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  • American Institute of Physics (AIP)  (9,330)
  • 1990-1994  (9,330)
  • 1991  (9,330)
  • 1
    Electronic Resource
    Electronic Resource
    Free-volume and pore size distributions determined by numerical Laplace inversion of positron annihilation lifetime data (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 4665-4670 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Positron annihilation lifetime spectroscopy is a powerful method for characterizing free volumes in a variety of materials. Correlations between positron annihilation rates and the size of free-volume regions in which o-Ps localizes are well described. Unfortunately, difficulties in the analysis of positron annihilation lifetime data have limited the approach to the determination of average lifetimes and average free volumes. Recent advances in the development of numerical integral transform methods now make it possible to extract continuous distributions of positron annihilation rates. The application of these methods to the determination of free-volume distributions is described. The variable transformations required to convert positron annihilation rate probability density functions (PDF) to the corresponding lifetime, radius, and free-volume PDFs are given and the approach is illustrated by application to positron annihilation lifetime data for amorphous polytetrafluoroethylene.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349057
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  • 2
    Electronic Resource
    Electronic Resource
    Photon-gated persistent spectral hole burning by donor-acceptor electron transfer in tetraphenylporphine/halogenated anthracene systems (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 4671-4678 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Photon-gated persistent spectral hole burning was observed for metal-free tetraphenylporphine with halogenated anthracene derivatives in poly(methylmethacrylate) or poly(ethylene) at liquid-helium temperatures. The hole formation yield was markedly dependent on the polymer matrices and the sample composition as well as on the gating wavelength. The irreversible broadening of holes in the systems measured by temperature cycling experiment was smaller than that by proton tautomerization of tetraphenylporphine. We concluded that the hole formation mechanism is donor-acceptor electron transfer on the basis of combined analysis of the matix and acceptor concentration dependence of hole formation yield, the action spectrum of the gating photon and the photoproduct spectrum.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349058
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  • 3
    Electronic Resource
    Electronic Resource
    Lasing of ArF with a microsecond electron beam (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 4694-4696 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: ArF(B-X) laser emission has been observed under long pulse electron-beam excitation at relatively low pump rate. ArF lasing only occurred with Ne buffered gas mixtures. Optimum performance of 1.93 J/l at an intrinsic efficiency of 1.35% occurred with an Ar/F2 mixture of 1/0.075% using a Ne buffer to 4.0 amagats. Total energy of 290 mJ in a 2-cm2 beam with a 1.0-μs full width at half maximum pulse width was recorded.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349061
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  • 4
    Electronic Resource
    Electronic Resource
    On the in-plane acceleration sensitivity of contoured quartz resonators supported along rectangular edges (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 4708-4714 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is shown that the resultant in-plane acceleration sensitivity of contoured quartz resonators symmetrically supported along rectangular edges always vanishes. This result is a consequence of symmetry and applies to any shape for which the support system is symmetric with respect to the center of the mode shape. When the active plate is supported on one side only, a state of flexure is induced in the resonator which degrades the in-plane sensitivity. This emphasizes the importance of a symmetric support system to achieve low acceleration sensitivities. The extensional and induced in-plane flexural biasing deformations are determined by means of our variational approximation procedure using the variational principles in which all conditions appear as natural conditions. The resulting biasing states are employed in an existing perturbation equation along with the mode shapes of the contoured resonators to calculate the nonvanishing acceleration sensitivities when the resonator is supported on one side only.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349063
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  • 5
    Electronic Resource
    Electronic Resource
    A liquid-metal free surface in a sliding electrical contact with a strong magnetic field (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 4741-4755 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A low-resistance electrical contact is provided by a liquid metal in a small gap between the perimeter of a rotating disk (rotor) and a static surrounding surface (stator). The liquid metal extends radially inward on both sides of the rotor to free surfaces with an inert cover gas, and there is a strong axial magnetic field. This paper presents results for the shape of the free surface and for the liquid-metal velocity and pressure adjacent to the free surface. The results depend on the magnetic-field strength, the surface tension, the wetting angle at the free-surface–solid intersections, and the voltage difference between the stator and rotor. The copper stator and rotor are perfect conductors compared to the liquid metal. Two cases are considered, with and without electrically insulating coatings on the sides of the rotor.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349066
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  • 6
    Electronic Resource
    Electronic Resource
    Threading dislocations in In-doped GaAs/Si (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 4770-4778 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The threading dislocation morphologies and characteristics in In-doped 3-μm-thick GaAs films grown by molecular-beam epitaxy on Si (100) substrates tilted toward the [110] orientation by 2° have been examined mainly using cross-sectional transmission electron microscopy. Most of the observed threading dislocations are 30°- and 60°-type dislocations along the 〈211〉 and 〈110〉 directions on inclined {111} planes. Also, screw-type dislocations running parallel to the [001] growth direction are frequently detected. No appreciable effect of homogeneous In doping throughout the films with a content between 2×1019 and 2×1020 atoms/cm3 on the reduction of threading dislocation generation is observed. However, an In content of 2×1020/cm3 (a misfit of 7.5×10−4) in GaAs films doped in the middle of growth is found to be effective for changing the dislocation direction on the {111} planes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349069
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  • 7
    Electronic Resource
    Electronic Resource
    Effect of void-size distribution on the Hugoniot state at low shock pressures (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 4790-4796 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In most theoretical and experimental investigations into the shock response of underdense solid media, the influence of the medium's mesostructure on the resulting pressure and degree of compaction has not been taken into account. In typical cases examined, shock pressures are well in excess of 1 GPa and this approach is clearly justified. However, at low pressures, calculations show that the distribution of void sizes can affect the final state achieved upon shocking the medium from a given initial porosity. This paper analyzes the response of porous aluminum to low pressure shocking and demonstrates a dependence of the final shocked state on the distribution of void sizes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349045
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  • 8
    Electronic Resource
    Electronic Resource
    Fracto-emission from embedded interfaces (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 4797-4807 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Fracto-emission is the emission of particles and photons during and after fracture of materials. The observed emissions include electrons, ±ions, neutral species in both ground states and in excited states, and visible photons. Here we examine primarily photon and electron emission during failure of axially loaded stainless-steel fixtures (e.g., rods) embedded in epoxy. The observed signals provide time resolved information on the sequence of events involving interfacial fracture preceding pullout. In addition, we examine the emissions during frictional pullout following debonding. Over a large range of strain rates this pullout exhibits stick-slip behavior. These effects model the processes of fiber/matrix debonding and fiber pullout in a brittle matrix composite.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.350339
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  • 9
    Electronic Resource
    Electronic Resource
    Determination of vacancy and self-interstitial contributions to gallium self-diffusion in GaAs (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 4823-4826 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A quantitative determination of the contributions of the triply negatively charged Ga vacancies (V3−Ga) and of the doubly positively charged Ga self-interstitials (I2+Ga) to the Ga self-diffusion coefficient in GaAs has been carried out. Under thermal equilibrium and intrinsic conditions, the V3−Ga contribution is characterized by an activation enthalpy of 6 eV for As-rich crystals and of 7.52 eV for Ga-rich crystals, while the I2+Ga contribution is characterized by an activation enthalpy of 4.89 eV for As-rich crystals and of 3.37 eV for Ga-rich crystals.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349048
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  • 10
    Electronic Resource
    Electronic Resource
    Viscosity of molten Pd77Cu6.5Si16.5 and the principle of corresponding states (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 4842-4845 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The viscosity of Pd77Cu6.5Si16.5 was measured from 995 to 1183 K by capillary flow method. In the experiment, fluxing of the molten specimen by boron oxide was necessary in order to inhibit the formation of crystallites at temperatures near its liquidus. It was found that the viscosity can be described very well by the following Vogel–Fulcher equation: ln η=−3.764+938.543/(T−726), where T is in K. No discontinuity in viscosity was observed at the thermodynamic melting temperature. Recently Tsao and Spaepen [Amorphous Materials: Modeling of Structure and Properties, edited by V. Vitek (The Metallurgical Society of AIME, New York, 1983), p. 323] determined the equilibrium viscosity of the same system near the glass transition temperature. It turned out that the viscosity data from the two regimes cannot be joined together by a single Vogel–Fulcher equation of constant parameters. Finally the present results were compared with those earlier viscosity data of Pd40Ni40P20 and Au77Ge13.6Si9.4 [D. E. Polk and D. Turnbull, Acta Metall. 20, 493 (1972)] to test the validity of the principle of corresponding states for atomic transport properties.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349051
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