Table of contents

In this work a theoretical and experimental thermal behaviour study of optical fibres with a high-purity SiO₂ core transporting concentrated radiative energy is carried out. A theoretical unidimensional model for the simultaneous transport of heat by conduction and radiation in optical fibres, including the heat losses by convection at the surface, is developed. This model considers a constant linear absorption coefficient and it is solved analytically. An experimental method to determine the linear coefficient of absorption is developed. The time evolution of the axial temperature distribution of two kinds of fibres is recorded and compared with the theoretical predictions. These experimental results validate the theoretical model proposed.

A systematic study has been carried out on the formation of an effective electron-injecting contact by depositing an LiF/Al bilayer on tris-(8-hydroxyquinoline) aluminium (Alq) in organic light-emitting diodes. Efficient electron injection is observed in both a LiF/Al bilayer cathode and an LiF-doped Al composite cathode. An analysis with ultraviolet photoelectron spectroscopy reveals a strong similarity in interface chemistry between LiF/Al and LiF-doped Al on Alq. Measurements with high-resolution electron energy loss spectroscopy show limited interfacial reaction of LiF on both Al and Alq, whereas a strong attenuation of the loss peak related to the Li-F stretch mode is observed after depositing an ultrathin Al film on Alq/LiF. The results indicate that the contact is formed as a consequence of chemical reaction with a reacted layer of 1 nm or less. Molecular orbital calculation suggests that the release of Li and subsequent reaction with Alq is thermodynamically allowed. The shallow-contact nature allows for much greater flexibility in the design of cathode structures and potential applications to various device configurations.

The problem of radiowave absorption and scattering by electrons (ions) moving about small particles is analysed. The solution of this problem gives a system of resonance orbits in which the electron can be present for a long period of time. Dissipative properties of this media are perhaps very high. An anomalous attenuation of electromagnetic (EM) waves in plasma containing small metallic particles has been observed. We attribute this phenomenon to the circular motion of ions around negative charged particles in resonance with the EM wave field.

A low current intensity study of a cutting plasma torch is presented. The operating gas is oxygen discharging in an air environment. A two-dimensional turbulent plasma model is developed with the commercial code Fluent 4.5. An experimental and a theoretical study are presented. Two configurations were used: one where the arc is transferred to a rotating anode 19 mm away and the other in a real cutting configuration (distance nozzle exit-workpiece around a few millimetres). In the first configuration, spectroscopic measurements are made and compared with the model. The supersonic plasma behaviour is shown with a Mach number of 1.5 at the nozzle exit. The turbulent effect on the mass fraction field is presented. It concerns the effects of turbulence on the presence of oxygen near the plate, and by a comparison of theoretical and experimental temperatures we conclude that the arc presents turbulent behaviour. In the second configuration, a power balance of the cutting process is presented above and in the thickness of the plate. The model shows that the most important contribution to the fusion process is due to convection, conduction and radiation terms.

A model of streamer propagation is developed, which accounts for heating and radial expansion of gas in the streamer channel. The model describes a stepped pattern of streamer propagation at applied voltage constant in time. The results of numerical simulation of positive streamer dynamics in a sphere-plane gap in SF₆ at 2 bar are presented.

As part of a complete theoretical description of the behaviour of the electric arc in the vacuum arc remelting process, a model has been developed for the column of plasma generated by a single cluster of cathode spots. The model combines a kinetic approach, taking into account the formation of the plasma in the cathodic region, and a hydrodynamic approach, describing the expansion of the plasma in the vacuum between the electrodes. The kinetic model is based on a system of Boltzmann-Vlasov-Poisson equations and uses a particle-type simulation procedure, combining the PIC (particle in cell) and FPM (finite point set method) methods. In the two-dimensional hydrodynamic model, the plasma is assimilated to a mixture of two continuous fluids (the electrons and the ions), each described by a system of coupled transport equations. Finally, a simplified method has been defined for calculating the electric current density and the energy flux density transmitted by the plasma to the anode. The results of the numerical simulation presented are consistent with a certain number of experimental data available in the literature. In particular, the model predicts a percentage of the electric power of the cluster transmitted to the anode (25%) in good agreement with the value indicated in the literature.

The heteroepitaxial yttria-stabilized ZrO₂ (YSZ) films on Si(001) substrates with various nano-grade thicknesses from 1.5 to 145 nm were prepared by pulsed laser deposition. Both the thermal coefficient mismatch and lattice constant mismatch effects on the high-thermal- and lattice-mismatch YSZ/Si(001) films have been studied mainly by the high-resolution x-ray diffraction. The curvatures and in-plane and out-of-plane lattice constants of film and substrate were measured by the high-resolution x-ray diffraction. The experimental radius of the overall curvature has a good agreement with the thermal coefficient mismatch model, indicating that the thermal residual stress ~1.7 GPa is the main stress source in this kind of film system. Furthermore, the film mosaic structure shows a fan-like uniform tilt with a radius in the inversed direction with respect to that of substrate. The huge tension stress of thermal coefficient mismatch provides an enhanced effect on the intrinsic mosaic dispersion of film geometrically. A possible misfit dislocation distribution was described for the huge mosaic tilt.

We report the far-infrared (FIR) reflection measurements of Pb_1-xSr_xSe thin films with different Sr concentrations grown by molecular beam epitaxy on BaF₂ substrates. Sr composition and lattice parameter have been assessed by x-ray diffraction. By fitting the FIR reflection spectra and by comparison with the results of binary PbSe and SrSe thin films, both PbSe-like and SrSe-like optical phonon reflection bands in ternary PbSrSe thin films have been identified. Also, the carrier concentration and mobility of PbSrSe thin films have been extracted from the theoretical calculation of the reflection spectra, and are found to be in agreement with reported results from Hall measurements.

We present band structure results for a new two dimensional (2D) rectangular array geometry of water (mercury) cylinders of square cross section in a mercury (water) host. The results show that the water/mercury system, consisting of low-density cylinders in a high-density host, is the most favourable configuration for obtaining large acoustic gaps. Otherwise, only very small stop gaps can be found for the mercury/water systems. For a given cylinder width value, the lowest band gap may not always have the maximum width, but at some value in both systems the lowest band gap will always have the largest width. The differences in the case of circular cylinders are also discussed.

Studies of the dynamics of liquid water in the pores of hydrating cement materials were performed by means of various nuclear magnetic resonance techniques, such as spin-echo T₂ relaxometry, echo-detected saturation-recovery T₁ relaxometry and pulsed field gradient diffusometry. While the diffusion coefficients and the transverse relaxation times were found to decrease monotonically with hydration time, the longitudinal relaxation time exhibits a transient minimum during the onset of the acceleration phase. Earlier explanations of the minimum can be ruled out on the basis of our experimental data and a new one is suggested. The findings are corroborated by observations in cement pastes with added fine particles.