Table of contents

Tungsten deposited with lanthanum hexaboride (LaB₆) is of interest, as LaB₆ is characterized by the properties of an excellent electron emitter. The authors report a field emission study of the LaB₆/tungsten system prepared by depositing LaB₆ in situ on to a tungsten tip. The observed maximum decrease in the average work-function was from 4.52 eV (clean tungsten) to 1.75 eV (LaB₆ deposited on tungsten). The field emission pattern of tungsten deposited with LaB₆ shows preferential adsorption around the (111) and (100) planes.

Stripe domains are supported in thin magnetic films which exhibit a sufficiently strong perpendicular magnetic anisotropy. Amorphous magnetic thin films of most rare-earth/transition-metal alloys prepared by sputtering or by evaporation commonly exhibit such a domain structure. Until now exceptions have been strong enough to give stripe domains. Some observations of such domains in amorphous GdCo films evaporated at oblique incidence are reported.

A method in which the refractive index and thickness of a transparent film are determined simultaneously from measurements of transmittance at normal incidence is presented. This has been applied successfully to films of Ta₂O₅.

A technique is described which allows a direct determination of the temperature dependence of the refractive index of transparent liquids and solids. It consists of an interferometric examination of a liquid or solid whose temperature changed in time linearly. For all the tested liquids except water, delta n/ delta t was found to be constant in the range 25-60 degrees C. The values delta n/ delta t determined for H₂O, C₆H₁₂ and CCl₄, and the value of delta ²n/ delta t² obtained for water, are in good agreement with literature.

Spreading resistance techniques have been used to measure the resistivity changes produced in n-type silicon irradiated by collimated beams of high-energy (>1 meV) protons and helium. The resistivity changes have been correlated with the ion dose and have provided cross-sectional profiles of beam intensity with 5 mu m resolution. Beam intensity has been shown to vary radially by more than an order of magnitude from the centre of the edges, and to be circumferentially non-uniform.

An exact analysis is presented of the gyrotropic waveguide magnetized along the direction of propagation. It is shown that in this case, the guide can support two sets of normal modes, both elliptically polarized, one right-handed the other left-handed. The basic wave equations of both sets of modes are derived. Similar conclusions are shown to apply for a DC field applied normal to the plane of the guide.

For pt.IV see ibid., vol.9, p.771 (1976). Changes in Taylor number, in wavenumber, and in drag are tabulated. The difficulty of assigning an appropriate viscosity is pointed out and the doubts that consequently arise as to whether stability is affected or not, are discussed. The appearance of striations around the inner cylinder at Taylor numbers less than critical are described. The possibility of overstability being observed as a fluctuation in torque is discussed; fluctuations in torque for both the elastic and inelastic fluids are described. The azimuthal instability (waves) are destabilized by the presence of macromolecules. The application of various equations of state to try to account for flow behaviour is described.

The authors describe experiments concerned with the limiting negative pressure (that is, tension) which water can sustain when stressed dynamically by propagating a pulse of tension through a vertical column of the liquid. The effect of boiling and deionization increased the ability of the water to withstand tension. A single sample of water was subjected to repeated tensile failure at regular time intervals and it was found that the breaking tension that could be sustained tended progressively to an upper limit of about 11.0 atm as this type of stressing proceeded. The breaking tension is a maximum at around the temperature corresponding to the maximum density of water.

A study has been made of spherical detonation waves in methane/oxygen/nitrogen mixtures. Using various masses M of solid explosive (Tetryl) as initiator, the detonatability limit in the (x,M) plane at atmospheric pressure has been established for mixtures of composition CH₄+20₂+_xN₂with 1.4 <or=x<or=5.53. The results cast doubt on a claim to have demonstrated spherical detonation in stoichiometric methane/air mixtures. Measured detonation velocities were all close to calculated Chapman Jouguet velocities.

The angle between the nematic director and an aligning surface in contact with a nematic phase is of importance in defining the mechanism of surface alignment of liquid crystals. A new and simple optical method of determining this tilt angle is described, and measurements are reported on a number of homologues of the series 4-n-alkoxy- and 4-n-alkyl-4'-cyano-biphenyl liquid crystals aligned by obliquely evaporated films. The tilt angle is found to depend in a systematic way on the liquid crystal material and also on the composition of the aligning layer.

A model has been developed which attempts to define in a rigorous manner the thermal evaporation and condensation of a binary semiconducting compound. Specifically it tries to characterize the deposition process in terms of the free energy of formation of the compound upon the depositing substrate. The model demonstrates how this free energy is then defined by both the temperature of the source from which the material is evaporating and the substrate temperature. The model is then tested by studying the growth behaviour of zinc selenide being deposited on to oxygen stabilized germanium surfaces. It is believed that the results demonstrate the influence that changes in the free energy of formation have upon the crystallographic structure and order of the deposited material.

A range of experimental evidence is presented showing that absorption and resorption currents in 125 mu m thick samples of the co-polymer(hexafluoropropylene-tetrafluoroethylene) are dominated by lateral charge transfer between the edge of the evaporated aluminium electrode and the surrounding uncoated polymer surface, rather than by bulk conductivity processes. These currents were found to increase proportionally with the electrode perimeter, rather than with the electrode area as would be expected if bulk conductivity processes were dominant. The surface conductivity mechanism accounts for at least 90% of the charge under the current-time plots.

Charging and discharging current transients in polypropylene films were measured over a wide range of temperatures and electric fields. Some limited studies of the behaviour of these current transients were also made with different electrode materials and sample thicknesses. The experimental evidence suggests that the absorption current in the temperature range 83-270K may be caused mainly by dipolar processes associated with structural motions. At temperatures above approximately 270K the absorption current may be attributed to a carrier hopping process.

Compensation effects present in polyacrylonitrile consisting of solid solutions of hydrated perchlorate salts in a PAN matrix are discussed. A conductivity mechanism is proposed and a relationship between the glass transition temperatures and the activation energies of the conductivity Arrhenius plots for the rubber-like high-temperature region is given.

A mass-action formulation is used to calculate the shape of the GaAs/Zn solidus at 1000 degrees C. The equilibrium constants are determined using experimental results for the solubility of zinc in GaAs. Recently published data on the electrical properties of annealed GaAs are also employed. It is assumed that the principal defects apart from substitutional zinc are charged and uncharged gallium and arsenic monovacancies. The concentrations of these defects are calculated for all points along the solidus line and hence the form of the solidus is derived. The results are compared with a previous theoretical determination for pure GaAs.

The current-electric field and transient current characteristics have been measured at high fields for a number of chalcogenide switching glasses, over a range of temperatures. Pulses between 2.5 and 50 ns wide were used so that it could be assumed that the Joule heating was adiabatic in the active layer of the devices tested. A high offstate current region associated with a rise in the current transient is observed at fields above a temperature-dependent critical field. This behaviour has been interpreted as the result of impact ionisation of Zener breakdown and, in the absence of a radial temperature gradient, can be considered as evidence supporting an electronic switching mechanism.

The electrical resistance isotherms have been measured during the interaction of hydrogen at T<or=295K with films of erbium evaporated in UHV (<or=10^-9 Torr). It is shown that the typical points where the isotherm changes its slope correspond to the phase boundaries of the Er-H₂ system, and that this agrees with present-day ideas on the role of interstitial hydrogen in the various phases. The use of evaporated films as samples has allowed the boundaries to be found down to 295K, and the alpha / alpha + beta ) boundary down to 130K. Previous experiments which used bulk samples substantially underestimated the room-temperature solubility limit of hydrogen in the metal.

The expansion of small, initially hemicylindrical domains on the surface of square-loop alloy tapes has been simulated numerically under conditions of (i) constant drive field H_A; and (ii) constant flux-reversal rate d phi /dt. The calculations treat each element of domain wall as being individually in dynamic equilibrium under the pressure of the applied and local eddy-current magnetic fields and the domain wall surface tension. Variation of the specific wall energy per unit area gamma with wall orientation is ignored, and the 'coercive field' H_c is also ignored. By expressing the equations of motion in reduced units, it has been possible to condense all constant H_A growth sequences onto a single sequence and similarly all constant d phi /dt sequences onto another.

A very stable direct-current source is described which employs a SQUID operating in the near-zero flux state. This is brought about by coupling to the SQUID via an intermediate superconducting loop, which carries a persistent supercurrent. Current stabilities as high as 2*10^-9h^-1 have been achieved and a new upper limit of 10^-26 Omega m for the resistivity of superconducting niobium has been set.

The effect of cobalt substitutions in manganese zinc ferro ferrites for inductor applications is discussed. Permeability-temperature curves are presented over the range -50 degrees C to 200 degrees C for compositions where the cobalt level is varied between 0 and 0.010 per formula unit, the Mn/Zn ratio and Fe²⁺ level being held constant at 3.5 and 0.105 per formula unit respectively. The anisotropy compensation model fails to give a satisfactory explanation of the above curves since the materials exhibit a uniaxial anisotropy, the degree of which is dependent on the thermal history of the samples. The effect causes the permeability to be unstable in time and the resulting high disaccommodation makes application of such materials unlikely.

An investigation of the major scattering processes involved when an incident electron suffers an appreciable change of momentum in a metal has been undertaken. The results of this study have been incorporated within a Monte Carlo framework, allowing calculations of energy and angular distributions of electrons, both transmitted and backscattered from films, to be made. The advantage of this treatment of the problem over earlier Monte Carlo work is that fundamental scattering processes are used rather than invoking mean energy-loss approximations, thus allowing an investigation to be made of the shortcomings of the theoretical treatments of the individual collision types.

The recombination velocity parameter S in the boundary conditions for diffusion of photoexcited electrons is expressed in terms of the quantum-mechanical reflectance of the interface and transport parameters for the interfacial region. The recombination velocity at the photoemitter-substrate interface in a transmission photocathode is expressed in terms of material parameters. Recent experimental data are analysed and factors favouring low recombination velocities are discussed. The ratio of the empirical surface escape probability P to the theoretical surface transmittance T may vary above or below unity. Physically, P/T>1 arises because an electron reflected back from the surface into the bulk has a significant probability of re-entering the bandbending region.

Experimental and theoretical studies of the variation in photoresponse of the Si:Cs-O negative electron affinity (NEA) photocathode with thickness of the Cs-O overlayer and acceptor concentration in the Si bulk, are reported. Resonance transmission peaks in the variation of photoyield with increasing overlayer thickness are predicted and observed. From a comparison of theoretical and experimental curves, the electron attenuation length in the overlayer is estimated to be about 50 AA. The theoretical analysis indicates that the optimum overlayer is metallic with a carrier concentration of about 2*10²¹ cm^-3, varying slightly with bulk acceptor concentration.