Abstract: Iron doped GaN layers were grown by metalorganic chemical vapor deposition (MOCVD) using ferrocene as the Fe precursor. Specular films with concentrations up to 1.7×1019 cm−3, as determined by secondary ion mass spectrometry, were grown. The Fe concentration in the film showed a linear dependence on the precursor partial pressure, and was insensitive to growth temperature, pressure, and ammonia partial pressure. Memory effects were observed, similar to Mg doping of GaN by MOCVD. The deep acceptor nature of Fe was used for growth of semi-insulating GaN films on sapphire substrates. A 2.6-μm-thick GaN film with a resistivity of 7×109 Ω/sq was attained when the first 0.3 μm of the film was Fe doped. X-ray diffraction rocking curves indicated high crystalline quality, very similar to an undoped film, showing that Fe doping did not affect the structural properties of the film. Fe doping allows for growth of semi-insulating GaN on sapphire without the high threading dislocation densities and/or high carbon leve...

Abstract: In this paper, a one-dimensional, steady-state agglomerate model was used to describe the functioning and the mass transport limitations of the cathode in the solid polymer fuel cell (SPFC). This mathematical model is then compared to experimental results obtained on cathodes in an SPFC. The following processes were considered: Tafel kinetics of the oxygen reduction reaction, proton migration, oxygen diffusion in the agglomerates, and diffusion of a ternary gas mixture vapor in the pores of the active layer and of the gas backing. The model shows that limitation by proton migration in the active layer or by oxygen diffusion in the agglomerates leads to a doubling of the Tafel slope at higher current densities. For those two types of transport limitations, the dependence of the reaction rate on the active-layer thickness, oxygen partial pressure, and relative humidity of the gas were simulated. When additional limitation due to slow gas phase diffusion appears, the double Tafel slope is distorted. A mathematical expression for the limiting current density due to this process is presented. By using this expression, it is possible to correct the polarization curves for slow gas phase diffusion. © 2002 The Electrochemical Society. All rights reserved.

Abstract: The interactions of carbon dioxide and carbon monoxide at moderately high pressures with clean iron surfaces have been studied using x-ray photoelectron spectroscopy (XPS) and static time-of-flight secondary ion mass spectrometry (ToF-SIMS). Clean iron surfaces were analysed after exposure to CO2 or CO gas having pressures of 10–104 Pa at 25 °C. Exposure to either gas results first in a thin surface film of FeO. In addition, this oxide is completely or partially covered with a carbonaceous layer. The thickness of the carbon layer is influenced by the type of gas used and its pressure. The ToF-SIMS studies using isotopically labelled 13CO2 under the same reaction conditions show that the labelled carbon is not enriched on the surface and therefore no direct reaction occurs between the labelled carbon and the surface. It is proposed that the influence of either gas on the carbonaceous layer formed may be indirect: these gases may alter the equilibrium partial pressures of other adsorbates on the walls of the vacuum chamber. Copyright © 2002 John Wiley & Sons, Ltd.

Abstract: Mist pyrolysis was used to prepare cerium oxide (CeO 2 ) fine powder. The average particle size was 200 nm. Using the powder, resistive oxygen gas sensors based on porous thick films were fabricated by screen printing and firing at 1473 K for 2 h in air. The microstructure of the fired thick film showed the neck growth and the random network structure of cerium oxide powder with no significant grain growth. The response time ( t 90 ) for oxygen gas sensors having an average particle size of 200 nm was one-tenth as long as that for those with 2000 nm particles after a sudden change of oxygen partial pressure, P (O 2 ), from 10 5 to 10 3 Pa in the temperature range of 873–973 K.

Abstract: Recent theoretical studies have concluded that the low formation enthalpies of intrinsic donor defects should preclude achievement of p-type conductivity in undoped ZnO grown in thermal equilibrium with a molecular oxygen reservoir. This letter demonstrates that reactive sputtering can produce intrinsic p-type ZnO, controlled by adjusting the oxygen partial pressure in the sputtering plasma. We report the properties of p–n homojunctions fabricated in this way, and characterize transport in the films by Hall measurements. Our finding of p-type conductivity in undoped ZnO grown with dissociated oxygen is qualitatively consistent with the effect of higher chemical potential of atomic oxygen reactant on defect formation enthalpies. This parallels to some degree the recent attention to nitrogen acceptor incorporation by means of dissociating nitrogen source gases.

Abstract: Crystalline titanium dioxide, TiO2, photocatalytic films were deposited by reactive r.f. magnetron sputtering on glass substrates without additional external heating. A pure metallic titanium target was sputtered in a mixture of argon and oxygen. The effect of the total pressure and the oxygen partial pressure on the deposition rate, the phase composition, the crystallinity, the surface morphology and the resulting photocatalytic properties was investigated. The films were characterized by X-ray diffraction, scanning electron microscopy and scanning probe microscopy. The photocatalytic activity was evaluated by the measurement of the decomposition of methylene blue under UV irradiation. The results showed that the crystalline anatase, anatase/rutile or rutile films can be successfully deposited on unheated substrate and their formation is dependent on the total pressure and the oxygen partial pressure. A schematic phase diagram was constructed. The surface morphology is strongly influenced by the total pressure and the anatase TiO2 films with a more open surface, a higher surface roughness and a larger surface area are formed at higher total pressures. The anatase films with such surface morphology deposited in the reactive sputtering mode exhibit the best photocatalytic activity.

Abstract: Yttrium-doped SrTiO3 (SYT) was assessed as an anode material for solid oxide fuel cells in terms of electrical conductivity, phase stability, redox behavior, chemical compatibility with yttria-stabilized zirconia (YSZ) and La0.8Sr0.2Ga0.8Mg0.2O2.8 (LSGM), thermal expansion coefficient, and fuel cell performance. With the optimized composition Sr0.86Y0.08TiO3−δ, the electrical conductivity was as high as 82 S/cm at 800 °C and oxygen partial pressure of 10−19 atm. A reversible change of conductivity was observed upon oxidation and reduction. The resistance to oxidation was enhanced by partially replacing Ti with transition metals such as cobalt. This material has high structural stability over a broad range of temperature (up to 1400 °C) and oxygen partial pressure (1–10−20 atm). No phase change was found for mixtures of SYT with YSZ or LSGM sintered at 1400 °C for 10 h. The thermal expansion of doped-SrTiO3 was determined to be compatible with that of YSZ and LSGM. A maximum power density of 58 mW/cm2 at 900 °C was obtained for single cells with the new anode.

Abstract: Electrical properties of the metallic oxide semiconductor, such as tungsten trioxide (WO3) thin films, deposited on SiO2/Si substrates by RF reactive magnetron sputtering system from a metallic tungsten target and argon–oxygen mixture gas have been investigated. This study is devoted to analyse the relationship between the electrical properties and the WO3 thin film deposition parameters (substrate temperature, oxygen partial pressure, annealing) and the sensitivity and stability of these WO3 gas sensors. The surface morphology evolution of these films has been investigated by atomic force spectroscopy (AFM). Two types of electrical measurements were performed: conductivity versus temperature and some tests under ozone at different temperatures. The activation energy evolution is correlated with the reactivity of surface sensors under oxygen partial pressure.

Abstract: Alloys of Al, Al-0.15Mg, and Al-12Sn made using air atomized aluminum powder and pressed to green densities of 75 to 98 pct were sintered under argon or nitrogen. Sintering in argon is only effective at high green densities when magnesium is present. In contrast, highly porous aluminum can be sintered in nitrogen without the need for magnesium. The oxygen concentration in the gas is reduced by the aluminum through a self-gettering process. The outer layers of the porous powder compact serve as a getter for the inner layers such that the oxygen partial pressure is reduced deep within the pore network. Aluminum nitride then forms, either by direct reaction with the metal or by reduction of the oxide layer, and sintering follows.

Abstract: The steady-state rate rCO2 of CO2 formation from CO + O2 on an epitaxially grown RuO2(110) single-crystal surface was recorded as a function of the partial pressures in the 10-7−10-6 mbar range. The RuO2(110) surface exposes singly coordinatively unsaturated “Ru-cus” sites and 2-fold coordinatively unsaturated “Ru-bridge” sites. Normally, the Ru-bridge sites are saturated by oxygen (O-bridge) which can be replaced by CO. The stage of the surface was controlled by vibrational spectroscopy (HREELS), and analysis of the kinetic data was based on previous information about the structural and adsorptive properties of this surface. Measurements of the rate as a function of temperature up to ≈350 K in a 1:1 mixture of CO and O2, each with 10-7 mbar partial pressure, revealed most remarkable agreement with data reported by Zang and Kisch13 with small supported RuO2 particles prepared from aqueous solution for 1 bar total pressure. Under these conditions of temperature and ratio of partial pressures the reaction w...

Abstract: A gas phase and surface chemistry study of inductively coupled plasmas fed with C4F6/Ar and C4F8/Ar intended for SiO2 etching processes was performed. Adding Ar to those fluorocarbon gases results in a strong increase of the ion current, by up to a factor of 5 at 90% Ar relative to the pure fluorocarbon gases. The fluorocarbon deposition rate is higher for C4F6/Ar than for C4F8/Ar, whereas the fluorocarbon etching rate is lower, and both quantities decrease as the amount of Ar is increased. For both C4F6/Ar and C4F8/Ar, the CF2 density is more than an order of magnitude greater than the CF density. The CF2 partial pressure decreases as more Ar is added to the C4F6/Ar plasmas. A comparison of these data with corresponding results obtained with C4F8/Ar shows that the CF2 partial pressure in C4F6 is higher for Ar-lean gas mixture than for C4F8/Ar. This remains true up to 40% Ar. Above 40% Ar the CF2 partial pressure in C4F8 is higher than for C4F6. The CF and COF2 partial pressures in C4F8 are higher than fo...

Abstract: The chemical diffusion coefficient and oxygen-transfer coefficients of selected compositions in the series $La_1-xSr_xCoO_3-delta$ were studied using the conductivity relaxation technique. Measurements were performed in the temperature range 600-850°C and oxygen partial pressure $10-4$ to 1 bar. Chemical diffusivity and oxygen surface transfer in the $La_1-xSr_xCoO_3-delta$ perovskites appear to be highly correlated. The general trend displayed is that both parameters decrease with decreasing pO2 below about $10-2$ bar at all temperatures. This is attributed to ordering of induced vacancies at low oxygen partial pressures. The observation that the correlation between both parameters extends even to the lowest $pO_2$ values in this study suggests a key role of the concentration of mobile oxygen vacancies, rather than of the extent of oxygen nonstoichiometry, in determining the rates of both processes. The characteristic thickness $L_c$, which equals the ratio of the chemical diffusion coefficient to the surface transfer coefficient, shows only a weak dependence on oxygen partial pressure and temperature. For different compositions $La_1-xSr_xCoO_3-delta$, $L_c$ is found to vary between 50 and 150 µm.

Abstract: The superoxide ion (O2•-) has been generated electrochemically from oxygen dissolved in two different solvent systems: (1) acetonitrile with tetraethylammonium perchlorate (TEAP) as the supporting electrolyte at elevated pressure and (2) in a room-temperature ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][HFP]), at atmospheric pressure. A high-pressure electrochemical cell with a quasi reference electrode was developed for elevated pressure tests. Increasing the partial pressure of oxygen in the first system increased the rate of superoxide generation because of the increased solubility of oxygen according to Henry's law. The subsequent addition of gaseous carbon dioxide enhances the rate of oxygen reduction in both systems but inhibits the reverse (oxidation) reaction of O2•- to O2. This later observation is consistent with the irreversible formation of a peroxydicarbonate ion, as has been postulated by others.

Abstract: The surface tension of molten tin has been determined by the sessile drop method at The surface tension of molten tin has been determined by the sessile drop method at temperatures ranging from 523 to 1033 K and in the oxygen partial pressure (P-O2) range from 2.85 x 10(-19) to 8.56 x 10(-6) MPa, and its dependence on temperature and oxygen partial pressure has been analyzed. At P-O2 = 2.85 x 10(-19) and 1.06 x 10(-15) MPa, the surface tension decreases linearly with the increase of temperature and its temperature coefficients are -0.151 and -0.094 mNm(-1) K-1, respectively. However, at high P-O2 (3.17 x 10(-10), 8.56 x 10(-6) MPa), the surface tension increases with the temperature near the melting point (505 K) and decreases above 723 K. The surface tension decrease with increasing P-O2 is much larger near the melting point than at temperatures above 823 K. The contact angle between the molten tin and the alumina substrate is 158-173degrees, and the wettability is poor.

Abstract: The dependence of the efficacy of an alternating current surface-discharge plasma display panel on the gas pressure is investigated for several Xe–Ne gas mixtures. In monochrome green 4 in. test panels the efficacy trends and emission spectra are examined for increasing gas pressure and/or Xe concentration. The measured panel efficacy and emission characteristics are compared with the results of a numerical discharge model. It is found that the discharge efficiency for the cell geometry used in present-day commercial products can be increased significantly by using a larger Xe partial pressure. An increase of the electron heating efficiency and of the Xe excitation efficiency contribute about equally to the efficacy increase. The contribution of the increasing Xe dimer radiation fraction to the efficacy improvement is relatively small. These findings are applied in a 4 in. color test display with a design that resembles the one used in present-day commercial products and contains a gas mixture of 13.5% Xe...

Abstract: We produced indium tin oxide (ITO) films on glass substrates by DMIBD technology and investigated the effect of secondary ion beam energy and oxygen partial pressure on the surface morphology, crystallinity and optical property of ITO films During deposition, substrate temperature was approximately 70 °C AFM measurements shown that the film deposited at 50 eV exhibits the characteristic of polycrystalline thin films with a larger grain structure than that of other conditions The XRD spectra indicated that the ITO films, which were produced in this study, have polycrystalline structures It means that surface adatoms had enough mobility to migrate on the substrate with transferred kinetic energy by colliding secondary Si − ions However, too energetic secondary ion beam bombardments decreased grain size and also roughened surface morphology due to radiation damage Optical transmittance of ITO film in the visible region was related with structural and surface morphological properties In this study, the highest optical transmittance of 85% at the wavelength of 550 nm and the lowest resistivity of 4×10 −4 Ωcm were obtained at the oxygen partial pressure of 13×10 −2 Pa and 50 eV, respectively

Abstract: Total gas pressure, oxygen, temperature and salinity were measured at 50 m on a mooring at the Hawaii Ocean Time series (HOT) station from January through September of 1997 and 1998 using a gas tension device (GTD) and a conductivity temperature depth and oxygen (CTD-O 2 ) sensor (GTD–CTD-O 2 instrument package). Our goal was to evaluate the precision and accuracy of the in situ total gas and oxygen measurements and to assess their utility for remotely determining net biological oxygen production in the euphotic zone. By calibrating the oxygen sensor approximately every month during periodic visits to the mooring it was possible to remotely measure the time history of the partial pressures of nitrogen and oxygen to within ±0.5% accuracy as assessed by independent determination of nitrogen concentration. Oxygen changed at 50 m by 2–4% in a series of episodes with durations of 1–3 months, indicating the discontinuous nature of net O 2 production and the probable decoupling of oxygen production and respiration in the euphotic zone. The GTD and oxygen measurements together can be used to distinguish the effects of physical processes and net biological oxygen production on the oxygen concentrations. The largest uncertainty in the coupled O 2 and total gas pressure measurements is the drift of the oxygen sensor, making frequent calibration presently a necessity. With more complete vertical coverage in the upper ocean it should be possible to use this approach to determine depth-integrated net oxygen production in the euphotic zone and upper thermocline respiration.

Abstract: The active oxidation, passive oxidation and bubble formation of CVD SiC were studied in O2 and CO2 at temperatures from 1650 to 2000 K The active oxidation rates in O2 increased with increasing oxygen partial pressure (PO2); however, those in CO2 showed the maxima at specific PO2 The passive oxidation kinetics in O2 were either linear–parabolic or parabolic depending on temperature and PO2, whereas that in CO2 was always parabolic The activation energies for the parabolic oxidation in O2 and CO2 were 210 and 150 kJ/mol, respectively, suggesting different rate-determining process in these atmospheres The bubble formation was controlled by temperature and PO2 being independent of oxidant gas species The linear and parabolic oxidation rates were accelerated by the bubble formation