Volume 28, Issue 5, 01 May 1957
Index of content:
28(1957); http://dx.doi.org/10.1063/1.1722792View Description Hide Description
Some early studies of the pinch effect, made with a view to its application to the problem of developing a thermonuclear reaction are here presented, together with experimental arrangements for producing pinched discharges in toroidal and straight discharge tubes. Dynamical properties of the pinch and its instabilities are demonstrated, and the possibility of stabilizing the latter is mentioned.
28(1957); http://dx.doi.org/10.1063/1.1722793View Description Hide Description
The dependence of ultrasonic attenuation and velocity on plastic deformation in 2Saluminum is compared at two frequencies, five and ten megacycles, and the comparison shows agreement as regards frequency dependence with what one can expect from dislocation damping theory.
28(1957); http://dx.doi.org/10.1063/1.1722794View Description Hide Description
Measurements have been made of the energy distribution of electrons penetrating thin samples of Al2O3. The observations were made using a 127° electrostatic analyzer. These results allow a determination of the dissipation of electron energy with depth when 2.5–10 kev electrons strike Al2O3. Results obtained indicate that for a given bombarding energy the average energy loss per unit thickness dW/dx remains almost constant over the entire penetration depth. However, dW/dx decreases slowly as the bombarding electron energy increases. These observations are in general agreement with those made by Ehrenberg and Franks for 10–40 kev electrons striking phosphors.
28(1957); http://dx.doi.org/10.1063/1.1722795View Description Hide Description
Deformation twins related to two separate (111) orientations have been observed in a coppersingle crystal deformed by tensile extension at 4.2°K. The unambiguous determination of the twin relationships is accomplished by a unique x‐ray technique which makes use of a single crystal adapter and Geiger counter methods. The adapter permits the detection and measurement of weakly diffracting reflections and the twin relationships are established by the unique application of geometrical data in a stereographic projection.
28(1957); http://dx.doi.org/10.1063/1.1722796View Description Hide Description
A method has been developed for obtaining a permanent three‐dimensional view of the flux pattern of a magnetic field by suspending microscopic magnetically‐responsive particles in a transparent plastic solution, applying the magnetic force to form the particles into a definitive pattern of the field, and allowing the plastic to set. After the plastic solidifies, the specimen may be cut and polished for observation and later study or reference. A variation of this technique provides for the preparation of views on transparent plates. The process lends itself readily to the study and resolution of problems in magnetics encountered in research and development. It also has possible applications in the fabrication of magnetic and electronic components.
28(1957); http://dx.doi.org/10.1063/1.1722797View Description Hide Description
A simplified method of calculating the crystallographic features of a diffusionless (martensitic) phase change is described and applied to the gold‐cadmium alloy cubic‐orthorhombic transformation. Use of the method requires a knowledge of the initial and final crystal structures and the specification of the crystallographic plane and direction of the inhomogeneous shear. It is based on the requirement that after partial transformation the interface between the two phases be free from stress on a macroscopic scale. The method involves representing on a stereographic net (a) the loci of directions in the original phase unchanged in length by the structure change per se and (b) the loci of directions unchanged in length by the inhomogeneous shear. The intersection of these loci determine possible undistorted interface planes. Excellent agreement with experimental observations is found when the method is applied to the cubic‐orthorhombic transformation in gold‐cadmium alloys.
28(1957); http://dx.doi.org/10.1063/1.1722798View Description Hide Description
Plastic deformation may be introduced in LiF single crystals by abrading, quenching, or bending. For x‐ray wavelengths of about 1.5 AU(CuKα) the diffracting power is increased about 4 times by any of the three methods. For shorter wavelengths of about 0.7 au(MoKα) the increase in diffracting power is about 4, 7, and 10 times for abrading, quenching, and bending, respectively. The bent crystals are more effective at shorter wavelengths because the plastic deformation is more uniform throughout the crystal while in the abraded crystal, all the deformation is concentrated in a layer extending only 0.002 inch below the surface. The extent and concentration of plastic deformation is shown by selective etching of dislocation sites.
28(1957); http://dx.doi.org/10.1063/1.1722799View Description Hide Description
The concentration dependence of the residual resistivity of Ni–Pd alloys might be expected to obey a simple Nordheim law since the number of holes in the d band and the number of electrons in the s band are approximately independent of composition. Measurements have indicated, however, that the maximum resistivity occurs at 70 atomic percent Pd instead of 50%. This behavior can be explained by the spin dependence of electron scattering processes associated with the fact that the alloys with greater than 3% Ni are ferromagnetic. Quantitative fit of the data is possible if it is assumed that the density of states at the Fermi energy in the d band is approximately proportional to the number of holes, rather than to the cube root of this number. Data on the resistivity and paramagnetic susceptibility of Ag–Pd alloys support this hypothesis. The relative strength of s‐d scattering to s‐s scattering must be assumed to be about 5:1.
28(1957); http://dx.doi.org/10.1063/1.1722800View Description Hide Description
28(1957); http://dx.doi.org/10.1063/1.1722801View Description Hide Description
Magnetic domain patterns have been observed on evaporated films of iron,cobalt,nickel, and several different alloys of these elements. The films were deposited in the presence of a magnetic field to establish a uniaxial direction of easy magnetization. It was found that the direction of easy magnetization, in films of all compositions, could be changed by re‐heat‐treating them in a magnetic field with a new orientation. This is of special interest because bulk specimens of the elements do not respond to heat treatment in a magnetic field.Nucleation and growth of domains were observed for fields applied at various angles to the uniaxial direction of easy magnetization in some of the films. Patterns were obtained on films ranging in thickness from approximately 50 A to 12 000 A. Domain walls in films ranging from 50 A to 500 A in thickness tend to aggregate in pairs.
28(1957); http://dx.doi.org/10.1063/1.1722802View Description Hide Description
Measurements of the integrated intensity of the superstructure reflections 100 and 110 from powders and a single crystal of CuAu3 give a long‐range order parameter S=0.87±0.04 for a temperature below Tc not well defined. At the composition CuAu3, Tc =199°C. With decreasing Au content, Tc increases. Compositions richer in Au than CuAu3 could not be ordered. No annealing treatment was found which would sharpen the superstructure reflections. The breadths correspond to antiphase domains about 50 A in size. Measurements of the diffuse intensity from a single crystal of CuAu3 quenched from 285°C, or held at temperatures of 250°C and 320°C, gave short‐range order coefficients qualitatively similar to those for Cu3Au. Compared with Cu3Au, CuAu3 shows a weaker first neighbor and a stronger second neighbor ordering.
28(1957); http://dx.doi.org/10.1063/1.1722803View Description Hide Description
The microwave breakdown electric field has been measured for pure air uncontaminated by discharge products (presumably oxides of nitrogen), and for nitrogen and oxygen separately. The breakdown field for pure air is significantly higher than that previously observed, in which cases such contamination could be suspected.
The breakdown field for pure air lies between those for nitrogen and oxygen. A calculation of the high‐frequency breakdown field in air is made from dc data, using the measured Townsend ionization coefficient, electron attachment coefficient, and average electron energy. The present data agree well with this calculation.
28(1957); http://dx.doi.org/10.1063/1.1722804View Description Hide Description
A new technique is described for measuring the charge distribution of an electron beam in a magnetic field. The method is especially applicable to the long, thin, high‐current beams commonly used in traveling wave tubes and backward wave oscillators. It consists of making the beam trace out its own characteristics by sweeping it across a pinhole in crossed electric and magnetic fields. The data are presented in the form of oscilloscope pictures of the actual charge distribution. Quantitative measurements of the charge distribution are also obtainable.
28(1957); http://dx.doi.org/10.1063/1.1722805View Description Hide Description
The field diffracted by an aperture of any shape is evaluated asymptotically for small wavelengths on the basis of several approximate diffraction theories. These are the Kirchhoff method, its two customary modifications, and W. Braunbek's new modification. In each case, a double integral over the aperture is evaluated asymptotically, and contributions from interior stationary points, edge stationary points, and corners of the edge are obtained. The contributions from points of each type, according to all the theories examined are exactly of the form recently deduced by J. B. Keller using his geometrical theory of diffraction. The edge stationary points and the corners correspond respectively to rays singly diffracted from edges and from corners. For edge‐diffracted rays the Braunbek diffraction coefficients alone coincide with those given by the geometrical theory of diffraction, but the Braunbek method does not apply to corner diffraction. None of these methods takes account of multiple diffraction as does the geometrical theory of diffraction.
28(1957); http://dx.doi.org/10.1063/1.1722806View Description Hide Description
Plots of flux tubes in axially symmetric static field problems are ordinarily obtained by rather tedious methods from plots of the equipotentials. The differential equation for the flux function in circular cylindrical coordinates does not have the same form as Laplace's equation. Hence, an ordinary wedge‐shaped electrolytic tank cannot be used directly to plot flux tubes in an axially symmetric potential field. A direct method is described making use of a coordinate system in which Laplace's equation has the same form as the flux equation in the cylindrical system. An electrolytic tank can then be set up which makes use of this coordinate system in plotting equipotentials which are the analog of the flux tubes in an axially symmetric problem.
28(1957); http://dx.doi.org/10.1063/1.1722807View Description Hide Description
Uniform thin films of carbon in the thickness range 100 to 2300 A have been prepared by arc‐evaporation in vacuum. Evaporation rates were about 50 A/sec. The electrical properties of these films suggest that unannealed arc‐evaporated carbon had less long‐range order and was, therefore, more truly amorphous in structure than any other form of carbon yet obtained. After the films had been heated to 1200°K, however, their properties were similar to ordinary fine‐particle carbon blacks, apparently as a result of increased ordering. The electrical resistivity of unannealed films followed Ohm's law, was not photosensitive, and could be well represented in the temperature range 77°K to 300°K by the equation R=AT−b , where b=5. When the carbonfilms were heated above room temperature, the resistivity and the exponent b were both decreased substantially. After the films had been annealed at 1200°K, the resistivity was reduced by a factor exceeding 800, and could be best represented, in the temperature range 77°K to 1200°K, by the equation R=α—βT. The values of the resistivity, temperature coefficient of resistivity, and thermoelectric power of the annealedfilms were then more nearly in agreement with the corresponding values for ordinary fine‐particle carbon blacks. Other properties and applications of the films are discussed.
28(1957); http://dx.doi.org/10.1063/1.1722808View Description Hide Description
Bombardment of a silica‐soda‐lime glass (nD =1.5246) by more than 5×1016 A + ions/cm2 with an energy of 33.5 kev reduces the reflection coefficient to 0.36 of its normal value for light of wavelength λ=0.6 μ. The change in reflection coefficient is attributed to the formation of an altered glass layer. For a bombardment by 40‐kev A+ ions, the layer has a thickness of 0.095 μ and an effective refractive index of 1.343. The depth of the layer is determined by the ion energy and agrees approximately with the theoretical range of the incident ions. The refractive index of the altered layer is determined by the integrated flux of positive ions and the type of ion. The magnitude of the positive ion flux employed here is shown to be orders of magnitude larger than the flux of primary ``knocked‐on'' atoms produced by fast neutron bombardment in a reactor.
28(1957); http://dx.doi.org/10.1063/1.1722809View Description Hide Description
A hydrodynamictheory for the flow of a viscoelastic fluid is formulated and applied. The viscoelasticity is specified in terms of the conventional and well‐known relaxation and retardation spectra. Time dependent recoverable strains are defined with respect to convected rotating coordinate axes. The theory is linear to the extent that these strains are small although the over‐all displacement gradients of the fluid may be large. The theory is applied to two‐dimensional steady‐state simple shear flow to predict the variation of the apparent viscosity and the steady‐state elastic compliance with shear rate.Pressureflow through a pipe of circular cross section has also been treated.
Previous work of a similar nature is reviewed.
28(1957); http://dx.doi.org/10.1063/1.1722810View Description Hide Description
A theoretical treatment of space charge effects in beam‐type magnetron amplifiers and oscillators is given. It is assumed that the beam is relatively thin and that the magnetic field is large. The ``cyclotron waves'' are not treated. A space charge parameter appears in this theory of magnetron‐type traveling‐wave interaction in a manner which is analogous to the manner in which QC appears in ordinary traveling‐wave interaction. A distinctive feature of the space charge waves in the magnetron case is that one increases along the beam and the other decreases along the beam. A simple physical explanation of this effect is given.
This theory is then used to determine the starting conditions of an M‐type backward wave oscillator. It is found that when the tube is long in space charge wavelengths there is an appreciable reduction of starting current. When the space charge parameter approaches zero, the solutions found here reduce to the usual two‐wave solutions.
28(1957); http://dx.doi.org/10.1063/1.1722811View Description Hide Description
Tetragonal BaTiO3 is transformed into cubic BaTiO3 as a result of irradiation with fast neutrons at a temperature of approximately 100°C. Sufficient atomic displacements are produced by bombardment with an integrated fast neutron flux of 1.8×1020 n/cm2 so that tetragonal single crystals (c 0=4.0349 A and a 0=3.9923 A) expand anisotropically to form perovskite‐type cubic single crystals with a 0=4.0824 A. The irradiated material remains cubic to the lowest temperature of measurement, 78°K, showing none of the low temperaturephase transitions of unirradiated BaTiO3, and after annealing at 1000°C the crystal remains cubic but with a reduced lattice parameter. X‐ray, thermal, and optical methods were employed in studying these effects.