摘要:

In this paper, the surface charge formation characteristics of thermoelectrets, prepared at different polarizing fields from a plant product, Boswellia Glabra, have been presented. A comparatively easy method for the fabrication of thermoelectrets of this meterial has been developed. The results obtained show that this plant product is a suitable material for electret preparation. Both the anode and cathode sides show the same sign of charge, without showing any charge reversal up to a polarizing field of 1000 V per mm thickness of the sample. The sample, prepared at zero field, indicates that the charge obtained with low‐field samples is not due to the molding charge. The results also fail to find experimental conformation of Gross's and Gemant's hypothesis concerning the production of homocharge with regard to the cathode sides of the low‐field Boswellia Glabra electrets.

ISSN:0021-8979    

DOI:10.1063/1.1658224

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Prebreakdown dc conduction and breakdown properties have been studied in thin‐film capacitors with dielectrics of CaF2, MgF2, NaF, and LiF. The results are compared mostly with theory developed for electronic breakdown in ionic crystals due to Fro¨hlich, Callen, and Forlani and Minnaja. Capacitors were formed on glass substrates with dielectric thicknesses from 700 to 30 000 Å and electrodes of Al. Between −200° and 100°C prebreakdown and breakdown conduction appear to be independent. The transition time between prebreakdown and breakdown conduction is 10−8–10−7sec. Except for some cases where the breakdown fieldFmax(≈106V/cm) displays an initial rise, an increase in temperature from −200 to 100 C is accompanied by a decrease inFmax. Only the initial rise can be interpreted in terms of theoretical developments by Fro¨hlich and Callen. An increase inFmaxis observed as the rate of voltage rise varies from 15 to 15 000 V/sec. The breakdown field also varies in most cases as w1/2, where w is dielectric thickness. This behavior is in agreement with the theory of Forlani and Minnaja, implying that impact ionization plays an important role in breakdown. A voltage threshold (≈15 V) for the cessation of breakdown which is independent of thickness and temperature is found for each material studied. This is consistent with the notion thatVminis the cutoff voltage of a gaseous arc. The above facts attribute an electronic nature to breakdown where, at the onset, the magnitude of current density is not altogether the critical factor. Evidence implies a period devoid of current enhancement just prior to breakdown during which the dielectric accumulates energy necessary to change to a gaseous state. It is the gas which affords the high conductance during breakdown.

ISSN:0021-8979    

DOI:10.1063/1.1658225

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The structural changes caused by explosive loading of a magnesium single crystal were investigated by using x‐ray Laue patterns, diffractometric measurements, and Fourier analysis of line profiles of x‐ray reflections. The results show that the magnesium single crystal was converted to polycrystalline metal with a high order of preferred orientation by explosive‐loading. The explosively loaded magnesium specimen consists of very small subgrains having an average size of about 500 Å. Experimental evidence tends to suggest that these subgrains are probably the result of instantaneous polygonization of an once highly strained, bent, and rotated lattice in the course of explosive‐loading. The residual strain associated with the subgrain structure is not as great as might be expected in view of the nature of loading.

ISSN:0021-8979    

DOI:10.1063/1.1658226

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A series of perfect single crystals of silicon have been irradiated with alpha particles from210Po to a maximum dose of 20.1×1015&agr;/cm2. The convoluted profiles obtained by means of a double‐crystal spectrometer in the (n, −n) arrangement, with a perfect unirradiated crystal on the first axis, exhibit a secondary peak on the low‐angle side with half‐width and peak intensity comparable to that of the main peak in most cases. The main peak does not seem to be much influenced by irradiation except for a slight depression and broadening. The dynamical theory as developed by Taupin for imperfect crystals has been applied to these results by assuming that the only effect of irradiation is to introduce a uniform change in the lattice constant of the crystal which is function only of depth normal to the crystal surface. The convoluted reflectivity of such an imperfect crystal is computed at the reflecting surface in the symmetrical Bragg case by using a profile for the damage as computed from the simple theory of collision based on Rutherford scattering and on the hard sphere model. A satisfactory agreement is found, on a qualitative basis, between computed and experimental profiles. The fact that the calculated profiles, within the limitations of the approximations involved, can reproduce the essential features of the experimental ones suggest that some coherence is preserved between the diffracted waves. The damaged region of the crystal, having a varying lattice parameter, gives rise to a secondary peak but exerts a negligible absorption effect on the main peak, which originates from the bulk.

ISSN:0021-8979    

DOI:10.1063/1.1658227

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The temperature dependence of elastic‐diffracted intensities for polycrystalline Al films has been investigated at 296° and 77°K as a function of crystal thickness in the range 600–2500 Å. A quantitative agreement is found with the predictions of the BJR two‐beam approximation of the dynamical theory. This agreement, however, appears to be best for Al in a range of thickness below the anomalous absorption distance or where kinematical theory should apply.

ISSN:0021-8979    

DOI:10.1063/1.1658228

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The self‐diffusion of titanium‐44 in TiCx(0.67≤x≤0.97) was studied in the temperature range 1920°–2215°C. The diffusion coefficients are composition independent and can be described by the expressionDTi*=(4.36−2.34+5.02)×104exp[−(176 400±3600)/RT] cm2/sec; titanium diffuses slower than the non‐metal atom by a factor of approximately 104in the temperature range investigated. The results support the concept of independent diffusion along the respective sublattice of each component.

ISSN:0021-8979    

DOI:10.1063/1.1658229

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The effect of open orbits on helicon propagation is investigated theoretically and experimentally. At the high field limit, the damping of helicons is found to be very sensitive to the amount of open orbit current, whereas the dispersion is only slightly affected. The low rf surface impedance of a copper single crystal is measured at 4.2°K in a magnetic field up to 55 kG. The field direction is oriented along or within a few degrees of the [001] axis. The result is compared with the calculated rf susceptibility based on the knowledge of several known open orbits. The standing wave pattern is observed whenBis parallel to the [001] axis. The ``effective Hall coefficient'' obtained from the dispersion relation agrees with other dc measurements. The attenuation whenBtilts away from the [001] axis is interpreted to be the damping effect caused by the two‐dimensional open orbit current. The average damping effect is smaller than that calculated from open orbits of known order.

ISSN:0021-8979    

DOI:10.1063/1.1658230

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A three‐term expression consisting of a constant plus two exponentials is given for the reflected field of a vertically polarized (Efield in the plane of incidence) plane wave in free space with a step‐function time variation incident obliquely on a plane imperfectly conducting surface. Curves computed with this expression are shown to be in good agreement with corresponding curves obtained numerically for a time range 0≤t≤10tr, wheretr, the relaxation time, is the ratio of permittivity to conductivity in the reflecting medium. Although the exponential time variation is limited to early times, it is shown that the three‐term approximation can be extended to times greater than 10trat the expense of introducing modified Bessel functions of zeroth order. The first approximation to the step‐function response is used to investigate the transient reflected field of a rectangular pulse: Explicit expressions are obtained for the reflected field, the energy of the reflected field, and the ratio of reflected to incident energy. The latter indicates that the ratio of reflected to incident energy for incident pulses less thantrin length is approximately equal to &agr;02, where &agr;0is the Fresnel reflection coefficient for a perfect dielectric.

ISSN:0021-8979    

DOI:10.1063/1.1658231

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Propagation coefficients and field configurations of the normal modes of an axially magnetized, plasma‐filled waveguide are treated by the Rayleigh‐Ritz variational technique. The results are placed in the form of a matrix eigenvalue problem. The eigenvalues are approximations to the propagation coefficients, and the eigenvectors contain Fourier coefficients of the electric fields of the normal modes expanded in terms of fundamental functions. This form is well suited to computer calculation and is used in a following paper to compute reflection and transmission coefficients of waveguide sections of finite length. The axially symmetric normal modes are examined in detail and are found to possess certain interesting properties that have not been fully recognized. First of all, these modes are reciprocal. Furthermore, under certain conditions they can, like ``helicon'' waves, be divided into ``ordinary'' (``cut off'') and ``extraordinary'' (``lossless'') modes. The axially symmetric modes are not circularly polarized, however, but instead change shape in a rather remarkable manner as they propagate.

ISSN:0021-8979    

DOI:10.1063/1.1658232

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The problem of mode conversion and scattering by a finite‐length section of magnetoplasma‐filled waveguide is formulated in matrix notation. This formulation becomes increasingly exact as the dimensions of the matrices (i.e., the number of modes considered) increases. The desired reflection‐ and transmission‐coefficient matrices are readily evaluated with a digital computer. Computations of TE010‐mode reflection coefficients indicate that good accuracy can be obtained with a fairly small (∼10) number of modes. Large errors result from considering only the incident (TE010) mode, however.

ISSN:0021-8979    

DOI:10.1063/1.1658233

出版商:AIP    

年代:1969

数据来源: AIP