摘要:

It is pointed out that the idealized problem of positron focusing considered by G. W. Hong has a simple analytical solution, but the properties of the exact solution do not agree at all with the approximate solution obtained by Hong's iterative procedure. In addition, the idealized problem considered by Hong completely ignores two factors which may play decisive roles in any practical focusing scheme.

ISSN:0021-8979    

DOI:10.1063/1.1710091

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The heat‐transport properties and heat capacity of UC, UP, and US were determined between room temperature and 600°C. These properties were measured by a transient technique with a laser as a heat pulse source. The thermal conductivity was calculated from the product of the thermal diffusivity, heat capacity, and density. At room temperature the thermal conductivity of the three compounds corrected to theoretical density was found to be 0.057, 0.033, and 0.021 cal sec−1cm−1°C−1for UC, UP, and US, respectively. At 600°C the value for UC decreased to 0.041 cal sec−1cm−1°C−1. The thermal conductivity of UP remained equal to its room‐temperature value and that of US increased to 0.029 cal sec−1cm−1°C−1. The room‐temperature heat capacity values for the three compounds investigated were all slightly above 12 cal mole−1°K−1. At 600°C the heat capacity of UC increased to 15.2 cal mole−1°K−1, whereas no appreciable changes were found for UP and US at this temperature. The experimental heat‐capacity values were used to formulate expressions of the typeCp=a+bT+cT−2with the aid of a computer program, and enthalpies and entropies of the compounds were calculated from 400° to 1000°K.The electronic contributions to the thermal conductivities of the three compounds were calculated by means of the Wiedemann‐Franz law with the Lorenz constant for metallic conductors. The limitations of this approach are discussed and an alternate method of separating the phonon from the electronic part of the thermal conductivity is presented. A value of 0.02 cal sec−1cm−1°C−1for the lattice conductivity alone was obtained. This value is close to the 0.01 to 0.02 cal sec−1cm−1°C−1obtained by using the Wiedemann‐Franz law on UP and US. The thermal conductivity of UC above 300°C can be ascribed to electrons if the traditional Lorenz constant is used. However, comparison of the thermal and electrical conductivity for the three compounds indicates that the Lorenz number when applied to UC should be lower, possibly because of the presence of optical scattering. It was found that the lattice resistance to the flow of heat in this group of compounds is essentially temperature independent and that calculations based on the classic models do not give good agreement with measured values.

ISSN:0021-8979    

DOI:10.1063/1.1710092

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The possible deformation behavior of spheres in a compact has been both theoretically analyzed and experimentally verified. The change in contact area radiusarelative to the particle radiusRhas been related to the bulk density and bulk strain for four possible modes of packing: simple cubic (Z=6), orthorhombic (Z=8), rhombohedral (Z=12), and body‐centered cubic (Z=8).An equation relating the above parameters can be represented byD−D0=32D0(a/R)2for different types of packings,DandD0being densities at anya/Randa/R=0, respectively. It has been shown experimentally by deforming lead spheres at room temperature, 50° and 100°C in a cylindrical die, that the over‐all deformation is similar to that of the orthorhombically packed monosized spheres. A change in the coordination numberZduring the deformation process was also observed and may account for the deviation from the theoretically predicted values.Similar experiments using sapphire spheres were carried out in the temperature range 1570°–1700°C, and the results showed that the deformation behavior was very similar to that of the lead spheres.

ISSN:0021-8979    

DOI:10.1063/1.1710093

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A transient interference experiment has been performed using the radiation from a normally spiking, pulsed ruby‐laser oscillator. In the experiment the laser output at timetis made to interfere with emission corresponding to an earlier time,t‐&tgr;, by means of an optical delay inserted into a two‐pinhole Young's interferometer. A light‐pipe photomultiplier oscilloscope system permits photographic recording of the interference fringes during a single spike of the laser emission, with a time resolution of less than 15 nsec. From the data obtained both the fringe visibility and the relative pattern phase of the interference pattern may be evaluated. Data are presented for both visibility and pattern phase for several delay times. The principal observation is that during a single laser spike the frequencyincreaseswith time. An analysis of the experiment and of possible causes for frequency variation of the laser with time results in the hypothesis that there is lattice strain in the ruby associated with the optical pumping to the excited state from which laser emission occurs. Linearity is assumed between strain and the number of excited ions. This permits a calculation to be made of the frequency change resulting from the strain and very reasonable agreement is obtained upon comparison with experiment. The visibility data are discussed but quantitative conclusions cannot be drawn.

ISSN:0021-8979    

DOI:10.1063/1.1710094

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Unidirectional solidification of the Fe‐FexSb eutectic resulted in the formation of aligned twelve‐sided iron rods in an FexSb matrix. The iron rods grew in the [111] direction and the FexSb matrix in the [001] direction. Half of the interfaces were parallel to the {110}Fe and {211}FexSb and the others were parallel to {211}Fe and {100}FexSb. Magnetic measurements on the controlled specimens showed that they had a much larger coercive force than bulk iron. The opening of the hysteresis loop is attributed to the small size and large aspect ratio of the iron rods.

ISSN:0021-8979    

DOI:10.1063/1.1710095

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Results of studies of Al‐Al2O3‐(Au or Al) thin‐film emission diodes are in agreement with a model based on electron injection by internal TF emission with subsequent energy loss in the insulating film, characterized by isotropic scattering with energy loss &Dgr;E=0.1 eV and mean free path &lgr;i=6 Å. The experimental results for Au overlayer films are found to be consistent with a modified ballistic transport model allowing weak elastic interactions, with a value for the electron‐electron scattering mean free path at 7.0‐V bias of &lgr;e∼47 Å. Low‐energy electron bombardment of the thin‐film sample is found to allow detection of overlayer‐film porosity, and a quantitative assessment of hot electron emission through holes in Au overlayer films is obtained. The results indicate that preferential electron emission through holes in pure Au films is not of significance over the entire range of Au film thicknesses studied; this conclusion is substantiated by results obtained with composite Al‐Au‐overlayer films.

ISSN:0021-8979    

DOI:10.1063/1.1710096

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Formal transport theory for heat conduction requires a constant thermal gradient over a small but finite distance ifQ=−KdT/dxis to be a good approximation. The kinetic model is used to show that this relation holds within 2% if the gradient is constant over a minimum range of ±5&Lgr;, where &Lgr; is the mean free path of electrons in metals. The usual differential equation of heat conduction cannot be applied to the absorption of blackbody radiation by metals if the absorption coefficient is high, since it overestimates the effect of thermal conduction and does not give the correct temperature distribution within the first few mean free paths from the surface.

ISSN:0021-8979    

DOI:10.1063/1.1710097

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A technique for the examination of the dynamic‐tensile‐strength characteristics of materials is presented. The dependence of tensile strength upon stress gradient, hence stress rate, is demonstrated for Al, Cu, Ni, and Pb. The results were tested and found to be self‐consistent in that once the dynamic‐tensile‐strength characteristics have been measured they can be incorporated into the numerical calculations, which then can be used to predict complex multilayer spall behavior even in layers of dissimilar materials.

ISSN:0021-8979    

DOI:10.1063/1.1710098

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

All possible linear electro‐optic retardation schemes for the twenty classes of linear electro‐optic crystals are synthesized in this paper under the single condition that light does not split directionally in a crystal. The result of this synthesis is tabulated in Table II. It gives the retardation and permissible polarization formulas for each class of crystals. It, therefore, may be used as a guide for designing linear electro‐optic devices. As an example, a linear transverse scheme for class 4¯2mcrystals (KDP, ADP) is selected from this table and verified experimentally. In this scheme, light is along (110) and electric field is along (001). This particular scheme has as advantages: no need for transparent electrodes and that the induced retardation can be increased by increasing the length‐to‐width ratio of the crystal. Table II depicts also the variations of retardation and permissible polarizations as the direction of light and electric field change under the above condition. This provides the reader with a foundation for synthesizing the electro‐optic‐polarization modulation and single‐side‐band‐suppressed‐carrier modulation (SSBSC). Again, as an example, a new scheme using cubic crystals for single‐side‐band and/or SSBSC modulation is proposed in this paper.

ISSN:0021-8979    

DOI:10.1063/1.1710099

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A theory is presented which provides insight into the effects of band‐bending on the energy distribution of photoemitted electrons. A ``band‐bending distribution function'' is introduced to relate the energy distribution of electrons when band‐bending is present to that when no band‐bending is present. Specific band‐bending distribution functions are presented for an exponential, a linear, and a more complicated dependence of potential on distance from the surface. Significant differences between the effects of upward and downward band‐bending related to apparent electron affinity and energy shifts in structure are discussed. The accuracy of the analysis improves for electron energies well above the threshold for escape. The density of surface states on homogeneous material required to produce observable effects due to band‐bending is estimated.

ISSN:0021-8979    

DOI:10.1063/1.1710100

出版商:AIP    

年代:1967

数据来源: AIP