Optimizing the performance of thermionic devices using energy filtering

Conventional thermionic power generators and refrigerators utilize a barrier in the direction of transport to selectively transmit high-energy electrons. Here we show that the energy spectrum of electrons transmitted in this way is not optimal, and we derive the ideal energy spectrum for operation in the maximum power regime. By using suitable energy filters, such as resonances in quantum dots, the power of thermionic devices can, in principle, be improved by an order of magnitude.Comment: 3 pages, 2 figure

Sprayable birefringent coating enables strain measurements on large surfaces

Birefringent coating for strain measurements on large surfaces contains constituents that can be premixed and sprayed as a single component with conventional paint spray equipment. Elevated temperatures are not required for spraying or curing of the coating material which has long pot life

A new comparison between solid-state thermionics and thermoelectrics

It is shown that equations for electrical current in solid-state thermionic and thermoelectric devices converge for devices with a width equal to the mean free path of electrons, yielding a common expression for intensive electronic efficiency in the two types of devices. This result is used to demonstrate that the materials parameters for thermionic and thermoelectric devices are equal, rather than differing by a multiplicative factor as previously thought

Concept study for a high-efficiency nanowire-based thermoelectric

Materials capable of highly efficient, direct thermal-to-electric energy conversion would have substantial economic potential. Theory predicts that thermoelectric efficiencies approaching the Carnot limit can be achieved at low temperatures in one-dimensional conductors that contain an energy filter such as a double-barrier resonant tunneling structure. The recent advances in growth techniques suggest that such devices can now be realized in heterostructured, semiconductor nanowires. Here we propose specific structural parameters for InAs/InP nanowires that may allow the experimental observation of near-Carnot efficient thermoelectric energy conversion in a single nanowire at low temperature

A Chandra View of the Normal SO Galaxy NGC 1332: II: Solar Abundances in the Hot Gas and Implications for SN Enrichment

We present spectral analysis of the diffuse emission in the normal, isolated, moderate-Lx S0 NGC 1332, constraining both the temperature profile and the metal abundances in the ISM. The characteristics of the point source population and the gravitating matter are discussed in two companion papers. The diffuse emission comprises hot gas, with an ~isothermal temperature profile (~0.5 keV), and emission from unresolved point-sources. In contrast with the cool cores of many groups and clusters, we find a small central temperature peak. We obtain emission-weighted abundance contraints within 20 kpc for several key elements: Fe, O, Ne, Mg and Si. The measured iron abundance (Z_Fe=1.1 in solar units; >0.53 at 99% confidence) strongly excludes the very sub-solar values often historically reported for early-type galaxies but agrees with recent observations of brighter galaxies and groups. The abundance ratios, with respect to Fe, of the other elements were also found to be ~solar, although Z_o/Z_Fe was significantly lower (<0.4). Such a low O abundance is not predicted by simple models of ISM enrichment by Type Ia and Type II supernovae, and may indicate a significant contribution from primordial hypernovae. Revisiting Chandra observations of the moderate-Lx, isolated elliptical NGC 720, we obtain similar abundance constraints. Adopting standard SNIa and SNII metal yields, our abundance ratio constraints imply 73+/-5% and 85+/-6% of the Fe enrichment in NGC 1332 and NGC 720, respectively, arises from SNIa. Although these results are sensitive to the considerable systematic uncertainty in the SNe yields, they are in good agreement with observations of more massive systems. These two moderate-Lx early-type galaxies reveal a consistent pattern of metal enrichment from cluster scales to moderate Lx/Lb galaxies. (abridged)Comment: 12 pages, 4 figures, accepted for publication in ApJ. Minor changes to match published versio

Temperature distribution and thermal anomalies along a flowline of the Greenland ice sheet

Englacial and basal temperature data for the Greenland ice sheet (GrIS) are sparse and mostly limited to deep interior sites and ice streams, providing an incomplete representation of the thermal state of ice within the ablation zone. Here we present 11 temperature profiles at five sites along a 34km east-west transect of West Greenland. These profiles depict ice temperatures along a flowline and local temperature variations between closely spaced boreholes. A temperate basal layer is present in all profiles, increasing in thickness in the flow direction, where it expands from about 3% of ice height furthest inland to 100% at the margin. Temperate thickness growth is inconsistent with modeled heat contributions from strain heating, heat conduction, and vertical extension of the temperate layer. We suggest that basal crevassing, facilitated by water pressures at or near ice overburden pressure, is responsible for the large temperate ice thicknesses observed. High-temperature kinks at 51-85m depth are likely remnants from the thermal influence of partially water-filled crevasses up ice sheet. Steep horizontal temperature gradients between closely grouped boreholes suggest the recent thermal influence of a moulin. These profiles demonstrate the ability of meltwater to rapidly alter ice temperatures at all depths within the ablation zone

Thermal tracking of meltwater retention in Greenland’s accumulation area

Poorly understood processes controlling retention of meltwater in snow and firn have important implications for Greenland Ice Sheet’s mass balance and flow dynamics. Here we present results from a 3 year (2007-2009) field campaign studying firn thermal profiles and density structure along an 85 km transect of the percolation zone of west Greenland. We installed one or two thermistor strings at 14 study sites, each string having 32 sensors spaced between 0 and 10 m depth. Data from our network of over 500 sensors were collected at 15-60 min intervals for 1-2 years, thereby recording the thermal signature of meltwater infiltration and refreezing during annual melt cycles. We document three types of heating of firn related to different mechanisms of meltwater motion and freezing, including heterogeneous breakthrough events, wetting front advance, and year-round heating from freezing of residual deep pore water. Vertically infiltrating meltwater commonly penetrates through cold firn accumulated over decades, even where ice layers are present at the previous summer surface and where ice layer thickness exceeds several decimeters. The offset between the mean annual air temperature and the 10 m firn temperature reveals the elevation dependency of meltwater retention along our transect. The firn is less than 10 degrees C warmer than the mean annual air temperature at the region where meltwater runoff initiates. During 2007-2009, runoff was limited to elevations lower than about 1500 m with no sharp runoff limit ; rather, the ratio of retention to runoff transitioned from all retention to all runoff across an about 20 km wide zone

An industrial application of the JPL ACTS with energy recovery

The JPL Activated Carbon Treatment System (ACTS) uses sewage solids derived from municipal wastewater treatment systems as a source of organic material for powdered activated carbons (PAC). The PAC is used for the COD removal from wastewater and as a filter aid in the recovery of additional sewage solids

Mapping subglacial surfaces of temperate valley glaciers by two-pass migration of a radio-echo sounding survey

High-resolution maps of the glacier bed are developed through a pseudo-three-dimensional migration of a dense array of radio-echo sounding profiles. Resolution of three-dimensional maps of sub-glacial surfaces is determined by the radio-echo sounding wavelength, data spacing in the field, and migration. Based on synthetic radio-echo sounding profile experiments, the maximum resolution of the final map cannot exceed one half-wavelength. A methodology of field and processing techniques is outlined to develop a maximum-resolution map of the glacier bed. The field and processing techniques valley glacier in south-central Alaska. The field techniques and the processing steps used on the glacier result in a map of 20 m x 20 m resolution