Abstract: The permeability of polydimethylsiloxane [PDMS] to acetone, nitrogen, and acetone/nitrogen mixtures has been determined at 28°C. In pure gas experiments, the permeability of PDMS to nitrogen was 245 × 10−10 cm3(STP) · cm/cm2 · s · cmHg and was independent of pressure. The permeability of PDMS to acetone vapor increased exponentially with increasing acetone pressure. PDMS is much more permeable to acetone than to nitrogen; acetone/nitrogen selectivity increases from 85 to 185 as acetone partial pressure in the feed increases from 0 to 67% of saturation. In mixed gas permeation experiments, the nitrogen permeability coefficient is independent of acetone relative pressure and is equal to the pure gas permeability coefficient. The acetone permeability coefficient has the same value in both mixed gas and pure acetone permeation experiments. Average acetone diffusivity in PDMS, determined as the ratio of permeability to solubility, decreases with increasing acetone concentration due to mild clustering of acetone in the polymer (because acetone is a poor solvent for PDMS) and changes in the polymer–penetrant thermodynamic interactions which influence diffusion coefficients. A Zimm–Lundberg analysis of the acetone sorption isotherm is also consistent with acetone clustering in PDMS. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 289–301, 1998

Abstract: The kinetics and the mechanism of the selective reduction of nitric oxides (NOx) by hydrogen is studied on a trimetallic Pt–Mo–Co/a-Al2O3 catalyst under oxidising conditions. This system is interesting in view of an exhaust gas control of power plants or lean-burn cars. It can be shown that the NO dissociation is the crucial step, dominating the overall reaction behaviour and that it depends on temperature and on the partial pressure of H2. With increasing temperatures the reaction reveals an autocatalytic behaviour resulting in bistability and hysteresis. At higher temperatures, where no bistability is found, the NO/H2 as well as the competing O2/H2 reaction occur only above a certain critical partial pressure of H2. The kinetics of the NO/H2/O2 reaction are established using a modified Langmuir–Hinshelwood model (T=142°C–160°C, yO2>4%) which takes into account the critical H2 partial pressure. The model describes the experimental data within ±15%. The determined activation energies are: 63 kJ/mol for the NOx consumption, 77 and 45 kJ/mol for the N2 and N2O formation, respectively, and 130 kJ/mol for the O2/H2 reaction. Adsorption enthalpies are determined to -59 kJ/mol for the adsorption of H2, -77 kJ/mol for the adsorption of NO and -97 kJ/mol for the adsorption of O2. An interesting feature of the reaction is the enhancement of the NO/H2 reaction by oxygen for low partial pressures of O2. This appears to be the first study where a promoting effect of oxygen for the NO/H2 reaction is found on a platinum supported catalyst.

Abstract: The tribological behavior of diamond-like carbon coatings (DLC) strongly depends on the chemical nature of the test environment. The present study proposes to explore the influence of water vapor and oxygen on the friction behavior of a hydrogenated DLC coating exhibiting ultralow friction in ultrahigh vacuum (friction coefficient below 0.01). Using a UHV tribometer, reciprocating pin-on-flat friction tests were performed in progressively increasing or decreasing partial pressures of pure oxygen and pure water vapor. The maximum gaseous pressures of oxygen and water vapor were 60 hPa and 25 hPa (1 hPa = 100 Pa), respectively, the second value corresponding to a relative humidity (RH) of 100% at room temperature. It was found that, for the pressure range explored, oxygen does not change the ultralow friction behavior of DLC observed in UHV. Conversely, water vapor drastically changes the friction coefficient at pressures above 0.5 hPa (RH = 2%), from about 0.01 to more than 0.1. Electron energy loss spectroscopy and in situ Auger electron spectroscopy have been performed to elucidate the friction mechanisms responsible for the tribological behaviors observed with the two different gaseous environments. In all cases no significant oxidation has been observed either inside the wear scars or in the wear debris particles. Ultralow friction is systematically associated with a homogeneous carbon-based transfer film. The higher friction observed at partial pressure of water vapor higher than 0.5 hPa, is associated with a thinner transfer film. Consequently friction seems to be controlled by the transfer film whose kinetics of formation strongly depends on the partial pressure of water vapor.

Abstract: Thin films of zirconium oxide were prepared by r.f. magnetron sputtering from a ZrO2 target. A systematic study has been made on the influence of the sputtering parameters (total pressure, oxygen partial pressure and r.f. power) on the film composition and on their structural and optical properties. The zirconia films crystallize either in the cubic or in the monoclinic phases depending on the sputtering gas. The crystallinity and the compactness of the films were found to increase with the kinetic energy of the sputtered particles. The stresses are compressive and become very important in thick films deposited at a high power density. Films are generally substoichiometric and their O/Zr atomic ratio was found to increase with the oxygen partial pressure. On the contrary, films deposited at high sputtering pressures (more than 5 Pa) contain an oxygen excess. This overstoichiometry results, as it was revealed by F. T. I. R. analyses, from the incorporation of water and hydroxyl groups into the ZrO2 structure. The optical constants (refractive index and extinction coefficient) vary also in a wide range with the deposition conditions. These variations were correlated mainly to structural properties.

Abstract: Active-to-passive oxidation transition in chemical vapour deposited β-SiC was investigated in the temperature range 1300≤T≤1700°C under low total pressures (100≤Ptot≤800 Pa) and relatively high linear gas flow rates (10≤Vgas≤60 m s1) by thermogravimetric analysis. For given T, Ptot and Vgas, the oxygen partial pressure at the transition, PtO2, corresponds to the value where the mass-loss rate per unit area of the oxidized sample, R, is maximum. Logarithms of PtO2 are linear functions of reciprocal temperature for given Ptot, and Vgas. Vgas has a significant influence on the position of the transition log(PtO2)–T-1 line. PtO2 is also slightly affected by an increase of Ptot from 100 Pa to 800 Pa. In passive oxidation at high temperatures (>1500°C), large bubbles form in the silica film which is then disrupted leading to a loss of material. In active oxidation, R significantly depends on Vgas: the kinetics is diffusion or mass transfer controlled under the conditions investigated in the present study. In both active and passive oxidation regimes, a mass loss of the test specimen is always observed; an explanation is proposed.

Abstract: The photoassisted monooxygenation of C6H12 to C6H10O and C6H11OH by molecular oxygen has been studied on TiO2 powder catalyst dispersed in neat C6H12 and in C6H12/CH2Cl2 mixtures. The composition of the mixed solvent has a strong influence on the selectivity of the process: an increase in the content of CH2Cl2 brings about both an enhancement in the rate of formation of mono-oxygenated products and a decrease in the production of CO2. At the same time, the alcohol to ketone ratio increases in the mixed solvent. An explanation of this behavior is proposed which is based on the observed decrease in the adsorption strength of intermediates (C6H11OH and radicals) as the solvent composition is varied from pure C6H12 to mixtures of it with increasing amounts of CH2Cl2. The results of experiments with different O2 partial pressures are reported. The process is unaffected for O2 partial pressures > 200 Torr. For lower values the formation of dicyclohexyl becomes significant and reaches a maximum at a pO2 of 60 T...

Abstract: Measurements of the thermoelectric power were applied to investigate the electrical conductivity in 5 mol % Sr-substituted LaPO 4 as a function of temperature (600 to 900°C), water vapor partial pressure (0.2 to 6 kPa), and oxygen partial pressure (4 to 100 kPa). Expressions of the thermoelectric power for materials conducting protons, oxide ions, and electronic defects were derived, based on a thermodynamic treatment of entropy production by heat and charge transfer. The experimental data for 5 mol % Sr-substituted LaPO 4 were interpreted in terms of protonic conduction, and some additional conduction, possibly by electron holes. In air, the proton transport number is unity at 600°C when p H2 O > 1 kPa, and at 700°C when p H2 O > 3 kPa. In wet atmospheres, the transported entropy of protons in this material was found to be nearly constant, 112 ± 2 J mol -1 K -1 at 600 to 800°C in wet atmospheres.

Abstract: To increase the rate or speed of formation of a thermal oxide film of a silicon carbide semiconductor device, the partial pressure of water vapor is controlled to within the range of 0.1 to 0.95 when a surface of silicon carbide is oxidized under a mixed atmosphere of water vapor and oxygen. In a pyrogenic oxidation method in which hydrogen and oxygen are introduced to perform thermal oxidation, the ratio of the flow rate of hydrogen to that of oxygen is controlled to within the range of 1:0.55 to 1:9.5. In another pyrogenic oxidation method in which hydrogen and oxygen are introduced to perform thermal oxidation, a large portion of an oxide film is formed while the ratio of the flow rate of hydrogen to that of oxygen is controlled to about 1:4.5, and a remaining portion of the oxide film is then formed while the ratio of the flow rate of hydrogen to that of oxygen is controlled to about 1:0.55.

Abstract: A method for maintaining the temperature of an oxygen-selective ion transport membrane within a desired temperature range includes providing an ion transport reactor with the oxygen-selective ion transport membrane. An oxygen-donating feed gas is delivered to a cathode side at a first temperature, at a first rate, and at a first oxygen partial pressure and a reactant gas is supplied to an anode side at a second temperature and a second rate. A physical condition is then established within the ion transport reactor that favors the transport of elemental oxygen through the oxygen selective ion transport membrane as oxygen ions. One or more process variables are then regulated to maintain the temperature of the oxygen selective ion transport membrane within the desired range.

Abstract: Contrasting behaviors are observed in InGaAs/GaAs island formation during vapor phase epitaxy: variation of group V partial pressures gives different critical thicknesses for the onset of the Stranski-Krastanow transformation, surface coverages, ratios between coherent and incoherent islands, and dissimilar morphologies upon annealing. The latter experiments show that small lens-shaped islands can be found in equilibrium if InGaAs surface energies are minimized, leading to the conclusion that AsH3 can raise surface energies and act as an impurity-free "morphactant."

Abstract: The transient reduction kinetics of surface rhodium oxide (Rh2O3) by gaseous H2 and CO have been probed in situ by surface-enhanced Raman spectroscopy (SERS). The Rh surfaces are ultrathin films electrodeposited onto SERS-active gold, enabling surface vibrational spectroscopic information to be obtained with high temporal resolution (≈1 s) at elevated temperatures (up to 500 °C) and under ambient-pressure flow-reactor conditions. Surface Rh2O3 is formed by heating Rh in flowing O2 at 1 atm and fingerprinted by an intense 530 cm-1 νRh-O feature. The reduction of such oxidized surfaces upon sudden exposure to either H2 or CO over a range of partial pressures (1−760 Torr) was monitored from the decay kinetics of the νRh-O band intensity. Surface oxide is readily reduced by both reductants, although striking differences in the observed kinetic behavior indicate the occurrence of distinct reaction pathways. In the case of H2, at low partial pressures (≤7.6 Torr) below 200 °C a temperature-dependent induction p...

Abstract: The absorption of dilute NOx gas was studied, at atmospheric pressure, in an industrial column packed with 25-mm plastic Pall rings The scrubbing liquid was a dilute hydrogen peroxide solution NOx removal efficiencies of 75−91% were obtained for gas mixtures with partial pressures of NOx in the range of 628−460 N/m2 A model was developed on the basis of equilibrium, chemical reactions, mass transfer in gas and liquid phases, and consideration of HNO2 and NO oxidation by H2O2 in the liquid phase The removal efficiencies calculated using this model fairly agreed with the experimental data The HNO2 formation in the gas phase largely determines the NOx absorption rate Its oxidation by H2O2 improves the NOx removal The influences of various parameters in the NOx removal efficiency are discussed from numerical simulations

Abstract: An automated system capable of accurate determinations of 0,, CO,, and N, partial pressures (p) in surface waters is described. The system is composed of a programmable data logger-controller, an infrared CO, analyzer, a polarographic 0, electrode, and a barometer linked to a submersed gas exchanger made of thin-walled (100 pm) silicone elastomer tubing. Oxygen electrode calibration and drift problems are eliminated by continuously referencing p0, measurements to atmospheric readings. pN, is calculated by subtracting p0, + pH,O + pC0, from total dissolved gas pressure. Complete (>99%) equilibration of internal gases with water pC0, occurs within 4 min. Equilibration half-times for p0, range from 7.1 min at 21°C to 9.4 min at 05°C. The equilibration half-time for pN, is 15.1 min at 15*C. Observations conducted in a small oligotrophic Shield Lake show that the epilimnion is usually supersaturated with both CO, (50-150 patm) and 0, (500-4,000 patm), with both gases exhibiting marked daily variations. The epilimnetic supersaturation (up to 2%) with respect to atmospheric 0, indicates that primary production slightly exceeds community respiration in the epilimnion. The system appears adequate for metabolic and gas flux studies, even in the most oligotrophic waters. Measurements of dissolved gas concentrations or partial pressures in surface waters are required in several fields of investigation in limnology and oceanography. Frequent needs for such measurements include the estimation of carbon fluxes across the air-water interface (Broecker and Peng 1984; Wanninkhof et al. 1987), the assessment of net ecosystem production (autotrophy) or respiration (heterotrophy) in lakes and rivers (Schurr and Ruchti 1977; Cole et al. 1994), and the monitoring of gas supersaturation and associated risks to aquatic life below reservoir spillways (Weitkamp and Katz 1980). These needs have stimulated a continuing interest in the design of automated systems for measuring dissolved gases in aquatic environments (Kelly et al. 1974; D’Aoust and Clarke 1980; DeGrandpre et al. 1995; Sellers et al. 1995). Determinations of the partial pressures, concentrations, and fluxes of CO, and 0, in surface waters can be used to document the production-respiration balance of lakes, rivers, and oceans at the ecosystem level. Gas fluxes (F) across the air-water interface can be estimated from differences in dissolved gas concentrations between surface water (C,) and air-saturated water (C,Y) using a relationship of the form F = K(C, - C,),

Abstract: In view of its potential application to high temperature heat energy storage, carbonation of CaO with various CO2 pressures is investigated in the temperature range 873–1173 K by using a thermogravimetric analyzer under atmospheric conditions. For a given temperature, although the reaction takes place for a partial pressure of CO2, P, higher than the equilibrium CO2 pressure of CaCO3, Pe, the reaction rate shows a slight increase with increasing P. On the other hand, for a given P, the rate increases rapidly with increasing temperature from 973 to 1023 K, above which it decreases gradually. By applying a grain model and assuming the presence of a reaction intermediate, a possible reaction rate equation has been developed.