Table of contents

The effects of Ag addition in Ti-doped La_0.67Ba_0.33MnO₃ (LBT) (abbreviated as LBT/Ag_x, where x is the nominal molar ratio of Ag) on the electric and magnetoresistance (MR) properties have been studied systematically. The results show that the resistivity of the samples decreases dramatically with increasing Ag addition and reaches its minimum for x = 0.27. The room-temperature MR ratio of LBT/Ag_x increases with increasing x and reaches its maximum at x = 0.27. The MR ratio at 280 K for the x = 0.27 sample at H = 10 kOe is as large as 41%, which is about 3.4 times larger than that for LBT. The Ag addition makes the LBT crystallites more perfect and uniform, and suppresses the magnetic scattering at the grain boundaries. The Ag conduction channels penetrating into the insulating regions can also lower the resistivity. The near room-temperature T_C and low resistivity of the Ag-added LBT are responsible for the large intrinsic room-temperature MR.

A ferroelectric and colossal-magnetoresistant material with a Ba_0.5Sr_0.5TiO₃ (BST)/La_0.67Sr_0.33 MnO₃ (LSMO) heterostructure was deposited epitaxially on (001) LaAlO₃ substrates through pulsed laser deposition. Measurements of the magnetic and ferroelectric properties show no obvious degradation of the LSMO layer after BST film deposition. For the BST film, as the temperature decreases from 300 to 20 K, both the remanent polarization and the coercive field increase monotonously up to 5.7 µC cm⁻² and 63.7 kV cm⁻¹, respectively. Meanwhile, the temperature dependence of the spontaneous polarization shows a second-order ferroelectric phase transition, and the Curie temperature is estimated to be about 220 K. Furthermore, in the temperature range from 20 to 300 K, the dielectric constant (E = 0) and the dielectric tunability (E = 200 kV cm⁻¹) exhibit similar temperature dependences, and show maximum values of 1252 and 45.3% at 200 K, respectively.

We report on a study of the experimental reflectance-difference (RD) line shape for CdTe near the E₁ and E₁ + Δ₁ spectral regions. A clear reflectivity component, as a major contribution near the E₁ and E₁ + Δ₁ spectral regions, to the measured RD CdTe line shape is found. No derivative-like, ε -like or 1/ε -like line shapes are observed for our sample. Results suggest that the experimental RD spectra cannot be explained by considering an anisotropic film on the surface of the material nor by a dislocation-induced piezo-optic effect.

A new model for thermoluminescence (TL) kinetics is presented, which is a natural generalization of the standard localized transition model. The model of semi-localized transitions (SLTs) allows trapped charge carriers to be excited to the conduction band. Analytic equations for SLTs are constructed using an enumeration technique which transforms concentrations of carriers to concentrations of states of localized trap–recombination centre pairs. Solving numerically the set of equations, we demonstrate some basic features of the SLT model. It is shown that SLT is able to produce so-called TL displacement peaks that were previously found in one-dimensional TL simulations.

In this study, thin films of Cu(In,Ga)Se₂ were prepared by the controlled reaction of thermally evaporated InSe/Cu/GaSe precursors with elemental Se vapour in vacuum. We indicate that this classical two-step growth process can be utilized to produce homogeneous single-phase chalcopyrite absorber films with superior structural properties. X-ray diffraction studies indicated that the interplanar spacing d₍₁₁₂₎ decreases linearly with an increase in the Ga/[In + Ga] atomic ratio due to homogeneous incorporation of gallium into the chalcopyrite lattice. Optical studies revealed the expected systematic increase in the band gap with increasing gallium concentration, once again confirming the monophasic nature of the alloys.

The floating potential acquired by a dust particle in an electronegative plasma is calculated on the basis of orbital motion limited theory. Calculations are presented for hydrogen and oxygen plasmas.

A method of in situ measurement of electrode work functions in free-burning argon arcs during operation is presented. This technique is based on the photoelectric effect ocurring at the surface of a tungsten cathode with the use of a pulse laser system consisting of a Nd–YAG laser and a dye laser. Three types of tungsten electrode, namely, pure W, W–2% ThO₂ and W–2% La₂O₃ are used in this work. Free-burning arcs are operated in argon at atmospheric pressure at currents of 100 A and 200 A. The effective work functions of pure W, W–2% ThO₂ and W–2% La₂O₃ electrodes during operation at a current of 200 A are found to be, respectively, 4.6, 2.8 and 3.0 eV from in situ measurements. These results are very close to the work functions of pure W, ThO₂ and La₂O₃ obtained from the literature. The in situ measurements, however, show that the effective work functions of pure W, W–2% ThO₂ and W–2% La₂O₃ electrodes for an 100 A arc are, respectively, 2.9 eV, 2.6 eV and 2.0 eV. It is shown that each effective work function for an 100 A arc clearly becomes lower than that for a 200 A arc for all types of tungsten electrodes.

Porous poly-silicon (PPS) samples, obtained by electrochemical anodization of p-type poly-silicon wafers, were doped with Pd by the electroless process. Rutherford backscattering spectroscopy shows that Pd has diffused a few micrometres into the PPS layer. Scanning electron microscopy and energy dispersive x-ray analysis results demonstrate the presence of Pd as dispersed clusters on the surface. The variation of the electrical resistance in the presence of dry air diluted with hydrogen at room temperature shows that Pd/PPS samples have the ability to sense hydrogen at levels down to several thousands of ppm. This value is far below the flammability limit of hydrogen gas. It was found that the increase in the time of anodization causes an increase in the penetration depth of Pd and, consequently, an increase in the sensitivity to hydrogen. By raising the device temperature to 60°C, the sensor response falls.

Zirconium titanate thin films were deposited on a p-type Si substrate using the pulsed excimer laser ablation technique and the thin films were either highly oriented along (020) or were of a polycrystalline nature, depending upon the processing conditions. A detailed structural characterization using atomic force microscopy, scanning electron microscopy, x-ray photoelectron spectroscopy (XPS) and x-ray diffraction of the highly oriented and polycrystalline ZrTiO₄ thin films were carried out. The XPS of highly oriented ZrTiO₄ thin films deposited on Si substrates was characterized. The O 1s spectrum did not show any splitting, indicating there is no possibility of adsorbed oxygens, and the single peak in O 1s is attributed to the oxygen ions in the lattice. Capacitance–voltage (C–V), conductance–voltage (G–V) and dc leakage current studies were done on both types of thin film. Secondary ion mass spectrometer studies revealed a reasonably sharper interface existing at the thin film/substrate for the polycrystalline films, compared with the highly oriented thin films. A higher dielectric constant and higher leakage current were observed for the highly oriented ZrTiO₄ thin films compared with polycrystalline thin films. The C–V and G–V studies were carried in the frequency regime of 1–100 kHz on both types of film at higher temperatures (300–500 K) so as to make the minority carriers of the Si substrate respond to the applied ac modulating signal. The activation energies calculated from the Arrhenius plot from C–V and G–V measurements were nearly equal to half of the silicon energy band gap, thereby suggesting a generation–recombination mechanism prevailing in the depletion region of the Si substrate through bulk traps. The interface traps were calculated using the high frequency method and observed to be higher for the polycrystalline films than for the highly oriented thin films. Dc leakage current studies were done in the accumulation region and both the highly oriented and polycrystalline thin films obeyed a bulk limited Poole–Frenkel type conduction mechanism. Finally, the electrical characteristics were correlated to the microstructure of zirconium titanate thin films.

Superconducting thin films of Nd_1−xCa_xBa₂Cu₃O_7−δ (x = 0.03 and 0.08) have been grown on single crystal SrTiO₃ substrates by pulsed laser deposition. The statistical methods of Experimental Design and regression analysis were used to optimize the film properties and to understand the correlation between the growth parameters and film properties. The orientation of the films was investigated by x-ray diffraction. The surface morphology of the films was examined by atomic force microscopy and scanning tunnelling microscopy. Qualitative and quantitative elemental analyses of the films were carried out using electron probe microanalysis. Micro-Raman spectroscopy was used to study the oxygen sublattice vibrations of the films. The effect of annealing on the superconducting transition temperature of the patterned films was also studied.

Hydrogenated diamond-like carbon (DLC) films were deposited on Si (100) wafers by a plasma enhanced chemical vapour deposition technique using CH₄ plus Ar as the feedstock. The friction and wear properties of the resulting films under different relative humidities, ranging from 5% to 100%, in a nitrogen environment, were measured using a ball-on-disc tribometer, with Si₃N₄ balls as the counterparts. The friction surfaces of the films and Si₃N₄ balls were observed on a scanning electron microscope, and investigated by x-ray photoelectron spectroscopy. The results showed that the friction coefficient increased continuously from 0.025 to 0.09 with increase in relative humidity from 5% to 100%, while the wear rate of the films sharply decreased and reached a minimum at a relative humidity of 40%, then it increased with further increase of the relative humidity. The interruption of the transferred carbon-rich layer on the Si₃N₄ ball, and the friction-induced oxidation of the films at higher relative humidity were proposed as the main reasons for the increase in the friction coefficient. Moreover, the oxidation and hydrolysis of the Si₃N₄ ball at higher relative humidity, leading to the formation of a tribochemical film, which mainly consists of silica gel, on the friction surface, are also thought to influence the friction and wear behaviour of the hydrogenated DLC films.

Uniformity of the printing process is one of the key parameters of nanoimprint lithography. This technique has to be extended to large size wafers to be useful for several industrial applications, and the uniformity of micro and nanostructures has to be guaranteed on large surfaces. This paper presents results of printing on 200 mm diameter wafers. The residual thickness uniformity after printing is demonstrated at the wafer scale in large patterns (100 µm), in smaller lines of 250 nm and in sub-100 nm features. We show that a mould deformation occurs during the printing process, and that this deformation is needed to guarantee printing uniformity. However, the mould deformation is also responsible for the potential degradation of the patterns.

The thermal conductivity and crystallization mechanism of alumina (Al₂O₃)/fluorescein sodium salt (FSS) composites prepared by the powder metallurgy method have been investigated by means of differential thermal analysis. The Kissinger method is applied to determine the crystallization kinetics from the endotherm peaks. The activation energy E and Avrami parameter n were calculated. The kinetic parameters (E and n) have made it possible to postulate the type of crystal growth exhibited in the crystallization process. The crystallization growth is found to be one-dimensional for the composite system. The thermal conductivity of the composite system was also determined by differential scanning calorimetry.

The nonlinear behaviour of an x-band microwave guide is investigated using special carbon loaded composite materials as partial filling. The measurements focus on the change in the phase shift of a low power guided microwave signal across a sample that is subjected to an intense microwave pump beam. The latter stimulates a nonlinear contribution to the dielectric permittivity of the composite material. The measurement technique involves an automated network analyser that determines an induced phase shift in the signal beam. The analysis of the results takes into account the spatial distribution of both the signal and pump wave fields, within an extensive quasi-linear model. The carbon-loaded material is demonstrated to be very nonlinear in the technologically important 8–12 GHz microwave frequency range.

We report for the first time the simultaneous measurement of electroluminescence and space charge (SC) distribution in low density polyethylene films under a high dc field, using a specially developed pulsed electro-acoustic apparatus. An interpretation of the observed behaviour is proposed with the help of a numerical model of transport including the effect of charge trapping and recombination. It is shown that the two complementary approaches give consistent information about SC dynamics and charge recombination. This new kind of set-up should open the way for studying hot carrier transport in insulating materials.

Non-first order (FO) kinetics models are of three types; second order (SO), general order (GO) and mixed order (MO). It is shown that all three of these have constraints in their energy level schemes and their applicable parameter values. In nature such restrictions are not expected to exist. The thermoluminescence (TL) glow peaks produced by these models shift their position and change their shape as the trap occupancies change. Such characteristics are very unlike those found in samples of real materials. In these models, in general, retrapping predominates over recombination. It is shown that the quasi-equilibrium (QE) assumption implied in the derivation of the TL equation of these models is quite valid, thus disproving earlier workers' conclusion that QE cannot be held under retrapping dominant conditions. However notwithstanding their validity, they suffer from the shortcomings as stated above and have certain lacunae. For example, the kinetic order (KO) parameter and the pre-exponential factor which are assumed to be the constant parameters of the GO kinetics expression turn out to be variables when this expression is applied to plausible physical models. Further, in glow peak characterization using the GO expression, the quality of fit is found to deteriorate when the best fitted value of KO parameter is different from 1 and 2. This means that the found value of the basic parameter, namely the activation energy, becomes subject to error. In the MO kinetics model, the value of the KO parameter α would change with dose, and thus in this model also, as in the GO model, no single value of KO can be assigned to a given glow peak. The paper discusses TL of real materials having characteristics typically like those of FO kinetics. Theoretically too, a plausible physical model of TL emission produces glow peaks which have characteristics of FO kinetics under a wide variety of parametric combinations. In the background of the above findings, it is suggested that the kinetics analysis of the TL glow curves should be carried out straightforwardly assuming FO kinetics.

This paper deals with the novel idea of controlling the electric stress in a hybrid air–solid dielectric insulation. In a parallel-plate electrode system, the electric stress can be reduced if the electrodes are covered with thick non-conducting dielectric coatings. Free charges are generated by microdischarges developing between the electrodes and are deposited at the dielectric surfaces. As a consequence, a counteracting electric field component results, which causes a reduction of the electric field in the air gap and an enhancement of the field in the dielectric coatings; i.e. the electric stress is forced into the dielectric coatings by the charges. A computation of the threshold voltage of the microdischarges is presented. The charge simulation technique is used for field calculation, irrespective of the thickness of the dielectric layer and the values of the charges deposited on the dielectric surfaces. The calculated threshold voltages are compared with those estimated before analytically and those measured for different gap lengths.

The optimal ecological performance of a Newton's law generalized irreversible Carnot refrigerator with the losses of heat resistance, heat leakage and internal irreversibility is derived by taking an ecological optimization criterion as the objective, which consists of maximizing a function representing the best compromise between the exergy output rate and exergy loss rate (entropy production rate) of the refrigerator. Numerical examples are given to show the effects of heat leakage and internal irreversibility on the optimal performance of generalized irreversible refrigerators.

Although numerical heat transfer and fluid flow models have provided significant insight about fusion welding processes and welded materials in recent years, several model input parameters cannot be easily prescribed from fundamental principles. As a result, the model predictions do not always agree with the experimental results. In order to address this problem, the approach adopted here is to develop and test a model that embodies a heat transfer and fluid flow sub-model and an algorithm for optimizing and learning the values of uncertain process variables from a limited volume of experimental data. The heat transfer and fluid flow sub-model numerically calculates three-dimensional temperature and velocity fields, the weld geometry and the shape of the solidified weld reinforcement surface during gas metal arc (GMA) welding of fillet joints. Apart from the transport of heat from the welding arc, additional heat from the metal droplets is also considered in the model. Alternative algorithms for optimization of uncertain welding variables are examined. The overall model is capable of estimating uncertain parameters such as the arc efficiency, effective thermal conductivity and effective viscosity from a limited number of data on weld geometry. Part I of this paper is focused on the details of the numerical model, optimization technique used and an examination of the important features of the model. In an accompanying article (part II), the application of the model to GMA fillet welding of mild steel is described.

In order to improve the reliability of numerical calculations of heat and fluid flow in fusion welding, a model was proposed in part I of this paper to determine the uncertain welding input parameters from a limited volume of experimental data. The application of the model for the complex gas metal arc (GMA) fillet welding of mild steel is reported in this part of the paper. The values of the arc efficiency, effective thermal conductivity and effective viscosity were evaluated as a function of the heat input. A vorticity-based mixing-length hypothesis was also used to independently calculate the values of the effective viscosity and effective thermal conductivity. Good agreement was obtained between the values of these parameters calculated using the two techniques, indicating the effectiveness of the proposed model in improving the reliability of heat and fluid flow calculations in fusion welding. The shape and size of the fusion zone, finger penetration characteristic of the GMA welds and solidified free surface profile computed using the optimized values of the uncertain welding parameters were in fair agreement with the corresponding experimental results for various welding conditions. In particular, the computed values of the leg length, the penetration depth and the actual throat agreed well with those measured experimentally for various heat inputs.

A general equation for the non-linearity parameter has been derived in terms of the compressibility factor, such that it can be evaluated using any hard sphere equation of state. Then the Carnahan–Starling equation of state was used to obtain B/A values. The method was found to be useful for determining the values of the ratio B/A for liquids in which a molecule can be regarded as a hard sphere. The results of the calculations obtained are compared with values in the literature for pure liquids, and satisfactory agreement is found.

The stability of a perfectly conducting viscous film flowing down an inclined plane at a moderate Reynolds number in the presence of an applied normal electric field is studied in the finite amplitude regime. Due to the presence of the interface, the normal electric field produces an additional stress, which affects the flow dynamics considerably. Using the momentum integral method a nonlinear evolution equation for the development of the free surface is derived. The study reveals that the electric parameter introduces instability into the flow field. Further, nonlinear studies show that both a subcritical unstable zone and a supercritical stable zone may exist along with unconditional stable and explosive zones, depending on the wavenumber versus Reynolds number plane.

This paper describes both an experimental study and a numerical investigation of the breakdown field in particle reinforced thermosets. The experimental study revealed that the increase of the conversion degree (α) of the epoxy matrix improved the breakdown voltages both in divergent and quasi-homogeneous fields. It was also observed that the presence of the second phase (mineral filler) dispersed within the polymeric host led to apparently contradictory results depending on the nature of the electric field. The neat matrix exhibited a higher field to breakdown than that of the composite in a quasi-homogeneous field configuration whereas the opposite behaviour was evidenced in a point–plane configuration. It thus appeared that the filler can either increase or decrease the breakdown voltage, depending on the nature of the electrode configuration. To further understand this behaviour, a deterministic numerical simulation of the dielectric breakdown that accounts for the organization within real composites was developed. This numerical method, based on the resistor–short breakdown model was improved to directly account for the real phase arrangement within the samples through scanning electron micrographs. The simulation furnished the explanation for the apparent disagreement, showing that the inorganic particles protect the composite in divergent fields through a mechanism similar in nature to the so-called barrier effect, whereas the same fillers were responsible for a local intensification of the electric field and thereby a reduction of the dielectric strength in a quasi-homogeneous field.

After exposure to ionizing irradiation, mineral zircon exhibits thermoluminescence (TL), which can be used to calculate the irradiation dose and the age of natural samples. Different aspects of the dating procedure based on zircon TL, including laboratory added irradiation at room and elevated temperatures, fading and preheat processes, have been considered within the framework of the kinetic model developed by our group earlier. It is shown that dose rate effects arising due to the difference between natural and laboratory irradiation can be removed by a suitable preheat treatment of the laboratory irradiated samples prior to the TL measurements. We demonstrate that the TL behaviour of zircon which has been irradiated under natural conditions can be reproduced by means of laboratory irradiation at some elevated temperature. This opens a promising possibility of (i) getting rid of dose rate effects and anomalous fading and (ii) reconstructing the dose dependence of the total TL signal emitted by samples irradiated under natural conditions. The model has been checked by simulating the dating procedures used for our coastal dune sand samples from Ameland (NL), the age of which is known from historical records; the results agree well with the available experimental data. The results of this investigation demonstrate the utility of the kinetic model for planning dating experiments and for development of new and innovative TL dating methods.

We report a systematic study of Pr_0.5Sr_0.5MnO₃ epitaxial thin films grown on (100)–LaAlO₃ single-crystal substrates using the pulsed laser deposition technique. Two different phases are observed depending on the deposition temperature (T_S) of the film, leading respectively to an out-of-plane lattice parameter of 0.3846 nm at low temperature (T_S < 600°C) and 0.3941 nm at higher temperatures (T_S > 670°C). Moreover, we find that the field-induced insulator-to-metal transition is correlated to the smaller lattice parameter and, thus, can be directly tuned by changing the deposition temperature of the film. Finally, we conclude that the growth conditions of the film (in particular the deposition temperature) directly influence the substrate-induced strain and, therefore, the transport properties.