Volume 33, Issue 7, 01 July 1962
Index of content:
33(1962); http://dx.doi.org/10.1063/1.1728919View Description Hide Description
The simultaneous diffusion of two isotopes of a given ionic impurity in an electric field is considered, and an improved method of analyzing the experimental data to obtain ΔD/D is discussed. (Here D is the diffusion coefficient of one of the isotopes, and ΔD the difference between those of the two isotopes.) As noted by Chemla, an electric field increases the accuracy with which ΔD/D can be determined. Equations are found giving this increase in accuracy as a function of field and diffusion time. If the ratio w 2α/w 2β of the jump frequencies of the two isotopes is known, relative values of the vacancy or interstitialcy jump frequencies near an impurity can be calculated from ΔD/D and μ/D, where μ is the drift mobility. As an example, data in the literature for Na22 and Na24 diffusing in KCl are considered. For self‐diffusion, w 2α/w 2β may be calculated from ΔD/D and known values of the correlation factor. When data in the literature are used to estimate w 2α/w 2β for Na22 and Na24 diffusing in NaCl, a value smaller than that predicted by classical rate theory is found.
33(1962); http://dx.doi.org/10.1063/1.1728920View Description Hide Description
The friction of sliding metal surfaces is often attributed to the formation and shearing of welded junctions between the surfaces. This paper describes a more detailed investigation of the phenomena which occur in the junctions. Experiments were performed with a hemispherically ended copper rider sliding against the surface of a large cylindrical copper drum. Observation of the sliding contact through the microscope shows that most of the time during sliding, the drum and rider are held apart by a wedge of displaced metal from the drum, which becomes trapped between them. Whenever a wedge escapes from between the drum and rider, another wedge forms rapidly to replace it. The wedges are also observed with several other combinations of metals, and with pairs of flat copper surfaces. Sections cut through the wedges and adjacent copper contact members were examined. It is found that the shearing of the metal by which the sliding proceeds occurs in a direction slightly inclined to the drum surface. This causes the formation and growth of the wedges. The shearing is primarily a continuous plastic deformation process somewhat similar to the shearing process by which the chip is generated in the cutting of ductilemetals with machine tools. It is possible that the adhesion between the rider and wedge may often be weaker than the junctions usually assumed to be responsible for friction. Wear debris is observed to be produced in a number of different ways.
33(1962); http://dx.doi.org/10.1063/1.1728921View Description Hide Description
A one‐dimensional, small‐amplitude analysis is presented of alternating‐current generation by a stationary electromagnetic circuit coupling to waves in a moving, perfectly conducting fluid. In a uniform applied dc magnetic field the waves in the fluid are either compressional waves if the time‐average velocity v̄ 0 is transverse to the dc magnetic field, or Alfvén waves if it is along the dc magnetic field. It is shown that, for velocities v 0 greater than the velocity of propagation c of the compressional or Alfvén waves, self‐excited oscillations may be produced in a resistively loaded coil coupling to the fluid. In a traveling‐wave circuit and in a resistively loaded distributed circuit coupling to the fluid, growing waves are found which result from a transformation of the fluid energy into electromagnetic energy. There is a close analogy between these power‐generation mechanisms and those employed in microwave electron‐beam tubes.
The results of two‐ and three‐dimensional analyses are summarized in order to show that the phenomena predicted by the simplified one‐dimensional analysis occur in less idealized geometries. Possible applications are briefly discussed with reference to ac power generation by using combustion gases or a flow of a high‐temperature plasma.
33(1962); http://dx.doi.org/10.1063/1.1728922View Description Hide Description
Fine rod and whisker‐type crystals of undoped CdS whose dark conductivity was of the order of 10−10 (Ω‐cm)−1 were bombarded with 100‐keV electrons. Following bombardment dark conductivity of the order of 10−3 (Ω‐cm)−1 was observed. Heat treatment to about 200°C reduced the conductivity to near its original value. Because of the high efficiency with which the electron beam raised the conductivity of the crystals, an explanation based on the redistribution of electrons and holes over existing levels rather than the production or removal of defects is required.
33(1962); http://dx.doi.org/10.1063/1.1728923View Description Hide Description
Dislocation multiplication and motion in NaCl was studied using etch pit and birefringence techniques. The observations show that dislocations in glide bands of NaCl are in the form of large concentric loops with sources staggered in the center of the bands. They propagate and multiply by cross‐slip and are braked through intersection jogs. The slip distance parallel to b is estimated to be 2–3 mm and that normal to b 0.5–1 mm, at a stress of 30 g/mm2 in crystals having 5×104 dislocations/cm2. ``Deformation bands'' form normal to the operating system during the latter part of stage I by ``glide polygonization'' of edge dislocations on closely spaced slip planes. These limit dislocation motion to shorter distances in stage II. Stage III appears when significant flow occurs on a secondary system oblique to the primary operating one.
33(1962); http://dx.doi.org/10.1063/1.1728924View Description Hide Description
The effects of surface sources, ozonized surface films, and surfacedissolution, on the nature of plastic flow in NaCl single crystals were studied using etch pit and birefringence techniques. Slip steps increase in depth and become rounded as the slip band broadens. This is also often accompanied by a bending of the steps as much as 3° off 〈100〉. These effects are explained in terms of both internal and surface cross‐slip mechanisms. Ozonization of NaCl for short periods restricts the action of surface sources and for longer times provides barriers to the egress of dislocations. The ``Suzuki effect'' (formation of a work‐hardened surface layer) is found to depend upon the existence of a multiplicity of surface sources such as is produced by polishing on wet silk. Cross‐slip of screw dislocations at the surface is found to be greater and flow more homogeneous when the surface is dissolved during deformation. This is proposed as a likely mechanism to explain the ``Joffe effect'' when atmospheric variables are eliminated.
33(1962); http://dx.doi.org/10.1063/1.1728925View Description Hide Description
The phase diagram for the Bi2Te3–Sb2Te3 pseudo‐binary system is of the solid‐solution type, where the distribution coefficient k is equal to unity at 33.3 and 66.7 mole % Sb2Te3. The c lat tice parameter remains essentially constant across the diagram at 30.49 Å for both slowly crystallized and quenched alloys. For quenched alloys the alattice parameter decreases almost linearly, from a value of 4.487 Å, for pure Bi2Te3, to a value of 4.275 Å, for pure Sb2Te3; however, a significant contraction from linear variation is found in slowly crystallized materials.EG diminishes in an essentially linear fashion from 0.16 eV, for pure Bi2Te3, to 0.12 eV at 24.2 mole % Sb2Te3 in both slowly crystallized and quenched materials.EG remains approximately constant from 24.2 to 66.7 mole % Sb2Te3 for slowly crystallized materials but continues to drop for quenched materials.
33(1962); http://dx.doi.org/10.1063/1.1728926View Description Hide Description
This paper presents a theoretical discussion of the results to be expected from magnetic aftereffect measurements on Ni containing a sufficient number of divacancies. Calculations are made of the stabilization energy for 109°−〈001〉 and 180°−〈110〉 Bloch walls, the walls expected to be most mobile at low magnetic fields. The results show features characteristic of the 〈110〉 symmetry of the divacancies that are not shared by centers with 〈100〉 symmetry: (1) The existence of two relaxation modes having different relaxation times and different interaction energies with the magnetization vector, and (2) the nonvanishing of the stabilization force on 109° walls at large Bloch wall displacements.
Some Properties of Dirty Contacts on Semiconductors and Resistivity Measurements by a Two‐Terminal Method33(1962); http://dx.doi.org/10.1063/1.1728927View Description Hide Description
The surface and bulk properties of semiconductors have been studied by a two‐terminal method using dirty contacts. These contacts are defined as ones that are easily applied and removed and that are separated from the bulk by surface states, oxides, adhered gasses, and chemical films. The method essentially involves measuring the resistance‐voltage (R‐V) characteristics from the millivolt range up to about 100 V, and from these data the sample resistivity can be obtained directly. The effect of work damaging or etching the semiconductor surface can be readily evaluated. By observation of the shape of the R‐V curves it is possible to separate the bulk from the surface effects, calculate the surface barrier height and thickness from tunnelingequations, and determine whether the barrier is a surface film or due to metal‐semiconductor contact potential difference. An application of the theory of electric field tunneling of Holm to the data of siliconcarbide gives values of about 2.4 eV for the barrier height and 13 Å for the barrier width of the film on the surface. Sufficient information is included on electrode techniques and pitfalls so that the experimentalist can readily make the measurements. Efforts were concentrated on siliconcarbide and silicon, but the techniques are applicable to all types of semiconductors.
33(1962); http://dx.doi.org/10.1063/1.1728928View Description Hide Description
Some unusual properties of the porous graphite contact on p‐type silicon are described. Current through the sample reaches a saturation level that is inversely proportional to the amount of adhered water vapor. Other gases such as ammonia, H2S, and HCl modify the shape or amplitude of the saturation current. There is a hysteresis loop in the low voltage region which is similar in appearance to that of a ferroelectric. The general method of measurement can be applied to studying the semiconductor surface as well as the contact phenomena. The possible applications include such devices as current regulators, humidity detectors, and surface‐barrier radiation detectors.
An electronic band model, which includes a trap‐dominated inversion layer, is presented to explain the phenomena. This model also integrates various conflicting theories of metal‐semiconductor contacts.
33(1962); http://dx.doi.org/10.1063/1.1728929View Description Hide Description
In a series of preceding papers connection has been made between the physics of the activated jump over a static potential barrier and related computing devices. This paper treats a more complicated system, namely, the tunnel diode, which stores information in one of two possible dissipative states. The activated jump between dissipative states is analyzed. An idealized physical model is used to find which of the two states is the really preferred one, and to also evaluate the rate at which fluctuations bring about an approach to this preferred distribution. To simplify the analysis, the case where the tunnel diode is in series with a vacuum diode, rather than a resistor, is emphasized. For typical germaniumEsaki diodes, activated jumps are improbable if the junction cross section exceeds 10−11 cm2.
33(1962); http://dx.doi.org/10.1063/1.1728930View Description Hide Description
So‐called front‐wall cells have been produced from CdS by inwards diffusion and subsequent selective etching away of the metal (Cu,Ag,Ni) producing the barrier layer. In particular, measurements were carried out of the dark current as a function of the voltage applied at different temperatures, of the shortcircuit current and the open‐circuit voltage as a function of tempering time, illumination and temperature; and the spectral sensitivity of the short‐circuit current was compared with the photoconductivity. It is shown that the photovoltaic effect can be explained not by a photoionization of electrons from the free metal but only by a p‐n photoeffect. On the assumption of an impurity conduction in the p region the properties of the p‐nphotovoltaic effect are discussed.
33(1962); http://dx.doi.org/10.1063/1.1728931View Description Hide Description
The yield phenomenon observed in dislocation‐free crystals of silicon is critically dependent on prior heat treatment at 1000°C. Since oxygen is always present in crystals grown from quartz crucibles, heat treatment at 1000°C is believed to result in silicon‐oxygen clusters. It is shown that these clusters are effective sites for the generation of dislocations. Since the velocities of dislocations in heat‐treated and unheat‐treated crystals is not altered, the lowered maximum stress is largely due to the increased numbers of dislocations generated at a relatively early stage of the deformation process.
33(1962); http://dx.doi.org/10.1063/1.1728932View Description Hide Description
The dust of NaCl and KCl ordinary monocrystals shows greater charge per particle surface area than the dust of whiskers of these materials. The main reason is a greater cleavage surface of monocrystals than the cleavage surface of whiskers. The electrification of the dust is also influenced by the presence of contaminations in the crystal.
33(1962); http://dx.doi.org/10.1063/1.1728933View Description Hide Description
When a single crystal is used in an x‐ray spectrometer to display the continuous spectrum of an x‐ray tube by Bragg diffraction, absorption edges characteristic of the atomic species in the crystal may be observed. The change in diffracted power across an edge is fundamentally due to a change in the atomic scattering, especially the imaginary part of the scattering, of the species whose edge it is. This produces a sharp anomaly in both the structure factor and the absorption factor of the reflection being used. The explicit effect of the absorption is a function of the perfection of the crystal face; the change in the structure factor depends uniquely on (hkl) for noncentrosymmetric crystals. Thus, the absorption edges may be used as a tool to study perfection of crystal faces and polarity of polar axes. The K edges of Ga and As seen in reflection from (111) and (1̄1̄1̄) faces of perfect and mosaic crystals of GaAs are discussed both theoretically and experimentally in some detail. The linear absorption coefficient of GaAs and Ge have been measured in the course of the work.
33(1962); http://dx.doi.org/10.1063/1.1728934View Description Hide Description
Two ferroelectricceramics,Pb (Zr0.52, Ti0.48)O3 with 1 wt % Nb2O5 and pure BaTiO3, were studied under shock wave loading. Plane shock waves having pressures ranging from 2 to 175 kbar were produced by high‐explosive driving systems. Measurements were made with a streak camera and shock‐detecting ferroelectric wafers. Both ceramicmaterials showed evidence of a two‐wave structure over a wide range of shock pressures. Results indicate that these two‐wave structures are characteristic of the elastic properties of the materials and are not due to phase transitions from ferroelectric to paraelectric. Pressure‐volume Hugoniot curves are presented for both materials. Electrical charge released to an external load by Pb (Zr0.52, Ti0.48)O3 with 1 wt % Nb2O5 was measured as a function of the pressure of a shock which was moving parallel to the axis of polarization. The primary mechanism in this reduction of polarization by shocks was found to be, not a phase transition or domain switching, but merely a reduction of the dipole moment by axial compression.
33(1962); http://dx.doi.org/10.1063/1.1728935View Description Hide Description
Measurements of critical current as a function of applied magnetic field were made on wire samples of Nb 25% at. Zr alloy at 4.22°K and 2.24°K. Curves are plotted for the cases for which the applied magnetic field is parallel to or perpendicular to the current direction. At 4.22°K, Hc (I → 0) for H perpendicular to I is found to be 70 kG, while for H parallel to I, it is found to be 82 kG. Reduction of the temperature to 2.24°K results in an increase of 13 kG in both cases. For H perpendicular to I, current densities of 4.4×104 A/cm2 are observed at 60 kG and 4.22°K, and at 73 kG and 2.24°K. The effective demagnetization factor of the current carrying elements is 0.6 π. All measurements were made in a dc magnetic field with quasidirect currents by observation of the first resistive voltage to appear across the sample.
33(1962); http://dx.doi.org/10.1063/1.1728936View Description Hide Description
Tables and plots are given of certain definite integrals which facilitate the calculation of an upper bound on the field attainable with a given configuration of magnetic material, when the volume occupied by magnetic material consists of a combination of solid and hollow circular cylinders. Such estimates are useful in the design of powerful electromagnets.
33(1962); http://dx.doi.org/10.1063/1.1728938View Description Hide Description
The behavior of magnetic domains in certain sections of single crystalyttriumirongarnet is related to growth history. Discontinuities in domain patterns coincide with lines of intersection of vicinal faces on the growthsurfaces of the crystals, and regions delineated by the discontinuities show differences in magnetic behavior. The relationship of these effects to oriented strains resulting from growth processes is considered.