8,280 research outputs found
Some notes on the Kruskal - Szekeres completion
The Kruskal - Szekeres (KS) completion of the Schwarzschild spacetime is open
to Synge's methodological criticism that the KS procedure generates "good"
coordinates from "bad". This is addressed here in two ways: First I generate
the KS coordinates from Israel coordinates, which are also "good", and then I
generate the KS coordinates directly from a streamlined integration of the
Einstein equations.Comment: One typo correcte
Using Cross-Linked SU-8 to Flip-Chip Bond, Assemble, and Package MEMS Devices
This paper investigates using an SU-8 photoresist as an adhesive material for flip-chip bonding, assembling, and packaging microelectromechanical systems devices. An important factor, when using SU-8 as an adhesive material is to control ultraviolet (UV) exposure during fabrication to maximize bond strength due to material cross linking. This approach is much improved over previous efforts where SU-8 bake times and temperatures where changed to alter material cross-linking. In this paper, bake times and temperatures were maintained constant and total UV exposure energy was varied. Once fabricated, bond strength was systematically tested to determine the tensile loads needed to separate bonded structures. The resulting separation force was shown to increase with UV exposure and ranged from 0.25 (5-s exposure) to 1.25 N (15-s exposure). The separation test data were then analyzed to determine the statistical significance of varying UV exposure time and its effect on SU-8 cross-linking and bond strength. The data show that total UV exposure dose is directly correlated with the bond strength of SU-8 bonded structures. By varying only UV dose, the separation force data exhibited a statistically significant dependence on SU-8 cross linking with a 5% probability of error. Further, SU-8 etch resiliency increased by approximately 40%-60% as cross linking was increased with UV exposures ranging from 5 to 15 s
Gravitational Collapse of Dust with a Cosmological Constant
The recent analysis of Markovic and Shapiro on the effect of a cosmological
constant on the evolution of a spherically symmetric homogeneous dust ball is
extended to include the inhomogeneous and degenerate cases. The histories are
shown by way of effective potential and Penrose-Carter diagrams.Comment: 2 pages, 2 figures (png), revtex. To appear in Phys. Rev.
Investigations of the effect of nonmagnetic Ca substitution for magnetic Dy on spin-freezing in Dy2Ti2O7
Physical properties of partially Ca substituted hole-doped Dy2Ti2O7 have been
investigated by ac magnetic susceptibility \chi_ac(T), dc magnetic
susceptibility \chi(T), isothermal magnetization M(H) and heat capacity C_p(T)
measurements on Dy1.8Ca0.2Ti2O7. The spin-ice system Dy2Ti2O7 exhibits a
spin-glass type freezing behavior near 16 K. Our frequency dependent \chi_ac(T)
data of Dy1.8Ca0.2Ti2O7 show that the spin-freezing behavior is significantly
influenced by Ca substitution. The effect of partial nonmagnetic Ca2+
substitution for magnetic Dy3+ is similar to the previous study on nonmagnetic
isovalent Y3+ substituted Dy2-xYxTi2O7 (for low levels of dilution), however
the suppression of spin-freezing behavior is substantially stronger for Ca than
Y. The Cole-Cole plot analysis reveals semicircular character and a single
relaxation mode in Dy1.8Ca0.2Ti2O7 as for Dy2Ti2O7. No noticeable change in the
insulating behavior of Dy2Ti2O7 results from the holes produced by 10% Ca2+
substitution for Dy3+ ions.Comment: 9 pages, 7 figures, 1 tabl
Integrating Nanosphere Lithography in Device Fabrication
This paper discusses the integration of nanosphere lithography (NSL) with other fabrication techniques, allowing for nano-scaled features to be realized within larger microelectromechanical system (MEMS) based devices. Nanosphere self-patterning methods have been researched for over three decades, but typically not for use as a lithography process. Only recently has progress been made towards integrating many of the best practices from these publications and determining a process that yields large areas of coverage, with repeatability and enabled a process for precise placement of nanospheres relative to other features. Discussed are two of the more common self-patterning methods used in NSL (i.e. spin-coating and dip coating) as well as a more recently conceived variation of dip coating. Recent work has suggested the repeatability of any method depends on a number of variables, so to better understand how these variables affect the process a series of test vessels were developed and fabricated. Commercially available 3-D printing technology was used to incrementally alter the test vessels allowing for each variable to be investigated individually. With these deposition vessels, NSL can now be used in conjunction with other fabrication steps to integrate features otherwise unattainable through current methods, within the overall fabrication process of larger MEMS devices. Patterned regions in 1800 series photoresist with a thickness of ~700nm are used to capture regions of self-assembled nanospheres. These regions are roughly 2-5 microns in width, and are able to control the placement of 500nm polystyrene spheres by controlling where monolayer self-assembly occurs. The resulting combination of photoresist and nanospheres can then be used with traditional deposition or etch methods to utilize these fine scale features in the overall design
PVDF-TrFE Electroactive Polymer Mechanical-to-Electrical Energy Harvesting Experimental Bimorph Structure
Research of electrostrictive polymers has generated new opportunities for harvesting energy from the surrounding environment and converting it into usable electrical energy. Electroactive polymer (EAP) research is one of the new opportunities for harvesting energy from the natural environment and converting it into usable electrical energy. Piezoelectric ceramic based energy harvesting devices tend to be unsuitable for low-frequency mechanical excitations such as human movement. Organic polymers are typically softer and more flexible therefore translated electrical energy output is considerably higher under the same mechanical force. In addition, cantilever geometry is one of the most used structures in piezoelectric energy harvesters, especially for mechanical energy harvesting from vibrations. In order to further lower the resonance frequency of the cantilever microstructure, a proof mass can be attached to the free end of the cantilever. Mechanical analysis of an experimental bimorph structure was provided and led to key design rules for post-processing steps to control the performance of the energy harvester. In this work, methods of materials processing and the mechanical to electrical conversion of vibrational energy into usable energy were investigated. Materials such as polyvinyledenedifluoridetetra-fluoroethylene P(VDF-TrFE) copolymer films (1um thick or less) were evaluated and presented a large relative permittivity and greater piezoelectric β-phase without stretching. Further investigations will be used to identify suitable micro-electromechanical systems (MEMs) structures given specific types of low-frequency mechanical excitations (10-100Hz)
Generating Static Fluid Spheres by Conformal Transformations
We generate an explicit four-fold infinity of physically acceptable exact
perfect fluid solutions of Einstein's equations by way of conformal
transformations of physically unacceptable solutions (one way to view the use
of isotropic coordinates). Special cases include the Schwarzschild interior
solution and the Einstein static universe. The process we consider involves
solving two equations of the Riccati type coupled by a single generating
function rather than a specification of one of the two metric functions.Comment: 4 pages revtex4, two figures, Final form to appear in Phys. Rev.
Enhancing the thermal performance of temporary fabric structures for the advanced energy efficient shelter system
The focus of this research is to characterize the thermal load on temporary fabric shelters deployed in the Middle East in order to establish realistic contract specification for the thermal performance of future shelters. Three different testing methods were utilized to evaluate shelter thermal performance. Small-scale tests allowed for economical comparisons of different shelter materials and configurations
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