Nickel Particle Size Research Articles

Synthesis and characterization of Ni magnetic nanoparticles in AlMCM41 host

Superparamagnetic nickel nanoparticles were prepared by incorporating nickel ion into AlMCM41 as a nanoreactor and then reduced with sodium borohydride or H 2 gas. Products were characterized by elemental analysis, transmission electron microscopy, X-ray powder diffraction, and magnetic susceptibility. The nickel particle size and blocking temperature depend on the reduction method.

Impact of the structure and reactivity of nickel particles on the catalytic growth of carbon nanofibers

Catalytically grown fishbone carbon nanofibers (CNF), are prepared by the decomposition of carbon-containing gases (CH 4, CO/H 2 or C 2H 4/H 2) over a silica-supported nickel catalyst and an unsupported nickel catalyst at 550 °C. It turns out that both the nickel particle size and the nature of the carbon-containing gas significantly affects the CNF growth process. We demonstrate that at the chosen temperature small supported nickel particles need a carbon-containing gas with a relatively low reactivity, like CH 4 or CO/H 2, to produce CNF. The resulting fishbone CNF have a uniform and small diameter (25 nm). The CNF thus synthesized hold great potential, e.g. as catalyst support material. However, the large unsupported nickel particles only produce CNF using a reactive carbon-containing gas, like C 2H 4/H 2. The CNF thus obtained show a variety of morphologies with a large range of diameters (50–500 nm). The CNF yield is a subtle interplay between the nickel particle size and consequently the exposed crystal planes on the one hand and the reactivity of the carbon-containing gas on the other.

Sol-gel processing of alkoxysilyl-substituted nickel complexes for the preparation of highly dispersed nickel in silicaElectronic supplementary information (ESI) available: Tables S1–S5 giving additional analytical data as described in the text and the exact quantities for the syntheses. See http://www.rsc.org/suppdata/nj/b1/b110612k/

Nanocomposites containing nickel nanoparticles in silica were obtained by sol-gel processing of Ni(NO3)2, Ni(OAc)2 (OAc = acetate), NiCl2, or Ni(acac)2 (acac = acetylacetonate), a complexing silane [(EtO)3Si(CH2)3NHCH2CH2NH2, (EtO)3Si(CH2)3NH2 or (RO)3Si(CH2)3NHCH2CH2NHCH2CH2NH2] and Si(OEt)4, followed by calcination of the metal-complex-containing xerogels in air and reduction in hydrogen. Two types of nanocomposite materials were prepared, both containing about 8–8.5 wt % of elemental nickel: (i) Ni/SiO2 nanocomposite powders in which the silica matrix is mainly formed from Si(OR)4, and (ii) Ni particles on pre-formed silica spheres. The latter were prepared by impregnating silica spheres with sols from the particular complexing silane–nickel salt combination followed by calcination and reduction. In both types of materials, the average nickel particle size is influenced by the composition of the precursor mixture, particularly the counter-ion of the employed nickel salt. The smallest particles (diameter below 3 nm) were obtained from Ni(NO3)2 [and Ni(OAc)2 for the powders], while the use of Ni(acac)2 and NiCl2 resulted in larger nickel particles. The variation of the nickel particle diameters was much larger for the powders than for the impregnated spheres.

Filamentous Carbon Growth on Nickel/Silica: Potassium and Bromine as Catalyst Promotors

In a previous Communication, we reported the catalytic growth of highly ordered carbon filaments during the hydrodechlorination of chlorobenzene over nickel/silica at the remarkably low temperature of 553 K. This low temperature growth was attributed to the presence of bromine and potassium on the catalyst surface, which facilitated a restructuring of the active sites leading to a destructive chemisorption of the reactant that preceded the dissolution and precipitation of carbon in an ordered fashion. In this paper, we examine the effect of bromine and potassium built into (by impregnation with KOH, HBr and KBr) a Ni/SiO2 parent sample on filamentous carbon production through ethylene decomposition. The parent catalyst delivered a low yield of carbon (0.2 g(C)g(cat)(-1)) and promoted the competing hydrogenation (to ethane) step to the same degree; hydrogenolysis to methane represented the only other significant reaction. The nature of the carbon deposition has been characterised by high-resolution transmission electron microscopy (HRTEM) and temperature-programmed oxidation (TPO). Impregnation with KOH resulted in little change in carbon yield but the hydrogenolysis step was completely suppressed. A combination of CO chemisorption/temperature programmed desorption has been employed to probe the electronic structure of the nickel sites where the presence of potassium limited CO uptake in contrast to surface bromine which served, through electron withdrawal, to strengthen the CO-Ni interaction. Incorporation of bromine into the catalyst resulted in a marked increase in nickel particle size and a dramatic enhancement of carbon yield; the sample treated with KBr delivered the appreciably higher carbon yield of 7.6 g(C)g(cat)(-1)). While the presence of bromine on the surface served to enhance carbon deposition, the additional incorporation of potassium raised the degree of order in the carbon growth.

Catalytic hydrodehalogenation as a detoxification methodology

Catalytic hydrodehalogenation is presented as a viable approach in the non-destructive treatment of concentrated halogenated aromatic gas streams to generate reusable raw material. Nickel loaded (from 1.5 to 20.3% w/w) silica catalysts have been used to hydrotreat a range of halogenated feedstock, where 473 K≤ T≤573 K: chlorobenzene, chlorotoluene, chlorophenol, bromobenzene, dichlorobenzene, dichlorophenol, trichlorophenol, pentachlorophenol. The long term (up to 800 h-on-stream) stability of these catalysts has been assessed where the changes in nickel particle size and morphology as a result of the prolonged catalytic step was probed by TEM; each catalyst irrespective of any loss of initial activity was fully selective in solely promoting dehalogenation. In the case of a polychlorinated feedstock, dechlorination can proceed in a stepwise manner to generate a partially dechlorinated product. Hydrodehalogenation appears to occur via an electrophilic mechanism where the presence of electron-donating substituents on the benzene ring enhances the rate of reaction. The reaction is shown to be structure sensitive over Ni/SiO 2 where the hydrodechlorination rates and ultimate yield of the parent aromatic from a polychlorinated reactant is favored by larger nickel particle sizes. A direct contact of the freshly activated catalyst with HCl or HBr gas induced an appreciable growth of the supported metal crystallites. Chlorobenzene hydrodechlorination was suppressed on a HCl or HBr treated Ni/SiO 2 which promoted instead the unexpected growth of highly ordered carbon filaments; this carbon growth is characterized by TEM and SEM. The dependence of the experimental hydrodechlorination and hydrodebromination rates on the gas phase aromatic partial pressure (in the range 0.02–0.1 atm) is adequately represented by a kinetic model involving a non-competitive adsorption of hydrogen and halogenated aromatic where the incoming aromatic reactant must displace the hydrogen halide from the catalyst surface.

Properties of Nanocrystalline Nickel Particles in Ni-Si0 2 Composites

Nickel-Silica nanocomposites with a nickel content equal to 10, 15, 20 wt% have been prepared by a sol-gel method starting from ethanolic solutions of tetraethoxysilane and nickel nitrate hexahydrate. After selation the samples were reduced in H-> flow at selected temperature (450 °C < T < 600 °C). The morphological, structural and magnetic properties were investigated by transmission electron microscopy (TEM), wide and small angle X-ray scattering (WAXS, SAXS), magnetic susceptibility in zero field cooled and field cooled mode (ZFC and FC), and magnetic hysteresis loop. Nanometric nickel particles are observed in all the investigated samples. TEM, WAXS and SAXS techniques indicate that the average nickel particle size grows slightly but almost regularly with the nickel concentration. TEM results moreover indicate that also the width of the particle size distribution, which can be simulated by log-normal functions, follows this trend. All the sample treated in hydrogen show superparamagnetic behaviour. The magnetisation falls to reach saturation up to highest measuring field of 70 kOe even at 3 K, while the observed coercivity Hc is much higher than the theoretical bulk one. Some uncertainty in the complete interpretation of the sequence of magnetic measurements is attributed to a progressive oxidation of the samples when these are air exposed.

XRD studies of evolution of catalytic nickel nanoparticles during synthesis of filamentous carbon from methane

The XDR technique was used for studying a series of high‐loaded (90%) nickel catalysts with silica as a textural promoter. These were catalysts for direct cracking of methane at 550°C. A relation between the initial average size of active catalyst particles, carbon yield and average methane conversion was demonstrated. Genesis of these catalysts was studied including oxide precursors, reduced catalysts prior to the reaction, as well as catalysts upon their contacting the reaction medium for various periods of time from 15 min to 2 h. The active catalyst particles were shown to merge or disperse at the outset of the reaction depending on their initial size. Anyway, close average sizes ranging from 30 to 40 nm were observed by the end of the first reaction hour for all the catalytic systems providing the carbon yield of 300–385 g/g Ni. The catalytic system was shown to self‐organize in the course of direct methane cracking, i.e., the catalyst particles transform in response to the reaction conditions. If the size of nickel particles cannot vary, these catalysts are inefficient for the given process.

Synthesis of Ni/SiO 2 catalysts through precipitation of silica-sols: effect of aging and Ca(Ba) additives

The preparation of Ni/SiO 2 catalysts by precipitation of silica sols with NH 4OH is described. Nickel introduction during precipitation of silica sols did not affect significantly the silica surface area, yielding amorphous Ni/SiO 2 catalysts in both calcined and reduced forms. This method of preparation results in Ni/SiO 2 catalysts showing homogenous distribution of components, surface areas ca. 255 m 2 g 1, and mean size of nickel particles between 4 and 6 nm. The effects of gel aging and the addition of Ca or Ba cations on the textural, structural and catalytic properties of Ni/SiO 2 catalysts were studied. Results are interpretated according to reported processes of dissolution–precipitation of silica occurring under basic conditions [R.K. Iler, The Chemistry of Silica, Chapters 3 and 4, Wiley, New York, 1979]. Alkaline earth addition to the nickel/silica catalysts decreased the surface area to 100–155 m 2 g −1. For reduced Ni/SiO 2–Ba and Ni/SiO 2–Ca catalysts, the X-ray diffraction (XRD) peak of (1 1 1) planes of metal nickel crystals was identified. Several XRD peaks not corresponding to any Ni, Si and Ba/Ca containing crystalline phase reported in the ICDD database were revealed. They were assigned to new crystalline phases including Ba and Ca in the second coordination sphere of nickel. Hydrogen adsorption on these new phases was stronger than on metal nickel. Variation of hydrogen chemisorption properties and reaction rate for benzene hydrogenation were observed with alkaline earth addition.

Combustion characteristics of the Ni3Ti-TiB2 intermetallic matrix composites

Combustion synthesis (SHS) of Ni3Ti-TiB2 metal matrix composites (MMCs) was selected to investigate the effect of gravity in a reaction system that produced a light, solid ceramic particle (TiB2) synthesized in situ in a large volume (>50 pct) of the liquid metallic matrix (Ni3Ti). The effects of composition, green density of pellets, and nickel particle size on the combustion characteristics are presented. Combustion reaction temperature, wave velocity, and combustion behavior changed drastically with change in reaction parameters. Two types of density effects were observed when different nickel particle sizes were used. The structures of the combustion zones were characterized using temperature profile analysis. The combustion zone can be divided into preflame, reaction, and after-burning zones. The combustion mechanism was studied by quenching the combustion front. It was found that the combustion reactions proceeded in the following sequence: formation of liquid Ni-Ti eutectic at 940 °C → Ni3Ti+NiTi phases → reduction of NiTi with B→TiB2+Ni3Ti.

Synthetic processes of uniform nickel aluminides by reactive infiltration and post hot-pressing of infiltrated precursors

Abstract The suitable condition for processing of uniform nickel aluminides by squeeze casting of aluminum melt into porous nickel preforms was investigated. The reactivity among nickel preforms and aluminum melt depended largely on the initial particle size of nickel. Uniform aluminides could be fabricated by in situ reaction at relatively low-temperatures when fine nickel particles are used. Large defects, which had an eutectic structure, were sometimes observed. These defects were formed by the subsequent infiltration into voids caused by rapid shrinkage due to self-propagating reaction. Dense and uniform aluminides could also be obtained by post hot-pressing of the synthesized aluminides at 1473 K. A large amount of fine Al 2 O 3 particles, which were formed by reaction with impurity oxygen, precipitated during the post hot-pressing.

Effect of Pd, Cu, and Ni additions on the kinetics of NiCl2 reduction by hydrogen

Differnetial thermal analysis (DTA) and thermal gravimetric analysis (TGA), at a heating rate of 10 °C/min, revealed a complete reduction of NiCl2 by hydrogen in a temperature interval of 375 °C to 450 °C. However, addition of 0.1 mass pct of Pd, Cu, or Ni to the sample caused the reduction to occur at considerably lower temperatures, in the rather narrow range of 315 °C to 370 °C. The activation energy of NiCl2 reduction by hydrogen (between 300 °C and 550 °C) without additives is 54 kJ/mol, and with Pd and Cu or Ni added, under isothermal conditions (from 260 °C to 380 °C), is 33 and 50 kJ/mol, respectively. These values confirm a positive effect of additives on the reduction kinetics. The positive effect of Pd is a consequence of the dissociation and spillover of hydrogen, whereas in the case of Cu and Ni(HCOO)2, it is manifested in a decrease in bonds energy in the nickel lattice because of good Cu solubility, and in the formation of artificial nickel nuclei that intensify the reduction, respectively. Scanning electron microscopy (SEM) analysis of nickel powders obtained under isothermal conditions shows relatively rounded spherical particles (0.321 to 0.780 µm in size) of powder samples with additives, and particles of irregular shape (2.085 µm mean size) of the sample without additives. This illustrates the positive effect of Pd, Cu, or Ni added in the reduction process, in decreasing the size of nickel particles and in the production of a more uniform particle shape.

Effect of thermal treatments on surface chemical distribution and catalyst activity in nickel on silica systems

The reducibility of several nickel on silica systems prepared by incipient wetness impregnation and precipitation-deposition has been studied using TPR and TG. The analysis of the results has allowed us to determine the minimum drying and reduction temperatures for both impregnation and precipitation catalysts. From an analysis of the obtained results, together with X-ray diffraction and the study of the reduction degree, dispersion and nickel particle size of catalysts activated at different reduction temperatures, conclusions on the nickel surface chemical distribution have been derived. An optimal reduction temperature for the catalysts has been determined from activity considerations in the hydrogenation of sunflower seed oil.

The influence of the reactant size on the micropyretic synthesis of NiAl intermetallic compounds

The effect of the nickel (Ni) and aluminum (Al) reactant particle size on the micropyretic synthesis of NiAl is studied in this article. A change in the low melting component (Al particle) size is noted to have a limited influence on the micropyretic synthesis conditions. However, a change in the high melting component (Ni particle) size not only influences the combustion temperature and propagation velocity, but also affects the final porosity and grain size of the synthesized products. The combustion mode is also noted to change from stable to unstable when the Ni particle size is increased. It is noted that a diffusion-type control mechanism is dominant for the rapid reaction sequence in the NiAl system. An atomistic mechanism of the Ni-Al micropyretic reaction is also proposed in this article. Following this model, analytical expressions are developed to relatc the variation of the Ni size to the NiAl formation rate with the imposed processing conditions during the micropyretic synthesis. The mechanism of the final grain formation and the grain size changes with changes in the processing variables is also discussed.

Morphology changes and deactivation of alkali-promoted Ni/SiO 2 catalysts during carbon monoxide hydrogenation

Deep morphological transformations undergone by unpromoted and alkali-promoted Ni/SiO 2, occurring during the first hours of CO hydrogenation at atmospheric pressure, are examined using temperature-programmed hydrogenation, magnetic measurements and surface area determination. It is observed, as a first step, that alkali addition results in an increase in the selectivities ofCO 2, C 2+. hydrocarbons and alcohols. This behaviour is similar to that reported elsewhere when the reaction is performed at 5 MPa. The stability of these systems varies with the sequence K>Na>Li>unpromoted. It is shown that catalyst reduction at 873 K results in sintering of the support when alkali promoters are present, which does not alter the nickel particle size so long as the promoter concentration is not too high. The surface area of the catalysts is unchanged after reaction. The deactivation of unpromoted samples results from nickel sintering, in contrast to that of alkalipromoted catalysts: this behaviour is discussed in terms of the stabilization of nickel subcarbonyl species by alkali promoters. Deactivation of promoted catalysts parallels the deposition of large amounts of carbon atoms chemically interacting with the nickel phase. In reaction conditions, alkali addition gives rise to a deep carbidization of the nickel phase (up to 85%) which probably accounts for the observed change of selectivity. Two types of carbon chemically interacting with nickel are detected when alkali promoters are added. Both of them can be hydrogenated at low temperature and can be related to surface and bulk carbides.

The role of catalyst activation in the enantioselective hydrogenation of methyl acetoacetate over silica-supported nickel catalysts

A range of Ni–SiO2 catalysts have been prepared by homogeneous precipitation–deposition and impregnation techniques and modified with aqueous solutions of R-(+)-tartaric acid (TA) to induce enantioselectivity in the asymmetric hydrogenation of a prochiral β-ketoester (methyl acetoacetate) to a β-hydroxyester ((R)-(−)-methyl 3-hydroxybutyrate). The effects of catalyst precursor preparation, nickel loading, and precalcination on the reduction of the precursor were examined and the nature of the resultant nickel particle size distribution is described. The influence of metal–support interactions on the uptake of TA was investigated and data on the corrosive metal leaching and the consequent changes in metal dispersion are presented. Modification of the catalysts with TA not only induced enantioselectivity, but also increased the turnover frequency by up to 15 times that observed for the unmodified surface. The amount of TA adsorbed and the fractional surface coverage by TA is related to the degree of enantiodifferentiation. The effects of variations in supported nickel metal particle sizes (in the overall range of 1.4–13.6 nm) on the reaction rate and enantioselectivity were examined; while the selectivity was independent of metal particle size, the rate of hydrogenation was found to be structure sensitive and a correlation between reaction rate sensitivity and particle size is presented.

Metal—support interaction effects in the liquid-phase selective hydrogenation of 1,4-butynediol with nickel catalysts supported on AlPO 4 and on other conventional non-reducible compounds

The liquid-phase selective hydrogenation of 1,4-butynediol has been carried out on nickel catalysts at 20 wt.-% supported on different AlPO 4, AlPO 4Al 2O 3 and AlPO 4-SiO 2 systems. Furthermore, some conventional non-reducible compounds, such as SiO 2, Al 2O 3, active carbon or a natural sepiolite were also used as supports. In addition, the effects of nickel loading and NiCu alloying were studied using carbon as the metal support. The relative adsorption constants, K T,D, were also obtained from the relative reactivities, R T,D, obtained in the consecutive hydrogenation process and the corresponding individual hydrogenation rates of 1,4-butynediol and 1,4-butenediol. These values were used to follow the changes in the electronic structure of supported nickel crystallites. Besides, structural defects of supported-nickel crystallites such as ‘twin faults’, ‘stacking faults’ and ‘microstrains’, were determined by X-ray diffraction line broadening analysis. These parameters were used for measuring the participation of geometric support effects in the catalytic activity of supported nickel catalysts. TEM measurements coupled with digital image processing were also used to measure nickel particle sizes. Results obtained in the correlation between kinetic data and structural defects in supported-nickel crystallites as well as with textural and acid—basic properties of the supports indicated that electronic and geometric effects were simultaneously developed throughout metal—support interactions.

Mathematical simulation and structural macrokinetics of the high-temperature synthesis of intermetallic compounds

In extending the results of [1], a theoretical study is made of the ignition temperature of a nickel-aluminum powder mixture as a function of the power of the external heating source, the dispersity of the refractory component, and the porosity of the powder mixture in the case of a volume reaction. The initial mixture is modeled by a set of spherical elementary cells whose dimension is determined by the range of nickel particle sizes, the mixture composition, and the porosity.

Relationship between atmospheric and urinary nickel in workers manufacturing electrical resistances using nickel oxide: role of the bioavailability of nickel.

The daily concentrations of nickel in total (ie inhalable) and respirable airborne dust (personal sampling) and in post-shift and pre-shift urine samples were monitored during five consecutive work days in 20 workers exposed to NiO in a workshop manufacturing electrical resistances. The individual daily atmospheric nickel concentrations ranged from 0.5 to 9586 micrograms Ni/m3 (geometric mean 22.9) for total dust and from 0.2 to 332 micrograms Ni/m3 (geometric mean 3.5) for respirable dust. The results of the urinary excretion of nickel suggested that the occupationally-related systemic absorption of nickel strongly differed in one subject (worker E) compared to the other 19 workers. In the latter group the nickel concentration in urine never exceeded 5 micrograms Ni/g creatinine, it did not differ between post-shift and pre-shift samples (geometric means: 1.1 versus 1.2 micrograms Ni/g creatinine), and it was only slightly higher than that measured in a group of 17 non-exposed subjects (mean 0.5 micrograms Ni/g creatinine; range 0.1-1.7); furthermore their nickel elimination in urine did not change during the days off or after two weeks of holiday. In worker E, the nickel concentration ranged from 21 to 101 micrograms Ni/g creatinine in post-shift urine, the next morning (after 16 h) it had dropped on average by 50 per cent, it decreased further during the days off, and amounted still to 4.4 micrograms Ni/g creatinine after two weeks of holiday. These divergent patterns of elimination of nickel in urine are most likely related to differences in the nature of exposure to airborne nickel involving both particle size and bioavailability of nickel. Worker E was exposed to NiO powder of 1-8 microns particle size resulting in nickel levels of the respirable fraction on average about 50 times that measured for the 19 other workers (3 micrograms Ni/m3). Transformation of the initial NiO powder into particles of 150 to 600 microns size and associated changes in physicochemical properties of NiO in the particles of the respirable fraction may explain why the urinary excretion of nickel in the 19 workers is hardly influenced by their occupational exposure to this metal. The pattern of urinary nickel elimination in worker E, however, most likely reflects very recent exposure to NiO, suggesting that the degree of bioavailability of nickel from this particular physicochemical form of NiO powder is much higher than that usually accepted for poorly soluble nickel compounds.

Structural and catalytic properties of several potassium-doped nickel/α-alumina solids

Studies of the chemical preparation, activation energies of reduction, temperature-programmed reduction (TPR), X-ray diffraction (XRD), X-ray photoelectron (XP) spectra and catalytic activities of several potassium-doped nickel/α-alumina catalysts have been carried out for the catalytic hydrogenation of hexanedinitrile, in a continuous process at 1 atm pressure, 443 K, and in the absence of ammonia. Activation energies of reduction for the potassium-doped non-stoichiometric NiO/α-alumina precursors were higher than those obtained for the potassium-free NiO/α-alumina and for the unsupported non-stoichiometric NiO. TPR results show that there is an inhibiting effect of α-alumina and potassium on NiO reduction. XRD measurements reveal the presence of α-alumina, NiO and Ni phases, the latter being present in larger quantities at higher reduction temperatures and lower potassium contents. XRD spectra also show fairly constant nickel particle sizes for all catalysts tested, XPS results confirm the inhibiting effect of potassium upon the reducibility of NiO. The XPS intensity ratios of surface Ni : NiO match the BET area ratios Ni : NiO of the samples. Conversions of the catalysed reactions decrease with increasing potassium content of the catalysts, probably because of poisoning of the latter with a Thorpe cycling formation of an enamine at high basic K2O contents. Selectivities towards 6-aminohexanenitrile (which may reach 100% at 85% conversion, thereby of potential interest for the manufacture of nylon-6,6) increase with potassium content of the catalysts. The less selective catalysts also yield 1,6-diaminohexane and azacycloheptane. A lack of oligomerization reactions is observed, and a mechanism is proposed.

カルシウムによる混合希土酸化物の還元拡散反応 還元拡散法によるニッケル‐ミッシュメタル系化合物の直接製造に関する研究 第１報

Aiming at the development of a less energy-consumptive process for producing hydrogen storag materials, an experimental study was carried out on the direct production of Ni-Rm (Rm: mixture of La, Ce, Pr and Nd) compounds from the mischmetal oxides by the reduction-diffusion (R-D) method. In the R-D process, a mischmetal oxide was reduced by metallic calcium at temperatures of 950, 1, 000 and 1, 050°C in the presence of nickel particles (size:-100, +200 mesh) and the reduced mischmetal elements diffused into the particles to directly form a Ni5Rm compound.An optical microscopic observation and an EPMA analysis of the reacted nickel particles revealed that three layers of Ni4Rm, Ni5Rm and Ni5Ca were formed topochemically from outside to inside the particle, and that the Ni5Ca layer was always sandwiched between the Ni5Rm layer and the unreacted nickel core, suggesting that a reaction of the diffused mischmetal elements with Ni5Ca to form Ni5Rm, Ni5Ca+ Rm=Ni5Rm+ Ca, occurred. Volume fractions of the Ni4Rm and Ni5Rm layers, which were determined for the particles with an average diameter of 200 pm, using an image analyzer, were found to rapidly increase with the reaction time, and the unreacted nickel disappeared in 135, 55 and 45 minutes at 950, 1, 000 and 1, 050°C, respectively.The experimental results were discussed based on the unreacted core model and a reactive diffusion mechanism, focusing on the role of Ni5Ca which enhanced the rate of formation of Ni5Rm.