摘要:

The location of implanted arsenic (As) atoms in silicon dioxide (SiO2) matrix is discussed based on avalanche injection experimental results. A model is proposed in which As atoms substitute for Si at two different sites in an oxygen‐deficient SiO2network. With an additional supply of oxygen, but one insufficient to achieve a fully oxidized system, As tends again to occupy Si sites but interstitial As atoms can also be present. In a fully oxidized network As is also oxidized, i.e., As is again incorporated on Si sites. In the frame of this model contradictory results of As diffusion in SiO2obtained by other authors can be understood in a consistent way. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360243

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The effects of water‐vapor ambients on the solid‐state epitaxial regrowth of ion‐beam‐amorphized, near‐surface layers on single‐crystal CaTiO3have been investigated using Rutherford backscattering‐channeling spectroscopy, time‐resolved reflectivity, and cross‐sectional transmission electron microscopy (TEM). The presence of water vapor in the annealing atmosphere increases the thermally induced epitaxial regrowth rate and, within the temperature range studied (400–550 °C), decreases the activation energy for this process. TEM micrographs from samples which were partially regrown in high‐H2O‐concentration atmospheres revealed uneven amorphous/crystalline interfaces with fluctuations on the order of 5–10 nm. Samples annealed in water‐vapor‐deficient atmospheres exhibited very flat interfaces after partial epitaxial regrowth. The morphologies of these interfaces are explained in terms of a segregation of hydrogen ahead of the regrowth interface. Additionally, it has been determined that the absence of oxygen does not affect the regrowth rate. Samples annealed in oxygen concentrations as low as 10−21atm exhibit growth rates that are identical to those measured for air‐annealed CaTiO3samples. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360244

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The effects of CH4/H2reactive ion etching (RIE) on the optical properties of an InP/InGaAs multiple‐quantum‐well structure have been investigated by low‐temperature photoluminescence (PL). The structure consisted of eight InGaAs quantum wells, lattice matched to InP, with nominal thicknesses of 0.5, 1, 2, 3, 5, 10, 20, and 70 monolayers, respectively, on top of a 200‐nm‐thick layer of InGaAs for calibration. The design of this structure allowed etch‐induced damage depth to be obtained from the PL spectra due to the different confinement energies of the quantum wells. The samples showed no significant decrease of luminescence intensity after RIE. However, the observed shift and broadening of the PL peaks from the quantum wells indicate that intermixing of well and barrier material increased with etch time. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360245

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A theoretical approach is proposed to estimate the effects of grain shape anisotropy on the overall response of polycrystalline piezoelectric ceramics. The self‐consistent approach is applicable to a polycrystal with arbitrary grain shapes, orientations, and anisotropy, but emphasis is placed on the effective bulk and shear moduli and the dielectric constant of a polycrystal consisting of randomly oriented piezoelectric grains. Because typical piezoelectric ceramics often contain substantial porosity, the effects of randomly oriented pores of various shapes on the overall electroelastic properties are considered. Analytical predictions are compared to simplified predictions which ignore the effects of piezoelectricity at the grain level and the effects of grain shape. Analytical predictions are in good agreement with measurements for unpoled, porous BaTiO3polycrystals. AFORTRANcomputer program developed to implement the theory is available upon request from the author. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360246

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Acoustic‐resonance spectroscopy was used to determine the complete elastic constants of a uniaxial‐fiber‐reinforced metal‐matrix composite: boron‐aluminum. This material exhibits orthotropic macroscopic symmetry and, therefore, nine independent elastic stiffnessesCij. The off‐diagonal elastic constants (C12,C13,C23), which contain large errors when measured by conventional methods, were especially focused on. Good agreement emerged among present results, a previous pulse‐echo study, and theoretical predictions using a plane‐scattered‐wave ensemble‐average model. Attempts to measure the internal‐friction ‘‘tensor’’ failed. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.360247

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In this paper we describe a method to determine absolute plane stresses in orthotropic materials from the angular dependence of ultrasonic velocities. No prior information on material anisotropy (texture) is required since the stress dependent elastic constants and stresses are found simultaneously from velocity data using least‐squares optimization. In the optimization algorithm no relation between the stress dependent elastic constants and stresses is assumed and therefore they are considered as uncoupled variables, but it is shown that the iteration process converges to the correct (coupled) values. The method is applicable for stress measurements in materials which have undergone a complicated (possibly plastic) history of loading and unloading; thus it can be used to determine both applied and residual stresses. To check the proposed technique we use synthetic sets of experimental data for different degrees of anisotropy and stress levels. Calculations using these synthetic data show that when the principal stress directions coincide with the symmetry axes the angular velocity data in the plane perpendicular to the stress plane may be used for reconstruction of both stress components. When the stress is off the symmetry axis, the shear and the difference of the normal stress components may be determined from the angular dependence of the velocities in the plane of stresses. In both cases the stress determination accuracy depends only on the accuracy of velocity measurements and is independent of stress level and material anisotropy. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.360248

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

An experimental method has been developed to use time resolved Raman spectroscopy as a microscopic probe of tensile deformation in shocked crystals. Tension is produced in the sample interior by the interaction of rarefaction waves generated in plate impact, uniaxial strain experiments. Optical fibers are used to transmit visible light from a pulsed laser to the sample and to transmit the scattered light to a recording system with 50 ns time resolution. The strain‐induced shift of the 464 cm1,A1line of &agr;‐quartz was measured. Results are presented for longitudinal stresses ranging from 54 kbar in tension to 60 kbar in compression. For each experiment, the 464 cm−1line exhibited a shift toward higher frequency in compression, followed by a return to the ambient position upon unloading, followed by a comparable shift toward lower frequency in tension. This latter shift correlates to a marked increase in the intertetrahedral (Si–O–Si) bond angle of quartz with tension. Comparison with earlier work and the analysis presented here suggests that there is also significant distortion of the SiO4tetrahedra and a mechanism for this distortion is proposed. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.360249

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The propagation of surface acoustic waves across the randomly rough surface of an anisotropic solid is studied. In the small roughness limit the dispersion relation for such waves is derived. The frequency dependences of the dispersion and attenuation of surface acoustic waves are obtained for the case when the wavelength &lgr; is long compared to the transverse correlation lengthaof the roughness. In this limit the numerical curves for the relative change of the phase velocity of surface acoustic waves versus the angle of propagation are calculated for the various cuts of anisotropic media. Crystals of cubic, tetragonal, trigonal, hexagonal, and orthorhombic symmetry, adopted in applications, are considered. The effects of piezoelectricity are taken into account through the approach of ‘‘partially stiffened elastic constants.’’ ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360250

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Pulsed laser melting experiments were performed on GexSi1−xalloys (x≤0.10) with regrowth velocities ranging from 0.25 to 3.9 m/s. Analysis of post‐solidification Ge concentration profiles, along with time‐resolved melt depth measurements, allowed determination of the liquid‐phase diffusivityDlfor Ge in Si and the dependence of the Ge partition coefficientkon interface velocityv. ADlof 2.5×10−4cm2/s was measured. Thekvsvdata were analyzed using various models for partitioning, including both the dilute and nondilute Continuous Growth Models (CGM). Extrapolating to zero velocity using the partitioning models, an equilibrium partition coefficient of approximately 0.45 was obtained. Best fitting of partitioning data to the nondilute CGM yields a diffusive speed of 2.5 m/s. These measurements quantify previous indications of partitioning observed in other studies of pulsed laser processed GexSi1−xalloys. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360251

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We examine thermal expansion of textured polycrystalline aggregates and focus on polycrystals containing hexagonal monocrystals. Our analysis is based on the coupling of the Voigt–Reuss–Hill averaging procedures for elastic constants, Roe’s [R. J. Roe, J. Appl. Phys.36, 2024 (1965)] framework for the mathematical description of the crystallite orientation‐distribution function (ODF), and the exact connections of Hashin [Z. Hashin, J. Mech. Phys. Solids32, 149 (1984)] and Schulgasser [K. Schulgasser, J. Mech. Phys. Solids35, 35 (1987)] between the thermal‐expansion coefficients of a polycrystalline aggregate and the corresponding polycrystal elastic constants, monocrystal elastic constants, and monocrystal thermal‐expansion coefficients. The results are simple expressions for the polycrystal thermal‐expansion coefficient in terms of the monocrystal elastic constants, thermal‐expansion coefficients, and the coefficients in the expansion of the crystallite ODF in a series of generalized spherical harmonics. We present numerical results for zinc and zirconium polycrystals with textures typical of rolling and extrusion processes. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.360734

出版商:AIP    

年代:1995

数据来源: AIP