Table of contents

In this communication, the plain and continuous epitaxial CoSi₂ was first synthesized on Si(111) wafers with metal vacuum vapour arc ion implantation without in situ heating or post-annealing. The measurements on the CoSi₂ layers showed that increasing the formation temperature and the implantation dose can improve the crystallinity of CoSi₂.

We present an investigation of the temperature dependence of the current-voltage characteristics of a single carrier poly[2-methoxy, 5-(2´-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) polymer light-emitting diode (LED) with indium tin oxide (ITO) and aluminium electrodes. We find that although injection effects are the dominant current limiting mechanisms, bulk transport effects also play a significant role due to the low charge carrier mobility. Tunnelling theory cannot account for the observations made and we find that the tunnelling barrier predicted by a simple Fowler-Nordheim analysis increases by more than a factor of two from 0.2 to 0.45 eV as the LED is cooled from room temperature to 13 K. We use a device model combining the interfacial currents of thermionic emission, tunnelling and interface recombination, with drift currents to describe successfully the measured current-voltage characteristics over a temperature range from 300 K to 100 K using the barrier height to injection and the field-dependent mobility as fitting parameters. Good quality fits are obtained over a wide range of temperatures. The field-dependent mobility parameters are found to have a temperature dependence characteristic of hopping in disordered solids with an activation energy of 0.75 eV for the zero field mobility. The barrier to injection for holes from the ITO anode is found to be lowered from 0.56 eV at 300 K to 0.41 eV at 100 K, which can in part be accounted for by the reduction of the band gap of MEH-PPV with decreasing temperature. The temperature dependence of the parameters is greater than values reported elsewhere, suggesting that approximating the thermally-activated, microscopic injection processes by a thermionic emission term overestimates the activation energy for charge carrier injection. We looked at the effect of photo-oxidation on device characteristics as a further test of the model and found that these modified characteristics are well described by a substantial reduction in mobility at constant barrier height.

A diagram is derived that visualizes the interplay of macroscopic thermal, mechanical, and magnetic effects in a diamagnetic crystal. The derivation is based on a Taylor expansion of a thermodynamic potential and on a theorem on the transformation behaviour of property tensors under time inversion. The interaction diagram provides a quick answer to the experimenter on the question of which linear and quadratic physical effects are possible in the given physical situation and how the measurement of an effect under study might be influenced by cross-effects.

The magnetic properties and magnetic-domain structures of nanocrystalline Sm₂Fe_14.5Cu_0.5Ga₂C_y (y = 0-2.0) and Sm₂Fe₁₅Ga₂C_1.0 alloys prepared directly by melt-spinning were studied and compared with the case of over-quenching and annealed ribbons. The coercivity reaches a maximum value at a wheel speed of v_s = 12.5-15 m s^-1 for Sm₂Fe_14.5Cu_0.5Ga₂C_y (y = 0-2.0), and 17.5 m s^-1 for Sm₂Fe₁₅Ga₂C_1.0. The coercivity increases significantly with increasing carbon content and reaches a maximum at y = 1.0-1.5. An increase in coercivity was also observed upon addition of a small amount of Cu. The maximum coercivity µ₀H_c at room temperature, obtained for the various ribbons at their optimal v_s value, is 2.2 T for Sm₂Fe_14.5Cu_0.5Ga₂C_1.5 and Sm₂Fe_14.5Cu_0.5Ga₂C_1.0, and 1.8 T for Sm₂Fe₁₅Ga₂C_1.0. The magnetic-domain structure was investigated by using magnetic force microscopy. The magnetic-domain-length scale is 100-300 nm and 200-500 nm for Sm₂Fe_14.5Cu_0.5Ga₂C_1.5 and Sm₂Fe₁₅Ga₂C_1.0 ribbons, respectively. Due to interactions between the grains, these values are larger than the grain size. In carbon-free Sm₂Fe_14.5Cu_0.5Ga₂ ribbons, the interaction strength between the magnetic tip and the sample is weaker than in Sm₂Fe_14.5Cu_0.5Ga₂C_1.5 ribbons. The phenomenon of remanence enhancement was observed in all ribbons with carbon.

We have investigated the relaxation of the polar order (P₁cos ) in a poled styrene-maleic anhydride copolymer with side groups of modified dispersed red chromophores - using pyroelectric studies - as well as the relaxation of the orientational order (P₂cos ) of the optical axes of the side groups previously induced by polarized light - using holographic techniques. Isomerization cycles of the chromophores cause in the latter case the formation of an optical grating. The relaxation processes were monitored within the glassy state. In addition we investigated molecular relaxation processes, in particular the glass relaxation, using dielectric techniques. The observation was that the relaxations of the pyroelectric response and of the holographic grating couple to the glass relaxation yet do not display identical kinetics within the solid glassy state, showing that the optical grating is less stable. The difference arises obviously from the excess free volume induced by the isomerization cycle in the optical storage experiment. The formation of this excess free volume is apparent from the formation of a surface relief pattern in the storage experiment. A straightforward conclusion is that the stability of the grating can be considerably enhanced if chromophores are used characterized by a significantly lower equilibrium concentration of cis-species during irradiation.

Quenching of O₂(¹_g) molecules both in the gas phase and on a reactor surface has been investigated in binary mixtures of hydrogen and oxygen by using the fast-flow quartz reactor and infrared emission spectroscopy. Rate constants of the O₂(¹_g) deactivation by H₂ and O₂ at room temperature have been determined to be (1.5±0.5) 10^-18 cm³ s^-1 and (1.6±0.2) 10^-18 cm³ s^-1, respectively. Heterogeneous quenching of O₂(¹_g) on quartz walls has been studied both in pure oxygen and in H₂:O₂ mixtures. A model of O₂(¹_g) heterogeneous quenching in binary mixtures has been developed and has allowed us to describe all observed features of singlet oxygen kinetics. Active surface complexes formed by `chemisorbed' atomic oxygen and `physadsorbed' molecules of O₂ and H₂ are assumed to be responsible for the O₂(¹_g) heterogeneous deactivation. It has been shown that the higher rate of O₂(¹_g) quenching in pure oxygen is connected with a quasi-resonant transfer of the O₂(¹_g) electronic excitation to physadsorbed oxygen molecules. The observed effect of the wall passivation by hydrogen is conditioned both by the absence of the similar resonance in the hydrogen surface complex and by the higher bond energy of H₂ in this complex. Bond energies of O₂ (3750±450 K) and H₂ (4050±450 K) in the surface complexes have been determined from the model parameters by fitting calculations to experimental results.

The results of an experimental investigation on a passively triggered vacuum spark device are presented. The diagnostics include the current, x-ray and optical emission measurements. The sharp dips in the current derivative signal indicate the occurrence of pinching at an early stage of the discharge (at current 5 kA). A well-confined plasma with a central hot region was recorded using a streak camera. The pinched plasma was observed to undergo kink-type oscillations with a time period of 10-15 ns. Repeated plasma fronts were seen to move from the anode to the cathode with an average velocity of 5 × 10⁶ cm s^-1. Soft x-ray emission having a radiation intensity of a few hundred mR per discharge was observed. The x-ray signals obtained using photodiodes showed multiple bursts. A soft x-ray pinhole camera recorded micro-pinches of 100 µm. The x-ray emitting regions were confined to the inter-electrode gap. The x-ray emission characteristics were influenced by the electrolytic resistance, which was connected across the spark gap to initiate discharge.

An analytical macroscopic model yielding the self-bias voltage dependence on the absorbed radio frequency (rf) power of an asymmetric capacitively coupled rf discharge has been worked out. The model considers -discharges and studies the contribution of the active electron current to the global energy balance. The model is applicable under gas-discharge conditions typical of technological etching devices and offers a reliable diagnostic method for determining the ion flux onto the powered electrode by using easily measurable quantities (the self-bias voltage and absorbed rf power). Theoretically calculated ion fluxes (or equivalently ion current densities) and self-bias voltages for low-pressure oxygen and SF₆-discharges are presented.

The formation of the electron distribution function (EDF) in the anode region of an inert gas discharge at low pressures and currents is analysed on the basis of non-local electron kinetics in spatially inhomogeneous fields. The deformations of the distribution function occurring in the axially extended, non-uniform quasi-neutral plasma disturbed by the anode and in the thin collisional sheath of space charge are studied. The EDF is modelled for the two field configuration observed in experiment. One is a constant electric field in the positive column ascending to a strong field due to space charge. The second configuration consists of weak field region between the positive column and the strong field in the anode sheath. The specific deformation of the electron distribution is demonstrated, in particular, its population with high-energy electrons in a strong field of space charge. Interesting kinetic phenomena, such as deformations of the EDF in the weak field region near the anode are found. Particular calculations are carried out for discharges in neon.

A new theoretical model for ozone generation from air is proposed. This model considers the dependency of the ozone dissociation rate by electron impact on the electric field strength, in addition to ozone decomposition processes through the reactions with nitrogen oxides. Assuming a stationary and uniform discharge in time and space, ozone concentrations obtained experimentally under various discharge gap widths and gas pressures are in good agreement with the analytical results. It is concluded that the discharge condition of a narrow gap and accordingly an optimized high gas pressure is one of the most effective means of efficient ozone generation for an air-fed ozone generator.

The aim of this study was to determine the effective thermal conductivity of a new insulating building material, called clayey aerated concrete (CAC), when used in a dry state. A series of six mixtures having different porosities obtained by adding aluminium powder to the clay-cement matrix was used in the investigation. The measurements of thermal conductivity have been performed inside a closely controlled climatic cell at ambient temperature (20±1 °C) using a `line source' method. The experimental results reveal that enhanced thermal performances can thus be obtained by the development of local materials, and their use in building insulation reveals very interesting properties as compared to other building materials. The study has been extended to the research of theoretical models capable of predicting the effective thermal conductivity of dry CAC considered as a two-phase system. Three models based on a geometric mean approach (Asaad model, Veerendra and Chaudhary model) and a effective continuous medium approach (Pande and Chaudhary model) were used. In order to predict effective thermal conductivity of elaborated material using such models, a correction term has been empirically determined and introduced in these models, taking into account the effect of a higher ratio of the thermal conductivity (_s/_f). A very good agreement has been observed between the analytical and experimental results, thereby confirming the model's ability to predict the effective thermal conductivity of dry CAC.

The previously developed hydrokinetic concept combining transport information at the hydrodynamic and kinetic levels is applied to educe transport approaches at various temporal and spatial scales. The hydrokinetic approaches are derived from the evolution process of the hydrodynamic parameters and kinetic distribution function (DF) influenced by scattering and variations in field. The DF resulting from this physics-based approach is described by a chosen finite set of moments (hydrodynamic parameters) whose characteristic scales therefore determine the evolution scale of the hydrokinetic DF. Formulation of the hydrokinetic approach at the momentum characteristic scale is presented. The approach is applied to the investigation of the electron DFs in silicon at 100 K and 300 K subjected to drastic changes in field. Monte Carlo (MC) simulations are used to examine the validity of the proposed hydrokinetic approaches. Major assumptions for the hydrokinetic approach at the momentum characteristic scale are verified numerically. Results show that, when properly incorporating influences of energy and velocity relaxation into the hydrokinetic DF, the approach can account for highly non-Maxwellian and ballistic behaviours of electrons in fast-transient situations. The proposed model is fairly simple and efficient, and results are in good agreement with those derived from the MC simulations in ultra-fast-transient situations.

The transient current response of molecularly doped polymers have been numerically modelled under space-charge-limited (SCL) conditions for the situation in which a step potential is applied to an ideal injecting contact. Under trap-free conditions, the transient SCL current pulse shape is found to be sensitive not only to the underlying field dependence of the injected carrier mobilities and diffusivities, but also to the magnitude of the applied step potential. A progressive reduction in the ratio of the peak current density j_p to the final steady-state magnitude j_ss is obtained by increasing either the field strength of the mobility or the relative amount of diffusion. It is demonstrated, however, that for times preceding the location t_p of the current peak, the rate of current increase displays a gradual transition from a super-linear to linear time dependence upon the introduction of diffusion. The diminishing observability of j_p/j_ss is accompanied by a shift in the position of t_p relative to the space-charge-free carrier transit time t₀. The classical fixed-mobility value t_p/t₀ = 0.786 is modestly reduced as the field strength of the mobility or amount of carrier diffusion is enhanced. The numerical predictions are compared with experimental SCL current transients obtained from hydrazone doped polyester samples fitted with gold contacts.

The MgSO₄:Dy, P and MgSO₄:Dy, P, Cu phosphors have been developed and studied by the thermoluminescence (TL) glow curves and dose responses to gamma radiation. The TL sensitivity of MgSO₄:Dy, P phosphor is higher than that of LiF:Mg, Ti. One main dosimetric peak at about 283.6 °C and a satellite peak at about 352.7 °C are involved in the TL glow curve of the MgSO₄:Dy, P which was measured with a heating rate of 5 °C s^-1 and deconvoluted by the general order kinetics equation to obtain the peak parameters and trap parameters of the elementary TL peaks. The dose responses to gamma rays in the dose range from 0.1 Gy to 20 kGy were obtained and fitted by the composite action dose response function to get the nonlinear characteristic parameters, one-hit factor R and characteristic dose D₀, which indicate that the TL dose responses for the two peaks of MgSO₄:Dy, P are supralinear. When a trace amount of Cu was co-doped into MgSO₄:Dy, P the TL glow peak at 352.7 °C was suppressed and the TL sensitivity and the nonlinearity of dose response for the main dosimetric peak at 283.6 °C critically depends on the concentration of doped Cu. The dose response of MgSO₄:Dy, P, Cu (0.1 mol%, 0.5 mol%, 0.01 mol%) phosphor is linear-sublinear. The MgSO₄:Dy, P, Cu (0.1 mol%, 0.5 mol%, 0.004 mol%) phosphor has a wider linear range in the dose response than MgSO₄:Dy, P (0.1 mol%, 0.5 mol%) and its TL sensitivity is close to MgSO₄:Dy, P (0.1 mol%, 0.5 mol%). It may be a candidate with which to measure radiation dose under higher dose circumstances.

The autocorrelation function (ACF) of a non-Gaussian random process, obtained by the memoryless nonlinear transformation of a Gaussian process with a known ACF, is calculated as a power series with coefficients expressed as one-dimensional integrals. In general these must be evaluated numerically; two analytically tractable special cases are also considered. In cases of practical interest the series has been found to converge rapidly. These results are then used in the simulation of a non-Gaussian process with a specified ACF, which can take values less than the square of its mean. Our approach is compared with other methods in the open literature. Examples are given of time series and random fields with gamma single-point statistics that provide controlled models of high-resolution radar clutter.

An irreversible cycle model of an absorption refrigerator operating between four temperature levels is established and used to analyse the performance of the refrigerator affected by the irreversibility of finite-rate heat transfer and the internal irreversibilities of the working substance. The specific cooling load of the refrigerator is taken as an objective function for optimization. The maximum specific cooling load is determined. Other main parameters of the refrigerator at maximum specific cooling load are calculated. The optimal distribution relation of the heat-transfer areas of the heat exchangers is derived. Several special cases are discussed in detail so that some important conclusions relative to absorption refrigerators in the literature may be derived directly from the present paper. Moreover, the influence of the internal irreversibilities on the performance of the system is analysed. The results obtained here can describe the optimal performance of a four-temperature-level absorption refrigerator affected simultaneously by the internal and external irreversibilities and provide the theoretical bases for the optimal design and operation of real absorption refrigerators operating between four temperature levels.

The first two equations of this recent review article have been found to be incorrect by a factor of two. The correct equations (1) and (2) are shown below.