Table of contents

The electronic energy levels of polyaniline (PANi) thin films have been investigated using UV/VIS/NIR spectroscopy and X-ray photo-electron spectroscopy. The experimental results have been compared with the latest theoretical band structure calculations for the polyemeraldine salt form of PANi, and also the various structural and electronic changes thought to occur during electrochemical oxidation of the polymer.

Molecular structure-preparation condition relationships are reported for electrochemically prepared polypyrrole-toluene sulphonate electrically conducting films. Polypyrrole films have been prepared from aqueous solutions employing a range of anodic potentials (0.5-1.6 V versus SCE) and at several temperatures. Consistently films grown using higher potentials and at lower temperatures exhibited greater electrical conductivities. The molecular organisation in these series of films was evaluated using quantitative X-ray scattering procedures. The X-ray scattering data show that polypyrrole molecules adopt a planar-type conformation in which the planes of the pyrrole moieties have a preferred orientation with respect to the electrode surface. This anisotropy is enhanced in samples prepared at low temperatures or at high anodic potentials, and such anisotropic films have greater electrical conductivity. Structural mechanisms for these observations are discussed.

The diffusion of oxidising dopant gases into polyacetylene films has been studied. The process is complicated because it is influenced by the doping reaction, by swelling of the polymer by the gas and by degradation reactions of the polymer with the counter-ions. Electrochemical measurements of anion and cation diffusion in doped polymer give diffusion coefficients comparable with those for gaseous dopants. Diffusion coefficients for ions in polythiophene are reported. The results are discussed in terms of potential applications of these polymers.

Photo-induced absorption (PA) is observed for the polydiacetylene PDA-10H in the infrared which is found to be consistent with the photogeneration of charged bipolarons. No comparable PA is seen in PDA-TS, a result that is consistent with photo-excitation leading to the formation of triplet excitonic bipolarons. Persistent effects are observed for PA in PDA-10H indicating a long carrier lifetime due to trapping. Electron correlation is significant and shifts the electronic transitions to lower energy in PDA-10H so that overlap of electronic and vibrational transitions occurs. This overlap leads to strong anti-resonances in the PA due to electron-phonon interaction.

Conjugated polymers such as polyacetylene form a group of model semiconductors in which the 'one-dimensional' character of the polymer strongly modifies the behaviour of charges added to the chains from that usually observed in three-dimensionally bonded semiconductors. First, the polymer chain can undergo local reorganisation of the pi -electron bonding in the vicinity of a charge added to it. This has the effect of localising the charge in a state that can be described as a soliton for the particular case of trans-polyacetylene, and more generally as a polaron. Secondly, the motion of charged excitations is highly anisotropic, with strong confinement to the chain through the 'polaronic' structural relaxation of the chain. This anisotropy strongly affects charge transport and charge separation following photo-excitation. The authors review some of the recent work in this area and present polarisation-dependent measurements of photoluminescence (PL) and photo-induced absorption (PA) on highly oriented films of polyacetylene and poly(phenylenevinylene). They find that charge separation, detected through PA from photogenerated solitons or bipolarons, is the result of inter-chain electron transfer, whereas intra-chain excitation leads to charge confinement, and decay by PL (for poly(phenylenevinylene)) and by non-radiative channels. They find furthermore that these non-radiative channels become more efficient as the length of the conjugated polymer chain increases, and consider that this occurs through rapid motion of the excited state to recombination centres. They have identified both monomolecular and bimolecular kinetics for these processes.

Co-polymers of styrene-maleic anhydride have been synthesised with a number of different loadings of azobenzene-containing side groups and formed into thin films by casting and by the Langmuir-Blodgett technique. By illumination with polarised white light, appropriately loaded materials cast as films could be transformed from isotropic to uniaxial structures that exhibited dichroism and birefringence of Delta n=0.21.

The authors review the results of extensive studies that have been reported by the author and by other workers on the preparation and properties of the conjugated polymer poly(p-phenylenevinylene) (PPV). The use of the sulphonium polyelectrolyte precursor-route synthesis gives considerable flexibility for materials processing and it is shown that high-quality samples of controlled morphology, crystallinity, orientation and length of uninterrupted conjugated sequences can be obtained through a suitable choice of preparation conditions. Where applicable, the experimental results are compared with those obtained from theoretical calculations and any discrepancies are discussed.

Electrochemically prepared films of polypyrrole have been studied as electrode coatings in an electrochemical cell with potassium ferricyanide as the electroactive species in solution. The electrode-solution interface was studied by AC impedance measurements over the frequency range 0.003-10⁵ Hz. A novel model circuit description of the electrode-electrolyte interface was devised, which consisted of seven elements (including a frequency-dependent Warburg impedance) for the bare platinum electrode and ten elements (including an additional Warburg impedance concerned with film oxidation and reduction) for the polypyrrole-coated electrodes. The values of the circuit element were calculated by computer fitting and excellent agreement was found between measured impedances and those predicted from the calculated circuit element values. Where parameters could be calculated from known physical properties of the system (e.g., the Warburg parameters) these agreed well with the fitted values. Model circuit element values were calculated for the bare platinum electrode and for polymer-covered electrodes for polypyrrole films of four different thicknesses. The most striking result is the large drop in the charge transfer resistance for the ferricyanide reaction when the electrode is covered with the polypyrrole film. The large drop in this value is partly explained by the increase in surface area (which from other parameters can be estimated to be 40-fold), but might additionally reflect electrocatalytic properties of the polypyrrole surface.

Measurements are reported on the mass spectrum of an Au-Si alloy liquid metal ion source with various alloy compositions and values of total source current. To obtain a high fraction of Si ions, it is best to use an alloy composition with a higher silicon content than that of the eutectic composition. It is shown that the fraction of Si²⁺ ions is independent of the total source current, whereas the Si⁺ fraction increases with increasing current. The eutectic composition and the temperature of the solidus line were determined to be (18.0+or-0.5) atomic per cent and (362+or-4) degrees C, respectively.

A comprehensive model of ozone generation in dielectric barrier discharges is presented. The model combines the physical processes in the micro-discharges with the chemistry of ozone formation. It is based on an extensive reaction scheme including the major electronic and ionic processes. The importance of excited atomic and molecular states is demonstrated. Theoretical limits are given for the ozone production efficiency and the attainable ozone concentration. The most important parameters influencing the performance of ozonisers are identified. All theoretical predictions are compared to measured data.

A new method for generating an image of the unwrapped phase is discussed which is based on real-zero conversion of the data. The images produced by this method are of value in reducing the effects of clutter and identifying strong scatterers.

A three-dimensional transmission line matrix (TLM) routine has been developed to model the effects of heat flow in structures comprising conducting metal films on insulating substrates. It is shown that the component of heat flow within the conductor itself is significant even for very thin layers of metal. The effects of thermal diffusivity are investigated within the millisecond time range and the implications for electrical fuse applications are discussed.

The technique of transmission line matrix (TLM) has been used to model the variation of temperature versus time for a silver conductor on silica and alumina substrates under conditions of near-adiabatic heating. Under conditions where changes in the thermal diffusivity of the substrate are large there can be a significant departure from the I²t^1/2 heating law.

A failure mode has been observed in metal film resistors when they are subjected to single shot high voltage pulses. The conductor fails at a high field point which may be inherent in the design or may be due to manufacturing defects. The localised nature of the failure leads to the conclusion that there is a closed loop mechanism: the decrease in thermal diffusivity of the structure with rising temperature leads to an increase in the rate of temperature rise.

The time evolution of the zero-field distribution functions and transport parameters of electrons injected into some atomic and (electronegative or non-electronegative) molecular gases are calculated. The method used is based on a numerical solution of a time-dependent Boltzmann equation written for the case of infinite space. The accuracy of this method is checked initially on model gases. The relaxation time in the short time scale is then obtained for noble gases (Ar, Xe, Ne and Kr) and compared to some data available in the literature. Finally, the authors show that the distribution function in SF₆/buffer gas mixtures is not Maxwellian as usually assumed.

The drift velocity and the longitudinal diffusion coefficient have been measured for Li⁺ ions in He and 296 K and for 7<or=E/n₀<or=180 Td, using a Tyndall-Powell type double-shutter drift tube with variable length. Above 40 Dt, the results disagree with previously published data. The disagreement can be traced to an inadequate analysis of the experimental arrival time spectra in the earlier works. The present data are however in good agreement with predictions based on ab initio interaction potentials, at high values of E/n₀.

The impurity level in the central plasma of fusion devices can be almost entirely controlled by the properties of the edge plasma. Progress on better understanding and control of the impurity problem requires improved diagnosis of the edge, including probes. The impurity fraction in the edge plasma is likely to be substantially greater than in the central plasma. The influence of high impurity levels on the interpretation of the probe data is examined. It is found that: (a) measurements of n_e and T_e using a Langmuir probe are not very greatly affected even by high levels of the type of impurities likely to be present in the edge plasma; (b) measurements of ion temperature using the combined Langmuir probe/heat flux probe technique can be compromised by the presence of impurities; (c) measurements of ion and electron temperature using a gridded energy analyser usually should not be compromised by the presence of impurities; (d) the measurement of hydrogenic density n_DT, using a trapping probe should not be greatly affected by even large impurity levels provided that the hydrogen is not released during exposure by sputtering or surface heating; (e) increased sputtering due to the presence of impurity ions can pose practical problems.

A low pressure (2 mu bar) silane plasma has been obtained with a hot cathode DC glow discharge and electrostatic confinement. The plasma parameters have been measured with a Langmuir probe and their dependence on confining potential for three different anode sizes have been obtained. The dependence of plasma parameters on discharge current has also been studied. It has been shown that the plasma density increases significantly with the confinement and this increase is attributed to the reflection of primary electrons at the confining electrode.

Ion currents collected at the metal wall connected to the cathode in a high-current (2 to 20 kA) vacuum arc are experimentally studied. Arcs are burned by exploding a Cu wire between a 60 mm diameter anode and a 30 mm diameter cathode made of various materials in a demountable chamber. The wall draws a larger ion current from a Cu cathode than from a Zn cathode and a smaller current than from a Mo cathode. The anode melting is caused for an 8 kA arc in a 4 mm gap between the Cu anode and the Mo cathode. A high-speed movie shows that the diffuse-arc mode appears in the same gap configuration between the Cu anode and the Zn cathode up to 15 kA. The anode-mode transition is found to occur when the collected ion current attains a certain value independent of the cathode material. The behaviour of the wall ion current associated with the ion generation between the anode and the arc plasma is briefly discussed, proposing a new idea for modelling the anode spot formation.

An asymptotic method for analysing steady corona discharges is described. Formulae are obtained for curves of current versus voltage for the discharge between coaxial concentric and accentric cylinders.

A wall-stabilised argon arc (35 A) submitted to recurring decay and reignition is disturbed during its decay phase by a high-voltage peak which includes some electrical parameter variations. The spectral observation of this event is presented. This study shows that the arc conductance variation for a short voltage pulse arises from electron heating.

The thermal conductivity of the amorphous alloy Fe₄₀Ni₄₀P₁₄B₆ is investigated with the aim to separate out the lattice contribution and to compare it with that of amorphous dielectrics. The problems of the influence of the radiative transfer of heat between the sample and its environment, which causes errors in the determination of thermal conductivity, are discussed. The authors found that the temperature dependence of the lattice thermal conductivity of the alloys is qualitatively the same as that of amorphous dielectrics.

Extrinsic photoconductivity measurements at 70 and 293 K in Cr-doped SI GaAs bulk crystals have been carried out in order to characterise the deep levels in this material. The photoconductivity spectra reveal two characteristic structures extended in the low-energy and the high-energy regions, respectively. It is shown that low-energy structure is connected with the Cr²⁺ deep acceptor level and the high-energy one with the EL2 deep donor level. The spectrum measured after an intentionally photo-induced transfer of EL2 into its metastable state confirms the authors' interpretation of the origin of the high-energy structure. They believe that their study is the first to identify the EL2 contribution to the photoconductivity spectra of Cr-doped SI GaAs. The optical ionisation energy with respect to the conduction band and the Frank-Condon parameter of Cr²⁺ centres are obtained from the experimental results.

Measurements of temperature dependences of conductivity for morphologically different samples of reticulate doped polymers containing dentrite-like microcrystals of a conducting charge-transfer (CT) complex demonstrate that there is no difference in the mechanism of charge carrier transport along dendrite branches, perpendicular to them or across interdendrite boundaries. Comparison of the conductivity behaviour, as measured along polymer films and perpendicular to the surfaces ('sandwich' samples), suggest that the same mechanism of conductivity operates in both directions. It is also demonstrated that lowering of the CT complex content, even to 0.003 vol. fraction, leads to some decrease of the conductivity but does not change its temperature dependence. The results demonstrate the high connectivity of the crystalline CT complex network and support the hypothesis that continuity of this network is due to submicroscopic dendritic structures scaling from microns, probably down to tens of angstroms.

Quantitative data are reported on the surface nitridation of titanium, zirconium and hafnium samples by multi-pulse microsecond-pulsed high-intensity TEA CO₂ laser irradiation in nitrogen atmospheres containing oxygen impurities. The possibility of controlling the competition between oxidation/nitridation processes by recording the reflectivity of the pulsed laser action zone in the light of a supplementary, low-power CW CO₂ laser source is revealed.

A certain kind of attractor solution of the primitive equation model has been examined. Lorenz has shown that in a nine-component shallow-water model a subset of the quasi-geostrophic set is the attractor set of some specific parameter setting. The Lorenz parameter r has been redefined and shown to vary with the Rossby number and diffusivity parameter. The results of Lorenz's numerical experiments are shown to be valid for middle latitudes but not for tropics.

The use of a point/plane gap (point radius rho =8 mm, gap length d=5 mm) to study Paschen's law in SF₆ is examined with reference to a recent experimental investigation. For the gas pressure range of interest (0.25<or=p/bar<or=4), the present assessment indicates that the reported breakdown voltages cannot be associated with this law. The conditions necessary to ensure Paschen breakdown data are discussed.

A new method of studying non-ferromagnetic particles (1 mu <d<60 mu m) using magnetic liquids is discussed. The technique is straightforward and makes particle counting and size analysis simpler and more accurate.

A technique to grow single crystals of the high-temperature superconductor YBa₂Cu₃O_{7- delta} is described. The crystals are rectangular tablets up to 2*2*0.2 mm³ in size.

Quenching rates corresponding to both different wheel speeds and different materials during the forming process of amorphous ribbons have been measured directly by an infrared thermal photography technique. The results show that this technique is applicable to the measurement of rapid quenching rates. The critical cooling rates are also determined.