improvement that the La2O3 in the catalyst not only functioned to restrain the decrease of the support surface area and reduce the sintering of nano-CaCO3, which could limit the decay of the sorption capacity and stability of the catalyst, but also increased the interaction between nickel oxide and the support, which improved the stability of the catalyst by increasing the dispersion of nickel grains and inhibited the growth of nickel grain size.

A Study on Ni–TiN Coatings Produced by Ultrasonic Pluse Electrodeposition

Nanocomposite Ni–TiN coatings were prepared by ultrasonic pluse electrodeposition and the effects of ultrasonication on the coatings were studied. X-Ray diffraction analysis was utilized to detect the crystalline and amorphous characteristics of the composite coatings. The surface morphology and metallurgical structure were observed by scanning electron microscopy, high-resolution transmission electron microscopy and scanning probe microscopy. The results show that ultrasonication has great effects on TiN nanoparticles in composite coatings. Moreover, the introduction of ultrasonication and TiN particles led to the formation of smaller nickel grains. The average grain diameter of TiN particles was 33 nm, while Ni grains measured approximately 53 nm.

Electrodeposition of Composite Coatings Containing AlN Particles in Nickel Deposit

Nanometer Ni-AlN composite layers were prepared by electrodeposited technology. The optimum technological parameters of nanometer Ni-AlN composite layers were obtained by experiments and analysis. We observed surface morphology and metallurgical structure of composite layers with scanning electron microscope (SEM) and high respective transmission electron microscope (HRTEM). The test results showed that the nanometer Ni-AlN composite layers prepared by proper ultrasonic-electrodeposited technology have better wear resistance and corrosion resistance. And the composite layers consist of AlN particles and nickel grains both nanometer-sized.

Grain growth in forsterite–nickel mixtures: Analogues of small parent bodies during early accretion

We report an experimental study on the grain growth kinetics and microstructural evolution of olivine–metal systems. 32 experiments have been performed on four different mixtures of forsterite (Fo) and nickel (Ni) (respectively in vol.%, 95:5, 80:20, 30:70 and 10:90). All experiments were carried out at high temperature (1440°C), at oxygen fugacity (fO2)∼3 log units below the Ni–NiO buffer and for durations between 2h and 21days. In aggregates containing isolated forsterite grains in a nickel matrix (Fo+Ni (10:90)) forsterite did not grow, and grain growth of nickel was abnormal. In all other cases, features of normal grain growth were observed and the data were fit to the classic grain growth law dn-d0n=kt. For all these aggregates, grain growth exponents are in the range 4.0<n<5.7 and are not correlated with the phase proportion in the mixture. On the other hand, grain growth of forsterite is slower and that of nickel faster as the proportion of nickel increases. Details of the normalised grain size distributions (NGSD) are used to infer that grain growth of forsterite occurred by grain boundary migration rate-limited by nickel grains. For nickel, two types of behaviour were observed. In aggregates containing 5 or 20vol.% nickel, grain growth occurred by “coalescence” strongly rate-limited by grain boundary migration of forsterite. In nickel-rich aggregates (Fo+Ni (70:30)), grain growth occurred by grain boundary migration. Comparison with studies from the literature shows that our experimental data are similar to those of Nichols and Mackwell (1991) for porous olivine aggregates, both Ni and pores apparently behaving as mobile minor phases. Extrapolation to time-scales relevant to the formation of ordinary chondrite parent bodies shows that mean grain sizes reached (Fo and Ni) are of the same order of magnitude (100μm to 1mm) as those observed in several types of meteorites. Iron rich phases (kamacite–taenite and/or troilite) in several H-chondrites are shown to present normal grain growth features and NGSD shapes similar to those in our experimental samples where nickel grains are not interconnected, i.e., Fo:Ni (95:5) and (80:20). The grain growth mechanism for the iron-rich phases in these meteorites may thus also occur by “coalescence” rate-limited by grain boundary migration of olivine.

The effect of grain refining and phosphides formation on the performance of advanced nanocomposite and ternary alloy coatings on steel

This paper is a continuation of our investigation into the designing of high performance anti-corrosion nanocomposite and ternary alloy Ni–P coatings via electroless deposition technique. It discusses the effect of nickel grain refining and phosphides formation on the coating performance over steel. Hard chromate coating has been reported as the most efficient widespread conversion coatings for many metallic substrates. However, the waste containing toxic chromate has many limitations due to the environmental consideration and health hazards. In this paper, newly developed (Ni–P–W) ternary alloy coatings and nano-scattered alumina (Ni–P–Al2O3) composite coatings were prepared by electroless deposition technique on low carbon steel. The optimum conditions under which such coatings can provide the highest micro-hardness and anti-corrosion characteristics to the steel substrate were determined. The coating performance was investigated using electrochemical impedance spectroscopy and polarization measurements. Ni–P–W alloy coatings showed the highest hardness and anti-corrosion properties compared with Ni–P–Al2O3 and Ni–P. Surface examination of Ni–P–W and Ni–P–Al2O3 using XRD, SEM-EDS and macroscopic images revealed grain refining of Ni grains. A protective layer of nickel phosphide and tungsten phosphide was formed over steel substrate due to Ni–P–W. While a protective layer of nickel phosphide enriched with aluminum oxide was formed due to Ni–P–Al2O3. The aim of this article is to deepen the current understanding of corrosion and protection of steel substrate in chloride containing environments and to provide a base for future research work in this field.

Orientation imaging using spatially resolved acoustic spectroscopy

The term 'orientation imaging microscopy' describes generic techniques for imaging the orientation of crystalline structures of heterogeneous media. Typically, scanning electron microscope techniques are used, such as electron backscatter diffraction or transmission electron microscopy. We have developed an acoustic technique that can perform equivalent measurements. The key to enabling a practical acoustic method is spatially resolved acoustic microscopy (SRAS), a robust laser ultrasonic technique for quantitatively determining the surface acoustic wave velocity at a high spatial resolution. Here we present quantitative determination of crystallographic orientation of large nickel grains by comparing the measured SAW velocity in multiple propagation directions, with a data base of calculated velocity surfaces for all orientations. As well as discussing the method of determining the crystallographic orientation details are also presented of the recent advances in the capabilities of the latest generation of instrumentation, including significant increase in the data acquisitions rate and the reduction in size and complexity of the SRAS instrument itself.

Quantification of grain boundary equilibrium segregation by NanoSIMS analysis of bulk samples

A technique for the quantification of equilibrium grain boundary segregation by high resolution secondary ion mass spectroscopy (NanoSIMS) on simple metallographically polished surfaces has been demonstrated for the model system of sulphur segregation to nickel grain boundaries. Samples of nickel containing 5.4 wt ppm of sulphur were annealed at different temperatures to achieve different equilibrium sulphur grain boundary concentrations, ranging from less than 1% to about 50% of a monolayer. Quantification was carried out from sulphur concentration profiles acquired across about 20 grain boundaries in each sample. An internal standard (nickel containing a known concentration of sulphur in solid solution) was used for calibration. It is found that, depending on the annealing temperature, the average grain boundary sulphur concentration ranges from 0.9 to 25.8 ng cm–2 (or 1.7 1013 to 4.8 1014 atoms cm–2), i.e. ~0.015 to ~0.43 monolayer. Thermodynamic analysis gives a segregation free energy of −97.8 kJ mol–1 and a grain boundary sulphur concentration at saturation of 26.7 ng cm–2 (or 5.0×1014 atoms cm–2), i.e. ~0.44 monolayer, in good agreement with previous measurements on this system. The limit of detection of the technique is shown to be as low as 0.24 ng cm–2 (or 4.5×1012 atoms cm–2), i.e. ~0.004 monolayer, with a counting time of only 10 min. Copyright © 2012 John Wiley &amp; Sons, Ltd.