Volume 34, Issue 5, 01 May 1963
Index of content:
34(1963); http://dx.doi.org/10.1063/1.1729586View Description Hide Description
The distribution of column lengths p(n)/Nc in a powder sample is given by the following combinations of the coefficients of the Fourier series expansion of the x‐ray diffraction line shape: .
34(1963); http://dx.doi.org/10.1063/1.1729587View Description Hide Description
Curves of thermal expansion parallel and perpendicular to the polar axis are shown for poled bariumtitanate and lead titanate zirconate ceramics. Anistropy is remarkably higher in the latter due to greater alignment by 90° switching during poling. The anisotropy disappears after heating through the Curie point, and is also virtually eliminated within a given temperature range below the Curie point after exposure to a temperature only slightly above this range. The contraction in the polar direction upon first heating is due largely to a decrease in the alignment of domains originally reoriented by 90° during poling. The reversible expansion on repeated heating is due to a combination of domain anisotropy and a reversible increase in nonpolar alignment during heating. The pyroelectric effect is also greatly reduced in subsequent heating, although piezoelectric constants are reduced only slightly. The pyroelectric effect is found to be largely primary in both bariumtitanate and lead titanate zirconate ceramics. The increase in nonpolar 90° domain alignment on second heating of the lead titanate zirconate causes the primary and secondary pyroelectricity to be of opposite sign near room temperature.
34(1963); http://dx.doi.org/10.1063/1.1729588View Description Hide Description
Lattice parameters and distribution coefficients for the Bi2Te3‐Bi2Se3 pseudobinary alloy system have been determined for several compositions across the phase diagram. The clattice constant (based on a hexagonal lattice) shows deviation from Vegard's law between 33% Bi2Se3 and pure Bi2Se3 with maximum deviation at 67% Bi2Se3. Thermal energy gap as a function of composition shows a maximum near 33% Bi2Se3 and a minimum near 67% Bi2Se3. It is postulated that at least two sets of energy band minima are present in this alloy system.
34(1963); http://dx.doi.org/10.1063/1.1729589View Description Hide Description
An experimental method is presented, for the study of glow discharges, that can provide data suitable for analysis in terms of the diffusion of a ternary mixture. Particular emphasis is given to the constriction of flow discharges in neon with increasing gas density. It is found that quantitative estimates of the onset of this phenomenon are now possible.
34(1963); http://dx.doi.org/10.1063/1.1729590View Description Hide Description
Low‐energy electron diffractionpatterns obtained from Ge(100) and Ge(111) clean surfaces are described and analyzed. The Ge(100) surface structure is the same as that observed with Si (100) surfaces. Parameters for the atom sites are reported. Structural units for both Ge and Si (111) clean surfaces are strongly suggested by the data reported for these surfaces. They have an atom defect of about 25% in the top layer. A wide variety of such structures can be proposed and three of them appear to have been observed. The importance of long‐range order on the density of semiconductor chemical ``surface states'' is emphasized and surface bands are discussed.
Low‐Energy Electron‐Diffraction Study of the Surface Reactions of Germanium with Oxygen and with Iodine. II34(1963); http://dx.doi.org/10.1063/1.1729591View Description Hide Description
The effects of pressure and temperature on the O–Ge and I–Ge surface reaction systems were studied by means of low‐energy electron‐diffraction with single‐crystal (100) and (111) surfaces. The results are very similar to those reported for silicon, but have a displacement to lower temperatures of about 350°C. On both surfaces the oxide films are amorphous, but well‐crystallized two‐dimensional iodide structures were obtained and the transitions to the iodides are ``reconstructive.'' The slopes for all four transitions from the ``clean'' surfaces correspond to energies of about 55 kcal/mole. Clustering phenomena are also discussed.
34(1963); http://dx.doi.org/10.1063/1.1729592View Description Hide Description
Strong magnetic fields are useful in work with high velocity charged particles by virtue of their ability to rotate the particle momentum vector. The degree of usefulness is measured by the extent to which the desired rotations can be produced at the proper positions in the field. It is frequently necessary to inject charged particles into magnetic fields at points remote from these proper positions, in which case the injection parameters which enable the particles to reach these positions must be specified. For the production of gyromagnetic radiation in axially symmetric magnetostatic fields the proper positions correspond to the high field regions near the symmetry axis. The present introductory study is concerned with delineating approximate injection parameter ranges which allow the particles to attain the regions of high magnetic field strength in the field whose axial variation is bell‐shaped. The single‐particle model is adopted, and the results of both qualitative and quantitative investigations are presented.
A general formulation of the axially symmetric, magnetostatic field, forbidden‐zone theory is given and the useful concept of the paraxial forbidden zones is introduced. This theory is then applied to obtain qualitative answers to the injection problem for the bell‐shaped field for both zero and nonzero values of the angular momentum constant. Quantitative relations for the zero angular momentum constant case are obtained using the bell‐shaped field paraxial trajectories. A discussion of the limitations of the approximations employed is included.
34(1963); http://dx.doi.org/10.1063/1.1729593View Description Hide Description
The magnetic field in the vicinity of the center of a magnet gap is analyzed. In measuring an angle by means of a Hall cell in the magnetic field of the gap, the inhomogeneity of the magnetic field due to the influence of the edge of the gap leads to an error. The maximum error of the Hall voltage due to this cause is shown to be (5/16)Ww2/R3 , where W is gap spacing, R is magnet face radius, and w is Hall cell width in direction across which VH is measured.
34(1963); http://dx.doi.org/10.1063/1.1729594View Description Hide Description
A particular solution of the linearized Boltzmann equation, applicable to longitudinal oscillations of a one‐dimensional plasma without collisions, is constructed. It has the properties that (i) the Vlasov dispersion relation (1) is satisfied rigorously; (ii) that for large values of time, the perturbation decays as 1/t ½; and (iii) that the solution may be singular at some points. The solution is not included in the theories of Van Kampen or Landau because of (iii), but it is in no sense a general solution of the initial value problem. The frequency at which damping is slowest is approximately twenty‐five percent above the plasma frequency.
34(1963); http://dx.doi.org/10.1063/1.1729595View Description Hide Description
The rf spectrum of a gaseous discharge in a strong magnetic field is measured with the aid of a fixed frequency receiver by sweeping the magnetic field by approximately two percent. Source of radiation and receiving equipment are described. The following results were obtained: (1) A series of emission lines around the cyclotron frequency. (2) An emission at a frequency which increases with increasing magnetic field strength and decreases with increasing external voltage. (3) A strong resonance when both frequencies coincide: the intensity increases by a factor of 100 and more.
The frequency spectrum is discussed in terms of a simple theory, which accounts for local space‐charge fields. Their spatial distribution is inferred from the frequency behavior of the emission lines. Finally, a mechanism is suggested which classifies the resonance effect as an energy transfer from a longitudinal plasma mode into e.m. radiation.
Effect of Fast‐Neutron Irradiation on the Pressure and Temperature Dependence of the Elastic Moduli of SiO2 Glass34(1963); http://dx.doi.org/10.1063/1.1729596View Description Hide Description
The low‐temperature ultransonic attenuation, the elastic moduli, and the temperature and pressure dependence of the moduli of SiO2 glass are modified by heavy fast‐neutron irradiation. A sample of Amersil optical‐grade fused silica was irradiated to an integrated flux density in excess of 5×1019 neutrons/cm2. In this sample the ultrasonic loss is decreased by about 85%, the shear modulus increased less than 2%, and the compressibility decreased by about 20%. The anomalous temperature dependence of the compressibility changed from −3.3×10−2% per °K before irradiation to −9.5×10−3% per °K after irradiation over a temperature range of from 100°K to 300°K. The anomalous pressure dependence of the compressibility changed from 7.35×10−2% per atm before irradiation to 2.86×10−2% per atm after irradiation over a pressure range from atmospheric pressure to 5×104 psi. Similar effects were found in the pressure and temperature dependence of the shear modulus. The effect of γ irradiation on the low‐temperature ultrasonic loss and on the temperature dependence of the sound velocity in this glass was also studied. No change in the ultrasonic attenuation was detected at an irradiation level several orders of magnitude larger than that required to eliminate effectively the comparable 50°K loss peak in synthetic quartz.
34(1963); http://dx.doi.org/10.1063/1.1729597View Description Hide Description
A single‐crystal x‐ray study of β′‐AgCd has confirmed that this alloy has the ordered CsCl structure and has shown that the order is essentially perfect at room temperature. The measurements illustrate the high sensitivity of single‐crystal techniques in such studies.
34(1963); http://dx.doi.org/10.1063/1.1729598View Description Hide Description
A method is developed for determining the thermal diffusivity of materials by heating with an electron beam. A solution of the heat transfer equation is obtained for the case of a finitely thick, infinitely wide slab, and the expression for the temperature‐wave phase shift is given. Using an experimental measurement of the temperature‐wave phase shift, values of diffusivity in Al2O3 and carbon were determined. The accuracy of the method should be within ±10%.
34(1963); http://dx.doi.org/10.1063/1.1729599View Description Hide Description
The growth of germanium dendrites containing two twin planes has been investigated. The solution of the heat flow problem in the stem of a growing dendrite being pulled from a melt permitted the design of experiments suitable for testing theory. The limiting growth velocity was determined as a function of melt undercooling for a number of twin spacings. Tip radii were measured on decanted interfaces, many of which were found to have parabolic sections. The data for these could be fitted with the theory for dendritic growth assuming a layer flow mechanism with a rate constant ≥3 cm/sec‐°C. The only adjustable parameter is the average interfacial energy which may be as little as 15% less than the expected value, although lower values result in a somewhat more satisfactory fit. All the dendrites observed appear to grow under the condition that the magnitude of the tip radii is determined by the presence and spacing of the twins, and not under the condition of maximum velocity.
Parametric Coupling of the Magnetization and Strain in a Ferrimagnet. I. Parametric Excitation of Magnetostatic and Elastic Modes34(1963); http://dx.doi.org/10.1063/1.1729600View Description Hide Description
A theoretical treatment is presented of the parametric coupling between the magnetization and elastic strain in a ferrimagnet produced by magnetic pumping. In this paper the parametric coupling between resonant magnetostatic and elastic modes is discussed. The case of a disk vibrating in a longitudinal mode coupled to the uniform precession mode is discussed in detail. The oscillation threshold for this simple geometry is calculated and shows clearly the dependence of the threshold on the properties of the modes and the magnetoelastic coupling coefficients. It has been found that the threshold calculated for the disk geometry agrees to within an order of magnitude with measurements of the threshold for this effect using single‐crystal YIG spheres.
Parametric Coupling of the Magnetization and Strain in a Ferrimagnet. II. Parametric Excitation of Magnetic and Elastic Plane Waves34(1963); http://dx.doi.org/10.1063/1.1729601View Description Hide Description
The parametric coupling between traveling electromagnetic and elastic waves in a ferrimagnet supplied by magnetic pumping is theoretically investigated. The pumping threshold is calculated for steady‐state wave propagation in the presence of loss for plane electromagnetic and longitudinal elastic waves in an infinite ferrite medium. This threshold is compared to the threshold for second‐order spin‐wave instabilities and it is found that these spin wave processes should not prevent the magnetoelastic process from occurring.
It is shown that this traveling wave system may be useful for amplifying electromagnetic or elastic waves.
34(1963); http://dx.doi.org/10.1063/1.1729602View Description Hide Description
GaAs grown in a horizontal Bridgman crystal growth apparatus to which oxygen has been added exhibits lower silicon content than that grown without oxygen. Material grown under oxygen additions of 10–20 Torr exhibits, at room temperature, carrier densities in the 2–4×1015 cm−3 range and mobilities between 7500–8650 cm2 V−1 sec−1.
Silicon concentrations computed from the reaction 4Ga+SiO2 → 2Ga2O+Si are compared with electrical determinations of donor densities and spectroscopic determinations of silicon concentrations with reasonably good agreement. It is concluded that suppression of SiO2dissociation at the walls of the silica reaction tube is the most important action of oxygen on GaAs properties although oxygen doping may play a role in the production of high resistivity GaAs.
Effect of Low‐Temperature Phase Changes on the Mechanical Properties of Alloys near Composition TiNi34(1963); http://dx.doi.org/10.1063/1.1729603View Description Hide Description
X‐ray diffraction and dilation studies have shown that alloys near the stoichiometric TiNi composition undergo transformation into the related phases Ti2Ni and TiNi3 at low temperatures. The main factors controlling these phase transformations are alloy composition, temperature, and mode of plastic deformation. In plastic deformation, tensile or compressive stressing produced separate and unlike decomposition phases; this finding was dramatically demonstrated by unique temperature‐sensitive dimensional changes in plastically deformed specimens. Changes of large magnitude in vibration damping have also been noted and appear related to variations in the phase equilibria of the system.
34(1963); http://dx.doi.org/10.1063/1.1729604View Description Hide Description
Experimental work on nickelfilms is presented which verifies the theory that the dc voltages accompanying ferromagnetic resonance in thin metallic films arise from the nonlinear electrical conduction terms in ferromagnetics involving the extraordinary Hall effect and the magnetoresistiveanisotropy. Each of a group of four nickelfilms of different thickness was subjected to microwave pulses in well‐defined configurations of local microwave fields, as required for quantitative comparison of theory and experiment, and the resulting dc pulses were studied as a function of applied magnetic field and of microwave power and field configuration. The results lead to a complete specification of the films' electrical and magnetic properties at microwave frequencies. The films studied showed low magnetization but similar relaxation times compared to bulk nickel. The microwave resistivity is usually larger than at dc and complex. The magnetoresistiveanisotropy has its bulk value, while the extraordinary Hall constant is very large.
34(1963); http://dx.doi.org/10.1063/1.1729605View Description Hide Description
The thermoelectric powers (TEP) of copper alloys containing dilute amounts of iron,cobalt, or chromium and up to 10 at. % manganese or nickel have been measured between 4.2° and 1000°K. Similar alloys of gold have been measured between 4.2° and 100°K. The individual solutes produce comparable effects in either solvent. In copper, 0.1% iron produces a minimum TEP of −14 μV/°K near 20°K. A similar level of cobalt produces a TEP of −10 μV/°K near 200°K. All of the solutes measured exhibited minima in the TEP vs temperature data, but nickel,manganese, and chromium are much less effective than cobalt or iron in altering the TEP of copper. The thermopower does not increase indefinitely with concentration. In gold,iron produces its maximum TEP at about 0.2 at. % concentration, cobalt at about 2%. Several of the alloys measured also exhibited maxima in the 4.2°K region, similar to results obtained by others with nontransition solutes. Resistivity counterparts for these TEP‐temperature curves have been published earlier.