Profiling Gene Expression to Distinguish the Likely Active Diazotrophs from a Sea of Genetic Potential in Marine Sediments

Nitrogen (N) cycling microbial communities in marine sediments are extremely diverse, and it is unknown whether this diversity reflects extensive functional redundancy. Sedimentary denitrifiers remove significant amounts of N from the coastal ocean and diazotrophs are typically regarded as inconsequential. Recently, N fixation has been shown to be a potentially important source of N in estuarine and continental shelf sediments. Analysis of expressed genes for nitrite reductase (nirS) and a nitrogenase subunit (nifH) was used to identify the likely active denitrifiers and nitrogen fixers in surface sediments from different seasons in Narragansett Bay (Rhode Island, USA). The overall diversity of diazotrophs expressing nifH decreased along the estuarine gradient from the estuarine head to an offshore continental shelf site. Two groups of sequences related to anaerobic sulphur/iron reducers and sulphate reducers dominated libraries of expressed nifH genes. Quantitative polymerase chain reaction (qPCR) and quantitative reverse transcription polymerase chain reaction (qRT-PCR) data shows the highest abundance of both groups at a mid bay site, and the highest nifH expression at the head of the estuary, regardless of season. Several potential environmental factors, including water temperature, oxygen concentration and metal contamination, may influence the abundance and nifH expression of these two bacterial groups

Efficiency-improvement study for GaAs solar cells. Final report, March 31, 1980-September 30, 1981

High-yield fabrication of good quality AlGaAs/GaAs concentration solar cells has been a limiting factor in widespread utilization of these high conversion efficiency (22 to 24%) photovoltaic cells. Reported is a series of investigations to correlate solar cell yield with substrate quality, growth techniques, layer composition, and metallization processes. In addition, several diagnostic techniques are described to aid in device characterization

Advanced photovoltaic concentrator cells. Quarterly technical progress report No. 1, August 28, 1979-November 30, 1979

First quarter activities for a project aimed at demonstrating the technical feasibility of advanced high efficiency concentrator solar converters are described. The goal of the program is to achieve 30% conversion efficiency with a converter operating at 30/sup 0/C under 500 to 1000 SUN AM2 illumination and 25% conversion efficiency with a converter operating at 150/sup 0/C under 500 to 1000 SUN AM2 illumination. The approach is to fabricate two cell, non-lattice matched, monolithic stacked converters using optimum pairs of cells having bandgaps in the range of 1.6 to 1.7 eV and 0.95 to 1.1 eV. The high bandgap cells are to be fabricated using MOCVD or LPE to produce the needed AlGaAs layers of optimized composition, thickness and doping to produce high performance, heteroface homojunction devices, The low bandgap cells are to be similarly fabricated from AlGaSb(As) compositions by LPE. These subcells are then to be joined into a monolithic structure by an appropriate thermal bonding technique which will also form the needed transparent intercell ohmic contact (IOC) between the two subcells. The activities this quarter have been largely focused on the development and study of low bandgap cell structures and attempts to develop suitable techniques for the thermal bonding operation

Advanced photovoltaic concentrator cells. Quarterly technical progress report No. 2, 1 December 1979-29 February 1980

This report describes second quarter activities for a project aimed at demonstrating the technical feasibility of advanced high efficiency concentrator solar converters. The goal of the program is to achieve 30% conversion efficiency with a converter operating at 30/sup 0/C under 500 to 1000 SUN AM2 illumination and 25% conversion efficiency with a converter operating at 150/sup 0/C under 500 to 1000 SUN AM2 illumination. The approach is to fabricate two cell, non-lattice matched, monolithic stacked converters using optimum pairs of cells having bandgaps in the range of 1.6 to 1.7 eV and 0.95 to 1.1 eV. The high bandgap cells are to be fabricated using MOCVD or LPE to produce the needed AlGaAs layers of optimized composition, thickness and doping to produce high performance, heteroface homojunction devices. The low bandgap cells are to be similarly fabricated from AlGaSb(As) compositions by LPE. These subcells are then to be joined into a monolithic structure by an appropriate thermal bonding technique which will also form the needed transparent intercell ohmic contact (IOC) between the two subcells. The activities this quarter have been largely focused on the development and study of low bandgap cell structures and attempts to develop suitable techniques for the thermal bonding operation

Effects of testing and storage environments on mechanical properties of Ni-plated and bare U-3/4 wt% Ti

It was found that storage environments with an adequate supply of oxygen can effectively minimize moisture corrosion of bare U-3/4 Ti. In particular, 0.75 cm/sup 3/ of dry air is calculated to protect 1 cm/sup 2/ of U-3/4 Ti for 20 years storage at room temperature. Consideration of the geometric details of U-3/4 Ti alloy specimens and the free volumes of air (and hence O/sub 2/) available can satisfactorily explain discrepancies in corrosion behavior between recent tests and previously reported data. The storage environment at 70/sup 0/C produces a minor strength increase in bare samples with increasing time. Decreases in ductility are observed for testing conditions of low temperature, low strain rate, and/or high humidity. Surface cracks occur under the same conditions conducive to corrosion, i.e., moderate temperatures, low strain rates, and high humidity. Significant increases in strength result under low-temperature and high-strain-rate conditions of tensile testing. Residual chloride contamination may be responsible for the occasional and otherwise unexplained large scatter in ductility for nominally similar specimens and test conditions. Nickel plating is observed to cause a statistically significant decrease in tensile strength, but no effect on the yield strength or ductility was observed and the presence of high explosive during the aging of tensile bars was observed to have no effect on mechanical properties