摘要:

The piezoelectric and dielectric properties of polarized lead titanate zirconate ceramics were measured between 4° and 77°K. The materials were found to retain strong piezoelectric activity at the lower temperature. For one type of ceramic the planar piezoelectric coupling in liquid helium was about 80% of its room temperature value. Its free relative dielectric constant, measured at 1 kc, rose almost linearly from 380 at 4°K to 1300 at room temperature. The mechanical losses decreased sharply at low temperatures, suggesting a reduction in domain wall mobility.

ISSN:0021-8979    

DOI:10.1063/1.1777175

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The problem of a spin one Weiss molecular field Hamiltonian with uniaxial anisotropy is solved exactly. The free energy contains magnetocrystalline anisotropy terms of all even order Legendre polynomials. TheP2coefficient varies as the third power of the magnetization rather well even for fairly large anisotropy, and over the entire temperature range. The higher order terms can be of both signs, and all change sign as the temperature is increased.

ISSN:0021-8979    

DOI:10.1063/1.1777176

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The tunnel cathode consists of a metal‐insulator‐metal sandwich in which the electrons tunnel through the insulator material. This paper presents an analysis of the noise associated with the space‐charge waves of a beam emitted from such a cathode. The beam noise temperature for a refrigerated tunnel cathode is shown to be 2730 &dgr;, where &dgr; is the value in volts of a built‐in energy window for the emitted electrons. The value of &dgr; is adjustable by means of a dc potential. Assuming a beam noise temperature of 30°K, the current density is calculated for a variety of cathode parameters. A discussion is presented of the significance of the parameters and of the difficulties which would be encountered in constructing such cathodes for low noise.

ISSN:0021-8979    

DOI:10.1063/1.1777177

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The thermoelectric figure of merit (z), resistivity, and Seebeck coefficient have been measured between 20° and 300°K on single crystals of several alloys in the range from 1% to 40% antimony in bismuth. These materials are semimetals (0 to 5% antimony) or small energy gap intrinsic semiconductors (5 to 40% antimony) and all arentype. The Seebeck coefficients and figures of merit are anisotropic, the larger values being those measured parallel to the threefold symmetry axis. In the 12% antimony alloy the largerzrises from 1.0×10−3/°K at 300°K to a maximum of 5.2×10−3/°K at 80°K and falls rapidly at lower temperatures. All of the alloys between 3% and 16% antimony have a maximumznear 5×10−3/°K at a temperature between 70°K and 100°K. The 5% antimony alloy has the highestzat room temperature (z=1.8×10−3/°K). In this material, the Seebeck coefficient is practically constant (S=−110±10 &mgr;v/°K) between 77° and 300°K and the ratio of the thermal to electrical conductivities is close to the theoretical Wiedemann‐Franz ratio above 100°K. As a result,zis inversely proportional to the absolute temperature (zT=0.52±0.05) between 100° and 300°K. In the 12% antimony alloy,Srises from −110 &mgr;v/°K at 300°K to −220 &mgr;v/°K at 20°K. A specimen of this material, doped with 0.01% lead, isptype below 42°K. A qualitative explanation of these results is given in terms of mixed conduction by electrons and holes having properties similar to those in pure bismuth. The use of these alloys (and semimetals in general) in thermoelectric refrigeration at low temperatures is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1777178

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

A photometric procedure for rapid determination of weight of isolated particles down to a size of 200 A is described. Under normal conditions of electron microscopy, weights of down to 10−18g can be determined with an inaccuracy of less than 10%. By lowering the accuracy or using more elaborate measures (such as very low accelerating voltage) one to two orders of magnitude for the lower weight limit can be gained. The method can now be applied to population studies of biologic particles, especially those of inhomogeneous and odd‐shaped entities. Individual losses of matter through the action of enzymes and the uptake of specific stains can be measured quantitatively. The method extends the possibilities of individual mass‐weight determination to the biologically important region of submicroscopic particles.

ISSN:0021-8979    

DOI:10.1063/1.1777179

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

A quantitative theory of dislocation multiplication in the early stages of glide band formation is proposed. It is based on the model of double cross slip suggested by W. G. Johnston and J. J. Gilman [J. Appl. Phys,31, 632 (1960)]. Moving screw dislocations are believed to undergo cross slip but return in a random fashion into slip planes parallel to the original slip planes. The edge dislocation parts created between the two cross slip events cannot follow the screw dislocations, thus creating dipole dislocation trails. If the distance between the dipole dislocations is large enough that they pass each other under the applied stress, a new dislocation loop is formed in the parallel plane and the original dislocation is restored completely in its glide plane. On this basis, expressions for the number of trails and for the number of new loops created directly and indirectly by a moving dislocation are derived. The results of the theory are found to be in good agreement with the limited available data, namely the measurements on dislocation multiplication in LiF by W. G. Johnston and J. J. Gilman [J. Appl. Phys.30, 129 (1959)].

ISSN:0021-8979    

DOI:10.1063/1.1777180

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The flow and recovery properties of nearly stoichiometric polycrystalline uranium carbide were studied in the temperature range 1500° to 1900°C at strain rates varying between 2×10−5/sec to 2×10−3/sec. The flow data seem to follow a relationship&egr;˙≅K&sgr;5exp(−37 500/RT), where&egr;˙is the strain rate,Ka proportionality constant,Rthe gas constant,Tthe absolute temperature, and &sgr; the flow stress at strain =0.001. A mathematical evaluation from recovery data of the activation volume associated with dislocation motion is presented and its potential to yield basic information on the mechanism of work hardening is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1777181

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The thermal conductivity, Seebeck coefficient, and resistivity were measured for a series of sintered cerium sulfide compounds of the thorium phosphide structure. The compounds were in the composition range CeSx(1.33 ≤x≤ 1.5) and were prepared with and without strontium sulfide doping. The electrical resistivity and Seebeck coefficient increased with temperature for all samples, similar in appearance to the temperature dependency of a metal. The addition of strontium sulfide to compositions for (x> 1.33) usually led to a decrease in the resistivity and Seebeck coefficient while maintaining the same temperature dependence. The thermoelectric ``figure of merit'' is given for one of the samples and some remarks are made on the conduction mechanism in the system.

ISSN:0021-8979    

DOI:10.1063/1.1777182

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

It is shown that film critical current, defined as the maximum current for which a film shows no resistance, can be strongly increased in one direction and strongly decreased in the other by the field from two current carrying conductors placed parallel to the film edges.The effect is shown to be due to mutual cancellation of the magnetic fields normal to the film created by the wire currents and the film current. It appears to make a flat film superconductive rectifier possible.It is proved that the symmetrical increase of critical current due to the proximity of a superconducting ``shield'' plane is due to a similar mechanism, and that fields normal to the film surface are largely responsible for the broad current induced transition of flat ``unshielded'' films.It is also shown that with mutually opposed wire currents the variation of critical current with wire current exhibits a significant hysteresis. This effect may have applications to digital storage.

ISSN:0021-8979    

DOI:10.1063/1.1777183

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The ideal magnetic circuit concept in which leakage and fringing flux are absent has not heretofore been realized because of the absence of magnetic insulators under normal conditions. Below the transition temperature, however, a superconductor is a perfect magnetic insulator and a superconducting enclosure of PbBi around the source of mmf and the air gap of an experimental electromagnet operating in liquid helium has been found to reduce leakage flux from 5.1 to 0.39 maxwells per maxwell of air gap flux. The small residual leakage flux observed is accounted for by leakage in the space occupied by the winding. No detectable fringing flux was observed outside the superconducting magnetic insulator surrounding the air gap. The flux within the air gap follows the contour of the superconducting enclosure and the degree of field uniformity within is determined by the precision of alignment of the enclosure surfaces. The large reduction in size and weight possible under ideal magnetic circuit conditions makes operation in liquid helium feasible. In a magnetic circuit for maser application, a reduction in weight of nearly 2 orders of magnitude was possible partly because of the reduction in air gap length resulting from the inclusion of the magnetic circuit in the cryostat and partly because of ideal magnetic circuit operation.

ISSN:0021-8979    

DOI:10.1063/1.1777184

出版商:AIP    

年代:1962

数据来源: AIP