Table of contents

The optical constants of spun-cast films of several polyfluorene derivatives were examined using a combination of reflection and transmission spectroscopic ellipsometry. Results for both unannealed films and films annealed at 150°C are presented. For some polymers, annealing allowed the polymer chains to re-organize such that the films became more isotropic. The optical constants of two common metal cathodes, calcium and aluminium, were investigated using reverse ellipsometry. Multi-layer structures incorporating both polymer and metal layers were also successfully modelled.

In this work, a model is developed to simulate the biological processes involved in nerve fibre transmission and subsequent muscle contraction. The model has been based on approximating biological structure and function to electrical circuits and as such was implemented on an electronics simulation software package called Pspice. Models of nerve, the nerve–muscle interface and muscle fibre have been implemented. The time dependent ionic properties of the nerve and muscle membranes have been simulated using the Hodgkin–Huxley equations and for the muscle fibre, the implementation of the Huxley sliding filament theory for muscular contraction. The results show that nerve may be considered as a fractal transmission line and that the amplitude of the nerve membrane depolarization is dependent on the dimensions of the fibre. Additionally, simulation of the nerve–muscle interface allows the fractal nerve model to be connected to the muscle fibre model and it is shown that a two sarcomere molecular simulation can produce realistic macroscopic muscle force profiles.

Dielectric relaxation spectroscopy was used to investigate the properties of residual water in lyophilized formulations of a proprietary tri-phosphate drug containing a sugar (trehalose, lactose or sucrose) or dextran. The dielectric properties of each formulation were determined in the frequency range (0.1 Hz–0.1 MHz) and temperature range (30°C–T_g). The temperature dependence of the relaxation times for all samples showed Arrhenuis behaviour, from which the activation energy was derived. Proton hopping through the hydrogen-bonded network (clusters) of water molecules was suggested as the principle mode of charge transport. Significant differences in dielectric relaxation kinetics and activation energy were observed for the different formulations, which were found to correlate with the amount of monophosphate degradation product.

This study investigates the temperature dependence of the low frequency dielectric properties (0.1 Hz–1 MHz) of hydrated globular proteins (namely, ovalbumin, lysozyme and pepsin). The study aims to reveal the mechanisms of water–protein interaction from the dielectric response of these model proteins. Two principle dielectric responses were observed for each hydrated protein, namely, an anomalous low frequency dispersion and a dielectric loss peak at higher frequency (called the ε₃ dispersion). The low frequency response conformed to a fractional power low of frequency, while the higher frequency response conformed to a Davidson–Cole model. The strength of both processes reached a maximum at a certain temperature within the experimental temperature range. This temperature is referred to as the percolation threshold (PT) and is thought to be associated with the percolation of protons between hydrogen-bonded water molecules. The relaxation times of the ε₃ dispersion conformed to Arrhenius behaviour at temperatures below the PT, from which an activation energy (ΔH) could be calculated. This activation energy is thought to be a measure of the concentration of available charged sites through which proton transport is facilitated. The structural fractal dimension in the hydrated protein system was also calculated, and enabled the approximation of the pathway for charge percolation in the protein matrix.

The conductivity of polyethylene oxide (PEO) is described with a three-dimensional hopping model considering electrostatic interactions between the ions. Ions fluctuate over energy-barriers in a multi-well potential. To decide whether positive or negative charges are responsible for this conductivity, the thermoelectric voltage is measured. The samples are embedded between two aluminium-electrodes. The oxide on the interface between the electrodes and the PEO serves as a blocking layer. The temperature of each electrode is controlled by a Peltier element. A temperature step is applied to one electrode by changing the temperature of one of the Peltier elements. Due to this temperature gradient, the mobile charges fluctuate thermally activated from the warmer side to the colder side of the sample. The direction of the measured thermoelectric voltage indicates the type of mobile charges. It is found that positive charges are mobile. Further, it is shown that the absolute value of the thermoelectric voltage depends on the energy-barrier heights in the multi-well potential.

The effectiveness of potentially powerful therapeutics, including DNA, is often limited by their inability to permeate the cell membrane efficiently. Electroporation (EP) also referred to as `electropermeabilization' of the outer cell membrane renders this barrier temporarily permeable by inducing `pores' across the lipid bilayer. For in vivo EP, the drug or DNA is delivered into the interstitial space of the target tissue by conventional means, followed by local EP. EP pulses of micro- to millisecond duration and field strengths of 100–1500 V cm⁻¹ generally enhance the delivery of certain chemotherapeutic drugs by three to four orders of magnitude and intracellular delivery of DNA several hundred-fold. We have used EP in clinical studies for human cancer therapy and in animals for gene therapy and DNA vaccination. Late stage squamous cell carcinomas of the head and neck were treated with intratumoural injection of bleomycin and subsequent EP. Of the 69 tumours treated, 25% disappeared completely and another 32% were reduced in volume by more than half. Residence time of bleomycin in electroporated tumours was significantly greater than in non-electroporated lesions. Histological findings and gene expression patterns after bleomycin-EP treatment indicated rapid apoptosis of the majority of tumour cells. In animals, we demonstrated the usefulness of EP for enhanced DNA delivery by achieving normalization of blood clotting times in haemophilic dogs, and by substantially increasing transgene expression in smooth muscle cells of arterial walls using a novel porous balloon EP catheter. Finally, we have found in animal experiments that the immune response to DNA vaccines can be dramatically enhanced and accelerated by EP and co-injection of micron-sized particles. We conclude that EP represents an effective, economical and safe approach to enhance the intracellular delivery, and thus potency, of important drugs and genes for therapeutic purposes. The safety and pharmaco-economic profile of EP compares favourably with other drug and DNA delivery methods.

We report the first use of GARField (STRAFI-like) magnetic resonance profiling (1D-MRI) to study skin hydration, both in vitro and in vivo. It is shown that, for in vitro samples, a high degree of reproducibility is achievable for skin samples from a single donor and that the signal intensity correlates well with equilibrium moisture content in the skin. In vivo, the method allows measurement of the influence of a moisturizer cream on water levels in the skin as a function of time.

Time domain dielectric spectroscopy has been used to study spherical erythrocytes, suspended in diluted phosphate buffered saline (PBS) buffers at varying concentrations of D- and L-glucose at 25°C. The osmolarity for each glucose solution was adapted, equalling that of a 63% PBS (183 mOsm). The strong effect of the electrode polarization was corrected using the fractal approach in time domain. For analysis of the dielectric properties of suspensions of erythrocytes, the Maxwell–Wagner model is used for small volume fractions. Values of the permittivity and conductivity of the cell membrane were obtained from a fitting procedure according to the one-shell model. The non-monotonic and specific response of membrane electric properties on D-glucose concentrations were observed, with a dramatic decrease around 12 mM. No changes of membrane properties have been observed in the presence of increasing concentrations of L-glucose, the biologically inactive enantiomer of D-glucose. The effect is thus specific to D-glucose. The possible mechanism of specific cell reaction to D-glucose is discussed in this paper.

Magnetic properties of Sm–Fe(Ti)–C(N)/α-Fe nanocomposite alloys prepared by mechanical alloying (MA) have been studied. The hysteresis loops of MA Sm₁₂Fe₇₄Ti₇C₇N_δ, Sm₁₀Fe₇₆Ti₇C₇N_δ and Sm_19.0Fe_66.7C_14.3 (i.e. Sm₂₀Fe₇₀C₁₅) made by different processes (annealing, nitriding and re-milling) were measured at the applied magnetic fields with different dH/dt at 400, 350 and 295 K. The main phase with hard magnetic properties is of TbCu₇-type. These three alloys have different behaviours of magnetization processes when the magnetic fields with dH/dt = 31.6 and 94.7 MA ms⁻¹ are applied. The magnetic viscosity S_v of Sm₁₂Fe₇₄Ti₇C₇N_δ, Sm₁₀Fe₇₆Ti₇C₇N_δ and Sm₂₀Fe₇₀C₁₅ is determined from hysteresis loops measured for different dH/dt at various temperatures. The magnetic viscosity S_v in the Sm–Fe(Ti)–C(N)/α-Fe alloys is approximately proportional to the coercive field _iH_c at the measuring temperature.

In the pressure range 10–100 mbar, the discharge development in SF₆ has been investigated in the photo-triggered excitation scheme for current density in the range 10–1000 A cm⁻² and pulse duration of about 60 ns. Thanks to the homogeneity of the photo-triggered discharge, a self-consistent zero-dimensional model can be used to predict the temporal evolution of the electrical parameters. From a detailed comparison between the experimental results and the modelling predictions, evidence for the occurrence of a two-step ionization process in high current density discharges in SF₆ is reported and the corresponding collision cross-section is estimated. Moreover, it is shown that the amount of the two-step ionization is directly correlated to the density of the electrical charge transferred to the plasma per unit surface. This two-step ionization becomes the main source of electron multiplication whenever the transferred charge per unit surface is higher than 15 μC cm⁻².

ZrO₂ thin films were fabricated in O₂ ambient and in N₂ ambient by pulsed laser deposition (PLD), respectively. X-ray diffraction revealed that films prepared at 400°C remained amorphous. The dielectric properties of amorphous ZrO₂ films were investigated by measuring the capacitance–frequency characteristics of Pt/ZrO₂/Pt capacitor structures. The dielectric constant of the films deposited in N₂ ambient was larger than that of the films deposited in O₂ ambient. The dielectric loss was lower for films prepared in N₂ ambient. Atom force microscopy investigation indicated that films deposited in N₂ ambient had smoother surface than films deposited in O₂ ambient. Capacitance–voltage and current–voltage characteristics were studied. The equivalent oxide thickness (EOT) of films with 6.6 nm physical thickness deposited in N₂ ambient is lower than that of films deposited in O₂ ambient. An EOT of 1.38 nm for the film deposited in N₂ ambient was obtained, while the leakage current density was 94.6 mA cm⁻². Therefore, ZrO₂ thins deposited in N₂ ambient have enhanced dielectric properties due to the incorporation of nitrogen which leads to the formation of Zr-doped nitride interfacial layer, and is suggested to be a potential material for alternative high-k (high dielectric constant) gate dielectric applications.

In this study, optical and photoluminescence (PL) properties of amorphous hydrogenated carbon (a-C : H) films are reported along with thermal annealing effects. The layers were prepared by dc glow discharge decomposition of methane (CH₄) at a frequency of 20 kHz. The optical Tauc gap (E_g) of as-deposited films can vary from 0.8 to 2.4 eV, depending mainly on reactor pressure. E_g increases by ∼0.1 eV for annealing temperatures, T_a⩽220°C, whereas in the range of T_a from 250°C to 300°C, a decrease of about 0.45 eV is observed, signalling the development of graphitic short range order as shown by dielectric losses measurements. Wide-gap (⩾1.8 eV) a-C : H films exhibit visible PL at room temperature. Annealing of a-C : H with E_g = 1.8 eV, from 100°C to 220°C, improves the PL intensity significantly, indicating a decrease of defects in the material. On the other hand, the dominant PL peak is now located in the whole 1.8–2 eV energy range, rather than at 1.87 eV for non-annealed films. This is likely assigned to the slight increase of E_g by ∼0.1 eV.

The electrical properties of a mixture of ethylene–propylene–diene monomer rubber and silicon carbide (SiC) have been measured as a function of filler concentration. It was found that mixtures containing angular SiC grains have a conductivity that displays not one, but two percolation thresholds. Different types of contacts between the conducting particles, being represented by edge and face connections, respectively, can explain the phenomenon. The two percolation thresholds are obtained at volume fractions of about 0.25 and 0.40, respectively. These values are higher than those predicted by theory, which can be explained by dispersion effects with only one phase being granular and the other being continuous. The value of the conductivity at the central plateau was found to be close to the geometric mean of the limiting conductivities at low and high concentrations. This is in good agreement with theory. With rounded SiC grains only one threshold is obtained, which is consistent with only one type of contact. The concentration dependence of the conductivity was simulated using a three-dimensional impedance network model that incorporates both edge and face contacts. The double-threshold behaviour also appears in the calculations. By dispersing the conducting particles more evenly than random, the thresholds are shifted towards higher concentrations as observed in the experiments.

The cycle model of a quantum refrigeration cycle working with many non-interacting harmonic oscillators and consisting of two isothermal and two constant-frequency processes is established. Based on the quantum master equation and semi-group approach, the general performance of the cycle is investigated. Expressions for some important performance parameters, such as the coefficient of performance, cooling rate, power input, and rate of the entropy production, are derived. Several interesting cases are discussed and, especially, the optimal performance of the cycle at high temperatures is discussed in detail. Some important characteristic curves of the cycle, such as the cooling rate versus coefficient of performance curves, the power input versus coefficient of performance curves, the cooling rate versus power input curves, and so on, are presented. The maximum cooling rate and the corresponding coefficient of performance are calculated. Other optimal performances are also analysed. The results obtained here are compared with those of an Ericsson or Stirling refrigeration cycle using an ideal gas as the working substance. Finally, the optimal performance of a harmonic quantum Carnot refrigeration cycle at high temperatures is derived easily.

It is pointed out that the simulation presented by Morrow and Blackburn for the 40 cm propagation of a positive streamer in a 50 cm point-to-plane air gap probably is seriously in error due to the neglect of gas heating and subsequent gas density reduction in the streamer channel. Also, the connection is established between the propagating high-field region of the simulated streamer and the well-known secondary streamer phenomenon caused by the Douglas-Hamilton and Mani attachment instability.

A response to the comments on `The stepped nature of lightning and the upward connecting stream' J. Phys. D: Appl. Phys.35 L69–73. We thank Professor Sigmond for his stimulating comments [1]. However, we disagree with the statement that the results are seriously in error; rather, we consider that the results are the solution obtained using a simple model showing the general nature of streamer propagation over large distances. More complex models can be envisaged, but cannot be solved at this time.