Adsorption of carbon monoxide on polycrystalline nickel films

The adsorption states of carbon monoxide on polycrystalline nickel films have been investigated by measuring the thermal desorption, the heat of adsorption, the change in resistivity, and the change in work function in dependence on coverage and temperature. It can be shown that there are two chemisorbed (β₂, β₂) and one weakly bound (γ) species. Desorption peaks appear at 170K, 310–360 K, and 460–490 K. The differential heat of adsorption is $\text{30}\text{kcal}\text{mole}$ at low coverages and approximately $\text{25}\text{kcal}\text{mole}$ between 0.3 and 0.6 monolayers. The resistivity of the nickel film is characteristically changed with increasing coverage, and there is a maximum of resistivity at half a monolayer. At low coverages the increase in the work function is proportional to the amount adsorbed; at a monolayer the total increase is 1.26 eV at 77 K and 1.46 eV at 273 K. The two chemisorbed species differ only in the structures they form in the adsorption phase, β₂ being the species that is stable at low coverages, β₁ being the species that is stable at high coverages. These results are in good agreement with those recently found for CO adsorption on single crystal surfaces.     

Formation of thin graphite films upon carbon diffusion through nickel

Experimental results on the synthesis of thin graphite films with the aid of annealing of nickel films on carbon substrate are presented. Highly oriented pyrolitic graphite is used as the substrate to provide structural quality of the deposited nickel film. It is shown that the cyclic annealing of the structure with intermediate cooling leads to crystallization of primary amorphous carbon into a film consisting of flakes of vertical graphene. The process of graphite formation is discussed.     

Decomposition of carbon monoxide on a (110) nickel surface

New investigations of the (110) nickel/carbon monoxide system have been made using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), mass spectroscopy and work function measurements. Room temperature adsorption of CO on the surface was reversible with the CO easily removable by heating in vacuum to 450°K. The CO formed a double-spaced structure on the surface which, however, was unstable at room temperature for CO pressures less than 1×10^?7 torr. Work function changes greater than + 1.3 eV accompany this reversible CO adsorption. Irreversible processes leading to the build-up of carbon, and under certain circumstances oxygen, on the surface were the primary concern of the measurements reported here. These processes could be stimulated by the electron beams used in LEED and AES, or by heating the clean surface in CO. The results of AES investigations of this carbon (and oxygen) build-up, together with CO desorption results could be explained on the basis of two surface reactions. The primary reaction was the dissociation of chemisorbed CO leaving carbon and oxygen atomically dispersed on the surface. The second reaction was the reduction of the surface oxygen by CO from the gas phase. The significance of the dissociation reaction to COdesorption studies is discussed.     

Shock compressibility of polycrystalline nickel aluminide

ABSTRACT

We present experimental data on Hugoniot and release isentropes of nickel aluminide obtained for shock pressures up to 83?GPa and make comparison with previously published calculation data. We estimate a Hugoniot elastic limit of samples, construct the EOS of nickel aluminide and investigate the Hugoniot position relative to the melting curve.     

Field emission properties and surface structure of nickel containing amorphous carbon

Nickel containing amorphous carbon (a-C:Ni) films have been deposited by filtered cathodic vacuum arc (FCVA) technique by introducing pure nickel into the graphite target. The field electron emission property of a-C:Ni was improved when compared to that of pure tetrahedral amorphous carbon (ta-C) by FCVA. The emission threshold field of a-C:Ni film is about 5 V μm⁻¹, whilst the threshold field of the ta-C film is about 13 V μm⁻¹. Raman spectroscopy suggests that the sp² clusters in the carbon film increase both in size and number when Ni is introduced. However, the emission was found to degrade to threshold fields beyond 20 V μm⁻¹ after the a-C:Ni film was left in ambient for a week. This observation is attributed to surface absorption of oxygen on the a-C:Ni film, as determined by X-ray Photoelectron Spectroscopy.     

SHG studies of magnetization effects on polycrystalline nickel surfaces

The influence of surface magnetization of polycrystalline nickel samples on Second-Harmonic Generation (SHG) in reflection was investigated using pleosecond laser pulses of 600 nm. The effect was measured for different polarization combinations with samples under normal conditions. For the azimuthal rotation of the magnetization anisotropies of (13±2)% and (19±3)% were observed for sP and pP polarization, respectively. The Kerr angle observed by. SHG was found to be an order of magnitude larger (4±1)^o than the linear Kerr angle. A hysteresis curve was recorded for pP polarization. No influence of the oxide layer was noticed.Paper presented at the 129th WE-Heraeus-Seminar on Surface studies by Nonlinear Laser Spectroscopies, Kassel Germany, May 30 to June 1, 1994     

Electrical transport properties of polycrystalline nickel vanadate

Investigations on a.c. and d.c. electrical conductivities, thermoelectric power and dielectric constant of nickel vanadate (NiV₂O₆) are reported in the temperature range from 300 to 800 K. The compound has been found to be ap-type semiconductor with an energy gap of 2.92 eV. Electrical conduction mechanism for this compound is discussed in terms of polaron theory.     

The maximal magnetic susceptibility of polycrystalline nickel

The two maxima in the susceptibility are shown to arise from irreversible displacement of the 182 ° and 90 ° boundaries; the activation energy arising from the internal stresses is found to be proportional to these.     

The reaction of carbon and water as catalyzed by a nickel surface

Many of the individual steps which make up the reaction of carbon and water to produce CO and H₂ were studied on a nickel foil surface using temperature-programmed reaction spectroscopy (TPRS), Auger electron spectroscopy (AES), and ultraviolet photoelectron spectroscopy (UPS). Surface graphite and carbide, two metastable surface carbon forms, were prepared by dehydrogeneration of C₂H₂ and served as reactant carbon. UPS of the graphite monolayer in contact with the metal yielded a valence electronic structure that could be interpreted in terms of the bulk band structure of graphite. The fully carbided Ni surface was active for H₂O dissociation with an estimated activation energy ≤ 5 kcal/mol. The reaction of graphitic carbon in contact with the nickel surface and adsorbed oxygen occurs directly without isolated prior breaking of carbon-carbon bonds. The estimated activation energy for the direct reaction was 44 kcal/mol. A different catalytic reaction cycle involving carbon-carbon bond breaking followed by oxidation of the carbide is energetically more demanding. The activation energy for direct carbon-carbon bond breaking was estimated to be between 65 and 70 kcal/mol. Following this demanding step, the reaction between carbidic carbon and oxygen proceeded with estimated activation energy of 31 kcal/mol.     

Coadsorption of carbon monoxide and hydrogen on polycrystalline rhodium

The coadsorption of carbon monoxide and hydrogen on polycrystalline rhodium filament has been studied by thermal desorption mass spectrometry. From a series of thermal desorption spectra of CO and H₂ from rhodium as a function of the exposure time to the gas mixture of CO and H₂, it is indicated that there are a single broad peak for CO and three peaks designated as β₁, β₂, β₃ for hydrogen. Thermal desorption of hydrogen is complex. CO and β₁-hydrogen coadsorb on the rhodium surface with their partial pressures in the initial stage of the exposure to the gas mixture and then the β₁ -hydrogen adsorbed on the surface is replaced by CO with the further exposure time. The kinetics for the replacement of β₁-hydrogen by CO may be discussed from the standpoint of the L-H reaction process. The β₂-and β₃-hydrogen are observed with a longer exposure time after the elimination of β₁ -hydrogen. It may be suggested that the β₃-hydrogen peak is attributed to the desorption of hydrogen absorbed in the bulk. The nature of β₂ -hydrogen is also briefly mentioned in possible implications.     

Growth of subgrains in strongly deformed polycrystalline nickel

The growth of subgrains is studied in strongly deformed 99·80% pure polycrystalline nickel. The observed zone of the specimen was heated by means of an illuminating electron-beam. It has been verified that the subgrains grow by the migration of their boundaries in steps of the order of 1 . In metals, the subgrains, the nuclei of which are already observed in the deformed material, grow first. Before a perfect subgrain is formed, its nucleus with diffusion subboundaries can always be found. The formation of subgrains by the coalescence of individual slightly misoriented zones is observed in some cases. The growth of subgrains in further stages however, does not, proceed in the above manner.The author is indebted to Dr. M. Rozsíval Dr. Sc., head of the electron microscopy laboratory of the ISSP for help in the work, to Dr. Míek C.Sc. for his interest and discussions on the problem, and to other colleagues of the department of physics of metals of the ISSP for critical comments.     

Grain-boundary diffusion of oxygen in polycrystalline ferrites

It is shown that layer-by-layer measurements of the electron-transfer activation energy in polycrystalline ferrites allow the profiles of depth distribution of oxygen introduced from the atmosphere into a ferrite at the stage of sintering or thermal treatment to be studied using a simple experimental technique. The analytical equations proposed here make it possible to determine the volume and grain-boundary diffusion coefficients of oxygen using a minimum number of adjustable parameters. Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 64–69, May, 1999.     

Domain-wall damping and diffusion in pure polycrystalline yig

The broadband frequency spectra (10 Hz < ∫ < 80 MHz) of the initial permeability allows one to observe simultaneously both the diffusants and domain-wall relaxations as function of temperature. Above room temperature no evidence of the diffusants can be seen in the spectrum, but they are still present and contribute to the damping of the domain wall.     

High-temperature creep of nickel under conditions of grain-boundary diffusion of impurities from the surface

Comparative investigations of the effect of diffusion streams of copper atoms (a weakly segregating impurity) and silver atoms (a strongly segregating impurity) from the surface in the high-temperature plastic deformation of nickel have been carried out. It has been established that in the high-temperature plastic deformation of nickel, when there are diffusion streams of copper and silver from the surface, there is a reduction in the creep resistance of nickel due to an increase in the contribution of grain-boundary slip to the overall deformation. Two stages, characterized by different values of the rate sensitivity factor m, are observed on the curve of the stress against the rate of deformation over a certain temperature range for each impurity. In the region of deformation rates of sec⁻¹, m≊0.2, and for sec⁻¹, m becomes less than 0.05. Institute of Physics of the Strength and Study of Materials. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 119–125, July, 1997.     

Release of a diamond-like film on a nickel surface irradiated simultaneously with carbon ions and electrons

Nickel samples at temperatures of 300–1000°C have been irradiated simultaneously with 10-to 30-keV C⁺ ions and 1-to 5-keV electrons. The release of implanted carbon atoms on the surface of a sample with the formation of a transparent carbon film with the prevailing sp ³ hybridization has been observed. The thickness of the film is several tens of nanometers. The formation of films is attributed to the acceleration of the formation of carbon structures in samples irradiated by accelerated electrons.