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Journal of Applied Physics

Issue 11 • Date Jun 2008

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Displaying Results 1 - 25 of 136
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

    Page(s): toc1
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  • Threshold switching and phase transition numerical models for phase change memory simulations

    Page(s): 111101 - 111101-18
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    A comprehensive numerical model for chalcogenide glasses is presented, coupling a physically based electrical model able to reproduce the threshold switching with a local nucleation and growth algorithm to account for the phase transition dynamics. The main ingredients of the chalcogenide physics are reviewed and analyzed through simplified analytical models, providing a deeper insight on the origin of the threshold switching mechanism in chalcogenide glasses. A semiconductorlike three-dimensional full-coupled numerical implementation of the proposed model is finally presented and its capabilities to quantitatively reproduce the key elements of the Ge2Sb2Te5 chalcogenide physics are demonstrated in the framework of phase change memory device simulations. View full abstract»

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  • Polarization-tunable electroluminescence using phase retardation based on photonic bandgap liquid crystal

    Page(s): 113101 - 113101-4
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    We have demonstrated polarization-tunable electroluminescence (EL) from organic light-emitting diodes (OLEDs) using a combination of polymeric cholesteric liquid crystal (PCLC) and nematic liquid crystal (NLC). By electrically controlling NLC alignment, the proposed EL device shows a continuous polarization tunability including two circular (right and left) and two linear polarizations without loss of light. This simple polarization-tunable EL device is based on photonic-device concept using spin-coatable PCLC. Hence, there is no limitation for choosing emissive materials with high internal efficiency. Also, its polarization tunability will be available in various optical devices and can extend OLED application to photonic technologies. View full abstract»

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  • Crystal-field study of Yb3+ doped LuVO4

    Page(s): 113102 - 113102-4
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    Raman active phonons of Yb doped LuVO4 ranging from 0.25% to 30% are detected and analyzed at low temperature in order to determine the doping effects on the crystalline quality and the phonon contribution to the crystal-field vibronics. Also, Yb3+ ion 2F7/2 and 2F5/2 crystal-field energy levels are measured as a function of doping by infrared absorption and luminescence. The appropriate crystal-field Hamiltonian parameters are determined and the presence of Yb3+Yb3+ magnetically interacting pairs, which play an important role in cooperative emission, is confirmed. View full abstract»

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  • Dose dependent x-ray luminescence in MgF2:Eu2+, Mn2+ phosphors

    Page(s): 113103 - 113103-5
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    In MgF2:Mn2+, Eu2+ phosphors, the x-ray excited luminescence from Eu2+ is decreased while the emission from Mn2+ is increased in intensity with the increase of x-ray dose. In MgF2:Mn2+, the luminescence is also increased, and in MgF2:Eu2+, the emission of Eu2+ is also decreased in intensity with the increase of x-ray dose. However, the intensity changes with x-ray dose in the singly doped MgF2:Mn2+ and MgF2:Eu2+ phosphors are much less than those in the doubly doped MgF2:Mn2+, Eu2+ phosphors. The increase of Mn2+ emission in intensity is likely due to the breakdown of the forbidden transition by the defects created by x-ray irradiation. No conversion of Eu2+ ions to Eu3+ ions was observed in MgF2:Eu2+ phosphors during x-ray irradiation. The decrease of Eu2+ emission in intensity in MgF2:Mn2+, Eu2+ must be closely related to the interaction and the en- ergy transfer to Mn2+ ions. The phenomenon observed is potentially interesting for the practical applications for radiation detection, as utilizing the ratio of the two emissions from Mn2+ and Eu2+ for radiation detection is more sensitive and more reliable than using emission intensity change only. View full abstract»

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  • Photo-electromotive-force from volume speckle pattern vibration with large amplitude

    Page(s): 113104 - 113104-6
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    We report an accurate mathematical model describing the photo-electromotive-force signal produced by a speckle pattern of light vibrating in the volume of a photorefractive crystal with a large transverse amplitude. Our model shows that, for vibrations much faster than the material response time, the first harmonic term of the photo-electromotive-force signal exhibits a maximum at a characteristic value of the vibration-amplitude-to-speckle-size ratio that depends on the dark-to-photoconductivity ratio in the material. The theoretical results are in good agreement with experimental data from a vanadium-doped photorefractive CdTe (CdTe:V) crystal under 1064 nm wavelength illumination. View full abstract»

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  • Signal restoration from atmospheric degradation in terahertz spectroscopy

    Page(s): 113105 - 113105-6
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    This paper presents a method of restoring signals in terahertz spectroscopy by removing the distortion from the observed terahertz signals. The distortion is generated by the absorption and scattering of gas molecules and water vapor in the atmosphere during the transmission of terahertz beams through the air from the source to the spectrometer. Such atmospheric degradation causes spurious spectral dips and peaks in the terahertz spectrum, which often obscure the spectral peaks specific to the material of interest. This fact makes it challenging to measure the terahertz spectroscopic signatures of objects in a humid air environment, even at a short distance of approximately 1 m. A terahertz signal restoration filter based on a nonlinear artificial neural network model effectively removes noisy absorption peaks in terahertz spectra caused by atmospheric degradation. View full abstract»

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  • Metal-oxide-semiconductor-compatible ultra-long-range surface plasmon modes

    Page(s): 113106 - 113106-7
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    Long-range surface plasmons traveling on thin metal films have demonstrated promising potential in subwavelength waveguide applications. In work toward device applications that can leverage existing silicon microelectronics technology, it is of interest to explore the propagation of surface plasmons in a metal-oxide-semiconductor geometry. In such a structure, there is a high refractive index contrast between the semiconductor (n≈3.5 for silicon) and the insulating oxide (typically n≈1.5-2.5). However, the introduction of dielectrics with disparate refractive indices is known to strongly affect the guiding properties of surface plasmons. In this paper, we analyze the implications of high index contrast in 1D layered surface plasmon structures. We show that it is possible to introduce a thin dielectric layer with a low refractive index positioned next to the metal without adversely affecting the guiding quality. In fact, such a configuration can dramatically increase the propagation length of the conventional long-range mode. While this study is directed at silicon-compatible waveguides working at telecommunications wavelengths, this configuration has general implications for surface plasmon structure design using other materials and operating at alternative wavelengths. View full abstract»

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  • Mode selection and phase locking of sidelobe-emitting semiconductor laser arrays via reflection coupling from an external narrow-bandwidth grating

    Page(s): 113107 - 113107-15
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    A phase locked array design, utilizing direct reflection feedback between adjacent cavities by an external grating, is analyzed and proposed. The narrow grating reflection bandwidth causes longitudinal mode selection, while the array geometry causes transverse wavenumber selection through the coupling strength. As a result, only one among the free running cavity eigenmodes can couple effectively into a phase locked collective eigenmode. The coupled array mode is experiencing the high reflectivity of the grating and surpasses the low gain of the free running modes, that experience only a much lower reflectivity from the cavity edge antireflective coating. These results suggest that in-phase locking and single mode operation can be achieved simultaneously through the use of an external narrow-bandwidth grating. View full abstract»

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  • Three-dimensional confocal Raman imaging of volume holograms formed in ZrO2 nanoparticle-photopolymer composite materials

    Page(s): 113108 - 113108-6
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    We describe physicochemical microanalyses of plane-wave volume holograms recorded in ZrO2 nanoparticle-photopolymer composite films with approximately 50 μm thickness. Transversal and longitudinal density distributions of compositions embedded in holograms are analyzed by a confocal scanning Raman microscope. Other microscopic measurements using an electron-probe microanalyzer and an atomic force microscope are also performed to examine hologram’s compositional and structural morphology near the surface. It is found that the induced density distributions of ZrO2 nanoparticles and the formed polymer are 180° out of phase with each other in holograms. This result shows a decisive evidence of the counterdiffusion of monomer molecules and ZrO2 nanoparticles in a film under two-beam holographic exposure. It is also found that when grating spacing is 5 μm or longer, a relief structure of a few 10 nm appears on a film’s surface and its height increases with the grating spacing. This result indicates a significance of the surface tension effect at shorter grating spacing and the imperfect compensation of local volume changes in the counterdiffusion process. View full abstract»

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  • Temperature sensitivity (T0) of tensile-strained GaAsP/(AlGa)As double-barrier separate confinement heterostructure laser diodes for 800 nm band

    Page(s): 113109 - 113109-7
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    The temperature sensitivity of the double-barrier separate confinement heterostructure (DBSCH) laser diodes (LDs), intended for high-power, low vertical beam divergence emission, was shown to be somewhat lower than that of the large optical cavity (LOC) devices of similar beam divergences. For the lowest beam divergences, it still remains considerably high, resulting in low characteristic temperatures T0 not exceeding 80 K in the LDs of the vertical beam divergences below 15°. In this work, the decrease in the T0 values of such tensile-strained GaAsP/(AlGa)As DBSCH LDs has been found primarily due to an increased, thermally activated occupation of the heavy-hole subband and, to a minor degree, occupation of the second quantum level, both giving rise to transitions that do not contribute to the optical gain. This is caused by the higher and more temperature-dependent at-threshold band filling for devices of reduced confinement factor (Γ) aimed at achieving low beam divergence. This phenomenon seems to be common for all the tensile-strained LDs and acts in addition to other mechanisms that decrease the T0, such as the carrier escape from a quantum well to surrounding layers. However, in DBSCH LDs no carrier escape has been detected, presumably because of their thin waveguide layers. This can explain the somewhat higher T0 values of these devices when compared to their LOC counterparts. View full abstract»

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  • Simulations of absorbance efficiency and power production of three dimensional tower arrays for use in photovoltaics

    Page(s): 113110 - 113110-10
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    The production of cheap energy from the sun will be a major research objective in the coming years. Major strides must be made in solar cell efficiency, including increasing absorbance efficiency of a cell by etching or texturing. In order to increase the absorbance efficiency of solar cells, we have developed a three-dimensional solar cell structure by depositing a cadmium telluride thin film overtop carbon nanotube towers. These towers act as both a scaffolding and an electrical interconnect. Multiple photon interactions as they reflect between these towers increase the absorption efficiency. We have developed a theoretical model and computer simulation to maximize the number of photon interactions due to the geometrical characteristics of the system (aspect ratio, spacing, size, shape, etc.). Simulated modeling has shown that by optimization of parameters, a three-dimensional cell can obtain up to a 300% increase in power production over traditional cells. View full abstract»

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  • Theory and simulation of surface plasmon excitation using resonant metal nanoparticle arrays

    Page(s): 113111 - 113111-8
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    We discuss a plasmonic coupling device consisting of a periodic array of ellipsoidal silver nanoparticles embedded in SiO2 and placed near a silver surface. By tuning the shape of the particles in the array, the nanoparticle plasmon resonance is tuned. The resulting resonantly enhanced fields near the nanoparticles, in turn, excite surface plasmons on the metal film. We have performed finite integration technique simulations of such a plasmon coupler, optimized for operation near a wavelength of 676 nm. Analysis of the frequency dependent electric field at different locations in the simulation volume reveals the separate contributions of the particle and surface resonance to the excitation mechanism. A coupled oscillator model describing the nanoparticle and the metal film as individual resonators is introduced and is shown to reproduce the trends observed in the simulations. Implications of our analysis on the resonantly enhanced excitation of surface plasmons are discussed. View full abstract»

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  • Dichotomy of the exciton wave function in semiconductors under intense laser fields

    Page(s): 113112 - 113112-11
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    We study the behavior of excitons in a semiconductor irradiated by a monochromatic, linearly polarized, intense laser field. By taking the finiteness of the hole effective mass into account and including the radiation field in a semiclassical manner, we solved the two-body quantum problem in the framework of a nonperturbative theory based upon the Kramers-Henneberger translation transformation for the Schrödinger equation. In the Kramers frame, the rapidly oscillating potential is expanded in a Fourier-Floquet series and, for laser frequencies high enough, only the zeroth-order term survives, the so-called “laser-dressed” potential. By applying the Ehlotzky’s approximation, this potential simplifies to a two-center potential that resembles that for the electronic motion in the H2+ molecule ion. The binding energy for an exciton in bulk GaAs under a nonresonant laser field is then computed by following a variational scheme we recently adapted from the linear combination of atomic orbitals-molecular orbitals method for the H2+ system. Similarly to the binding energy in H2+ in the separated-atoms limit, we found that, instead of vanishing, the exciton binding energy tends to a quarter of the excitonic Rydberg energy with the increase of the laser intensity. We also trace a correlation between this residual binding and the dichotomy of the excitonic wave function in the large dressing parameter limit, which indicates the possibility of excitons becoming stable against ionization. View full abstract»

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  • Mask pattern interference in AlGaInAs selective area metal-organic vapor-phase epitaxy: Experimental and modeling analysis

    Page(s): 113113 - 113113-8
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    We studied selective area growth modeling and characterization of the AlGaInAs material system. We used a three-dimensional vapor phase diffusion model to extract the effective diffusion lengths of Al, Ga, and In species from measured thickness profiles of the three binaries AlAs, GaAs, and InP. Our growth conditions yield to 50, 85, and 10 μm for Al, Ga, and In, respectively. Based on these values, we achieved a precise prediction of AlGaInAs thickness, composition, band gap, and biaxial strain variations in different selective area growth conditions. Particular attention was paid to the influence of neighboring cells in the case of high mask density. This configuration occurs in practical component mask layout. High mask density leads to interferences between masked cells and enhances the effect of the long diffusion length of aluminum and gallium species. Then, the biaxial strain is tensile shifted and the band gap is blue shifted in the vicinity of a mask, compared to reference material features grown away from the mask. High-resolution micro-photoluminescence and optical interferometer microscopy measurements confirmed the validity of simulated band gap and thickness variations for both bulk and multi-quantum well layers. View full abstract»

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  • Negative transconductance in apertured electron guns

    Page(s): 113301 - 113301-5
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    Passing an electron beam through an aperture can serve to reduce the beam current or change the transverse beam profile. For a sufficiently intense beam, space charge will drive a radial expansion of the beam, which may cause the current passing through the aperture to decrease even though the current arriving at the aperture is increasing. When a gridded electron gun is used, this may be expressed by stating that the transconductance of the apertured gun is negative. Here, we explain this effect and explore some of the key factors governing when it can occur and influencing its strength. View full abstract»

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  • Plasma deposition of elastic wear resistant Si–C coatings on nickel-titanium for biomedical applications

    Page(s): 113302 - 113302-9
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    A bilayer system consisting of carbon and silicon has been developed to coat metals such as copper, stainless steel, nickel, and especially NiTi with an elastic, wear, and corrosion resistant film. The plasma deposited film system exhibits excellent adhesion on all metals under investigation. This adhesion is promoted by silicide formation. Additionally, the film system shows good performance under extremely high load induced via cavitation admission. Tensile tests reveal high elasticity of the film system, in that it can be elongated up to 5% before occurrence of damage of the film. The intrinsic stress inside this film system is reduced compared to pure diamondlike carbon films. Both good chemical bonding and reduced stress lead to good adhesion of the film systems on metals. View full abstract»

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  • Two-carrier transport and ferromagnetism in FeSe thin films

    Page(s): 113501 - 113501-6
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    α- and β-FeSe thin films were grown by metal organic chemical vapor deposition. Compared to the other parameters, the growth temperature shows decisive influence on the phase transition of the FeSe samples. In temperature-dependent electrical measurements, n-type to p-type reversion was observed for both the α- or and β-FeSe samples. Furthermore, the p-type character of the films becomes more and more obvious with increasing the Se/Fe atomic ratio in the samples. Ferromagnetism was observed in the α-FeSe films although which is not supported by calculation on density of states. The ferromagnetic character shows significant dependence on Se/Fe atomic ratio in the films and was attributed to the Fe vacancies or Fe clusters in the α-FeSe thin films. The magnetic domain and hysteresis loop of the β-FeSe thin films are also studied. View full abstract»

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  • Numerical characterization of the Ga interstitial self-diffusion mechanisms in GaAs

    Page(s): 113502 - 113502-5
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    Recent diffusion experiments by Bracht and Brotzmann [H. Bracht and S. Brotzmann, Phys. Rev. B 71, 115216 (2005)] show that the dominant charge states for GaI diffusion in GaAs should be 0 and +1 with surprisingly high enthalpy barriers of 5.45±0.12 and 5.80±0.32 eV, respectively. Using the activation-relaxation technique, coupled with the ab initio code SIESTA, we identify possible migration pathways and barriers for these two charge states. In the +1 charge state, we observe two different migration paths between tetrahedral sites surrounded by Ga atoms (tetra[Ga-Ga]), implicating either a tetrahedral interstitial surrounded by As atoms (tetra[Ga-As]) or a <111>-oriented split configuration, with total barriers of 1.4 and 1.3 eV, respectively. Including formation energy, the enthalpy barriers that we find are lower than the experimentally derived values. We analyze the possible causes and the significance of this discrepancy and offer a partial explanation based on the correction method used for finite-size effects. View full abstract»

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  • Emissivity-corrected power loss calibration for lock-in thermography measurements on silicon solar cells

    Page(s): 113503 - 113503-8
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    This paper describes power loss calibration procedures with implemented emissivity correction. The determination of our emissivity correction matrix does neither rely on blackbody reference measurements nor on the knowledge of any sample temperatures. To describe the emissivity-corrected power calibration procedures in detail, we review the theory behind lock-in thermography and show experimentally that the lock-in signal is proportional to the power dissipation in the solar cell. Experiments show the successful application of our emissivity correction procedure, which significantly improves the informative value of lock-in thermography images and the reliability of the conclusions drawn from these images. View full abstract»

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  • Photoluminescence characteristics of quantum dots with electronic states interconnected along growth direction

    Page(s): 113504 - 113504-5
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    We report the excitonic photoluminescence (PL) characteristics in multiple stacked quantum dots (QDs) fabricated by using a strain compensating technique. The PL characteristics of QD excitons vary according to the spacer layer thickness; with decreasing spacer layer thickness, the PL intensity decreases and the PL decay time becomes longer. Furthermore, the intensity ratio of the transverse-magnetic to transverse-electric modes in the PL emission from the cleaved edge surface increases. As the spacer layer thickness decreases, the degree of overlap of the electron envelope functions owing to tunneling becomes larger, which consequently interconnects the QDs along the growth direction. This interconnection induces a large change in the oscillator strength of the QD excitons and the PL characteristics. Therefore, we concluded that the optical characteristics can be controlled drastically by changing the spacer layer thickness. View full abstract»

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  • Development of pulsed laser-assisted thermal relaxation technique for thermal characterization of microscale wires

    Page(s): 113505 - 113505-9
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    A transient technique is developed to measure the thermal diffusivity of one-dimensional microscale wires. In this technique, the thin wire is suspended over two copper electrodes. Upon fast (nanosecond) pulsed laser irradiation, the wire’s temperature will quickly increase to a high level and then decrease gradually. Such temperature decay can be used to determine the sample’s thermal diffusivity. To probe this temperature evolution, a dc is fed through the wire to sensor its voltage variation, from which the thermal diffusivity can be extracted. A 25.4 μm thin Pt wire is characterized to verify this technique. Sound agreement is obtained between the measured data and reference value. Applying this pulsed laser-assisted thermal relaxation technique, the thermal diffusivity of multiwall carbon nanotube bundles and microscale carbon fibers is measured. Detailed analysis is conducted to study the effect of the wire embedded in the paste/base on the final measurement result. View full abstract»

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  • Calculations of potential functions and thermophysical behaviors for La62Al14Ni12Cu12 and Cu46Zr44Al7Y3 bulk metallic glasses

    Page(s): 113506 - 113506-5
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    Bulk metallic glasses La62Al14Ni12Cu12 and Cu46Zr44Al7Y3 were prepared by copper mold suction casting. The thermal-physical behaviors of bulk metallic glasses were investigated by means of x-ray diffraction, differential scanning calorimetry, ultrasonic techniques, and dilatometry. By calculating the Mie potential function from experimental data, the values of potential function powers (m and n) and related physical parameters such as the mean binding energy, etc., are obtained. Thus, unlike what some people have done by assuming values of m-n (7–14), the values of average nearest-neighbor separation r0 and effective depth of pair potential Φ0 can be obtained from calculated values of m and n from Mie potential functions and they agree very well with the results in the literature. The calculations can be well consistent with the thermophysical behaviors by comparing the two amorphous alloys. In addition, it was enhanced that the effective depth of pair potential correlated with the glass-forming ability of bulk metallic glass. View full abstract»

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  • Shock compression of condensed matter using Eulerian multimaterial method: Applications to multidimensional shocks, deflagration, detonation, and laser ablation

    Page(s): 113507 - 113507-13
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    The reactive flow analysis of high energy material is performed using hydro shock compression of condensed matter (SCCM) tool that is being developed for handling complex multimaterial dynamics involving energetic and inert matters. Typically, the reacting flows of high energy materials such as fires and explosions give rise to strong nonlinear shock waves and high strain rate deformation of metallic confinements at unusually high pressure and temperature. In order to address difficulties associated with analyzing such complex systems, we have developed a suite of modeling capabilities for elegantly handling large gradients and high strain rates in solids as well as reactive shock waves present in gaseous phase. Mathematical formulation of explosive dynamics involving condensed matter is explained with an emphasis on validating and application of hydro-SCCM to a series of problems of high-speed multimaterial dynamics in nature. A detailed numerical description of a level-set based reactive ghost fluid approach is reported in a separate paper. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Meet Our Editors

Editor
P. James Viccaro
Argonne National Laboratory