42,600 research outputs found
Improved first order interpolator
Data compression method enables first order interpolator to operate at higher speeds. Method requires same number of additions and subtractions but fewer multiplications than the conventional method
Methane Beneath Greenlands Ice Sheet Is Being Released
Methane is a potent greenhouse gas, but a complete accounting of global methane sources and sinks is still ongoing. Sediments beneath glaciers and ice sheets harbour carbon reserves that, under certain conditions, can be converted to methane. However, the formation and release of such methane is an unquantified component of the arctic methane budget. Lamarche-Gagnon et al. present direct measurements of dissolved methane in proglacial discharge from a land-terminating glacier of the Greenland Ice Sheet. This proglacial discharge was supersaturated with methane and had diffusive methane fluxes similar to other terrestrial rivers for the duration of the summer measurement period. Their results suggest that variability in proglacial discharge is associated with methane release from subglacial environments, implicating both the form and evolution of the subglacial hydrologic system as a newly discovered control in the arctic methane cycle.The study by Lamarche-Gagnon et al. is an important example of how the cryosphere can interact with the surrounding Earth system in unexpected and potentially significant ways. Characterizing the ability of subglacial sediments to convert and store methane and the ability of the subglacial hydrologic system to export this methane to the atmosphere, through both modeling and observational studies, are critical steps in improving our knowledge of the sources and sinks of arctic methane and better constraining their future changes
Volume Fraction Determination in Cast Superalloys and DS Eutectic Alloys by a New Practice for Manual Point Counting
Volume fraction of a constituent or phase was estimated in six specimens of conventional and DS-eutectic superalloys, using ASTM E562-76, a new standard recommended practice for determining volume fraction by systematic manual point count. Volume fractions determined ranged from 0.086 to 0.36, and with one exception, the 95 percent relative confidence limits were approximately 10 percent of the determined volume fractions. Since the confidence-limit goal of 10 percent, which had been arbitrarily chosen previously, was achieved in all but one case, this application of the new practice was considered successful
The Geometry of Niggli Reduction I: The Boundary Polytopes of the Niggli Cone
Correct identification of the Bravais lattice of a crystal is an important
step in structure solution. Niggli reduction is a commonly used technique. We
investigate the boundary polytopes of the Niggli-reduced cone in the
six-dimensional space G6 by algebraic analysis and organized random probing of
regions near 1- through 8-fold boundary polytope intersections. We limit
consideration of boundary polytopes to those avoiding the mathematically
interesting but crystallographically impossible cases of 0 length cell edges.
Combinations of boundary polytopes without a valid intersection in the closure
of the Niggli cone or with an intersection that would force a cell edge to 0 or
without neighboring probe points are eliminated. 216 boundary polytopes are
found: 15 5-D boundary polytopes of the full G6 Niggli cone, 53 4-D boundary
polytopes resulting from intersections of pairs of the 15 5-D boundary
polytopes, 79 3-D boundary polytopes resulting from 2-fold, 3-fold and 4-fold
intersections of the 15 5-D boundary polytopes, 55 2-D boundary polytopes
resulting from 2-fold, 3-fold, 4-fold and higher intersections of the 15 5-D
boundary polytopes, 14 1-D boundary polytopes resulting from 3-fold and higher
intersections of the 15 5-D boundary polytopes. All primitive lattice types can
be represented as combinations of the 15 5-D boundary polytopes. All
non-primitive lattice types can be represented as combinations of the 15 5-D
boundary polytopes and of the 7 special-position subspaces of the 5-D boundary
polytopes. This study provides a new, simpler and arguably more intuitive basis
set for the classification of lattice characters and helps to illuminate some
of the complexities in Bravais lattice identification. The classification is
intended to help in organizing database searches and in understanding which
lattice symmetries are "close" to a given experimentally determined cell
The Geometry of Niggli Reduction II: BGAOL -- Embedding Niggli Reduction
Niggli reduction can be viewed as a series of operations in a six-dimensional
space derived from the metric tensor. An implicit embedding of the space of
Niggli-reduced cells in a higher dimensional space to facilitate calculation of
distances between cells is described. This distance metric is used to create a
program, BGAOL, for Bravais lattice determination. Results from BGAOL are
compared to the results from other metric-based Bravais lattice determination
algorithms
Application of the SEM to the measurement of solar cell parameters
Techniques are described which make use of the SEM to measure the minority carrier diffusion length and the metallurgical junction depth in silicon solar cells. The former technique permits the measurement of the true bulk diffusion length through the application of highly doped field layers to the back surfaces of the cells being investigated. It is shown that the secondary emission contrast observed in the SEM on a reverse-biased diode can depict the location of the metallurgical junction if the diode has been prepared with the proper beveled geometry. The SEM provides the required contrast and the option of high magnification, permitting the measurement of extremely shallow junction depths
Observation of hypersonic phononic crystal effects in porous silicon superlattices
Brillouin light scattering experiments were carried out on porous silicon superlattices with modulation wavelengths in the range 37–167 nm. Phonon frequencies deduced from the Brillouin spectra show good agreement with those obtained from an elastic continuum model for a system with one-dimensional periodicity. Evidence for the existence of a hypersonic phononic bandgap and zone-folded longitudinal acoustic phonons is reported
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