摘要:

The attenuation of plane elastic waves in the megacycle region is investigated for several different propagation directions in single crystalline zinc. Results are in qualitative agreement with the losses predicted theoretically due to dislocation damping and thermoelastic effects, and in addition losses not accounted for by the available theory are observed.

ISSN:0021-8979    

DOI:10.1063/1.1723400

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

The work functions of silicon single crystals have been obtained by measuring the contact potential differences between the crystals and a gold reference whose work function was measured by the Fowler method. The measurements were carried out in high vacuum (p<10−9mm Hg). For nonfloating‐zone silicon, the work function was higher for samples which were radiation quenched from 990°C than it was for samples which were annealed at 500°C. This effect was not observed for floating‐zone material and was presumably a function of occluded oxygen. The work functions for the different faces after ion‐bombardment cleaning were in the sequence (100)>(110)>(111). The values after quenching were: (100), 4.92 ev; (110), 4.89 ev; (111), 4.77 ev. The values after annealing were: (100), 4.82 ev; (110), 4.70 ev; (111), 4.67 ev. The work function of the (111) face of a 2000‐ohmcm,p‐type, floating‐zone sample was 4.73 ev after quenching or annealing. Changes in both surface and volume properties were observed when silicon was heated above 1000°C in contact with quartz. Oxygen exposures of 5×10−5mm Hg min resulted in work function increases which were larger on the annealed surfaces than on the quenched ones. This was observed for both floating‐zone and oxygen‐doped material and is therefore a property of the silicon itself. Photoelectric evidence indicated a diffusion of oxygen into the silicon (1) when silicon was heated in oxygen, (2) when silicon was heated at 990°C in high vacuum after O2adsorption, and (3) when silicon with residual surface films from chemical etching was heated at 990°C prior to ion bombardment. Hydrogen exposures of 7.5×10−4mm Hg min resulted in work‐function decreases only in the presence of a heated filament. Nitrogen exposures of 4×10−6mm Hg min resulted in no work‐function changes. Heating silicon in high vacuum at 990°C for about 100 hours developed thermal‐etch patterns which were rectangular on the (100) face, hexagonal on the (110) face, and triangular on the (111) face. Heating at 1080°C for 24 hours caused further etching which developed the same triangular patterns on the (111) face but left the (100) and (110) faces with a sandblasted appearance.

ISSN:0021-8979    

DOI:10.1063/1.1723401

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

A general formalism, applicable to a wide class of missile trajectory optimization problems, is obtained by straightforward application of standard methods of the calculus of variations. In all cases, a point particle model is used for the missile. It is shown that the steering and burning programs are determined always by the same differential equations, different optimization problems corresponding simply to different boundary conditions. Results found previously by assuming the gravitational force field uniform during powered flight are obtained as special cases and the corrections to these due to variations in the force field are discussed. The extensions to variable mass and thrust programs, variable burning times, etc., are also included.

ISSN:0021-8979    

DOI:10.1063/1.1723403

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

The internal friction in pure copper, in copper doped with nickel and gold, and in magnesium, subjected to various amounts of cold work was measured at 40 kc from 4°K to 300°K. Cold work produces a Bordoni relaxation peak in copper at approximately 80°K. The peak position is independent of the amplitude of vibration for strain amplitudes less than 3×10−5. Suitable annealing reduces and eliminates the peak, as does the presence of relatively large quantities of impurities. Increased cold work and impurity additions also systematically reduce the strain amplitude of the internal friction. A peak was found in cold‐worked magnesium at 20°K and also a small peak in crystalline quartz at 39°K. A comparison of present theories with these results is given.

ISSN:0021-8979    

DOI:10.1063/1.1723404

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

Noise temperatures at 3000 Mc were compared with the corresponding electron temperatures for the positive columns of some cold cathode discharges in argon. The noise temperatures were measured using a superheterodyne receiver of low noise figure, by comparing the noise outputs of the various tubes with that from a commercial fluorescent lamp of measured noise output, a rapid comparison between known and unknown noise powers being effected by means of a hand‐operated wave‐guide switch. The noise temperature of the fluorescent lamp in its mounting was measured by comparison with an absolute standard consisting of an incandescent tungsten filament lamp fitted in a suitable wave‐guide holder. Electron temperatures were measured by the Langmuir single‐probe method. It is concluded that for most of the tubes measured, noise temperature, and electron temperature are in close agreement as required by the theories.

ISSN:0021-8979    

DOI:10.1063/1.1723405

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

The electron emission from tungsten by bombarding helium atoms of energy ranging from 300 to 3500 ev has been measured for a surface with less than a monolayer and with multilayers of adsorbed gas. The yield expressed in electrons per particle rises almost linearly from about zero at 300 ev to 0.42 electrons/particle at 3500 ev for a monolayer of adsorbed gas. Further adsorption of gas gives a much increased yield. These yields are compared with the ion under like conditions. The distributions of the normal velocity and a component of the tangential velocity are compared for the ion and neutral atom on hot and cold targets.

ISSN:0021-8979    

DOI:10.1063/1.1723406

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

Positive and negative corona characteristics of air and nitrogen are determined experimentally for coaxial cylindrical electrode geometry with gas temperatures from 300°K to 1100°K and with pressures from 0.1 to 8 atmospheres. Over this range, positive corona characteristics appear to be functions of gas density only. On the other hand, negative corona characteristics depend on gas temperature as well as gas density. At high temperatures both positive and negative coronas show instabilities which are pressure dependent.

ISSN:0021-8979    

DOI:10.1063/1.1723407

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

A theory for the tensile strength of amorphous plastics is presented. It extends a previous theory so as to include the effects of imperfections and chain relaxations. The result obtained shows that the time taken for a plastic to break,tc, under constant stress,F, is given byF=−Bln(tc)+C,whereBandCare molecular constants. The experimental values found for these constants are shown to be consistent with their molecular interpretation. This extended theory shows that the effect of chain relaxations is the major time dependent factor as far as strength is concerned.

ISSN:0021-8979    

DOI:10.1063/1.1723408

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

In the preparation of shadowed specimens for electron microscopic examination, it is often required to estimate the contrast to be expected with different thicknesses of films of various metals, different aperture angles, and beam potentials. This communication describes a graphical method for this purpose.

ISSN:0021-8979    

DOI:10.1063/1.1723409

出版商:AIP    

年代:1958

数据来源: AIP

摘要:

Fluctuations in the electrical conductivity of an extrinsic semiconductor arising from fluctuations in the growth conditions of the crystal can be eliminated by adding to the melt a critical concentration of a suitable donor or acceptor. Useful solute combinations are those for which one of the two characteristics; (a) the conductivity type and (b) the sign of (1−k), wherekis the distribution coefficient, must be opposite and the other the same for the two solutes. The critical ratio of melt concentrations, (C2/C1), is equal to (dk1/df)/(dk2/df), wherefdenotes growth rate, and subscript 1 denotes the major solute. Expressions are derived for the critical ratio, for the effects of deviations from it, and for the optimum characteristics of solute combinations.

ISSN:0021-8979    

DOI:10.1063/1.1723410

出版商:AIP    

年代:1958

数据来源: AIP