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Journal of Applied Physics

Issue 1 • Date Jan 1962

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Displaying Results 1 - 25 of 118
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Thermionic Current Transmission in a Strong Magnetic Field

    Page(s): 1 - 2
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    In an earlier analysis of the effect of magnetic fields on thermionic diodes, numerical results indicated that under certain conditions the fraction of emitted electrons reaching the collector equals the sine of the angle at which the magnetic field intersects the emitting surface. An analytic proof of this relation has since been found and is presented here. View full abstract»

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  • Spectroscopic Measurements of Temperatures and Densities in a Cesium Plasma

    Page(s): 3 - 10
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    The ion, electron and atom number densities, and electron temperatures of a cesium plasma generated in a diode have been measured by spectroscopic techniques. The theoretical bases for these techniques are outlined. The errors involved in applying these theories to heavy atom collision‐dominated plasmas are discussed. Working equations for the electron, ion and atom temperatures, and number densities are developed. Among the working equations developed and applied are those for the determination of the electron number density from the continuum intensity and Stark broadening. Temperatures in the cesium plasma were 0.7 ev with ion and electron number densities of the order of 1015 cm-3. The agreement of cesium pressure computed from the spectroscopically determined temperatures and number densities and that from vapor pressure curves strongly suggest the existence of thermal equilibrium. The agreement in the values of electron number densities determined from continuum intensity and Stark broadening measurements justify the approximations made in applying these theories to the cesium plasma. View full abstract»

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  • Tensile Behavior of Lithium Fluoride

    Page(s): 11 - 14
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    Tensile stress strain curves were obtained for single crystals of lithium fluoride. The specimens were elongated at room temperature along a 〈100〉 axis. The number of slip‐band sources was controlled by surface treatment. Both the yield stress and the flow stress in the plastic range were found to depend on the number of active slip bands. The shapes of the stress‐strain curves were related to the behavior of individual dislocations in lithium fluoride. View full abstract»

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  • Frequency Spectra of Low‐Frequency Fluctuations in a Plasma

    Page(s): 15 - 20
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    Observations have been made of the low‐frequency fluctuations occurring in hot‐cathode dc mercury‐vapor discharges at a pressure of approximately 1 μ. A spectrum analyzer was employed to study frequency spectra from three widely differing experimental tubes and to determine the effects of variations in tube geometry, electrode spacing, and anode current. Similar measurements were also made at points along a long discharge column with the aid of a capacitive probe sliding on the outside of the tube. The character of the spectrum is markedly dependent on tube geometry, substantially independent of column length and position along the column, and may be coarse or contain relatively pure frequency components. Two mechanisms observed involved frequency variation proportional to the square and cube roots of anode current, and possible explanations are given. There is, however, evidence for the existence of other basic mechanisms of generation. View full abstract»

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  • Impedance Characteristics of a Mercury‐Vapor Plasma

    Page(s): 20 - 25
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    Measurements have been made of the impedance/frequency characteristics of a hot‐cathode, dc, mercury‐vapor plasma at a pressure of approximately 1 μ. The experimental tube contained a directly‐heated cathode from a commercial mercury‐vapor rectifier and provided a discharge column 80 cm long. Bridge techniques were used to obtain impedance loci extending to 2 Mc/sec for several values of anode current. These appeared to normalize, the magnitudes of the components being approximately inversely proportional to current. A basic equivalent circuit can be deduced, its parameters being dependent on physical processes occurring in the discharge. Prominent frequency components of the low‐frequency voltage fluctuations present in the tube appear to be closely related to the impedance characteristics. View full abstract»

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  • Evidence for a Selenium Donor Level above the Principal Conduction Band Edge in GaSb

    Page(s): 26 - 28
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    Low‐temperature Hall coefficient and resistivity data on selenium‐doped GaSb are interpreted under the assumption that selenium introduces a donor level above the principal [000] conduction band minimum. The postulated level is associated with electrons in [111] valleys localized in hydrogen‐like orbits around selenium atoms. The results at 77°K are consistent with the assumption that impurity scattering is dominant and that a selenium donor level is located about 0.07 ev above the principal conduction band edge at 77°K. The low‐temperature data on tellurium‐doped samples are also discussed. View full abstract»

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  • Pulsed Probe Measurements

    Page(s): 29 - 33
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    A pulsed probe has been used to measure certain parameters of a time varying plasma in mercury vapor. Langmuir probe characteristics have been obtained by pulsing the probe voltage to successively higher values for μsec intervals at specific times relative to a repetitive discharge pulse. Curves showing the time dependence of the plasma potential, electron temperature, and electron density relative to this pulse are presented. The probe characteristics do not exhibit a sharp break at the plasma potential. This is related to a disturbance of the plasma by the probe. A novel and precise technique for determining the actual plasma potential is described. This technique depends upon the abrupt appearance of a spike on the leading edge of the probe current pulse. View full abstract»

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  • Mechanical Behavior of Sapphire Whiskers at Elevated Temperatures

    Page(s): 33 - 39
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    The tensile behavior of sapphire whiskers was measured at temperatures ranging from 25–2030°C. At room temperature, the tensile strength of the whiskers is size dependent and varies from several hundred thousand psi to 1 500 000 psi. With increasing temperature the strength decreases and becomes less size dependent. The whiskers exhibit delayed fracture above 630°C, the time to fracture being exponentionally related to the applied stress. Delayed fracture occurs in both hydrogen and oxygen atmospheres and hence is not believed to be due to stress corrosion. The mechanical behavior of the whiskers has been rationalized by assuming that they are essentially dislocation free and that failure occurs by thermally activated crack propagation preceded, at high temperatures, by localized dislocation nucleation. View full abstract»

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  • Elastic Wave Propagation in Piezoelectric Semiconductors

    Page(s): 40 - 47
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    A plane elastic wave propagating in a piezoelectric crystal may be accompanied by longitudinal electric fields which provide an additional elastic stiffness. When the crystal is also semiconducting, these fields produce currents and space charge resulting in acoustic dispersion and loss. A linear theory of this effect is developed, taking into account drift, diffusion, and trapping of carriers for both extrinsic and intrinsic semiconductors. Conductivity modulation sets an upper limit on strain amplitude for a linear theory. The directional characteristics and the magnitude of the effects are illustrated for CdS and GaAs. The Appendix treats the interaction of an arbitrary acoustic plane wave with the electromagnetic fields in a piezoelectric crystal (based on a treatment by Kyame [J. J. Kyame, J. Acoust. Soc. Am. 21, 159 (1949); 26, 990 (1954).]) and further shows explicitly that only the effects of longitudinal electric fields need be considered. View full abstract»

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  • Comparison of Several Methods for the Photographic Detection of Thermal Neutron Images

    Page(s): 48 - 55
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    Several methods of photographic detection of thermal neutron images have been studied and compared with regard to photographic speed, relative neutron‐gamma response, and image sharpness qualities. All these comparisons have been made using a low gamma content, monochromatic neutron beam (1.05 A) having an intensity of 3×105 neutrons/cm2‐sec. The fastest speed, best neutron‐gamma response of the direct exposure methods, and good image sharpness have been obtained by using a direct exposure method employing a B‐10 loaded scintillator and type F x‐ray film. Gadolinium and rhodium intensifier screens with type KK film have also yielded good results by the direct exposure method (film and intensifier screens exposed to the neutron beam together). Results are also reported for the use of indium, cadmium, silver, and gold screens. The transfer method, in which the photographic film is exposed only to the radioactive decay radiation of an image‐carrying metallic screen, and not to the neutron beam itself, has been found useful not only because no effect on the image is produced by gamma radiation in the neutron beam, but also because improved image sharpness has been obtained. In the cases of indium and gold (the two materials which have been found useful for the transfer method in the neutron flux available for this study), the improved image resolution obtained with the transfer method has been shown to result from the fact that the high energy prompt (n,γ) radiation is not detected on the film. View full abstract»

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  • Elastic Moduli of Cadmium Telluride

    Page(s): 56 - 59
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    The elastic moduli of cadmium telluride at 25°C have been determined by measurement of ultrasonic wave velocities in the frequency range 40–300 Mc/sec. Preparation of the crystal specimens is described, and factors affecting the accuracy of results are considered. The question of possible piezoelectric stiffening and its effect on measured wave velocities is also considered. Based on a density of 5.854 g/cm3 as computed from the lattice constant (and taken as exact with respect to uncertainty estimates) the zero field elastic moduli are: Modulus Value d/cm2 Uncertainty in % c11 5.351×1011 0.15 c12 3.681 0.2 c44 1.994 0.15 View full abstract»

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  • A Solution to the Frenkel‐Kontorova Dislocation Model

    Page(s): 60 - 62
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    An exact solution to the nonlinear difference equation which describes a static configuration in the Frenkel‐Kontorova dislocation model is given. The solution is in terms of a power series which is proved convergent. Previously, only approximate solutions had been obtained by replacing the difference equation by a differential equation. A numerical comparison is made between the exact and the approximate solutions and the Peierls energies they give. View full abstract»

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  • Kinking and the Fracture of Ionic Solids

    Page(s): 62 - 67
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    A single crystal undergoing plastic bending develops constraints due to the gradient in lateral contraction across the beam. These constraints result in lateral stresses which may be relieved by the process of anticlastic kinking. Anticlastic kink boundaries in rocksalt structure solids consist of arrays of {121}〈110〉 edge dislocations formed by the interaction of two systems of {110}〈110〉 glide dislocations, one system being responsible for slip in the main part of the crystal beam, the other confined to its corner. Temperature affects the structure of kink boundaries and their subsequent role in initiating fracture. I. At high temperatures (∼0.3 Tm). The resultant edge dislocations in the boundary can move over their {121} slip planes and the kinks become sharp. The resultant dislocations are ineffective barriers to slip and the crystals are ductile. II. At low temperatures (∼0.1–0.2 Tm). The resultant edge dislocations are immobile and the kinks consist of a diffuse array. The resultant dislocations provide strong barriers to slip and cracks nucleate at the kink boundary. III. At very low temperatures (∼0.1 Tm). Fracture occurs before the second set of {110}〈110〉 glide dislocations have been activated to generate anticlastic kinks. Relaxation of the lateral stress results in a complex fracture. View full abstract»

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  • Work Function Variation of Metals Coated by Metallic Films

    Page(s): 67 - 73
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    A theoretical correlation is derived to account for the variation of the work function of refractory metals coated by metallic films for all degrees of coverage. This correlation is based on an extension of the concept of electronegativity to composite surfaces and use of Pauling's rule about electronegativity and dipole moment of complex molecules. A series of theoretical curves is given for different crystallographically ideal surfaces of refractory metals such as W, Mo, and Ta coated by Cs, Sr, Ba, and Th. The derived correlation is compared with available experimental data for which the experimenters specify the exact conditions under which the experiment is performed and excellent agreement between theory and experiment is established. It is shown for the first time that the maximum work function variation does not necessarily always occur either when a full monolayer is reached or at a definite fractional coverage. The exact position of the maximum variation is a function of the adsorbate and substrate materials and the type of the substrate surfaces. It is also shown that it is erroneous to characterize a monolayer as the point at which maximum emission (or maximum work function variation) is achieved because such a maximum may be flat and extend from half a monolayer to one monolayer. View full abstract»

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  • Electrical Conduction Mechanism in Ultrathin, Evaporated Metal Films

    Page(s): 74 - 82
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    The electrical conduction mechanism in the film plane of ultrathin, evaporated metal films was investigated. These films consist of a planar array of many small discrete islands. The conduction process consists of, first, charge carrier creation which is thermally activated and involves charge transfer between initially neutral particles, and, second, the drift velocity of these charges in an applied field. Charge transfer between particles occurs by tunneling. The following features were predicted and can be verified experimentally: the conductivity depends exponentially on reciprocal temperature, and it should be independent of field at low fields. Deviations from the exponential temperature dependence can be understood in terms of a spectrum of activation energies, while deviations from Ohm's law at high fields can be explained readily in terms of a field dependent activation energy. View full abstract»

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  • Preparation and Some Properties of Tantalum Carbide

    Page(s): 82 - 87
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    Small tantalum carbide objects may be made by heating the objects made from tantalum in dilute hydrocarbon atmospheres at temperatures ≫2000°K. Carburization proceeds quite rapidly above 2300°K by diffusion of carbon initially deposited on the surface into the metal, principally along grain boundaries, to form a thin layer of Ta2C which is quickly converted to the TaC. The latter has a tensile strength of 21–25 kg/mm2, a work function φ=3.17 ev, an emissivity of 0.40–0.46 between 2400° and 3500°K, rising with increasing temperature, and a resistivity ρ=102+0.0178 T (°K) ohm cm between 1400° and 3500°K. View full abstract»

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  • Electron Optics in Accelerating Tubes

    Page(s): 87 - 90
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    The focusing properties of the multi‐stage system of weak lenses, such as in the accelerating tubes, are considered. The lens system is treated as one thick lens in the paraxial ray approximations. By assuming that the field distribution of each lens is the same, the simple expressions for the focal lengths and the locations of the focal points of this lens system as a whole are given. View full abstract»

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  • Progress in Field‐Emission Work‐Function Measurements of Atomically Perfect Crystal Planes

    Page(s): 91 - 95
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    The field electron microscope has been used previously to measure the work function of thermally smoothed single‐crystal tungsten planes. In this work the field ion microscope is used to create atomically perfect single‐crystal planes by field evaporation and then to observe and preserve these clean surfaces. The slope of the field emission Fowler‐Nordheim plot then gives the ratio of the three halves power of the work function to the field. Two techniques have been developed in order to obtain a second relationship between work function and field: (1) temperature dependence of field‐emission current density and (2) energy distribution of field‐emitted electrons. While investigating temperature dependence it was found that a reinterpretation of the Gel'berg et al. data for the work function temperature dependence of molybdenum up to 1100°K revealed the following unexpected cubic temperature dependence: φT0+0.44×10-10T3 ev. View full abstract»

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  • Cranberg Hypothesis of Vacuum Breakdown as Applied to Impulse Voltages

    Page(s): 96 - 99
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    If the Cranberg hypothesis for breakdown in vacuum is valid, measurements made using impulse and dc voltages may yield different results depending in part upon the rate of rise of the impulse and the nature of the attachment of the clumps to the parent electrode. It is assumed that metallic clumps are instantaneously detached from an electrode surface at a voltage dependent upon gap length as described by Cranberg. Consideration of the rise in impulse voltage occurring during the transit time of the clump to the opposing electrode yields the following results. For an over-volting impulse of constant rise rate the breakdown voltage of a vacuum gap depends upon the ⅚ power of the gap length in the limiting case of large gap length and clump radii, and fast rise impulses. For the same limiting conditions, with an impulse of constant rise time, breakdown voltage was found to be related to the 52 power of the gap length. A brief comparison of calculated results with experimental data in the literature indicates limited, although not conclusive, support for the Cranberg hypothesis in its application to impulse breakdown in vacuum. View full abstract»

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  • Nature of the Valley Current in Tunnel Diodes

    Page(s): 100 - 111
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    An explanation of the ``excess current'' up to and including the valley point, observed in tunnel diodes, is suggested in terms of tunneling between impurity band states of the heavily doped p and n regions. Density‐of‐states curves are constructed, based on several different models of impurity bands, and the tunneling current between such bands is computed and compared with experiment. The effect of different base materials and impurities on the valley current is discussed. Some aspects of impurity band formation in heavily doped semiconductors are considered, and possible modifications and difficulties of the present approach are indicated. The possibility of obtaining information about impurity band structure from tunnel diode measurements is discussed. Experimental evidence is presented about the position of the Fermi level in heavily doped semiconductors and is shown to agree with that postulated by the present model. The Appendix gives the theory of locating the Fermi levels from measurements of the temperature variation of the diode characteristic. View full abstract»

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  • Saturation Effects in Ferrite Frequency Doublers Operating in the Uniform Precession Mode

    Page(s): 112 - 114
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    Three distinct saturation mechanisms occurring in ferrite frequency doublers are described. An approximate nonlinear equation of motion, including the effect of the reaction field, is obtained and solved for a particular case. The effects of the system parameters on conversion efficiency are discussed. Restrictions on the validity of the analysis due to transverse and longitudinally pumped spin waves are examined. View full abstract»

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  • Current Noise in Pyrolyzed Polyacrylonitrile

    Page(s): 114 - 117
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    Current noise measurements of the organic semiconductor polyacrylonitrile pyrolyzed in nitrogen at temperatures from 550° to 900°C reveal conductivity fluctuations similar to those attributed to carrier fluctuations in inorganic semiconductors. The current noise spectra exhibit characteristic time constants of 10-2, 3×10-4, and 5×10-5 sec which are independent of pyrolysis temperature and are tentatively associated with the molecular structure leading to conductivity. The observed noise magnitude suggests the existence of a large number of independent sub‐volumes which increase in size with treatment temperature. Samples pyrolyzed above 700°C exhibit 1/f noise that may be indicative of a different molecular structure. View full abstract»

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  • Direct Observation of Outward‐Progressing Flux Change in Tape‐Wound Cores at Power Frequencies

    Page(s): 118 - 120
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    This brief preliminary report shows by a simple experiment that major flux changes in tape‐wound, magnetic‐amplifier‐type cores begin near the core's inner perimeter and progress outward, at power frequencies, as has been predicted [R. C. Barker, Trans. Am. Inst. Elec. Engrs. 79‐I, 482 (1960), with accompanying discussion]. It also shows, when the core is operated on a minor hysteresis loop, that at the start of every odd half‐cycle of excitation additional small inward‐progressing flux changes occur near the outer boundary of the region in which flux was reversed during the preceding even half‐cycle. This reduces slightly the hysteresis loss in the odd half‐cycles, and the resulting energy unbalance causes the minor loop to cling to one end of the major loop—a phenomenon well known in rectangular‐loop materials. The cause of the energy unbalance was hitherto unknown, however. The method used for the experiments is observation of the voltage that appears between two probes touched to the side of the core when the core is excited by a conventional winding. This method is known to other investigators but scantily reported in the literature. It is not analyzed here, but some experimental justification is given. View full abstract»

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  • Base Region Transport Characteristics of a Diffused Transistor

    Page(s): 120 - 125
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    A one‐dimensional analysis is given on the minority carrier transport characteristics of a transistor base region containing an arbitrary drift field distribution. This field is assumed to enhance—or retard—the motion of minority carriers between an emitter and collector junction, thereby modifying the influence of bulk recombination mechanisms. Base region transport efficiency is established in terms of the transistor current gain when an ideal emitter junction is assumed. Applications of this analysis are demonstrated by establishing the base region transport efficiency for diffused transistors. Two types of structures have been analytically investigated: the alloy‐diffused transistor, containing a diffused collector junction and an alloy‐type emitter; and the double‐diffused transistor constructed entirely by diffusion techniques. For practical semiconductor devices, a comparison of these two structures has shown negligible differences in their base region transport efficiency and, furthermore, the drift mechanisms within these diffused devices have little influence upon their one‐dimensional current gain. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Editor
P. James Viccaro
Argonne National Laboratory