Table of contents

The triode electrostatic boundary value problem is examined for plane geometry and with special attention to the penetrating electric field. Based on special solutions (stripes and circles) the authors analyse the grid with rectangular holes treat its limiting cases and develop an interpolation formula.

Scattering of 633 nm photons from silica aerogels with densities 71, 105 and 143 kg m^-3 is investigated. A polarised He-Ne laser is used as a light source. Two types of data sets are determined: (i) total specific extinction coefficients for the porous specimens and (ii) the scattered intensity as a function of scattering angle theta , with 20 degrees <or= theta <or=160 degrees . The derived correlation lengths a_c are in the range 10-25 nm and show a systematic decrease with density. The extinction coefficients E are in the range of 10 m^-1; here no systematic variation of E with density is observed.

A common method for making computer-generated holographic elements (CGHOES) is to display the CGHOE on a high-resolution photoplotter and subsequently photographically reduce it to a 35 mm negative film. The predominant problem in making grey-level-modulated CGHOES using this technique is to ensure linearity in the high-resolution plotter and the photographic reduction process. This paper describes how two-step methods can be linearised using standard densitometric procedures frequently applied to evaluate black-and-white photographic recording materials. Another familiar problem encountered when recording CGHOES is the appearance of moire patterns in the plot from the plotter. The sources of these and the means to eliminate them are discussed. Finally, the authors examine how a surface relief on the 35 mm negative film influences the filtered output image of a Fourier processor.

In penetration welding with a laser, a plasma is formed in the keyhole. In the energy exchange process it is to be expected that a substantial part of the power is absorbed directly at the walls of the keyhole, but another exchange mechanism is the interaction of the laser beam with the plasma. A simple model of this part of the process is described and its properties are investigated. It is found to be consistent, and estimates are obtained for the parameters. The existence of a linking intensity is deduced. This relates the power absorbed by the workpiece per unit thickness of the workpiece to the laser power available at each cross section of the keyhole.

The propagation of coupled modes in a viscous incompressible dusty fluid confined between two perturbed parallel plates is considered. The treatment is confined to first-order perturbation theory in order to simplify the algebra. The multiple-scales approach yields information on the transition curve separating passbands from stopbands as well as the interaction equations governing the amplitudes and phases of the coupled modes.

Drift velocities were measured for ions of both polarities in unpurified cylinder SF₆, at pressures ranging from 50 to 700 Torr. The ions were produced in electron avalanches occurring in the high-field region close to a protrusion on one electrode of a uniform-field discharge gap, and the drift velocities were measured in the uniform-field region, where the electric field strength was below the minimum value required for avalanche growth. Negative ions were studied over the E/p range from 33 to 100 V cm^-1 Torr^-1: their behaviour suggests a possible transition in the nature of the ion in the range 64-82 V cm^-1 Torr^-1, but this is not entirely clear. Positive ions were studied over the E/p range from 40 to 120 V cm^-1 Torr^-1 and their drift velocity was found to vary linearly over this range. Empirical equations are given for the measured values. It appears that these may safely be extrapolated to higher pressures provided that the compressibility of the gas is taken into account. The presence of variable amounts of impurities did not affect the results significantly.

The voltage transient technique has traditionally been limited to high-pressure (p>500 Torr) discharges so that electron diffusion can be neglected. In much previous work image terms have also been omitted from the analysis of the experimental data. An analysis of the voltage transient has been developed which includes all image terms and both electron and ion diffusion. This analysis is applied to data in nitrogen gas, and the results have been compared with previous values obtained by other techniques. It is established that the measurement of voltage transients can be used to obtain electron transport parameters over a wide range of gas pressure and of E/N so long as the present full analysis is used.

A Q-switched ruby laser, focused by means of a spherical-cylindrical lens combination, was used to irradiate massive planar carbon targets. Intensity on target was maintained at 4*10⁹ W cm^-2 as the line focus length-to-width aspect ratio was varied. Measurements made with charge-collecting Faraday cups indicate a collimation of both integrated ion number flux and total ion energy in a plane orthogonal to the laser line focus length, the extent of collimation being dependent upon the line focus aspect ratio. Results are compared with a simple axisymmetric potential fluid flow model.

The characterisation of surface-wave-produced discharges is extended to the case where there is a neutral density gradient along the discharge. Experimental results for a low-pressure helium discharge at 2.45 GHz with a gas flow are presented. In order to model the electron density along the discharge, a law giving the variation of the mean power theta needed to maintain an electron with the neutral density is experimentally established. A good agreement is obtained between the measured and calculated electron densities. Furthermore the variations of theta are explained by a simple energy balance model.

The theory of the positive column at moderate pressures in electronegative, i.e. electron-attaching, gases is treated as a boundary layer problem, and the zeroth- and first-order terms are evaluated. The structure of the electron-positive-ion boundary layer is revealed and discussed. Comparison is made with other recent work and good agreement found for high attachment rates.

Multiple-beam Fizeau fringes in transmission have been applied to gamma -irradiated blue Cashmeline fibres, to determine their refractive indices for plane-polarised monochromatic light at gamma =5461 AA, vibrating parallel and perpendicular to the fibre axis, and the birefringence over the range from 5 Mrad up to 200 Mrad.

A fractal model for intergranular and transgranular fractured surfaces of cracked metals is suggested on the basis of the micromechanism of fracture. The fractal dimensions of the fractured surfaces are expressed in terms of grain sizes and the average distances between two large inclusions. It is shown that the fractal number of the fractured surfaces decreases with increasing fracture toughness, and it is qualitatively consistent with observations.

Thermally stimulated currents (TSC) in a-Si_xC_1-x:H films with various silicon contents (x) prepared by the RF (13.56 MHz) plasma CVD method showed a peak in the temperature range from 130 to 170 degrees C, and this peak tended to saturate as the poling field increased. The results obtained were well explained in terms of the two-site hopping model. The TSC analysis revealed the average hopping distance, the density and the barrier height of two-site pairs, which depended upon the silicon content (x) of the specimen.

Charge exchange measurements have been made on polystyrene and PTFE films in contact with mercury. The experiments were carried out in a purified atmosphere of nitrogen. Surface charge accumulation was observed on the polymer films upon repeated contacts. The surface charge saturation level could be altered and even reversed by using mercury samples of different purity for the contacts. Each mercury sample drove the surface charge to a characteristic saturation level, regardless of the previous contact history of the polymer sample. Surface charge densities as high as 200 nC cm^-2 have been observed on polystyrene.

Poly-2-chloro-p-xylylene (PCPX) thin films prepared from the vapour phase showed a negative temperature dependence of high-field current at higher fields (>6 MV cm^-1). The electroluminescence was also observed above 3 MV cm^-1. Both high-field current and EL in a PCPX film were lower than those in a PPX film. PCPX film showed a higher breakdown strength than PPX film. These results are explained in terms of the suppression of collision ionisation and its suppression by chlorine atoms.

Thermally stimulated depolarisation measurements have been carried out on P(VDF/HFP) copolymers and alpha -form PVDF homopolymer. Samples were polarised for 30 min under an electric field of 0.5 MV cm^-1. The poling temperature was varied between 10 and 100 degrees C. Three peaks were observed: two between room temperature and 130 degrees C, and a third always present at the poling temperature. Calorimetric measurements have shown an endothermic peak at this temperature. The origin of these three peaks are discussed.

Single crystals of potassium hydrogen phthalate, commonly known as potassium acid phthalate (KAP) have been grown from aqueous solutions. The dielectric constant, loss tangent, specific heat and pyroelectric coefficient were determined as a function of temperature. Frequency response of the dielectric constant and tan delta has been studied over a frequency range of 1-100 kHz. It is found that both the dielectric constant and tan delta decreases with increase in frequency. Anomalies in the dielectric constant and tan delta have been observed in the vicinity of decomposition temperature. The specific heat (C_p) and the pyroelectric coefficient have been determined as a function of temperature. The pyroelectric coefficient is found to increase gradually with increase in temperature up to about 440 K and it is consistent with the variation of C_p with temperature. The figure of merit which has been calculated at 320 K by combining pyroelectric, dielectric and thermal properties of the crystal is 1.79*10^-9 C cm J^-1.

Being different from the traditional TL phosphor, LiF:Mg, Ti, LiF:Mg, Cu, P-TLD is sublinear in its response to gamma rays instead of supralinear. A model is proposed in this paper to explain theoretically the origination of nonlinearity of LiF-TLD. It is shown that there exist non-emission competitive centres which influence the intrinsic parameters of crystal as well as the nonlinearity of the gamma -ray response. Experimental results agree with the model.

Thermoluminescence kinetics is discussed using a model with one type of recombination centre and an arbitrary number of trapping centre types, assuming that electron retrapping is dominant. A distinctive feature of the suggested method is that an integral equation is obtained for the one-time function z(t) through which both the intensity of the thermoluminescence curve (TLC) and the concentration of charge carriers are expressed. Using a series of approximations, which are analysed in detail, analytical solutions of the integral equation are obtained for an arbitrary elementary peak in a complex TLC consisting of a number of slightly overlapping peaks. The comparison of the theoretical results with the numerical ones obtained by the Runge-Kutta method shows that the obtained analytical expression is precise for the first peak and describes the following TLC peaks satisfactorily. The possibility of using the results of the present study for computer division of a complex TLC into elementary components, even in the case of considerably overlapping peaks, is also discussed.

The eigenmodes of a spherical resonator, with a perfectly conducting wall and filled with a homogeneous isotropic chiral medium, have been identified and studied. In particular, the roots of the dispersion equation have been given as functions of beta /a, where a is the cavity radius and beta is the chirality parameter. Application as a microwave circuit element is suggested.

Solar control coatings, required for architectural glazing applications in warm climates, must provide controlled optical transmission ( approximately 10-50%) of the solar radiation in the visible region and should reflect efficiently in the infrared (>0.7 mu m) region to create a cool interior in the buildings. Thin films of PbS and Cu_xS on glass substrates, deposited from chemical baths, are shown to possess excellent solar control characteristics-superior or comparable to the metallic solar control coatings. For example, for an acceptable range of integrated optical transmittance ( approximately 10-20%) in the visible region, the integrated infrared reflectance for AM2 solar spectrum for the different glazings are: PbS coated glass, 50%; Cu_xS coated glass, 14%; stainless steel/Cu coated glass, 25% and tinted glass, 4%. The Cu_xS and PbS coatings also have the advantage of giving pleasant reflected colours (golden, purple, blue, etc), which improves the cosmetic appearance. This paper presents the basic requirements of solar control coatings and provides a comparison of the characteristics of PbS and Cu_xS coatings against commercially available coatings.

Ba_0.9Ca_0.1TiO₃ ceramics have been fabricated using powders obtained by calcining mixtures of (i) BaCO₃, CaCO₃ and TiO₂ and (ii) Ba_0.9Ca_0.1CO₃ and TiO₂. The precursor Ba_0.9Ca_0.1CO₃ was prepared by a wet technique to ensure homogeneous mixing of Ba and Ca ions at the atomic level. DTA analysis of solid state thermochemical reactions in the two types of mixtures shows that the temperature of reaction completion is 140 degrees C lower for the mixture containing the precursor carbonate solid solution. XRD results indicate better chemical homogeneity for powders obtained by the new route. A powder diffraction pattern for Ba_0.9Ca_0.1TiO₃ has been theoretically computed assuming partial substitution of Ba²⁺ by Ca²⁺. The theoretically predicted intensities are in good agreement with those experimentally observed for powders prepared by the precursor route only. Better chemical homogeneity with respect to the distribution of Ca²⁺ ions leads to more pronounced diffuse phase transition in Ba_0.9Ca_0.1TiO₃ ceramics.

Non-ohmic characteristics of ZnO varistors are obtained by diffusing Bi₂O₃ and/or Sb₂O₃ into a ZnO-based matrix. The varistors derived from metal oxide diffusion have a smaller non-linearity parameter, lower threshold field, larger dielectric constant and larger dissipation factor than conventionally processed varistors. The difference between these two types of varistors is discussed on the basis of the Schottky barrier model. The low threshold field thus obtained makes the metal oxide diffusion process a potential approach in fabricating low-voltage varistors.

Peoglos (1988) reports forces on portions of a current loop due to the remainder of the loop which are in excellent agreement with the direct integration of Ampere's original force law. Peoglos has not, however, confirmed the non-physical absurd Biot-Savart law: it predicts ambiguous, and thus non-testable, results for the force on Ampere's bridge. It predicts merely fortuitously the correct Ampere force on a straight portion, since only lateral forces are measured. The force is not generally given by 2F_x=I²dL/dx and could not, in any case, distinguish between the Ampere and Biot-Savart laws.

The authors describe the use of stimulated Raman scattering as a probe of fuel injection sprays. The presence of morphology-dependent resonances convolved with molecular spectral information permits simultaneous droplet sizing and component concentration measurement, and provides good discrimination between liquid and gas phases. The different features of droplet non-linear Raman spectra are described in typical fuels. Multimode scattering peaks in alkanes mean that it is easier to monitor aromatics in a fuel mixture where the molecular and morphological peaks are distinct from one another.

A recent theoretical analysis of the effect of non-linear elasticity on the toughness of biological materials as measured by tear tests emphasises the importance of considering energy storage in the legs of the test piece. This is supported by reference to practical results on tearing biological tissues.

The theoretically predicted phenomenon of isotropisation of an intensive monoenergetic beam (MEEB) was shown for the first time in interactions with plasma oscillations, as a result of probe investigations in a non-collisional low-voltage discharge plasma in helium. The MEEB relaxation was shown to take place gradually: at first the process of isotropisation under constant beam energy is indicated by the energy relaxation of a weakly anisotropic beam to a state with a plateau-shaped electron velocity distribution function. The effective cross section of quasi-elastic electron-plasmon 'collisions' was estimated.

This letter makes a plea for consistency in the use of the terms plasma, sheath and presheath in bounded low pressure discharges.

A simple method for estimation of the effective fast-neutron dose received by the crystalline silicon during neutron transmutation doping process has been proposed. It applies standard optical absorption measurements in the near-infrared range at room temperature, taking into account the dependence of the divacancy-related absorption peak versus fast-neutron fluence.

Thick superconducting YBa₂Cu₃O_{7- delta} and SrBiCaCu₂O_x films on alumina substrates were prepared using a screen printing method. The films were post-processed by a zone-melting technique which caused limited grain growth in the Y-Ba-Cu-O films but greatly enhanced the texture of the Sr-Bi-Ca-Cu-O films. The process eliminated the gaps between the film's grains and significantly reduced the porosity. The films did not melt incongruently, and maintained their superconducting properties after processing.

Thin single-crystal Ni foils, annealed in a vacuum of 10^-2 Torr at temperatures between 550 and 950 degrees C, have been used as positron moderators in the transmission mode. Efficiencies of low-energy positron production of up to 6.5*10^-4 have been observed with a characteristic energy spread of 0.3 eV.