Table of contents

Low density polyethylene 500 µm thick samples have been submitted to a 15 MV m^-1rms ac 50 Hz field. No charge accumulation in the samples has been observed after a few hours of polarization. This has been attributed to the recombination of the majority of electrons and holes alternately injected each half cycle. To confirm this assumption, a space charge has been injected in the sample by using a dc electrical stress. Coupled measurements of space charge and electroluminescence have been made under a single and a double alternation ac 50 Hz field. Results have shown that the decrease of the installed charge at the interfaces is partially due to the recombination with injected carriers.

In this review, I will describe the combination of pseudopotentials and density functional theory to determine the electronic structure of matter. This combination, called the pseudopotential-density functional method (PDFM), represents the most popular technique for examining a wide range of structural and electronic properties. I will illustrate applications of the PDFM to problems of current interest: nanostructures and other complex confined systems.

Dispersion of surface-wave-produced plasmas and field behaviour outside the discharges are studied under the influence of collisions and radial and axial density inhomogeneities, the latter density, both in WKB and eigenfunction treatment, is carried through for a cylindrical discharge configuration. The trends of joint influences can be approximated by an extrapolation from WKB treatment, avoiding the large numerical effort of an accurate two-dimensional code. Some consequences for radiophysics diagnostics are pointed out. Whereas standard methods of phase measurements are only weakly affected, amplitude measurements are more subject to various influences. Therefore, potential extensions of methods using test waves of various frequencies can yield additional and improved information on the collision frequency and on the value of the radial inhomogeneity index. A procedure is suggested and exemplified with measurements at a helium discharge.

HoH_yexhibits dramatic changes in both structural and optical properties as yis varied from zero to three by hydrogen loading. This work reports on the effect of such loading upon epitaxial single-crystal Ho films grown by molecular beam epitaxy on (110) Nb (11 0) Al₂ O₃substrates. Upon loading, the film undergoes a structural transition from hcp metal ( -phase, H in solid solution) to the fcc dihydride ( -phase). There is a further transformation between the dihydride phase and the hexagonal trihydride ( -phase). Three films of HoH_y , where, nominally, y= 0,2,3, were studied by XRD, AFM, UFM, SEM and TEM. Triangular networks of features of width ~300 nm and height ~20-30 nm that align with the [0 1], [ 01] and [1 0] directions of the dihydride sample are seen on the surface of both the dihydride and the trihydride samples, but have a much greater density on the surface of the latter. Such features are not present on the as-grown metal layer. In situcontrolled-environment transmission electron microscopy (CETEM) studied the effects of hydrogen loading within the dihydride phase, in cross sectional geometry. During loading, slip within the dihydride phase was observed. The hypothesis that the triangular features are due to the precipitation of the di- and trihydride compounds, with expanded lattices, on the slip planes within the - or -phase host crystal is discussed.

Photoreflectance spectra have been measured to determine the E₁and E₁ +₁energies in ZnTe at temperatures Tbetween 77 and 300 K using an Ar⁺ -ion laser as a modulation light source. The measured photoreflectance spectra can be successfully explained by an excitonic line shape. The temperature dependence of the E₁and E₁ +₁critical-point parameters (energy, amplitude and broadening parameter) have been analysed using the Varshni equation and an empirical expression of the Bose-Einstein type.

A comment correcting this article has been published: Molina, Espinosa-Luna and Adachi 2004 J. Phys. D: Appl. Phys.37 2047

Thin films of ferroelectric PbTiO₃(PT) and Pb(Zr_0.5 Ti_0.5 )O₃(PZT) as well as antiferroelectric PbZrO₃(PZ) have been prepared on LaNiO₃ /SiO₂ /Si substrates by nebulized spray pyrolysis (NSP) of metal-organic precursors. The metallic LaNiO₃(LNO) electrode layer was also deposited by NSP. The ferroelectric films obtained show satisfactory morphology and desirable dielectric properties. Typical values of the coercive field, remnant polarization and dielectric constant (300 K) for the PT/LNO/SiO₂ /Si film are 170 kV cm^-1 , 22 µC cm^-2and 210, respectively, with the corresponding values for the PZT/LNO/SiO₂ /Si film being 120 kV cm^-1 , 13 µC cm^-2and 540, respectively. The PZ/LNO/SiO₂ /Si film shows typical antiferroelectric characteristics including the electric-field induced reversible antiferroelectric - ferroelectric transition. The various films deposited on LNO/SiO₂ /Si by NSP are comparable in all respects to those prepared on Pt/Ti/SiO₂ /Si by the same technique.

Titanium dioxide (TiO₂ ) nanocrystals are prepared by a hydrolysis process of tetrabutyl titanate. Nanocrystal samples with various sizes of 6.8-27.9 nm are obtained after annealing from 100 to 650 °C. The crystal structures and the average particle sizes are examined using x-ray diffraction. Raman scattering was employed to investigate the evolution of the anatase phase in the nanocrystals during annealing. Phonon confinement and non-stoichiometry effects are responsible for the blueshift and broadening of the lowest-frequency E_gRaman mode. The influence of interfacial vibrations on the Raman linewidth is also discussed.

Fe₄₀ Al₂₄ O₃₆(atomic ratio) nano-structured granular films were prepared by rf reactive sputtering, and the microstructure, electrical and magnetic properties as well as the tunnelling magnetoresistance (TMR) effect were studied systematically. It is found that the samples consist of small Fe particles with the diameter around 2-4 nm separated by an Al₂ O₃dielectric matrix, and the conductance mechanism is thermally-activated tunnelling. The samples display superparamagnetic properties at room temperature. Considering the size distribution of metal particles, the magnetization and TMR curves can be fitted with the Langevin formula and the Inoue model quite well. TMR is caused by spin-dependent tunnelling, it is closely related to the structure and magnetic state of the samples. Under the optimal conditions, the TMR value reaches a maximum of 5.7% at room temperature under 1.6 T magnetic field.

The magnetoresistive behaviour of La_0.67 Ca_0.33 MnO₃thin films deposited by ultrasonic spray pyrolysis on YSZ substrate has been investigated at low applied dc magnetic fields. The highest magneto-resistance ratio (MR ) of -17.75% has been observed at 77 K for applied dc magnetic field, H_dc= 3 kG. The variation of the MRwith temperature has been investigated and it has been observed that at lower temperatures the increase in MRis very rapid. This MR -Tbehaviour shows that the magneto-resistance has its origin in the grain boundaries. The variation of the MRwith H_dcshows a faster increase in the MRat lower applied dc fields. Surface morphological explorations employing scanning electron microscopy show the presence of microcracks throughout the films surface and also that these films are micro-crystalline, having an average grain size of ~125 nm.

The effect of Co addition combined with W on the microstructure and magnetic properties of nanocomposite magnets with composition of (1-xwt%)Nd_8.55 Fe_84.49 W_0.60B_0.636 +xwt% Co, prepared by mechanical alloying, has been investigated. The addition of W can increase the unit cell volume of soft and hard phases in nanocomposite magnets, and cause the anisotropy field of the hard phase component to increase. Therefore, the addition of W can improve coercivity. On the other hand, addition of Co leads to a decrease of the grain size of the -Fe soft phase component in nanocomposite magnets annealed at temperatures lower than 670 °C. It leads further to a decrease of the optimal annealing temperatures, an increase of the remanence and maximum magnetic energy product, and a slight decrease of the coercivity. With increasing the content of Co, the dependence of remanence J_ron annealing temperature T_achanges from a convex shape, giving a maximum, to a monotonically decreasing one. These phenomena have been explained in terms of the variation of grain growth and exchange coupling between the grains, due to the addition of Co and W.

A computer code simulating the interaction-by-interaction discrete slowing down process of heavy charged particles (i.e. protons, -particles, and bare heavier ions of z <10) of speeds greater than about 0.3-1 MeV amu^-1in water medium is presented. Along with the primary particle, the transport of all secondary (and subsequent generations) electrons is explicitly followed down to the minimum electronic excitation potential of a water molecule. Monte Carlo sampling techniques were applied to accurate probability distribution functions characterizing the particle-molecule interaction. The entire secondary electron energy spectrum has been modelled by a semi-empirical approximation combining the leading term of Bethe's asymptotic cross section with an appropriate binary collision formula. The elastic scattering cross section was modelled by a modified Rutherford formula that accounts for screening, while impact excitation was modelled by an empirical formula which exhibits the correct (Born-Bethe) asymptotic behaviour. Binding effects for the most important molecular transitions are explicitly accounted for by partial inelastic cross sections. By following the histories of all particles, the code scores the spatial coordinates of all elastic and inelastic collision events, as well as the partitioning of energy loss to the various inelastic channels.

The laser beam absorption lengths of CO₂laser and a high-power diode laser (HPDL) radiation for the ordinary Portland cement (OPC) surface of concrete have been determined. By employing Beer-Lambert's law the absorption lengths for concrete of the CO₂laser and the HPDL radiation were 470±22 µm and 177±15 µm, respectively.

A numerical simulation was undertaken on the neon isotope separation in a dc discharge through a narrow capillary. The mass transport phenomenon of neutral particles as well as ions in the positive column was treated by the direct simulation Monte Carlo (DSMC) method, whereas electrons were treated as background particles in the simulation to evaluate the effects of electron collisions onto the neutral particles, by applying the concept of a hybrid particle simulation. The numerical results qualitatively agreed with existing experimental results, concerning not only the isotope separation phenomena, but also the pressure difference between the region of the anode and that of the cathode. It was also found that the neutral particles of the positive column flow to the anode side near the axially central region due to the electron impact, whereas they flow to the cathode direction near the capillary wall due to the ion wind.

Deviations from ionization equilibrium caused by diffusion in the near-cathode region of an atmospheric-pressure argon arc discharge are studied. The model comprises the continuity and transport equations for electrons, atoms, and singly-, doubly- and triply-charged ions. A numerical solution of these equations is obtained in a wide range of electron temperatures in the layer. Distributions of densities of the species across the layer and the ion currents to the cathode are presented. It is shown that in the whole range of the electron temperatures the major contribution to the total ion current to the cathode is due to singly-charged ions. A comparison is presented of the obtained numerical results with those given by a model with only singly-charged ions.

This work deals with the study of deviations from thermal equilibrium in high-pressure mercury discharges operating in ac mode. A one-dimensional time-dependent fluid model assuming a non-equilibrium plasma (two-temperature code) has been developed. This code solves, self-consistently, the set of hydrodynamic equations describing the discharge plasma, coupled with an equation describing the power supply circuit. This code is based on a finite-element method and it has been optimized in order to keep CPU times realistic.

Our calculations at an industrial 50 Hz frequency, validated by using experimental data from the literature, allow us the possibility to better understand the mechanisms that are responsible for deviations from thermal equilibrium within an ac cycle. Furthermore, a parametric study allows us to study in detail the influence of some key parameters, such as frequency and mercury load on those deviations.

A simple formulation and solution strategy for the Saha equation is introduced. The formulation discriminates between the cases in which either the pressure or the number density of heavy particles is known. This discrimination allows the method to be generalized to include all problems of practical interest, as well as to clarify ambiguities found in other formulations in the literature. The present method overcomes restrictions imposed on other competitive techniques and takes into account all possible formulae for nonideality corrections. In most practical cases the solution of the nonlinear set of the Saha equations is reduced to the simple problem of solving a single transcendental equation.

Results are presented of friction and wear measurements during diamond polishing using new instrumentation designed and built in our laboratory. The wear rates are found to be highly anisotropic, depending both on the crystallographic plane and on the direction along which sliding occurs. Friction measurements performed during polishing on a {100} plane in the 100 direction (the direction of the highest wear rate on the plane) and the 110 direction (the direction of the lowest wear rate) exhibit similar trends to those occurring in friction at low speed on the {100} face, with the 100 direction having a higher friction coefficient than the 110 direction. It was further found that the variation of friction with pressure in the 100 and the 110 directions are different, pointing to the fact that two wear mechanisms operate in the two principal directions. The results are consistent with, and provide macroscopic evidence in support of, a model of mechanically induced transformation of diamond to sp²hybridized carbon occurring on the diamond surface during polishing along the direction in which the wear rate is high. In contrast to this, the wear process proposed for the 110 direction is one of micro-fracture, in agreement with earlier studies of the polished surface and the wear debris. By explaining the wear mechanism in terms of two different types of processes operating in the two directions, we are able to understand the high anisotropy observed.

Two grades of nuclear graphite, an isotropic graphite (GCMB) and an anisotropic graphite (PGA), were thermally oxidized to high weight losses. Average values of the coefficient of thermal expansion (CTE) in the temperature range 20-600 °C for GCMB and PGA in the parallel direction were unaffected by thermal oxidation up to 60% and 50% weight loss, respectively. The average CTE values for the PGA samples in the perpendicular direction were also unaffected by oxidation in the range 10-50% weight loss, although a slight increase in the CTE in the early stages of oxidation was observed. A new model based upon a continuous network of material, able to transmit thermal strains, which persists after oxidation, was proposed to explain the insensitivity of the CTE to oxidation. The transmission of thermal strains in the continuous network model was considered as a percolation phenomenon. Application of percolation theory to the effect of thermal oxidation on electrical conductivity of oxidized GCMB graphite suggests that the percolation threshold occurs at very high levels of oxidation that are in excess of 95% weight loss.

Discharge currents of low-density polyethylene after a polarization under an ac 50 Hz field have been investigated in both 100 µm thick films and 1 mm thick slabs. These currents have been recorded against the polarization field (10-40 MV m^-1rms), temperature (30, 50 and 70 °C) and the time of polarization (2-200 min). Excellent reproducibility has been observed, except at 70 °C, for which a physical change in the sample's morphology has been suggested. No differences were observed when the polarization time was increased from 2 min to 200 min. The voltage has been switched off after a specified polarity and the results have shown that the sign of the discharging currents can be directly related to this polarity. The effect of the injection process can be neglected as shown by a preliminary measurement made with a novel set-up. These discharge currents have been attributed to the presence of a space charge build-up during the polarization. We propose a model in which carriers are predominantly produced within the bulk. This indicates that these carriers could be activated from donor-like states or could originate from the ionization of impurities which move under the influence of the field and create microscopic charge packets. A numerical simulation has revealed that these currents can result from charge motion and diffusion. We discuss the theoretical results, which are in reasonable agreement with our discharge current measurements.

Quartz undergoes very significant luminescence sensitivity changes after high temperature annealing (0-1200 °C), with particular enhancement occurring between the phase transition temperatures 573 and 870 °C. In order to understand why this occurs, high frequency electron paramagnetic resonance (EPR), operating at 90 GHz, has been used to monitor the structure and population of defects in natural sedimentary quartz, following annealing and -irradiation. The results are compared with the optically stimulated luminescence (OSL) data of the same samples. It is shown that: (i) the structure and population of the dominant [AlO₄ ]⁰recombination centres are largely unaffected by the annealing process; (ii) the oxygen vacancy E´ centres are destroyed when annealed at temperatures between the phase transitions and; (iii) the numbers of both [TiO₄ /H⁺ ]⁰and [TiO₄ /Li⁺ ]⁰donors increase between 400 and 700 °C. Photo-EPR spectra are presented, providing evidence that both the Ti associated donors and Al acceptors are directly involved in the OSL process. The heat-induced changes in the population of these EPR defects is mirrored in part by the change in the luminescence sensitivity of several OSL components. Evidence is also presented suggesting that E´ may act as non-radiative centres competing in the OSL process.

The low-temperature annealing effect on electron field emission of tetrahedral amorphous carbon (ta-C) films was investigated. Ta-C films prepared by a filtered arc deposition technique were annealed at 400 °C for 60-120 min in a nitrogen atmosphere. The density and sp³content in as-grown film were 3.3 g cm^-3and ~90%, respectively. After annealing, no obvious changes in film density or sp³content were observed. The threshold electric field of electron emission of annealed ta-C films increased. Through annealing the clustering of sp²bonded carbon would occur, resulting in bandgap reduction and the eventual increase of the emission barrier. After a given annealing period, the degree of clustering increased, and was responsible for the threshold electric field increasing further. The results implied that the control of the sp²bonding configuration in addition to the sp³bonding is important in obtaining good emission properties for ta-C films.

Stationary solitary signals with eigenfrequencies in the gap of the linear spectrum are considered for a bi-inductance electrical transmission line. The evolution of these localized waves is studied in the phase plane and analytical expressions are obtained. Moreover, theoretical and numerical examinations of the stability of these nonlinear waves which appear as kink or antikink solitons, show that they are long-lived nonlinear excitations of the bi-inductance line.