Table of contents

Electro-rheology is the phenomenon in which the rheology of fluids is modified by the imposition of electric fields. In this Review the nature of the fluids active in this role are presented and the influence of important variables such as shear rate, field strength, field frequency, temperature and fluid composition are described. Also discussed are the dielectric behaviour of dispersions in flow since this is of direct bearing on the phenomenon of electro-rheology. Various models for electro-rheology are presented and assessed. The authors also describe how recent developments in new classes of electro-rheological fluids have resulted from a clearer understanding of the probable electrodynamic origins of the effect. The Review concludes with an overview of the practical applications that electro-rheology has and may have in the future.

The electromagnetic field scattered from an array of one-dimensional inhomogeneous, lossless dielectric slabs is calculated by an approximate method embodying surface, volume and multiple-scattering effects. Resolution cells (or pixels) within the array are assumed to be independent of each other. The scattered fields from a line of pixels are linearly combined with complex phase shifts appropriate to a simulated range direction. Range lines are then arranged in parallel strips to simulate an image plane that is directly related to the original test object. In this manner, the complex back-scattered field from several simple objects is approximated and used to produce amplitude images which demonstrate the effects of inhomogeneity and multiple scattering. Attention is paid particularly to the regime where the scale size of the object being illuminated is comparable to the wavelength. The authors discuss the relation of this simulation to existing imaging systems.

The authors point out that the all-angle eikonal picture recently proposed by Perrin and Chiapetta (1985) does have a small-angle approximation implicit in it. Numerical comparisons for optical scattering from perfect homogeneous dielectric spheres are presented. The results confirm that the eikonal picture does involve a small-angle approximation. Comparisons of the eikonal picture predictions are also made with another small-angle approximation, namely the high-energy approximation.

When holographic interferometry is applied to the determination of the deformation of an opaque body, it is usually assumed that the displacement of each point is very small. In this paper, the authors discuss the basic concepts that open the way to an investigation of large deformations by means of holographic interferometry.

The use of holography in the optical inspection and measurement of three-dimensional objects, now known as hologrammetry, places considerable emphasis on geometric precision and high resolution in the reconstructed real image. A discussion of the factors which dictate image fidelity in reconstruction must include an assessment of potential sources of optical aberration. In this paper the authors consider the aberrations introduced when the refractive index of the medium in which the subject is located differs from that in which the real image is replayed, addressing specifically their quantitative evaluation in relation to underwater hologrammetry.

A Cerenkov free-proton laser is proposed. Analyses shows that the interaction length of a Cerenkov free-proton laser (CFPL) may greatly exceed that of a Cerenkov free-electron laser (CFEL). The grain of a CFPL can thus be expected to be higher than that of a CFEL. In addition the energy loss in a CFPL due to bremsstrahlung is much smaller than that in a CFEL, so the requirement on the beam quality can be reduced.

Taking advantage of a new non-linear technique, the authors have analysed the pulse-to-pulse fluctuations of the mode size and pulse length of the picosecond pulses delivered by a mode-locked neodymium-doped yttrium aluminium garnet laser. The technique can be exploited to process, in real time, laser pulses independently of their pulse length.

Laser heating of field-desorption emitters is evaluated against conventional resistive heating by comparing mass spectra of organic polymers (polypropylene glycol, polystyrene and polymethyl methacrylate). The temperature dependence of such spectra allows comparison at equivalent temperatures of the mass spectrometric sensitivities obtained with the two heating methods. No significant differences in sensitivity are observed. The power required for laser heating is 85% of that required using resistive heating to produce equivalent field desorption conditions.

The one-dimensional Stefan problem of heat conduction is solved including evaporation cooling i.e. for conditions similar to those occurring in vacuum arcs, when the current constricts. For high values of heating power density a quasi-stationary state is found, i.e. when the melting depth does no longer increase despite further heating. The dependence of this maximum on the heating power density and duration is discussed for copper.

The authors analyse experimentally the relation between the length scales characterising the heterogeneity of a porous medium and the tracer dispersion coefficient D at a velocity U. Experiments are performed on two-dimensional lattices of millimetric channels. For a partly connected network with a percolation geometry they find D=Ul_d log(Ul_d/D_m)l_d is close to the percolation correlation length and much larger than the individual channel size. This variation is in agreement with the Saffman model for mechanical dispersion. Experimental curves are non-Gaussian and no U² contribution to D associated with zero-velocity zones could be detected because of the long exchange times with them. For a well connected network with channels of variable width, one has D=Ul_d+U² tau ;l_d is close to the individual channel length, and tau is close to the value for Taylor dispersion in capillary tubes of similar diameter.

A criterion for the positive-ion velocity at the boundary of an electronegative plasma is discussed. The case considered is that in which the negative-ion density obeys the Boltzmann relation. The appropriate physical solution is determined in circumstances where a triple-valued mathematical solution is found. The work has much in common with models of two-electron-temperature plasmas. Under conditions where the theory is valid, the ratio of negative-ion density to positive-ion density can be determined from Langmuir probe characteristics.

In the past, it has been presumed that the main-gap breakdown in a trigatron is caused by a precursor spark channel occurring either at the trigger gap or between a trigger electrode and oppositely located main electrode. Based on the detailed analysis of voltage and current waveforms at the trigger gaps of single- and double-mode gas trigatrons, the authors have proposed an interaction between the main gap and the trigger gap under favourable conditions. This interaction gives rise to two or more auxiliary spark channels which develop between different over-volted electrode pairs in a predictable manner. The enhanced UV illumination and electric field distortion as a result of these multiple auxiliary arcs lead to the development of a streamer breakdown between the main electrodes.

The effective ionisation coefficient alpha /p₂₀(= alpha /p₂₀- eta /p₂₀, alpha and eta being respectively the Townsend first ionisation coefficient and the electron attachment coefficient) and the total secondary ionisation coefficient gamma _T for SF₆-air mixtures have been measured by the steady-state Townsend method for 150 V cm^-1 Torr^-1 <or=E/p₂₀<or=2000 V cm^-1 Torr^-1. The results show that alpha /p₂₀ decreases with decreasing SF₆ percentage partial pressure k for E/p₂₀<or=220 V cm^-1 Torr^-1 and increases with increasing k above this E/p₂₀ value. The alpha /p₂₀-values of the present mixtures are summarised in a form of formula for the prediction and simulation of the ionisation current development and breakdown in the mixtures. The inverse current plots according to Gosseries are studied for the present measurements, but they do not give any evidence of pronounced non-equilibrium which could cast doubt on the validity of the measurement at high E/p₂₀.

The dependence of the minimum initial electron density necessary to form a volumetric discharge in a high-pressure CO₂ laser on the pressure has been estimated. The electron density produced by a sliding discharge at pressures up to 8 atm has been measured. The measured electron density is higher than the minimum value of the initial electron density. The frequency of the channels in a multi-channel sliding discharge at high pressure has also been investigated.

A study has been made of the electrical properties of CdTe thin films prepared by chemical vapour deposition (CVD) using the direct combination of cadmium and tellurium on foreign substrates as a function of the deposition parameters. The experimental results indicate that the conductive type and the carrier density are not dependent on the Cd/Te molar ratio in the gas reaction mixture or on the doping concentration; they are controlled mainly by substrate temperature. The films are p-type at substrate temperatures above 550 degrees C and n-type at substrate temperatures below 520 degrees C. The carrier density varies with the substrate temperature. These conclusions are significantly different from previous reports on CdTe films prepared by the same method. In addition, the effect of different substrate materials on the electrical properties is discussed. A comparison between two kinds of CdTe films prepared by close-spaced vapour transport and by chemical vapour deposition using the direct combination of Cd and Te has been made. A convenient method of CdTe films deposited by CVD has been provided.

CW CO₂ laser annealing of boron-doped polycrystalline silicon at various substrate temperature, from room temperature to 600 degrees C, is reported. The overall improvement in charge carrier conduction of polycrystalline silicon is shown to occur at a substrate temperature of 200 degrees C, where the maximum improvement is observed in the charge carrier mobility. These changes in the electrical properties of polycrystalline silicon are correlated with the structural modifications occurring due to laser annealing, as observed by scanning electron microscopy and X-ray diffraction. The simultaneous changes in the electrical and structural properties of polycrystalline silicon are thus carefully studied and reported.

The relationship between photoconductivity sigma _ph and light intensity l expressed in the form sigma _ph varies as l^beta with beta in the range of 0.5<or= beta <or=1.0 was first explained by Rose. This paper presents an analysis to generalise Rose's original expression in two ways. Firstly, the requirement for an exponential distribution of gap states between the Fermi level and the conduction band edge has been relaxed to include the contribution of gap states at any energy level. Secondly, an occupation factor has been added to take into account to dependence of the recombination rate on the densities of the free electrons and holes. The analysis is based on asymmetrical gap-state distributions and the charge neutrality condition and it shows that the odd values of beta may occur in solids with gap states distributed discretely or continuously.

Photoconductive CdS:Cu, Cl thin films have been prepared by immersion of dip-deposited CdS films in an aqueous solution containing CdCl₂ and CuCl₂ followed by a high-temperature bake in air. The films have a high photosensitivity in the visible range and show photo-electronic behaviour typical of such films prepared by traditional techniques. Evidence of coalescence of the grains as a result of the CdCl₂ treatment was found.

Composite catalysts have been specifically designed for use in a reactor which uses electromagnetic power to heat the catalyst. The DC and AC conductivity ( sigma ) of materials comprising high-alumina cement, alumina and one of the mixed conductors K_xWO₃ (0.5<or=x<or=0.7) or K_0.5Mg_0.8Ti_0.2O₂ was studied between 400 and 900 K in inert and reducing atmospheres. For the K_xWO₃ composite, pre-reduction caused an increase in sigma _e but a decrease in sigma _i above 600 K at 1592 Hz. For the second composite, slight decreases in both sigma _e and sigma _i were observed after reduction and K⁺ ions dominated the loss mechanism at all temperatures. O^2- ions may also contribute to conductivity above 600 K. Detailed examination of the alkali bronze composite between 10^-2 to 10⁴ Hz as a function of temperature revealed a behaviour typical of many solid dielectrics. A change in loss mechanism from a region of dominant lattice response, to one dominated by thermally activated charge hopping was observed. Both materials have sufficient AC conductivity to absorb energy from an RF field and act as efficient catalysts for the steam reforming of heavy oils at 1000 K. An enhanced selectivity towards higher production of olefins may be related to the mobility of electrons and ions being much higher than those in conventional reforming catalysts.

The mechanical and electrical properties of elongated polypyrrole film have been investigated. The maximum elongation ratio of electrochemically prepared polypyrrole film increased on decreasing the elongation speed and reached 0.55. The simultaneous measurement of tensile stress and electrical resistance of the film during elongation has also been carried out. The approximately constant resistivity of the film by weight or volume was observed during measurement at slow elongation speeds; this provides evidence that the polypyrrole is a low-dimensional conductor. The electrical conductivity of the elongated and relaxed films increased in proportion to the square of the elongation ratio. The films with an elongation ratio of over 0.25 exhibited anisotropy in the conductivity.

The authors describe a novel technique for investigating the motion of free charge in insulators. It involves measurements of the charge transfer in a series of contacts with a (preferably liquid) metal and, simultaneously, the mean distance of the charge from the insulator surface. The technique can detect charge migration over very small distances and it provides a powerful means of elucidating mechanisms of conduction; there is no possibility of confusing free-charge motion with polarisation changes due to e.g. dipole orientation. Their experiments on polymers reinforce previous suggestions that the polymers contain both permanent traps and 'shallow' traps which retain charge for a relatively short time. They suggest, however that this distinction may reflect different degrees of isolation of the electron traps in a relatively loose and mobile network of polymer molecules, rather than differences in their energy.

Inter-facial processes and charge-transport mechanisms in a segmented resistor have been investigated using single- and double-layer resistive test structures. Microstructure studies of the cross section of the specimens supported by elemental mapping using energy- and wavelength-dispersive X-ray analysis show that, in dual-layer structures, RuO₂ agglomerates in a thin region near the interface and the current flow takes place principally through this region. From the Arrhenius plots of the specimens combined with Mott's T^1/4 law, it is found that the conduction mechanism at the junction of two cofired thick resistive films is governed by the mechanism of the lower-sheet-resistivity film.

Tungsten trioxide (WO₃) films prepared by spray pyrolysis at various temperatures and on various SnO₂:F substrates were investigated by means of X-ray diffraction and the results correlated to the electrochromic behaviour of the films. Films deposited onto polycrystalline SnO₂:F films at a substrate temperature of 420 degrees C showed an electrochromic transmittance change of 90% in the near infrared. This strong colouration was associated with a randomly oriented film structure and a large crystallite size of the WO₃ films.

The rectified Barkhausen noise envelope, induction rate dB/dt signal, and hysteresis curve have been measured simultaneously for an annealed and plastically deformed mild steel specimen. The two peaks in the Barkhausen noise envelope of the annealed specimen occurring in the knee regions of the hysteresis curve are due to domain nucleation and annihilation, while the single-peak in the Barkhausen noise envelope of the deformed specimen at the coercive field is due to the dislocation pinning occurring during irreversible wall displacement. The increase of the coercive field with the plastic strain is attributed to the increase in the dislocation density. The strain dependence of the Barkhausen noise energy is explained in terms of the energy released during domain nucleation and annihilation, and the hysteresis loss in terms of pinning effects of a moving wall. The variations of the frequency spectrum with the plastic deformation and magnetising frequency are characterised by the Barkhausen noise exhibiting a clustering and overlapping nature.

The AC potential drop technique is widely used in non-destructive testing for sizing surface-breaking cracks in metals but, in order to find the through-thickness penetration of a crack, its inclination to the surface is needed as well as its size. By modelling the electric field around an inclined crack in the extremes of both large and small skin depth, it is shown in this paper that the asymmetry in the voltage signal across the crack can be used to find the angle of inclination. Experimental results obtained from a range of metals confirm the theoretical findings.

Some important aspects of a little-known equation in electrostatics are discussed. The equation in question expresses the relationship between the field parameters and the differential geometry of equipotential surfaces. It is suggested that the equation be referred to as Green's differential equation.

This Letter corrects the erroneous impression, created in a recent publication (see V. Peoglos, ibid., vol.21, p.1055, 1988) that in the authors' experiments with an electrodynamic impulse pendulum they measured the magnitude of the magnetostatic force applied to the pendulum.

The results of an experimental study of the formation of micropinches (MP) in a plasma focus discharge in the presence of small impurities of heavy gas are reported. MPS of 10-100 mu m in size are observed and the X-ray spectra of multicharged ions of the impurity (Xe) are registered.

A coaxial plasma plug igniter is shown to function as a gas pump when operated repetitively at frequencies up to 30 Hz. Measurements of the rate of gas entrainment when the igniter is injected into various tubes are in excellent agreement with previous measurements of the momentum imparted to the surrounding gas by an igniter. Steady velocities of the order of 1 m s^-1 are achieved in the vicinity of the igniter. It is believed that this ability to generate steady flows may be useful in furnaces and in some other combustion applications in that it should improve ignition performance with fuel rich or stratified fuel-air mixtures either by directly drawing in air via a tube or by stirring the mixture in the combustion chamber.

A method is presented by which the proportion of substituting Ge⁴⁺ ions that are on octahedral sites can be determined in epitaxial magnetic garnet films. For (YSmCa)₃(FeGe)₅O₁₂ composition films 6.5% octahedral substitution was found.

A gas field ion source with a fine protrusion, a 'supertip', on top of a W (111) emitter tip has been operated for a period of 170 h at an angular current density of 8 mu A sr^-1 He⁺. The current stability, at a level of 2.5 nA, has been found to be typically of the order of +or-1%, with very little occasional need for operator intervention.