摘要:

The size of mosaic blocks in lead powder was determined on the basis of the primary extinction effect of x rays. Integrated intensities of nine reflection lines in the Debye‐Scherrer spectrum with CuK&agr;radiation were measured with a Geiger counter spectrometer. The mosaic size was calculated by comparing these values with theoretical values of the intensities. Correction for absorption of x rays in the specimen powder was especially important because of the large absorption coefficient of lead. The order of magnitude of the mosaic size was found to be 10−4cm. From this value, the dislocation density was calculated to be of the order of 108cm−2. The dislocation density decreased slightly when the specimen was annealedin vacuoat 150°C for a long time.

ISSN:0021-8979    

DOI:10.1063/1.1728412

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The current‐voltage characteristics of high‐pressure (0.25–300 mm) rare‐gas diodes have been measured. Data have been taken on diodes containing xenon, argon, and helium as the ambient gas, and tungsten, tantalum, rhenium, thoriated tungsten, and an oxide cathode as filamentary cathodes. Some unexpected phenomena have been observed and some of these general results can be summarized as: (1) The current vs voltage characteristics of argon‐ and xenon‐filled diodes, at gas pressure above 1 mm, do not obey any space‐charge law if the cathode temperature is above 2400°K. (2) The above violation of space‐charge relations in argon‐ and xenon‐filled diodes is found with tungsten, tantalum, or rhenium filaments, which can be operated above 2400°K. When thoriated tungsten or the oxide cathode, which operate below 2400°K, are used as filaments, the current‐voltage characteristics of the diode follow a space‐charge relation (current‐voltage characteristics independent of temperature). (3) The current vs voltage characteristics of helium‐filled diodes, however, do obey the space‐charge relations at all filament temperatures available with the present cathode materials. (4) Some very unusual early breakdown phenomena (breakdown at potentials below ionization potential) were observed in these high‐pressure rare‐gas diodes employing a hot cathode. These data can be explained qualitatively by postulating the existence of thermally generated xenon or argon gas ions at temperatures in the range of 2400°K. An attempt will be made to justify this assumption by a semiquantitative theoretical treatment based on Saha's thermal ionization theory rather than the surface ionization theory of Langmuir and Kingdon.

ISSN:0021-8979    

DOI:10.1063/1.1728413

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

A consideration of the crystal structures of simple binary compounds shows that they can be represented by closest packings of the larger anions in which two kinds of interstices are available for occupation by the metal atoms. In hexagonal closest packings, the octahedral voids form continuous chains by sharing opposite faces while the tetrahedral voids form isolated pairs. In cubic closest packings, each kind of void shares faces only with unlike voids. The specific diffusion paths available in these compounds depend, therefore, on the manner in which the voids are occupied. Continuous diffusion paths comprised of normally unoccupied voids exist in ZnO and &agr;‐ZnS type structures so that voidal diffusion can take place without requiring defect formation. Similarly, voidal diffusion can occur in BiO3, CrCl3, and CdI2type structures. Conversely, all possible continuous diffusion paths are blocked by metal atoms in the NiAs, NaCl, and antifluorite‐type structures so that vacancy or interstitialcy mechanisms are necessary to account for diffusion.

ISSN:0021-8979    

DOI:10.1063/1.1728414

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The effect of available diffusion paths on activation energies are considered for silver iodide, zinc oxide, and bismuth selenide. It is shown that the energy in &bgr;‐AgI should be nearly twice that in &ggr;‐AgI because the Ag atoms in tetrahedral sites first must be displaced to octahedral voids before voidal diffusion can occur in the beta modification. The 2:1 ratio between the self‐diffusion energies of Zn determined by radioactive tracer and electrical conductivity measurements in ZnO is similarly explained. It is also shown that the difference between the activation energies for diffusion in BiSe and Bi2Se3can be used to determine the formation energy of vacancies in BiSe. The agreement between these predictions and experimentally determined values attests that qualitatively accurate explanations can be based on considerations of the crystal structure.

ISSN:0021-8979    

DOI:10.1063/1.1728415

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Low‐resistivityn‐type GaAs crystals with silicon donors are compensated with diffused copper to produce high‐resistivity crystals in a manner which is amenable to semiquantitative description in terms of a simple thermodynamic model. The high‐resistivity GaAs:Cu crystals are subjected to photoelectronic analysis, including room temperature Hall and photo‐Hall measurements, to obtain information about the effects of deep‐lying imperfections on the properties of the initialn‐type GaAs. In addition to three deep donors previously reported, five acceptors are revealed. A 0.42‐ev acceptor level, when compensated, provides a long electron lifetime resulting in highn‐type photosensitivity at low temperatures. Evidence for effects on the electron mobility is obtained for compensated deep donor levels, important mainly in high‐resistivityn‐type material, and for compensated acceptors lying 0.22 ev above the valence band, important mainly at low temperatures.

ISSN:0021-8979    

DOI:10.1063/1.1728416

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

A flash method of measuring the thermal diffusivity, heat capacity, and thermal conductivity is described for the first time. A high‐intensity short‐duration light pulse is absorbed in the front surface of a thermally insulated specimen a few millimeters thick coated with camphor black, and the resulting temperature history of the rear surface is measured by a thermocouple and recorded with an oscilloscope and camera. The thermal diffusivity is determined by the shape of the temperature versus time curve at the rear surface, the heat capacity by the maximum temperature indicated by the thermocouple, and the thermal conductivity by the product of the heat capacity, thermal diffusivity, and the density. These three thermal properties are determined for copper, silver, iron, nickel, aluminum, tin, zinc, and some alloys at 22°C and 135°C and compared with previously reported values.

ISSN:0021-8979    

DOI:10.1063/1.1728417

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The switching time and the coercive field in ferroelectric BaTiO3are known to depend on crystal thickness. A model along the following lines is proposed to explain the experimental results: on the crystal surface there is a thin layer of electrically biased and mechanically strained BaTiO3which has a much smaller dielectric constant than the bulk. It is proposed that the electrical bias and the mechanical strains in the surface are caused by a Schottky exhaustion layer. This model and the model proposed by Drougard and Landauer are discussed and their predictions are compared with experimental results.

ISSN:0021-8979    

DOI:10.1063/1.1728418

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The problem of determining the effective thermal conductivity of a two‐phase system, given the conductivities and volume fractions of the components, is examined. Equations are described which have been proposed as solutions to this problem, including those of Maxwell, de Vries, and Kunii and Smith, the weighted geometric mean equation, and an equation based on a three‐element resistor model found applicable to the analogous electrical conductivity problem. Experimental results are presented for five unconsolidated samples: three quartz sand packs, a glass bead pack, and a lead shot pack. The method of conductivity measurement using the transient line heat source (thermal conductivity probe) is described. Data are reported showing the variation of effective thermal conductivity with porosity, solid particle conductivity, saturating fluid conductivity, and the pressure of the saturating gas. From considerations based on the kinetic theory of gases, it is shown that the characteristic dimension of the pore space, with respect to heat conduction in the gas occupying this space, is smaller than the mean particle diameter by a factor of roughly 100. The thermal conductivity equations which best represent the observed data are those of de Vries, and Kunii and Smith, and a slightly modified version of the resistor model equation.

ISSN:0021-8979    

DOI:10.1063/1.1728419

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Measurements have been made of the effective thermal conductivity of porous sandstones. The method is based on the transient heating effect resulting from use of a line heat source. Data are presented for six sandstones ranging in porosity from 3 to 59% and show the variation of thermal conductivity with porosity, the conductivity of the saturating fluid, the pressure of the gas filling the pore space, and overburden pressure. The results are compared with those previously obtained for unconsolidated sands. All samples, except one, exhibited a lower thermal conductivity when saturated with a gas at atmospheric pressure than when saturated with a liquid of the same conductivity as the gas. An explanation for this effect, in terms of the kinetic theory of gases, is advanced and substantiated by other data. Finally, the validity of certain equations for the thermal conductivity of two‐phase systems is examined; the weighted geometric mean of the two constituent conductivities is found to agree well with the measured effective conductivities.

ISSN:0021-8979    

DOI:10.1063/1.1728420

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The ratio of the capture cross section of a sensitizing center for a photoexcited hole to the subsequent capture cross section for a free electron, inn‐type photoconductors, can be determined from the measurement of the thermal quenching of photoconductivity as a function of excitation intensity. The magnitude of the ratio gives information about the charged state of the sensitizing center. A summary of many such measurements on seven different photoconductors of type II–VI or III–V supports the hypothesis that doubly charged sensitizing centers are present in some of these materials, but that singly charged sensitizing centers are present in the majority.

ISSN:0021-8979    

DOI:10.1063/1.1728421

出版商:AIP    

年代:1961

数据来源: AIP