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Journal of Applied Physics

Issue 7 • Date Oct 1996

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Displaying Results 1 - 25 of 102
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • A universal optimum work rate potential for continuous endoreversible Carnot heat engine cycles

    Page(s): 3619 - 3621
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    The theory of finite‐time thermodynamics for continuous endoreversible cycles gives rise to an optimum efficiency at maximum power output of η=1-(TL/TH)0.5 in contrast to the upper limit of η=1-(TL/TH) obtained from infinite‐time thermodynamics. It is shown here that, additionally, for continuous Carnot cycles, the finite‐time optimum rate of work output (W˙opt), is exactly half of that obtained for infinite‐time reversible cycles (Carnot work rate, W˙rev) operating between the same temperature limits (i.e. W˙opt=1/2W˙rev). This expression is true for both unoptimized as well as optimized mass flow rate conditions. The formulation used in the analysis is universal for all working substances for cycles using linear heat transfer laws. © 1996 American Institute of Physics. View full abstract»

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  • Studies of vibrating atomic force microscope cantilevers in liquid

    Page(s): 3622 - 3627
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    An atomic force microscope (AFM) design providing a focused spot of order 7 μm in diameter was used to analyze the motion of vibrating cantilevers in liquid. Picking an operating frequency for tapping mode AFM operation in liquid is complex because there is typically a large number of sharp peaks in the response spectrum of cantilever slope amplitude versus drive frequency. The response spectrum was found to be a product of the cantilever’s broad thermal noise spectrum and an underlying fluid drive spectrum containing the sharp peaks. The geometrical shape of transverse cantilever motion was qualitatively independent of the fluid drive spectrum and could be approximately reproduced by a simple theoretical model. The measurements performed give new insights into the behavior of cantilevers during tapping mode AFM operation in liquid. © 1996 American Institute of Physics. View full abstract»

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  • Performance study of polycapillary optics for hard x rays

    Page(s): 3628 - 3638
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    In order to investigate the feasibility of using Kumakhov capillary x‐ray optics for high energy x‐ray applications, measurements have been performed on the behavior of capillary optics from 10 to 80 keV. Transmission efficiencies of straight polycapillary fibers of different types have been measured as a function of source location and x‐ray energy. The measurements are analyzed using a geometrical optics simulation program, which includes roughness and waviness effects. Despite the low critical angle for total external reflection at high energies, capillary x‐ray optics appear promising for many hard x‐ray applications. Transmission measurements at high energies have also proven to be a very sensitive tool in capillary quality analysis. © 1996 American Institute of Physics. View full abstract»

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  • Metal/Al2O3: A new class of x‐ray mirrors

    Page(s): 3639 - 3645
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    We have examined sputtered metal/Al2O3 multilayers (metal: Co, Pt, W) with respect to their suitability as x‐ray mirrors in high temperature environments. The rf‐sputtering technique leads to layered structures with interface roughnesses of only 0.2 nm as confirmed by x‐ray scattering and transmission electron microscopy. In situ resistance measurements characterize the percolation process and indicate the lower thickness limits for the chosen materials which lead to minimum modulation periods of ≊2 nm. The samples were characterized after stepwise annealing up to 1000 °C. Reflectivity values of nearly 100% at the first satellite‐reflection were found in the case of Pt and W (for λ=0.15418 nm). The most stable W/Al2O3 multilayer reflects 70% of the primary beam even after annealing at 900 °C. © 1996 American Institute of Physics. View full abstract»

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  • Mass, charge, and energy separation by selective acceleration with a traveling potential hill

    Page(s): 3646 - 3655
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    A traveling electric potential hill has been used to generate an ion beam with an energy distribution that is mass dependent from a monoenergetic ion beam of mixed masses. This effect can be utilized as a novel method for mass separation applied to identification or enrichment of ions (e.g., of elements, isotopes, or molecules). This theory for mass‐selective acceleration is presented here and is shown to be confirmed by experiment and by a time‐dependent particle‐in‐cell computer simulation. Results show that monoenergetic ions with the particular mass of choice are accelerated by controlling the hill potential and the hill velocity. The hill velocity is typically 20%–30% faster than the ions to be accelerated. The ability of the hill to pickup a particular mass uses the fact that small kinetic energy differences in the lab frame appear much larger in the moving hill frame. Ions will gain energy from the approaching hill if their relative energy in the moving hill frame is less than the peak potential of the hill. The final energy of these accelerated ions can be several times the source energy, which facilitates energy filtering for mass purification or identification. If the hill potential is chosen to accelerate multiple masses, the heaviest mass will have the greatest final energy. Hence, choosing the appropriate hill potential and collector retarding voltage will isolate ions with the lightest, heaviest, or intermediate mass. In the experimental device, called a Solitron, purified 20Ne and 22Ne are extracted from a ribbon beam of neon that is originally composed of 20Ne:22Ne in the natural ratio of 91:9. The isotopic content of the processed beam is determined by measuring the energy distribution of the detected current. These results agree with the theory. In addition to mass selectivity, our theory can also be applied to the filtration of an ion beam according to charge state- or energy. Because of this variety of properties, the Solitron is envisioned to have broad applications. The primary application is for the enrichment of stable isotopes for medical and industrial tracers. Other applications include mass analysis of unknown gases (atomic and molecular) and metals, extracting single charge states from a multiply charged beam, accelerating the high energy tail in a beam or plasma with a velocity distribution, and beam bunching. View full abstract»

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  • Characterization of laser‐driven shocks of high intensity using piezoelectric polymers

    Page(s): 3656 - 3661
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    Piezoelectric elements made of polyvinylidene‐fluoride‐trifluoroethylene copolymer P(VDF/TrFE) have been used to measure the pressure induced on the rear surface of metallic foils irradiated by infra‐red laser pulses of 1.5 ns duration, with intensities up to 3 TW/cm2. The application of such piezoelectric materials under those conditions is new, and a special effort has been made for their improvement and characterization under plate impact loading. Then, the laser experiments have been carried out. First, peak pressures of 0.5–7.5 GPa transmitted in the copolymer have been derived from the peak voltage measured at each shot, using simple assumptions. Next, a more accurate analysis of the data, involving computer simulations, has provided the pressure profiles characterizing the laser shocks driven on the front surface of the targets, over a wide range of laser intensities. Peak pressures of 7–60 GPa on the irradiated surface have been inferred from that analysis. The results have been compared to values obtained from a classical scaling law on one hand, and to predictions of a laser‐matter interaction simulation code on the other hand. An overall coherence has been obtained, despite some discrepancies that have been discussed. © 1996 American Institute of Physics. View full abstract»

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  • Large electro‐optic modulation effect observed in ion‐exchanged KTiOPO4 waveguides

    Page(s): 3662 - 3667
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    We report the observation of an anomalously large electro‐optic modulation effect in potassium titanyl phosphate (KTiOPO4) waveguides fabricated under various conditions. An interferometric method was used to measure the effective electro‐optic coefficient in the waveguides by measuring the phase retardation of an optical mode when a voltage was applied across the substrate. We observed that at low modulation frequency the effective electro‐optic coefficient in the waveguides can be higher than the bulk value by a factor up to about 100. A space charge model relating the mobile charge, space charge field, and observed enhancement in the electro‐optic modulation is proposed to explain the mechanism of this effect. Calculation for dc steady state, small signal analysis, and large signal simulation is compared to the experimental observation. Possible device application and long term effects on system operation are also discussed. © 1996 American Institute of Physics. View full abstract»

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  • Electronic structure of second harmonic generation crystal Li3VO4

    Page(s): 3668 - 3673
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    The origin of second harmonic generation (SHG) of Li3VO4 was investigated from the viewpoint of the band structure by using the tight‐binding method. The tight‐binding parameters were optimized to reproduce the density of states (DOS) obtained from x ray photoelectron spectroscopy and the optical band gap. Although Li3PO4 has the same crystal structure as Li3VO4, it shows no SHG. To explain the difference in optical nonlinearity we compared the electronic structures of Li3VO4 and Li3PO4, in particular at the bottom of conduction band (CB) and the top of valence band (VB), since they are known to play a primary role in SHG. In Li3PO4, the bottom of CB consists of P 3s and O 2p orbitals and the top of VB is composed of O 2p orbitals. These electronic structures result in a relatively low DOS at the bottom of CB and a wide band gap in Li3PO4. On the other hand, in Li3VO4, both bottom of CB and top of VB are composed of V 3d and O 2p. The preferential contribution of V 3d orbitals to the band edge states leads to a high DOS at the bottom of CB, a narrow band gap and delocalization of electrons on V–O bonds. We conclude that these electronic structures are responsible for the high optical‐nonlinearity of Li3VO4. © 1996 American Institute of Physics. View full abstract»

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  • Third‐order nonlinear optical properties of phthalocyanine and fullerene

    Page(s): 3674 - 3682
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    We studied the third‐order nonlinear optical properties of phthalocyanine and fullerene in solution and in thin film form through third‐harmonic generation measurement. X‐ray diffraction measurement and scanning electron microscopy observation were also conducted to investigate the relationship between nonlinear optical and structural properties of the thin film. The nonlinearities depend on the π‐conjugated length and the introduction of a central metal atom. We clarify the factors for forming suitable thin films by organic molecular beam deposition. It is shown that both the crystal structure and molecular alignment can be controlled to enhance the nonlinearities of the phthalocyanine thin film, thereby providing the maximum third‐order susceptibility of 3.0×10-10 esu. We show that nonlinearities of the thin films can be estimated experimentally from those of solutions. © 1996 American Institute of Physics. View full abstract»

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  • Diffractive phase elements by electron‐beam exposure of thin As2S3 films

    Page(s): 3683 - 3686
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    Variable‐dose electron‐beam exposure of thin amorphous As2S3 films is studied as a potential fabrication technique of index‐modulated diffractive optical elements for the near infrared. The relationship between the electron dose and the phase delay is determined using a coarse grating structure, which eliminates the influence of volume diffraction and proximity effects. The effective refractive index change is determined by comparing experimental and calculated zeroth‐ and first‐order diffraction efficiencies, taking into account the small but detectable surface modulation. © 1996 American Institute of Physics. View full abstract»

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  • Parameter space representation of liquid crystal display operating modes

    Page(s): 3687 - 3693
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    A parameter space approach is used to represent the operation of all liquid crystal displays. The Jones matrix nematic is used to analyze the static optical properties of liquid crystal displays with any twist angle, cell thickness, natural birefringence, and polarizer angles. All the usual display modes such as 90° TN, ECB, OMI, SBE, and STN can be visualized on a simple parameter space diagram. Besides its pedagogical values, this formulation has the advantage of simplicity in showing the physics and operating conditions of supertwist as well as low twist liquid crystal displays. It can also be used to analyze novel situations such as reflective displays which do not require any back polarizer. © 1996 American Institute of Physics. View full abstract»

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  • Concentration of atomic hydrogen in the ground state in a CH4‐H2 microwave plasma

    Page(s): 3694 - 3698
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    Ground state atomic hydrogen (H:1s 2S1/2) concentrations in the CH4‐H2 microwave plasma have been measured using actinometry. These measurements have been made over a wide range of plasma conditions including power inputs of 100–800 W and pressures of 0.5–60 Torr. A trace amount of argon was added to serve as an inert reference gas for concurrent optical emission measurements, in which optical emission intensities from the Hγ line (2p2P0–5d2D) at 434 nm and the Ar* line (4s′[1/2]°-4p′[1/2]) at 750 nm were recorded. © 1996 American Institute of Physics. View full abstract»

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  • Self‐consistent kinetic modeling of low‐pressure inductively coupled radio‐frequency discharges

    Page(s): 3699 - 3704
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    An efficient method for solving the spatially inhomogeneous Boltzmann equation in a two‐term approximation for low‐pressure inductively coupled plasmas has been developed. The electron distribution function (EDF), a function of total electron energy and two spatial coordinates, is found self‐consistently with the static space‐charge potential which is computed from a 2D fluid model, and the rf electric field profile which is calculated from the Maxwell equations. The EDF and the spatial distributions of the electron density, potential, temperature, ionization rate, and the inductive electric field are calculated and discussed. © 1996 American Institute of Physics. View full abstract»

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  • Cl2/Ar and CH4/H2/Ar dry etching of III–V nitrides

    Page(s): 3705 - 3709
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    Electron‐cyclotron‐resonance (ECR) and reactive ion etching (RIE) rates for GaN, AlN, InN, and InGaN were measured using the same reactor and plasma parameters in Cl2/Ar or CH4/H2/Ar plasmas. The etch rates of all four materials were found to be significantly faster for ECR relative to RIE conditions in both chemistries, indicating that a high ion density is an important factor in the etch. The ion density under ECR conditions is ∼3×1011 cm-3 as measured by microwave interferometry, compared to ∼2×109 cm-3 for RIE conditions, and optical emission intensities are at least an order of magnitude higher in the ECR discharges. It appears that the nitride etch rates are largely determined by the initial bond breaking that must precede etch product formation, since the etch products are as volatile as those of conventional III–V materials such as GaAs, but the etch rates are typically a factor of about 5 lower for the nitrides. Cl2/Ar plasmas were found to etch GaN, InN, and InGaN faster than CH4/H2/Ar under ECR conditions, while AlN was etched slightly faster in CH4/H2/Ar plasmas. The surface morphology of InN was found to be the most sensitive to changes in plasma parameters and was a strong function of both rf power and etch chemistry for ECR etching. © 1996 American Institute of Physics. View full abstract»

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  • Microwave plasma characteristics during bias‐enhanced nucleation of diamond: An optical emission spectroscopic study

    Page(s): 3710 - 3716
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    A negative bias applied to a nondiamond substrate at the initiation of microwave plasma‐enhanced chemical‐vapor deposition of thin‐film diamond can lead to diamond nucleation, high crystalline density, and an improved level of crystallographic alignment. In this work, optical emission spectroscopy has been used to study changes in the chemical species within the plasma that occur as a result the applied bias to a tungsten substrate. The ratio of C2 to CH species detected changes considerably as does the atomic hydrogen intensity as the bias is applied. Both effects appear to be greatest near the substrate surface. The results are discussed in terms of possible origins for the bias‐enhanced nucleation process. © 1996 American Institute of Physics. View full abstract»

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  • Binding of cobalt and iron to cavities in silicon

    Page(s): 3717 - 3726
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    The chemisorption binding of Co and Fe to cavity walls in Si was quantitatively characterized in the temperature range 973–1273 K in order to evaluate the efficacy of cavities for impurity gettering. The cavities were formed by He ion implantation and annealing. Then, with the solution concentration of Co or Fe being held at the solid solubility through prior formation of excess metal‐silicide phase, the equilibrium number of metal atoms bound to the cavities was measured. Using this information in conjunction with published solubilities, a binding free energy relative to interstitial solution was extracted. The binding free energies for cavity‐wall chemisorption of Co and Fe were found to be less than those for precipitation of the respective silicide phases, a reversal of the ordering previously observed by us for Cu and Au. Nevertheless, model calculations indicate that the chemisorption mechanism is important together with silicide precipitation for cavity gettering of all four elements. The results of this work, taken with the known thermal stability and the anticipated device‐side compatibility of cavities, suggest that these sinks will prove attractive for gettering. View full abstract»

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  • Dose‐rate effects in GaAs investigated by discrete pulsed implantation using a focused ion beam

    Page(s): 3727 - 3733
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    The dependence of the retained lattice damage upon dose rate was investigated by focused ion beam (FIB) implantation of 210 keV Si++ into GaAs at room temperature. The as‐implanted and postannealed states were characterized by ion channeling and Hall‐effect measurements, respectively. Dose‐rate effects arise from stabilizing interactions between populations of defects produced by different ions, and these experiments were designed to probe the time constants of those interactions. In the context of dose‐rate experiments, direct‐write FIB represents a much more controllable means of implantation over conventional broad beams since the exact timing of dose delivery may be precisely defined and varied. In this work, the final implanted dose was achieved by the successive application of individual flux pulses of constant intensity but of varying duration td and repetition period tr. A consistent trend toward a greater concentration of displaced atoms directly after implantation and a higher sheet resistance after annealing was observed for longer td and for shorter tr. This effect did not manifest itself simply in terms of the average current density, Javg∝td/tr. Furthermore, it was observed on all time scales accessible in this experiment, suggesting that the important self‐annealing mechanisms have a wide range of time constants, from less than 1 μs to more than 1 s. A heterogeneous model of damage nucleation is discussed whereby the defect track of an individual ion event self‐anneals until it is overlapped by a following event. © 1996 American Institute of Physics. View full abstract»

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  • The active dopant concentration in ion implanted indium tin oxide thin films

    Page(s): 3734 - 3738
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    The effect of oxygen ion implantation on the electrical and optical properties of Sn‐doped In2O3 (ITO) thin films, sputter deposited from a planar magnetron source on glass substrates, is described. The films were characterized as a function of the implanted dose (3×1013–1×1016 O+ cm-2) by Hall effect, resistivity, and optical transmission measurements. The dependencies observed are explained in terms of the deactivation of the Sn dopant and the removal of oxygen vacancies. In this way an estimate of the amount of electrically active Sn contributing to the carrier density in as‐deposited films was obtained. Furthermore, the accompanying changes in the band gap with decreasing free‐carrier density could be explained quantitatively in terms of the Burstein–Moss effect. © 1996 American Institute of Physics. View full abstract»

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  • Phosphorus‐related donors in 6H‐SiC generated by ion implantation

    Page(s): 3739 - 3743
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    Aluminum‐doped 6H‐SiC epilayers were implanted with phosphorus and subsequently annealed in a temperature range from 1400 to 1700 °C. The annealing behavior of implanted phosphorus atoms was studied by the Hall effect, admittance spectroscopy, and photoluminescence. Phosphorus acts as a shallow donor. Two ionization energies of (80±5) meV and (110±5) meV are determined, which are assigned to phosphorus atoms residing at hexagonal and cubic lattice sites, respectively. Assuming first‐order kinetics, the annealing process results in an activation energy of the phosphorus donors of 2.5 eV. A set of four lines at a wavelength of about 420/421 nm is observed in the low temperature photoluminescence spectra; the intensity of these lines increases in parallel with the electrical activation of phosphorus donors by raising the annealing temperature. It is proposed that these lines are phosphorus‐related. © 1996 American Institute of Physics. View full abstract»

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  • The Eδ and triplet‐state centers in x‐irradiated high‐purity amorphous SiO2

    Page(s): 3744 - 3749
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    High‐purity silicas synthesized by the chemical‐vapor‐deposited soot remelting method were studied by electron‐spin‐resonance techniques after being irradiated by x rays at 77 K or higher temperatures. The spectra of the Eδ center including its 29Si hyperfine splitting, and the triplet‐state center, were measured using two different detection modes. The effects of x‐ray dose, thermal annealing, hydrogen treatment, and impurities were examined; the Eδ and the triplet‐state centers have a similar dependence on all these parameters, indicating that they share a common precursor. These centers are found only in low OH, oxygen‐deficient samples. There appears to be no correlation with chlorine impurities. The measured intensity of the Eδ center’s 29Si hyperfine signal indicates that approximately four Si atoms are involved. A model for this center and the triplet‐state center is proposed. © 1996 American Institute of Physics. View full abstract»

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  • A positron lifetime investigation of InP electron irradiated at 100 K

    Page(s): 3750 - 3756
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    Positron lifetime investigations have been made on variously doped InP samples irradiated at 100 K with 2.5 MeV electrons. Three irradiation‐produced positron lifetimes were found: 240±10, 265±10, and 330±20 ps which are, respectively, ascribed to VP, VIn, and VP∙VIn vacancies in close association with interstitials. Total introduction rates of these defects were in the range of 0.6–1.2 cm-1. Observation of the defects depends on the position of the Fermi level. In n‐type materials no evidence could be found for VP∙PI, while in p‐type material VIn∙InI was not observed. Annealing studies up to 300 K show that VP∙PI anneals slightly below 300 K, while VIn∙InI anneals in part around 125 K, but a sizable fraction remains at 300 K. Divacancies begin annealing at 125 K, but some can persist to 300 K. © 1996 American Institute of Physics. View full abstract»

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  • Shock‐induced transformations of carbyne

    Page(s): 3757 - 3759
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    The influence of the shock wave loading on films of amorphous carbyne was explored. Chemically synthesized films were deposited on copper disks of 1 mm thickness. X‐ray and transmission electron microscopy examination of 10 and 150 μm carbyne films recovered after shock loading at pressures of 18, 26, and 36 GPa were carried out. It was detected that 10 μm films partially transformed to carbyne crystals with a size of about 1 μm. Thick films of carbyne transformed to a mixture of an amorphous carbon phase and nanocrystalline cubic diamond. © 1996 American Institute of Physics. View full abstract»

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  • Measurement of stiffness coefficients of anisotropic materials from pointlike generation and detection of acoustic waves

    Page(s): 3760 - 3771
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    A scanned point source‐point receiver technique, based on laser generation and detection of acoustic waves, is used to measure the stiffness coefficients of anisotropic materials. The striking effects that anisotropy gives rise to are analyzed and, when possible, advantage is taken of them. The processing developed for recovering the coefficients is presented and applied starting with simulated or experimental signals. A silicon crystal, for which acoustic wave focusing induced by anisotropy is critically sensitive, is first studied. To provide an accurate interpretation of these waves, the two‐dimensional problem considering a line source is discussed, before analyzing the point source generation. Secondly, a manufactured composite material is characterized by means of this noncontact technique. By scanning a symmetry plane, four coefficients of the stiffness tensor are then recovered with good reliability. © 1996 American Institute of Physics. View full abstract»

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  • Nanoindentation investigation of the Young’s modulus of porous silicon

    Page(s): 3772 - 3776
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    Young’s modulus of porous silicon samples, with porosity ranging from 36% to 90%, is measured by the nanoindentation technique. The analysis of the nanoindentation data, including the specific problem linked with porous materials, is presented. The Young’s modulus values Ep thus obtained appear to be drastically dependent on the porosity and on the doping level (p or p+ type). The dependence of Ep versus the relative density (for a series of p+ type samples) is quadratic, in good agreement with the model of Gibson and Ashby developed for cellular materials. This also shows that highly porous silicon layers exhibit very low Young’s modulus (for a porosity of 90% it is about two orders of magnitude smaller than that of the nonporous material). © 1996 American Institute of Physics. 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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Editor
P. James Viccaro
Argonne National Laboratory