Study of hydrogen chemisorbed on Raney-nickel by neutron inelastic spectroscopy

The motion of chemisorbed hydrogen on the Raney-nickel surface was studied by neutron inelastic spectroscopy. The peaks found at low energy transfers (below 320 cm^?1) are nearly identical to the spectrum of lattice frequencies of pure nickel. This means that each hydrogen atom is bound to only one nickel atom. The mean square amplitude of the bound proton was found to exceed that of nickel by 0.04 ± 0.02 Ų. A broad band found at 1120 cm^?1 (800 cm^?1 in the case of deuterium) is attributed to motions of hydrogen atoms relative to the nickel surface. An interpretation of this band is given in terms of harmonic approximation. An analysis of the shape of the elastic line has shown that no broadening could be detected with our instrument. This leads to an upper limit for the diffusion constant of the protons, D<5×10^?7 cm²/s, at room temperature.     

Diffusion of nickel in single- and polycrystalline Cr2O3

Chromia protective layers are formed on many industrial alloys to prevent corrosion by oxidation. Their role is to limit the inward diffusion of oxygen and the outward diffusion of cations. A number of chromia-forming alloys contain nickel as a component, such as steels, FeNiCr and NiCr alloys. To ascertain if chromia is a barrier to outward diffusion, nickel diffusion in chromia was studied in both single crystals and polycrystals in the temperature range 900–1100°C at an oxygen pressure of 10^?4 atm (argon + 100 ppm O₂). A nickel film of ～35 nm thick was deposited on the chromia surface and, after diffusing treatment, nickel penetration profiles were established by secondary ion mass spectrometry (SIMS). Two diffusion domains appear in polycrystals, the first domain is assigned to bulk diffusion and the second is due to diffusion along grain boundaries. For the bulk diffusion domain and diffusion in single crystals, using a solution of Fick's second law for diffusion from a thick film, bulk diffusion coefficients were determined at 900 and 1000°C. At the higher temperature, a solution of Fick's second law for diffusion from a thin film could be used. For the second domain in polycrystals, Le Claire's model allowed the grain boundary diffusion parameter (αD _gb δ) to be established. Nickel bulk diffusion does not vary significantly according to the microstructure of chromia. The activation energy of grain boundary diffusion is slightly greater than the activation energy of bulk diffusion, probably on account of segregation phenomena. Nickel diffusion was compared with cationic self-diffusion and with literature data on Fe and Mn heterodiffusion in the bulk and along grain boundaries. All results were analyzed in relation to the oxidation process of stainless steel.     

Rapid nickel diffusion in cold-worked carbon steel at 320–450 °C

The diffusion coefficient of nickel in cold-worked carbon steel was determined with the diffusion couple method in the temperature range between 320 and 450 °C. Diffusion couple was prepared by electro-less nickel plating on the surface of a 20% cold-worked carbon steel. The growth in width of the interdiffusion zone was proportional to the square root of diffusion time to 12,000 h. The diffusion coefficient (D_Ni) of nickel in cold-worked carbon steel was determined by extrapolating the concentration-dependent interdiffusion coefficient to 0% of nickel. The temperature dependence of D_Ni is expressed by D_Ni = (4.5 + 5.7/?2.5) × 10^?11 exp (?146 ± 4 kJ mol^?1/RT) m²s^?1. The value of D_Ni at 320 °C is four orders of magnitude higher than the lattice diffusion coefficient of nickel in iron. The activation energy 146 kJ mol^?1 is 54% of the activation energy 270.4 kJ mol^?1 for lattice diffusion of nickel in the ferromagnetic state iron.     

Bulk-to-surface precipitation and surface diffusion of carbon on polycrystalline nickel

The time evolution of the KLL Auger spectrum of carbon as a function of temperature is used to derive the kinetics of the surface diffusion and bulk-to-surface precipitation of carbon on polycrystalline nickel. The results show that the activation energy for the surface diffusion of carbon atoms on polycrystalline nickel is 6.9 ± 0.6 $\text{kcal}\text{mole}$, and the activation energy for bulk-to-surface precipitation is 9.4 ± 0.6 $\text{kcal}\text{mole}$. The dependence on the surface diffusion coefficient D_s (cm²s^?1), on the absolute temperature T can be represented, over the experimental temperature range, 350–425° C, by: $\text{ln}\text{D}{}_{\mathrm{s}}\text{= 10.27 ?}\text{3568}\text{T}$.     

Electron beam effects in auger analysis of physisorbed xenon

Adsorbed xenon on evaporated films of nickel and platinum has been analysed by Auger electron spectroscopy. The primary electron beam is shown to cause some surface heating resulting in a displacement of the isotherms. This temperature effect is the same for both metals and is due to the limiting thermal conductivity of the glass substrate. A further effect, the electron induced desorption (EID) of xenon is evidenced by a distortion of the isotherms at low equilibrium gas pressures. This effect is more clearly observed on platinum because the coverage is higher than on nickel. The EID cross-section for xenon on both metals is found to be 1 × 10^?17cm². The attenuation of the metal Auger peaks by the xenon overlayer is found to be less for platinum than for nickel. This difference is attributed to a lower packing density of xenon on the platinum surface.     

Anomalous mass transport in the vicinity of the interface of two metallic media

Diffusion of nickel in copper was studied experimentally in a temperature range of 250–375°C in the “coating (nickel)-metal (copper)” system using a radioactive isotope ⁶³Ni. Anomalously high values of the diffusion coefficients and an anomalously low value of the activation energy were found. To explain the effect, a new way of describing the diffusion phenomena in the vicinity of the interface of two metallic media is suggested, which takes into account the presence of high gradients of chemical potentials near the boundary. Based on the principles of nonlinear thermodynamics of irreversible processes, a system of differential equations of diffusion in the vicinity of the interface was obtained. Analysis of the kinetics of the diffusion zone formation revealed that chemical-potential gradients significantly accelerate the diffusion process in the vicinity of the interface. A comparison of the calculated kinetics of the formation of a diffusion zone with that obtained upon the experimental investigation of diffusion shows their qualitative agreement.     

A 1H NMR study of the effects of Ni additions on the behavior of hydrogen in β-vanadium hydride

The behavior of hydrogen in the hydrides of vanadium metal containing small amounts of nickel was studied by ¹H NMR spectroscopy. FT spectra and spin-spin and spin-lattice relaxation times for VH_0.77, V_0.95Ni_0.05H_0.73, and V_0.90Ni_0.10H_0.65 were measured at temperatures between 77 and 400 K. On the addition of nickel the number of hydrogen atoms on O₂₂ sites decreases and the superstructure of hydrogen is altered. Different effects of nickel on hydrogen diffusion are observed above and below about 200 K, and, therefore, the mechanism of hydrogen diffusion is assumed to change at this temperature.     

Carbon Sphere@Nickel sulfide core-shell nanocomposite for high performance supercapacitor application

The Carbon sphere@Nickel sulfide core-shell nanocomposites for different mole ratios of Carbon sphere (0:1; 0.5:1 and 1:1) have been synthesized by a facile low temperature water-bath method without any further calcination. XRD studies on the core-shell nanocomposites show that characteristic peaks associated with rhombohedral phase structure of nickel sulfide have been retained. TEM morphology presents the interlinked core-shell of Carbon sphere@Nickel sulfide composite with grass-leaf dexterity for better ionic diffusion. BET study confirms the formation of mesoporous structure with high surface area. The existence of elements and its electronic configuration is noted through XPS. The electrochemical studies on pristine nickel sulfide and its Carbon sphere@Nickel sulfide core-shell composites reveal that Carbon sphere@Nickel sulfide (0.5:1) exhibits high specific capacitance of 1022?F?g^?1 at a current density of 1?A?g^?1. It shows good cyclic performance even beyond 4000 consecutive charge/discharge cycles at a relatively high current density of 20?A?g^?1 with the ～83% of retention.     

Superficial migration of tritium physisorbed on monocrystalline nickel (111)

The aim of this study is the measurement of superficial migration coefficient of tritium physisorbed on monocrystalline nickel without chemisorbed sublayer. The chosen crystalline orientation was (111) because it offers the greatest concentration of adsorption sites per square centimeter. A clean surface sample is obtained by mechanical polishing, chemical etching and finally ionic bombardment by high purity argon gas. The pressure in the experimental vessel is maintained below 10^?9 torr, by liquid helium cryopumping after zeolite sorption pumping.A little spot of adsorbed tritium is produced by introduction of a finite amount of tritium gas on the clean surface of the nickel sample through a stainless steel tube. Temperatures of nickel and of the gas introduction tube are respectively regulated at 5 K and 35 K. Tritium is used as a radioactive marker and its 10 keV β-radiation is measured by a channeltron type detector which permits the localization of the deposit without acting on the surface. We observed that tritium sorbed at 5 K is quite immobile (at the time scale of our experiment). After heating up to a fixed temperature T chosen between 10 K and 20 K, the deposite profile variation in function of time is observed to determine the superficial diffusion coefficient D. For the values of T from 13 K to 20 K, D varies from 10×10^?6 to 150×10^?6 cm² sec^?1. A diffusion activation energy of 200 cal mole^?1 is deduced from the exponential increase of the curve. A vibrational frequency can be evaluated to 3×10¹² sec^?1. The rate of desorption permits the evaluation of sorption energy at about 1800 cal mole^?1 in good agreement with usual results concerning physorption of H₂ on metals.     

Surface diffusion of hydrogen on Ni(100) studied using laser-induced thermal desorption

The surface diffusion coefficient for hydrogen on Ni(100) at low coverage has been measured between 223 and 283 K. A pulsed laser is used to desorb hydrogen from a small, well defined, region on the surface without perturbing the ambient surface temperature. Hydrogen from nearby regions on the surface migrates into the vacant area and the time required to refill that area is determined by subsequent laser-induced-desorption measurements. The diffusion coefficient is obtained using an equation derived from Fick's Second law with non-stationary boundary conditions. The temperature dependence of the diffusion coefficients yields a value of 4.0 ± 0.5 kcal/mol for the energy barrier to diffusion. A value of roughly 3 × 10¹³ s^?1 for the site-to-site hopping frequency is derived when the pre-exponential for diffusion is fit to a random-walk mechanism.     

Mechanism of the transport of nickel along a Si(111) surface in the presence of adsorbed cobalt atoms

The mechanism of the transport of nickel along a Si(111) surface in the presence of adsorbed cobalt atoms is established by LEED and Auger electronic spectroscopy. The mechanism consists of diffusion of nickel atoms through the bulk and segregation of the atoms on the surface during annealing. This mechanism of nickel transport plays the governing role at temperatures below 700°C, where nickel transport along clean silicon surfaces is not observed. It is found that the nickel segregation factor is what determines the lowest temperature at which nickel transport is observed on clean silicon surfaces. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 6, 423–425 (25 March 1999)     

Diffusion of sulfur in nickel oxide single crystals

The diffusion of sulfur in nickel oxide single crystals has been investigated over the temperature range from 1000 to 1250°C. The measured data were found to deviate markedly from the error function complement dependence for diffusion from a constant source. The deviation is attributed to the migration of sulfur by the “double mode simultaneous diffusion mechanism.” The faster mode diffusion is suggested to be via nickel vacancies, and the slower mode diffusion is suggested to be via oxygen vacancies. The diffusivities for faster mode are given by D_f = 2.94 exp[? 86.6 kcal/RT] cm² sec^?1 and, the slower mode, D_s = 1.08 × 10^?9 exp [?32.8 kcal/RT]cm²sec^?1.     

Grain boundary diffusion of Cu in TiN film by X-ray photoelectron spectroscopy

In this study, the grain boundary diffusion of Cu through a TiN layer with columnar structure was investigated by X-ray photoelectron spectroscopy (XPS). It was observed that Cu atoms diffuse from the Cu layer to the surface along the grain boundaries in the TiN layer at elevated temperature. In order to estimate the grain boundary diffusion constants, we used the surface accumulation method. The diffusivity of Cu through TiN layer with columnar structure from 400 °C to 650 °C is D_b≈6×10⁻¹¹exp(−0.29/(k_BT )) cm²/s. Received: 18 May 1999 / Accepted: 8 September 1999 / Published online: 23 February 2000     

Oxidation of nickel and transport properties of nickel oxide

Oxidation kinetics of high purity nickel, as well as the nonstoichiometry and chemical diffusion in nickel oxide have been studied as a function of temperature (1373-1673 K) and oxygen pressure (10-10⁵ Pa) using microthermogravimetric techniques. In order to eliminate the possible participation of grain boundary diffusion in scale growth at lower temperatures, the oxidation rate measurements have always been started at the highest temperature (1673 K), when coarse-grained scale was formed, and the temperature and pressure dependence of the oxidation rate was determined by step-wise lowering the temperature of such pre-oxidized sample. Nonstoichiometry and the chemical diffusion coefficient in Ni_1−yO have also been determined on such coarse-grained oxide samples, obtained by complete oxidation of nickel at highest temperature (1673 K). It has been found, that under such conditions oxidation of nickel follows strictly the parabolic rate law, and the parabolic rate constant of this reaction is the following function of temperature and oxygen pressure: The results of nonstoichiometry measurements, in turn, may be described by the following relationship Finally, chemical diffusion coefficient in Ni_1−yO has been found to be independent on oxygen activity, indicating that the mobility of point defects in this oxide does not depend on their concentration, being the following function of temperature: It has been shown, that the parabolic rate constants of nickel oxidation, calculated from nonstoichiometry and chemical diffusion data are in excellent agreement with experimentally determined k_p values. All these results clearly indicate that the predominant defects in nonstoichiometric nickel oxide (Ni_1−yO) are double ionized cation vacancies and electron holes and the oxide scale on nickel growths by the outward volume diffusion of cations.     

Temperature effect in a montmorillonite clay at low hydration—microscopic simulation

The effect of temperature in the range 0–150°C was studied for homo-ionic montmorillonite clays with Na⁺ and Cs⁺ compensating ions in low hydration states. Monte Carlo and molecular dynamics simulations were employed to provide both static and dynamic information concerning the interlayer ions and water molecules, and emphasis was laid on the temperature activation of the diffusion coefficients. Principal structural changes were limited to the interlayer water phase. In the monohydrated systems, neither of the cations was seen to enter into the hexagonal cavities of the clay. Cs⁺ exhibited clear site-to-site diffusion between sites allowing coordination to six oxygen atoms of the clay sheets, this behaviour persisting to high temperatures. Preferential sites for the Na⁺ counterion were much less well-defined, even at low temperatures. The behaviour of the water phase in the monohydrated states was similar for the two ions. A rapid approach to bulk dynamics was seen in the transition from monohydrated to bihydrated Na-montmorillonite. A detailed quantitative comparison of the temperature activation of diffusion for a two-dimensional water phase and three-dimensional bulk water is presented for the first time.     

Evolution of the optical properties of a silver-doped phosphate glass during thermal treatment

The optical properties of a silver-doped phosphate glass have been monitored during thermal processing at several fixed temperatures by in situ optical microspectroscopy. Silver nanoparticle (NP) formation and growth processes were assessed by analysis of surface plasmon resonance spectral features. Nucleation and growth processes were distinguished, which appeared temperature and time dependent. While nucleation was favored at low temperatures, relatively high temperatures promoted NP growth by silver diffusion. Photoluminescence spectra acquired along with optical absorption data indicated a continuous reduction of Ag⁺-Ag⁰ pairs concomitant with NP precipitation, suggesting their role as nucleation centers. The work of Ag NP formation and the activation energy for silver diffusion were estimated.     

Surface oxidation of NiCo alloy: A comparative X-ray photoelectron spectroscopy study in a wide pressure range

Oxidation of NiCo alloy has been studied under two pressure regimes, 5 × 10⁻¹⁰ and 5 × 10⁻¹ bar, by X-ray photoelectron spectroscopy (XPS). The aim of this work is to investigate the synergetic effect between the two alloy components during the initial stages of oxidation. The results showed that at low oxygen pressure, segregation and preferential oxidation of cobalt takes place, while oxidation of nickel is largely suppressed. The species dominating the surface is CoO but small amount of metallic cobalt still remains even after prolonged oxidation at 670 K. At 0.5 bar O₂ pressure, alloy oxidation was found to be temperature depended. From 420 K to 520 K, cobalt is completely transformed to CoO and the Ni:Co atomic ratio at the surface approaches a minimum, similar to the observations at low pressure regime. However, at higher temperatures (from 520 K to 720 K), nickel is re-segregated on the surface, in the expense of cobalt, while CoO is further oxidized to Co₃O₄. At this temperature range formation of mixed Ni-Co-O spinel-like oxides is probable as supported by the characteristic modifications of the Ni 2p_3/2 photoelectron peak and the increase of the Ni:Co atomic ratio.     

Thermal decomposition of nickel carbide thin films

Thin films of nickel carbide are produced by evaporating fourteen atomic layers (1.8 × 10¹⁶ atoms cm^?2) of nickel onto the (0001) surface of graphite at room temperature. The presence of nickel carbide is indicated by the characteristic carbide Auger electron signal. LEED shows that no ordered structural change takes place on the graphite (0001) surface when nickel carbide is produced in this manner. Isothermal heating of the sample leads to an irreversible change of the carbon Auger signal. The times required for this change range from 150 min at 150°C to 45 min at 185°C. The times required for decomposition yield an activation energy for decomposition equal to 50 kJ mole^?1. The decomposition of nickel carbide thin films obeys zero order kinetics. Depth profiling of the film after decomposition reveals that the observed decomposition is limited only to the top two or three atomic layers.     

Une détermination du coefficient d'autodiffusion de surface en présence d'une couche d'adsorption à l'aide de pointes àémission de champ (nickel sur tungsténe)

The tip blunting technique to measure the surface self-diffusion of clean metals (A. Piquet, Vu Thien Binh, H. Roux, R. Uzan and M. Drechsler) is extended to study the influence of an adsorption layer on diffusion. The system studied is nickel on tungsten. The increase of the apex radius is measured by means of FEM characteristics. In the temperature range used (1200–1500 K), the nickel monolayer (1.16 × 10¹⁵ atoms/cm²) is maintained by compensation of desorbed Ni atoms with a continual flux from an evaporation source. The adsorption life time between 1350 and 1500 K decreases from 850 to 16 s. The conservation of the degree of coverage leads to a method to determine the desorption activation energy of nickel (E_d = 4.56 eV/atom). The surface self-diffusion data of tungsten with a nickel monolayer are found to be D₀ = 3 × 10^?3cm/²s and Q_s = 1.9 eV/atom, compared to the clean tungsten data D₀ = 1 cm²/s and Q_s = 3.1 eV/atom. The Ni monolayer increases the surface self-diffusion coefficient by a factor 160 at 1200 K and 20 at 1500 K. The results are discussed with respect to nickel activated sintering of tungsten powders.     

A Diffusion Model for Spin-Spin Relaxation of Compartmentalized Water in Wood

A diffusion model for spin-spin relaxation of compartmentalized water with a surface relaxation was verified for lumen water in wood. Spin-spin relaxation measurements were carried out on water in redwood sapwood, spruce sapwood, and spruce compression-wood samples, which possessed different cell-lumen radius distributions as measured by scanning electron microscopy. For the redwood sample, NMR measurements were made for seven temperatures between 4 and 55°C over which the average lumen-water T₂ decreased from 177 to 103 ms. The lumen-water theory and experiment were in agreement, and evidence of higher-order relaxation modes, theoretically predicted for low temperatures, was found. This model was extended to two water regions to characterize the surface relaxation in terms of the spin-spin relaxation and diffusion coefficient of the cell-wall water and the partition coefficient. Using the extension and measurements of the spin-spin relaxation times and relative populations of lumen and cell-wall-water, estimates for cell-wall-water diffusion in a maximally hydrated redwood varied from 0.92 × 10⁻⁶ cm²/s at 4°C to 5.89 × 10⁻⁶ cm²/s at 55°C. The activation energy for cell-wall-water diffusion in redwood sap-wood in this temperature range was 6700 cal/mol, about 40% higher than the free-water value of 4767 cal/mol.