摘要:

It has been established that LiTaO3can be made resistant to laser‐induced inhomogeneities in the index of refraction at power levels as high as 500 W/cm2. This is accomplished by annealing LiTaO3in an electric field of 250 V/cm at a temperature of 700°C for ½ h and then cooling the crystal to room temperature with the field on at a rate of 100°C/h. The susceptibility to laser‐induced index changes in LiNbO3is reduced by this treatment but not to the same extent as in LiTaO3. A mechanism is proposed to explain the observed reduction in susceptibility to damage.

ISSN:0021-8979    

DOI:10.1063/1.1710070

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Samples of polypropylene were cold drawn to a maximum extension of five times their original length and then annealed, under no load, at 160°C. Eighty percent of the extension was recovered on annealing. Samples were then redrawn and annealed again repeating the procedure through five drawing cycles. While the first drawing proceeded by neck formation and propagation, subsequent drawings were homogeneous. One hundred percent of the extension in the second and subsequent cold drawings was recovered on annealing. In general the polypropylene became more rubbery with each subsequent cold drawing in the sense that less of the drawing extension was retained after removing the drawing load and the modulus of drawn material decreased while its elastic extensibility increased. Micrographs of sections cut from the polypropylene showed a spherulite structure which was retained throughout the drawing and annealing. After several cycles a distinct Maltese‐cross pattern was seen when viewing the sections between crossed polars. Such a pattern is not generally seen with polypropylene spherulites.

ISSN:0021-8979    

DOI:10.1063/1.1710071

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Data are presented for tests which were conducted on Nb‐33% Zr superconducting cable. The method of testing is described and provides a reliable method for predicting the maximum stable current and field which can be expected for a magnet wound from the cable.

ISSN:0021-8979    

DOI:10.1063/1.1710072

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

This paper contains calculations of (a) the effect of a circular hole on sheet‐resistance readings as measured by a four‐point probe; (b) a solution to the generalized four‐point probe measurement on a thin circular disk, regardless of the shape, position, and orientation of the probe. The first analysis is intended to aid in the estimation of the effect of pinholes in thin films, and also in certain thin‐film circuits where holes have been deliberately made in the substrates. The greatest effect on a linear and evenly spaced four‐point probe is found when a hole is just touching one of the potential probes. It is shown that an effect of up to +100% or −40% can result from a hole whose diameter equals the probe spacing, depending upon its position relative to the probes. Analytical expressions are developed for several limiting cases, including that of a straight‐edge boundary, in which case the results coincide with earlier work. The second analysis can be used to calculate the sheet resistance of a thin semiconductor wafer, and to evaluate its uniformity. For certain special cases, the present analysis coincides with results obtained elsewhere.

ISSN:0021-8979    

DOI:10.1063/1.1710073

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The variation in the ac output current in a coherent optical‐detection process as a function of orientation has previously been analyzed for point sources. The work is extended here to include finite sources, finite bandwidth, random‐phase sources, and rough surfaces. The general expression for two finite sources in the Fraunhofer region indicates that the directivity is the product of the radiation patterns of the individual sources multiplied by the directivity of the detector due to point sources. For typical lasers the bandwidths are so narrow that the directivity is not practically affected by the finite width. ``White'' surfaces are shown to be equivalent to uniform surfaces with a random phase which is statistically independent from point to point. These surfaces are shown to have effectively the same directivity as far‐field point sources.

ISSN:0021-8979    

DOI:10.1063/1.1710074

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

An equation has been derived for the exposure‐dose rate due to radiation emitted by x‐ray tubes and transmitted through media which absorb all characteristic radiation. The parameters involved in this expression are the operating voltage and current of the tube and the linear‐absorption coefficient and thickness of the absorbing media. Experimental verification of the equation shows that exposure‐dose rates can be calculated within ±6% on a relative basis.

ISSN:0021-8979    

DOI:10.1063/1.1710075

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

An experimental investigation of the average range of fission fragments in pyrolytic graphite was performed by exposing coupons of this material bearing a thin electromagnetically deposited layer of235U to thermal neutrons. For the experiments, reliable and reproducible sectioning techniques were developed for the determination of ranges in thick targets with variable structure. The procedures used to obtain the range average in terms of the experimental data are described in detail, and an empirical formula for the calculation of ranges in pyrocarbons is given.Analytical expressions were generated for describing the penetration of the recoil‐fission fragments in the target material and evaluating various theoretical models of electronic and nuclear‐stopping mechanisms in a reference structure. It was found that for carbons of homogeneous structure and with void sizes smaller than Bohr's straggling parameter, the penetration curves could be obtained as convolutions of a kernel of the form of a normal‐distribution curve. For the thin extended sources used in this investigation, the various integrations performed mitigate straggling effects so that the only important parameter for data correlations is the average range, which was obtained for four mass chains in six different experiments. The values so obtained compared favorably with Lindhard's and Frank's microscopic models for the calculation of the average range in carbon.

ISSN:0021-8979    

DOI:10.1063/1.1710076

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Space‐charge‐limited (SCL) current of holes has been observed in high‐resistivity silicon at room temperature through proper sample design and suitable choice of an upper limit for electric fields applied in the sample. Joule heating and hot‐carrier effects, which limited the observation of SCL current in previous work, were hardly noticeable in our measurements. Current—voltage characteristics were measured and found to contain an Ohmic portion followed by a region that obeyed the theoretical space‐charge‐limited‐current lawJ=(9/8)&egr;&mgr;pV2/L3. This behavior has been verified for various samples differing in lengths by a factor of 10. The value of the incremental capacitance of the samples, when suitably corrected for fringing capacitance, was found to agree with the value (¾)Cpredicted by Shao and Wright for SCL currents, whereCis the capacitance due to the sample geometry. Values of current were observed to be much greater on some samples than those predicted by the single carrier space‐charge‐limited‐current law. The excess current was identified with double‐injection behavior. The double injection was caused by a surface condition that allowed electrons to be injected at the negatively biased contact. Comparison with the capacitance—voltage characteristic and transient response behavior of double‐injection structures aided in identifying the nature of the excess current.

ISSN:0021-8979    

DOI:10.1063/1.1710077

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The resistance of thin continuous (100–1000 Å thick) Au, Ag, Cu, and Al films as influenced by super‐imposed thin layers of conducting, semiconducting, and insulating materials has been studied. An overlay of SiO or Permalloy increases the resistance of Au films, but has little effect on Ag films. An overlay of Ge increases the resistance of Ag films by about 25%, but has negligible effect on Al films. The resistance of Cu films first increases with the thickness of the Ge and Cr overlay and then decreases rapidly to a saturation value. The changes in the resistance are markedly dependent on the thickness of the overlay; saturation values are obtained for an average thickness of less than 10 Å. Changes are observed for both diffuse and specular films. The observed changes are a function of the ratio of the thickness to the mean free path of conduction electrons and can be explained only by a change in the amount of diffuse scattering at the surface rather than in the specularity parameter. The data suggest that the formation of a new interface modifies the nature of the surface potential making both enhanced as well as reduced scattering of electrons possible and thus an increase or decrease of the resistance can result.

ISSN:0021-8979    

DOI:10.1063/1.1710078

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Silicon self‐diffusion coefficients were determined by studying the diffusion of31Si into silicon crystals of various degrees of perfection and doping. For intrinsic silicon, the self‐diffusion coefficient can be represented byD=9,000 exp− (5.13 eV/kT) cm2/sec. Doping above intrinsic levels increases the diffusion coefficient. It it is assumed that silicon diffuses by means of vacancies, which act as acceptors, the influence ofn‐type doping can be attributed to the increase in total vacancy concentration caused by the excess electrons through a mass‐action principle. It is concluded that the vacancy mechanism is the most probable for silicon diffusion. The data of this investigation would indicate that the vacancy‐acceptor level is about 0.34 eV below the conduction band in silicon.

ISSN:0021-8979    

DOI:10.1063/1.1710079

出版商:AIP    

年代:1967

数据来源: AIP