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The electrical characteristics of a hydrogen sensor utilizing a metal/AlN/Si(111) structure, which behaves similarly to a metal-insulator-semiconductor-type capacitor, have been investigated. The AlN layer, which acts as the insulator, was deposited by plasma source molecular-beam epitaxy. The device employed alternating gates of Pd and Al, and an Al back contact, all grown by magnetron sputtering. The Pd gate structures were found to be sensitive to hydrogen, down to parts per million concentrations in the surrounding flow, while the Al gate structures showed no response to hydrogen. Both structures show similar capacitance versus voltage as well as conductance versus voltage curves. Therefore the combination of Al and Pd gates can be utilized in a balanced hydrogen sensor structure. Furthermore, the frequency dependence of the capacitance and conductance curves clearly indicated the presence of series resistance effects, but with multiple time constants, requiring a complex equivalent circuit representation. Evidence was found for the presence of carrier traps at the AlN/Si interface, with energy levels distributed throughout the silicon band gap, and with an estimated surface density ranging between 8×10[sup 14] and 1.5×10[sup 15] m[sup -2] eV[sup -1]. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Methylesters and ethylesters of fatty acids were synthesized using homogeneous CH(3)ONa and CH(3)CH(2)ONa, anion exchanged resin, and CaO-La(2)O(3) catalysts. Methanol, ethanol, and methanol/ethanol mixtures were used as the alcohol feed for transesterification of soybean oil. With a homogeneous catalyst (CH(3)ONa) there was essentially no difference in conversion rates between methanolysis and ethanolysis in batch reactions. However, with a heterogeneous resin and CaO-La(2)O(3) catalysts, significant differences in the conversion rates between the methanolysis and ethanolysis were observed. The formation rate of methylesters over a CaO-La(2)O(3) catalyst was higher than that of ethylesters, which may be attributable to a steric hindrance effect. Conversely, with a heterogeneous resin catalyst, the conversion rate of ethylester was higher than that of methylesters which may be attributable to the surface hydrophobicity of the anion exchanged resin. When the transesterification of soybean oil was carried out with an equimolar methanol/ethanol mixture, the yield ratio of methylester to ethylester formed within the first 30 min was 2.6 for the homogeneous catalyst (0.3% CH(3)ONa), and 3.4 for the heterogeneous CaO-La(2)O(3)catalyst. These differences in selectivity are likely due to both the higher reactivity of methoxide and to a steric hindrance effect of ethoxide on the catalyst surface. In addition, the transformation of methylester to ethylester was observed when a methanol/ethanol mixture was used., ((c) 2010. Published by Elsevier Ltd. All rights reserved.)