Theoretical investigation of the adsorption and diffusion of hydrogen on the surface and in the bulk of the intermetallic compound Mg2Ni

The intermetallic compound Mg₂Ni as a potential material for hydrogen storage has been investigated theoretically. The sorption and diffusion of a hydrogen atom in the bulk and on the surface of this material, as well as the step-by-step process of dissociative chemisorption of a H₂ molecule on the surface, have been considered. The dependence of the sorption energy of atomic hydrogen on the structural characteristics of the intermetallic compound Mg₂Ni has been analyzed.     

Plasma-electrolytic formation, composition and catalytic activity of manganese oxide containing structures on titanium

In the present work, we report the data about formation of TiO₂-rutile or TiO₂ and Mn₂O₃, Mn₃O₄ containing oxide structures on titanium in aqueous electrolytes by means of plasma-electrolytic deposition. The layers formed are characterized by X-ray diffraction, electron probe microanalysis and scanning electron microscopy methods. The PEO coatings on titanium formed in sodium tetraborate solution contain the TiO₂ stabile rutile modification that is important when utilizing such a structure as a catalyst carrier. Manganese acetate adding into the electrolyte leads to formation of layers that contain Mn₂O₃, Mn₃O₄ and TiO₂-rutile in outer region. The manganese content in the surface layer depends on the formation conditions as well as on manganese acetate concentration in the electrolyte. Catalytic activity of the layers in CO → CO₂ reaction is studied in the static and flow conditions. The manganese-containing layers obtained possess the catalytic activity in CO → CO₂ oxidation reaction at the temperature range of 250-350 °C. The catalytic activity depends on the concentration and surface distribution of manganese as well as on the layers morphology.     

Absence of superconductivity down to 80 mK in graphite intercalated BaC6

Following recent theoretical and experimental investigations of superconductivity in the graphite intercalated compounds CaC₆ and SrC₆, we report on a very low temperature magnetisation study on BaC₆ down to 80 mK. The data show no trace of superconductivity even at fields as low as 0.7 Oe. Using a McMillan parametrisation of the BCS parameters, we conclude that the Coulomb pseudopotential is expected to be as large as 0.19 in both BaC₆ and SrC₆, i.e. 40% larger than in CaC₆. As an alternative scenario, we argue that extrinsic effects such as intercalant disorder may depress superconductivity in both BaC₆ and SrC₆, as in the case of CaC₆.     

Enhancement of in-plane magnetic anisotropy in (111)-oriented Co0.8Fe2.2O4 thin film by deposition of PZT top layer

The CoFe₂O₄ and Co_0.8Fe_2.2O₄ single layer (CFO) as well as PZT/CoFe₂O₄ and PZT/Co_0.8Fe_2.2O₄ bilayer thin films were grown using the pulsed laser deposition technique on Pt(111)/Si substrates at 600 °C. All films had a perfect (111)-orientation and the degree of orientation of CFO films was improved by the deposition of a PZT top layer. Precision X-ray diffraction analysis (avoiding the shift of peaks due to sample misalignment) revealed that the CFO films on Pt(111)/Si substrate were under an out-of-plane contraction and the deposition of a PZT top layer led to the increase in the out-of-plane contraction. The (111)-oriented CFO single layer films had a strong in-plane magnetic anisotropy as a result of orientation as well as the stress-induced magnetic anisotropy. The magnetic properties of CFO film were altered by the deposition of a PZT top layer leading to the enhancement of in-plane magnetic anisotropy. The enhanced in-plane magnetic anisotropy was more detectable in PZT/Co_0.8Fe_2.2O₄ rather than PZT/CoFe₂O₄ bilayer film, which could be expected from its higher magnetocrystalline as well as magnetostriction constants.     

Formation of nitric oxide dimers on MgO-supported gold particles

We present density functional theory (DFT) calculations on the formation of nitric oxide dimers (N₂O₂) on Au atoms, dimers and trimers adsorbed on regular O^2 ? sites and neutral oxygen vacancies (F_s sites) of the MgO(100) surface. The study of the N₂O₂ species is of great interest since it has been detected in the NO reduction reaction as an intermediate towards the formation of N₂O. We found that the coupling of a NO molecule with a previously adsorbed one on Au/MgO is energetically favorable on Au₁ and Au₃, but unfavorable on Au₂. The stability of N₂O₂ is in direct relation with the amount of charge taken from the support. Furthermore, one of the N―O bonds can be activated as a result of the attraction between the negatively charged NO dimer and the ionic oxide surface. In fact, for Au₁ anchored on the F_s site a barrierless reaction occurs between N₂O₂ and a third NO molecule, forming adsorbed N₂O and NO₂.     

Radiolysis of Methyliodide-Methylene Chloride Liquid Solutions

The production of iodine as a result of gamma radiolysis of methyliodie-methylene chloride liquid solutions was studied as a function of dose and solvent composition. The effect of varying the composition of the CH₃I–CH₂Cl₂ mixtures and the amount of energy absorbed by the mixture on initial G(I₂) were investigated in aerated solutions at ambient temperature. The results have been discussed in view of mechanism based on ion-molecule reactions.     

Electrical and structural modifications of TZP

The stability range of the metastable tetragonal phase in the ZrO₂-Y₂O₃-TiO₂ and ZrO₂-Y₂O₃-Fe₂O₃ systems was investigated by XRD at room temperature. The solid solubility limit of TiO₂ was found to be as high as 20 mol%, while that of Fe₂O₃ does not exceed 0.8 mol%. Impedance measurements show a decrease of the total conductivity, bulk conductivity and grain boundary conductivity as a result of the TiO₂ and Fe₂O₃ addition. Dc-polarization measurements using the Hebb-Wagner technique were applied to determine the partial hole and electron conductivities of TiO₂ and Fe₂O₃ co-doped samples. These show a slightly higher hole conductivity as compared to pure TZP and a remarkably higher electron conductivity as compared to TiO₂ or Fe₂O₃ doped samples. The Hebb-Wagner curves are interpreted according to a model which considers the addition of mixed valence ions. The influence of the minority charge carriers on the charge-transfer resistance is investigated. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 23–19, 1997.     

Importance of pre-ionisation for the non-chain discharge-pumped HF laser

The importance of pre-ionisation for the non-chain discharge-pumped HF laser is studied through experiments on an X-ray photo-triggered laser using mixtures of Ne, SF₆, and ethane. The discharge dynamic in Ne/SF₆ mixtures or pure SF₆, as well as the stabilisation effect induced by C₂H₆ and consequences for the laser performance, are investigated for pre-ionisation electron density values, n_eo, ranging from 10⁶ cm^-3 up to 10⁹ cm^-3, as well as for the so-called discharge self-breakdown mode. Without ethane, the minimum n_eo value which is needed to complete 100% homogeneous charge deposition in the plasma is a very sharply increasing function of the SF₆ pressure. This hinders performance optimisation when the molecule used to react with F-atoms, for instance H₂, has no effect on the discharge dynamic. The minimum ethane partial pressure that is needed to stabilise the discharge depends on n_eo, the pumping pulse duration, the deposited electric charge, and the SF₆ pressure. Discharges in Ne/SF₆ can be much more efficiently stabilised by addition of a small amount of ethane than by an increase of n_eo. A pre-ionisation density as low as 10⁶ cm^-3 is sufficient to achieve the maximum laser energy value, but total suppression of the pre-ionisation has a detrimental effect on the active medium homogeneity. Received: 30 May 2000 / Revised version: 9 October 2000 / Published online: 9 February 2001     

Molecular beam epitaxy of GaN on a substrate of MoS2 layered compound

The feasibility of MoS₂ layered compound as a substrate for GaN growth was investigated. GaN films were successfully grown on MoS₂ by plasma-enhanced molecular beam epitaxy and the crystal quality of GaN on MoS₂ was compared with that on Al₂O₃. For GaN grown on MoS₂ substrate, it was found that the surface flatness observed by atomic force microscopy, stress in the film measured by Raman spectroscopy, optical properties measured by photoluminescence spectroscopy, and threading dislocation density observed by transmission electron microscopy show superior properties compared with that grown on Al₂O₃. These results suggest the layered compound such as MoS₂, which has no dangling bonds on the surface and has lattice mismatching of 0.9% to GaN, has high potential for a substrate of GaN growth. Received: 1 March 1999 / Accepted: 8 March 1999 / Published online: 26 May 1999     

Preparation and Investigation of the Properties of W/CeB<Subscript>6</Subscript>/W Heterostructure as a Sensitive Element of Single-Photon Thermoelectric Detector

The possibility of W/CeB₆/W heterostructure preparation on Al₂O₃, AlN, Si, and W substrates by electron-beam evaporation method was investigated. The conditions for preparation of W thin films on dielectric substrates and CeB₆ films, as well as of stoichiometric CeB₆ films on W films, dielectric and tungsten substrates are determined. The reflection spectra of W films, the results of X-ray diffractometry, X-ray microanalysis, and electron microscopy of W and CeB₆ films are presented. W/CeB₆/W heterostructures of various configurations and sizes are produced. It was shown by means of computer simulation that at the detection of 6–50 eV photons, a detector with W/CeB₆/W heterostructure-based sensitive element may provide microvolt level signal at terahertz count rate. The obtained results serve as the basis for creation of a prototype of a sensitive element of single-photon thermoelectric detector.     

Combustion synthesis of magnesium ferrite as liquid petroleum gas (LPG) sensor: Effect of sintering temperature

We report synthesis of Spinel type magnesium ferrite (MgFe₂O₄) material by a simple, inexpensive combustion method with glycine as a fuel and their application as a gas sensor for reducing gases (LPG, Acetone, Ethanol, Ammonia). The dependence of reducing gas sensing properties on the structural and surface morphological properties has been studied as an effect of sintering temperatures. The structural and surface morphological properties were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The MgFe₂O₄ were highly oriented along (311) with the spinel type crystal structure. The SEM observation reveals that porous morphology decreases due to the grain growth as sintering temperature increases. The mechanism of reducing gas sensing by the MgFe₂O₄ pellets is explained on the basis of adsorbed oxygen on the sensor surface. The selectivity and maximum response of 71% to 2000 ppm of LPG was observed at 698 K with the (MgFe₂O₄) material sintered at 1173 K.     

Elemental and chemical-state mapping of boron carbide fracture surfaces

Samples of commercial B₄C and of reactive hot-pressed B₉C were fractured in situ in a scanning Auger microscopy (SAM) system. The B₄C fracture surfaces showed small areas of pore structures, as well as larger relatively smooth areas. Impurities (Cu, Si, S, Ca, N, O and Cr) were found in trace amounts in the pore areas. SAM maps were made of most of these impurities. The nitrogen was found by Auger lineshape in point Auger electron spectroscopy scans to be in the compound BN. Carbon was found on the surface in both boron carbide and as graphite. SAM was used to seperately map B in B₄C and B in BN, as well as C in B₄C and C in graphite, in a pore region. In the smooth areas of B₄C fracture surfaces fewer impurities were found. The carbon occured in B₄C and in localized carbon-rich areas which had graphite surface layers. In contrast, only B and C were found on the B₉C fracture surfaces; but the distributions of these elements were not homogenous. Carbon was found both in elemental form as graphite and chemically bound to boron as a carbide. In comparison with the graphite regions on the B₄C surfaces, the graphite areas on the B₉C fracture surfaces were not localized. The total surface area was nearly balanced between graphite and carbide regions. Argon ion bombardment revealed the graphite areas to be less than 100 nm thick.     

Morphologies of C60 deposited on a substrate

The morphologies of deposited C₆₀ on a copper substrate from a C₆₀-benzene solution were evaluated by using Scanning Electron Microscopy. It was found that the forms of deposited C₆₀ were dependent on the aggregation of C₆₀ in its original solution. During natural vaporization of benzene solution on a copper substrate, independent C₆₀ molecules in the solution assembled into crystal C₆₀ on the substrate, whereas the aggregates of C₆₀ in the solution were remained as amorphous C₆₀ particles on the substrate. However, if spinning was employed during vaporizing solvent, the C₆₀ aggregates were destroyed, leading to the formation of crystal C₆₀. Furthermore, the speed of spinning a substrate can tune the size and shape of deposited C₆₀.     

On the temperature dependence of positron lifetimes in tin and the transition from theβ- to theα-phase

The N₂, O₂, H₂O, and CO₂ molecules that have condensed on the surface of a pyroelectric tourmaline crystal were degassed successively by means of electron bombardment. The temperature dependence of the electrostatic field strength on the specimen surface was observed by electron diffraction; it decreased as the degassing advanced. The tourmaline surface behaved as a gas Chromatographic adsorbent.     

Generation of a stable surface concentration of amino groups on silica coated onto titanium substrates by the plasma enhanced chemical vapour deposition method

This report describes the amino functionalisation of the surface of plasma enhanced chemically vapour deposited silica films (PECVD-SiO₂), which were coated onto titanium substrates. Amino groups were linked to PECVD-SiO₂ via 3-aminpropyl triethoxysilane (APTES). We showed that the APTES functionalised PECVD-SiO₂ surfaces contained a high packing density of amino groups (67-92 NH₂ groups per nm²), indicative of a multilayered and highly cross-linked APTES film. 65-66% of the original surface concentration of APTES was retained on the PECVD-SiO₂ surface after incubation under physiological conditions, indicating that APTES films are relatively stable on PECVD-SiO₂ in these environments. The stability of the amino groups obtained on PECVD-SiO₂ in this study is much higher compared to other hydroxyl-bearing materials, such as titanium. Therefore, PECVD-SiO₂ films may find use as functional biomaterial coatings and as intermediate adhesion layers in silanisation processes.     

Interface morphologies and magnetization characteristics of Co70Fe30 thin films deposited on conjugated polymer thin films

The surface and interface morphology and magnetization characteristics of Co₇₀Fe₃₀ thin films deposited on bare glass and p-Si/SiO₂ substrates and on conjugated polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films on such substrates have been studied by atomic force microscopy and magneto-optic Kerr effect. It was found that the average absolute magnitude of the coercive field of Co₇₀Fe₃₀ correlates with the roughness of the underlayer prior to Co₇₀Fe₃₀ deposition. P3HT deposited on p-Si/SiO₂ substrates possesses an increased surface roughness as compared to the p-Si/SiO₂ surface, but displays a decreased surface roughness as compared to the one of a bare glass substrate.     

Effect of acids on the gamma-radiolysis of frozen aqueous systems of some organic compounds

Abstract

ESR spectra of γ-irradiated frozen aqueous solutions of a number of organic compounds such as alcohols, ether, acetone and tetrahydrofurans have been examined in the presence and absence of mineral acids such as H₂SO₄. The presence of the acid is found to cause an intensification of the organic radical ESR spectra as compared with the acid free solutions. Also, the presence of the organic compounds in frozen aqueous H₂SO₄ suppresses the formation of both H-atoms and SO₄ ^? radical ions. These results have been explained on the basis of reactions of the electrons and holes, or excitons, primarily formed by the action of radiation on the substrate ice.     

The fabrication of ReS2 flowers at controlled locations by chemical vapor deposition

Here we report a metal induced nucleation to realize the growth of ReS₂ flowers at controlled locations. The ordered arrays of ReS₂ flowers have been successfully prepared on SiO₂/Si substrate using Pt metal dots as nucleation sites and S, NH₄ReO₄ powders as precursors by a chemical vapor depostion method. The NH₄ReO₄ powders are used as the rhenium sources. The ReS₂ flowers are grown above the pre-patterned Pt dots, Raman and transmission electron microscopy measurements indicated that the prepared ReS₂ flowers have excellent crystalline quality.     

SERS and XPS observation of the adsorption behavior of NO and NO2 on a silver powder surface

The influence of H₂O on the adsorption behavior of NO or NO₂ on a silver powder surface was studied by SERS and XPS at room temperature. Water vapor was found to be responsible for the adsorption of NO on the silver powder surface. When surface species such as Ag₂O are present on the surface, some of the NO₂ molecules are adsorbed on the surface species to produce NO^-₃, whereas NO molecules are adsorbed on a different site to produce NO^-₂.