Table of contents

The paper presents the results of investigation of the electrical breakdown in low pressure gases when the secondary electrons released from the cathode play the dominant role in the initiation of electrical breakdown. The secondary electrons are created by the charged and neutral species formed during the previous breakdown and discharge as well as by γ-rays. Electrical breakdown investigations are based on the measurements of electrical breakdown voltage and electrical breakdown time delay for gas-filled tubes with spherical electrodes with diameters much larger than an interelectrode distance. Stochastic nature of both the breakdown voltage and time delay are discussed and their distributions based on experimental data are shown. The methods for the determination of static breakdown voltage are also analysed. The influence of different parameters (overvoltage, cathode material and its surface purity, gas pressure, glow current, etc) on time delay are studied. A special attention is paid to the memory effect in various gases that depends on the positive ion recombination times, catalytic recombination times in the case of nitrogen and hydrogen, as well as metastable states deexcitation times in noble gases. The analysis of this effect is done by memory curves on the basis of which the presence of long-lived neutral active states can be followed to their very low concentrations when cosmic and environment radiation play the dominant role in electrical breakdown initiation.

The phase transformation and magnetic properties of nanocomposite Nd₂Fe₁₄B/α-Fe magnets with W and Co additions, according to the composition formula (1-x wt%-y wt%) Nd_8.6Fe₈₅B_6.4 + x wt% W{} + {} y wt% Co, prepared by mechanical alloying, have been systematically studied. It has been observed that separation and decomposition of the B-rich phase Nd_1.1Fe₄B₄ occur during crystallization of the amorphous state of the alloys after annealing at different temperatures. The addition of W can promote separation of the B-rich phase, and some B-rich phase remains at a relatively high annealing temperature. The addition of Co increases the content of the B-rich phase. A small amount of W atoms can enter the lattice of the hard magnetic phase, substituting for Fe atoms, and cause the coercivity of the magnets to increase. However, an excessive addition of W degrades magnetic properties due to the increase of B-rich phase. Addition of Co decreases the crystallization temperature of the hard phase and refines the grain size of the soft phase at a relatively low annealing temperature. In addition, the addition of Co is favourable for improving the exchange coupling between nano-grains of the hard magnetic and the soft magnetic phases. The optimum grain size of the soft magnetic phase has been calculated. The calculated values of the optimum grain size of the soft phase fairly agree with experimental results only for nanocomposite magnets with a soft magnetic phase volume fraction χ close to 30%. Further, the B-rich phase dilutes the inter-grain exchange interaction resulting in a decrease of the coercivity for magnets with χ close to 30%. These observations imply that the exchange coupling between the nano-grains of the soft and the hard phases constitutes a predominant control mechanism of coercivity for nanocomposite magnets with χ close to 30%.

An Ising model is presented for a simple cubic-type structure, to determine the magnetic properties for superlattices of periodic A_k(A_pB_1-p)₁B_h formula consisting of k layers of spin-½ A ions, h layers of spin-½ B ions and single layer disordered alloy interfaces between them. The interface layers are characterized by random arrangements of A and B ions so that (A_pB_1-p)₁ is a two-dimensional thermodynamically stable alloy. The magnetic properties studied concern notably the phase diagrams and the sublattice magnetizations of these systems. The model is general and can be used for ferro- or anti-ferromagnetic A-B exchange couplings. In this paper the A-A and B-B exchange couplings are considered ferromagnetic with no loss of generality. An effective field theory is employed to calculate the properties of these systems. We apply the method to various Fe_k(Fe_p Tb_1-p)₁Tb_h superlattices. The needed exchange constants for the two-dimensional alloy interface are calculated in the framework of this model, and used to calculate these properties for different concentrations 0⩽p⩽1, for these superlattices. The architecture and concentration dependence of the magnetic properties is an important feature of this paper, allowing a useful experimental analysis of similar systems.

The coupled wave theory of Kogelnik has given a well-established basis for the comprehension of how light propagates inside a volume hologram. This theory gives a good approximation for the diffraction efficiency of both volume phase holograms and volume absorption holograms. Mixed holograms (phase and absorption) have also been dealt with from the point of view of the coupled wave theory. In this paper we use Kogelnik's coupled wave theory to give quantitative information about the mechanisms which produce mixed gratings in photographic emulsions. In particular, we demonstrate that mixed amplitude-phase gratings are recorded on photographic emulsions when fixation-free bleaching techniques are used to obtain volume holograms. We will prove that the oxidation products of the bleach can give rise to an absorption modulation at high values of exposure and high concentrations of potassium bromide in the bleach bath. We will also give quantitative data regarding the absorption created by these oxidation products.

In this paper, delayed photoluminescence (DPL) and infrared-stimulated luminescence (ISL) spectra of Eu²⁺ and Sm³⁺ in CaS : Eu, Sm have been investigated. It is found that Eu²⁺ and Sm³⁺ show different characteristic luminescence in DPL. The explanation is based on the fact that Eu²⁺ ideally have the same local environment in the lattice position while Sm³⁺ have a more complicated local environment due to charge compensation. By studying ISL spectroscopy of CaS : Eu, Sm, different ISL behaviour has also been found. Since defects are formed in CaS : Eu, Sm by replacing divalent calcium with trivalent samarium in the lattice position, the ISL in CaS : Eu, Sm is thought to be contributed by combination of Sm³⁺ itself and defects formed in charge compensation.

The relaxation of a nonequilibrium distribution of electrons in a mixture of SF₆ and Ar is studied. In this paper, electron-SF₆ and electron-inert gas elastic collisions, vibrationally inelastic collisions between electrons and SF₆, as well as the electron attachment reaction with SF₆ are included in the analysis. The time-dependent electron energy distribution function is determined from the Boltzmann equation and the energy relaxation times are determined. The coupling of the thermalization process and the attachment process are discussed in detail. The results from the calculations (for 300 K and a total pressure of 1 Torr) are analysed analogous to experimental studies, and the methodology of the experimental reduction of the data is studied.

This paper presents the first application to an argon atmospheric plasma of a very recent derivation of a two-temperature (2T) transport properties theory, based on the Chapman-Enskog method expanded up to the fourth approximation, where only elastic processes are considered. The kinetic electron temperature T_e is assumed to be different from that of heavy species T_h, chemical equilibrium being achieved. This new theory, where electrons and heavy species are coupled, allows one to determine 2T diffusion coefficients which was not the case of the previous ones.

First, basic definitions of transport fluxes are recalled and a binary diffusion coefficient approximation is defined which involves an asymmetric relationship between these coefficients. Second, a particular care is taken in choosing the most recent data of potential interactions or elastic differential cross sections in order to determine the collision integrals. Third, a convergence study of transport coefficients is led to evaluate the influence of the non-equilibrium parameter θ = T_e/T_h on this convergence. It is shown that changing θ does not modify the convergence of transport coefficients. Moreover, ordinary and thermal diffusion coefficients, electrical and electron translational thermal conductivities as well as viscosity are displayed as functions of the electron temperature for different values of θ = T_e/T_h. It is pointed out that the non-equilibrium parameter θ has a non-negligible influence on transport coefficients. Besides, recently, it has been shown that the 2T simplified theory of transport properties, very often used in modelling, does not allow one to achieve mass conservation. Consequently, a comparison is presented between the 2T simplified theory and this new approach. Significant differences are found in the electrical conductivity and the electron translational thermal conductivity.

Three different types of tungsten-based cathodes (thoriated, lanthanized and ceriated) were used in plasma torches operating at different current intensities ranging from 30 to 210 A. For each one of the current intensities used, the erosion rate was measured for each of the cathode types, thereby obtaining for them the curves describing the dependence of the erosion on the current intensity. The effects of erosion on the cathode surface were explored by scanning electron microscope and energy-dispersive x-ray spectrometer.

On each of the types of cathodes studied, the cathodic erosion had a maximum for a critical value I* of the current intensity. The value of this critical current intensity depended on the dopant used. The following values were obtained: I_Th* = 50 A for thoriated tungsten, I_Ce* = 80 A for ceriated tungsten and I_La* = 110 A for lanthanized tungsten.

Several novel ceramic processing technologies (e.g. oxide ceramic melting and spheroidization) using an atmospheric pressure microwave plasma torch were recently developed in our lab. Understanding the processes and optimization requires complete characterization of the plasma as a function of operating condition. As a first step, a non-intrusive spectroscopic method was employed to map rotational (gas), electron and excitation temperatures and electron densities of the afterglow region of microwave generated atmospheric plasmas with and without alumina particle aerosol. Two-dimensional spatially resolved mapping of rotational (gas), excitation and electron temperatures and electron densities as a function of operating conditions during material processing were developed. It was shown that the passage of an aerosol dramatically changes the structure of the afterglow. Also the non-equilibrium nature of microwave generated atmospheric argon plasma was confirmed, suggesting that only multi-temperature models are capable of modelling this region of the plasma.

A hollow-cathode device has been shown to operate as a plasma reflector for radar electronic beam steering using helium in the 0.2-0.5 Torr pressure range. Compared to former experiments, the use of this light gas reduces significantly spurious sputtering effect on the cathode materials. In a previous paper, a semi-quantitative physical model was developed to calculate the time evolution of the sheet reflectivity from the experimental current I_d(t) measured across the discharge. A self-consistent, numerical, stationary model is now developed to describe the main physical mechanisms that govern the hollow-cathode source. The model describes the coupling between the high-voltage collisional sheath and the magnetized plasma through the hollow cathode. The cold electron creation rate includes the efficiency of ionization from the fast secondary electrons emitted from the surface of the cathode, lowered by the three-body recombination process in volume and by the ejection of a part of these fast electrons out of the cathode plasma. As the recombination rate scales as T_e^-9/2, the energy balance of the electrons must be solved precisely, so that the collisional-radiative exchanges in Helium are included in the model. The results are then compared to the experimental V-I characteristics for different pressures of the neutral gas; there is good agreement between the theoretical plasma model and the experiment.

An electrical corona discharge at atmospheric pressure in positive point-to-plane configuration creates an electric wind from the point to the plane which, within a few seconds, generates an axisymmetrical vortex in a closed cylindrical vessel (ring vortex). A first model for the establishment of this vortex was recently published and our study is an evolution based on an improved physical model. It does not include any empirical time scaling parameter, and takes into account the successive ionizing fronts as a varying electrical force due to the propagation of a sharp electrical field maximum acting on the space charge distribution along the interelectrode gap. After the front propagation (50 ns), the remaining positive ions drift (during 100 µs) in the slowly restoring electric field.

Numerical simulations show that the very first streamers and remaining positive ions rapidly push the gas along the very narrow discharge volume, creating a small ring vortex at the cathode around the discharge axis. The following ones have a cumulative effect leading to the enlargement of the vortex. In a few seconds, a quasi-stationary distribution is obtained, which is very close to the experimentally observed steady state.

The early oxidation stages of hydrogen-terminated single-crystalline (100) silicon exposed to a diluted N₂ : N₂O atmosphere at 850°C for different durations have been studied by x-ray photoemission spectroscopy, following the evolution of the Si 2p signal. Evidence is given that the usual analysis, in terms of five pairs of peaks attributed to silicon in the oxidation states from 0 to +4, does not account for the observed Si 2p signal. An explanation for silicon in unusual oxidation states is proposed.

The role of water in lipases, a class of proteins endowed with a large external hydrophobic surface, is not yet fully understood. To analyse the water-related structural properties and the possible implications for the protein functionality, three experimental techniques such as water sorption isotherms, thermally stimulated depolarization currents (TSDCs), and Fourier transform infrared spectroscopy were applied to pellets and/or films of lipase from Candida lipolytica, prepared at very low hydration degrees h, ranging between 0.003 and 0.457 g_water/g_protein. Two main broad TSDC bands (a weak one peaking at T≅160 K and a strong one at T≅260 K) were detected. The peak amplitudes and positions critically depend not only on the water content, but also on the previous hydration history of the sample. FTIR spectra monitored: (1) the amplitude and position changes of the characteristic optical absorption bands (amide A, B, I, and II) as a function of the humidity level, (2) the presence of a considerable amount of β-sheet structure at high hydration degrees, and (3) a conformational transition induced by drastic dehydration treatments. Complementary sorption isotherms, performed by means of a gravimetric method, showed a marked hysteresis in the lipase-aqueous solvent interaction. The whole set of results provides a model for the initial steps of the lipase hydration kinetics. At h = 0.009, 13 water molecules are buried in the macromolecule, probably bound to the peptidic backbone. At h = 0.037 all polar and charged free groups are hydrated. At higher h a solution phase begins, and at h = 0.457 about 660 water molecules are accommodated around the protein, giving rise to three to four complete layer coverages.

Modelling results are presented concerning the laser full-penetration welding characteristics. The effects of welding speed, Marangoni convection and natural convection on melt flow and heat transfer are all included in the modelling, and thus a three-dimensional (3-D) approach is employed. Comparison of the present 3-D modelling results with corresponding two-dimensional ones shows that besides the welding speed, Marangoni convection also plays critical role in determining the temperature distribution in the workpiece and melt flow in the weld pool and cannot be ignored even for the full-penetration welding of a thin plate. A method is described concerning how to use the present 3-D modelling results to estimate the keyhole radius or predict the energy efficiency in the laser full-penetration welding.

This paper is devoted to studies of the accuracy of approximate equations for the reflection function of semi-infinite turbid media. It is assumed that the probability of photon absorption β by a particle is low. We derive quadratic and cubic terms of the expansion of the reflection function with respect to (β)^1/2. Our results show that the cubic term makes a great improvement in the accuracy in the region of values of the similarity parameter that are not small.

Experimental determinations versus pressure of the nonlinear acoustic parameter B/A have been conducted for methanol, 1-butanol and 1-octanol in the pressure range 0-50 MPa and temperature range 303.15-373.15 K. These measurements proceed from an experimental technique based on a phase comparison method allowing to measure the change in sound speed with the pressure for an isentropic process. The value of B/A is found to decrease with increasing pressure and seems to be an increasing function of temperature. A comparison with the data determined numerically by the classical thermodynamic method has also been performed.

The ac electrical properties of acetylene black composites mixed into ethylene butylacrylate copolymer (EBA) and into poly (methyl methacrylate) (PMMA) have been measured in thermal cycling and isothermal annealing experiments. The results show that changes in electrical properties are due to rearrangement of gaps between the carbon black aggregates. This has been concluded using an exponent z that relates the critical frequency ω_c denoting the crossover of the conductivity from the dc-plateau to its frequency-dependent part to the dc conductivity, σ_dc, according to ω_c ∝σ_dc^z. Below the melting range of EBA and the glass transition of PMMA z is about one corresponding to strong variation of the conductivity and weak dependence of the permittivity on the gaps. Above the melting range of EBA z is about 1.5, indicating strong dependence of both the conductivity and the permittivity on the gaps, as predicted by percolation theory. This was not found in the PMMA composites above the glass transition. We conclude that the polymer matrix affects the nature of the gaps between carbon black aggregates, either allowing their size to vary continuously (z about 1) or letting them open and close (z about 1.5).

Two kinds of carbon nanotubes, multiwalled bamboo-like and well-aligned straight ones are obtained on quartz substrate by chemical vapour deposition using Fe-phthalocyanine. The bases of these nanotubes include nanoparticles with conical and cylindrical shapes, respectively. The former are found in smooth areas and the latter in ragged areas. Growth processes of these nanotubes are interpreted to be controlled by the degree of wetting of liquid metal particles in those areas. Strong warping observed in the graphitic shells of straight nanotubes is well rearranged by heat treatment at 2800\r{}C, giving shell spacing equivalent to the interlayer distance of a turbostratic graphite.

An effective thermal conductivity in layer melt crystallization was explored based on a model considering inclusions inside a crystalline layer during crystal growth, molecular diffusion of inclusions migration due to temperature gradient and heat generation due to recrystallization of inclusions in the crystalline layer. The effective thermal conductivity increases with time, in general, as a result of compactness of the layer. Lower cooling temperature, i.e. greater supercooling, results in a more porous layer with lower effective thermal conductivity. A similar result is seen for the parameter of melt temperature, but less pronounced. A high concentration of the melt results in a high effective thermal conductivity while low concentration yields low effective thermal conductivity. At higher impurity levels in the melt phase, constitutional supercooling becomes more pronounced and unstable growth morphologies occur more easily. Cooling rate and Reynolds number also affect the effective thermal conductivity. The predictions of an effective thermal conductivity agree with the experimental data. The model was applied to estimate the thermal conductivities of the crystalline layer during layer melt crystallization.

In this paper we apply variational calculus procedures for the optimization of a Curzon-Ahlborn thermal cycle under the so-called modified ecological criterion. Our result for the optimum efficiency is the same that Velasco et al (2000 J. Phys. D33 355) previously obtained by means of the method of the saving functions. Besides, we show that both the saving functions and the modified ecological criteria are equivalent.

Please see attached pdf for article.