Table of contents

Measurements of the transverse mode beat frequency of two different length TEA CO₂ lasers employing plane-plane resonators are presented for varying resonator diameters. The results are in agreement with theoretical predictions.

Using currently accepted ideas the authors investigate whether ergodic regions of finite volume can interpret cross-field electron thermal conduction in Tokamaks. A novel application of well-known results from the theory of heterogeneous mixtures suggests that field line ergodicity cannot plausibly explain the observed thermal conduction.

The anomalous temperature dependence of the yield stress and of the associated activation volume observed below about a quarter of the Debye temperature in metallic crystals is shown to be explicable if one takes into account the temperature dependence of the heat content of the lattice.

A recently proposed model to explain the supralinearity, sensitisation and PTTL in LiF TLD-100 is discussed in detail. It is shown that the predictions of the model are not borne out by the experimental results, and hence the model in its present form is inadequate.

A formula is obtained for the temperature dependence of onset evaporation field F, namely 1/F=b+CT¹2/, where b and C are constant for a chosen onset flux. The formula is based on a parabolic surface-atom bonding well, and should be applicable in a limited temperature range to materials that field evaporate via a Gomer-type escape mechanism. A good fit with experiment is found for tungsten and molybdenum in the temperature range 60K to 150K. Deviations from linearity below 60K may be a clear indication of heavy-ion tunnelling.

Field evaporation is assumed to be a principal emission mechanism for a liquid metal ion source, but emitter apex fields calculated on the basis of the conventional shape hypothesis (rounded Taylor cone) do not seem high enough to make field evaporation possible. A numerical estimate is made of the degree of field enhancement provided by the presence of a microprotrusion. Enhancement factors of about 10 or more seem possible, and these values would be sufficient to render field evaporation a plausible emission mechanism. Alternatively, the Taylor cone apex may be drawn out into a cusp.

Adjustable niobium Josephson point contacts, mounted in fundamental-mode waveguide cavities, have been used as mixers in an experimental heterodyne receiver with a 450 GHz laser as the local oscillator. It was possible to estimate separately the various factors, such as intrinsic mixer conversion efficiency, mixer noise, and input coupling efficiency that contribute to the overall receiver sensitivity. Mixer performance agreed reasonably well with qualitative predictions of resistively-shunted-junction theoretical models, and there was broad quantitative agreement for some aspects of performance. Spurious, and potentially very misleading non-heterodyne responses to thermal signals were observed under some regimes of applied DC bias and local oscillator power. The lowest overall receiver noise temperature measured was 2100K (double side band), although reduction to 1000K should be possible with quite simple improvements. The intrinsic mixer noise temperatures were usually in the range 500-1000K (single side band), and local oscillator power required by the receiver was usually less than one microwatt. This performance makes these mixers competitive as the basis of very sensitive near-millimetre-wave receivers.

The effects of the additive gases nitrogen and hydrogen on the gain, electron density, discharge voltage and positive-ion spectra of the plasmas of room-temperature sealed-off CO-He-Xe gas laser mixtures have been examined. Hydrogen affects the electron kinetics little, but its addition is always detrimental because the vibrationally excited CO is relaxed by V-T collisions. The effect of nitrogen is more complex; there is a beneficial CO-N₂ vibrational pumping channel and a detrimental effect due to increased discharge voltage and hence gas heating. It is not a necessary additive for high-power laser operation.

A light-scattering technique has been developed to discriminate between fibrous and nonfibrous airborne particulate matter. No sampling is required and the method is intrinsically independent of fibre orientation in the plane orthogonal to the incident light beam. Calculations indicate that the method may also work satisfactorily even for fibres at almost end-on incidence. A modification of the method is described which simultaneously monitors particle velocity. This is essential if the fibre concentration is to be measured.

The eikonal and other approximation methods used to describe light scattering from non-absorbing transparent long fibres are compared with the exact theory. Fibre diameters in the range 0.20-4.00 mu m are considered. Graphical comparisons between the methods are presented for values of the refractive index in the range 1.05-1.15. The implications of these results to fibre sizing are briefly discussed.

The ultrasonic velocities in a polycrystalline aggregate of cubic crystals with orthorhombic symmetry are derived. Use is made of the formalism of Roe (1965, 1966) for treating the texture of polycrystalline aggregates. It is shown how information about the crystallite orientation distribution function can be derived from ultrasonic velocity measurements. This enables the construction of ultrasonic pole figures, which may be compared with those obtained from neutron diffraction. Application is made to the effect of texture on ultrasonic propagation in austenitic welds.

A Monte Carlo method for studying the steady-state behaviour of charged species under the influence of a non-uniform electric field is described. This method, using a null-collision technique based on determination of the increase in kinetic energy between two collisions, avoids calculation of the time variations of the phase space coordinates of the charged species. The electron behaviour in the cathode region of a glow discharge has been analysed using this method; spatial variations of the energy and angular electron distribution functions, and of the macroscopic electron parameters are discussed.

The authors studied the extent to which Maxwellian and bi-Maxwellian series expansions can describe plasma flows characterised by non-Maxwellian velocity distributions. The problem they considered was the steady-state flow of a weakly ionised plasma subjected to homogeneous electric and magnetic fields, and both polarisation and hard-sphere collision models were used. Monte Carlo simulations provided a basis for determining the adequacy of a given series expansion. They found that, in general, the bi-Maxwellian-based 16-moment expansion for the velocity distribution function is better suited to describing anisotropic plasmas than the Maxwellian-based 20-moment expansion. They also found that for large electric fields the distribution function displays toroidal and bean-shaped characteristics, depending on the collision-to-cyclotron frequency ratio.

The distribution function at low energy, u<or=15 eV, has been calculated for an electron-beam generated argon plasma. The Rockwood formalism (1973) has been used and discretised with non-equal energy steps. The calculations included all the interactions between electrons and ground or excited states of atomic and molecular argon, and used the results obtained in a previous paper (see J. Bretagne et al., 1981) for the high-energy part of the distribution. The optical nature of the various transitions has been taken into account. The calculated distribution function is compared with a Maxwellian distribution for different time-dependent energy ranges. The significant departures from this distribution are discussed.

Measurements of the plasma efflux from a pulsed plasma jet igniter show that, contrary to previous expectations, plasma leaves the jet cavity in a substantially supersonic condition. Associated measurements of the electric field of the arc within the cavity and of the time taken for plasma to be expelled from the cavity show, together with the efflux measurements, good agreement with established arc theory and with a theory of plasma expansion based on a model in which the jet cavity is considered to act as the high-pressure section of a conventional shock tube. The implications of this model to the efficiency of a plasma igniter are discussed.

Mixed solution-grown two-phase NaCl-KCl crystals are found to be structurally different from the melt-grown single-phase (NaK)Cl crystals. In the mixed crystal, quench hardening increases steadily with temperature which contrasts with the appearance of the plateau at 250-450 degrees C in pure NaCl and pure KCl crystals. These studies suggest that more than one mechanism is predominant in the mixed crystal. It is found that the interactions between the interphase diffused ions acting as impurities in the other phase and the dislocations, as well as the misfit strains introduced by these ions, contribute mainly to the hardening mechanism. Etching studies carried out also supported this finding.

The lattice parameters of the compound cadmium stanni-diphosphide (CdSnP₂) have been determined at different temperatures in the range 28 to 536 degrees C. From these data, the thermal expansion and the tetragonal distortion have been evaluated at various temperatures. The linear thermal expansion coefficients along the a and c axes are estimated to be alpha _a=6.55*10^-6 K^-1 and alpha _c=2.75*10^-6 K^-1 respectively and the axial ratio c/a, changes with a coefficient of -3.63*10^-6 K^-1. This study indicates the anisotropy between the axial expansion coefficients ( alpha _a> alpha _c) and an increase in the tetragonal distortion, 2-(c/a), with temperature. These results have been interpreted in terms of the thermal expansion of the Cd-P and Sn-P bonds of the CdSnP₂ lattice.

Voltage-current characteristics of a glass-metal particulate composite of the V₂O₅-P₂O₅-Bi₂O₃ system, especially the negative resistance region prior to the on-state, have been computed using a particle stretching model. The theoretical curves are found to be in reasonable agreement with experimental data. Specimens having effective thicknesses of the order of 50 mu m show threshold switching. This has been explained on the basis of the magnitude of the relaxation time for the temperature profile to decay within the sample after the removal of the electric field.

Considered the transport properties of p-type polysilicon using a three-dimensional spherical model and compared the results with experimental data and a one-dimensional model commonly used. It was observed that the main differences between the two models exist after the crystal grains are totally depleted. This happens to be a region where reliable data (especially for carrier density) are scarce. From the theoretical analyses, the three-dimensional model seemed to be as good as the one-dimensional model in the high doping regime and for the resistivity data. Some significant differences existed between theory and experiment for the activation energies based on either of the two models and it is suspected that gross inhomogeneity could be the cause.

When insulators are repeatedly touched on the same spot by a metal, it is found that the charge transferred to them increases on each contact. This contact-charge accumulation has been noted by many authors and is surprising and unexplained. The present authors report an experimental investigation of the charge accumulation on hard crystalline insulators. They show that the charge accumulation can occur as the result of a small electrical conductivity in the insulator, and propose an explanation for the charge accumulation in these materials.

The influence of the electric field dependence of the differential conductivity on superconducting state stability in multifilamentary superconducting composites is studied theoretically. The obtained stability criterion allows one to determine the upper limit of the values of transport current and background electric field in the superconductor. To illustrate the theory, the superconducting state stability is studied in a wire in a range of sufficiently high magnetic fields.

Using standing-wave techniques, the dielectric constant ( epsilon ') and the dielectric loss ( epsilon ") of dilute solutions of N,N-dimethylformamide (DMF) in benzene, dioxane and carbon tetrachloride have been measured at 9.987 GHz at 25, 35, 45 and 55 degrees C. Following the single-frequency concentration variational method, the dielectric relaxation time ( tau ) and the dipole moment ( mu ) at various temperatures (25, 35, 45 and 55 degrees C) have been evaluated. From these studies the monomer structure of DMF in various solutions (benzene, dioxane and carbon tetrachloride) has been inferred and for a DMF-in-benzene solution a solute-solvent type of molecular association has been found. The energy parameters ( Delta F_epsilon , Delta H_epsilon and Delta S_epsilon ) for the dielectric relaxation of DMF in benzene, dioxane and carbon tetrachloride solutions have been calculated and compared with the corresponding viscosity parameters. A comparison between these two sets of parameters shows that the dielectric relaxation, like the viscous flow, can be considered as a rate process.

The dielectric behaviour of isocyanate-terminated polymers is reported in this paper as a function of frequency (100 Hz-100 kHz), temperature (30-90 degrees C) and the concentration of the curing agent, 4,4'-methylene bis(ortho-chloroaniline) (MOCA) (1.2, 3.5 and 4.7%) which was used in preparing the polymer from the toluene di-isocyanate and castor oil. The dielectric constant and the loss were found to decrease as the MOCA concentration was increased. Distinct loss peaks were obtained. The influence of the curing agent on the dielectric properties of the polymer is discussed in detail. The most probable relaxation times were calculated from the complex arc plots of epsilon 'and epsilon ". From these arc plots it is established that the distribution of relaxation times becomes more and more symmetric with the increasing temperature. Activation energies were calculated. Maxwell-Wagner-Sillars interfacial polarisation is proposed as the probable mechanism.

Single crystal GaP epilayers can be grown successfully by metallorganic vapour phase epitaxy on Si substrates by thermal decomposition of a gas mixture of triethylgallium (TEG) and phosphine (PH₃). The optimum growth temperature and the optimum mole ratio of P and Ga were 660 degrees C and 13 to 1, respectively. The as-grown GaP layer was n-type. The concentration and mobility of GaP layers at room temperature were in the range 3*10¹⁶-8.5*10¹⁶ cm^-3 and 50-100 cm² V^-1 s^-1, respectively. I-V and C-V characteristics of this heterojunction device were measured. The leakage current was a few tenths of a mu A and the breakdown voltage was 12 V. A linear relationship of C^-2.08 and voltage can be obtained. There is a wide wavelength range of photo-response (from 0.5 mu m to 1.2 mu m) for this diode. The dependence of open-circuit voltage and short-circuit current on light intensity were also measured.

Addison et al. (1962) and Vedder and Wilkins (1969) have proposed two alternative mechanisms to explain the oxidation which occurs in layered silicates. A Mossbauer study of tri-octahedral micas (iron-rich biotites) after heat treatment from 400 degrees C-1200 degrees C under different conditions has been performed. An attempt was made to explain the results using the mechanism of Vedder and Wilkins. It was found that a large amount of oxygen in contact with the mica grains was necessary for the mica to decompose completely.

An effective energy of activation for arc erosion has been deduced from mass loss rates observed on oxidised copper cathode surfaces exposed to arcs of long duration rotating in atmospheric pressure air. An analysis of rate processes expected to be associated with the observed low value for the effective activation energy excludes the involvement of ion migration or the filamentation of pure copper across the oxide layer in the cathode erosion process in an oxidising medium. Independent confirmation is obtained from a comparison of mass loss rates on oxidised cathodes differing in metallic composition, but subject to common operating conditions for pulsed stationary arcs in air. A causative correlation is indicated between erosion rates and properties of the pure metal in the cathodes, rather than with properties of their corresponding oxides. This trend further implies that oxide contributions to electrode erosion occur on timescales which preclude commonly accepted averaging procedures over the lifetime of the arc.

Annealed copper targets were impacted by 5 mm diameter hardened steel spheres at a velocity of 130+or-20 m s^-1 and at an impingement angle of 20+or-0.5 degrees . Target temperatures were varied over the range 70-1350K. Evidence for melting was found throughout the temperature range. At low temperatures melting was insufficient to produce detectable mass loss and was only found in localised shear bands. At temperatures within 200K of homogeneous melting, mass losses as great as 0.8% of the impacting sphere were measured. In this latter range, evidence of considerable melting was observed in craters, and deposits of copper were found attached to the spheres. The deposits were composed of unusually small crystallites (grain size approximately=10 mu m) and appeared to have been formed by melting followed by rapid quenching.