Table of contents

In this article we present the results of an experimental study of negative corona pulses in air at atmospheric pressure. New, more complex waveforms of pulses in discharge on trees and plants (pine, spruce) are found and compared with waveforms measured in a classical negative corona with metal electrodes. It is also found that the repetition rate of negative plant pulses is much lower than in the case of classical `metal' Trichel pulses. The pulses are less regular and fluctuations of amplitudes and periods are large.

For the last decade it has been possible to cool atoms to microkelvin temperatures (~1 cm s^-1) using a variety of optical techniques. Light beams provide the very strong frictional forces required to slow atoms from room temperature (~500 m s^-1). However, once the atoms are cold, the relatively weak conservative forces of static electric and magnetic fields play an important role. In our group we have been studying the interaction of cold rubidium atoms with periodically magnetized data storage media. Here we review the underlying principles of the forces acting on atoms above a suitably magnetized substrate or near current-carrying wires. We also summarize the status of experiments. These structures can be used as smooth or corrugated reflectors for controlling the trajectories of cold atoms. Alternatively, they may be used to confine atoms to a plane, a line, or a dot and in some cases to reach the quantum limit of confinement. Atoms levitated above a magnetized surface can be guided electrostatically by wires deposited on the surface. The flow and interaction of atoms in such a structure may form the basis of a new technology, `integrated atom optics' which might ultimately be capable of realizing a quantum computer.

Interaction of CO₂, Nd:YAG, excimer and high-power diode laser radiation with the surface of a common mild steel (EN8) was found to effect changes in the wettability characteristics of the steel, namely changes in the measured contact angle of certain liquids. Such changes were identified as being due to modifications to (i) the surface roughness, (ii) changes in the surface oxygen content and (iii) changes in the surface energy of the mild steel. However, it was found that changes in the wettability characteristics of the mild steel were predominantly influenced by the surface roughness. To a much lesser extent surface O₂ content is also thought to play a role. The work has shown that the wettability characteristics of the selected mild steel could be controlled and/or modified with laser surface treatment. Moreover, the findings of this work strongly indicate the existence of a relationship between the change of the wetting properties of the mild steel and the laser wavelength.

Neutron reflectometry has been used to study liquid metal-ceramic interfaces in situ. A high-temperature facility designed and commissioned for use on the SURF reflectometer at the ISIS beamline, Rutherford Appleton Laboratory, is described. Results from a study of two Sn-Ti alloys on sapphire up to 1173 K are presented and the interfacial composition established by interpretation of the recorded reflectivity profiles. Overall the results are in good agreement with previously published work inferred from the analysis of solidified structures and the neutron reflection technique, established as a unique method for the high temperature analysis of buried solid/liquid interfaces.

Results of the investigations carried out on particulate silver films, deposited at a rate of 0.4 nm s^-1 on poly(4-vinylpyridine) coated substrates held at temperatures in the range 440-490 K in a vacuum of 8 × 10^-6 Torr are reported in this paper. A morphological study by scanning electron microscopy and x-ray diffraction was further confirmed by optical absorption studies. The shift in the plasmon resonance towards longer wavelength is ascribed to polymer-metal particle interactions. Electrical resistances immediately after deposition and at room temperature show a regular dependence on the thickness for the films in the temperature range studied. The films deposited at lower substrate temperatures show a lower negative temperature coefficient of resistance when compared to films deposited at higher substrate temperatures. X-ray photoelectron spectroscopy studies indicate the formation of a sub-surface particulate structure at higher substrate temperatures.

Photoreflectance (PR) spectra have been measured to determine the fundamental-absorption-edge (E₀) structure of hexagonal CdSe single crystal for both Ec and E|| c polarizations at temperatures T between 77 and 300 K. The measured PR spectra are found to be successfully explained by an excitonic line shape for temperatures T below ~200 K and by a three-dimensional one-electron line shape for T>200 K. No clear evidence of the exciton interference effect has been observed within our experimental temperature range T = 77-300 K. The temperature dependence of the E₀-exciton parameters (exciton energy, amplitude and broadening parameter) has been determined and analysed using the Varshni equation and an empirical expression of Bose-Einstein type.

The insertion of cobalt species in 2 µm thick nickel and iron matrixes is performed by electrodeposition. The influence of the bath or sample composition and the growth velocity on the morphology and the interface pattern of the resulting alloys are studied following the behaviour of the root-mean-square surface roughness . It is shown here that pure Ni and Fe electrodeposits exhibit high values. Low Co concentrations in each of the matrices prompts a sensitive decrease of this parameter until a typical _min value is reached. This value is affected by the growth velocity v and the chemical nature of the alloy. also appears to be connected to the textural pattern of the sample surface. The increase of v in the deposition of pure Ni and Fe destroys the preferred growth orientation of these materials, which is accompanied by an increasing movement of . The presence of Co²⁺ species in the deposition bath reveals that the = f(v) curves of the resulting Co_xNi_(1-x) and Co_xFe_1-x samples always exhibit a critical point (´´_min,v_min) corresponding to a change in the preferred growth orientation of the alloy system, without structural modification. The scaling analysis of the pure metal and alloy surfaces obtained at v = 5.47 µm s^-1 always present a self-affine fractal pattern. Ni and Fe lead to one roughness exponent while the insertion of Co in each of the two matrices engenders two roughness exponents. The main exponent values obtained with the Co, Fe and Co-Fe samples are consistent with the local non-conservative Kardar-Parisi-Zhang dynamics while those of Ni and Co-Ni interface growths are in agreement with the local conservative model.

We report on the design and performance of a megagauss generator at the Humboldt High Magnetic Field Centre. The installation uses the fast capacitor discharge into single-turn coils to produce microsecond pulses of 260 T in 8 mm diameter suitable for low-temperature experiments and 310 T in 5 mm diameter. Simple concepts are applied to substantiate crucial design principles for the generator and the choice of components. The implementation of a high-power/low-inductance circuit operated at 60 kV and other technical questions are discussed in detail. The analysis of experimental field and current data is based on numerical simulations of the magnetic pressure and the nonlinear flux diffusion. It was carried out in order to investigate the limitations and characteristics of the Berlin generator in particular and the single-turn coil technique in general. The paper focuses primarily on field, generator and coil parameters, which can be used for performing routine, reproducible scientific experiments at low temperatures.

In this paper the magnetic field dependence of the resonant frequency, f_res, and frequency of maximum absorption, f_max, for a ferrofluid sample, for polarizing fields both perpendicular to and parallel with the axis of a coaxial cell, are presented for the first time.

Measurements of the complex magnetic susceptibility, () = ´()-i´´() as a function of frequency, f (Hz), over the range 100 MHz-6 GHz and external polarizing field, H (kA m^-1), up to approximately 100 kA m^-1, are obtained by means of the coaxial transmission line technique. The presence of ferromagnetic resonance in the ferrofluid, (at a frequency f_res) is found for all values of the polarizing field and the influence of demagnetizing effects on f_res is accounted for. The dependence of f_res on H is found to be linear and an average value of internal anisotropy field, , and the corresponding magnetic anisotropy constant, , are determined.

The susceptibility component profiles obtained for both cases of polarization are similar to those obtained in an analogous theoretical study of dielectrics subjected to an external electric field.

The structure and magnetic properties of Ho₂(Co_1-xFe_x)₁₇ compounds with x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8 or 1.0 have been investigated by x-ray diffraction (XRD) and magnetization measurements. XRD patterns demonstrate that all samples are single phase with the hexagonal Th₂Ni₁₇-type structure. The substitution of Fe for Co initially leads to a increase in the lattice constant a and unit cell volumes v and then a small decrease. The Curie temperatures decrease monotonously with increasing Fe concentration. Saturation magnetization increases with x and then decreases, attaining a maximum at x = 0.2. The magnetic anisotropy phase diagram of Ho₂(Co_1-xFe_x)₁₇ compounds has been determined from the temperature dependence of magnetization and the XRD patterns of magnetically oriented powder samples. The spin-reorientation temperature of Ho₂(Co_1-xFe_x)₁₇ compounds is found to decrease with increasing Fe content for x<0.3 and then increase for x>0.3, attaining a lowest value of 248 K at x = 0.3.

In this paper we calculate the intensity emitted by different atomic lines and by the (B ³_u^--X ³_g^-) system of S₂. We show the influence on intensity of the deviation from thermal equilibrium and of the hypotheses on different temperatures (atomic excitation temperature T_ex^at, rotational temperature T_rot, vibrational temperature T_vib, diatomic excitation temperature T_ex^mol, translation temperature of free electrons T_e-^tr and translation temperature of heavy species T_h^tr). From this method, we show that from optical emission of atomic line and diatomic molecule spectra, the deviation from thermal equilibrium and consequently the composition of plasma can be determined.

The work presented is entirely devoted to the coupling between electrochemistry and hydrodynamics. It takes place in the magnetohydrodynamics (MHD) of a seawater context for direct propulsion or flow control. The experimental measurements reported were carried out in the small seawater tunnel of our laboratory using seawater with a NaCl concentration of 35 g l^-1. The first aspect of our study is the possible effect of electrolysis micro-bubbles on the flow, for instance, leading to a possible modification of the turbulent boundary layer or the effect of the flow on bubble characteristics. The second aspect is the effect of the flow conditions on the electrode working conditions, for instance the effective electroactive species and the electrode potential evolution. The main conclusions are that the electrolysis micro-bubbles do not affect the flow in the domain considered, but, in contrast, flow conditions act on bubble size and distribution. On the other hand, the competition between electroactive species (i.e. anodic reactions) is entirely controlled by the flow. Which means, from a practical point of view, that there is no opportunity to select an anode material that enables the selection of oxygen evolution instead of chlorine in the conditions prescribed by seawater MHD applications.

The oscillatory behaviour of a glow positive corona in pure nitrogen is observed experimentally. The regular self-oscillations of the corona current and the light emission from the glow ionization region, as well as the averaged volt-ampere characteristics of this discharge, are studied comprehensively within gas pressure range 10-765 Torr. It is revealed that the positive glow corona generates a soft x-radiation. This radiation plays a crucial role in the photo-ionization of nitrogen. Furthermore, it provides a fast positive feedback for electron avalanches through the ionization region of the corona. The results of numerical modelling of the glow corona under free-running oscillations are also presented.

The use of alternating current (ac) arc plasma involves a problem of instability of the arc plasma at current zero point. This paper describes the effects of ambient temperature and of superimposing a pulse current on stability of the ac argon arc plasma; the pulse current (10 A) is superimposed on an ac current only at each current zero point. The ac arc plasma is generated between a tungsten electrode of a plasma torch and a copper counter electrode in a chamber. The ac current is 100-500 A_rms and the arc plasma length is 40 mm. As the ambient temperature increases from 300 K to 900 K, the stored energy in the arc plasma increases and the required supply voltage to stabilize the arc plasma decreases by about 20% (from 270 to 220 V_rms). Additionally, superimposing a pulse current on an ac current causes the required supply voltage to decrease by about further 20% (from 220 to 180 V_rms).

We have investigated the effect of constriction on the characteristics of low-pressure glow discharges in argon. In a series of experiments four discharge tubes with plane-parallel electrodes of different diameters (D = 31.4, 20, 10 and 5 mm) and same electrode separation (L₀ = 45 mm) were studied. The discharges were surrounded by floating-potential metal tubes. We measured voltage-current characteristics of the discharges and recorded the spatial intensity distribution of selected spectral lines (Ar-I 750.3 nm, 811.5 nm and Ar-II 476.5 nm) in the electrode gap at current densities 0.2 mA cm^-2j 1 mA cm^-2. We also observed copper lines in the spectrum originating from the sputtering of the copper cathode, even at these relatively low discharge current densities. The `effective surface' of the cathode - that actively participates in discharge operation - is always smaller than the cathode surface. This results from the radial electric field formed in the cathode dark space due to the accumulation of charges on the metal tube surrounding the discharge. Using a simple model we could calculate the effective cathode diameter (which determines the current density for given current). Taking into account the current density obtained this way, the voltage of the tubes was found to obey the V = f(j/p²) scaling relation. The increasing loss of charge on the metal wall with decreasing tube diameter also resulted in structural changes in the discharge. While at large diameter the cathode dark space and the negative glow filled the interelectrode gap, at low D a positive column-like part was also formed.

The intensities of ArI emission lines 750.4 and 811.5 nm and of H (hydrogen appearing as a small admixture to the discharge in Ar) were measured along the column of a stationary, surface wave sustained plasma. Simultaneously, electron densities along the discharge were determined, probing the spatial phase characteristics of test surface waves. Using test waves of different frequencies, the evaluation of electron densities was based on the dispersion relation, which connects densities with axial wavenumbers of the m = 0 mode. The emitted line intensities plotted against the electron density, particularly those of H and the 750.4 nm line, reveal that the discharge economizes on dissipated RF power in fulfilling the local energy and particle balance. A theoretical model of the diffusion dominated plasma confirms the conclusion drawn from the spectroscopically observed trends. According to this model, the lowering of the RF field intensities is correlated with the transition from ground-state excitation/ionization (prevailing at low electron densities) to additional, less power consumptive, stepwise excitation/ionization, which is enabled due to enlarged densities of metastables.

A dc arc discharge was generated between graphite (C) and molybdenum (Mo) electrodes at 25 kPa of He gas ambient for a period of about 1 s in order to reveal the relation between nanotube growth and arc discharge phenomena. Numerous multiwall carbon nanotubes and nanocapsules were observed at the cathode spot area of the C-cathode. However, almost no nanotubes were observed at the C-anode, Mo-anode, and Mo-cathode. These results clearly indicated that the nanotubes were grown by the cathode spot of the C-cathode. Based on the results, a logical explanation of the growth of soft-core containing nanotubes and a hard-shell in the usual arc with a C-cathode and C-anode electrode system was presented.

Band tailing refers to the effects of impurity states in and near to the band edge in a heavily-doped semiconductor. In the present study, the Fermi energy, screening length and the impurity screening potential are determined for a heavily-doped parabolic band semiconductor. The effects of unperturbed and perturbed bands are both considered in the evaluation of the entities. In the perturbed band, the density-of-states technique, earlier used by Kane, and the E- dispersion relation technique for band tailing, obtained by us, are applied. Our results on Fermi energy are compared with the results of Kane's model and with those for the unperturbed band for a typical semiconductor of n-type GaAs. We find that our model provides much lower values at a low concentrations of 10¹⁷-10¹⁸ cm^-3, as compared to the other two cases and finally merges with the unperturbed band case at a higher doping conditions. On the other hand, the Kane's model offers much higher values over the doping range studied here.

The thermal conductivity, thermal diffusivity and specific heat per unit volume of three types of porous insulating solids were measured, simultaneously, by the transient plane source (TPS) technique. The samples, namely, fibreglass, closed-cell foam and foam insulation, were subjected to external pressure, taking into consideration the compressibility of the samples and the requirements of the TPS technique. These materials are used for insulation for temperature control of air-conditioned space and therefore are helpful in reducing energy losses. Although all three samples are good insulators, having very small thermal conductivity values (~0.03-0.05 W m^-1 K^-1), closed-cell foam insulation shows insignificant changes in thermal conductivity for the selected pressure range (8-15 kPa). Therefore it is a better choice for thermal isolation. For the other materials investigated, the thermal conductivity value increases slightly with external pressure.

Hydrogenated amorphous silicon carbide (a-Si_1-xC_x:H) films were prepared by use of the preparation system possessing a hydrogen plasma chamber and a silane/methane plasma chamber (the hydrogen radical chemical vapour deposit (CVD) method) and their electrical properties before and after light-soaking were investigated. The photo- and dark conductivities both before and after light-soaking increased with increasing rf power on the hydrogen plasma side; however, the photosensitivity remained unchanged. The activation energy of the dark conductivity before light-soaking decreased with increasing rf power on the hydrogen plasma side, while that after light-soaking increased. The defect density decreased with increasing rf power on the hydrogen plasma side. The hydrogen radical CVD method was found to be useful for preparing a-Si_1-xC_x:H films with high photoconductivity and low defect density. The electrical properties obtained in this study were explained by the two-energy-level model. Although the hydrogen radical CVD method mainly reduced the defect density of the lower energy level, it also reduced that of the upper energy level.

Hydrophilic polymeric network structures have exhibited an ability to absorb large quantities of water, if available, at subzero (Celsius) temperatures. While the rate of water uptake is lower when compared to that at ambient temperatures, the equilibrium water uptake is comparable. This has been attributed to temperature-dependent changes in the physicochemical parameters of the solvent (water) and the polymer, which then favours increased interactions of the solvent molecules with the hydrophilic sites within the polymeric network. The results obtained show that the temperature range of application of hydro-active polymers may be extended to subzero temperatures. In other words, it may now be possible to find applications for hydrophilic polymeric materials in processes operating at subzero temperatures and where there is need to manage and control available water.

The fluorescence polarization of rhodamine B is used as an environment probe to study the microstructural evolution of silica gel during sol to gel to gel glass transitions. The wet gel is an amorphous porous solid of which organics and solvents (water and ethanol) exist in the pores. During gelation and heat treatment, the solvents evaporate gradually and the inorganic network of the matrix forms and strengthens as a result of the collapse of pores and the structural relaxation. The rhodamine B molecules lose mobility gradually and are finally steadily isolated and adsorbed into the tiny pores of the inorganic network, which makes the rotational diffusion of rhodamine B molecules difficult and increases the fluorescence anisotropy and polarization of rhodamine B.