摘要:

The generation of the longitudinal waves in the frequency range up to the microwave region was observed in the GaAs Gunn diode when the applied field exceeded the threshold. This ultrasonic excitation in the Gunn diode is caused by the lattice deformation by the high‐field domains through the piezoelectricity of GaAs. The two possible excitations, in the bulk and at the surface, are discussed using simple models. It is speculated that a properly oriented Gunn diode may be applicable to the narrow beam‐like ultrasonic generator.

ISSN:0021-8979    

DOI:10.1063/1.1658536

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The liquid‐wedge technique has been used to excite ultrasonic surface waves on various solids; using this technique, only a suitable noncorrosive liquid need come into contact with the solid, and no acoustic power need be coupled into the bulk of the solid. An optical probe was used to measure the surface‐wave pattern excited; data obtained from this probe are presented, and results for the surface‐wave attenuation on stainless steel are given.

ISSN:0021-8979    

DOI:10.1063/1.1658537

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Very thin films of evaporated carbon have been deposited on various metallic thin film and single‐crystal substrates to form metal/carbon/metal sandwiches. Electrical conductivity measurements through the carbon, over a range of temperatures from room temperature to below 1°K, show that conduction is a process of complicated mechanisms dependent on the presence of both structural and impurity defects. At low temperatures, conductivity appears to be independent of temperature, and the carbon films may act as tunneling barriers of effective heights 30–100 meV. These heights can be engineered by suitable choices of evaporation and substrate conditions. Explanation of conduction in these films remains speculative.

ISSN:0021-8979    

DOI:10.1063/1.1658538

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Starting with the traditional time‐invariant parabolic‐cylinder model to represent the growth of a platelet‐shaped crystal in a pure supercooled melt, the effect of thermal imbalance at the solid‐liquid interface is calculated. In particular, the heat field equations are solved for the initial excess velocity distribution at the interface, on the assumption that the shape constraint is suddenly replaced by the constraint of heat conservation. The computed results resemble those found for the paraboloidal needle crystal. In particular, the hypothetical parabolic interface tends to bulge with a pronounced peak at one radius behind the leading edge of the platelet crystal. The maximum velocity principle, when applied to this model, yields a square‐law relation between the dimensionless tip velocity and the supercooling for pure ice and water. The results parallel those calculated by Horvay and Cahn for the isothermal model. The curvature effect of surface tension and the effect of the molecular attachment kinetics are found to lower the dimensionless tip velocity by about 20% below that in the isothermal model, uniformly over the range of supercooling considered.

ISSN:0021-8979    

DOI:10.1063/1.1658539

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Color‐center transformations in pure and dosimetry grade LiF have been studied over the wavelength range 105–750 nm. Both impurity‐ and irradiation‐induced optical absorption were measured, the latter by a relatively high x‐ray exposure at room temperature. Thermal and optical treatments were chosen to induce transformations which are easily related to the thermoluminescence (TL) process. The results demonstrate the following for dosimetry LiF: The TL‐associated 310‐nm absorption results from trapped electrons. The TL process intimately involvesV3centers (113‐nm absorption band). The centers causing the 225‐nm band (Z3band) and those causing the 137‐nm impurity band may both be involved in the sensitization (and supralinearity) phenomenon. A complete model for the TL is presented and discussed. Not so important to the TL is a broad apparently intrinsic absorption from 120–200 nm which is induced by irradiation in both pure and dosimetry LiF.

ISSN:0021-8979    

DOI:10.1063/1.1658540

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The effect of dislocations on both integrated intensities and rocking‐curve half‐widths has been studied in an (n, −n) double‐crystal spectrometer arrangement on copper crystals of 103<N<108dislocations per cm2. The results show that the rocking curve provides a suitable means of characterization of the density and the type of dislocations in this range. The changes in half‐width of the rocking curve and the integrated intensity produced by the dislocations can be described satisfactorily by the model of independently scattering perfect crystal blocks up to a dislocation density of 2×105/cm2, and at higher dislocation densities, lattice tilts produced by the dislocations appear to be the main source of line broadening.

ISSN:0021-8979    

DOI:10.1063/1.1658541

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Electro‐opticgcoefficients, which relate induced birefringence and polarization via the quadratic electro‐optic effect, are found to have the valueg11−g12≈g44≈ 0.011±0.008 m4/C2for 12 oxygen‐octahedra ferroelectrics having Pb in the perovskiteAsite. Measurements on four such ferroelectrics were made using ceramic compositions of the series (Pb, Ba) (Zr, Ti)O3which has a diffuse phase transition and a maximum quadratic electro‐optic coefficient ofR33−R13=3.4×10−16m2/V2at room temperature. To explain why the measuredgis 10 times smaller than those compiled and computed by DiDomenico and Wemple for other oxygen‐octahedra ferroelectrics, the optical‐index dispersion energy &egr;dand the birefringence dispersion constantKwere determined. Values of &egr;d=25 eV andK=−0.5 are as found for the other ferroelectrics. A smallergmust therefore arise from a smaller polarization potential coefficient &bgr;, which is defined with respect to energy shifts and polarization of the BO6perovskite octahedra. Such a definition can lead to a smaller &bgr; in Pb‐containing ferroelectrics because the higher polarizability and displacement of the Pb ion contribute polarization which is not properly accounted for. In any case, thegcoefficient can no longer be considered as a universal constant for oxygen‐octahedra materials.

ISSN:0021-8979    

DOI:10.1063/1.1658542

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Models for initial‐, intermediate‐, and final‐stage densification under pressure have been developed, which explicitly include both the surface energy and applied pressure as driving forces. For the initial stage, the time dependences and size effects given by the integrated equations are identical to those reported earlier for surface energy (alone) as the driving force. The only modification is that the surface energy (&ggr;) is expanded into (&ggr;+PaR/&pgr;), wherePais the applied pressure andRis the particle radius. For the intermediate stage of the process, the Nabarro‐Herring and Coble creep models may be adapted to give approximate (∼4×) densification rates for lattice and boundary diffusion models, respectively. In these cases the complex driving force is written as: (Pa/D+&ggr;k), whereDis the relative density, andkis the pore surface curvature. At the final stage of the process those models are invalid; an alternate model is developed based on diffusive transport between concentric spherical shells which will give a better assessment of the time dependence of densification high density (>95%); the driving force is (Pa/D+&ggr;k) in this case also. Because of the fact that the pore size is some unknown function of density, the rate equations cannot be integrated without further information. It is shown that of the various relations which have been assumed in development of models for hot pressing, for the effective stress in relationship to the applied stress and the porosity, (Pa/D) is the only form which satifies the criteria demanded by self‐consistency in generation of steady‐state diffusion models.

ISSN:0021-8979    

DOI:10.1063/1.1658543

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The diffusion of uranium and thorium in columnar pyrocarbons between temperatures of 1450° and 2065°C has been studied using the usual thin‐layer depletion technique. Constant coefficients are obtained that are in good agreement with results reported elsewhere for uranium diffusion in the ⟨a⟩ direction at actinide concentrations less than about 0.15‐mg actinide/cm3. Diffusion coefficients increase at higher actinide concentration. The anisotropic pyrocarbons are well oriented with the basal planes aligned parallel to the deposition surface. Diffusion coefficients parallel to the deposition surfaces are about three orders‐of‐magnitude greater than those in the perpendicular direction. The difference is due primarily to the pre‐exponential term in the usual Arrhenius equation. Autoradiographic studies of specimens in which diffusion occurred perpendicular to the deposition surface show segregation of actinides to growth cone centers or boundaries depending on whether diffusion occurs along or opposite the direction of growth of the pyrocarbon. The segregation is attributed to rapid diffusion parallel to the deposition surface coupled with the roughened nature of the surface. The results are discussed in terms of previously proposed models for actinide diffusion in columnar pyrocarbons.

ISSN:0021-8979    

DOI:10.1063/1.1658544

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

A two‐term expression consisting of a constant plus the product of an exponential and complementary error function is given for the transient reflected field of a horizontally polarized (Efield normal to the plane of incidence) plane wave in free space with a step‐function time variation incident obliquely on a plane imperfectly conducting surface. Curves computed with this expression are shown to be in good agreement with corresponding curves obtained numerically for all values of the time. Asymptotic expressions valid in early and late time are also given.

ISSN:0021-8979    

DOI:10.1063/1.1658545

出版商:AIP    

年代:1970

数据来源: AIP