12,506 research outputs found
On the nature of pressureāinduced coordination changes in silicate melts and glasses
Progressive decreases in the SiāOāSi angles between cornerāshared silicate tetrahedra in glasses and melts with increasing pressure can lead to arrangements of oxygen atoms that can be described in terms of edgeā or faceāshared octahedra. This mechanism of compression can account for the gradual, continuous increases in melt and glass densities from values at low pressure that indicate dominantly tetrahedral coordination of Si to values at several tens of GPa that suggest higher coordination. It also can explain the unquenchable nature of octahedrally coordinated Si in glasses, the absence of spectroscopically detectable octahedrally coordinated Si in glasses until they are highly compressed, the gradual and reversible transformation from tetrahedral to octahedral coordination in glasses once the transformation is detectable spectroscopically, and the fact that this transformation takes place in glass at room temperature. It may also have relevance to pressureāinduced transformations from crystalline to glassy phases, the difficulty in retrieving some metastable high pressure crystalline phases at low pressure, and the observed differences between the pressures required for phase transformations in shock wave experiments on glasses and crystals
Thermal analysis of conceptual designs for GPHS/FPSE power systems of 250 We and 500 We
Thermal analyses were performed for two distinct configurations of a proposed space nuclear power system which combines General Purpose Heat Source (GPHS) modules with the state of the art Free-Piston Stirling Engines (FPSEs). The two configurations correspond to systems with power levels of 250 and 500 W(sub e). The 250 W(sub e) GPHS/FPSE power system utilizes four GPHS modules and one FPSE, and the 500 W(sub e) contains eight GPHS modules and two FPSEs. The configurations of the systems and the bases for selecting the configurations are described. Brief introductory sections are included to describe the GPHS modules and free piston Stirling engines. The primary focus of the thermal analyses is on the temperature of the iridium fuel clad within the GPHS modules. A design goal temperature of 1573 K was selected as the upper limit for the fuel clad during normal operating conditions. The basis for selecting this temperature limit is discussed in detail. Results obtained from thermal analysis of the 250 W(sub e) GPHS/FPSE power system indicate fuel clad temperatures which slightly exceed the design goal temperature of 1573 K. The results are considered favorable due to the numerous conservative assumptions used in developing the thermal model and performing the thermal analysis. To demonstrate the effects of the conservatism, a brief sensitivity analysis is performed in which a few of the key system parameters are varied to determine their effect on the fuel clad temperatures. It is concluded that thermal analysis of a more detailed thermal model would be expected to yield fuel clad temperatures below the design foal temperature limiy 1573 K
Biodiversity's big wet secret: the global distribution of marine biological records reveals chronic under-exploration of the deep pelagic ocean
Background: Understanding the distribution of marine biodiversity is a crucial first step towards the effective and sustainable management of marine ecosystems. Recent efforts to collate location records from marine surveys enable us to assemble a global picture of recorded marine biodiversity. They also effectively highlight gaps in our knowledge of particular marine regions. In particular, the deep pelagic ocean - the largest biome on Earth - is chronically under-represented in global databases of marine biodiversity.
Methodology/Principal Findings: We use data from the Ocean Biogeographic Information System to plot the position in the water column of ca 7 million records of marine species occurrences. Records from relatively shallow waters dominate this global picture of recorded marine biodiversity. In addition, standardising the number of records from regions of the ocean differing in depth reveals that regardless of ocean depth, most records come either from surface waters or the sea bed. Midwater biodiversity is drastically under-represented.
Conclusions/Significance: The deep pelagic ocean is the largest habitat by volume on Earth, yet it remains biodiversity's big wet secret, as it is hugely under-represented in global databases of marine biological records. Given both its value in the provision of a range of ecosystem services, and its vulnerability to threats including overfishing and climate change, there is a pressing need to increase our knowledge of Earth's largest ecosystem
Numerical modeling tools for chemical vapor deposition
Development of general numerical simulation tools for chemical vapor deposition (CVD) was the objective of this study. Physical models of important CVD phenomena were developed and implemented into the commercial computational fluid dynamics software FLUENT. The resulting software can address general geometries as well as the most important phenomena occurring with CVD reactors: fluid flow patterns, temperature and chemical species distribution, gas phase and surface deposition. The physical models are documented which are available and examples are provided of CVD simulation capabilities
Deep drilling into a Hawaiian volcano
Hawaiian volcanoes are the most comprehensively
studied on Earth. Nevertheless, most
of the eruptive history of each one is inaccessible
because it is buried by younger lava
flows or is exposed only below sea level. For
those parts of Hawaiian volcanoes above sea
level, erosion typically exposes only a few
hundred meters of buried lavas (out of a total
thickness of up to 10 km or more).Available
samples of submarine lavas extend the time
intervals of individual volcanoes that can be
studied. However, the histories of individual
Hawaiian volcanoes during most of their ~1-million-year passages across the zone of melt
production are largely unknown
Angular and Polarization Response of Multimode Sensors with Resistive-Grid Absorbers
High sensitivity receiver systems with near ideal polarization sensitivity
are highly desirable for development of millimeter and sub-millimeter radio
astronomy. Multimoded bolometers provide a unique solution to achieve such
sensitivity, for which hundreds of single-mode sensors would otherwise be
required. The primary concern in employing such multimoded sensors for
polarimetery is the control of the polarization systematics. In this paper, we
examine the angular- and polarization- dependent absorption pattern of a thin
resistive grid or membrane, which models an absorber used for a multimoded
bolometer. The result shows that a freestanding thin resistive absorber with a
surface resistivity of \eta/2, where \eta\ is the impedance of free space,
attains a beam pattern with equal E- and H-plane responses, leading to zero
cross polarization. For a resistive-grid absorber, the condition is met when a
pair of grids is positioned orthogonal to each other and both have a
resistivity of \eta/2. When a reflective backshort termination is employed to
improve absorption efficiency, the cross-polar level can be suppressed below
-30 dB if acceptance angle of the sensor is limited to <60degrees. The small
cross-polar systematics have even-parity patterns and do not contaminate the
measurements of odd-parity polarization patterns, for which many of recent
instruments for cosmic microwave background are designed. Underlying symmetry
that suppresses these cross-polar systematics is discussed in detail. The
estimates and formalism provided in this paper offer key tools in the design
consideration of the instruments using the multimoded polarimeters.Comment: 22 pages, 15 figure
Asynchronous multiple-access channel capacity
The capacity region for the discrete memoryless multiple-access channel without time synchronization at the transmitters and receivers is shown to be the same as the known capacity region for the ordinary multiple-access channel. The proof utilizes time sharing of two optimal codes for the ordinary multiple-access channel and uses maximum likelihood decoding over shifts of the hypothesized transmitter words
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