Table of contents

The variation of resistivity and dielectric constant with temperature at high and low frequencies has been investigated for the polycrystalline systems CeCrO₃ and Ca_0·1Ce_0·9CrO₃. At high frequencies the activation energy of resistivity was found to be 0·22±0·01 eV for both materials. This activation energy is probably the energy required for p-type controlled valence conduction between Cr³⁺ and Cr⁴⁺ ions. Using Koops' theory for dispersion in inhomogeneous materials, expressions were obtained for the temperature dependence of resistivity and dielectric constant at high and low frequencies. The experimental results are shown to be consistent with this theory.

Lattice-parameter-temperature relationships have been examined for iron-cobalt solutions containing up to 12 wt.% cobalt for temperatures up to approximately 900°C, with particular attention being paid to the Curie-point regions. Cobalt additions progressively reduced the magnitude of the lattice-parameter anomaly previously reported for iron by Ridley and Stuart until at 12 wt.% cobalt no anomaly was observed. The results showed that volume changes resulting from the ferro-magnetic-paramagnetic transition had no significant effect on the room-temperature lattice parameters of iron-cobalt alloys, which show a marked deviation from Vegard's law.

Weak-field magnetoresistance measurements have been made on p-type PbTe-CdTe alloys for CdTe concentrations of 1·0, 1·5, 2·5 and 3·2 mol.%. At 300°K the effect of CdTe on the magnetoresistance coefficients is very small, whereas at 77°K the effect is quite large, the coefficients showing a pronounced increase. This has been interpreted as a variation in the scattering relaxation time anisotropy and has been discussed in terms of known possible scattering mechanisms.

An experimental investigation of the electro-optical processes of the Thorn image-retaining panel is made in order to explain the production of retained luminescent images on excitation with visible or near-infra-red radiations. It is concluded that image production and retention is due to the release of positive holes in the phosphor layer, which diffuse to Schottky barriers between phosphor grains and the underlying semiconducting enamel. The holes held at the barrier by the applied field reduce the barrier width, permitting electron flow into the phosphor grains. Subsequent excitation of electroluminescence is by impact ionization and a continuing supply of holes for the image retention. This requires modification for electron-beam excitation of the panel.

A quantitative assessment is made of the conversion efficiency of infra-red emitting phosphors. It is shown that for exitation from a ZnS (Mn, Cu) d.c. electro-luminescent cell at 0·58 μm the efficiency is 18% and 8% for 60/40 CdS/ZnS-10⁻⁴ g/g Cu and CdS-10⁻³ g/g Ag respectively, for outputs over the range 0·65−1 μm. A donor-acceptor pair model is proposed.

Spike-like magnetic domains play several important parts in the magnetization processes occurring in a hysteresis cycle, especially in a material magnetized in an easy direction of magnetization, such as transformer sheet.

This theory is applicable to the spike domains observable in bulk magnetic materials but not, unfortunately, to thin magnetic films.

A comparison was made between the calculated shape of a spike domain and that of such domains in 3% silicon-iron, the latter being available in the form of photographs. As far as could be determined from the photographs the calculated shape was an exact fit to the observed shape.

Critical current density measurements, transmission electron microscopy and chemical analysis have established a correlation between critical current density J_c, the oxygen content in Nb₃Sn and the presence of a fine dispersion of precipitate. As the oxygen content increased from <50 to 1000 p.p.m. by weight, J_c increased from 1 to 6 × 10⁵ A cm⁻² at 46 kG. A fine precipitate (particle size similar 50 Å) was observed for all oxygen contents greater than 160 p.p.m. It is concluded that the precipitate particles act as flux-pinning centres in Nb₃Sn. No correlation was observed between nitrogen content and J_c.

Electrical breakdown between a pair of electrodes immersed in a flowing plasma corresponds to the transition between low- and high-current conduction régimes and has been attributed to many causes. In the work reported here, which involved cold electrodes in contact with the plasma produced by an electromagnetic shock tube, it was found that the minimum breakdown voltage corresponded to an average electric field in the cathode sheath of about 2 MV cm⁻¹ over a wide range of plasma conditions. This was probably adequate to initiate the formation of non-thermionic cathode spots, and the voltage-current characteristics in the high-current régime were consistent with a field-emission theory of this mechanism, originally suggested by Smy. Moreover, electric fields in the plasma, sheath thicknesses and mean free paths were such that alternative explanations of the high-current régime were reasonably eliminated.

The construction and some operational characteristics of an argon laser in which the discharge operates between a water-cooled copper anode and a plasma jet which acts as a cathode is described. With 50 A and 550 G extra cavity powers of 4½ W (6 lines, multimode) are obtained in a tube 80 cm × 5 mm with an unoptimal cavity.

The growth of a positive ion sheath has been measured in a mercury plasma of density 10¹⁰-10¹¹ cm⁻³. The sheath was formed on a plane electrode, the potential of which changed at a rate of up to 3 kv μs⁻¹ and reached a maximum value of 10 kv. The thickness of the sheath is compared with various theoretical models and good agreement is found between experiment and theory. An expression is derived for the current flowing in the dynamic sheath system and compared with experiment.

Measurements of the drift velocity and lateral diffusion of positive potassium ions in nitrogen at room temperature (290-300°K) are reported for values of E/N (E is the electric field strength in v cm⁻¹, N the gas number density in cm⁻³) up to 320×10⁻¹⁷ v cm². The zero-field reduced mobility of the ions was found to be 2·53±0·02 cm²v⁻¹s⁻¹, the mobility of the ions increasing above E/N = 30×10⁻¹⁷ and decreasing above E/N = 220×10⁻¹⁷. The ions are shown to have remained within 5% of thermal equilibrium with the gas molecules for E/Nless, similar25×10⁻¹⁷. The variations of drift velocity and lateral diffusion with E/N are compared with other available data.

Electrical conduction studies of pure hexane and decane liquids are reported using a novel electrode system of crossed wires with a droplet of the liquid freely suspended between them. Volumes of liquid no greater than 5 × 10⁻⁴ cm³ can be studied, the drop being kept continuously under close microscopic observation. Very stable currents are obtained because dust particles in the gap can easily be detected and eliminated. Current-stress relationships have been obtained up to stresses as high as 800 kv cm⁻¹ and little or no direct correlation with electrode material has been found, although with air-saturated liquid the anode appears to influence the conduction. The conduction in outgassed liquid is more than three orders of magnitude greater than that of air-saturated liquid suggesting, in the latter case, a strongly blocking anode and the accumulation of space charge. Photoconduction which occurs readily in outgassed liquid, even with light of wavelengths as long as 6000 Å, is inhibited by the presence of dissolved air. With air-saturated decane the current at constant stress has been found to increase markedly with increased electrode spacing over the range 4-47 μm. It is reasonable to conclude that some degree of collision ionization occurs in this case.

The d.c. and a.c. electrical properties of rubber have been investigated as a function of temperature to determine the mechanism for charge transport through the bulk and the nature of the charge carriers involved. Hopping appears to be the predominant mechanism, although a transition from hopping-type to an effectively band-type conduction may occur. High bulk polarization is present and peaks probably associated with the reorientation of atoms along the chains occur, particularly at lower frequencies.

The bi-directional and the directional-conical transmittances and reflectances of various materials used for sampling aerosols by filtration were measured, using directional incident light. Observations were made with white light and, in some instances, with light filtered through broad-band optical filters. The results are interpreted in terms of a one-dimensional model which incorporates a term taking account of absorption, as well as one involving scattering.

The size, shape and stability of microscopic molten metal bridges of iron, platinum, gold, nickel, tungsten and molybdenum have been studied in the current range 10-100 A and the rupture voltages measured in each case. High-speed photographs were also obtained, and these gave information concerning the development and rupture of the molten metal bridges and the post-rupture micro-plasma.

The accuracy of thermal diffusivity measurements by the flash method on specimens containing cracks has been assessed from a theoretical treatment of heat flow through a specimen containing a model crack. It was found that the thermal diffusivity as measured by the flash method is probably within 5% of the value derived from a steady-state heat-flow method. Measurements on Armco iron specimens containing restricted heat-flow paths confirmed this conclusion.

Assuming that the refractive index in a medium depends exponentially on the height, an expression for the focusing factor in a stratified medium has been obtained by extending earlier work. The special cases of an isotropic source and a vertical dipole antenna have been discussed in detail.

The weighted geometric mean of maximum and minimum thermal conductivities, proposed by Chaudhary and Bhandari for calculating effective thermal conductivities of porous media, has been applied to unconsolidated dry materials. The relation is tested with values obtained experimentally by Smith for Miami silt loam, and fair agreement is found.

Measurements have been made of the instantaneous electron temperature in the positive column of a mercury-argon discharge operated at a constant discharge current of 0·23 A, from 150 to 6000 Hz a.c. supplies. Modulation of the electron temperature with twice the frequency of the power source was obtained in the same phase, but with a small percentage modulation, as compared with that of the light output.

An account is given of a series of experiments directed at examining the variation with velocity of depth and volume of hole produced in semi-infinite targets of aluminium by impact at normal of long rods of hardened and tempered steel.

A model is proposed which can explain the appearance and properties of the photovoltaic effect during the formation of tellurium layers deposited obliquely in a vacuum. The following phenomena have been accounted for: (i) the increase and saturation of the photovoltaic effect with the film thickness; (ii) the increase of the electrical conductivity with the same parameter; (iii) the decrease of the photovoltage and the increase of the electrical conductivity under the influence of air; (iv) the ohmic character of the resistivity.

The elastic compliances of KBr have been measured over the range 20-650°C using an audio-frequency resonance method. The compliances S₁₁, −S₁₂ and S₄₄ all increase markedly in this range. Values extrapolated to 0°K and to the melting point have been found to be in close agreement with other reported data.

The simultaneous study of thermoluminescence and thermally stimulated conductivity has been carried out on a CaS-Bi phosphor. It has been found that maxima for thermally stimulated conductivity occur at higher temperatures than for thermoluminescence. Attempts have been made to show that this may be explained by the fact that the factor μτ is an increasing function of temperature.

A method is described for reducing the loss due to laser rod strain bire-fringence, and increasing the output energy, in an electro-optically Q-switched laser system. The optical arrangement uses the strain birefringence to couple photons out of the cavity, which is formed by two 100% reflecting mirrors. Typical increases in output of about 50% have been obtained with 3 in. and 2 in. Nd in CaWO₄ laser systems.

A new experimental technique has been developed to investigate condensation and collapse of vapour bubbles in a sub-cooled liquid, and of the pressure waves which are generated in the process. It appears particularly suited to the study of opaque liquids such as liquid sodium, when visual observation is impossible.

Initial tests with water at about 20°C have generated condensation shocks of 650 b with the collapse of as little as 100 cm³ of vapour and 1250 b with 500 cm³ of vapour. There is evidence of very brief and extremely high pressure transients, beyond resolution with the measuring apparatus used for the initial experiments.

The classical effect of `collapse' and `rebound' has been observed by high-speed ciné photography through the transparent walls of the apparatus.

A qualitative description of the apparatus is given as well as a selection of some of the results.

The K absorption edges of zinc, manganese and iron have been recorded photographically in the spinel ZnMnFeO₄, using a bent-crystal x-ray spectrograph. The positions of these edges have been compared with those in some bivalent and trivalent compounds. This comparison shows that the valence states of zinc, manganese and iron in this spinel are 2, 3 and 3 respectively.