Table of contents

Atmospheric-pressure, non-equilibrium plasmas are susceptible to instabilities and, in particular, to arcing (glow-to-arc transition). Spatially confining the plasma to dimensions of 1 mm or less is a promising approach to the generation and maintenance of stable, glow discharges at atmospheric-pressure. Often referred to as microdischarges or microplasmas, these weakly-ionized discharges represent a new and fascinating realm of plasma science, where issues such as the possible breakdown of 'pd scaling' and the role of boundary-dominated phenomena come to the fore. Microplasmas are generated under conditions that promote the efficient production of transient molecular species such as the rare gas excimers, which generally are formed by three-body collisions. Pulsed excitation on a sub-microsecond time scale results in microplasmas with significant shifts in both the temperatures and energy distribution functions associated with the ions and electrons. This allows for the selective production of chemically reactive species and opens the door to a wide range of new applications of microplasmas. The implementation of semiconductor and microelectronics and MEMs microfabrication techniques has resulted in the realization of microplasma arrays as large as 250,000 devices. Fabricated in silicon or ceramics with characteristic device dimensions as small as 10 µm and at packing densities up to 10⁴ cm⁻², these arrays offer optical and electrical characteristics well suited for applications in medical diagnostics, displays and environmental sensing. Several microplasma device structures, including their fundamental properties and selected applications, will be discussed.

Magnetoimpedance (MI) and hysteresis loops were studied in electroplated Ni_79.1Fe_20.9/Cu, Ni_75.64Fe_24.26Ru_0.10/Cu, Ni_79.51Fe_20.31Ru_0.18/Cu and Ni_75.99Fe_23.66Ru_0.34/Cu wires. Scanning electron micrographs showed that crack-free electroplated wires were obtained with a smooth surface. The average grain size varied between 3 and 10 µm for all samples. It was found that circumferential anisotropy was induced in all samples during the electroplating process. The maximum MI response was 459% at a frequency of 90 kHz, 361% at 110 kHz, 157% at 100 kHz and 114% at 340 kHz for Ni_79.1Fe_20.9/Cu, Ni_75.64Fe_24.26Ru_0.10/Cu, Ni_79.51Fe_20.31Ru_0.18/Cu and Ni_75.99Fe_23.66Ru_0.34/Cu wires, respectively.

Sm(Co_balFe_0.1Cu_xZr_0.03)₇ (x = 0.0–0.25) ribbons have been prepared by melt spinning. The effects of annealing parameters on coercivity and its temperature dependence have been studied systematically. It is found that the melt-spinning technique remarkably improves the magnetic properties and simplifies the annealing process. The high-performance precipitation-hardened magnets can be obtained by only short-time ageing and slow cooling from 850 to 400 °C, without the standard solid solution. More interestingly, the temperature coefficient of coercivity of the ribbons can be tuned through adjustments of the processing parameters.

The effect of ghosting (radiation-induced changes in sensitivity) in amorphous selenium (a-Se) is a serious hurdle to enhancing parameters of flat-panel imagers on the base of this material. We present the results of experimental investigation of ghosting by measuring the kinetics of current generated by short x-ray pulses in a-Se as well as the theoretical model explaining the phenomenon in terms of recombination of holes with trapped electrons. An alternative model assuming re-distribution of electric field in a sample due to space change phenomena is also analysed theoretically and is shown to contradict the experimental data. By comparing theoretical formulae with experimental results, the lifetimes and mobilities of both types of carriers are determined.

The correlation between ultraviolet and infrared emission in Ni-doped SiO_x (1 < x < 2) films has been investigated. Due to the presence of nickel, the maximum intensity of UV emission was obtained after annealing at 800 °C, which was explained by the formation of oxygen excess defects in the oxireduction reaction between Ni and SiO_x. However, the intensity of infrared emission increases with the increase of annealing temperature and so does the mean size of SiO₂ clusters characterized by scanning electron microscopy. An oxireduction reaction model was proposed to explain the correlation between ultraviolet and infrared emission in this system. The mean size of Si nanocrystals annealed at 1000 and 1100 °C was obtained from Raman spectra, which is in good agreement with the estimation by the PL peak shifts, based on quantum confinement effects.

The purpose of the present study was to examine the possibility of laser-machining of CuInSe₂-based photovoltaic devices. Therefore, ablation thresholds and ablation rates of ZnO, CuInSe₂ and Mo thin films have been measured for irradiation with nanosecond laser pulses of ultraviolet and visible light and subpicosecond laser pulses of a Ti : sapphire laser. The experimental results were compared with the theoretical evaluation of the samples heat regime obtained from numerical calculations. In addition, the photo-electrical properties of the solar cells were measured before and after laser-machining. Scanning electron microscopy and energy dispersive x-ray analyses were employed to characterize the laser-induced ablation channels. As a result, two phenomena were found to limit the laser-machining process: (i) residues of Mo that were projected onto the walls of the ablation channel and (ii) the metallization of the CuInSe₂ semiconductor close to the channel. Both effects lead to a shunt in the device that decreases the photovoltaic efficiency. As a consequence of these limiting effects, micromachining of CuInSe₂-based solar cells was not possible with nanosecond laser pulses. Only subpicosecond laser pulses provided selective or complete ablation of the thin layers without a relevant change in the photoelectrical properties.

We report the results of a detailed study of the spectral and temporal properties of visible emission from three different GaN-based ultraviolet (UV) light emitting diodes (UV LEDs). The primary UV emission in the 360–380 nm band decays rapidly (less than 1 µs) following switch-off; however, visible luminescence (470–750 nm) with a decay lifetime of tens of microseconds was observed at approximately 10⁻⁴ of the UV intensity. For applications of UV LEDs in time-resolved fluorescence (TRF) employing lanthanide chelates, the visible luminescence from the LEDs competes with the target Eu³⁺ or Tb³⁺ fluorescence in both spectral and temporal domains. A UV band-pass filter (Schott UG11 glass) was therefore used to reduce the visible luminescence of the UV LEDs by three orders of magnitude relative to UV output to yield a practical excitation source for TRF.

In this paper, we propose a novel weighted method to improve the inclined pass-band problem that is intrinsic to surface acoustic wave (SAW) filters with slanted finger interdigital transducers (SFITs). By the dispersive characteristics of SAW in layered piezoelectric substrates, the present method could make the inclined pass-band flatter or the bandwidth wider. We derived a coupling-of-modes model for the calculation of the frequency responses of layered SAW devices with SFITs and calculated the dispersive characteristics of an AlN/silicon layered piezoelectric substrate, such as the phase velocity and the electromechanical coupling coefficient. Then we utilized the dispersive electromechanical coupling coefficient to flatten the inclined pass-band and the dispersive phase velocity to widen the bandwidth. Two designed examples of wide-band layered SAW filters using SFITs on AlN/silicon substrates are presented to prove the validity of the proposed method: one is designed to flatten the inclination of the pass-band; the other, to widen the bandwidth. Results show that the dispersions of layered piezoelectric media are favourable for improving the performance of SFIT-based SAW filters.

Photoluminescence (PL) properties of ZnO thin films on Si substrate with and without an indium tin oxide (ITO) buffer layer, prepared under different oxygen partial pressures in the sputtering gas, were studied. It was found that PL characteristics of ZnO thin films depend on oxygen partial pressure and substrate, and the PL peak in the ultraviolet region has a strong red-shift with increasing excitation intensity on the glass and Si substrates with the ITO buffer layer, and the PL intensity increases with the increasing measuring cycle. Enhanced luminescence efficiency of ZnO thin films on the substrates with ITO buffer layer measured at different cycles can be obtained by thermal annealing.

Two laser-aided techniques for electric field (EF) measurements in a plasma have been realized. The first technique is based on the use of Stark splitting of high-lying levels of atomic hydrogen, whereas the second technique is based on the use of Stark mixing of the Λ-doublet energy sublevels of boron monohydride molecules. Both diagnostic techniques have been applied to the measurements of EFs in a hollow cathode discharge. The results of the EF measurements obtained by both techniques are in good agreement. The strength of the EF in a hollow cathode discharge was in the range of 1000–2600 V cm⁻¹ for the dark space region and in the range of 200–300 V cm⁻¹ for the negative glow region.

We have investigated the unresolved transition array (UTA) emission around 13.5 nm from solid density tin and tin doped foam targets. Extreme ultraviolet (EUV) spectral measurements were made in the wavelength region 11–17 nm using a transmission grating spectrograph and the EUV in-band conversion efficiency was measured using an absolutely calibrated EUV calorimeter. The aim of this work was to optimize the UTA emission with the proper density of tin dopant in low-Z foam targets. The addition of tin as an impurity leads to a reduction in the plasma continuum and narrowing of the UTA compared with fully dense tin targets. Our studies indicate that the required percentage of tin for obtaining bright in-band spectral emission is less than 1%.

Zn nanoclusters were formed in highly-pure amorphous silica slices by 160 keV Zn ions implantation with a dose of 2 × 10¹⁷ ions cm⁻². The Zn implanted sample was then implanted with F ions at 40 keV with 2 × 10¹⁷ ions cm⁻². TEM and HRTEM studies indicate some core–shell nanoclusters have been formed in the Zn/F sequentially implanted sample. An extended x-ray absorption fine structure spectrum of the Zn/F sequentially implanted sample shows the existence of both Zn and ZnO. The oxygen atoms in the substrate are replaced by implanted F ions, producing O₂ molecules, which partially oxidize the already formed Zn nanoclusters and form ZnO nanoshells. A bond length contraction of ZnO and Zn nanoclusters with different sizes was observed, which can be explained by the surface tension of nanoclusters and the compression stress from surrounding substrate effects. The latter was confirmed by the Fourier transform infrared attenuated total reflection spectra.

This paper describes numerical simulations of dielectric dispersion in biological cells of complex geometry to which analytical approaches are not applicable. A numerical technique based on the three-dimensional finite difference method (3D-FDM) has been developed to calculate the equivalent complex permittivity of a system including a single cell or periodically arranged cells in a continuous medium. It has been tested with a spherical cell model, whose equivalent complex permittivity is calculated from analytical formulae. The agreement between the dielectric spectra calculated by 3D-FDM and from the analytical formulae was quite satisfactory with volume fractions of cells up to 0.5. Furthermore, dielectric spectra were simulated for cells in cell division, i.e. a spherical cell divides into two spherical cells via a doublet-shaped cell with a narrow cytoplasmic junction.

Characterizations of layered vanadyl phosphate, VOPO₄ · 2H₂O, intercalated with conducting polypyrrole (PPY) have been carried out employing x-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and high resolution transmission electron microscopy. Plate like structures are observed in scanning electron micrographs. The UV spectrum reveals the presence of PPY within nanocomposites in bimodal distribution. The room temperature conductivity of the nanocomposites is about 1000 times superior to the host vanadyl phosphate. Small polaron tunnelling is observed for smaller content of PPY. Electrical dc conductivity at low temperature is dominated by Mott's one-dimensional variable range hopping for the higher concentration of PPY.

Ultrasonic measurement of polymer cure has been demonstrated to be a method that can provide material property information for coatings or bonding applications. Often the polymer thickness required in real applications is significantly less than typical ultrasonic wavelengths that may be practically available or suitable for direct amplitude measurements of reflection or transmission coefficients. The cure behaviour of thin layers will be different from that of thicker samples as the cross-linking reaction is usually exothermic and strongly influenced by heat flow in the system. By applying a thin layer of polymer in the thickness range 10–100 µm to an aluminium substrate with thickness of approximately 1 mm we are able to monitor the effects of adhesive cure on the resonant modes of this two-layer system. We describe a simple model that does not take into account ultrasonic attenuation and then an improvement to this model that does take into account ultrasonic attenuation in the polymer through viscoelastic behaviour. We then compare the results of both models with our experimental data. Experimental results show good correlation with the models and help to explain the complicated behaviour observed even when using the simpler model without attenuation. Including attenuation in our model does affect the frequency of the resonant modes, but where this effect is significant the attenuation of that mode is so high that the mode is not observed experimentally. The potential to extract the ultrasonic properties of the polymer layer from these measurements is discussed.

Charging and discharging currents have been measured in a diglycidyl ether of bisphenol-A epoxy resin with and without silica fillers, below and above its glass transition temperature T_g = 65 °C. Both transient and steady-state current densities have been analysed. The average applied fields ranged from 3 to 35 kV mm⁻¹ with a sample thickness of 0.5 mm. Above T_g, transient currents suggested a phenomenon of charge injection forming trapped space charges even at low fields. Steady-state currents confirmed that the behaviour was not Ohmic and suggested Schottky-type injection. Below T_g, the current is not controlled by the metal–dielectric interface but by the conduction in the volume: the current is Ohmic at low fields and both transient and steady-state currents suggest a phenomenon of space-charge limited currents at high fields. The field threshold is similar in the filler-free and the filled resin. Values in the range 12–17 kV mm⁻¹ have been measured.

During laser spot welding of many metals and alloys, the peak temperatures on the weld pool surface are very high and often exceed the boiling points of materials. In such situations, the equilibrium pressure on the weld pool surface is higher than the atmospheric pressure and the escaping vapour exerts a large recoil force on the weld pool surface. As a consequence, the molten metal may be expelled from the weld pool surface. The liquid metal expulsion has been examined both experimentally and theoretically for the laser spot welding of 304 stainless steel. The ejected metal droplets were collected on the inner surface of an open ended quartz tube which was mounted perpendicular to the sample surface and co-axial with the laser beam. The size range of the ejected particles was determined by examining the interior surface of the tube after the experiments. The temperature distribution, free surface profile of the weld pool and the initiation time for liquid metal expulsion were computed based on a three-dimensional transient heat transfer and fluid flow model. By comparing the vapour recoil force with the surface tension force at the periphery of the liquid pool, the model predicted whether liquid metal expulsion would take place under different welding conditions. Expulsion of the weld metal was also correlated with the depression of the liquid metal in the middle of the weld pool due to the recoil force of the vapourized material. Higher laser power density and longer pulse duration significantly increased liquid metal expulsion during spot welding.

Isotactic polypropylene (i-PP) films containing additives such as the commercial α -nucleation agent NA11 and the anorganic filler particles CaCO₃ and Al₂O₃ were biaxially stretched. As a result, the films assume a cellular morphology with oblong cavities extending in the direction of the film elongation. In the present study, stretched films of 50 µm thickness with additive concentrations of 0.05–10 mass per cent were charged with a corona method to potentials of 400 or 500 V. The stability of the charges was tested isothermally at temperatures of 90 and 120 °C and by means of thermally stimulated discharge (TSD) experiments. The isothermal measurements show, for the above additives with concentrations higher than about 0.3%, a reduction of the charge decay with increasing additive concentrations. Compared with reference films of pure PP, the potential decay of the films containing additive concentrations of 10% is significantly reduced. Correspondingly, the TSD measurements indicate a shift of the main discharge peak to higher temperatures up to the melting temperature. Generally, the voiding and thus the stability also increases with the stretching ratio. These improvements of the charge stability are attributed to the barrier effect of the cavities. The results are of interest with respect to the various applications of PP electrets, such as ferroelectret devices and air filters.

Nitrile butadiene rubber (NBR) structure foam of different apparent densities was obtained by using different concentrations of foaming agent, azodicarbonamide, ADC/K. The true stress–strain characteristics, in case of compression, of foamed samples were measured. It was found that the theoretical values predicted from the simple blending model are in more agreement with the experimental results than those from the square-relationship model. The effect of cyclic loading–unloading and dissipation energy of rubber foams was studied. The results also indicated that foams with low density exhibited a small hysteresis. The electrical properties were found dependent on the foaming agent concentration. This study was assisted by Mott and Gurney equation. The effect of compressive strain on the electrical conductivity of rubber foams was studied. The free current carrier mobility and the equilibrium concentration of charge carrier in the conduction band were produced as functions of compressive strain. The results also indicate that there is a linear variation between pressure and conductivity for all samples, which means that these samples can be used as a pressure sensor. At a certain concentration of foaming agent (7.5 phr) a change of electrical conductivity by more than three orders is observed at 20% compression strain.

The thermal conductivity and the assessment of moisture effect on building materials are essential for the calculation of the thermal loads on houses. Building materials such as simple units e.g. bricks, tiles, cement plasters, mortar and ground soils are investigated in this work. In the eastern coastal province of Libya, old buildings have thick walls (more than 50 cm thick made of mixed clay and stones) and consequently have good capacitive insulation. On the other hand, the relatively new houses have thin walls and need the addition of insulating materials. Unfortunately, these new houses were constructed without having enough technical data on the thermal properties of building materials and thermal loads were not considered. This leads to uncomfortable living conditions during hot and humid summers and cold and wet winters. This article reports the thermal conductivity values of three types of locally produced building materials used in the construction of a typical Libyan house envelope and gives suggestions to improve the thermal performance of such envelopes. The transient plane source technique (TPS) is used to measure the thermal conductivity of these materials at an average room temperature of 25 °C. The TPS technique uses a resistive heater pattern (TPS element) that is cut from a thin sheet of metal and covered on both sides with thin layers of an insulating material. The TPS element/sensor is used both as a heat source and as a temperature sensor. This technique has the dual advantage of short measuring time and low temperature rise (around 1 K) across the sample. This will prevent a non-uniform moisture distribution that may arise when the temperature difference across the wet samples is maintained for a long time. In addition, the flat thin shape of the TPS element substantially reduces the contact resistance between the sample and the sensor. More details about the TPS technique are included.

Transmission of sound through a flexible electrorheological (ER) layer, in which ER fluids are sandwiched between two plastic sheets and a single face of grating electrodes is employed, is investigated experimentally. The periodic grating electrodes provide an electric field that is approximately parallel to the surface plane of the ER layer, so the sound propagation direction is treated as vertical to the electric field. It is found that the transmitted sound pressure level (SPL) can be modulated (debased or/and augmented) by the electric field. For one kind of ER material, the intrinsic resonant frequency of the ER layer is determined by the face size of the ER layer, the resonant state can be tuned via the electric field. For a face size of 90 × 90 mm² the transmitted SPL tends to increase with increasing electric field, but for a face size of 20 × 20 mm² it tends to decrease with increasing electric field, while for a face size of 40 × 40 mm² it both increases at higher frequencies and decreases at lower frequencies. The viscoelasticity in ER fluids and the complex vibration modes on the ER layer may be responsible for the experimental results. It is thought that the investigated thin ER layer is usable for tunable acoustic devices.

Nanocrystalline titanium dioxide films with anatase structures were prepared on glass by pulsed laser deposition. Our results demonstrated that deposition temperature and oxygen pressure were important parameters in optimization of the microstructure and conductivity of TiO₂ anatase film. The conductivity of the TiO₂ film increased as the substrate temperature increased. It did not, however, increase further even though the depositions were performed above 600 °C. It was also confirmed that the conductivity showed a unique dependence on ambient oxygen pressure, which demonstrated the significant influence of oxygen pressure on the carrier concentration in thin oxide films.

An experimental and theoretical study on the role of the nitrogen gas stream, exiting from a conventional conical nozzle tip during a laser welding process, has been carried out. A mathematical model has been used, based on the Navier–Stokes equations which express fundamental conservation laws of mass, momentum and energy for a compressible fluid. Numerical simulations of the gas stream colliding onto a plane surface have been performed showing the effects of variations of inlet gas pressure, nozzle exit diameter and standoff distance on the density and Mach number contours, axis pressure of the gas jet and plate pressure produced on the workpiece surface. Laser welding experiments have been performed on carbon and stainless steel specimens, by varying the process parameters in the same range as in the simulations and keeping constant the incident power and the travel speed. Two different gas stream regimes were found, namely sonic and subsonic, which were experimentally verified to produce cutting and welding conditions, respectively.

Weld performances have been evaluated in terms of bead width, penetration depth and melted area. Nozzle standoff distance was found to have a negligible influence, while the exit diameter and the flow rate significantly affect the weld results. The numerical predictions allowed an explanation of the experimental results yielding useful suggestions for enhancing the weld quality, acting simply on the shielding gas parameters.

The objective of the study is to experimentally determine the information content of lightning optical emissions through clouds. Clouds affect the amplitude of lightning signals and the apparent dimensions of the optical source. Multiple scattering from the cloud media also alters the shape of the temporal profile of the lightning signal. The goal is to provide accurate estimates of the arrival time delay and temporal pulse width broadening of output signals emitted from clouds for different cloud and lightning parameters. Experiments conducted in the laboratory yield a temporally broadened pulse with an overall decrease in the peak and a delay in the pulse rise time. Parameters such as optical thickness of the cloud medium and the scattering coefficients are varied to simulate different cloud properties. The experimental results are compared with a transient radiative transfer formulation solved using the discrete ordinate method. The practical implications of this research will be improved reliability on prediction of weather conditions, defence applications and geophysical applications like atmospheric studies.

An analytic expression describing the complex voltage measured between the pickup points of a four-point probe, in contact with the surface of a half-space conductor, is derived. The driving current is assumed to be time harmonic. There are two contributions to the measured voltage. One arises from the potential drop due to electric current flowing in the conductor. The other arises from induction in the loop of the pickup circuit. Both terms are obtained by integrating analytic expressions for the electric field, derived previously, along appropriate paths. Theory is compared with experimental data for co-linear and rectangular arrangements of the probe points, and very good agreement is obtained.

A previously developed model for simulation of recoil-pressure induced melt displacement during laser pulse interaction has been upgraded to include the restraining effect of surface tension. The results of numerical simulations of melt displacement/ejection during laser welding and drilling using this enhanced model are presented. In particular, the dependences of the threshold pulse energies for melt displacement and melt ejection as functions of laser pulse duration, beam radius and beam intensity distribution are computed and analysed.

The pulsed electric field (PEF) treatment of liquid and pumpable products contaminated with microorganisms has attracted significant interest from the pulsed power and bioscience research communities particularly because the inactivation mechanism is non-thermal, thereby allowing retention of the original nutritional and flavour characteristics of the product. Although the biological effects of PEF have been studied for several decades, the physical mechanisms of the interaction of the fields with microorganisms is still not fully understood. The present work is a study of the dynamics of the electrical field both in a PEF treatment chamber with dielectric barriers and in the plasma (cell) membrane of a microbial cell. It is shown that the transient process can be divided into three physical phases, and models for these phases are proposed and briefly discussed. The complete dynamics of the time development of the electric field in a spherical dielectric shell representing the cellular membrane is then obtained using an analytical solution of the Ohmic conduction problem. It was found that the field in the membrane reaches a maximum value that could be two orders of magnitude higher than the original Laplacian electrical field in the chamber, and this value was attained in a time comparable to the field relaxation time in the chamber. Thus, the optimal duration of the field during PEF treatment should be equal to such a time.