摘要:

Measurements of the dark electrical conductivity &sgr;, and Hall coefficient were made on sintered spectroscopically pure zinc oxide powder samples over a temperature range from 100°K to 625°K and at room temperature on zinc oxide crystals containing lead impurities, using both the usual dc potentiometer‐probe method and an ac (4000 cps) set. In this reproducible range, ln&sgr; is not linear with 1/Tfor the sintered samples but exhibits maxima occurring at higherTand &sgr; values, the higher the sintering temperature; the electronic carrier concentration of the different samples, calculated from the Hall data, indicates thermal ionization of 1.6×1016to 9.0×1017donors/cm3lying 0.017 to 0.045 ev below the conduction band, with increasing ionization energy and increasing donor concentration, in general, occurring for higher sintering temperatures. Conductivity measurements are also made on sintered samples from 300°K to 1040°K using the ac (4000 cps) voltmeter‐ammeter method; above about 650°K, ln&sgr;versus1/Tis linear and in agreement with the results of previous workers with ionization energies varying from 1.4 to 2.4 ev. Back reflection x‐ray patterns indicate an increase in the lattice spacing after sintering; the change can be correlated with the presence of interstitial zinc atoms. The forbidden gap is taken to be about 3 ev, corresponding to an ultraviolet absorption edge and luminescence peak at 3.2 ev; the yellowing of worked or highly conducting or heated samples is attributed to a shift of this absorption edge into the visible. Room temperature impedance measurements show a frequency dependent reactance and resistance attributable to capacitive shunting of the high resistivity grain boundaries (&ngr;∼107cps). It is shown that the low frequency conductivity of the sintered samples is determined by the conductivity and relative thickness of the grain boundary material while the Hall coefficient depends mainly on the carrier concentration in the semiconducting grains; uncritical use of low frequency and dc results thus would lead to spuriously low values of the conductivity and mobility in relation to the true properties of the grains.

ISSN:0021-8979    

DOI:10.1063/1.1700063

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

The complex relationship between body profile and pressure distribution is important in the design of high‐speed undersea bodies. For slender axisymmetric bodies integration of the equations for irrotational flow has been accomplished, the coordinates of the body profile being related to the pressure distribution through coefficients of Legendre polynomials. The limitations and applications of this approximate analysis have been demonstrated through comparisons with preassigned conditions and with the results of experiments. Computed values closely approach those preassigned or measured throughout the central portion of the body if the maximum diameter does not exceed three‐tenths of the length. The analysis is accordingly useful in designing bodies with low coefficients of drag and low incipient cavitation numbers.

ISSN:0021-8979    

DOI:10.1063/1.1700064

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

The motion of suspensoid particles relative to that of the solvent resulting from polarization forces produced by an inhomogeneous electric field is defined as ``dielectrophoresis.'' It is analogous to the related phenomenon of electrophoresis, in which motion of suspensoid particles is produced by the action of an electrostatic field on the charged particles.From a consideration of theoretical calculations and from experimental observations it is concluded that the phenomena of ``dielectrophoresis'' and ``dielectro‐precipitation'' can be observed under rather ideal conditions, though it is ordinarily often difficult to do so because of the presence of the more easily produced electrophoresis or ion‐type migration of charged particles.The usefulness of dielectrophoresis (and/or electrophoresis) for removing suspended solid particles from polymer solutions during analysis is described.

ISSN:0021-8979    

DOI:10.1063/1.1700065

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

The classical Sommerfeld‐Runge method for deriving the laws of geometric optics in isotropic inhomogeneous media from Maxwell's equations is applied to problems in ``weakly'' anisotropic and inhomogeneous media. This leads to a generalized Hamilton‐Jacobi partial differential equation for the phases of the two waves (eikonals) which can propagate with finite velocity in such a medium. If one restricts oneself to a limited class of stress distributions, one derives a very powerful method of solving axially and spherically symmetric problems which would be very difficult to treat by the direct solution of the classical equations of elasticity. The Maxwell‐Neumann law is derived in variational form so that, in principle, one can estimate in any given case the error involved in the conventional assumption that the paths are straight lines instead of the actual Fermat paths.The last section treats the so‐called general problem of photoelasticity in which the principal stress directions in the plane of the wave front rotate arbitrarily as the latter propagates through the medium. Again the Sommerfeld‐Runge method gives one a solution which claims that if the rate of rotation is small in a wavelength one can ignore the contributions of the rotation to the integrated phase. An experiment is proposed to check this claim.

ISSN:0021-8979    

DOI:10.1063/1.1700066

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

Converging shock waves offer interesting possibilities of attaining very high temperatures and pressures. A theoretical treatment by G. Guderley which we have confirmed and extended by the method of characteristics indicated that the strength of a strong converging cylindrical or spherical shock varies inversely with a power (0.396 for &ggr;=1.4) of the surface area of the wave, thus becoming very great close to the center of convergence. The experimental production of high temperatures and pressures by means of these converging shocks depends on their ``stability'' of form. A converging wave is said to be stable if it approaches perfect cylindrical or spherical shape, thus damping out random disturbances as it propagates. The experimental work of L. G. Smith on Mach reflection is applied to show that these converging waves are stable for the shock range (M≤2.4) covered by his experiments. Smith's work and the theoretical work of Lighthill indicate that the stability decreases greatly at high Mach numbers.The simplest experimental method of achieving a cylindrical converging shock is by the use of a shock tube with a converging channel. This, however, results in the hottest region of the gas being in close thermal contact with the cold walls. An axially symmetric shock tube has been designed and constructed which produces a complete converging cylindrical shock rather than just a sector and in which the region of convergence is comparatively well isolated thermally from the walls. It has been found possible to converge a moderate strength shock wave (M=1.7) sufficiently to produce considerable luminosity at the center of convergence. Schlieren photographs are presented showing various phases of the formation and stability of these converging waves.

ISSN:0021-8979    

DOI:10.1063/1.1700067

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

A rigorous formula for the radiation field of a helical antenna is derived on the assumption of an empirical current distribution obtained experimentally by Kraus. For a helix of several turns this formula yields results very nearly the same as those obtained from an approximate method by Kraus, and it has the additional advantages of greater simplicity of computation and applicability to helices of nonintegral numbers of turns. It appears that this empirical current distribution is of a most fortuitous form, but no theoretical basis for it has yet been found.

ISSN:0021-8979    

DOI:10.1063/1.1700068

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

The theory of air‐coupled flexural waves in a floating ice sheet is derived for the case of an impulsive point source situated either in the air or in the water. It is found that new branches are introduced to the dispersion curve of flexural waves as a result of coupling to compressional waves in the atmosphere. Experimental data are briefly reviewed.

ISSN:0021-8979    

DOI:10.1063/1.1700069

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

An investigation of the effects of rolling and annealing on the orientation of individual grains in silicon ferrite strip yielded results which led to the formulation of a theory for the appearance of the ``magnetic'' orientation in the final processed strip. It is proposed that, in a fine‐grained strip of silicon ferrite, those grains which are already in the ``magnetic'' orientation before rolling will keep that mean orientation during rolling but become more highly strained than other grains, and therefore, they will become the portions of the cold‐rolled strip with highest free energy. In the annealing process these highest energy regions will recrystallize first, becoming lowest energy regions which will be able to consume the now higher energy regions surrounding them. Since these nuclei have the ``magnetic'' orientation, the final product also attains the ``magnetic'' orientation.

ISSN:0021-8979    

DOI:10.1063/1.1700070

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

Previous researches on the helix have made simplifying approximations about the nature of the boundary conditions. In this paper the field equations and boundary conditions are formulated exactly. They are then solved by an expansion in powers of the ratio of the thickness of the wire to the distance between turns.The method used consists of introducing a new coordinate system, which is such that a helical wire of circular cross section is a surface in which one coordinate is constant. Maxwell's equations and the electromagnetic boundary conditions are expressed in terms of this system. Since nonorthogonal coordinates are involved, the equations cannot be solved exactly; but a perturbation procedure may be applied as indicated above.The result of the analysis is to show that there is a principal mode, which propagates with the free space velocity of light in the direction of the wire. The characteristics of this mode are studied, and they are compared successfully with experiment.

ISSN:0021-8979    

DOI:10.1063/1.1700071

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

An experimental investigation has been made of the distributions of current and charge amplitude and phase on cylindrical antennas and on the driving lines. The presence of a stub support modifies the distributions on the antenna only near the junction of the antenna, line, and stub; however, for antenna loads near antiresonance, the distributions on the line as well as the impedances of the structure are altered considerably.A corrective network is defined for the terminal region of the junction. Impedances of theoretical models of antennas, the network, and conventional transmission line equations may then be used to predict the apparent impedances of the structure.

ISSN:0021-8979    

DOI:10.1063/1.1700072

出版商:AIP    

年代:1951

数据来源: AIP