Volume 34, Issue 8, 01 August 1963
Index of content:
34(1963); http://dx.doi.org/10.1063/1.1702702View Description Hide Description
A study was made of (110) fracture in LiF using birefringence and etch‐pit techniques. Fracture always occurred parallel to birefringence bands, and it is suggested that the stress of these bands aids (110) crack propagation. Surfaces formed by (110) fracture appear to be flat. If (100) steps are present, as suggested by Kuznetsov, they are less than 100 Å in height. By comparing etch‐pit density data with birefringence data, it is estimated that only about one in twenty etch pits was due to single, unpaired dislocations. All the rest were probably caused by dipoles.
Theoretical Analysis of Plastic Deformation in Superlattices Based on the Body‐Centered Cubic Structure34(1963); http://dx.doi.org/10.1063/1.1702703View Description Hide Description
A theoretical analysis has been made of the friction stress opposing the movement of both superlattice slip dislocations and twinning dislocations which arises from their interaction with antiphase domains in B2‐, DO3‐, and L21‐type superlattices, all of which are based on the body‐centered cubic structure. In the case of superlattice slip dislocations, this interaction results in the well‐known maximum in yield stress at some critical antiphase domain size. Twinning dislocations, on the other hand, require stress levels about twenty times greater than the maximum value calculated for superlattice dislocations; however, this friction stress decreases for domains smaller than the critical size. In both DO2‐ and L21‐type superlattices, second nearest neighbor as well as first nearest‐neighbor ordering energies are found to play an important role in the resistance to dislocation motion. These results agree well with the limited number of experimental observations obtained by other investigators with the Fe3Al superlattices.
34(1963); http://dx.doi.org/10.1063/1.1702704View Description Hide Description
The electron‐voltaic effect in commercial siliconp‐n junctions is used to determine the distribution of radiation defects with penetration distance for a 500‐keV electron beam in n‐type silicon. It is shown that the concentration of radiation defects falls off far more rapidly than previously reported by Vavilov et al., who used changes in resistivity to determine the distribution. Their experimental data are reinterpreted and found to be in reasonable agreement with the electron‐voltaic data. Attempts are made to explain the experimental data by constructing radiation defect vs depth curves using various relations for the probability of defect formation as a function of electron energy, and for electron energy as a function of penetration distance. Reasonable agreement exists between such theoretical curves and the experimental data.
34(1963); http://dx.doi.org/10.1063/1.1702705View Description Hide Description
The forced vibration problem of a finite sphere with an exciting source embedded inside it has been treated. The source is assumed to oscillate sinusoidally and the complex dilatational modulus is used to describe the motion inside the sphere. It is found that if the outer surface of the sphere is restrained from radial displacement, the ratio of the amplitude of a midpoint (between the outer surface of the sphere and the inner exciting source) to the amplitude of a point on the exciting source could be used as an indication of the dilatational properties of the sphere. Therefore, from the criterion of maximum amplitude ratio in conjunction with the frequency, the dilatational properties of a testingmaterial can be determined. This method can be applied to viscoelastic fluids as well as solids.
34(1963); http://dx.doi.org/10.1063/1.1702706View Description Hide Description
Electrical charges developed during plastic deformation of sodium chloride were investigated. Experiments are chosen (orientation and shape of the crystals; direction of applied stress) to enable the edge and screw components of dislocation loops to emerge separately on two different pairs of the external surfaces of the specimen. The experimental results indicate that an electrical charge is associated only with the edge components of dislocation loops. It is also found that in tensile experiments of a bent specimen (accidentally or intentionally) negative charge appears on the concave side and positive charge appears on the convex side, and the negative charge is larger, in absolute value, than the positive charge, in the early stages of deformation. A model is proposed to explain these results.
34(1963); http://dx.doi.org/10.1063/1.1702707View Description Hide Description
The effect of preferred orientation on the Hall effect in titanium was examined in an attempt to resolve the inconsistent results of previous investigators. Three specimens of iodide titanium were prepared with different textures and the Hall coefficient of each was measured between 4.2° and 295°K. The Hall coefficient was found to depend on both temperature and preferred orientation. At room temperature, it was determined to be −1.8×10−11m3/C in two specimens and +1.2×10−11m3/C in a third one. From the x‐ray analysis of the texture and the Hall coefficient data, it was concluded that the positive component of the Hall coefficient was associated with the hexagonal axis of titanium, being parallel to the magnetic field. An approximate calculation estimated the principal components of the Hall tensor to be R ∥ = +4.2×10−11 and R ⊥ = −7.7×10m3/C, where R ∥ is the Hall coefficient when the magnetic field is parallel to the c axis and R ⊥ the Hall coefficient when the magnetic field is perpendicular to the c axis. The values of R ∥ and R ⊥ given above can account for the spectrum of previously published Hall values for titanium.
34(1963); http://dx.doi.org/10.1063/1.1702708View Description Hide Description
Etch pits on (001) surfaces of calcium tungstate crystals, formed on treatment with hot dilute solutions of hydrochloric acid, were identified to be dislocationetch pits. The technique is able to follow the motion of dislocations during annealing by the formation of new sharp‐bottomed pits adjacent to flat‐bottomed pits at the initial dislocation positions. Inclusions, both solid and gaseous, are an important source of dislocations in crystals of calcium tungstate grown from the melt.
34(1963); http://dx.doi.org/10.1063/1.1702709View Description Hide Description
Evidence for the existence of ultrasonic internal conical refraction in rocksalt and calcite single crystals was obtained by observing the behavior of shear waves along one of the axes of threefold symmetry in each crystal. A method of selective absorption was used in conjunction with pulse‐echo ultrasonic attenuation measurements. Absorbing material was pasted at various spots on the bottom of the specimen opposite the transducer to map the energy density in the wave. Positions of maximum flux coincided with predictions when the shear wave was not perturbed by misorientation‐induced ultrasonicdouble refraction.
34(1963); http://dx.doi.org/10.1063/1.1702710View Description Hide Description
Internal friction in a series of V‐H alloys has been measured over the temperature range −190° to +210°C. The hydrogen compositions varied from 1.2 to 14.5 at.% and two frequencies, 75 and 270 cps, were used. For each frequency a single peak was observed having an amplitude which increased essentially linearly with hydrogen concentration. This is suggestive of stress‐induced ordering of the hydrogen atoms in the vanadium‐hydride lattice. Comparison of the activation energy and temperature of this peak with that reported by the authors for the tantalum‐hydrogen system indicates that the hydrogen atoms are located in the tetrahedral interstices.
34(1963); http://dx.doi.org/10.1063/1.1702711View Description Hide Description
The formation of point defects during plastic deformation and their subsequent interactions with dislocations in high‐purity copper was studied using a low‐temperature strain‐aging technique. The point‐defect‐dislocation interaction gave rise to a small yield point whose magnitude depended on the temperature and time of aging after deformation. The annealing kinetics indicated that two types of point defects were formed by deformation in concentrations which depended on the prestrain and prior history of the specimen. A mechanism for the point‐defect annealing was proposed to account for the experimental results and was used to calculate the diffusion‐activation energy of the defects in the lattice and along the dislocations. Based on this mechanism, the defects were tentatively identified as the interstitial and the divacancy. The temperature dependence of the yield point was examined and was shown to be consistent with an elastic interaction due to the dilatation around the point defects. The results indicated that the point defects retained their identity on arrival at the dislocation rather than becoming incorporated into the dislocation structure.
34(1963); http://dx.doi.org/10.1063/1.1702712View Description Hide Description
A method for the measurement of dc and ac components of number density in cylindrical gas‐discharge columns has been studied. The technique is based on a dipole resonance, occurring below the electron plasma frequency. It is excited by an rf signal transmitted along a stripline enclosing the plasma column. By working on the side of the resonance curve, a fluctuating output signal is obtained corresponding to the density fluctuations. The simple theory of the resonance has been extended to take account of stripline size, of the glass surrounding the column, and of a radial variation in number density. The method is considerably more sensitive than cavity perturbation techniques, and the results compare reasonably well with number densitymeasurements made by other methods.
34(1963); http://dx.doi.org/10.1063/1.1702713View Description Hide Description
Observations of dc and ac components of electron number density have been made along the positive columns of hot‐cathode, mercury‐vapor discharges, at a pressure of about 1 μ Hg. Direct measurements were made by a microwave method involving a dipole resonance of the column, while indirect measurements were made from the accompanying light output and, in the case of fluctuating components, by capacitive pickups sliding on the outside of the tube. The frequency spectra of number density, light output, and potential fluctuations generated by the tube itself were found to be similar, and their variations in amplitude along the column were found to be closely related. For injected ac signals of variable frequency, phase and amplitude variations in the number density are shown to agree closely with the theoretical predictions of Crawford. Unsuccessful attempts were made to excite low‐frequency fluctuations by injecting electrons into the positive column from an auxiliary cathode.
Some Preliminary Results on Defects in Irradiated UO2 Single Crystals as Revealed by Transmission Electron Microscopy34(1963); http://dx.doi.org/10.1063/1.1702714View Description Hide Description
Vapor‐grown single crystals of nearly stoichiometric UO2 have been thinned for transmission electron microscopy. The as‐grown crystals show a dense pattern of precipitates and may contain loops due to quenched‐in vacancies but contain hardly any dislocations. Two models for the production of loops are proposed. Dislocation networks (small angle boundaries) are produced on annealing the crystals. Irregular patterns of dislocations arise when sufficient thick parts of the crystal foils are treated with heat shocks in the microscope suddenly increasing the beam current. After exposure to 1015 nvt the crystal foils show fission tracks, short segments of straight dislocation lines and black dots, which are partly distributed at random and partly are aligned on fission tracks. Interaction between fission tracks and dislocations has been observed. The conditions for contrast production on fission tracks have been investigated. The experimental findings can be explained by a model of a displacement field around the fission tracks.
34(1963); http://dx.doi.org/10.1063/1.1702715View Description Hide Description
Controlled experiments have been conducted to isolate the processing parameters which produce bright electroluminescence in hexagonal ZnS:Cu, Cl phosphors. Crystal structure, in particular, was studied as it is affected by the mechanical, thermal, and chemical treatments of this processing. The correlation between electroluminescent brightness and stacking disorder which has often been suggested has been more quantitatively established. It was possible only partially to separate the effects of the entry of activator (Cu) from those of faulting upon electroluminescence. It was found that these two phenomena are intimately related in their importance for high brightness.
The nature and extent of the disorder has been investigated by precision x‐ray diffractometry of the powders and by single‐crystal x‐ray diagrams of individual granules from them. Each granule has been found to be heterogeneous to xrays so that the analysis of the powder patterns is not straightforward. Much, or most of the disorder is obscured in the background of powder patterns and shows itself clearly only on single‐crystal diagrams. The latter have been analyzed according to Jagodzinski's model based on two independent fault parameters; the fits obtained with this model between calculated and observed x‐ray scattering are impaired by heterogeneity. The analysis discloses that the disorder in the homogeneously scattering‐faulted regions may be an order of magnitude larger than indicated by the line breadth of the parent phase, which indicate faulting frequencies seldom exceeding 3%. Coherence in both the transformed and residual parent phase is favored by faulting in the presence of activator during the processing which produces bright EL. This coherence is lacking, even when faulting is extensive, if the disorder occurs without activator; under such conditions EL is practically not observed.
Conclusions reached in the past about the effect of structure upon EL, and also upon birefringence in ZnS phorphors, should be re‐examined for the validity of the x‐ray diffraction work on which they may have been based, inasmuch as in the course of the present work it has been shown that high precision diffractometry and single granule diagrams are needed to obtain an adequate picture of the disorder. Film diffraction may be entirely misleading.
34(1963); http://dx.doi.org/10.1063/1.1702716View Description Hide Description
Tensor components have been derived for the matrix relating entropy flux and gradient of electrochemical potential to gradient of temperature and electrical current. The entries have been specified in terms of six basic galvano‐thermomagnetic transport coefficients; the Casimir—Onsager reciprocity conditions have been utilized to reduce the number of independent coefficients. These relationships have also been employed to extend the Bridgman—Heurlinger identities to anisotropic materials. Using these equations along with the phenomenological relations, the characteristics for galvano‐thermomagnetic Nernst generators operating under adiabatic conditions have been formulated. The results are compared with the equations which are valid for the isothermal case.
Theory of Galvano‐Thermomagnetic Energy Conversion Devices. V. Devices Constructed from Anisotropic Materials34(1963); http://dx.doi.org/10.1063/1.1702717View Description Hide Description
Operating characteristics of galvano‐thermomagnetic generators and refrigerators for any anisotropicmaterial are summarized in tabular form for both the longitudinal (thermoelectric) and transverse (Nernst) devices and for isothermal and adiabatic operating conditions. In the most general case, device performance is governed not only by the figure of merit, but also by an anisotropy factor. This factor involves off‐diagonal elements of the Nernst and Seebeck tensors for Nernst devices and diagonal elements of the Nernst and Seebeck tensors for thermoelectric devices. It is believed that the ``Umkehr'' effect (such as observed in Bi) is due to a nonvanishing diagonal component of the Nernst tensor.
34(1963); http://dx.doi.org/10.1063/1.1702718View Description Hide Description
The effect of specimen size on the residual electrical resistivity has been measured for six types of superpurity aluminum. The specimens were strips with thicknesses between 1.5 and 0.01 mm. The results are compared with Fuchs' theory for the resistance of thin films. The measurements on the four purest materials gave good agreement with theoretical curves, but the variations in the constant ρ·l (bulk resistivity times bulk mean free path) were much larger than expected from the experimental error. The measurements on the two least pure materials could not be fitted to a theoretical curve, although the individual points are reproducible.
34(1963); http://dx.doi.org/10.1063/1.1702719View Description Hide Description
With the use of x‐ray diffraction and etch‐pit techniques, the imperfections in electrodepositedcoppersingle crystals were compared to corresponding substrate imperfections. Areas of local polycrystallinity in the deposit were found to be associated with areas of misorientation in the substrate. The deposit perfection is influenced by the interaction of both dislocations and impurities in the substrate. With special purification of the electrodeposition solution, a deposit could be produced with a degree of perfection higher than that of the substrate.
34(1963); http://dx.doi.org/10.1063/1.1702720View Description Hide Description
The melting and crystallization of tetragonal phosphorus pentoxide has been investigated by the use of a microscopic technique. Superheating of about 50° above the melting temperature was possible. Melting was heterogeneous and began only at free surfaces or grain boundaries but never within a crystal. At a supercool or superheat of 10°, the respective rates were approximately 5 and 10 A/sec. The dependency of melting and crystallization on temperature, time, crystal direction, morphology, and preparation history was studied. Under ideal conditions, the observed rates were in almost quantitative agreement with those calculated from a simple model based primarily on viscosity considerations.
34(1963); http://dx.doi.org/10.1063/1.1702721View Description Hide Description
The viscosity of liquid phosphorus pentoxide at temperatures both above and below the melting point of the tetragonal crystal has been determined by the falling sphere method. At the melting temperature of 580°, the viscosity is 5.15×106 P and the activation energy for viscous flow is 41.5 kcal/mole. These values suggest that similar to liquid B2O3 or SiO2, the melt of tetragonal P2O5 can be classified as a network liquid. From qualitative observations on melting behavior and a consideration of the refractive indices of the crystalline and glassy phases, hexagonal P2O5, on the other hand, gives a molecular liquid on fusion. The ``hexagonal'' liquid, however, is unstable and undergoes rapid polymerization to give the network liquid.