Volume 40, Issue 1, 01 January 1969

Analysis of Thermal Desorption Spectra
View Description Hide DescriptionThe behavior of detector signals corresponding to the thermal desorption of gas from a solid surface for integral‐order kinetics is investigated with respect to variations of the parameters describing (1) the kinetics of the desorption process itself, and (2) the interaction of the desorbed gas with the vacuum system. A computational approach is used because the equations describing the simple model used are, generally, analytically intractable. A computer‐driven display and a data‐logging system facilitate not only the analysis indicated above, but also curve fitting so that the kinetic parameters can be extracted from experimental desorption data containing distortions caused by gas scattering from structures in the vacuum system.

Magnetic Shielding by Superconducting Nb‐40Zr‐10Ti Hollow Cylinders
View Description Hide DescriptionThe magnetic shielding by the heat‐treated superconducting Nb‐40 at.% Zr‐10 at.% Ti hollow cylinder, 5 mm in inner diameter, 9 mm in outer diameter, and 25 mm in length, was investigated at 4.2°K in applied axial magnetic field. The dependence of the magnetic shielding capacity upon the field rate of increase was examined for the samples subjected to the solution treatment at 1100°C for 5 h in vacuum of 10^{−6} Torr and to the heat treatment at 700°C for 3, 10, and 100 h after solution treatment. The magnetic shielding capacity of the sample, heat treated at 700°C for 100 h, was over 17.5 kOe as the magnetic field was applied with its rate of increase of 0.45 kOe/min. A possible phenomenological model estimating the maximum or minimum magnetic shielding capacity was developed by assuming that the flux front in the Bean critical state moves isothermally or adiabatically toward the interior of the hollow cylinder. The calculated value agreed well with the measured value for each sample.

Effect of Hydrostatic and Shock‐Wave Compressions on the Magnetization of a 31.4 at.% Ni–Fe Alloy
View Description Hide DescriptionThe saturation magnetization of a 31.4 at.% Ni–Fe alloy has been measured as a function of hydrostatic pressure between 0 and 26 kbar, and as a function of shock‐wave compression between 2.5 and 112 kbar. From the hydrostaticpressure measurements, a zero‐pressure value of the pressure derivative of the saturation magnetization (∂lnσ_{ s }/∂P) = −3.15×10^{−2} kbar^{−1} is found. A marked departure from linearity of σ_{ s }(P) is seen at higher pressures. This is considered to be due to the slope of σ_{ s }(T) becoming steeper and steeper at the ambient temperature as the Curie temperature is depressed by pressure. The shock‐wave compression experiments yield a value of (∂ lnσ_{ s }/∂P) = −2.8×10^{−2} kbar^{−1} in the 15–40 kbar stress range.

An Approximate Theory of Skin‐Effect Acoustic Generation in Conductors
View Description Hide DescriptionAbeles has shown that a marked acoustoelectric coupling can occur due to the penetration of a high‐frequency electric field into a conductor. Calculations are presented here of the coupling due to the spatial phase shift between the Coulomb force on the lattice ions and the reaction force of the electrons, and due to the localized electronreaction on the surface. The latter is largest for diffuse electron reflection, and is the most important coupling mechanism at frequencies above 30 GHz. Efficiencies are fairly low, being in the 10^{−2} to 10^{−3} range under ideal conditions; however, they remain of this magnitude up to the lattice vibration frequency. The model used is approximate: a classical free‐electron model, idealized cases of fixed electron free paths with either uniaxial or isotropic electron motion being considered. For specular surface reflection of electrons, the exact model of Quinn is available for comparison, and it is shown that the results are very similar. Single and multilayered film structures are considered in addition to the semi‐infinite conductor.

Perturbation of Microwave Cavities by Lossy Dielectrics and Plasmas
View Description Hide DescriptionThe frequency shift and Q change of a microwavecavity caused by lossy dielectrics or plasma spheres or cylinders is calculated. The necessity for this analysis is prompted by the use of microwavecavities to measure the expansion of nearly spherical plasmas created by focused high‐power lasers. The conditions required for the perturbing volume to behave as if it were a perfect conductor are examined. It is found that much lower conductivities (or permittivities) are required when the perturbing volume is placed in a cavity region where electric energy is predominant.

Microwave Measurements on Laser‐Produced Blast Waves
View Description Hide DescriptionThe expansion of a laser‐produced blast wave is measured by creating a laser spark at the center of a microwavecavity. The radius measurement as a function of time is accomplished by measuring the cavities' resonant frequency shift. In addition, the blast‐wave surface temperature and electron density can be deduced by measuring the change in Q of the cavity. The results are in general agreement with ideal blast‐wave theory.

Poisoning of LaB_{6} Cathodes
View Description Hide DescriptionElectron‐emission density for LaB_{6}cathodes is compared with that for other common cathode types. Equations for describing the poisoning mechanism are presented. Poisoning is related to the gas pressure,cathode temperature, and heat of adsorption. Posioning was measured with a flowing gas system which maintained a constant pressure in the vicinity of the cathode for varying adsorption rates. Poisoning rates were determined by the cathode temperature and gas pressure. An equilibrium poisoned emission level was reached within a few minutes for each cathode temperature and gas pressure. Poisoning gases used were oxygen, carbon dioxide, air, hydrogen, nitrogen, and argon. Oxygen was the most active poisoning gas and argon the least active. No poisoning results until a critical pressure is reached; the emission then decays rapidly with increasing pressure. Resistance to poisoning increases with increasing cathode temperature. At a cathode temperature of 1570°C, the critical poisoning pressure for oxygen is about 5×10^{−5} Torr. At 1400°C, the critical poisoning pressures are as follows: 2×10^{−6} Torr for oxygen, 2×10^{−5} Torr for carbon dioxide, 5×10^{−5} Torr for air, and greater than 10^{−2} Torr for hydrogen, nitrogen, and argon. On a comparative basis, the critical poisoning pressure for LaB_{6}cathodes is several orders of magnitude higher than the poisoning pressure for oxide and impregnated cathodes.

The Magnetic Susceptibilities of Some Rare Earth Silicides and Germanides with the D8_{8} Structure
View Description Hide DescriptionThe magnetic susceptibilities for neodymium,gadolinium and dysprosium germanides and silicides were measured between about 60° to about 700°K. Plots of 1/χ vs T show a Curie‐Weiss behavior, but deviations from it are observed below room temperature. The deviations become smaller with decreasing temperature and in most instances tend to approach the asymptote just above the transition temperature. These compounds have the D8_{8}structure and a ferromagnetic spin ordering for rare earth atoms in the crystallographic 6(g) sites, and an antiferromagnetic ordering in the 4(d) sites is consistent with the observed behavior.

Electron Flow in Gas Diodes. I. Transition from Inertia‐Limited Flow to Mobility‐Limited Flow
View Description Hide DescriptionThe transition from inertia‐limited flow (vacuum) to mobility‐limited flow (high pressure) in gas‐filled diodes is studied theoretically by taking velocity moments of the Boltzmann equation for the electron‐velocity distribution function. It is shown that the momentum‐transfer equation can be integrated when ν_{ c }(C), the frequency of elastic collisions between electrons and gas atoms, is independent of the electron speed c, and the hydrostatic‐pressure term is neglected. The resulting current‐voltage (J‐V) curve, which is valid for all gas pressures, reduces to the proper vacuum law (J ∝ V ^{3/2}) at extremely low gas pressure and to the proper high‐pressure law (J ∝ V ^{2}) at high gas pressure, while it is a mixture of the two laws for intermediate gas pressures. The importance of the ratio ν_{ c }/ν_{ p }, where ν_{ c } is the average value of ν_{ c }(C) and ν_{ p } is the electron‐plasma frequency, is emphasized. It is shown that the current is inertia limited for ν_{ c }/ν_{ p }<1, and is mobility limited for ν_{ c }/ν_{ p }>1. It is shown further that mobility‐limited flow divides naturally into two cases, according to whether the electrons retain the energy imparted to them by the electric field or whether this energy is given up in elastic collisions with atoms. The former situation, called the low‐pressure case, prevails when (m/M)^{1/2}ν_{ c }/ν_{ p }<1, and the latter, called the high‐pressure case, prevails when (m/M)^{1/2}ν_{ c }/ν_{ p }>1, where m/M is the ratio of electron mass to atom mass.

Electron Flow in Gas Diodes. II. Mobility‐Limited Flow for Collision Frequency Proportional to Electron Speed
View Description Hide DescriptionThe voltage (V) dependence of the mobility‐limited electron current density (J) in a gas‐filled diode is calculated for low pressure [(m/M)^{1/2}ν_{ c }/ν_{ p }]<1 <ν_{ c }/ν_{ p } as well as for high pressure [1<(m/M)^{1/2}ν_{ c }/ν_{ p }], where m/M is the ratio of electron mass to atom mass, ν_{ c } is the average frequency of elastic collisions between electrons and gas atoms, and ν_{ p } is the electron‐plasma frequency. It is assumed that ν_{ c }(C) is proportional to the electron speed c, which corresponds to the case of an energy‐independent mean free path. It is shown that J should be proportional to V ^{3/2} p ^{−1} d ^{−3} for low pressure, and to V ^{3/2} p ^{−1/2} d ^{−6/2} for high pressure, where p is the gas pressure and d is the (planar) diode spacing. Corresponding expressions for cylindrical geometry are derived and compared with experiment.

Nonlinear Photovoltaic Effect in Silver Bromide
View Description Hide DescriptionThe photovoltage of silver bromide single crystals has been measured at room temperature as a function of light intensity and wavelength. A two‐part photoresponse was observed: a fast part with a risetime in the millisecond range, followed by a slower part with a response time of seconds. The fast response was nonlinear, the voltage varying approximately as the ⅔ power of the incident light intensity. A model based on a Dember‐type effect was formulated to explain the results. Electrons are assumed to diffuse from the surface into the bulk, owing to their own concentration gradient, while holes are trapped near the surface to form interstitialsilver ions which then move under the influence of the field of the electrons. The resulting differential equation was then solved with the aid of a computer. The slow response was ascribed to the effect of photodecomposition products on the initial photovoltage.

Line Integral Expressions of Interaction Energy of Dislocation Loops in Anisotropic Materials
View Description Hide DescriptionThe interaction energy of two dislocation loops in anisotropic materials is expressed by the Fourier transform of line integrals along the dislocation loops. The result can also be used for the expression of the self‐energy of one dislocation loop. The Fourier integrals are performed for the cases when the elastic constants differ by a small amount from the isotropic elastic constants.

Electronic Deposition of Sprayed Suspensions of Protein for Electron Microscopy
View Description Hide DescriptionThe deposition of microdroplets from a nebulized aerosol is enhanced by the application of an electric field. Electronic deposition facilitates the electron microscopic sampling of the aerosol by capturing those microdroplets that are electrically charged or whose dimensions are too small and therefore elude the more conventional methods of collection. The subsequent deposition of the microdroplets in the system described is a function of the strength of the electric field and the size of the individual microdroplets, and therefore negates the use of forced‐air impaction of the microdroplets onto the collecting surface. Electronic deposition as applied to the collection of nebulized suspensions of particulate protein renders the resulting microdroplets (0.25 to 2.0 μ diameter size range) and their contents accessible to sampling under conditions suitable for high‐resolution electron microscopy.

Single‐Crystalline Elastic Constants of ZrCo_{2} and HfCo_{2} in the Temperature Range 4.2°–300°K
View Description Hide DescriptionThe elastic constants of single crystals of the cubic Laves phases ZrCo_{2} and HfCo_{2} were determined by the pulse‐echo technique over the temperature range 4.2°–300°K. Debye temperatures for the two materials were computed from extrapolations of the elastic constants to 0°K. The ratio of these Debye temperatures is such as to indicate that the primary difference between the two materials is a difference in vibrational amplitudes resulting from the mass difference between zirconium and hafnium. Further substantiation of a close similarity of interatomic force interactions in the two materials is obtained by comparison of appropriate combinations of lattice parameters and force constants.

High‐Density Saturation Effects of the Dislocations in n‐Type Germanium
View Description Hide DescriptionThe dislocation effects on electrical resistivity and Hall coefficient at the liquid‐nitrogen temperature were examined in n‐type germanium specimens whose dislocation density, distribution, and nature were determined by means of optical and electron microscopy. In particular, the high dislocation density regions were considered, where the space‐charge cylinders and the low‐mobility rings around the dislocations overlap. Three stages were revealed, the first connected with the bulk conductivity, the second with the conductivity in the low‐mobility regions, the third with the prevalence of the minority hole conductivity. The interpretation of the experimental data led to the following determinations: the specific volumes of the space‐charge cylinders and the low‐mobility rings around the edge dislocations; the ratios between low mobility and bulk mobility of the electrons; the degree of the contamination specific of the deformations; the saturationhole concentrations and mobilities. Further, the availability of differentiated specimens with prevalence of either screw or edge dislocations made it possible to resolve the effects of the two types of dislocations.

Barrier Heights and Contact Properties of n‐Type ZnSe Crystals
View Description Hide DescriptionBarrier‐height measurements performed on ultrahigh‐vacuum‐cleaved and on chemically etchedZnSe crystals have shown that for most metals the barrier height increases linearly with its electronegativity. The highly reactive metalsMg, Ca, and Ba were found to be an exception, showing a decrease in barrier height with increasing electronegativity. Aluminum, when diffused into ZnSe from a filmdeposited onto a vacuum‐cleaved surface, was found to produce a low‐resistivity surface layer, followed by a highly compensated region in the crystal. This effect is suggested to arise from copious generation of Zn vacancies at the Al‐doped crystal surface, and their influx, ahead of the Al, into the crystal.

Theory of the Thermal Breakaway of a Dislocation from a Row of Equally Spaced Pinning Points
View Description Hide DescriptionThis paper gives a comprehensive description of the breakaway of a dislocation from a row of equally spaced pinning points, under the combined action of stress and temperature, within the model of Teutonico, Granato, and Lücke (TGL). The method is to obtain an algebraic solution, for a general pinning force, using, in turn, the continuous‐pinning approximation of TGL and the ``independent‐joint approximation''; the conditions of validity of these approximations are investigated in detail. For β«1, the pinning is effectively continuous; here, β=L_{c}U _{0}/Cr ^{2}, where L_{c} is the distance between pins, U _{0} is the maximum binding energy between a dislocation and a pin, C is the tension of the dislocation, and the pinning force has its maximum at a displacement of order r, the ``range.'' For β»1, the independent‐joint approximation holds (except near the mechanical breakaway stress). Then major breakaway (breakaway from the whole row) is activated at a single pin down to the stress σ_{ s } = (4CU _{0}/b ^{2} L_{c} ^{3})^{1/2}, where b is the Burgers vector. As the stress drops further, a saddle configuration still exists for breaking the first pin, but the dislocation must surmount higher saddle points to break subsequent pins. Major breakaway is activated over an increasing number of pins as the stress decreases, and for σ«σ_{ s } the pinning becomes ``quasicontinuous.'' A paradox encountered by TGL is thus resolved.

Transverse Magnetoresistance in Thin Epitaxial Films of PbS: Evidence for a Layered Distribution of Charge Carriers
View Description Hide DescriptionRoom‐temperature transverse magnetoresistance measurements on n‐type PbSfilms are reported. The results are different for magnetic fields perpendicular and parallel to the film faces. This anisotropy is sensitive to ambient and is understandable in terms of a model consisting of an n‐type bulk region and p‐type surface space‐charge layer.

Time‐Domain Analysis and Measurement Techniques for Distributed RC Structures. I. Analysis in the Reciprocal Time Domain
View Description Hide DescriptionThe impulse response in distributed‐parameter systems results in a spectrum of delta functions in the time domain for LC structures. It is shown that for RC structures the impulse response results in a spectrum of Poisson‐derived functions in the reciprocal time domain. This isomorphism is exploited for the analysis of RC devices to establish a calculus of Poisson‐derived functions and a transform technique which is essentially the Laplace transform. Applications of this technique to thin‐film devices and minority‐carrier devices are examined and lead to the determination of the transient response for one‐ports and two‐ports that are subject to a wide range of terminations. Illustrative examples of excitations include the impulse, the step, and the ramp, as well as signals generated by the distributed parameter devices themselves. Significant advantages accrue from a spectral description of RC devices in the reciprocal time domain: (1) conceptual, by establishing a physical interpretation based on diffusion phenomena; (2) analytical, by developing a systematic approach to transients paralleling the network function approach in the frequency domain; (3) computational, by expressing the response by a rapidly converging spectrum of functions.

Time‐Domain Analysis and Measurement Techniques for Distributed RC Structures. II. Impulse Measurement Techniques
View Description Hide DescriptionTransient analysis for uniform RC structures is considered in this paper. A method is presented for determining the parameters of such structures. The measurements are obtained using impulse excitations in open‐circuit and short‐circuit configurations. The theoretical results obtained predict fairly the experimental results.