GaN resistive hydrogen gas sensors

GaN epilayers grown by organometallic vapor phase epitaxy have been used to fabricate resistivegas sensors with a pair of planar ohmic contacts. Detectible sensitivity to H2 gas for a wide range of gas mixtures in an Ar ambient has been realized; the lowest concentration tested is ∼0.1% H2 (in Ar), well below the lower combustion limit in air. No saturation of the signal is observed up to 100% H2 flow. Real-time response to H2 shows a clear and sharp response with no memory effects during the ramping cycles of H2 concentration. The change in current at a fixed voltage to hydrogen was found to change with sensor geometry. This appears to be consistent with a surface-adsorption-induced change of conductivity; a detailed picture of the gas sensing mechanism requires further systematic studies

Eggshell catalyst and methods of its preparation

An eggshell catalyst useful for a Fischer-Tropsch (FT) synthesis or other reactions comprises a homogeneously dispersed transition metal and a promoter situated in an active phase in a precisely selected outer region of a catalyst pellet. The active phase region is controlled to a specific depth, which permits the control of the catalysts selectivity, for example, the size of the hydrocarbon chains formed in the FT process. A method of preparing these eggshell catalysts involves a non-aqueous synthesis where polar and non-polar solvents of relatively low vapor pressure are employed to define the depth of penetration of metal species in a refractory oxide substrate, which is followed by fixing and activating metallic catalytic species in the structure by calcination of the catalyst particles

Use of x-ray photoelectron spectroscopy and cyclic polarization to evaluate the corrosion behavior of six nickel-chromium alloys before and after porcelain-fused-to-metal firing

Statement of problem. Nickel-chromium casting alloys rely on a surface oxide layer for corrosion resistance to the oral environment. Porcelain-fused-to-metal (PFM) firing procedures may alter the surface oxides and corrosion properties of these alloys. Changes in alloy corrosion behavior affect metal ion release and therefore local and/or systemic tissue responses. Purpose. The aim of this study was to evaluate changes in alloy surface oxides and electrochemical corrosion properties after PFM firing. Material and methods. The electrochemical corrosion behavior of 6 commercial nickel-chromium alloys was evaluated in the as-cast/polished and PFM fired/repolished states. Surface chemistries of the alloys were analyzed by x-ray photoelectron spectroscopy. Results. Results indicated an increase in corrosion rates after PFM firing and repolishing for alloys containing 14% to 22% Cr and 9% to 17% Mo. This increase in corrosion rates was attributed to a decrease, caused by the PFM and repolishing process, in the Cr and Mo levels in the surface oxides of these alloys. The PFM firing and repolishing process did not alter the corrosion behavior of the alloys containing lower levels of Cr and Mo and/or Be additions in their bulk composition. These alloys exhibited low levels of Cr and Mo surface oxides in both test conditions. Si particles became embedded in the surfaces of the fired alloys during repolishing and may have contributed to the changes in surface oxides and the corrosion behavior of some alloys. Conclusion. The effects of PFM firing and repolishing on Ni-Cr dental casting alloy surface oxides and corrosion properties appear to be alloy dependent. (J Prosthet Dent 2000;84:623-34.)

Metallurgical, surface, and corrosion analysis of Ni-Cr dental casting alloys before and after porcelain firing

Objectives: A porcelain veneer is often fired on nickel-chromium casting alloys used in dental restorations for aesthetic purposes. The porcelain-fused-to-metal (PFM) process brings the temperature to over 950 °C and may change the alloy\u27s corrosion properties. In this study, the metallurgical, surface, and corrosion properties of two Ni-Cr alloys were examined, before and after PFM firing. Methods: Two types of alloy were tested-a high Cr, Mo alloy without Be and a low Cr, Mo alloy with Be. Before the PFM firing, specimens from both alloys were examined for their microstructures, hardness, electrochemical corrosion properties, surface composition, and metal ion release. After the PFM firing, the same specimens were again examined for the same properties. Results: Neither of the alloys showed any differences in their electrochemical corrosion properties after the PFM firing. However, both alloys exhibited new phases in their microstructure and significant changes in hardness after firing. In addition, there was a slight increase in CrOx on the surface of the Be-free alloy and increased Mo-Ni was observed on the surface of both alloys via X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). This might be one of the reasons why both alloys had increased Ni and Mo ion release after firing. Significance: The PFM firing process changed the alloys\u27 hardness, microstructure, and surface composition. No significant changes in the alloys\u27 corrosion behavior were observed, however, the significant increase in metal ion release over a month may need to be further investigated for its clinical effects. © 2007 Academy of Dental Materials

Study of surface chemical changes and erosion rates for CV-1144-O silicone under electron cyclotron resonance oxygen plasma exposure

CV-1144-O silicone thin films were irradiated in an electron cyclotron resonance oxygen plasma, which is a simulation of the low earth orbital environment. A crude equivalence between this plasma system and the low earth orbital environment was determined by measuring Kapton weight loss in the plasma and comparing to Kapton weight loss in space experiments. Changes in optical properties and erosion rates under ultraviolet light and atomic oxygen radiation were studied using in situ spectroscopic ellipsometry (SE). The erosion rate at the beginning of the plasma exposure was significantly faster than that at later stages. Approximately one third of the total silicone thickness was etched away within 1 h, which according to the equivalence experiment, corresponds to about two months in low earth orbit. The refractive index of silicone in the visible range increased during the exposure, indicating that the film was being densified. Optical constants (both before and after plasma exposure) were determined by ex situ spectroscopic ellipsometry in the ultraviolet– visible–near-infrared (0.7–8.5 eV) and IR (200–7000 cm-1) ranges. Also, SE was used to map thickness and uniformity before and after radiation. Regression fits using Lorentzian and Gaussian oscillators as parametric models for the optical constants were excellent, and the major absorption peaks in the IR region were identified. The before- and after-radiation spectra showed significant decreases in CH3-associated peaks and increases in SiOx-associated peaks