Volume 40, Issue 13, 01 December 1969

EPR Study of the Structure of CsPbCl_{3}
View Description Hide DescriptionStudies of the EPR of dilute Mn^{2+} in CsPbCl_{3} have been carried out over the temperature range 77°–340°K. Crystalline phase changes as indicated by changes in the Pb site symmetry were evident at 320°K (46.5°C) and 185°K. The first of these has been reported previously while the latter is apparently a new observation. Between 185° and 320°K, the EPR spectra can be assigned to two and possibly four crystallographically equivalent sets of magnetically inequivalent sites, the spectra of the ith set derivable from the spin Hamiltonian.Typical values of the crystal field parameters are: A = −87.5 G, B = −86.5 G, and a≈1 G; all sensibly temperature independent, while D and E are temperature dependent. For example at 195°K, D _{1,2,3,4}=45 G and E _{1}= −E _{2}=7.2 G, E _{3}=6.4 G, and E _{4}= −5.0 G. Both D and E tend continuously to zero at 320°K. Thus, the individual site symmetries are evidently orthorhombic and the overall crystal symmetry is apparently orthorhombic or nearly so. Some discussion of possible crystal space groups is included.

Structure and Properties of Sputtered Ta–Al_{2}O_{3} Cermet Thin Films
View Description Hide DescriptionTa–Al_{2}O_{3} cermet thin films were produced by diode sputtering. Cathodes were fabricated by powder techniques, plasma spraying and as mechanical composites. Film compositions of 1–20 wt% Al_{2}O_{3} produced resistivities of 250 to 25 000 μΩ·cm and TCR's of +100 to −450 ppm/°C, respectively. The films showed good thermal stability and could be anodized. Electron microscopy and x‐ray analyses suggest a film structure consisting of long columnar grains of bcc Ta, approximately 100 Å in diameter, surrounded by Al_{2}O_{3}. A quantitative analysis is made of tunneling currents in an idealized film. Resistivity‐composition and resistivity‐temperature data are explained in terms of this analysis. The height of the Al_{2}O_{3} barrier between grains is estimated to be 0.23 eV for an effective electron mass of 1/9. Barrier thicknesses are estimated at 15–30 Å.

Ionic Conductance of Potassium Azide in the Solid State
View Description Hide DescriptionNew experimental results on the dcelectrical conductance, between room temperature and 250°C, of potassium azide single crystals and pellets and potassium azide doped with barium azide are described. These data have been interpreted as involving cation current carrying species. Numerical values for the enthalpy of migration and enthalpy of defect pair formation are deduced. These are, respectively Δh _{1}=0.79±0.05 and h=1.43±0.05. Isothermal dcconductance data are also presented and imply that the charge carriers are involved in the thermal decomposition processes of KN_{3}.

Dislocation Pile‐Up in Half‐Space
View Description Hide DescriptionIf an obstacle exists in the vicinity of the free surface of a half‐space and a stress field is applied in such a manner that dislocations are pushed towards the obstacle, an array of dislocations then piles up into an equilibrium distribution against the obstacle. The distributions of dislocations are obtained by the Wiener‐Hopf technique for the edge and screw dislocations. The total strength of dislocations (Burgers vector multiplied by the number of dislocations) distributed in the distance L is calculated as 0.92π(1−v)σ^{ A } L/G for edge dislocations and 2σ^{ A } L/G for screw dislocations, where G, v are the shear modulus and Poisson ratio respectively and σ^{ A } is the applied stress. The result can be applied to crack problems. The above two numbers for the total strength of dislocations give the crack openings at the free surface for the extensional mode and the antiplane shear mode of fracture, respectively.

Effect of Annealing on the Substructure of Cold‐Worked Cu‐8.5 At. % Ge
View Description Hide DescriptionThe annealing behavior of Cu‐8.5 at.% Ge filings has been studied by means of x‐ray diffraction. The shifts in the positions of the powder pattern peaks from deformed and partially annealed samples can be accounted for by stacking faults and changes in the layer spacings at the stacking faults. No evidence was found for extrinsic faulting. Isothermal annealing treatments in the range 180°–240°C showed that the deformation stacking faultsannealed out in a two‐stage process. The first stage was associated with recovery while a kinetic study showed the second stage to be connected with recrystallization. The rms microstrains obtained by Fourier analysis techniques annealed out in a manner similar to the stacking faults. The particle size as determined by Fourier analysis remained constant during isothermal annealing until the recrystallization stage where it increased sharply. Analysis of the peak asymmetries during annealing showed that the number of twin faults remained constant at the value for the cold‐worked state until the recrystallization stage where a small increase occurred followed by a rapid decrease toward zero.

Diffraction Contrast Profiles of Voids
View Description Hide DescriptionDiffraction contrast profiles have been calculated using the two beam approximation for a thin crystal foil which contains a spherical void. The influence of the foil thickness, foil orientation, void radius, and void depth on the total contrast, a combination of both the strain contrast due to distortion of the foil by the void and the thickness contrast due to the absence of scatteringmaterial within the void, was examined, the lengths being expressed in units of ξ_{ g } ^{*} =ξ_{ g }(1+w ^{2})^{−1/2} where w=S _{ g }ξ_{ g } with S _{ g } the deviation from the exact Bragg condition and ξ_{ g } the extinction distance. Maximum strain contrast occurs for the profiles of both beams at depths d ^{*}= (N/2)ξ_{ g } ^{*} (N an integer); the black and white sides of these profiles alternate as the depth changes by 1/2ξ_{ g } ^{*}. The amplitude of the strain contrast is maximum at thicknesses t ^{*}=Nξ_{ g } ^{*} and (N+1/2)ξ_{ g } ^{*} for the scattered beam and the transmitted beam respectively. In either case, the inbetween thicknesses give a smaller strain contrast for which the white sides of the profiles are suppressed for voids within an extended region of the foil. This region of small amplitude is centered on the midplane of the foil for w=0, but shifts for only the scattered beam toward the entrance (exit) surface as w increases positively (negatively); a much larger amplitude can then occur near the opposite surface. Although the strain contrast for either beam is largest for void radius R _{0} ^{*}=1/4ξ_{ g } ^{*}, the thickness contrast at w=0 dominates the image of the void at this and larger radii. For , the strain contrast, when maximum, becomes the dominant image forming mechanism. As the foil is rotated away from the exact Bragg condition, w≠0, the thickness contrast can firstly become quite small compared to the strain contrast even at R _{0} ^{*}=1/4ξ_{ g } ^{*}, but upon further rotation it predominates once again as in the case w=0 while showing an image of opposite character to the former case.

Measurement of the Integrated X‐Ray Intensities of Ge and Si
View Description Hide DescriptionThe integrated Bragg intensities of the (111) reflections of perfect crystal Ge and Si have been measured with a higher accuracy than heretofore. Ge was measured using x‐ray wavelengths of 0.56 and 1.54 Å, and Si using 1.54 Å. Comparison of the scattering factors so obtained with those measured previously by the same method or by the Pendellösung fringe method leads us to the conclusion that there is not a systematic difference between the two methods, as has been suggested. We find that the reflectivity in the wings of the Bragg peak is much lower than the calculated value, and this effect would have to be taken into account if accuracies of the order of 0.1% are envisaged. We also measured Si (222) and found results in agreement with other reflection measurements, but not with recent transmission measurements.

Analysis of Laue Diffraction Images from Deformed Lattice Planes
View Description Hide DescriptionAn extension of the method for calculating the shapes of diffracting planes from the shape of the observed Laue spots is described in this paper. The method can be used for determining the shapes of the Laue spots diffracted from ideally perfect and deformed sets of planes. The method has been applied for calculating the shapes of several Laue spots by considering the planes to be ideally perfect. The calculated shapes are then compared with the shapes observed on Laue photographs.

Cylindrical Antennas Immersed in Arbitrary Homogeneous Isotropic Media
View Description Hide DescriptionThe cylindrical antenna is potentially useful as a probe for studying the properties of any material in which it is immersed. Examples of possible media are the earth, plasmas, and organic tissues. Such applications presuppose a knowledge of the detailed behavior of the currents and charges along an antenna in a medium with very general properties. In this paper the integral equation for the current along a highly conducting center‐driven cylinder of length 2h and radius a when immersed in a homogeneous isotropic medium is solved analytically and simple, and quite accurate formulas are obtained for the distribution of current, the distribution of charge, and the admittance. The lengths 2h may range from zero to over a wavelength; the radius a is assumed to satisfy the condition a≪h. The real effective conductivity of the medium may extend from zero to arbitrarily large values, the real effective permittivity from large negative values (as in certain plasmas) to large positive values (as in ordinary dielectrics like water).

Scattering Obliquely Incident Microwaves by a Moving Plasma Column
View Description Hide DescriptionThe problem of the interaction of obliquely incident microwaves with a plasma column which is moving uniformly in the axial direction is treated analytically. An arbitrary polarization for the incident plane wave is assumed. Two methods in solving this problem are presented. Extensive numerical results for the scattered energy in the backward and broadside directions and the angular distribution of the scattered energy are obtained for various interesting ranges of the parameters involved. It is found that cross‐polarized field components are induced even at normal incidence when the plasma medium is moving with respect to the observer and that cross‐polarized field components disappear at an incident angle θ_{0}=sin^{−1}(v _{ z }/c), where v_{ z } is the velocity of the moving plasma and c is the speed of light in vacuum when the plasma column is imbedded in free space.

Experimental Study of Scattering from a Plasma Column
View Description Hide DescriptionExperiments in microwavescattering from the positive column of a low‐pressure (1.2 mTorr) discharge in mercury under steady‐state conditions are described. A free‐space environment is simulated with an anechoic chamber and a unique cw cancellation method. Both amplitude and phase of the scatteredelectric field for both polarizations in the 2.6‐ to 5.85‐GHz frequency range are measured, so that diffraction characteristics, radar cross sections, frequency ratio, and phase shifts are determined. A calibration procedure using silver‐plated cylinders provides a higher degree of accuracy than has been attainable heretofore. Diffraction measurements show that, for plane‐wave incidence, the scattering is purely dipolar with a 95% confidence interval, except for overdense scattering where a significant monopole term characteristic of conductors is observed. Theoretical comparisons yield an estimate of the electron density distribution function and the damping of the Tonks‐Dattner resonances, as well as providing a test of the various theories. The results of some auxiliary experiments to find the average electron density in the column and the effects of wall and mercury reservoir temperatures upon the observed results are also given.

Statistical Properties of Light Scattered by Particles Suspended in a Turbulent Fluid
View Description Hide DescriptionSecond‐ and fourth‐order statistical properties of radiation scattered by particles in a turbulent fluid are theoretically investigated. They turn out to be of a non‐Gaussian type so that the measurements of the statistics of the scattered field can furnish information not obtainable by means of a spectrum analysis. In particular, our results are related with the possibility of measuring a Lagrangian type of velocity correlation function, generalized to different fluid elements.

Ion Emission from Rhenium and Lanthanum Hexaboride Coated Rhenium Filaments
View Description Hide DescriptionWhen heated, rhenium, like many other metals emits positive ions of the alkali metals. However the emission properties of these ions from rhenium are found to be very different from those observed for tungsten. In addition, the ion emission from rhenium can be greatly reduced relative to that of tungsten by short term vacuum annealing at sufficiently high temperatures. Lanthanum hexaboride coatedrhenium filaments also emit positive alkali ions in a manner very similar to that of bare rhenium. The implications, both practical and theoretical of these results are discussed.

Photoinjection into SiO_{2}: Use of Optical Interference to Determine Electron and Hole Contributions
View Description Hide DescriptionThere is some ambiguity in the interpretation of internal photoemission experiments because of the possible contribution of hole emission from the anode in addition to the cathode electron emission. Furthermore, correction of the photocurrent spectra for optical intensity variations is essential to determination of photoelectric yields. Optical interference is useful as a tool in ascertaining current contributions from the two electrodes since wavelengths of intensity maxima at one electrode correspond approximately to minima at the other. In this work an analysis technique is presented whereby the absorbed intensities are accurately calculated, the absolute quantum yields are determined assuming no hole photocurrent contribution, and the resulting spectra are examined for evidence of such currents. Internal photoemission measurements have been made on Au–SiO_{2}–Si structures, and the technique has been applied to obtain quantum yields and look for evidence of hole emission. It is shown that hole photocurrents (if present) are negligible compared to electron currents and in particular for photoemission from Si into SiO_{2} the quantum yield for holes must be at least two orders‐of‐magnitude smaller than the electron quantum yield. The first determination of absolute quantum yields for Si–SiO_{2} and Au–SiO_{2} emission is presented, and the barrier energies are found to be 4.30 and 4.15 eV respectively. The optical problem has been solved for a range of oxide thicknesses and it is shown that intensity variations are not negligible even for very thin (∼500 Å) films.

Electrohydrodynamic Ion Source
View Description Hide DescriptionAn electrohydrodynamic (EHD) technique is used to generate ions from liquid metals.Liquid metal is fed to the tip of a capillary needle emitter with a voltage difference applied between the emitter and an extractor electrode to produce an intense electric field at the liquid surface.Electrostatic forces overcome surface tension forces to produce ions by field emission. When using liquid cesium, time‐of‐flight mass analysis showed the ion current to be primarily Cs^{+} with a small percentage of Cs_{2} ^{+} and Cs_{3} ^{+}. Electron currents of over 1 mA have been produced by operating the emitter at 2 kV negative. Besides cesium, alkali ion beams have been generated using NaK/cesium alloy and sodium. Calculations show that liquid metals of low work functions appear more suitable for production of atomic ions while higher work functionsmetals may produce multiatomic ions and charged droplets in addition to atomic ions.

Critical Pressures of the Positive Corona Discharge between Concentric Cylinders in Air
View Description Hide DescriptionIf, in a positive corona discharge in air, the voltage is raised at constant gas pressure over a range of pressures beyond atmospheric, the sparkover voltage is found to exhibit a maximum and then decline, eventually to coincide with the corona‐starting voltage. The pressure at this point of intersection, beyond which sparkover alone obtains, without antecedent corona, is termed the critical pressure, and the associated gas density the critical density. The critical characteristics of dry air at room temperature are here determined in terms of electrode dimensions for a coaxial wire‐tube system. The critical relative density δ_{cr} (air density relative to atmospheric conditions) is shown to fit the empirical relation,where r is the wire radius, k an increasing function of tube radius, and c is a constant. The sparkover voltage V _{cr} corresponding to the critical density is given empirically by,where R is the tube radius and A and B are constants of the gas. The unattainability of positive corona above the critical density is shown not to be absolute as hitherto supposed. Substantial corona currents are possible behond δ_{cr} provided that a suitable path in voltage‐current‐pressure space is followed in circumventing the critical density.

High‐Speed Impact between a Liquid Drop and a Solid Surface
View Description Hide DescriptionThe dynamics of high‐speed impact between a compressible liquid drop and a solid surface are reviewed. Previous estimates for the maximum impact pressure have been based on one‐dimensional approximations. This paper presents a two‐dimensional approximation, adapted from a closely related analysis of the oblique impact between two solid plates. This is valid only for the ``initial'' phase of the impact during which the expanding shock front generated by the impact still remains attached to the target surface, and no lateral outflow takes place. The derivations assume a linear relationship between shock velocity and particle velocity change across the shock front. Numerical results are presented for water and sodium, and can be generalized as follows: The contact pressure remains substantially equal to the one‐dimensional pressure until the contact angle φ at the edge has reached about half of its critical value, at which the assumed model beaks down and lateral outflow must initiate. As this critical condition is further approached, the contact edge pressure increases progressively, and its critical value P_{c} is taken as the maximum impact pressure. The ratio P_{c} /ρ_{0} C _{0} V _{0} always exceeds about 2.75 exhibiting a minimum in the vicinity of V _{0}/C _{0}=0.2, where ρ_{0} and C _{0} are the density and acoustic velocity of the liquid, and V _{0} is the impact velocity. These pressures are considerably higher than have been heretofore supposed, but circumstantial experimental evidence supports the present results.

Relaxation Model for Heat Conduction in Metals
View Description Hide DescriptionA time‐dependent relaxation model for the heat flux in metals is derived from the quantum mechanical form of the Boltzmann transport equation. In the derivation, the manipulation of the nonlinear integral term of the Boltzmann equation is simplified by assuming that the phonons are in thermal equilibrium at all times and by using the Lorentz approximation to treat free electron‐phonon interactions. The relaxation model for the heat flux is found to yield a damped wave equation for the temperature and as a result, the speed of propagation for heat is shown to be finite instead of infinite as implied by the Fourier model for the heat flux. Approximate expressions for the thermal conductivity and the isothermal electrical conductivity are derived in the appendices in order to estimate the magnitude of the relaxation times and to obtain an expression for the Lorentz number. In general it is found that the thermal and electrical relaxation times are not equal although they are estimated to be at the same order‐of‐magnitude, 10^{−14} sec for the common monovalent metals. The Lorentz number is found to be a function of the ratio of the relaxation times and as a consequence the difference in the relaxation times may account, at least in part, for the derivation of the experimental Lorentz number from the usual theoretical value based upon equal relaxation times.

Tensile Properties of Polymer‐Filler Composites
View Description Hide DescriptionThe strain energy and stress concentrations in filled polyethylene are analyzed and the effect of fillers on the ultimate tensile strength of these composites is discussed. It is shown that the usual two‐phase model is inadequate to describe filler‐polyethylene composites when the filler has a high surface energy. Since a high surface energy substrate is known to generate a transcrystalline region, at the polymer matrix‐filler interface, which differs mechanically from the bulk polymer, a three‐phase model is proposed and analyzed. The agreement between theory and experiment lends credence to the three‐phase model.

Cataphoresis in the Helium‐Cadmium Laser Discharge Tube
View Description Hide DescriptionSide light measurements were made to determine the density distribution of cadmium in a discharge tube similar to that used in the helium‐cadmium laser. The tube had an electrode at either end and a single cadmium source near the center. The discharge causes the cadmium vapor to flow toward the cathode, the flow being greater at higher currents. At currents greater than 20 mA almost no cadmium is found a few cm from the cadmium source in the discharge region between it and the anode. In contrast to this, large amounts of cadmium are found in the discharge region between the source and the cathode. It is concluded that an approximately uniform density of metal vapor may be maintained in a helium‐cadmium laser by placing a single cadmium source near the anode and passing currents greater than 50 mA through the tube. A very simple theory gives a qualitative explanation of the gross features of the density distribution curves.