Volume 38, Issue 10, 01 September 1967
Index of content:
38(1967); http://dx.doi.org/10.1063/1.1709019View Description Hide Description
The problem of two‐stream instability in plasma is studied by specifying the importance of initial magnetic field associated with the motion of the charged particles and the boundary effects. In Part I the accurate initial steady state is studied when the streams of electrons and ions move with different uniform speeds in plasmas with plane and cylindrical geometry. In Part II, in order to show the effects of finiteness and inhomogeneity of the system, small transverse plasma oscillations are studied in the case of plane plasmas. The role of plasma‐sheath oscillations at the boundaries is found to be very important in driving the instabilities associated with the electromagnetic modes. The numerical estimates of the growth rates of the instability are given for the specific case of the physical data in discharge tubes.
38(1967); http://dx.doi.org/10.1063/1.1709020View Description Hide Description
A method is presented for solving problems of one‐dimensional heat flow in regions with plane, spherical, or cylindrical boundaries. It is based on the physical concept of a uniform continuous‐source distribution on the boundary, whose strength varies with time in such a way as to meet the prescribed boundary condition. Examples of the method are given, including the solution for the temperature in a half‐space with an initial steady gradient of temperature and the boundary condition (∂T/∂x) − w(∂T/∂t) − hT = − hθ.
38(1967); http://dx.doi.org/10.1063/1.1709021View Description Hide Description
The H‐I c characteristics of Nb‐15 at. % Zr‐45 at. % Ti alloys are experimentally discussed in relation to metallurgical structure, particularly, precipitation. Peak effect may be due to the random distribution in barrier heights and the interaction between flux lines at the magnetic fields near H c2.
38(1967); http://dx.doi.org/10.1063/1.1709022View Description Hide Description
The magnetic field,Hfj , of the first flux jump during the initial magnetization of synthetic high‐field superconductors, composed of indium in porous glass, has been measured as a function of the temperature and the field‐sweep rate dH/dt. The behavior is reasonably well described by Hfj ∝log(dH/dt). At low values of dH/dt, log Hfj is approximately proportional to 1/(dH/dt). It is shown that under rather special conditions the flux‐jumping behavior depends upon the condition of the sample surface.
Acoustic‐Wave Propagation in Tetragonal Crystals and Measurements of the Elastic Constants of Calcium Molybdate38(1967); http://dx.doi.org/10.1063/1.1709023View Description Hide Description
A complete analysis is given of the propagation of acoustic waves in crystals of the tetragonal system. The determination of the elastic constants from measurements of longitudinal‐ and shear‐wave velocities in selected directions is discussed and experimental results for calcium molybdate are presented. The elastic constants in units of 1011 dyn/cm2 at 25°C are found to be
38(1967); http://dx.doi.org/10.1063/1.1709024View Description Hide Description
This paper describes an investigation of the diffraction of 100–500‐Mc/sec longitudinal elastic waves in anisotropicmaterials, using the techniques of optical (Bragg) scattering, which allow direct measurement of ultrasonic intensity distributions along the path of the beam. Such measured distributions for beams propagating along pure mode axes in crystal quartz,silicon, and potassium bromide are shown to agree very well with intensity distributions predicted using a diffraction integral derived from a rigorous solution of the related problem of electromagnetic diffraction in uniaxially anisotropic media. In addition, it is demonstrated that cylindrical crystal surfaces, symmetric about the proper pure‐mode axis, are appropriate excitation surfaces for cylindrically focused ultrasonic beams. The focusing properties of thin‐film CdS and ZnO transducers evaporated on cylindrical surfaces of crystal quartz and silicon are described and compared with the properties of focused beams in isotropic materials.
38(1967); http://dx.doi.org/10.1063/1.1709025View Description Hide Description
The initial response of solid models to low‐Mach, high‐mass‐flow shock waves was analyzed. A rapid‐response piezoelectric load cell was mounted in a series assembly behind the models to directly measure resultant loading. The load history was analyzed and correlated with model geometry and shock‐wave velocity for rectangular wood models of various length and weight and for the effects caused by changing the front surface material. The front surface of the model was in some cases bare and in other cases covered with plastic foam or thin foamrubber. It was observed that the coatings produced increased loading effects. The maximum loading force was determined for various overpressures and found to be in linear relationship to a dimensionless ratio involving shock‐tube and shock‐wave pressure parameters.
Correlations of Field‐Emitter Ring‐Rate Transients and Temperature Variations of Surface‐Tension Anisotropies38(1967); http://dx.doi.org/10.1063/1.1709026View Description Hide Description
Transients in the ring‐rate changes, which take place when the temperature of a field‐electron emitter is varied, have been observed for several crystal directions on bcc, fcc, and hcp crystals. It is proposed that these transients are the result of changes in the size of small surface facets which are in turn related to anisotropies in the surface tension.
38(1967); http://dx.doi.org/10.1063/1.1709027View Description Hide Description
Gravimetric and Seebeck‐effect measurements have been made on CoO single crystalsdoped with Li in the temperature range of 900°–1200°C and partial pressures of oxygen up to 1 atm.
For low‐doped crystals and high oxygen pressures, the gravimetric and Seebeck‐effect results confirm a simple model by which the positive‐charge carriers are given at equilibrium by the singly ionized vacancies and the Li ions. By this model the concentration of holes is increased, and the concentration of vacancies is decreased by the addition of Li.
The results at low pressures of oxygen for the low‐doped samples as well as the results in all the range of oxygen pressures for the heavier‐doped samples show that the model has to be enlarged, taking into account the contribution of Co interstitials to the defect structure of CoO(Li) and that of electrons to the conductivity in CoO(Li).
38(1967); http://dx.doi.org/10.1063/1.1709028View Description Hide Description
Ionic diffusion currents measured in a metal‐insulator‐metal structure yield an activation energy of 1.60 eV at 550°K for alumium ions diffusing into alumium oxide. Aluminum evaporated at 10−10 Torr on aluminum oxide, formed by chemical reaction of pure oxygen with the base electrode, produces a structure with a graded alumium‐concentration profile at the lower interface and a relatively abrupt aluminum‐concentration profile at the upper interface. Thermal activation of aluminum ions from the electrodes produces a transient current of diffusing metal ions when an external short circuit prevents charge accumulation on the electrodes.Analysis of the diffusion‐current data of aluminum into aluminum oxide gave values of 1.60 eV for activation energy and 5.8×10−19 cm2/sec for the diffusion constant.
38(1967); http://dx.doi.org/10.1063/1.1709029View Description Hide Description
A moments method is applied to the non‐self‐similar problem of the diffusion of radiation from a cavity, in the case that the transport of energy is purely radiative. The energy in the cavity is assumed to be introduced instantaneously, and the diffusion into the surrounding wall, particularly the motion of the radiation front, is followed for subsequent times. The moments method used has the characteristic that it gives, for the physical model assumed, exact results for the position of the front in both the short‐ and long‐time limits for all geometries.
Both plane and spherical cavities are treated in detail. Series solutions which apply in the limits of short and long times are given. Equations describing the first‐order departure of spherical radiation flow from plane flow are derived, and the short‐time solution is given. In plane geometry all quantities of interest can be written in parametric form in terms of elementary functions, and numerical results are presented in this case. Spherical geometry yields a more complex parametric solution, which is also given.
38(1967); http://dx.doi.org/10.1063/1.1709030View Description Hide Description
A study has been made of the thermoelectric powers (S) and electrical resistivities (ρ) of CrFe alloys over the concentration range 0 to 48.2 at. % Fe and temperature (T) range from 25° to 1000°C. The plots of S vs T and ρ vs T move in an orderly progression from that of pure Cr to the curve for the Cr51.8Fe48.2 alloy with no anomalies occurring. One might think that S should change sign as the alloy concentration passed through the region where Beck et al. observed their ``spike'' in the electronic specific heat, but no such phenomenon was observed.
38(1967); http://dx.doi.org/10.1063/1.1709031View Description Hide Description
The full Navier‐Stokes equations are solved numerically in cylindrical coordinates in order to investigate the splash of a liquid drop onto a flat plate, into a shallow pool, or into a deep pool. Solution is accomplished with the Marker‐and‐Cell technique using a high‐speed computer. Results include data on pressures, velocities, oscillations, droplet rupture, and the effects of compressibility. They also show how the technique can be applied to a wide variety of other complicated fluid flow problems involving the transient behavior of a free surface.
38(1967); http://dx.doi.org/10.1063/1.1709032View Description Hide Description
A variety of optical absorption, emission and luminescence excitation spectra have been measured in an attempt to identify the centers involved in the thermoluminescence of commercial LiF:Mg. It is concluded that the principal trapping centers consist of a hole trapped near various groupings of Mg2+ ions and vacancies. The optical absorption bands of these centers occur in the 3100–3800 Å region which contains several absorption bands corresponding to different geometries of the centers. It is suggested that the 2200‐Å band arises from Mg2+ ion‐vacancy complexes which have captured two holes. During thermoluminescence, holes are transported from traps to emitting centers. The luminescent center appears to be the F center both in an isolated position and when adjacent to a complex involving Mg2+ ions.
38(1967); http://dx.doi.org/10.1063/1.1709033View Description Hide Description
An investigation has been made of p‐n junction devices performing under conditions when the depletion layer was subjected to sudden generation of extreme carrier densities. Significant local distortion to the layer was identified, arising from the fact that the instantaneous high carrier density grossly changed the effective conductivity of the depletion layer. These findings are shown to be of particular significance when such junction devices are employed to analyze intense radiation phenomena.
38(1967); http://dx.doi.org/10.1063/1.1709034View Description Hide Description
The photoconductive response of several semiconducting diamonds was measured in the spectral range 0.8 to 4.7 μ with crystal temperatures of 4.2° and 77°K. Photoconductive minima, most of which are not observed in the optical absorptionspectrum of these diamonds, were recorded at 3.6, 3.5, 2.72, 2.65, 2.48, 2.41, 2.38, 2.35, 1.97, 1.87, 1.83, 1.78, 1.56, 1.44, 1.21, 1.05, 0.92, and 0.83 μ, and minor features were recorded at 2.44, 2.25, 2.02, 1.33, and 1.13 μ. Five activation energies, 0.21, 0.30, 0.37, 0.52, and 0.7 eV, for semiconducting diamonds have been observed. Good correlation is found between the photoconductive minima and the five activation energiesEAi (i=1,⋯, 5) through the terms En = EAi +nEph wherein Eph = 0.165 eV, the energy of a Raman‐frequency phonon, and n takes integral values from 1 though 8. The minima may be explained as follows. Holes are excited to the valence band by the incident radiation. They lose energy mainly through the emission of Raman‐frequency phonons. When the energy relations are proper, holes are left in excited bound states at acceptor sites and do not contribute to the conduction process.
38(1967); http://dx.doi.org/10.1063/1.1709035View Description Hide Description
Quantitative measurements of the kinetics of nucleation in some simple substrate‐overgrowth systems have been made using improved in situelectron microscopy techniques. The nucleation of bismuth and silver on evaporated carbon and the nucleation of bismuth on evaporated SiO substrates were studied as a function of substrate temperature and impinging flux. The results were analyzed in terms of the phenomeno‐logical theory of nucleation and in terms of Walton's atomistic model of condensation from the vapor phase. Both theoretical concepts led to specific conclusions concerning such nucleation parameters as the number of atoms in the critical nucleus n* and the free energy of desorption ΔG des. The substrate temperature and impingement flux dependence of the maximum number of deposit particles that can be accommodated on the substrate surface were determined and tentatively interpreted on the basis of nucleation‐limited surface migration. Some nucleation induction time observations were made and were related to the clustering kinetics of adatoms, the growth process of individual nuclei, and the instrumental limitations of the detection method.
38(1967); http://dx.doi.org/10.1063/1.1709036View Description Hide Description
Isothermal changes in long‐range order S, x‐ray line shifts due to stacking faults Δ, and x‐ray particle size from Fourier analysis of line shapes D were measured for filed samples of Ni3Al. Recovery temperatures between 200° and 270°C were investigated. The recovery of all three measured properties appeared to depend upon the mobility of vacancies in this temperature range. The evaporation of vacancies from faulted loops appeared to supply the vacancy flux for ordering and particle‐size changes. Fault shrinkage was found to cease quite abruptly between 215° and 200°C; at 200°C faulting initially increased, during which ordering did not occur, but after some 20 h the faulting began to anneal out and somewhat later ordering proceeded. An activation energy of 1.65 eV was found for the ordering reaction; this compares with a value of 1.5 eV for Em v in nickel.
38(1967); http://dx.doi.org/10.1063/1.1709037View Description Hide Description
Previous treatments of stress‐pulse‐activated piezoelectric and ferroelectric stress gauges and power generators neglect the secondary stresses produced by circuit‐enforced fields in not‐directly‐stressed parts of the transducer. It is pointed out that under short‐circuit conditions these secondary stresses generate strong effects in materials with high electromechanical coupling factors. To demonstrate this, a thin plate activated by a plane rectangular low‐amplitude stress front is considered. A rigorous theoretical analysis is sketched for this configuration. It is found that short‐circuit current pulses generated by the stress‐front transit are not rectangular, but vary exponentially in time. The exponent is proportional to the square of the electromechanical coupling factor and depends on the mechanical boundary conditions. Supporting experiments are reported.
38(1967); http://dx.doi.org/10.1063/1.1709038View Description Hide Description
Decay of the electron distribution injected into a polymeric solid at 2MeV was investigated by measuring the charge released during a triggered discharge. From the decay time constant τ=ρε, resistivity ρ was calculated. For polymethylmethacrylate, the variation of ρ with temperature yielded an activation energy or trap depth W≈0.6 eV. Discharges resulting in Lichtenberg figures were also produced in polystyrene and polypropylene. Trap depths were estimated to be W≈0.6 eV for polystyrene and W≳0.4 eV for polypropylene.