Table of contents

We describe a microelectrode device, fabricated using photolithography and laser ablation, that combines the electrokinetic effects of travelling wave dielectrophoresis and electrorotation. Here it has been used to concentrate and then assay the viability of Cryptosporidium parvum oocysts.

The effect of imaging environments and imaging parameters on the lattice-resolved AFM images of mica surfaces has been investigated. As the force-contrast of an AFM image is determined by the tip - surface interactions, the appearance of the AFM images changes accordingly when the force applied to the tip, the scanning direction, or the imaging liquid used is changed. All these factors can modify the details of the tip - surface contact situation so as to affect both the vertical repulsive and the direction-dependent frictional forces, resulting in the superposition of local distortions and other anomalies onto the atomically resolved images of the mica surface.

single crystals have been grown and characterized by experimental techniques such as high-resolution transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy and optical and electrical measurements. The samples were prepared in single-crystal form from a melt. The structural analysis indicates that has a hexagonal structure, and confirms the high quality of the produced single crystals. Quantitative information on electrical and optical properties of single-crystalline was obtained by investigating the resistivity and photoluminescence as a function of the temperature and excitation intensity.

Angle-resolved x-ray photoelectron spectroscopy (ARXPS) has been used to study surface compositional changes in GaAs (100) as a consequence of 1 to 5 keV ion bombardment. Prior to ion bombardment, the ARXPS measurements showed that neglecting surface contamination, the composition of the GaAs surface was close to its stoichiometric value of 1:1. After ion bombardment, the oxide layer was efficiently removed. At steady state the altered layers induced by 1 - 5 keV ion bombardment were, on average, Ga-rich up to the sampling depth of the ARXPS technique. The ARXPS measurements also showed that the depth profile of the altered layer was a function of ion energy. The altered layer induced by 1 keV ion bombardment was inhomogeneous as a function of depth and appeared richer in Ga on the surface than in the subsurface region, that by 3 keV ion bombardment was homogeneous and that by 5 keV ion bombardment was less Ga-rich on the surface than in the subsurface region. The results are discussed in the context of preferential sputtering, radiation-enhanced diffusion/segregation, and altered layer thickness dependence on ion energy.

Reflection polarization transformations by a given sample depends both on the composition and surface state and on the illumination angle. In the present work, the evolution of polarization transformation given by dielectric samples and metallic surfaces has been studied. The initial metallic surface was polished up to m and then progressively debased. The polarimetric characteristics are described by using the Mueller matrix, which provides the depolarization index, namely the mean depolarization power, and the degree of polarization for all the pure incident states of polarization. The polarization of each sample is measured for various angles of incidence. The noise is reduced by a statistical method to optimize the matrix elements. The results obtained are first presented in a global matrix form and then the depolarization phenomenon is analysed. In the last step the studied samples are characterized and classified.

A marked improvement is observed in the performance of a pulsed microwave-excited slab laser when the gas is permitted to expand following a discharge pulse. Gas pressure changes following a discharge pulse indicate heating followed by expansion and contraction stages. A sequence of over ten reciprocation cycles is observed after each pulse.

Two modes of operation (open and closed) were investigated. At high repetition rates, gas movement in and out of the discharge space in the open structure leads to a faster overall cooling and better laser performance than for a fully confined gas. At low repetition rates, no difference in performance was noted between the open and closed structures. Conduction cooling across a thin sheath is thus considered sufficient at low repetition rates.

An average output power of 20 W with 10% efficiency and about 1 kW peak power were obtained from the open structure at a gas pressure of 28 kPa and 1.5 kHz pulse repetition rate. The absence of expansion cooling in the closed structure resulted in about a 40% reduction in the output power and efficiency.

The results of this research suggest a new architecture for pulsed lasers that provides markedly faster gas cooling than that achieved for a stagnant gas in an identical discharge geometry.

The behaviour of , the saturation intensity averaged over the optical cavity width w of a transverse-flow laser, is discussed based on a numerical analysis of a laser of that type. It is shown that for any w the mean saturation intensity is almost independent of the cavity position x along the flow and remains nearly unaffected by the optical power extraction upstream of the cavity. Thus, similarly as for non-flow lasers, it can be used, in conjunction with the small-signal gain coefficient averaged over the cavity width, for prediction of the optical power available from a transverse-flow laser and optimization of the optical cavity position and width. For axial-flow lasers the available optical power can be predicted in the same way if the beam path along the discharge is taken for w.

A theoretical analysis of the energy balance in the laser - metal interaction zone is carried out. The heat transfer due to the recoil-pressure-induced melt flow is taken into consideration. It is shown that, for the absorbed laser intensities typical in welding and cutting, the recoil pressure induces high-velocity melt-flow ejection from the interaction zone. This melt flow carries away from the interaction zone a significant portion of the absorbed laser intensity (about 70 - 90% at low laser intensities); thus, convection-related terms can be ignored neither in calculations of the energy balance in the interaction zone nor in calculations of the thermal field in the vicinity of the weld pool or cutting front.

The thermal conductivity and diffusivity of lithium sulphate have been measured simultaneously, using the transient plane source technique over the temperature range 300 - 900 K. The thermal conductivity decreases slowly up to about 640 K, whereupon a distinct rise occurs, indicating the onset of a pre-transitional behaviour, which causes a continuous growth of the conductivity up to the structural phase transition at 851 K, whereupon a very sharp increase occurs. A similar behaviour has been observed for the thermal diffusivity, for which a very sharp dip occurs at the transition point due to the exceptionally large transition enthalpy. The pre-transitional behaviour of heat transport is associated with the librational disorder of the sulphate anions known from Raman scattering studies of both phases (and neutron scattering from the cubic phase), whereas the translational disorder of lithium cations is of hardly any importance. It is thus possible to link the `paddle-wheel' concept of ion migration in the cubic phase to the enhancement of heat transport observed in the `pre-transition' region, as well as to the large difference in heat-transport rates between the monoclinic and cubic phases.

A previous study concerning the drag force on a spherical particle in a plasma flow is extended to a more general case of combined specular and diffuse reflection at the sphere surface. It is shown that the drag force decreases with increasing fraction of specular reflection and that the effect of the fraction of specular reflection on the drag force acting on a non-evaporating particle is less significant than that on an evaporating one.

Plasma arc cutting can be characterized in terms of two distinct speeds. At cutting speeds above , the plasma jet does not cut through metal plate. At speeds below , the molten metal from the kerf sticks to the bottom of the plate, forming the so-called dross. In the first part of this work, both speeds, and , have been measured in a wide range of cutting parameters (currents, metal thicknesses and nozzle orifice diameters) for oxygen plasma arc cutting of steel. In the second part of the work, models by which to calculate and are presented. Comparison of calculated and measured values of allowed us to obtain the efficiency with which the arc power is consumed by the cutting process. It is shown that the efficiency rises as the cutting speed increases. It is suggested that the speed separating dross-producing and dross-free modes of cutting corresponds to a specific value of the Weber number. Calculations performed according to this hypothesis agree well with our measurements of .

A semi-analytic method of solution of one-dimensional electron and ion fluid transport equations for a steady-state magnetized discharge has been developed by generalizing a previous analysis of an unmagnetized steady-state glow discharge. A discharge normal to the magnetic field can operate in one of two modes, a positive space charge (PSC) mode, characterized by a strong cathode fall, and a negative space charge (NSC) mode, characterized by a broad anode fall. The transition from the PSC mode to the NSC mode as the magnetic field is increased or the pressure is reduced is demonstrated. The results are in qualitative agreement with results from a one-dimensional (in space) particle-in-cell simulation. The relation of these results to experiments in discharges such as magnetrons is discussed.

According to theory, a radial component of magnetic field in the driving region of a vacuum arc centrifuge should increase the force and the angular velocity. Experiments with different magnetic field configurations of a vacuum arc centrifuge have been carried out. Langmuir probe measurements have been made as well as cross-correlation measurements of angular velocities. Based on correlation measurements, the column angular velocity increased in the presence of a radial field. This increase was accompanied by a reduction in column radius and ion density. It is concluded that vacuum arc centrifuge designs with a radial magnetic field component should have an improved separative performance.

A theory of x-ray diffraction in crystals with electron density modulated due to acoustic wave propagation is developed for Bragg geometry of scattering. Expressions, describing modifications of diffraction spectrum as well as an integrated diffraction intensity are derived and analysed in certain extreme cases. Special attention is paid to the behaviour of diffraction intensity under a strong acoustic field applied to a crystal. The crucial role of x-ray absorption in acoustically induced diffraction effects is established. It was shown that acoustic modulation results in significant increase of diffraction intensity up to a limiting value, which is inversely proportional to the x-ray absorption coefficient.

Electron attachment to trichlorotrifluoroethane (1, 1, 2-) was investigated in buffer gases of and Ar using a high-temperature electron swarm apparatus. The negative ion intensity was also measured as a function of electron energy using an electron beam apparatus. The electron attachment rate constant, , was measured in the mean electron energy range, , 0.043 to 4.7 eV and over a temperature, T, range 300 to 700 K in the electron swarm experiments. The electron attachment rate constant was found to first increase slightly with increasing temperature and subsequently decrease for K. Our room-temperature electron beam study showed that 1, 1, 2- attaches electrons predominantly via dissociative negative ion states and revealed that the electron attachment cross section exhibits three main peaks: one at eV producing , a second one at eV due to and a third one at eV also producing . The electron attachment cross sections obtained using an electron swarm-unfolding technique show that the peak at eV observed in the electron beam study is actually composed of three peaks; one at eV and two more at higher electron energies but below 1.5 eV.

We describe the development of a comprehensive theory of thermoluminescence (TL) supralinearity and sensitization, the unified interaction model (UNIM), based on both radiation absorption stage and recombination stage mechanisms. The UNIM incorporates both the track interaction model (TIM) for heavy charged particles (HCPs) and the defect interaction model (DIM) for isotropically ionizing gamma rays and electrons, in a unified and self-consistent conceptual and mathematical formalism. The model is applied to explain the unique features of gamma-induced supralinearity and sensitization of peak 5 in LiF:Mg,Ti, especially the strictly linear, then supralinear behaviour and the dependence of the supralinearity on ionization density (gamma ray energy and particle type). Both features arise from a localized trapping entity (the track for HCPs, spatially correlated trapping centres and luminescent centres (TC/LC pairs) for gamma rays and electrons, which dominate the dose response at low dose and are not subject to intra-track competitive processes, thus leading to linear dose response behaviour. The decreasing efficiency of the competitive processes relative to the luminescence recombination processes, as a function of dose, leads to the supralinear behaviour. The decrease of the supralinearity with decreasing gamma ray energy (increasing ionization density) arises from the increasing probability of the TC/LC pair to simultaneously capture an electron - hole pair, leading to geminate recombination not subject to competitive processes. The UNIM is shown to be capable of yielding excellent fits to the experimental data with many of the variable parameters of the model strongly constrained by ancillary optical absorption and sensitization measurements.

This work presents new results on Cu - Ni oxides obtained by IR thermochemical laser processing of controlled mixtures of metal salts. The CuO and NiO oxides were characterized by several analytical methods, such as x-ray photoelectron spectroscopy, x-ray diffraction and IR spectrometry. Compared with the reaction products obtained in classical thermal decomposition, as well as with mixtures of the respective oxides, the laser-produced oxides exhibited peculiarities: different oxide fractions and smaller dimensions of crystallites were obtained in relation to the classical treatment, while the chemical composition was characterized by the presence of minority phases of metal suboxides.

A non-uniform AC electric field induces a motion in polarizable particles, called dielectrophoresis. The force responsible for this motion is governed by the dielectric properties both of the suspending medium and of the particles, as well as the geometry of the field. The dielectrophoretic properties of sub-micrometre latex spheres have been studied using micro-fabricated electrode structures. The electric field geometry for electrodes used in the measurements has been solved using numerical analysis. Measurements of the dielectrophoretic properties of the spheres have been made over a range of medium conductivities and applied field frequencies and strengths. Comparisons between the observed behaviour and that expected from theory are presented.

Dielectric and conduction effects in electrorheological (ER) fluids with ohmic conductivity of the host liquids and with an applied AC electric field are considered. It is found that the conductivity ratio , the dielectric constant ratio and the applied field frequency f are three important parameters which determine the ER effect. If , a stronger ER response is obtained with a low applied field frequency than with a high frequency. If , the opposite occurs. Empirical equations are given for the attractive force between the particles in an ER fluid and for the shear yield stress. The calculated current density is independent of the applied field frequency when the frequency is smaller than a critical value, but becomes a linear function of the frequency beyond the critical value. The predicted attractive force of two nearly touching polymer spheres in mineral oil is in good agreement with that measured; also the predicted shear yield stress of some common ER suspensions is in accord with measurements.

Theoretical models of the noise spectra of MESFETs have been researched based on experimental data with the aim of obtaining a correlation with the fluctuating conductivity and transconductance dispersion in terms of frequency, bias, temperature and surface state. The results obtained correspond well with the experimental data. Moreover, we show the influence of the transconductance dispersion on the surface state and some sources of 1/f noise.