Volume 38, Issue 12, 01 November 1967
Index of content:
Determination of the Pressure of the Barium I‐II Transition with Single‐Stage Piston‐Cylinder Apparatus38(1967); http://dx.doi.org/10.1063/1.1709184View Description Hide Description
The pressure of the barium I‐II transition has been determined in a modified single‐stage piston‐cylinder apparatus to be 55.0±0.5 kbar at 22°C. Three barium samples of different purity were used and no significant variation of the transition pressure found. The temperature coefficient of the transition pressure is 0–0.015 kbar/°C near room temperature. The effect of the presence of dissolved hydrogen on the transition pressure is discussed.
38(1967); http://dx.doi.org/10.1063/1.1709185View Description Hide Description
If absorption in the reflector is low, ultrasoft x rays exhibit a well‐defined critical angle of total reflection. Experiments with simple apparatus verify critical angles calculated for paraffin of θ c ≈4° for carbon radiation (λ=44.7 Å) and θ c ≈6° for boron radiation (λ=67.7 Å). Utilization of this concept in a low‐resolution (10<λ/Δλ<20) instrument for the chemical analysis of carbon and boron by a paraffin reflector and of nitrogen, oxygen, and fluorine by a lithium fluoride mirror is suggested.
38(1967); http://dx.doi.org/10.1063/1.1709186View Description Hide Description
Experimental data on the heat generated during torsional oscillations of polymethylmethacrylate tubes are presented. Various factors which affect the internal heat generation, including the amplitude and frequency of oscillation and the ambient temperature are investigated. A comparison of the mechanical energy added to a specimen during a cycle of deformation and the resulting heat indicates that some energy is ``stored'' during the process. In addition to the detail of the response during a single cycle of deformation, the longtime response of the material to constant strain‐amplitude oscillations is investigated. The data demonstrate that thermomechanical coupling effects can be extremely important.
38(1967); http://dx.doi.org/10.1063/1.1709187View Description Hide Description
The anharmonic decay of a high‐frequency longitudinal elastic wave at low temperatures, such that ℏω> KT, is calculated by perturbation theory, using a Grüneisen parameter as measure of the anharmonicity. The attenuation at very low temperatures varies as the fifth power of frequency, and is small at frequencies below 1012 Hz.
38(1967); http://dx.doi.org/10.1063/1.1709188View Description Hide Description
Acceleration of a microwave‐energized plasma by expansion in a diverging magnetic field is studied. Results of a theoretical collisionless particle analysis are reviewed. An experimental accelerator is described and its pertinent conditions are considered. Measurements of plasma beam shape and potential are compared with the theoretical predictions. The conclusions are that, while some experimental characteristics (beam shape and potential profile) agree with theory,collisions in the experimental device prevent it from attaining operating conditions predicted by collisionless theory.
38(1967); http://dx.doi.org/10.1063/1.1709189View Description Hide Description
Avalanchegeneration of minority carriers was studied under pulsed‐field conditions in thermally oxidizedsilicon surfaces. In the doping range between 5×1016 and 1×1018 cm−3avalanche breakdown voltages and fields are in agreement with step‐junction results. For lower doping, breakdown voltages are below the expected values. Avalanche light‐emission observations revealed this effect to be due to field concentration at the contact edges. In the higher doping range between 1018 and 1019 cm−3 minority carriers are produced by Zener tunneling. Band bending for tunneling is 1.1 V, independent of doping.
38(1967); http://dx.doi.org/10.1063/1.1709190View Description Hide Description
When a field greater than that required to produce Gunn instabilities is applied to gallium arsenide containing free electrons in concentrations of 1017 cm−3 or more, excess current rather than current saturation is seen. Impact ionization due to high‐field domains causes a substantial increase in carrier density. The processes involved in the generation and recombination of these excess carriers are analyzed here. Recombination occurs by competing radiative and nonradiative mechanisms, with long‐term trapping also playing a role. Measurements of the time dependence of excess conductivity and recombination radiation, the spectral distribution of the radiation, and its intensity as a function of excess carrier concentration, allow the generation rate and recombination cross sections to be deduced. The generation rate is consistent with that expected from flat‐topped domains. Cross sections for the capture of both holes and electrons by the recombination centers dominant under these conditions are of the order 10−17 cm2. The emitted radiation is strongly self‐absorbed; correction for this factor shows that most of the original emission has energy greater than the bandgap. The direct recombination cross section is deduced to be near 5×10−18 cm2, which is in reasonable agreement with that derived from the van Roosbroeck‐Shockley theory.
38(1967); http://dx.doi.org/10.1063/1.1709191View Description Hide Description
The theory of hot electrondiffusion is developed for the case of space‐charge‐limited current flow in insulators or weakly dopedsemiconductors. To compare the results of the calculation with experiments on siliconn +‐π‐n + structures by Denda and Nicolet, a crude theory of the uniform‐field hot‐electron effect in n‐type silicon is developed.
38(1967); http://dx.doi.org/10.1063/1.1709192View Description Hide Description
A consistent electronic theory of adhesion is developed relating to the contact of bodies of different nature.
The strength (as well as energy) of adhesion is calculated for the contact of crystal bodies (semiconductors,metals) as well as for the contact of amorphous bodies whose surface is saturated by donor and acceptor functional groups. The theory developed allows us to calculate the electrostatic component, when certain (experimentally determined) physiochemical characteristics of studied objects are known. The theory also determines the temperature dependence of the adhesion strength. The main inference is that cases of electrostatic component reading values of the order of 108−109 (dyn/cm2) are fairly realistic. There follows from the theory the role of surface conditions as the chief factor of high adhesion. The theory suggests the ways for conditioning the adhesion via chemical modification of the surfaces (sometimes also of the bulk) of contacting bodies, and indicates which characteristics of solid bodies are to be studied when investigating the adhesion.
38(1967); http://dx.doi.org/10.1063/1.1709193View Description Hide Description
The reflectance spectra of the cubic fluoride perovskites KMgF3, KMnF3, KCoF3, KNiF3, RbMnF3, and mixed crystals of K (Mg0.8:Ni0.2) F3 and K (Mg0.5:Ni0.5) F3 have been measured from 4000−40 cm−1 at 300° and 85°K. The spectra of all crystals at both temperatures have been fitted by means of a classical dispersionanalysis to yield the frequencies, oscillator strengths, and damping constants of the three ir active lattice vibrations. An assignment of the normal mode frequencies to particular vibrations has been made. The high‐ and low‐frequency dielectric constants of these materials have been determined from the reflectance measurements. The isotropic polarizabilities of Mg2+, Mn2+, Co2+, Ni2+, and Rb+ have been calculated. The transverse effective charge at 300°K has been obtained in order to determine the strength of second‐order dipole moments due to charge deformation during lattice vibrations.
38(1967); http://dx.doi.org/10.1063/1.1709194View Description Hide Description
The transmission spectra of thin single‐crystal samples of KMgF3, KMnF3, KCoF3, KNiF3, KZnF3, and K (Mg0.5:Ni0.5) F3 have been investigated from 2.5–400 μ (4000−25 cm−1) at 300° and 85°K. A large number of bands have been observed in each material and these have been interpreted in terms of multiphonon processes at critical points in the lattice dispersion curves. The average energies of the phonon branches at the edge of the Brillouin zone have been deduced from the observed infrared spectra.
38(1967); http://dx.doi.org/10.1063/1.1709195View Description Hide Description
A solid‐to‐solid phase transition has been induced in antimony by an explosive‐produced shock wave of about 130 kbar. Details of the transition were determined by measurement of the free—surface motion with electrical pin contactors. The first plastic wave pressure varied from 108 kbar for the thinnest samples to 91 kbar for the thickest samples. This variation is felt to be due to the sluggishness of the transition, the material requiring about 0.6 μsec to relax into its higher‐density phase. The transition pressure was found to be 88 kbar. Hugoniot data are presented in conjunction with existing higher‐pressure dynamic data.
38(1967); http://dx.doi.org/10.1063/1.1709196View Description Hide Description
This paper develops geometrical optics ray techniques for problems of transient electromagnetic wave propagation in inhomogeneous, lossless, dispersive, dielectric media. The method results in a series expansion of the fields about the wavefronts. The theory is applied to solve a few illustrative problems dealing with wave propagation in a cold isotropic plasma. Special attention is given to the fact that in all physical media the wavefronts must propagate at the speed of light in vacuum. This physical requirement does not seem to have been incorporated into the mathematical models used in previous works dealing with geometrical optics techniques for solving transient electromagnetic wave problems.
38(1967); http://dx.doi.org/10.1063/1.1709197View Description Hide Description
The thermal conductivity, Seebeck coefficient, and electrical resistivity have been measured as a function of temperature for various impurity concentrations and alloy compositions. n‐type alloys, doped with tellurium to give room‐temperature resistivities of 0.7−2×10−3Ω·cm, provided the optimum figure of merit, which reached a maximum of 1.15×10−3 deg−1 at 600°K. This represents an improvement by over a factor of two above the best values for n‐type Ge‐Si alloys below 800°K. In contrast, the figure of merit for p‐type alloys are generally inferior to that of Ge‐Si alloys at all temperatures.
38(1967); http://dx.doi.org/10.1063/1.1709198View Description Hide Description
Before self‐locking of modes can be accomplished in a laser, all adjacent longitudinal modes must have the same frequency separation. It can be shown that the dispersion associated with the laser transition generally causes longitudinal modes to have slightly different frequency separations. For homogeneously broadened lines these differences are indeed very small, but for inhomogeneously broadened lines they become significant. An equidistant mode separation can be produced when the mode oscillations shift slightly in frequency and lock onto nonlinear polarization source terms produced by the interaction of three modes. The question as to whether mode locking is actually achieved depends upon the relative size of the required frequency shift and the amplitude of the nonlinear source terms. Among other variables, the nonlinear polarization is proportional to the fourth power of the dipole matrix element between the two states in question, and thus depends strongly on their radiative lifetime. For example, in Nd3+‐doped glass the matrix element between the laser states is too small for locking, but a short‐lifetime saturable absorber inserted into the cavity will produce self‐locking.
38(1967); http://dx.doi.org/10.1063/1.1709199View Description Hide Description
Re‐emission coefficients for SiO2films and for Si filmsdeposited through rf sputtering have been measured as a function of pressure and input power. Values for SiO2 were 0.86−0.30 and for Si 0.45−0.07. High re‐emission coefficients correlated with negative potentials on the deposit surface, which, in turn, caused resputtering of the deposit by positive ions drawn from the glow discharge. The existence of additional mechanisms for re‐emission was established for rf‐sputtered silicondeposited at zero substrate bias. The re‐emission coefficient of dc‐sputtered silicon was found to have values comparable to those of rf sputtering.
38(1967); http://dx.doi.org/10.1063/1.1709200View Description Hide Description
Expressions are derived for calculating radiant heat transfer rates in laminar arrays of high‐emissivity fibers having diameters much larger than the wavelengths of the emitted radiation. New experimental results are given for a sample composed of 280‐μ fibers, with the mean sample temperature near or below room temperature, so the dominant wavelengths of fiber emission are of the order of 10 μ. The data show general agreement with the theory, and establish clearly that the rate of radiant heat transfer is proportional to the cube of the mean absolute temperature of the sample. A brief review of published data suggests that the derived expressions also give correct functional dependence on insulation parameters for finer fibers having diameters equal to, or less than, the wavelengths of the emitted energy. But the absolute magnitude of the values reported for finer fibers is generally larger than that predicted by the large‐diameter expression, and in some cases is larger by a whole order of magnitude. Reflection, diffraction, and fiber transparency effects are probable causes of the higher values. Since these effects are neglected in this analysis, it is suggested that the expressions given here may represent a lower limiting value for smaller‐diameter fibers.
38(1967); http://dx.doi.org/10.1063/1.1709201View Description Hide Description
Precise cell dimensions of GaAs,GaAs0.59P0.41, GaAs0.5P0.5, and GaP have been determined from x‐ray powder diffraction patterns from −62° to 200°C. Within the temperature range investigated, the constant thermal coefficients of expansion are, respectively, 6.86×10−6, 7.81×10−6, 5.91×10−6, and 5.81×10−6°C−1. The cell dimension of pure GaP (impurity less than 3.0 ppm) is 5.4495±0.0001 Å at 24°C.
38(1967); http://dx.doi.org/10.1063/1.1709202View Description Hide Description
Detailed studies have been carried out to define the phase relations in the Te‐rich region of the Bi‐Tl‐Te system. Isopleths were constructed for the Bi2Te3‐Tl2Te3 and BiTlTe2‐Te sections, and a schematic diagram is suggested for the projections of the liquidus lines and the invariant planes. The phase behavior can be characterized by the cooling reactions: L+Bi2Te3→BiTlTe2+Te at 325°C, and L→Tl2Te3+BiTlTe2+Te at 223°C. No evidence was found to support the existence of the compound ``BiTlTe3'', as reported by Borisova et al.
38(1967); http://dx.doi.org/10.1063/1.1709203View Description Hide Description
A general, unified solution of the wave excitation due to electric‐current sources, magnetic‐current sources, fluid‐flux sources, and mechanical‐body sources in a compressible plasma which may be anisotropic and inhomogeneous is presented. The Maxwell—Eulerequations are reformulated through linear operator and generalized transform techniques into an equivalent matrix integral equation. The dispersion relation can be obtained from the kernel of the integral equation. When the medium is homogeneous, this integral equation has an ideal kernel and the explicit solution can be easily obtained. Equivalence relations between different types of sources are obtained from the forcing function of the integral equation, which can be employed to express the field excited by one type of source in terms of the field excited by another type of source. Generalized telegraphist's equations are also derived in due course.
Some dispersion curves, and asymptotic solutions for the radiation field from a point current source oriented in the direction of a constant magnetic field, are presented in graphical form. A proper ionospheric plasma is assumed for this calculation, which combines some of the results obtained for anisotropic cold‐plasma problems on the one hand and some of the results obtained for isotropic warm‐plasma problems on the other hand.