5,278 research outputs found
An Analysis of Limited Maritime War
Limited maritime war is a promising area for investigation for two reasons. First, it is one of the two major nonstrategic-nuclear war scenarios upon which the naval planning of the United States is based
Joining of thirty three percent by weight random glass fibre reinforced polystyrene using variable frequency microwave
[Abstract]: This paper extends the range of applications for Variable Frequency Microwave (VFM) (2 ā 18 GHz) facilities to joining thirty three percent by weight glass fibre reinforced polystyrene composite [PS/GF (33%)]. With a given power level, the composite was exposed to various exposure times to microwave irradiation. The primer or coupling agent used was 5-minute two-part adhesive containing 100% liquid epoxy and 8% amine, i.e. Araldite, which was more readily microwave reactive than the composite itself. Bond strengths of the joints were lap shear tested and results were compared with those obtained using fixed frequency (2.45 GHz) microwave processing. The VFMF was operated under software control, which provided automatic data logging facilities. The maximum lap shear bond strength of joint was 430 N/cm2 using variable frequency microwave facility while that obtained by fixed frequency microwave configuration was only 331 N/cm2. The former is nearly 30% stronger than the latter
Permittivity measurement of thermoplastic composites at elevated temperature
[Abstract]: The material properties of greatest importance in microwave processing of a dielectric are the complex relative permittivity Epsilon = Epsilon' - jEpsilon'', and the loss tangent, tan Delta = Epsilon'/Epsilon''. This paper describes two convenient laboratory based methods to obtain Epsilon', Epsilon'' and hence tan Delta of fibre-reinforced thermoplastic (FRTP) composites. One method employs a microwave network analyser in conjunction with a waveguide transmission technique, chosen because it provides the widest possible frequency range with high accuracy. The values of the dielectric constant and dielectric loss of glass fibre reinforced (33%) low density polyethylene, LDPE/GF (33%), polystyrene, PS/GF (33%), and Nylon 66/GF (33%), were obtained. Results are compared with those obtained by another method using a high-temperature dielectric probe
Probing the size and density of silicon nanocrystals in nanocrystal memory device applications
Structural characterization via transmission electron microscopy and atomic force microscopy of arrays of small Si nanocrystals embedded in SiO2, important to many device applications, is usually difficult and fails to correctly resolve nanocrystal size and density. We demonstrate that scanning tunneling microscopy (STM) imaging enables a much more accurate measurement of the ensemble size distribution and array density for small Si nanocrystals in SiO2, estimated to be 2-3 nm and 4 x 10^(12) - 3 x 10^(13) cm^(-2), respectively, in this study. The reflection high energy electron diffraction pattern further verifies the existence of nanocrystallites in SiO2. The present STM results enable nanocrystal charging characteristics to be more clearly understood: we find the nanocrystal charging measurements to be consistent with single charge storage on individual Si nanocrystals. Both electron tunneling and hole tunneling processes are suggested to explain the asymmetric charging/ discharging processes as a function of bias
Design approaches and materials processes for ultrahigh efficiency lattice mismatched multi-junction solar cells
In this study, we report synthesis of large area
(>2cm^2), crack-free GaAs and GaInP double
heterostructures grown in a multi-junction solar cell-like
structure by MOCVD. Initial solar cell data are also
reported for GaInP top cells. These samples were grown
on Ge/Si templates fabricated using wafer bonding and ion
implantation induced layer transfer techniques. The double
heterostructures exhibit radiative emission with uniform
intensity and wavelength in regions not containing
interfacial bubble defects. The minority carrier lifetime of
~1ns was estimated from photoluminescence decay
measurements in both double heterostructures.
We also report on the structural characteristics of
heterostructures, determined via atomic force microscopy
and transmission electron microscopy, and correlate these
characteristics to the spatial variation of the minority
carrier lifetime
Ab initio Potential-Energy Surfaces and Electron-Spin-Exchange Cross Sections for H-O2 Interactions
Accurate quartet- and doublet-state potential-energy surfaces for the interaction of a hydrogen atom and an oxygen molecule in their ground states have been determined from an ab initio calculation using large-basis sets and the internally contracted multireference configuration interaction method. These potential surfaces have been used to calculate the H-O2 electron-spin-exchange cross section; the square root of the cross section (in a(sub 0)), not taking into account inelastic effects, can be obtained approximately from the expressions 2.390E(sup -1/6) and 5.266-0.708 log10(E) at low and high collision energies E (in E(sub h)), respectively. These functional forms, as well as the oscillatory structure of the cross section found at high energies, are expected from the nature of the interaction energy. The mean cross section (the cross section averaged over a Maxwellian velocity distribution) agrees reasonably well with the results of measurements
Hydrogen-evolution characteristics of NiāMo-coated, radial junction, n+p-silicon microwire array photocathodes
The photocathodic H_2-evolution performance of NiāMo-coated radial n+p junction Si microwire (Si MW) arrays has been evaluated on the basis of thermodynamic energy-conversion efficiency as well as solar cell figures of merit. The NiāMo-coated n^(+)p-Si MW electrodes yielded open-circuit photovoltages (V_oc) of 0.46 V, short-circuit photocurrent densities (J_sc) of 9.1 mA cm^(ā2), and thermodynamically based energy-conversion efficiencies (Ī·) of 1.9% under simulated 1 Sun illumination. Under nominally the same conditions, the efficiency of the NiāMo-coated system was comparable to that of Pt-coated n+p-Si MW array photocathodes (V_oc = 0.44 V, J_sc = 13.2 mA cm^(ā2_, Ī· = 2.7%). This demonstrates that, at 1 Sun light intensity on high surface area microwire arrays, earth-abundant electrocatalysts can provide performance comparable to noble-metal catalysts for photoelectrochemical hydrogen evolution. The formation of an emitter layer on the microwires yielded significant improvements in the open-circuit voltage of the microwire-array-based photocathodes relative to Si MW arrays that did not have a buried n^(+)p junction. Analysis of the spectral response and light-intensity dependence of these devices allowed for optimization of the catalyst loading and photocurrent density. The microwire arrays were also removed from the substrate to create flexible, hydrogen-evolving membranes that have potential for use in a solar water-splitting device
Moored velocity measurements on the edge of the Gulf-Stream recirculation
Mean velocities, eddy kinetic energies and time series of velocities at several depths are presented from moored current meters for the 15 month period May 1978 to July 1979 as part of the POLYMODE Local Dynamics Experiment, centered at 31N, 69Ā°30\u27W. The mean velocities in the upper one kilometer are toward the southwest at 2.4 to 4.4 cm/s, consistent with Worthington\u27s (1976) description of the mean circulation of the North Atlantic...
Hot Stars and Cool Clouds: The Photodissociation Region M16
We present high-resolution spectroscopy and images of a photodissociation
region (PDR) in M16 obtained during commissioning of NIRSPEC on the Keck II
telescope. PDRs play a significant role in regulating star formation, and M16
offers the opportunity to examine the physical processes of a PDR in detail. We
simultaneously observe both the molecular and ionized phases of the PDR and
resolve the spatial and kinematic differences between them. The most prominent
regions of the PDR are viewed edge-on. Fluorescent emission from nearby stars
is the primary excitation source, although collisions also preferentially
populate the lowest vibrational levels of H2. Variations in density-sensitive
emission line ratios demonstrate that the molecular cloud is clumpy, with an
average density n = 3x10^5 cm^(-3). We measure the kinetic temperature of the
molecular region directly and find T_H2 = 930 K. The observed density,
temperature, and UV flux imply a photoelectric heating efficiency of 4%. In the
ionized region, n_i=5x10^3 cm^(-3) and T_HII = 9500 K. In the brightest regions
of the PDR, the recombination line widths include a non-thermal component,
which we attribute to viewing geometry.Comment: 5 pages including 2 Postscript figures. To appear in ApJ Letters,
April 200
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