Table of contents

The breakdown voltage in diluted SF₆ was studied at elevated temperatures up to 4000K. The measurements were carried out in a diaphragm shock tube behind the reflected shock wave. The temperature dependence of the electric strength of pure and diluted SF₆ may be described by two factors E_c=E_c0( rho / rho ₀) chi (T), i.e. density variation and change in molecular state and/or chemical composition. chi (T) is characterised by three linear regions and disappears at 2900K.

The importance of the value of the metal first ionisation potential in a low temperature region (T<8000K) is shown through a theoretical investigation of plasmas contaminated by various metals, for a temperature range between 5000K and 14000K.

A new method for obtaining surface-conductive polymer systems, related to the reticulate doping technique, is presented. The surface layer of a non-conductive macromolecular material doped with 0.2-1.0 wt.% of a conductive charge-transfer complex is treated with solvent vapour to induce precipitation of the initially molecularly dispersed additive. Under proper conditions a conductive network of the precipitated 'organic metal' is formed in this layer making the material surface conductive.

The effect of critical fluctuations in magnetisation near the Curie point and far from the phase transition on the sputtering yield of the magnetic materials Ni and Gd is examined. The inclusion of the spin configuration of the nearest atoms around an atom sputtered from a surface has been found to result in an increase of the target sputtering yield by about 1.5 times compared with the ferromagnetic state.

In order to describe the response of an RF SQUID to current flowing in a signal coil to which it is inductively coupled, the authors introduce the idea that currents flowing in the SQUID body are divided into two components, which they term coil-screening and flux-screening currents. They develop a more precise definition of what is usually called the mutual inductance of the SQUID, and obtain an expression for it. For the case of the two-hole SQUID, this expression is compared with the experimentally known relationship. Several other useful relations for the SQUID are obtained. Using these concepts they have analysed the dependence of the period of the response on the SQUID geometry. Experimental results for two-hole and toroidal SQUIDS support this analysis.

By comparison of experimental K_e,0 factors determined in reactor positions with greatly different thermal-to-epithermal neutron flux ratios, with theoretically calculated K_e,0 factors, it is possible in some cases to select the best values from literature data (n, gamma ) activation cross-section and for absolute gamma intensities. Eighteen different isotopes were considered.

For pt.I see ibid., vol.17, pt.1 (1984). The transverse Cartesian components B_y and B_z of the magnetic field of cylindrical saddle-shaped coils are evaluated in a series expansion. The terms of the series are written in the form A_lmnx^ly^mzⁿ, where the coefficients A_lmn are functions of the coil size and shape parameters R₀, Z₀, Phi ₀ (R₀ and 2Z₀ are the radius and height of the cylinder defining the geometry of the saddle-shaped coil, Phi ₀ is the angle of the arcs). The y component of the magnetic field is determined to the fourth order (l+m+n=4); the z component was evaluated including the sixth-order terms. The existence or non-existence of the second-order terms determine the limiting properties of B_y and B_z for r to 0. the analysis of the second-order terms as a function of the coil parameters reveals some general properties of the magnetic field at the coil centre.

A rectangular flat-roof and plane mirror combination type of microwave open resonator, with a mirror spacing of a single half-wavelength ( lambda =12.5 mm), is investigated experimentally in the context of molecular beam masers. The flat-roof resonator possesses the property of a radiation field that is extensive in the direction of the mirror crease, but concentrated in the transverse direction by an amount dictated by the roof-top pitch, down to a value of about a wavelength. The associated reduction of diffraction loss leads to an increase of Q value of almost twice that of a flat mirror resonator of similar dimensions. Resonances are easy to find and are insensitive to rotations of the roof edge about the plane mirror. The resonator is tunable over several GHz. A lifting of the frequency degeneracy of mode polarisation is reported. the application of the flat-roof resonator is illustrated by results obtained by its incorporation into a high-resolution molecular beam maser spectrometer and oscillator. It could also find an application in pulsed molecular beam Fourier transform spectroscopy.

When penetration welding is performed with a laser, the boundary of the keyhole formed in the weld-pool near to the surface of the workpiece experiences considerable variation of temperature; this variation could affect the dynamics of the flow in a number of ways. A simple mathematical model for the temperature distribution is investigated, and solutions obtained. These confirm that for values of the Peclet number of order one the variation of temperature at the keyhole surface can be very substantial.

This paper uses continuum theory to investigate the onset of convective instabilities in long pitch homeotropic cholesterics. A numerical method of solution is described which is capable of solving the problem for heating from both above and below. The possibility of both stationary and oscillatory instabilities is considered and the effects of the thermomechanical coupling parameters, peculiar to cholesterics, upon the critical threshold are investigated.

Measurements of the ratio of radial diffusion coefficient to mobility, D_r/ mu , have been obtained for carbon monoxide, nitrogen and hydrogen. A pulse technique has been used covering a wide range of E/N from 28 to 5650 Td. The values of D_r/ mu vary between 0.8 and 17 eV in carbon monoxide, between 1.2 and 16 eV in nitrogen and between 1 and 12 eV in hydrogen. Comparison has been made with results of a Monte Carlo computer simulation. For all the gases, fair agreement has been obtained with the theory except for the highest of range E/N. This discrepancy has been explained by the presence of a non-equilibrium electron energy distribution in the experimental measurements.

Using a pulsed Townsend technique, the density-normalised ionisation coefficients, alpha /N, positive ion drift velocities, v₊, and longitudinal diffusion coefficients, D₊, have been measured in SF₆ over the E/N range from 909 to 5450 Td, and gas number density N from 0.23 to 2.3*10²² m^-3; E is electric field intensity. The alpha /N values were found to be generally higher than others reported previously. The ion mobilities were in fairly good agreement with those derived from theory by Brand and Jungblut (1985), who assigned them to the SF₅⁺ species. To the authors' knowledge, no previous ND₊ data have been reported.

The electron drift velocity, W, and the longitudinal diffusion coefficient as a ratio of the electron mobility D_L/ mu , have been measured in helium gas over the electron swarm energy range 20 Td<or=E/N<or=100 Td, using the photon flux technique. The uncertainty observed in these data is significantly larger than that previously noted in the work in the molecular gases hydrogen and nitrogen. Furthermore, the nature of the uncertainty in the experimental results leads to the postulate that W and D_L/ mu are spatially dependent. A Monte Carlo simulation of the travelling electron swarm in helium supports this contention.

High-resistivity GaAs doped with boron and aluminium has been subjected to (i) 2 MeV electron irradiations, (ii) fast n⁰-irradiations up to 10¹⁹ cm^-2 and (iii) subsequent anneals up to 700 degrees C. At each stage, infrared absorption measurements have been made of the localised vibrations of boron present in a wide range of defects. Prominent annealing stages have been revealed at 200, 230, 400 and 650 degrees C. The most interesting feature is the observation of boron site switching from B_Ga to B_As and back again. This behaviour is similar to that found in GaP. The effects are more pronounced the higher the initial dose of irradiation. Defects involving aluminium complexes have not been observed.

When studying trapping centres in ultra-high-purity, quasi-intrinsic semiconductors by deep level transient spectroscopy (DLTS), considerable errors may be introduced in applying the original small theory. In the present work expressions are presented, which allow a more accurate determination of deep level concentrations with DLTS in the case of: (i) high relative trap concentrations r_T (0<t_T<or=1); (ii) the application of filling pulses covering only part of the depletion region; (iii) high resistivity material, giving rise to appreciable values of the diode Q-factor, as is the case for high-purity germanium (HP-Ge) in particular. A general trap concentration formula is proposed, which is suitable for routine calculations of r_T in high resistivity material. It is furthermore shown that under certain conditions, a C-V analysis of HP-Ge diodes leads to the correct value of the net ionised impurity density. This way of obtaining the residual shallow level density, as compared to Hall measurements, is discussed.

The AC conductivity sigma ( omega ) and dielectric constant epsilon ' of lithium niobate single crystals have been measured along the ferroelectric c-axis in the temperature range 77-700K and in the frequency region 0.1-100 kHz. At low temperatures, below 400K, the AC conductivity can be expressed as sigma ( omega )=A omega ^s, where the slope s is close to unity and decreases with increasing temperature. The dielectric constant in this temperature region shows a weak frequency and temperature dependence. At higher temperatures, above 400K, the AC conductivity shows a strong temperature dependence and weak frequency dependence. The dielectric constant in this region also shows a strong frequency dispersion. The mechanism of conduction in low and high temperature regions is discussed in the light of existing theoretical models.

Flat-band voltage shifts in MOS capacitors irradiated by a kilovolt electron beam have been measured. The relatively low dose conditions employed resulted in the data points being obtained under non-equilibrium conditions and therefore sensitive to the exact manner in which the bias voltage across the sample was cycled. All the results obtained can be explained by considering the transient growth and decay of hole space-charges at the oxide interfaces with the metal electrode and the silicon substrate. No evidence was found to support the role of excitons in generating hole space charge at the oxide/silicon interface as postulated by previous authors.

The electrical conductivity associated with energy barriers in a polycrystalline semiconductor is derived, taking into account both tunnelling and thermionic emission. Numerical calculations are compared with conductivity data of polycrystalline CdS films, and the contribution of each transport mechanism is determined in a wide temperature interval.

An electron beam evaporation method is used to prepare In₂O₃ films with and without Sn doping. It is shown that highly transparent and conducting films can be prepared at substrate temperature as low as 200 degrees C. The characteristic feature of such films is their high carrier density and high infrared reflectivity. The lowest resistivity is found to be 2.4*10^-4 Omega cm with a carrier concentration of 8*10²⁰ cm^-3 and mobility of about 30 cm² V^-1 s^-1 at the doping level of 4 mol.% SnO₂. These polycrystalline films show a highly preferred orientation. On the basis of Hall measurements and structural data, sources of scattering in these films are suggested.

A model for magnetisation reversal is developed in which a small conical or parabolic spike domain grows across a spherical particle and opens out into a cylindrical reverse domain which then expands. Examination of the total particle energy shows the presence of initial energy barriers to spike growth and energy minima during the expansion of the cylinder. The energy minima are used to construct theoretical magnetisation loops for SmCo₅ particles.

The complex permittivity of double-distilled degassed water has been measured with a typical accuracy of +or-0.06% in both epsilon ' and epsilon " at 2 degrees C steps from 10 to 40 degrees C at 9.356 GHz. This accuracy is limited by the estimated +or-0.05 degrees C thermal homogeneity of the sample and not by the +or-0.01% precision of the instrumentation system. The values of epsilon ' and epsilon ", as well as the attenuation alpha and phase beta per unit length coefficients, have been fitted to the empirical polynomial expressions epsilon '(T)=44.73(17)+1.430(24)T-0.0318(10)T²+0.000230(14)T³ epsilon "(T)=41.96(17)-0.407(23)T-0.0083(10)T²+0.000132(13)T³ alpha (T)=560.0(1.8)-9.92(25)T+0.044(11)T²+0.00024(14)T³ beta (T)=1426.1(2.1)+14.23(29)T-0.354(12)T²+0.000275(17)T³. It is found by fitting the values of epsilon '(T) and epsilon "(T) in this temperature interval to the Cole-Cole equations using the empirical functions for epsilon ₀(T), epsilon _infinity (T) and tau (T) that remarkably good agreement is obtained with the known values of alpha , Delta H and epsilon ₀(0).