Index of content:
Volume 32, Issue 4, 01 April 1961
32(1961); http://dx.doi.org/10.1063/1.1736048View Description Hide Description
Self‐diffusion coefficients for both In114 and Sn113 as tracers have been measured in the liquid In‐Sn alloy at several concentrations. The temperature range investigated is the range 200°–600°C. The dependence of diffusiveness on temperature at constant composition can be represented by the customary Arrhenius equation for all the compositions investigated. The variation of diffusiveness with composition at constant temperature is also examined, and it gives evidence of a large difference in the behavior of the two tracers.
Effects of Doping Additions on the Thermoelectric Properties of the Intrinsic Semiconductor Bi2Te2.1Se0.932(1961); http://dx.doi.org/10.1063/1.1736049View Description Hide Description
An alloy of 70 mol % Bi2Te3−30 mol % Bi2Se3, or Bi2Te2.1Se0.9, is the intrinsic semiconductor of the pseudobinary system Bi2Te3–Bi2Se3. This V–VI alloy was doped with lead (Group IV) and iodine (Group VII) separately and together. The effects of the dopants are analogous to those produced by Group III or Group V impurities in Group IV elemental semiconductors, the lower Group impurity producing a p‐type material and the higher Group impurity an n‐type material. Plots of the Seebeck coefficient, electrical conductivity, and thermal conductivity are given as a function of impurity concentration and show that the separate effects of the impurities are countered when the impurities are in the lattice together in the same amount.
Doping was also done with silver and iodine, separately and together. The results indicate that the silver is in the lattice interstitially (lead and iodine substitutionally) and that the type material produced is dependent also on how the impurity atom enters the lattice.
32(1961); http://dx.doi.org/10.1063/1.1736050View Description Hide Description
The theory of the direct Fourier analysis with finite sampled data is developed. Formulas for planning a measurement for a specified resolution and accuracy are obtained. A quantitative comparison with autocorrelation analysis is given. It is shown that direct Fourier analysis can be applied equally well to attain comparable resolution and accuracy with the same amount of data. The computer requirements are also comparable in both cases.
32(1961); http://dx.doi.org/10.1063/1.1736051View Description Hide Description
It is shown that through the use of the infrared absorption technique to measure the added carrier density in a semiconductor bulk adjacent to a metal contact coupled with simultaneous measurement of the floating potential of the contact, a reliable determination of the semiconductor surface barrier height under the metal contact can be obtained.
32(1961); http://dx.doi.org/10.1063/1.1736052View Description Hide Description
Absorption of radio signals by cyclotron resonance is shown to be possible within a very narrow layer of the atmosphere (20 km thick just below 100 km). For a plane‐polarized signal only 25% of the power can be absorbed. Better results are obtained (50%) with a circularly polarized radio wave. The absorbable power is limited by the effects obtained so that an electron density of 3000 cm−3 and an electron temperature of 1600°K cannot be exceeded. The reason for this is the weakness of the earth's magnetic field.
32(1961); http://dx.doi.org/10.1063/1.1736053View Description Hide Description
The boundary conditions for a radio signal incident normally on a layer of plasma are discussed. It is shown that 10 different modes must in general propagate in the layer, instead of the usually assumed six. The 10 modes are obtained when the Langevin form of the force is used in Boltzmann's equation.
32(1961); http://dx.doi.org/10.1063/1.1736054View Description Hide Description
A method is developed whereby electrical conductivity and dielectric constant of semiconducting and dielectric materials without contacting electrodes can be measured. The principle of this method is that a specimen suspended in a rotating field with a fine fiber is rotated by the torque proportional to the electrical conductivity or the imaginary part of its complex dielectric constant, and the torque exerted on it by a linearly polarized field is proportional to the real part of its dielectric constant.
An analysis of the method and some preliminary measurements of conductivity, photoconductivity, the dielectric constant of cadmium sulphate crystals and the dielectric loss of the lamella containing CdS powder are presented. The latter shows the photodielectric effect.
32(1961); http://dx.doi.org/10.1063/1.1736055View Description Hide Description
The nature of double cross slip of screw dislocations as induced by a locked parallel screw dislocation, that of edge dislocations as induced by a locked edge dislocation through climb, and that of the cross climb of edge dislocations as induced by another edge dislocation through slip are studied in the light of the interaction of the mutual stress fields of the dislocations. Some quantitative calculations of the multiplication of a single loop and of the growth of slip bands in LiF are given; it is found that agreement with the experimental results of Johnston and Gilman [W. G. Johnston and J. J. Gilman, J. Appl. Phys. 30, 129 (1959); 31, 687 (1960)] is satisfactory. A mechanism for linear hardening at low stresses is suggested.
32(1961); http://dx.doi.org/10.1063/1.1736056View Description Hide Description
Measurements of conductance, lifetime, change in contact potential with light, and contact potential have been carried out on bombardment‐cleaned siliconsurfaces and during the adsorption of molecular oxygen and atomic hydrogen. In the case of oxygen adsorption, the work function increased linearly with coverage. A change of 0.35 ev was obtained in going from θ=0 to θ=1. Very small changes in the transport properties were observed. Hydrogen atoms produced an initial decrease in work function of 0.1 ev for coverages below θ=0.35. From θ=0.35 to θ=1.0 the work function was increased by 0.3 ev. The changes in the transport properties were substantial and indicated a downward movement of the energy bands at the surface by about 0.08 ev. In the clean condition, the valence band edge was 0.12–0.14 ev below the Fermi level at the surface compared to 0.36 ev in the interior. The effect of hydrogen adsorption is discussed in terms of the adsorption data previously obtained on this system.
32(1961); http://dx.doi.org/10.1063/1.1736057View Description Hide Description
The response of a thermocouple circuit functioning as a Peltier cooler to time‐varying currents was determined by assuming that the current density could be represented by the sum of a dc and a time‐varying component. The time‐varying component took the form of either an impulse applied at time t 0>0, a square pulse lasting from t 0 to t 1, a step increase in the current at time t 0, or a sinusoidal ripple superposed on the dc current. The increased current results in an initial thermal cold spike at the cold junction but the time‐average temperature difference between the junctions is reduced unless the dc current is well below the optimum value. The possibility of using such thermal spikes in a very long wavelength infrared communications system or in synchronous detection is discussed. In the case of a sinusoidal ripple the temperature difference between the junctions may either follow the fluctuations in current or may not, depending on the time constant of the couple and the frequency of the ac signal. In the latter case the only effect is a reduction in the temperature difference between junctions by the additional Joule heating due to the ac component.
32(1961); http://dx.doi.org/10.1063/1.1736058View Description Hide Description
It is shown that d(θΔH)/dT=0 is a necessary condition for the attainment of Carnot cycle efficiency, (T 1–T 2)/T 1, by a thermally regenerated fuel cell system in which ideal gases are reacted in a fuel cell to convert chemical to electrical energy, and are then regenerated for recycling by thermal dissociation of the product of the reaction, also assumed to be an ideal gas. In d(θΔH)/dT, T is the temperature, ΔH is the change in enthalpy for the reaction in the fuel cell, and θ is the degree of reaction, or the fraction of the road from zero to 100% product which is covered by the reaction when chemical equilibrium is reached. Formulas are given for calculating the theoretical efficiency. Curves show calculated efficiencies for hypothetical thermodynamic data chosen to illustrate the effect of thermodynamic properties on efficiency. Temperature‐entropy diagrams are used to explain why the efficiency reaches a maximum and then decreases when T 2 is held constant and T 1 is increased. The efficiency is pressure independent when the reaction does not involve change in the number of molecules, pressure dependent when it does.
32(1961); http://dx.doi.org/10.1063/1.1736059View Description Hide Description
The thermoelectric parameters α, 1/ρ, and 1/(K−Ke ) of oriented polycrystals increase as the freezing rate decreases and as the temperature gradient in the liquid at the interface increases. The effects are primarily attributed to the variation in the degree of microsegregation produced in the crystals as a function of the freezing conditions.
32(1961); http://dx.doi.org/10.1063/1.1736060View Description Hide Description
A helium gun was used to propel metal projectiles against targets of the same materials. Pressures produced by impact were varied by varying projectile velocities. The method produced pressures up to somewhat over 100 kbar, thus overlapping the lower part of the range of pressures which can be produced by detonation of high explosives. Of particular interest was the range of pressures intermediate between the highest which can be produced statically and the lowest which can be produced by explosives.
A condenser micrometer was used to detect motion of the free surface of the target. This method permitted examination of the motion in greater detail than could reasonably be accomplished by other methods which have been used. Elastic and plastic waves were observed in iron and in bismuth. Assuming the Rankine‐Hugoniot shock conditions, pressures and compressions were computed. Pressure‐compression data for iron were obtained for intermediate pressures not previously investigated. Phase transitions in bismuth were observed for pressures up to about 40 kbar.
32(1961); http://dx.doi.org/10.1063/1.1736061View Description Hide Description
The theory of magnetic viscosity due to solute atom pairs has been verified on several bcc alloys. The results on Fe‐Si alloys with Si content up to 6% wt show that solute atom pairs give magnetic viscosity. If the strong effect of the temperature is taken into account, the induced anisotropy energy deduced from the viscosity field can be considered in agreement with the value measured directly from the magnetization curves. The results on a series of Fe‐Al alloys up to 25 at. % Al, where order occurs, further confirm the theory. The viscosity field is, with reasonable agreement, proportional to the number of solute atom pairs, as deduced from studies on ordering of the alloy and from the internal friction resulting from corresponding stress‐induced ordering.
32(1961); http://dx.doi.org/10.1063/1.1736062View Description Hide Description
A theory of the current flow across grain boundaries in n‐type germanium is given. In the temperature range where carrier generation in the space charge region can be neglected and for donor concentrations in the bulk larger than 1014/cm3, the current is carried essentially by electrons crossing the barrier, the zero bias conductance is independent of the donor concentration and is given by G 0=2.2·108 Te −φ0/kT . The apparent activation energy φ0 is directly related to the barrier height. The current for applied voltages which are large compared to kT/q fails to saturate. The deviation for saturation is related to the density of states in the boundary band. At sufficiently low temperatures the carrier generation in the space‐charge region is the rate‐determining process for the current flow across the boundary.
32(1961); http://dx.doi.org/10.1063/1.1736063View Description Hide Description
The current flow across grain boundaries in n‐type germanium has been measured in the temperature range of 350−200°K. Precisely oriented bicrystals have been grown for this study with 4, 6, and 25° tilt boundaries and 6° twist boundaries. The experimental data were found to be in good agreement with theoretical calculations given in the accompanying article [R. K. Mueller, J. Appl. Phys. 32, 635 (1961)]. The current across the boundary is mainly carried by electrons crossing the barrier, and it increases with decreasing boundary angle. The activation energy was found to be 0.71±0.01 ev for all boundaries. A lower limit of NB ≥1013 (cm−2 ev−1) was found for the density of boundary states of 6 and 25° boundaries. No significant difference between tilt and twist boundaries has been observed.
32(1961); http://dx.doi.org/10.1063/1.1736064View Description Hide Description
The operation of a new class of devices employing the principle of tunnel emission is discussed. It is shown that a controlled electron source may be obtained with the use of a metal‐insulator‐metal diode structure where the second metal layer is very thin. A triode geometry may be secured by the addition of an additional insulator and a metal collector layer. Limitations on the operating frequency, current density, and current transfer ratio of such devices are discussed. Experimental results on diode and triode are discussed. Experimental results on diode and triode structures which employ several materials are presented. Successful triodes and vacuum emitters have been realized with the use of Al2O3insulating films. Experiments using Ta2O5 are described, and the results are discussed.
32(1961); http://dx.doi.org/10.1063/1.1736065View Description Hide Description
The motion of irradiation‐produced defects in germanium has been observed under the influence of the electric field of a reverse‐biased p‐n junction. The defects were created with 1‐Mev electrons or Co60gamma rays, and the spatial distribution of the defects was obtained from the bias dependence of the junction capacity. Motion in the electric field was observable between 40–70°C and corresponds to negatively charged centers with a mobility of 2.6×10−14 cm2/v sec at 65°C. The activation energy of the mobility is about 1 ev. The centers are thought to be those which have previously been observed to anneal thermally. No observable effect was found for those defects which anneal at lower temperatures. Tentative interpretations are given.
32(1961); http://dx.doi.org/10.1063/1.1736066View Description Hide Description
Isothermal and step‐annealing data were obtained and show that the frequency factor is ∼1014/sec for processes having activation energies beyond the peak of the distribution but some orders of magnitude lower and possibly dispersed for the portions of the distribution at activation energies less than the peak.
32(1961); http://dx.doi.org/10.1063/1.1736067View Description Hide Description
The conductivity of the hot gases from propane‐oxygen flames containing potassium has been measured in a coaxial graphite cell. In the temperature range 1800–2400°K with potassium concentrations of 0.01–1.0% by weight, the measured conductivity agrees with that calculated from the Saha equation and a ``free‐path'' kinetic equation. A constant value of 1×10−15 cm2 was used as the cross section for collision between electrons and the combustion products in this calculation. At higher potassium concentrations, up to 7½% by weight, the electron‐potassium atom cross section becomes important but a value of about 10−15 cm2 gives fair agreement with the data. At low temperatures and high cell voltages, current saturation resulting from the limited electron emissivity of the graphite was observed.