Volume 29, Issue 8, 01 August 1958
Index of content:
29(1958); http://dx.doi.org/10.1063/1.1723390View Description Hide Description
Experiments on the neutrons produced in deuterium gas during a high‐current pulsed discharge are described. With 15 kv applied to a straight discharge tube 30 cm long, neutrons are produced during a 0.1 μsec pulse ∼1.4 μsec after gas breakdown. The yield of 107 to 108neutrons per discharge depends on gas purity and the discharge tube wall material. The axial asymmetry in neutron energy indicates that the neutrons are produced in reactions whose center of mass moves preferentially towards the cathode.
29(1958); http://dx.doi.org/10.1063/1.1723391View Description Hide Description
Wire probe contacts are found to be diodic upon first touching CdS, but can be permanently changed from diodic to ohmic by the passage of a moderately intense electric current pulse. This result obtained for ten metals, which work‐function‐wise bracket CdS, supports the idea that an exhaustion barrier exists on the surface and contradicts proposals that ohmic contact is solely dependent upon the relative metal and CdSwork functions. It is postulated that the current pulse punctures the exhaustion barrier, thereby permitting electrons to tunnel more freely and in greater numbers.
29(1958); http://dx.doi.org/10.1063/1.1723392View Description Hide Description
Cinematographic observations have been made of crack propagation under well‐defined boundary conditions in rubbers at speeds up to 30 m/sec. The fracture‐markings showed resemblances to those obtained with metals, plastics, and glass, and could be related directly to the corresponding speed of fracture‐propagation. In close analogy with Schardin's observations on glass, a noncrystallizing rubber (GR‐S) showed a mode of crack propagation in which the fractured surfaces were visually smooth and the speed was about one quarter of the speed of longitudinal elastic waves. A crystallizing rubber (natural rubber) did not show this mode of propagation under the present test conditions. It is suggested that the modes of solid fracture can be usefully classified in three categories: (i) slow propagation, generally with smooth surfaces, obtained by careful control of the boundary conditions; (ii) propagation at intermediate rates with rough surfaces, involving correspondingly greater energy consumption; and (iii) fast propagation with smooth surfaces, the rate of propagation being limited by the speed of elastic waves in the material in accord with Mott's theory.
Application of the Ion Bombardment Cleaning Method to Titanium, Germanium, Silicon, and Nickel as Determined by Low‐Energy Electron Diffraction29(1958); http://dx.doi.org/10.1063/1.1723393View Description Hide Description
The ion‐bombardment cleaning method has been successfully applied to the (100) faces of germanium,silicon, and nickel, and to the (0001) face of titanium. Conditions and precautions necessary for the production of clean surfaces are described. Tests have been made for contamination from the ambient during the cleaning procedure for germanium. It has been shown that contamination approximating one‐half monolayer does not occur under the conditions which were obtained, and that the method is capable of producing surfaces which are atomically clean.
Results for clean (100), (111), and (110) germanium faces and the (100) silicon face indicate that the atomic positions in the surface planes are not the same as the corresponding positions in the bulk structure. For (100) nickel and (0001) titanium, the positions of the atoms in the surface planes are in agreement with x‐ray data. In the case of titanium, an unidentified surface structure was present until after several cycles of alternate heating and ion bombardment. In the case of nickel, a double‐spaced surfacemonolayer and in some cases a single‐spaced, simple‐square structure, attributed to carbon, was present until after several cycles of alternate heating and ion bombardment.
29(1958); http://dx.doi.org/10.1063/1.1723394View Description Hide Description
The adsorption of oxygen, carbon monoxide, and nitrogen on a tungsten ribbon filament at 300°K has been measured by means of the flash filament technique. An omegatron rf mass spectrometer has been used to measure pressure. It is shown that oxygen pressures cannot be measured satisfactorily with ionization gauges of the hot‐tungsten‐filament type. The total amounts of gases adsorbed by tungsten are 5.7×1014 molecules/cm2 for oxygen, 5.8×1014 for carbon monoxide, and 1.8×1014 for nitrogen. The sticking probabilities of these gases on the uncovered surface are 0.65 for oxygen, 0.62 for carbon monoxide, and 0.42 for nitrogen. All of the carbon monoxide and nitrogen desorb from the heated filament as the gases CO and N2; less than 1 percent of the oxygen desorbs as O2. The filament pumps oxygen at a rate of 9 liters/cm2 sec at temperatures above 2000°K.
29(1958); http://dx.doi.org/10.1063/1.1723395View Description Hide Description
A distribution of surface asperities is proposed for the case in which both mating surfaces are rough and asperities are in the form of right circular cones. The assumption of this particular asperity form is suggested by roughness measurements of ground steel, brass, and aluminum which yielded similar results for stylus traversing parallel, 45°, and perpendicular to lay. Considering the mating cones perfectly plastic and distributed in the proposed manner, bulk deformation‐load is related to compliance.
The essential finding is that the load‐compliance characteristics for mating surfaces having comparable roughness have slopes much steeper than those predictable by previous theories, all of which involve the assumption that one of the surfaces is smooth and rigid. A simple experiment is described in which load and compliance are measured. Results seem to support the plausibility of the proposed theory. Moreover, this plausibility shows promise of the calculation of other bulk‐quantities such as frictional resistance.
29(1958); http://dx.doi.org/10.1063/1.1723396View Description Hide Description
Two simple formulas are given which enable the total mass absorption coefficient of air to be readily calculated. These formulas, which are derived from published theoretical values of the total coefficient, are applicable to the energy ranges 0.01 to 100 Mev and 0.05 to 10 Mev, and yield results which agree with the theoretical values to within about 5% and 1%, respectively. Comparison is made with values calculated from existing empirical formulas which cover energies up to about 1 Mev.
29(1958); http://dx.doi.org/10.1063/1.1723397View Description Hide Description
If the surface area of a liquid is large compared to its volume, the normal freezing temperature of the liquid will be altered. In a porous material a liquid can therefore exist in equilibrium, below its normal freezing temperature. However, such a liquid will be unstable with respect to bulk solid. This instability provides the driving force for frost heave in soils.
29(1958); http://dx.doi.org/10.1063/1.1723398View Description Hide Description
Some of the more interesting effects of nonuniform electric fields are described in this report. Experimental and theoretical studies show the effects to be rather striking for particles larger than molecular size. The results show that the effect can be used to produce a fairly efficient pumping action of nonconducting liquids, to cause continuous and easily measureable separations in coarse suspensions, to cause selective precipitation, and to produce mixing.
By this means, liquids may be thrown several feet into the air with an electromechanical efficiency of about 25%. A separation factor of at least 2.5 in continuous separatory operation may be produced in a suspension of polyvinyl chloride in carbon tetrachloride‐benzene mixture. Suspensions of polar materials in less polar liquids may be either dispersed or precipitated. In one interesting ``demonstration'' type experiment, drops were ``hung'' in mid‐air.
29(1958); http://dx.doi.org/10.1063/1.1723399View Description Hide Description
This paper discusses the effect of pressure‐diffusion flux upon the concentration distribution of gas mixtures in flow fields. The equation of concentration is formulated for a binary gas mixture in which the mass ratio is large and the concentration of the lighter gas is very small. An asymptotic solution to the steady‐state equation of concentration is given for an irrotational and incompressible flow.
As an illustration, the diffusive separation (i.e., deviation from the original homogeneous state) of a mixture of helium and nitrogen along streamlines at the entrance to a long straight channel is calculated. It is assumed that the pressure inside the channel is 90% of that in the free stream, and that the diffusion coefficient of the mixture corresponds with the atmospheric conditions at 80‐km altitude.
29(1958); http://dx.doi.org/10.1063/1.1723400View Description Hide Description
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.
29(1958); http://dx.doi.org/10.1063/1.1723401View Description Hide Description
The work functions of siliconsingle 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<10−9 mm 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 5×10−5 mm 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 O2 adsorption, and (3) when silicon with residual surface films from chemical etching was heated at 990°C prior to ion bombardment. Hydrogen exposures of 7.5×10−4 mm Hg min resulted in work‐function decreases only in the presence of a heated filament. Nitrogen exposures of 4×10−6 mm 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.
29(1958); http://dx.doi.org/10.1063/1.1723403View Description Hide Description
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.
29(1958); http://dx.doi.org/10.1063/1.1723404View Description Hide Description
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.
29(1958); http://dx.doi.org/10.1063/1.1723405View Description Hide Description
Noisetemperatures at 3000 Mc were compared with the corresponding electron temperatures for the positive columns of some cold cathode discharges in argon. The noisetemperatures 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 measurednoise output, a rapid comparison between known and unknown noise powers being effected by means of a hand‐operated wave‐guide switch. The noisetemperature 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,noisetemperature, and electron temperature are in close agreement as required by the theories.
29(1958); http://dx.doi.org/10.1063/1.1723406View Description Hide Description
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.
29(1958); http://dx.doi.org/10.1063/1.1723407View Description Hide Description
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.
29(1958); http://dx.doi.org/10.1063/1.1723408View Description Hide Description
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 by ,where B and C are 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.
29(1958); http://dx.doi.org/10.1063/1.1723409View Description Hide Description
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.
29(1958); http://dx.doi.org/10.1063/1.1723410View Description Hide Description
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), where k is the distribution coefficient, must be opposite and the other the same for the two solutes. The critical ratio of melt concentrations, (C 2/C 1), is equal to (dk 1/df)/(dk 2/df), where f denotes 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.