Table of contents

The thermal conductivity of electrochemically etched porous silicon (PS) layers was determined over a wide temperature range (T = 35 - 320 K) using the dynamic technique. Both the doping level of the silicon wafers (p and ) and the porosity P of the porous layers (P = 64 - 89%) were varied. The measured thermal conductivities were three to five orders of magnitude smaller than the values for bulk silicon. Furthermore, they increase with increasing the wafer doping level and with decreasing the porosity P of the layers. For all investigated PS layers the thermal conductivity increases with temperature. The results are discussed in terms of a simple model for heat conduction in PS based on the phonon diffusion model.

From the optical emission of and radiative states in the negative glow of DC pulsed discharges at pressures 1 - 4 Torr (133 - 532 Pa) and current densities , it has been determined that the vibrational excitation of the state deviates from a Boltzmann distribution and strong intensities of the second positive system (especially from v = 1) were also observed in the afterglow. Such strong emission is interpreted in terms of the growing of the vibrational distribution in the post-discharge region as the gas temperature is reduced. The ) states are mainly produced by electron collisions on ions under discharge conditions and quickly disappear in the afterglow as a result of - electron recombination.

UV - visible absorption, steady state and time-resolved fluorescence techniques were used to investigate the aggregation behaviour in Y-type hemicyanine Langmuir - Blodgett (LB) multilayers. Hemicyanine molecules formed H-aggregates in LB films leading to blue shifts of the absorption and fluorescence bands with respect to those of monomeric hemicyanine in chloroform solutions. The aggregates in LB films were dissociated by mixing with arachidic acid. The fluorescence decay profiles show that two main species (monomer and aggregate) exist in the LB films.

Electrical impedance measurements over the frequency range 10 Hz to 1 MHz are reported for fatty acid Langmuir - Blodgett (LB) films containing an organometallic tetrabutylammonium Ni(dmit) complex. Experiments were undertaken using both in-plane and out-of-plane electrode configurations. In the latter case, a relationship between the AC conductivity and frequency of the form was found, with n = 0.51 for a film containing a 0.31 molar fraction of fatty acid. This n value was found to decrease with an increase in the amount of the charge-transfer compound. For the in-plane AC measurements, the conductance of the sample was dominated by the DC conductivity of the LB film.

(Fe,Ti) - N films containing phase were prepared on Si and NaCl single-crystal substrates by facing target sputtering (FTS). An analytical transmission electron microscope was used for the structural studies prior to and during in situ annealing of the samples. The as-deposited films with 3 - 5 at.% Ti contained and phases and films with 15 - 18 at.% Ti comprised phase and TiN phase. The phase was the dominant phase in the two samples. It existed stably during in situ annealing from room temperature to C, and remained stable when the samples were cooled down to room temperature. It is found that proper Ti addition can stabilize the phase to a temperature of C. The saturation magnetization of the two annealed samples was 2.68 and 2.46 T respectively, which is larger than that of pure iron.

The helical Cerenkov effect results from the helical electron motion in a medium caused by a magnetic field B. For B values of up to 100 T in strength helical Cerenkov radiation occurs in the visible spectrum for which an analytical expression for the number of emitted photons per unit path length is derived. In general, however, for a non-mono-energetic beam at a fixed radiation angle and a fixed arbitrary frequency, one may have simultaneous contributions from the helical Cerenkov effect and radiation occurring below and above the threshold of the helical Cerenkov effect with the fundamental frequencies and their harmonics. When B increases we come to the asymptotic regions where the number of harmonics on both sides of the threshold first decreases towards the respective fundamental frequencies and, eventually, disappears altogether. Also, a general asymptotic limit, in which there is no radiation below and above the threshold of the helical Cerenkov effect and only the helical Cerenkov effect remains at any angle of radiation and for any medium, we argue, should occur at about T for the visible spectrum.

The characteristics of a segmented hollow-cathode He - 781 nm laser are modified by varying the cathode to anode area ratio (C/A), in steps from 3.0 to 0.33, for a fixed bore diameter of 4 mm. The discharge voltage for a given current to pressure ratio, and consequently the small-signal gain and multimode output power for a given current, are significantly increased when C/A is reduced. For a given gain in excess of 20% , the input power decreases slightly as C/A is reduced. However, for a given multimode output power, the input power is almost independent of C/A (0.33 to 2.0). For a gain of 60% the current density decreases from 450 to 340 mA when the area ratio is reduced from 3 to 0.5.

A general cycle model including several major irreversibilities existing usually in real heat transformer systems is established and used to investigate the optimal performance of a heat transformer affected by the irreversibilities of finite-rate heat transfer between the working fluid and the external heat reservoirs, heat leak losses of the heated space, and internal dissipation of the working fluid. The coefficient of performance and the specific heating load of the cycle system are used in turn as objective functions for optimization. The maximum values of these parameters are calculated for a given total heat-transfer area of heat exchangers. The dimensionless specific heating load versus coefficient of performance curves describing the general performance characteristics of heat transformers are presented. The practical operating region of the heat transformer is discussed.

A relativistic electron beam propagating through an annular plasma sheath is subject to a transverse plasma - electron coupled electrostatic instability. From the linearized fluid equations, the beam - sheath interaction is resolved into three coupled equations. The corresponding wakefield is computed and the asymptotic linear evolution is noted. For illustration, numerical examples are given for a plasma accelerator employing such a sheath. While the coasting beam scalings are quite severe at low energy, single-bunch instability growth can in fact be reduced to nil for a very high-gradient accelerator.

Assuming local thermodynamic equilibrium (LTE), the total radiation which escapes from a - plasma is calculated in the temperature range between 5000 and 30 000 K. The evaluation of the net emission coefficient is performed by solving the radiative transfer equation in a cylindrical isothermal plasma taking into account line broadening phenomena. The radiation emission of methane depends mainly on the carbon species, whereas the presence of hydrogen in the mixture leads to a radiation decrease especially at high temperatures.

The macroparticle (MP) flux distribution in a quarter-torus filtered vacuum arc deposition system was experimentally investigated and compared with theoretical predictions based on a previously developed model. The average MP flux was measured by collecting the MPs generated from a Ti cathode on substrates located at different radial positions on the entrance and the exit planes of the quarter-torus filter, and measuring the MP size distribution function using an automated microscopic image analysis system.

The average MP flux along the torus centre line decreased by a factor of 50 from the entrance to the exit, while the average MP flux decrease near the outer wall of the torus was more moderate. The flux near the outer wall was greater by a factor of three than the centreline flux. Increasing the toroidal magnetic field from 4 to 10 mT resulted in no substantial change in the MP flux on the centreline, while near the outer wall it decreased by a factor of two. The experimental results on the MP flux in the near outer wall region are consistent with electrostatic reflection of the charged MPs by the charged wall.

In this work an attempt is made to understand theoretically the trigger mechanism of a Pierce-type hydrodynamic instability found experimentally in thermionic discharges at low pressure. Particle-in-cell simulations are used to obtain detailed information about plasma parameters such as phase space and potential distributions. In contrast to earlier studies, the simultaneous influence of the ion dynamics, collisions with neutrals, and the sheath capacitance is taken into account. This is done by adopting the Lagrangian description of a general Pierce diode model developed previously. The new results obtained are threefold. First, few ion - neutral collisions prevent the coupling between electron and ion dynamics on the electronic time-scale thus removing oscillatory growing Pierce - Buneman modes. Second, the classical Pierce-diode supplemented by an external capacitance to account for the damping effect of the sheath on the Pierce instability explains the existence of stable equilibria. It further reveals the nature of the bifurcation at the instability threshold, i.e. a Hopf bifurcation is the trigger mechanism of the non-linear relaxation oscillations observed in both simulation and experiment. Third, the transient behaviour prior to the instability onset is found to be qualitatively described within the hydrodynamic model.

This paper is devoted to the study of the phenomenon of arc re-striking in low-voltage circuit breakers. The arc re-strike can be described as a sudden re-appearance in the arcing contact region when the arc had been situated in the quenching chamber a few tens of microseconds before. Our experimental investigations have established that the critical arcing contact region is still crossed by a so-called residual current of the order of several amperes. A gas temperature of around 4000 K was derived both from fine electrical measurements and from a molecular spectroscopy technique just before the occurrence of the arc re-strike. We also demonstrate that the re-strike takes place through the growth of the remaining current of several amperes in the arcing contact region. A numerical approach was carried out with a two-dimensional hydrodynamic code. This was found able to describe the arc movement in the model circuit breaker throughout a high-current-interruption operation and, notably, to simulate the arc re-strikes. The simulation exhibits the role of the flow of gas evaporated from the wall in the process of maintaining a slightly conductive medium in the arc ignition region.

The determination of plasma parameters in RF capacitive discharges enhanced by an external magnetic field using emission spectroscopy, double Langmuir probes and thermocouple methods is reported. The gas temperature, rotational temperature of CH radicals, electron temperature, and charged particle and atomic hydrogen densities as functions of the discharge parameters are determined. The system operating parameters can be adjusted in the following ranges: gas pressure in the discharge chamber 0.1 - 10 Pa; discharge power 80 - 200 W; magnetic field strength 0 - 200 G. Pure methane and pure argon, as well as mixtures of methane and argon in various proportions were used as working gases. In addition, carbon films were deposited on glass substrates and their optical properties were investigated. It is shown that the external magnetic field has a significant influence on both plasma parameters and properties of the deposited films.

Quantitative x-ray diffraction investigations were performed on ultrafine-grained copper samples with various crystallite sizes processed by severe plastic deformation. In the x-ray diffraction patterns, common features such as the relative maximum intensity, considerable broadening, long tails, centroid positions of Bragg reflections shifted to larger diffraction angles and increased background integrated intensities are revealed. The evolution of the shape of the Bragg reflections, background integrated intensity, microstrain, dislocation density, lattice parameter and atomic displacement with decreasing crystallite size was investigated. The obtained results are analysed and discussed in terms of the structural model of ultrafine-grained materials processed by severe plastic deformation.

A novel heterojunction high-frequency source is described. It operates based on a polarization-induced limit cycle oscillation of the trapped-hole charge in one of the barriers of a double-barrier heterostructure with staggered band-gap alignment. A physically based mathematical model of quantum transport rate equations yields the dynamical behaviour of this device structure. The condition for the existence of autonomous device oscillation is derived. The amplitude of the trapped-charge oscillation increases with the applied bias. A proof of the step function characteristic of the time-averaged values of the trapped hole charge in the barrier as a function of bias is given. These results yield agreement with the experimental measurements of the `parallelepiped' hysteresis of the average current - voltage characteristic of an AlGaSb/InAs/AlGaSb heterostructure. Thus a class of novel high-frequency oscillators useful for high-bandwidth applications is introduced.

The thermoluminescence excitation spectrum of single crystals of was measured and its convolution with solar spectral irradiances reaching the ground was estimated from a radiation-transfer model. The same crystals were used to register direct solar ultraviolet-C radiation reaching the ground at Madrid (N, W, 700 m ASL) on clear-sky days. By comparing the ultraviolet dose thermoluminescence sensitivity with the measured dose of solar irradiation, we concluded that, although UV-C irradiation at ground level is six orders of magnitude smaller than the UV-B, the solar UV-C radiation at ground level is nonetheless detectable and can be measured using dosemeters.

Two sets of silicon detectors were irradiated with 1 MeV neutrons to different fluences and then characterized. The first batch were ordinary p - i - n photodiodes fabricated from high-resistivity silicon, while the second batch were gold-doped power diodes fabricated from silicon material initially of low resistivity . The increase in reverse leakage current after irradiation was found to be more in the former case than in the latter. The fluence dependence of the capacitance was much more pronounced in the p - i - n diodes than in the gold-doped diodes. Furthermore, photo current generation by optical means was less in the gold doped devices. All these results suggest that gold doping in silicon somewhat suppresses the effects of neutron irradiation.