摘要:

A new type of electromechanical coupling is postulated to explain the polarization signals resulting from the shock loading of nonpiezoelectric materials. The driving force is viewed as being proportional to the gradient of the stress. The electromechanical coupling constant is found for an ionic solid by deriving the effect for a linear ionic chain. A stress gradient of 1011dyn/cm3is found to give rise to a voltage gradient of 10 V/cm.

ISSN:0021-8979    

DOI:10.1063/1.1714210

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

By measuring voltage drop in xenon flashtubes the resistivity of intense xenon plasmas is found to bear an inverse square root relation to the current density. A weak dependence on pressure is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1714211

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The total ultraviolet radiation from self‐sustained Townsend discharges in hydrogen has been measured by means of a sodium salicylate wavelength shifter and a photomultiplier tube. The excitation coefficient per unit pressure &dgr;0/phas been determined forE/p=80−250 V/Torr‐cm. &dgr;0/pas a function ofE/ppasses through a maximum forE/p≈150 V/Torr‐cm. The pressure dependence of the radiation can be ascribed to quenching with a quenching pressure equal to 20 Torr. An attempt has been made to evaluate separately the excitation coefficient for the 21&pgr;uradiation by subtracting other uv contributions from the total light intensities observed.

ISSN:0021-8979    

DOI:10.1063/1.1714212

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The history of motion of charge carriers generated in a gap by ionizing radiation is investigated. The study is limited to cases where the field‐induced motion of these carriers overcomes their inertia and diffusion; the motion is thus completely collision‐dominated. The severity of the space‐charge effect is described by a single nondimensional parameter &ggr;. When &ggr; is not much smaller than unity, usual linearization procedures are inapplicable. The nonlinear problem is solved for two limiting cases. In the first, the electric current is carried by the free electrons and ions as originally created. In the second, the electric current is carried by positive and negative ions, the latter formed by electron attachment. The method of characteristics is employed to solve the equations of motion and exact analytical and numerical solutions are presented. It is shown that if &ggr;>2 the nonlinear effects of the induced charges are important and depart appreciably from calculations based on linear approximations.

ISSN:0021-8979    

DOI:10.1063/1.1714213

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The modes of screw instability in a positive column of a nitrogen gas discharge have been studied. Under a certain condition, the critical magnetic field for them=2 mode has been found to be smaller than that for them=1 mode. In addition, the regular oscillations have been observed at the magnetic field larger than the critical one. These phenomena can not be described by the results of Kadomtsev and Nedospasov which are calculated from the small‐amplitude theory. In this paper a theoretical explanation for these phenomena is proposed based on the conception that only the instability which has the maximum growth rate can grow while other instabilities are masked.

ISSN:0021-8979    

DOI:10.1063/1.1714214

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

In an anisotropic medium there are only certain directions along which elastic waves can propagate in pure longitudinal and transverse modes. For the determination of third‐order elastic coefficients from sound speed measurements in stressed crystals it is desirable to know these modes. Using a method due to Borgnis the pure mode directions are determined for all crystal point groups belonging to the orthorhombic, tetragonal, cubic, rhombohedral, and hexagonal systems. The eigenvalue problem is solved for each of these directions, and the polarization vectors and the wave speeds are tabulated.

ISSN:0021-8979    

DOI:10.1063/1.1714215

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Third‐order elastic coefficients can be derived from sound speed measurements in statically stressed crystals. The necessary relations between experimental data and elastic coefficients are here presented for orthorhombic, tetragonal, cubic, rhombohedral, and hexagonal crystal classes. For each symmetry a sufficient number of wave modes and stress systems is selected to determine all third‐order coefficients and to allow experimental cross checks. All the modes considered are pure longitudinal or transverse. They are selected so as to minimize the number of specimen orientations required, and following experimental procedures for uniaxial static stress, only wave modes propagating at right angles to the stress direction are considered.

ISSN:0021-8979    

DOI:10.1063/1.1714216

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

A complete theory of the lens‐shaped intersection of Kossel lines in the transmission pseudo‐Kossel method is developed. The general expression gives the lattice parameter with respect to the characteristics of the interaction of two arbitrary conics, i.e., conics corresponding to arbitrary diffracting planes and arbitrary wavelengths. Geometrical considerations involving the conics are developed and from these, the errors involved in lattice parameter measurement are evaluated. Lattice parameter variations of 10−5can be easily measured with suitably chosen conic intersections. The precision to be expected as a function of measurement errors is expressed as a sensitivity ratio and tabulated for an Fe‐3wt.%Si alloy. Several methods of reducing measured lengths to lattice parameter data are evaluated.The IBM 7090 computer has been used to find suitable intersections for precise lattice parameter measurement. The computer supplies the wavelengths and conics to be used in the transmission pseudo‐Kossel method. Consequently, a long and tedious experimental investigation is avoided. Examples using Fe‐3wt.% Si are given; the lattice parameter for this alloy is found to be 2.86268±0.00003 Å. Proper radiations for investigation of Ge and diamond are indicated.

ISSN:0021-8979    

DOI:10.1063/1.1714217

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Experimental data are presented for the thermal propagation of a normal‐superconducting boundary along wires of Nb‐25%Zr, Nb‐37%Zr, and Nb‐51%Zr. The thermal propagation velocity was measured as a function of current at 4.2°K in constant longitudinal and transverse magnetic fields up to the upper critical fieldHc2as well as in a zero magnetic field for temperatures between 2.5° and 11°K. Previous models which describe thermal propagation phenomena neglected the temperature dependences of the specific heat, thermal conductivity, and electrical resistivity. A more exact model is proposed here which takes into account the temperature dependences of these quantities, and all the present experimental data are in good agreement with this model. From a measurement at 4.2°K of the dependence of the thermal velocity on magnetic field, the high‐field transition temperatures as a function of magnetic field are calculated for temperatures above 4.2°K. In conjunction with the theory, the linear dependence of the thermal velocity on current implies a weak current dependence for both the transition temperature and superconducting‐state specific heat in Nb‐Zr alloys up to 3×105A/cm2. A method is discussed for using thermal propagation techniques to determine the superconducting‐state specific heat in the presence of known transport currents and magnetic fields.

ISSN:0021-8979    

DOI:10.1063/1.1714218

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Using an ultrasonic pulse echo technique operating at frequencies ∼10 Mc/sec, we have obtained the following elastic constants for cubic PbF2: C11=8.88×1011dyn/cm2,C12=4.72×1011dyn/cm2,C44=2.454×1011dyn/cm2at 27°C. From these data a Debye &thgr; of 221.2°K was calculated.

ISSN:0021-8979    

DOI:10.1063/1.1714219

出版商:AIP    

年代:1965

数据来源: AIP