Silicon nanowires as efficient thermoelectric materials

Thermoelectric materials interconvert thermal gradients and electric fields for power generation or for refrigeration. Thermoelectrics currently find only niche applications because of their limited efficiency, which is measured by the dimensionless parameter ZT-a function of the Seebeck coefficient or thermoelectric power, and of the electrical and thermal conductivities. Maximizing ZT is challenging because optimizing one physical parameter often adversely affects another. Several groups have achieved significant improvements in ZT through multi-component nanostructured thermoelectrics, such as Bi(2)Te(3)/Sb(2)Te(3) thin-film superlattices, or embedded PbSeTe quantum dot superlattices. Here we report efficient thermoelectric performance from the single-component system of silicon nanowires for cross-sectional areas of 10 nm x 20 nm and 20 nm x 20 nm. By varying the nanowire size and impurity doping levels, ZT values representing an approximately 100-fold improvement over bulk Si are achieved over a broad temperature range, including ZT approximately 1 at 200 K. Independent measurements of the Seebeck coefficient, the electrical conductivity and the thermal conductivity, combined with theory, indicate that the improved efficiency originates from phonon effects. These results are expected to apply to other classes of semiconductor nanomaterials.     

Brood parasitism and nest survival of Brown-headed Cowbird hosts at high-elevation riparian sites in the eastern Sierra Nevada, California

Deciduous riparian ecosystems in the western United States provide habitat for a higher density of breeding birds than reported for any other avian habitat type and provide habitat for more breeding bird species than adjacent uplands. On the east slope of the Sierra Nevada, riparian ecosystems make up Molothrus ater )—an obligate brood parasite that forages on bare ground and feedlots but typically commutes to distinct shrubland or woodland habitats for breeding. We examined nest survival, brood parasitism, breeding phenology, and causes of nest failure for birds at North Lake and Rock Creek: 2 high-elevation (>2500 m) riparian breeding habitats adjacent to recreational development and within cowbird commuting distance to additional potential foraging sites. Nest survival tended to be higher for host species at Rock Creek than for those at North Lake, but parasitism rates were not significantly different between plots. Of 21 open-cup nesting species, 12 were parasitized. We found the highest rate of parasitism (92%) for Warbling Vireos ( Vireo gilvus ) at North Lake, and parasitism contributed to lower total nest survival there (14%). For nearly all species, parasitized nests were less successful and produced fewer young than nonparasitized nests. However, predation was the leading cause of complete nest failure across all species and contributed to the lowest total nest survival estimates for Western Wood-Pewees ( Contopus sordidulus, 11%) and Dusky Flycatchers ( Empidonax oberholseri, 15%) at North Lake and for Dark-eyed Juncos ( Junco hyemalis, 15%) at Rock Creek. Nest survival was relatively high for Western Wood-Pewees (41%) at Rock Creek and for Yellow Warblers ( Dendroica petechia, 47%) at North Lake. We noted whether the arrival of pack animals at pack-station corrals contributed to variation in cowbird numbers at corrals or in parasitism rates at the 2 sites. Cowbirds occupied corrals before and after pack-stock arrival, and most host clutches were completed prior to pack-stock arrival at nearby corrals, suggesting that the presence of pack animals did not directly affect cowbird host species.