Showing papers on "Atmospheric temperature range published in 1992"

Temperature Dependence of the Electrode Kinetics of Oxygen Reduction at the Platinum/Nafion® Interface—A Microelectrode Investigation

Abstract: Results of a study of the temperature dependence of the oxygen reduction kinetics at the Pt/Nafion interface are presented. This study was carried out in the temperature range of 30-80 C and at 5 atm of oxygen pressure. The results showed a linear increase of the Tafel slope with temperature in the low current density region, but the Tafel slope was found to be independent of temperature in the high current density region. The values of the activation energy for oxygen reduction at the platinum/Nafion interface are nearly the same as those obtained at the platinum/trifluoromethane sulfonic acid interface but less than values obtained at the Pt/H3PO4 and Pt/HClO4 interfaces. The diffusion coefficient of oxygen in Nafion increases with temperature while its solubility decreases with temperature. These temperatures also depend on the water content of the membrane.

Textured aluminum‐doped zinc oxide thin films from atmospheric pressure chemical‐vapor deposition

Abstract: Aluminum‐doped zinc oxide films have been deposited on soda lime glass substrates from diethyl zinc, triethyl aluminum, and ethanol by atmospheric pressure chemical‐vapor deposition in the temperature range 367–444 °C. Film roughness was controlled by the deposition temperature and the dopant concentration. The films have resistivities as low as 3.0 × 10−4 Ω cm, infrared reflectances close to 90%, visible transmissions of 85%, and visible absorptions of 5.0% for a sheet resistance of 4.0 Ω/⧠. The aluminum concentration within doped films measured by electron microprobe is between 0.3 and 1.2 at. %. The electron concentration determined from Hall coefficient measurements is between 2.0 × 1020 and 8.0 × 1020 cm−3, which is in agreement with the estimates from the plasma wavelength. The Hall mobility, obtained from the measured Hall coefficient and dc resistivity, is between 10.0 and 35.0 cm2/V s. Over 90% of the aluminum atoms in the film are electrically active as electron donors. Scanning electron microscopy and x‐ray diffraction show that the films are crystalline with disklike structures of diameter 100–1000 nm and height 30–60 nm. The films have the desired electrical and optical properties for applications in solar cell technology and energy efficient windows.

Systematic evolution of temperature-dependent resistivity in La2-xSrxCuO4.

Abstract: The in-plane resistivity (ρ ab ) of La 2-x Sr x CuO 4 has been studied over a wide temperature (4-1000 K) and composition range (0 0.2), ρ ab follows a novel power-law dependence, ρ∞T 1.5 , over the entire temperature range up to 1000 K

Surfactant-induced layer-by-layer growth of Ag on Ag(111)

Abstract: We have investigated the epitaxial growth of Ag(111) by x-ray reflectivity experiments. In the temperature range from 175 to 575 K the growth mode changes gradually from three-dimensional to step flow, but at no temperature was layer-by-layer growth observed. Submonolayer deposits of Sb on the starting surface dramatically alter the growth behavior. With Sb the Ag grows layer by layer over the entire temperature range investigated (225--375 K), resulting in smoother surfaces at the lowest growth temperatures. We propose that Sb decreases the barrier for interlayer diffusion.

High-temperature elasticity of iron-bearing olivines

Abstract: The first high-temperature data on the nine adiabatic elastic moduli for iron-bearing olivine are reported. These measurements are on two single-crystal specimens of natural olivine at ambient pressure and from room temperature to a maximum of 1500 K. The two specimens contain 8 and 9 modal percent fayalite, which required the oxygen fugacity be controlled at high temperature to preserve their chemical stability. The rectangular parallelepiped resonance apparatus was adapted to buffer the specimens from the atmosphere with a mixture of CO and CO2 gas. A small increase (∼1–2 GPa) in the adiabatic bulk modulus of each specimen, over that of end-member forsterite, was found. The data are high quality to extreme temperatures, with good agreement found when comparing the temperature derivatives of the elastic moduli of the two specimens. Neither specimen exhibits measurable nonlinear temperature dependence in the computed isotropic bulk and shear moduli, which is in contrast to published forsterite data. The temperature derivatives of the isotropic bulk modulus KS are (−1.69, −1.80) × 10−2 GPa K−1 for the two olivine specimens, and the shear modulus G derivatives are (−1.38, −1.36) × 10−2 GPa K−1. These derivatives are only slightly larger in magnitude than |(∂KS/∂T)P| = 1.56 × 10−2 and |(∂G/∂T)P| = 1.30 × 10−2 GPa K−1 found previously for iron-bearing olivine over a very small temperature range. There are also no significant differences between the temperature derivatives found here and the average derivatives of end-member forsterite from data retrieved over a slightly larger temperature range. Several dimensionless parameters have been calculated from these results and are discussed in view of systematics which bear on high-pressure phases in Earth's transition zone. One result from these systematics related to the seismic velocities in the Earth, and especially the shear wave velocities, is that an olivine content of less than 50% is implied at the 400-km discontinuity if Earth's upper mantle is isochemical. Furthermore, the substitution of almost 10% iron for magnesium at the forsterite end of the olivine solid solution series has little effect on the dimensionless parameters or on the temperature derivatives of the elastic moduli at high temperature.

Calibration of the ruby R1 and R2 fluorescence shifts as a function of temperature from 0 to 600 K

Abstract: Recent work by Gupta and Shen [Appl. Phys Lett. 58, 583 (1991)] has shown that in a nonhydrostatic environment, the frequency of the ruby R2 line provides a reliable measure of the mean stress or pressure. When using the frequency of either the R1 or R2 line to measure pressure at nonambient temperature, it is necessary to know the temperature dependence of the line shift. Unfortunately, the shift of the R2 line with temperature has not been reported. The ruby R1 and R2 fluorescence shifts have been determined as a function of temperature from 15 to 600 K. Both can be fitted very well to the simple cubic forms R1(T) =14 423+4.49×10−2T−4.81×10−4T2+3.71×10−7T3 cm−1 and R2(T)=14 452 +3.00×10−2T−3.88×10−4T2+2.55×10−7T3 cm−1. From 300 to 600 K the shifts fit well to linear functions of temperature. In addition, it is found that the R1‐R2 splitting changes by about 3 cm−1 over the 600 K temperature range. Linewidths were found to vary both with temperature and from sample to sample.

Magnetism in Cr thin films on Fe(100)

Abstract: The spin polarization of secondary electrons from a Cr film on Fe(100), measured with scanning electron microscopy with polarization analysis, oscillates as a function of Cr thickness with a period near two atomic layers, consistent with incommensurate spin-density-wave antiferromagnetism in the Cr. The position of a phase slip due to incommensurability varies reversibly by 14 layers over the temperature range of 310 to 550 K. The Cr surface magnetic moment persists well above the Neel temperature of bulk Cr

Apparatus for zone melting recrystallization of thin semiconductor film

Abstract: An apparatus for zone melting a thin semiconductor film comprises an first laser for heating the thin semiconductor film, at least one additional laser for heating an insulating substrate, a first temperature detecting device for detecting the temperature of a melted portion of the thin semiconductor film, and a second temperature detecting device for detecting the temperature of a solidified portion of the thin semiconductor film. The apparatus further comprises a first controller for controlling an output of the first laser so as to maintain the temperature of the melted portion in a first predetermined temperature range, and a second controller for controlling an output of the additional laser so as to maintain the temperature of the solidified portion in a second predetermined temperature range.

Temperature Dependence of Thermoelectric Properties of Mg 2 Si 0.6 Ge 0.4

Abstract: The thermoelectric properties of Sb-doped n-type and Ag-doped p-type Mg 2 Si 0.6 Ge 0.4 samples were investigated in the temperature range 300 to 900 K. The scattering factor r was determined to be 0.0 by measuring the temperature dependence of the Hall mobility. Thermoelectric power and electrical conductivity were measured and discussed on the basis of the degenerated carriers. The thermal conductivity was evaluated from the values measured at 300 K.

2223 Phase Formation in Bi(Pb)─Sr─C─a─Cu─O: II, The Role of Temperature—Reaction Mechanism

Abstract: The formation mechanism, the temperature range for the growth, and the thermal stability of the 2223 phase in Bi(Pb)─Sr─Ca─Cu─O have been investigated using DTA/TG, XRD, SEM/EDAX, TEM, EPMA, four-probe resistance and acsusceptibility measurement. The formation of the 2223 phase was found to follow a dissolution–precipitation process. A 2212 phase first reacts with the liquid phase formed via an incongruent melting of the Ca2PbO4 phase, and a dissolution of CaO and CuO takes place. The 2201 phase, which has the largest negative free energy, is then precipitated from the melt; the nucleation and growth of the 2223 phase are subsequently developed by the reaction between the 2201 phase precipitates and ions of Ca2+ and Cu2+ present in the liquid phase. The 2223 phase is formed at temperatures in the range 827°C T < 856°C. The optimum temperature Tf for the formation of 2223 phase is 845°± 5°C. The 2223 phase is thermodynamically unstable at temperatures above 856°C.

Low-frequency dielectric response of PbMg1/3Nb2/3O3

Abstract: An investigation was carried out on both the linear and nonlinear parts of the complex dielectric susceptibility in the frequency and temperature ranges 10-3-103 Hz and 100-450 K, respectively. The analysis of the imaginary part of the dielectric susceptibility has revealed the existence of a broad spectrum of relaxation times in the above temperature range. The temperature dependence of the most probable frequency of the spectrum was obtained. For temperatures T

Simulated Diurnal Range and Variability of Surface Temperature in a Global Climate Model for Present and Doubled C02 Climates

Abstract: The variability of surface temperature simulated by a global climate model with a simple mixed-layer ocean is analyzed. The simulated diurnal and seasonal ranges of temperature are compared with observation, as is the day-to-day and interannual variability of temperature. The qualitative changes in these quantities due to doubling atmospheric carbon dioxide concentration are also presented. The simulation of the seasonal cycle of surface temperature has a cold bias in much of the extratropics, including central Europe, even allowing for the difficulties in comparing grid-box surface temperatures with station temperature at screen height. The simulated diurnal range of temperature for present-day climate is similar to that observed, though the diurnal cycle in the model in midlatitudes is generally less than observed. On doubling C02, the diurnal range over land decrease by 0.3°C whereas mean temperatures increase by 6.3°C (global average over land). In CO2-doubling experiments with a one-dimensio...

Electrical and thermal properties of chalcogenide glass system Se75Ge25-xSbx

Abstract: The dc and thermal conductivities of five different compositions of the chalcogenide glass system Se75Ge25−xSbx have been studied in a temperature range below Tg. The dc conductivity results indicate that each composition has a single activation energy in the considered temperature range. The coefficient of thermal conductivity increased linearly with temperature below Tg for the compositions investigated. The increase of Sb content in the chalcogenide glass system leads to an increased coefficient of electrical conductivity σ, an increased coefficient of thermal conductivity ψ, and to a decreased activation energy Eσ and pre-exponential factor σ0. The observed compositional dependencies of σ and Eσ have been correlated with the increase of weak bond density and the decrease of covalent bond density in the structure of the compositions investigated with increasing Sb content at the expense of Ge content. The decrease in σ0 and the increase in ψ has been, respectively, correlated with the decrease in mobility and the increase in phonon velocity.

Active to passive transition in the oxidation of silicon carbide at high temperature and low pressure in molecular and atomic oxygen

Abstract: The active to passive transition in the oxidation of sintered α-SiC has been determined by a thermodynamic approach and then experimentally under flowing air at high temperatures (1673–1973 K) and low pressures (oxygen partial pressure from 200–2100 Pa). Then, the physico-chemical behaviour of samples was compared for two different environments such as air-plasma atmosphere generated by microwaves, and a molecular atmosphere. The thermodynamic calculation does not predict any variation of the oxidation transition when changing the chemical state of oxygen (O2 or O) but experimentally the domain characterized by the formation of a passive layer of silica is extended to lower pressure under atomic oxygen (for the same temperature range) than in the case of molecular oxygen.

Electrochemical, hydrothermal, and electrochemical-hydrothermal synthesis of barium titanate thin films on titanium substrates

Abstract: Well-crystallized polycrystalline films of cubic BaTiO3 have been synthesized on Ti metal substrates by electrochemical, hydrothermal, and electrochemical-hydrothermal reaction of a Ti metal with a saturated solution of Ba(OH)2 in the temperature range 80 °C−200 °C. The films can be produced in the electrochemical process by the anodization of a Ti substrate in Ba(OH)2 solution at pH = 13 and current densities of the order of 10–50 mA/cm2. The films can also be synthesized by the hydrothermal reaction of Ba(OH)2 and a Ti substrate without electrical bias. A combination of these two techniques reduces the reaction time to 30–45 min. The films were found to have good adherence on the substrate and films of thickness up to 2 μm could be formed within a reaction time of 45 min. Capacitance measurements have yielded a dielectric constant of 300. The films prepared by the three different techniques have been compared for crystallinity and microstructure. The thickness of the films increased with temperature and time of reaction. The crystallinity decreased with extended time of reaction for a given temperature. It is likely that a reactive intermediate such as Ti(OH)4 (aq.) is formed in all the three cases which leads to the formation of crystalline BaTiO3.

Elevated-temperature strength of an Al88Ni9Ce2Fe1 amorphous alloy containing nanoscale fcc-Al particles

Abstract: An Al 88 Ni 9 Ce 2 Fe 1 alloy with ultrahigh tensile fracture strength exceeding 950 MPa in the temperature range from room temperature to 573 K was found to be obtained by rapid solidification in the structural state where the nanoscale fcc-Al particles without internal defects disperse homogeneously in the amorphous matrix. The ultimate tensile strength is as high as 1560 MPa at room temperature and 970 MPa at 573 K

Measurement of elastic properties of single-crystal CaO up to 1200 K

Abstract: The elastic moduli of a single-crystal calcium oxide, CaO, are measured in the temperature range from 300 to 1200 K (1.8 times of the Debye temperature) by the resonant sphere technique (RST). The lowest 18 modes are identified in the frequency range from 0.6 to 1.4 MHz for the vibrating spherical specimen, which is 5.6564 mm in diameter and 3.3493 g/cm3 in density at room temperature, and the resonant frequencies are traced as a function of temperature. The adiabatic elastic moduli are determined in the present temperature range from the observed frequencies by inversion calculations. Most of the elastic moduli, except forC 12 modulus, decrease as temperature increases. The temperature curves ofC s andC 44 moduli cross at 372 K. This means that the CaO specimen has an isotropic elasticity at the temperature. The temperature derivatives (∂C 11/∂T) P and (∂C s/∂T) P become slightly less negative with temperature increase and (∂C s /∂T) P and (∂C 44/∂T) P are almost constant. Combining the present elastic data with thermal expansion and specimen heat capacity data of CaO, we present the temperature dependence of thermodynamic parameters important in the studies of earth's interior.

Silicon temperature measurement by infrared absorption. Fundamental processes and doping effects

Abstract: The fundamental absorption mechanisms in silicon at 1.30 and 1.55 mu m have been investigated in the temperature range of 500-800 degrees C. For lightly doped wafers in this temperature range, the absorption at 1.55 mu m is by free carriers. and that at 1.30 mu m is predominantly by bandgap absorption. The effect of heavy substrate doping on infrared absorption at an elevated temperature has also been studied, and it was found that doping has little effect below levels of 7*10/sup 17/ cm/sup -3/. Above that level, the temperature dependence of free carrier absorption strongly affects the transmission as a function of temperature. The knowledge of the fundamental absorption processes is then used to predict the ultimate temperature ranges over which the technique will be useful. >

H2-induced changes in electrical conductance of β-Ga2O3 thin-film systems

Abstract: H2-induced changes of electrical conductivity in polycrystalline, undoped β-Ga2O3 thin films in the temperature range of 400–650° C are described. The sheet conductance of these films depends reversibly, according to a power law σ□ ∼ p 1/3, on the partial pressure of hydrogen in the ambient atmosphere of the Ga2O3 film. A bulk vacancy mechanism is excluded by experiments and it is shown that the interaction is based on a surface effect. Changes in conductance are discussed to result from the formation of an accumulation layer due to chemisorption on the grain surfaces. Typical coverages are determined to be approximately 10−4 ML for pH2=0.05 bar and T=600° C. A possible explanation of the σ□ ∼ p 1/3 power law is provided.

Characteristics of nanophase TiAl produced by inert gas condensation

Abstract: Nanophase TiAl, with grain sizes in the range of 10–20 nm, was synthesized by magnetron sputtering in an inert gas atmosphere and consolidated, in situ, under vacuum. The properties of the powders and sintered compacts were studied by transmission electron microscopy, scanning electron microscopy, calorimetry, Rutherford backscattering, and x-ray diffraction. Samples compacted at 1.0 GPa at room temperature had a large fraction of amorphous phase, while samples compacted at the same pressure and 250 °C were predominantly the equilibrium γ phase. An enthalpy change of 22 kJ/g-atom was measured during a DSC scan over the temperature range 125–450 °C, which is approximately the range over which crystallization occurs. Nearly full density could be achieved by sintering at 450 °C without significant, concomitant grain growth. The Vickers microhardness of these samples at room temperature and at −30 °C revealed an inverse Hall–Petch relationship at small grain sizes, 10–30 nm, and the usual Hall–Petch behavior at larger grain sizes. A small component of indentation creep was also observed. The maximum hardness is 4 times larger than that of a cast TiAl specimen of the same composition. The Vickers hardness was also observed to decrease rapidly with temperature above 200 °C.

Deposition of Boron Doped Zinc Oxide Films and Their Electrical and Optical Properties

Abstract: Boron doped zinc oxide films were deposited by atmospheric pressure chemical vapor deposition in a laminar flow reactor from diethyl zinc, tert-butanol, and diborane in the temperature range from 300 to 420 o C. When the deposition temperature was above 320 o C, both doped and undoped films have highly oriented crystallites with their c-axes perpendicular to the substrate plane

Measurement of refractive indices and thermal refractive-index coefficients of LiNbO(3) crystal doped with 5 mol. % MgO

Abstract: The principal refractive indices and the thermal coefficients for a LiNbO(3) crystal doped with 5 mol. % MgO have been measured by a minimum-deviation method for the wavelengths of 0.53975, 0.6328, 1.0795, and 1.3414, microm in the temperature range 20-154.5 degrees C. The constants of modified Sellmeier equations are all given in the described temperature range. The measured results were used to calculate the type I critical phase-matching angles for 1.0795- and 1.3414-microm second-harmonic generation at room temperature and the noncritical phase-matching temperature for 1.0795-microm second-harmonic generation for LiNbO(3) crystal doped with 5 mol. % MgO. The values obtained agree well with the experimental results.

Optimization of growth conditions of vapor deposited Mo/Si multilayers

Abstract: The dependence of the layer structure and interfaces of e‐beam deposited Mo/Si multilayers (ML) on growth conditions has been studied The substrate temperature was varied over a range of 300–600 K at deposition rates of 1 and 3 A/s The structure of the ML was determined using small‐angle x‐ray scattering, large‐angle x‐ray scattering, and cross‐sectional high‐resolution electron microscopy The variation of the normal incidence reflectivity was measured as a function of soft x‐ray wavelength using synchrotron radiation We found that the lateral extent of the Mo crystallites and the thickness and roughness of interlayers are a sensitive function of the substrate temperature The morphology shows a lesser dependence on the deposition rate The formation of the Mo‐on‐Si interlayer is partially thermally activated with an activation energy of ∼02 eV We propose that the rate limiting mechanism is surface diffusion during growth At the optimal deposition conditions, of a substrate temperature of ∼525 K and a deposition rate of 1 A/s, the ML structure was found to have a smoothness and maximum normal incidence reflectivity comparable to the best sputtered Mo/Si films

Rheological properties of Y-Si-Al-O-N glasses — elastic moduli, viscosity and creep

Abstract: Three oxynitride glasses from the Y-Si-Al-O-N system and differing in their N/O ratio were studied in the 800–1000 °C temperature range. Their viscosities were measured using a threepoint bending test through the glass transition domain. For a given temperature, 4.8 wt % N2 enhances the viscosity by three orders of magnitude in comparison with the corresponding oxide glass. Nitrogen also improves creep resistance. The activation enthalpy for creep, aboveT g, is of the same order as those measured for silicon nitride ceramic (∼900 kJ mol−1). The elastic moduli were determined by ultrasonic techniques, from room temperature up to 1200 °C, which allowed calculation of the free activation enthalpy for viscous flow. Owing to the sharp decrease of shear modulus in the glass transition domain, the free activation enthalpy (∼500 kJ mol−1) greatly differs from the activation enthalpy usually measured in creep studies.