Table of contents

The transport coefficients of SF₆ have been recalculated in the range of reduced field between 35 and 225 V cm^-1 Torr^-1 using the latest cross-section data. While fitting of calculated and experimental ionisation coefficients has been used to determine missing data on excitation cross-sections, calculations of the drift velocity and the diffusion and attachment coefficients provided a means of checking the consistency of the model and the data. The calculated drift velocity proved to be about 20% lower than the experimental value. The diffusion coefficients agreed well but the attachment coefficients compared poorly with measured values. Good fitting was obtained by reducing the width of the SF₆^- attachment cross-section to 30% and increasing the high-energy attachment cross-sections by about 30%.

It is possible that too little attention has been given to the role of time in cavitation experiments. In this letter a summary is given of some of the time effects which have been reported in the literature. First, it seems that the rate at which tension is applied to the test liquid is an important factor. Secondly, in some experiments, there are incubation times 'during which nothing happens'. Thirdly, if a liquid is first subjected to pressurisation before being subjected to tension, the time duration of this precompression is important. Lastly, in repeated dynamic stressing, there is evidence of a 'recovery' time in the cavitation behaviour of the liquid. Some results from recent work are also reported and discussed.

Measurements of the dielectric constant and pyroelectric coefficient of alpha -nylon 11 have been carried out in the temperature range from -20 degrees C to 120 degrees C. The temperature dependence of both the dielectric constant and pyroelectric coefficient was found to correlate with an anomalous temperature dependence of the thermal expansion coefficient determined for the polar direction in the alpha crystal. The results are discussed in terms of a diffuse ferroelectric-paraelectric transition.

Measurements of the coefficient of linear thermal expansion of pure polycrystalline silicon between 80 and 280K were made with a polarisation interferometer and a platinum resistance thermometer. The results agree with those of Lyon et al. (1977) within 0.02*10^-6 K^-1 over the whole range.

A survey is given of the different techniques for generating high magnetic fields at various time scales; the influence of the current distribution on the construction of a high field magnet is shown and the effect of the temperature rise as well as that of the forces is demonstrated. Magnetometry in pulsed fields is discussed by comparing different techniques; the influence of the construction of the pick-up system upon sensitivity is calculated. The value of a high field device is demonstrated on different selected results. Analysis methods which can be applied to high field magnetisation are also given. Different approaches are developed for the determination of the anisotropy of polycrystalline materials. Examples are the law of approach to saturation (LAS), the singular point detection technique (SPD) as well as field induced transitions (e.g. FOMP). For a band magnet the occurrence of a metamagnetic transition is demonstrated on materials like Co(S_xSe_1-x)₂. In the case of ferrimagnetic systems various descriptions based on molecular field models are developed. The existence of a spin-flop transition is demonstrated even for polycrystalline materials by applying the theory graphically.

The aims were to understand and to learn how to control potentially destructive stresses which limit the performance of anvils in high pressure optical and X-ray cells. The calculations show that the largest compressive and shear stresses are near the working face of the anvil and, hence, that plastic flow will begin there, as has been observed. The largest tensile stresses are at the base, above the unsupported light port. Since diamond, a brittle material, is liable to failure in tension, these stresses are potentially destructive. The largest principal tensile stress is in a hoop direction: consequently diamond anvils are liable to basal failure by radial cracking, with ring cracking from tensile radial stresses important also. This type of failure has been observed in diamond anvil cells. In the light of these results, mechanisms of anvil failure are discussed and illustrated with photographs. Attention is drawn to the importance of fatigue as a cause of failure in diamond.

The properties of parasitically coupled standing-wave antennae on thin dielectric substrates have been studied from their scattering characteristics at 891 GHz (337 mu m). Measurements of total and back-scattering cross-sections on arrays of dipole antennae are described. Arrays of dipole elements, in which each element is terminated by a length of short-circuited coplanar transmission line, are also investigated. The electrical properties of the coplanar transmission lines are found and the ability of standing-wave antennae to couple energy to them efficiently at this high frequency is verified.

The high-Q microwave properties of a sapphire dielectric ring resonator in the frequency ranges 8-12 GHz and 18-22 GHz at both room temperature and low temperature are discussed. The Q of the resonator is limited significantly below the loss tangent limit, but the frequency stability is excellent when compared with other types of resonator. The results indicate that the sapphire ring resonator should be well suited for use in long-term, high-stability frequency standards, and should also be useful for phase noise stabilisation of microwave sources.

One frequently uses the Ritz variational method to determine the coefficients of series obtained by separation of the variables when solving boundary value problems of electrostatic fields. In this paper an improvement to the variational method for determining the coefficients is proposed. When the solution of the problem is obtained numerically the calculational procedure for the new method is much simpler than that of the old, and the accuracy of the solution using the new method is better than that obtained using the least-squares method. By taking a problem of two-dimensional electrostatic fields as an example, the authors are able to understand the numerical calculational procedure of the new method. The results are compared with the true solution. The new method is applied to two-cylinder electron lenses and the electron optical characteristics are made clear.

A method to determine the variation of the average refractive index in a ruby laser caused by the direct coupling of Cr³⁺ ions to the average strains of the crystal lattice is presented. The method involves creation of a spatial variation of the population inversion density in the laser rod, and then measurement of the resulting dynamic divergence of the laser. The results obtained are in reasonable agreement with the values determined by other methods reported previously.

The problems of illumination symmetry in laser-imploded targets is studied using a two-dimensional fluid computer model. The thermal smoothing of 'hot spots' in the laser illumination is found to be much reduced for short laser wavelengths because of the smaller separation of critical density from the solid.

A small volume of an initially uniform plasma is heated with a CO₂ laser focused to an intensity of 1.3*10¹¹ W cm^-2. The evolving thermal and rarefaction wave is measured by Thomson scattering. Comparison of these measurements with a one-dimensional simulation shows that the heat flux reaches 15% of the free-streaming limit.

Spatially modulated laser beams at 1.05 mu m and 0.53 mu m were used to accelerate mylar disc targets. The response of the target was smoothed by the plasma and the degree of modulation observed on the rear of the target, by pulsed X-ray shadowgraphy, was proportional to 1/I lambda ^1.5. Code comparisons show this is due to thermal conduction in the absorption region and smoothing in the coasting phase of the target motion.

On the basis of an analysis of the boundary layer, expressions for the ion saturation current density on a cylindrical probe have been deduced in connection with the conditions for the dense plasma flow. It has been shown that both the local ion saturation current density and its circumferentially averaged value are inversely proportional to the square root of the probe diameter. The experimental and computational results of the effect of probe diameter on the ion saturation current density at the surface of the cylindrical probe are consistent with the prediction of the simplified analytical solution. Expressions for the specific heat flux on the cylindrical probe are also given.

The ionic transparency of a grid submitted to an ionised gas flow under various electrical and mechanical conditions is evaluated. A mathematical model is used, which gives simple analytical expressions for both the electrical field and the ionised gas flow. An agreement is found with experimental results for a flow obtained from a corona discharge.

The theory of electron multiplication in gases at large values of E/P is extended to cases where the characteristic energies of individual electrons are of the order of 10² eV or more. The first Townsend ionisation coefficient is found analytically and is compared with existing experimental data.

A non-equilibrium, two-phase model for a plane, one-dimensional system is used to calculate the pressure and cavitation development in water containing free gas as cavitation nuclei. Two cases are considered, (i) the field between a vibrating horn and a stationary specimen which is placed at a small distance from the horn, and (ii) a water column in a flat-bottomed tube which is accelerated by axial impact on the tube. The calculations are based on data corresponding to experimental investigations and the calculated developments are compared with the observed cavitation processes.

A relationship between the lattice and thermodynamic Gruneisen parameters for a polymer has been examined and analysed in the case of several polymers. An expression for the lattice Gruneisen parameter and the interchain heat capacity ratio in terms of the cubical thermal expansion coefficient has been obtained. Calculated data on the Gruneisen parameter, interchain heat capacity ratio and interchain isobaric heat capacity taken up by the molecular vibrations for 18 polymers are presented. Also, a relation is derived between ultrasonic absorption and the lattice Gruneisen parameter, which is found to be remarkably similar to the result for phonon-phonon interaction even though that mechanism is not expected to be applicable in polymers. The calculated value of ultrasonic absorption is in excellent agreement with the experimental value for a polymer for which the necessary literature input data are available.

A fairly simple diffusion mechanism is described in which the concentration of the diffusing species is relatively high and the self-diffusion coefficient of the host atoms is small. Diffusion profiles are computed and it is shown that the anomalous 'double profile' often demonstrated by semiconductor systems can be obtained by this means. It is suggested that the mechanism may be operative when sulphur diffuses into GaAs at high temperatures, and computed profiles are fitted as closely as possible to experimental profiles obtained previously for this system. In the course of the fitting procedure estimates are made for the concentration of arsenic vacancies, the diffusion coefficient of sulphur and the self-diffusion coefficient of arsenic in GaAs at 1120 degrees C.

Interpretation of DLTS measurements performed on Al-SiO₂-Si structures with low densities of interface states (<10¹⁰ cm^-2 eV^-1) is re-examined critically on the basis of some original deep-level transient current spectroscopy (DLTCS) measurements. A comparison of DLTCS spectra obtained in the depletion and accumulation modes of operation suggests an interference between the transient capacitance of the oxide and that of the space-charge region of the semiconductor. Possible mechanisms of charge relaxation in the gate oxide are discussed, mainly in connection with the available data on time-resolved charge transport in a-SiO₂. Suggestions are made as to how to depress the interferences or, alternatively, how to distinguish between the two types of transients.

Layers of PVK and of a PVK-TCNQ charge transfer complex at a component weight ratio of 25:1 have been investigated with respect to the AC conductivity versus frequency and temperature. In the high-frequency range the AC conductivity could be described by a general formula sigma ( omega ) approximately omega ^s(T) with the exponent s decreasing with increasing temperature. The results obtained have been interpreted with relation to Pike's theory (1972) regarding charge carrier hopping between the localised states and on the dipole polarisation mechanism.

The AC field-effect theory developed by Garrett (1957) for free semiconductor surfaces is applied to the interface states of MOS devices. Frequency and temperature dependence of the field-effect mobility for interface states in Si-SiO₂ is measured in the frequency range 5 kHz to 100 kHz and the temperature range 230K to 400K. The temperature dependence of relaxation times of interface states is found to obey Rupprecht's theory (1960) in the limited temperature range giving the energy level of interface states at 0.1 eV below the conduction band and an effective capture cross-section of the order of 10^-20 cm².

A method of characterisation of superconducting materials is presented, using an ultrastable oscillator at 10 GHz. The method provides precise and continuous measurement of the surface resistance, through the study of the noise spectrum of the oscillator, which is phase-locked to a superconducting cavity. Comparison is also made between differently processed resonators.

The temperature dependence of the complex dielectric permittivity of ten primary amines (from propylamine to dodecylamine) has been measured at frequencies of 2 MHz and 3.26, 9.23, 15.05 and 35.25 GHz. From the analysis of the experimental results the authors conclude that the Davidson-Cole model is the most suitable to describe the dielectric relaxation phenomena in those amines. For this model they obtain the usual relaxation process characteristic parameters; that is to say the relaxation time, tau ₀, and the constant beta which expresses the typical asymmetry of the relaxation times' zero-distribution function. The influence of the chain length and the behaviour of the NH₂ group in the relaxation process is shown. Finally, the dielectric behaviour of the amines has been compared with that of their analogous alcohols.

In the context of thermoluminescence emission, first-order kinetics is defined formally as the case where every electron, having escaped from a trap, recombines with a trapped hole without first being re-trapped. Rather surprisingly, most studies of the kinetic order of thermoluminescence report first order. Two previously recognised conditions leading to pseudo first-order kinetics are discussed, and the likelihood of their occurrence is assessed. A third condition involving the trapping of each electron within the Onsager radius of its geminate trapped hole, and geminate recombination, is described; this will lead to pseudo first-order kinetics, even in the presence of extensive re-trapping, provided the irradiation dose does not exceed approximately 30 Mrad. It is shown that this condition can be expected to arise in many dielectrics.

A strong dependence of the intensity of the thermoluminescence (TL) output of peaks 5 (208 degrees C), 7 (266 degrees C) and 10 (422 degrees C) on temperature (T_irr) (in the range 25-295 degrees C) at which the phosphor samples were irradiated was observed in LiF (TLD-100) in the dose range covered by the authors (1.7*10²-2.5*10³ Gy). Below the dose level of 1.26*10³ Gy, the intensity of peak 10 was 35-75% higher for high T_irr compared with the room-temperature (25 degrees C) irradiation. Above 1.26*10³ Gy, a reversal was observed in which the decrement in TL intensity of peak 10 increased with T_irr. In general, the TL intensities of peaks 5 and 7 decrease with increasing T_irr. The effect of T_itt on the sensitisation of peaks 5 and 7 was also studied. The sensitisation of peak 7 depends on the intensity of peak 10. The sensitisation of peak 5 at high T_irr is more than that at 25 degrees C for pre-doses below 1.26*10³ Gy and a reversal is then observed from this dose onwards. The authors explain increase of TL intensity and S/S₀ at high temperatures as due to increased elimination of competitors to luminescent centres. The decrease in the intensity of TL and S/S₀ with increasing T_irr is explained as due to the increased radiation damage to luminescent centres.

Several techniques for measuring the porosity and pore-size distribution in hardened cement paste (HCP) have given conflicting results. This is partly because these methods require drying or pre-treatment of specimens. Small-angle neutron scattering (SANS) allows investigation of the fine-pore porosity without any such pre-treatment. Measurements using the Pluto SANS spectrometer indicate a bimodal distribution of near-spherical pores with diameters of 5 and 12 nm. The former have a well peaked size distribution and account for 1-1.5% porosity. The pores are relatively unaffected by the water-to-cement ratios investigated although drying has the effect of collapsing the structure.