Lam T.T.,The Aerospace Corporation
International Journal of Thermal Sciences | Year: 2010
Most studies of heat transfer pertaining to a planar medium subjected to time-varying and spatially-decaying laser incidence along with external surface cooling are based on the diffusion theory (parabolic heat conduction equation), an approximation that implies a non-physical infinite speed of energy transport. In this study, temperature distributions within one-dimensional plates subjected to the aforementioned heating and cooling conditions, but with thermal energy propagation at a finite speed, are presented. Incident energy that is partially absorbed at the external surface is transferred through the plate by conduction, while the remaining energy is convectively cooled to the environment. The present investigation will examine three different time characteristics of the incident heat sources which include a continuously operating, a pulsed and an oscillatory laser source. The temperature results were obtained by using a simple and concise finite-difference algorithm based on the Godunov scheme, a method developed for the solution of resulting characteristic equations that govern thermal wave propagations within the solid. © 2010 Elsevier Masson SAS. All rights reserved.
Mahajan V.N.,The Aerospace Corporation
Applied Optics | Year: 2010
The classical aberrations of an anamorphic optical imaging system, representing the terms of a powerseries expansion of its aberration function, are separable in the Cartesian coordinates of a point on its pupil. We discuss the balancing of a classical aberration of a certain order with one or more such aberrations of lower order to minimize its variance across a rectangular pupil of such a system.We show that the balanced aberrations are the products of two Legendre polynomials, one for each of the two Cartesian coordinates of the pupil point. The compound Legendre polynomials are orthogonal across a rectangular pupil and, like the classical aberrations, are inherently separable in the Cartesian coordinates of the pupil point. They are different from the balanced aberrations and the corresponding orthogonal polynomials for a system with rotational symmetry but a rectangular pupil. © 2010 Optical Society of America.
Walterscheid R.L.,The Aerospace Corporation
Journal of Geophysical Research: Space Physics | Year: 2013
The group velocity of a wave packet is defined in terms of the central wave number. Because of scale-dependent dissipation, the central wave number migrates to lower values as transient wave packets propagate through dissipative media. The importance of this effect is greater the broader the bandwidth of the wave packet is or the more localized it is spatially. For modest localization, the migration of the central wave number significantly accelerates the vertical motion of wave packets and allows propagation to significantly greater altitudes before packets are substantially dissipated. Wave packets may experience significant attenuation due to dispersion before they begin to experience strong viscous dissipation. © 2013. American Geophysical Union. All Rights Reserved.
Nemanick E.J.,The Aerospace Corporation
Journal of Power Sources | Year: 2014
The electrochemistry of non-aqueous lithium-oxygen (Li-O2) batteries on both reduction and oxidation was investigated using carbon black and single-walled nanotube (SWNT) microcavity electrodes. Two oxidative peaks were observed on oxidation (charge). The first peak at 3.0-3.7 V vs. Li/Li + was limited in storable charge (∼8 × 103 C cm-3). The oxidation potential of this first peak was independent of Li2O2 thickness (as measured by total charge on reduction) and assigned to interfacial Li2O2, lying between the bulk of the Li2O2 and the carbon surface. A second peak between 4.0 and 4.6+ V showed significant discharge product thickness dependence on oxidation potential and was assigned to bulk Li2O2, away from the carbon surface. On reduction (discharge), deposition of the interfacial Li2O2 showed a significant overpotential indicated by the ratio of interfacial/bulk Li2O2 formed at varying reduction potentials. SWNT were shown to effectively decrease the Li-O2 average charge overpotential for a given charge by increasing the electrode surface area available for formation of interfacial discharge products. © 2013 Elsevier B.V. All rights reserved.
Didziulis S.V.,The Aerospace Corporation |
Butcher K.D.,California Lutheran University
Coordination Chemistry Reviews | Year: 2013
The electronic structure of the early transition metal carbides and nitrides is linked to their remarkable physical properties and their surface chemistry. In this review, we focus on experimental studies of the electronic structure and surface adsorption properties of these rock-salt structured materials. A straightforward molecular orbital framework is used to understand the surface chemical interactions of the materials, primarily the stoichiometric (001) surfaces of TiC and VC, with small molecule adsorbates. This framework is then expanded to include more comprehensive theoretical treatments of the surface adsorption, with a particular emphasis on recent density functional theory results. The adsorbates reviewed include CO, NH 3, O 2, H 2O, SO 2, methanol, methyl formate, and ethanol. This overview reveals that the properties of these materials are heavily influenced by two factors: highly covalent bonding interactions between the metal and carbon species and the total number of electrons present, as the added electron per formula unit in either VC or TiN relative to TiC, populates low lying metal 3d levels that are formally unoccupied in TiC. This results in materials that appear to be d 0 (TiC), d 1 (VC, TiN) or d 2 (VN), with actual charges on the atoms that are close to ±1. These influences are apparent in valence band photoemission data obtained on the (001) surfaces. The surface chemistry trends with probe molecules such as CO and NH 3 can be predicted based on coordination chemistry principles, with the σ-donor ammonia molecule, for example, have very similar interactions with TiC and VC, while CO adsorption is measurably stronger on the VC surface due to π-backbonding interactions. More comprehensive surface models are needed to probe surface reactions as they are heavily influenced by neighboring carbon and metal atoms on the (100) surface, with the added d-electron density on the metal in VC or TiN enabling stronger surface bonding with reaction intermediates than is found on TiC. © 2012.