Table of contents

Phase transitions of an electrorheological (ER) fluid were studied by measuring its shear specific acoustic impedance with increasing electric field. Two nematic phases were found for the first time in experiment and they confirmed the theoretical prediction. It is proved that measuring acoustic impedance is a unique way to study the structure formation in non-transparent ER fluids where the optical techniques are not available. The measuring results also demonstrate that ER fluids have potential of being active impedance matching devices.

Several applications of optical fibres for sensor and telecommunications require the polymer jacket to be stripped from the fibre, e.g. for the inscription of Bragg gratings. Here, we report an alternative approach to chemical and mechanical fibre jacket stripping based on laser ablation. It is shown that excellent quality removal is possible using the ultraviolet excimer and infra-red CO₂laser, provided the latter is tuned to a suitably strong absorption band in the polymer. Characterization of the ablation properties of the acrylic jacket material, and the optical and mechanical properties of the stripped fibre are reported.

We present new results on a single-layer organic alloy light emitting diode. The device shows significant improvement in lifetime at room and elevated temperatures. The improvement is attributed to the elimination of the heterointerface and the minimization of the formation of unstable tris-(8-hydroxyquinoline) aluminum (Alq₃ ) cations. The efficiency is comparable to those of heterojunction counterparts. Balanced bipolar carrier injection and transport are made possible by adjusting the alloy composition and doping.

Lead lanthanum titanate (PLT) thin films with excess PbO (from 0 to 20 mol%) were prepared by a metal-organic decomposition process. The ferroelectric properties and current-voltage (C -V ) characteristics of PLT films were investigated as a function of the excess PbO. Abnormal ferroelectric and C -Vproperties were observed in PLT films with excess PbO. The polarization against applied electric field (P -E ) hysteresis loops were pinched before saturation of polarization of the films, and C -Vcurves had four peaks instead of the two peaks found in the normal C -Vcurves. The abnormal level of the hysteresis loops and C -Vcurves deteriorate with increasing concentrations of excess PbO in the films. Electron probe microanalysis has revealed that there is excess Pb in PLT thin films. Auger electron spectroscopy has detected that the Pb accumulates at the interfaces between the thin film and the bottom electrode. Meanwhile, transmission electron microscopy has found that PbO nanocrystals on the interface between the PLT thin film and the bottom electrode, and clusters of vacancies and interstitials, in particular, exist in the PLT grains. Therefore, a part of the excess PbO may accumulate at the domain wall of the grains and the grain boundaries and the interface between the bottom electrode and film during the thermal annealing process of the films. Meanwhile, the oxygen vacancies of the grains will increase with the increasing concentration of the excess PbO in the films. The excess PbO and oxygen vacancies act as pinning centres and have a strong pinning effect on the domains. When the poling voltage is not large enough, part of the domains can overcome the force of the pinning, and abnormal ferroelectric and C -Vproperties were observed.

A method is described for calculating the impedance of conductor structures comprised of ferromagnetic and non-magnetic layers of arbitrary cross sectional shape with transverse dimensions which are small compared to the conductors length. The method is exemplified on a conductor of axial symmetry enclosed in a high permeability film of uniaxial anisotropy along the conductor axis. To find the impedance the full boundary value problem for the vector potential is solved, allowing driving and eddy currents in both the conductor and the magnetic film, and taking into account the dynamics of the magnetization governed by the Landau-Lifshitz equations. The results obtained are used to model the giant magneto impedance effect in ferromagnetically coated wires.

The effect of the substitution of Sm, Nd and Ce for Gd on the structure and magnetic properties of (Gd_1-x R_x )₃ Fe₂₈ Nb (R = Sm, Nd and Ce) compounds have been studied. The single-phase regions for R = Sm, Nd and Ce correspond to the composition range of 0x 0.2, 0x 0.5 and 0x 0.4, respectively. All of the compounds crystallize in the Nd₃ Fe_29-x Ti_x -type structure. The lattice parameters a , b , cand Vincrease with the increase of the Sm and Nd content, while decrease with an increase of the Ce content. Substitution of Sm, Nd and Ce for Gd leads to a decrease of Curie temperature (T_C ) and an increase of the saturation magnetization (M_S ). The compounds (Gd_1-x Sm_x )₃ Fe₂₈ Nb and (Gd_1-x Nd_x )₃ Fe₂₈ Nb exhibit planar anisotropy perpendicular to the [102] direction, the anisotropy field H_Aincreases with the substitution content, while in compounds (Gd_1-x Ce_x )₃ Fe₂₈ Nb, the planar anisotropy field H_Adecreases with an increase of the Ce content. (Gd_0.6 Ce_0.4 )₃ Fe₂₈ Nb exhibits an intermediate easy axis behaviour.

The magnetic properties of densely packed composite granular materials are strongly affected by interactions between the grains. This paper addresses the influence of the interactions, as a function of the packing density, on the initial susceptibility. Our calculations are based on systems with realistic physical microstructures which are free from an artificially imposed lattice. The introduction of particle elongation via demagnetizing factors due to prolate spheroids is shown to enhance the susceptibility and give results which are in good agreement with experimental observations of composite soft materials prepared by dispersing large Fe particles in a non-magnetic matrix.

In the present work the propagation of a fast ionization wave (FIW) within a long discharge tube filled by electronegative gas is investigated both experimentally and theoretically. In the theoretical model, the existence of the radial potential profile, the dependence of plasma parameters at each space point on processes in the whole tube and the temporal transformation of electron energy distribution function are taken into account. The parameters of the plasma in the wave front and behind it, the energy deposition in the discharge and the attenuation of the wave during FIW propagation are obtained. The problem of the initial electron concentration is considered.

Analytical expressions for the thermophoretic forces acting on two typical non-spherical objects, i.e. a cylinder and a flat plate, immersed in a plasma are presented for the extreme case with a thin plasma sheath and at great Knudsen numbers. A combined specular and diffuse reflection at the object's surface is included in the analysis. It is shown that the thermophoretic force is proportional to the surface area of the cylinder or the plate. The thermophoretic force increases with an increasing specular-reflection fraction of gas particles at the cylinder's or plate's surface. This prediction is appreciably different from that obtained previously for the case of sphere (Chen Xi 1999 Plasma Chem. Plasma Process.1933), where the thermophoretic force acting on a non-evaporating spherical particle was demonstrated to be independent of the specular-reflection fraction of gas particles at the sphere's surface. Some suggestions are thus proposed concerning how to check the present kinetic-theory analysis and to determine the values of the specular-reflection fraction of gas particles at an object's surface through the thermophoretic force measurements.

Using the Monte Carlo method, we study the behaviour of electrons as they move under the influence of a mirror magnetic field and a uniform electric field in positive column of a helium dc gas discharge. The symmetrical axis of the mirror magnetic fields is in a direction parallel to the uniform electric field. Some types of collisions (elastic, excitation and ionization collision) are considered. Graphs showing the electron density and the energy distributions, the angular and the energy distributions at the end of the positive column, and the percentage of electrons that reach the wall and the end of the positive column are presented. The results indicate that the mirror magnetic field can control the electron transport behaviour in the positive column. In the presence of the mirror magnetic field the electrons are restricted in the middle part of the positive column and the electron energy is reduced; the electron scattering angles are extended by the mirror magnetic fields. These results are in good agreement with experimental results.

The energy distribution function of the electron beam produced in a transient hollow cathode discharge is measured using a method which is based on the collector self-biasing, allowing an access very close to the extraction hole. The distribution obtained is a mainly polyenergetic one extending over a range of up to some tenths of kiloelectronvolts. It is composed both of runaway electrons, whose energies we assume are equal to the instant discharge voltage, and a main part composed of a monotonically decreasing energy distribution. The ratio between these two populations is around 10% and strongly increases with distance from the extraction hole.

Multiwall carbon nanotubes (MWCNTs) were fabricated by a new cathodic vacuum arc method, in which the anode itself was the vacuum chamber and was inert. The cathodic vacuum arc discharge was performed at 0.5 Pa of He and H₂gas atmospheres using a dc arc current of 100 A. The cathode materials were pure graphite and various metal-containing graphite electrodes. Microscopic observation revealed that MWCNTs were produced at the cathode crater generated by the cathode spot and on the surface of macrodroplets emitted from the cathode spot. A diamond-like carbon film that embedded the macrodroplets with MWCNTs was also deposited.

A review is made of the theories proposed for quantitative assessment of ozone generation in dc-energized electrostatic precipitators. The underlying simplifying assumptions adopted by these theories are discussed. Among the most important assumptions is ignoring the ozone destruction which takes place at high ozone generation rates. Therefore, a modification is proposed to account for ozone destruction in assessing the generation of ozone. Subsequently, a linear relationship between the ozone generation rate after being modified and the discharge power is obtained by using the present experimental results or the data published previously.

Nd substituted Tl-1212 type phase high-temperature superconductors with nominal starting composition Tl(Sr_1-x Nd_x )₂ CaCu₂ O₇(x= 0-1.0) have been prepared and characterized. The x-ray diffraction patterns indicate that Nd substitution for x= 0.1-0.5 showed a dominant 1212 phase and a 1201 minor phase. This system showed a metallic normal state behaviour for x 0.4 and semiconducting-like behaviour for x 0.5. Scanning electron micrographs indicate a similar grain size for the x= 0.1, 0.2 and 0.3 samples which is approximately 2-4 µm. The transport critical current density, J_c , for the superconducting samples (x= 0.1, 0.2 and 0.3) at various temperatures together with a self-field approximation model indicates that the characteristic length (L_c ) associated with the pinning force becomes lower with Nd substitution.

In this paper a model for the calculation of major and minor hysteresis loops in particulate systems is described. For the case of a particulate system with aligned easy axes, the model is based on a numerical solution of the rate equation which was modified to take into account particle size distribution and interaction effects. In addition, calculations of minor loops for systems with a distribution of orientation of easy axes have been made. In the model, interaction effects are represented using a mean field approach, i.e. H_iMwhere Mis the average magnetization of the system. The results show that for the non-interacting case, no minor hysteresis loops occur. Interaction formalism in terms of a mean field approach gives minor hysteresis loops only when H_iis positive. This result suggests that the interaction field should be proportional to M²so as to predict minor loops at positive and negative values of average magnetization similar to experimental observations. In general, the interaction field is an unknown function and should be expressed as a series expansion in terms of M . For the prediction of minor loops we assume that the interaction field is dominated by the quadratic term.

In the applications of thermoluminescence (TL) in dosimetry and archaeological dating, it is usually assumed that the measured TL depends on the total dose applied and it is independent of the dose rate. Thus, calibration of a TL specimen is usually performed at a significantly higher dose rate than that of the dose to be determined. A few experimental accounts in the literature report on dose-rate dependences of TL intensity for a given total dose. One theoretical work published gave a numerical solution of the simultaneous differential equations governing the filling of traps and centres at different dose rates. In the present work, the numerical solution is extended so that it includes the other important stage of TL, namely the heating phase. It is shown that with a rather simple model of one trapping state and two kinds of recombination centres, dose-rate effects may occur. An appropriate choice of the relevant parameters indeed yields one emission, which increases with increasing dose rate, whereas another emission decreases with the dose rate with a constant total dose, in agreement with an experimental result in quartz quoted in the literature.

A new method for the determination of the bulk lifetime of minority carriers in silicon is presented. By applying a dry polymer (poly-epichlorhydrin-co-ethyleneoxide-co-allyl-glycylether with iodide/iodine) to silicon and exerting a negative potential to the silicon, an increase of the lifetime of photogenerated charge carriers (from 25 to 92 µs), measured by microwave conductivity techniques, is observed. This effect is caused by the electric field, which pulls minority carriers into the interior of the silicon wafer so that surface recombination can only continue on the back contact of the wafer. It is shown how, from this electropassivation effect, the bulk lifetime of silicon (0.81 ms in this case) and the effective diffusion constant can be obtained. Space-resolved images of minority carrier lifetimes show that it is difficult to get a homogeneous polymer-silicon contact at the whole wafer surface. Minority carrier lifetime against potential curves show that only polished silicon surfaces can be perfectly electropassivated. The procedure presented has potential for development as a technique for wafer characterization.

The UV-spectrum of a high-current slipping surface discharge radiation has been investigated experimentally by means of a liquid chemical actinometry technique. The photon fluxes emitted in the hard UV ( <190 nm) and soft UV (190 nm< <430 nm) regions were determined by the H₂ O₂yield in irradiated distilled water and by the Fe II yield in an irradiated solution of K₃ Fe(C₂ O₄ )₃ , respectively. The results obtained lead to the conclusion that the spectrum differs from the Planckian one by the presence of an `enriched' short-wavelength ( <190 nm) component.

A rate-equation model with greatly improved quantitative rigour is detailed for the CW optogalvanic effect on the 1s-2p transitions of neon atoms in the positive column of a dc normal glow discharge. This work constitutes one part of a wider complementary programme which also includes CW laser collisionally-induced fluorescence, optical emission spectroscopy and optical absorption spectroscopy for the excited-state populations, all employing the same atomic and discharge data set. Our aim has been to produce a theoretical model and test it with stringently collected data, to demonstrate that tunable laser CW optogalvanic spectroscopy (OGS) can provide a truly quantitative diagnostic of the excited-state kinetics in low-temperature discharges. The model is deliberately restricted to just six essential perturbed rate equations, four for the 1s states, one for the charged particles and one for the discharge current. Our formulation, via the 1s and 2p CW-induced pump rate perturbations, allows very direct identification of the important excited-state kinetics producing the OGE. The principles and scope of our theory are demonstrated for a neon filling pressure of 2.0 Torr and currents of 1-10 mA, based on fitting the model for three carefully selected transitions, the 1s₅ -2p₄(594.5 nm), 1s₄ -2p₄(609.6 nm) and 1s₅ -2p₉(640.2 nm). Results show that the magnitudes of the CW optogalvanic effect on the 1s-2p transitions are strongly dependent on the discharge pumping rates of the 1s states and their coupling, and confirms that cascade effects must be included in the 1s excitation rate coefficients.