Adsorption and solution of H2, D2, N2, O2 and CO by (100) Ta

Sticking coefficients, saturation densities, and solution rates of gases on (100) Ta are obtained by comparing with results on (100) W using Auger electron spectroscopy and flash desorption. Hydrogen has a lower sticking coefficient on (100) Ta than on polycrystalline Ta, but solution occurs readily even at 78°K. Differences between H₂ and D₂ are observed for both adsorption and solution. Nitrogen is confined to the surface of Ta for T < ≈500°K, and adsorbed nitrogen dissolves with an activation energy of ≈2.5 kcal mole^?1 upon heating to higher temperatures. The saturation density of O₂ at 300° K is approximately twice that on (100) W. The first monolayer dissolves at ≈500°K but the second dissolves or desorbs only at much higher temperatures. Carbon monoxide adsorbs without solution of either species at 300°K. At ≈500°K carbon dissolves completely leaving oxygen which desorbs at much higher temperature.     

On the kinetics of oxygen adsorption on a Pt(111) surface

The kinetics of O₂ adsorption on a clean Pt(111) surface were investigated in the temperature range 214–400°C. The oxygen coverage was measured by CO titration as well as Auger electron spectroscopy both of which show the same dependence on O₂ exposure. The initial sticking coefficient on clean Pt(111) is 0.08–0.10 and decreases exponentially with increasing oxygen coverage. For θ > 0.23 a (2 × 2)-O LEED pattern was observed. The highest oxygen coverage obtained was approximately 0.45. A theoretical model was proposed which correlates the coverage dependence of the sticking coefficient with adsorbate interactions in the chemisorbed state. These interactions cause a coverage dependent activation energy of adsorption assuming the existence of a precursor state. Experiments dealing with the effect of carbon contamination on the sticking coefficient showed that the initial sticking coefficient decreases with increasing carbon coverage.     

Interactions du rhénium à haute température avec N2O sous basse pression

Starting at room temperature, N20 adsorption on rhenium proceeds dissociatively. Oxygen atoms remain on the surface while nitrogen molecules are desorbed. The overall process is characterized by an initial sticking coefficient value equal to 0.3 at 298 °K. In stationary conditions, and in a higher temperature range (> 1200°K) rhenium trioxide and oxygen atoms are the reaction products, depending on oxygen coverage on the surface. When the oxygen coverage is low, atomization, characterized by a reactive sticking probability of 0.2 is the only observable process. All the results are consistent with a model, previously proposed for the system oxygen-rhenium and oxygen-transition metals. The main differences in reaction rates between rhenium and oxygen or N₂O are interpreted in terms of saturation coverages.     

The oxidation and co reduction kinetics of a platinum surface

A thermogravimetric technique was used to study the oxidation and CO reduction kinetics of Pt as a function of temperature, T. The measurements were performed on Pt powder (particle sizes 0.8–2.5 μm) at 1 atm pressure. After exposure to 1 atm of oxygen at 600 K for 1 h, the total uptake of oxygen by the powder amounted to less than one oxygen atom per Pt surface atom and followed a logarithmic growth law. For 400 < T < 600 K, the logarithmic rate constant, K₀, could be described by an Arrhenius law with an apparent activation energy of 2.6 kcal/mole. Above 600 K, K₀ slowly decreased, an effect believed to be associated with the dissociation of the oxide between 650 and 850 K. Oxidation isotherms were calculated using the low pressure oxygen sticking coefficient data of Hopster et al. The calculated and measured oxidation isotherms were found to be in remarkable agreement. The CO reduction data were more difficult to analyze but showed that the reduction rate had a stronger temperature dependence (~11 kcal/mole) than the oxidation rate. For427 < T<487 K, the general time scale of the reduction process was 10–50 min. Typical durations of the oxidation and CO reduction processes are consistent with the periods observed in studies of the oscillatory rate of CO oxidation over a Pt catalyst.     

Positron annihilation study of alpha-irradiated stainless steel SS 302 and Cobalt

Abstract

Positron lifetime and Doppler broadening measurements have been done to study the α-induced defects in stainless steel SS 302 and polycrystalline cobalt. For stainless steel the samples have been thin and the effect of the presence of helium on defect kinetics has been avoided by allowing the 30 MeV alpha particles to pass through the samples. The presence of impurity (carbon) atoms in steel is found to play an important role in the trapping and detrapping of vacancies in the temperature range 200°C to 450°C. Formation of vacancy-impurity complexes and their dissociation around 500°C have been observed. A steady decrease of the positron parameters has been seen beyond 700°C and they attain those of the reference sample around 1000°C. While annealing alpha irradiated cobalt we find migration of vacancies and rearrangement of dislocation loops below 650 K and then dissolution around 900 K. Helium-vacancy complexes form in the region 600 to 1000 K, leading to the growth of the He-bubble above 1000 K. The trapping model analysis shows strong interaction between He and vacancy clusters in the temperature region 600 to 1000 K.     

Measurements of bulk-to-surface segregation of impurities in electrodeposited NiCr alloys by auger electron spectroscopy

Auger Electron Spectroscopy (AES) measurements of the non-equilibrium segregation of K to the surface of an electrodeposited NiCr alloy of 20%Ni-80%Cr composition are reported. It is shown that a surface enrichment of K takes place on the surface to a maximum of 8% coverage in the temperature range 400–550°C. The process is initially diffusion-connrolled, with an activation energy of 17 ± 3 kJ/mol. An electrodeposited alloy of approximately 80%Ni-20%Cr composition showed a considerable C-enrichment at the surface in the temperature range 300 to 600°C. A cast alloy of the same composition showed a similar C-enrichment in the temperature range 300 to 455°C. The electrodeposited alloy of 20%Ni-80%Cr showed no such C-enrichment, due to the segregation of both K and Cr to the surface in the range 300 to 600°C and the formation of chromium carbide precipitates.     

Specific features of electrical properties of porous biocarbons prepared from beech wood and wood artificial fiberboards

This paper reports on comparative investigations of the structural and electrical properties of biomorphic carbons prepared from natural beech wood, as well as medium-density and high-density fiberboards, by means of carbonization at different temperatures T _carb in the range 650–1000°C. It has been demonstrated using X-ray diffraction analysis that biocarbons prepared from medium-density and high-density fiberboards at all temperatures T _carb contain a nanocrystalline graphite component, namely, three-dimensional crystallites 11–14 Å in size. An increase in the carbonization temperature T _carb to 1000°C leads to the appearance of a noticeable fraction of two-dimensional graphene particles with the same sizes. The temperature dependences of the electrical resistivity ρ of the biomorphic carbons have been measured and analyzed in the temperature range 1.8–300 K. For all types of carbons under investigation, an increase in the carbonization temperature T _carb from 600 to 900°C leads to a change in the electrical resistivity at T = 300 K by five or six orders of magnitude. The dependences ρ(T) for these materials are adequately described by the Mott law for the variable-range hopping conduction. It has been revealed that the temperature dependence of the electrical resistivity exhibits a hysteresis, which has been attributed to thermomechanical stresses in an inhomogeneous structure of the biocarbon prepared at a low carbonization temperature T _carb. The crossover to the conductivity characteristic of disordered metal systems is observed at T _carb ? 1000°C.     

Messung der Verweilzeit von He-Atomen an Glasoberflächen

Mean sticking times of helium on a glass surface are determined at very low pressures from nonstationary molecular flow through glass capillaries. The temperature range covered is 13.8 °K to 20.4 °K. Resulting sticking times are of the order of 10^?7 to 10^?5 sec. They show a characteristic dependence on temperature and pressure. These measurements can be interpreted by means of a simple model: He-atoms mostly are bound to the surface with an adsorption energyE of 229 cal/mol?0.01 eV (±20%). However with a probability of 10^?4 the energy is 530 cal/mol?0.023 eV (±6%). In both cases sticking times τ follow the equation τ=τ₀exp(E/RT) where τ₀ is about 10^?9 to 10^?10 sec.     

Nature of room-temperature ferromagnetism from undoped ZnO nanoparticles

ZnO nanoparticles were synthesized by a sol–gel calcination process following being annealed in air at 400, 600, 800 and 1000°C. X-ray diffraction pattern and X-ray photoelectron spectroscopy show that all the samples present a typical wurtzite structure and no other impurity phases are observed. Room-temperature ferromagnetism from all the samples is confirmed by the vibrating sample magnetometer measurements, which shows that the RTFM decreases with the annealing temperature increasing from 400 to 800°C, and then became larger for the sample annealed at 1000°C. According to the Raman scattering spectra and electron paramagnetic resonance, the RTFM of samples annealed at 600, 800 and 1000°C might be related to the oxygen vacancy related defects. However, the RTFM from the sample annealed at 400°C, presenting nearly the same value as that of the sample annealed at 1000°C, could originate with the interstitial zinc defects associated with XPS and photoluminescence analyses.     

Effect of radiation-induced defects on the dielectric response of lead scandoniobate ferroelectric ceramics

The effect of ionizing gamma irradiation on the frequency and temperature dependences of the permittivity ε’(T) for lead scandoniobate relaxor (disordered) ferroelectric ceramics before and after thermal annealing was investigated by low-and infralow-frequency dielectric spectroscopy. It was demonstrated that the irradiation leads to a shift in the temperatures of the maxima in the dependence ε’(T) and the temperatures of the maxima in the dependence of the dispersion depth Δε’(T) toward the high-temperature range, as well as to the suppression of high-temperature (～ 300–400°C) polarization relaxation. After annealing at a temperature T ～ 600°C, the properties of the material were partially recovered.     

Interaction of H2O with a clean and oxygen precovered Ni(110) surface studied by XPS

The adsorption and reaction of H₂O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature (T<150 K), a multilayer adsorption of H₂O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H₂O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T=150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O_ads… HOH species on the surface. For T>350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H₂O into O_ads and H₂. The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state.     

Displacive surface phases formed by hydrogen chemisorption on W {001}

A detailed LEED study is reported of the surface phases stabilised by hydrogen chemisorption on W {001}, over the temperature range 170 to 400 K, correlated with absolute determinations of surface coverages and sticking probabilities. The saturation coverage at 300 K is 19(± 3) × 10¹⁴ atoms cm^?2, corresponding to a surface stoichiometry of WH₂, and the initial sticking probability for both H₂ and D₂ is 0.60 ± 0.03, independent of substrate temperature down to 170 K. Over the range 170 to 300 K six coverage-dependent temperature-independent phases are identified, and the transition coverages determined. As with the clean surface $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$ displacive phase, the c(2 × 2)-H phase is inhibited by the presence of steps and impurities over large distances (～20 Å), again strongly indicative of CDW-PLD mechanisms for the formation of the H-stabilised phases. These phases are significantly more temperature stable than the clean $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$, the most stable being a c(2 × 2)-H split half-order phase which is formed at domain stoichiometries between WH_0.3 and WH_0.5. LEED symmetry analysis, the dependence of half-order intensity and half-width on coverage, and I-V spectra indicate that the c(2 × 2)-H phase is a different displacive structure from that determined by Debe and King for the clean $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$. LEED I-V spectra are consistent with an expansion of the surface-bulk interlayer spacing from 1.48 to 1.51 Å as the hydrogen coverage increases to ～4 × 10¹⁴ atoms cm^?2. The transition from the split half-order to a streaked half-order phase is found to be correlated with changes in a range of other physical properties previously reported for this system. As the surface stoichiometry increases from WH to WH₂ a gradual transition occurs between a phase devoid of long-range order to well-ordered (1 × 1)-H. Displacive structures are proposed for the various phases formed, based on the hypothesis that at any coverage the most stable phase is determined by the gain in stability produced by a combination of chemical bonding to form a local surface complex and electron-phonon coupling to produce a periodic lattice distortion. The sequence of commensurate, incommensurate and disordered structures are consistent with the wealth of data now available for this system. Finally, a simple structural model is suggested for the peak-splitting observed in desorption spectra.     

Transport phenomena in superionic Na<Subscript><Emphasis Type="Italic">х</Emphasis></Subscript>Cu<Subscript>2 − <Emphasis Type="Italic">х</Emphasis></Subscript>S (<Emphasis Type="Italic">х</Emphasis> = 0.05; 0.1; 0.15; 0.2) compounds

The electronic and ionic conductivity, the electronic and ionic Seebeck coefficients, and the thermal conductivity of Na _x Cu_2 ? x S (x = 0.05, 0.1, 0.15, 0.2) compounds were measured in the temperature range of 20–450 °С. The total cationic conductivity of Na_0.2Cu_1.8S is about 2 S/cm at 400 °С (the activation energy ≈ 0.21 eV). Over the studied compounds, the composition Na_0.2Cu_1.8S has the highest electronic conductivity (500–800 S/cm) in the temperature range from 20 to 300 °С, and the highest electronic Seebeck coefficient (about 0.2 mV/K) in the same temperature range is observed for Na_0.15Cu_1.85S composition; the electronic Seebeck coefficient increases abruptly above 300 °С for all compounds. The thermal conductivity of superionic Na_0.2Cu_1.8S is low, which causes high values of the dimensionless thermoelectric figure of merit ZT from 0.4 to 1 at temperatures from 150 to 340 °С.     

Anomalous temperature dependence of the energy gap of AgGaS2

The lowest energy gap Eg of AgGaS₂ in the temperature range from 4.2 to 300° K was determined from the reflectivity, photoluminescence and absorption measurements. Below ～ 80° K the temperature coefficient of the energy gap is +6 × 10^-5eV/°K. Above ～80° K the sign of the coefficient reverses and the value is -1.8 × 10^-4eV/°K. The positive value is explained with the lattice dilation effect being the dominant mechanism for the energy gap variation at lower temperatures than ～80°K.     

Investigation of the optimal annealing temperature for the enhanced thermoelectric properties of MOCVD-grown ZnO films

In this study, we demonstrate the optimization of the annealing temperature for enhanced thermoelectric properties of ZnO. Thin films of ZnO are grown on a sapphire substrate using the metal organic chemical Vapor Deposition (MOCVD) technique. The grown films are annealed in an oxygen environment at 600–1000°C, with a step of 100°C for one hour. Seebeck measurements at room temperature revealed that the Seebeck coefficient of the sample that was not annealed was 152 μV/K, having a carrier concentration of N _D ~ 1.46 × 10¹⁸ cm^–3. The Seebeck coefficient of the annealed films increased from 212 to 415 μV/K up to 900°C and then decreased at 1000°C. The power factor is calculated and found to have an increasing trend with the annealing temperature. This observation is explained by the theory of Johnson and Lark–Horovitz that thermoelectric properties are enhanced by improving the structure of ZnO thin films. The Hall measurements and PL data strongly justify the proposed argument.     

Die Temperaturabhängigkeit des Bandabstandes von Bleiselenid,gemessen an photoleitenden Schichten

The temperature dependence of spectral distribution of photoconductivity was measured on evaporated polycrystalline layers of lead-selenide in the range from 80 to 300 °K. The method ofBardeen, Blatt andHall was used, to calculate the band gap for direct and indirect transitions. A linear positive temperature coefficient was obtained for both transitions. The values areβ _dir=+(4.5±0.2) · 10^?4 eV/°K andβ _ind=+(3.0±0.2)· 10^?4eV/°K.     

Interaction kinetics of As2 and Ga on {100} GaAs surfaces

A modulated molecular beam technique, using mass spectrometric detection of desorbed species, has been applied to a study of the kinetics of Ga and As₂ interactions on {100} GaAs surfaces. In the temperature range 300–600 K a surface association reaction occurs, leading to the desorption of As₄ by a first order process. Above 600 K, As₂ is lost from the substrate itself by a dissociation reaction, which gives rise to a temperature dependent Ga adatom population, and this in turn results in a temperature dependent As₂ sticking coefficient which can reach unity.     

Direct inelastic and trapping desorption scattering of N2 from polycrystalline W; elementary steps in the chemisorption of nitrogen

Scattering of N₂ from a clean polycrystalline W surface is studied with a time-of-flight molecular beam apparatus. The time-of-flight spectra are used to characterize the N₂-W energytransfer and condensation, allowing inferences to be made about the initial steps of N₂ chemisorption, thought to proceed via a molecular precursor state. The sticking coefficient on our sample for N₂ to chemisorb to an atomic nitrogen bound state was 0.5 ± 0.1 5 for a 600 K beam and a 450 K surface temperature. Unreacted N₂ scattered into direct and trapping-desorption channels. The direct channel is shown to be entirely inelastic with temperature independent differential energy accommodation coefficients that average 0.46 for normal and specular scattering at 45° incidence angle. The fraction of trapping-desorption scattering diminishes significantly with increasing surface and beam temperature. The observed decrease in sticking coefficient with increase in surface temperature is shown to be due to a diminution of the N₂ condensation coefficient as well as an increase in desorption of the N₂, recursor relative to its migration-chemisorption.     

Temperature dependent core photoemission in Ce24Co11

We present Ce 3d photoemission results (XPS with Al Kα) in the temperature range 100–660°K. The mixed valence behaviour of Ce is very clear with an increase of the valence at lower temperature. A model analysis (of the Gunnarsson and Schönhammer type) gives the weight of the ?₀ configuration equal to 0.19 at 300°K and equal to 0.23 at 100°K. This soft temperature dependence is discussed in connection with the temperature dependence of magnetic properties and with the chemistry of Ce intermetallics.     

Thermal conductivity of high-porosity cellular-pore biocarbon prepared from sapele wood

This paper reports on measurements (in the temperature range T = 5–300 K) of the thermal conductivity κ(T) and electrical conductivity σ(T) of the high-porosity (～63 vol %) amorphous biocarbon preform with cellular pores, prepared by pyrolysis of sapele wood at the carbonization temperature 1000°C. The preform at 300 K was characterized using X-ray diffraction analysis. Nanocrystallites 11–30 Å in ize were shown to participate in the formation of the carbon network of sapele wood preforms. The dependences κ(T) and σ(T) were measured for the samples cut across and along empty cellular pore channels, which are aligned with the tree growth direction. Thermal conductivity measurements performed on the biocarbon sapele wood preform revealed a temperature dependence of the phonon thermal conductivity that is not typical of amorphous (and X-ray amorphous) materials. The electrical conductivity σ was found to increase with the temperature increasing from 5 to 300 K. The results obtained were analyzed.