Table of contents

A large magnetoresistance (MR) has been observed in Ni_1-xRh_xS (x = 0, 0.02, 0.04 and 0.06) with MR = 1530% at 268 K for x = 0, MR = 820% at 224 K for x = 0.02, MR = 680% at 167 K for x = 0.04 and MR = 580% at 98 K for x = 0.06 in a magnetic field of 4 T. The large MR is due to magnetic field-induced magnetic and electrical transition from the antiferromagnetic nonmetal phase to the paramagnetic metal phase.

An enhancement to a recently introduced pulse width modulation sampling force probing technique is proposed using a square wave modulation method. The proposed method significantly simplifies the implementation of the pulse width modulation approach at high frequencies, thus further enhancing the measurement temporal resolution. The technique is demonstrated by the measurement of a 500 Mbit s^-1 waveform with a resolution of 50 ps. An analysis of the voltage sensitivity suggests that the proposed technique has reached the performance limits of pulse-sampling force probing methods, and that any further enhancement in the temporal resolution requires probes of smaller dimensions.

The piezo-, pyro- and ferroelectric properties were investigated for extruded polyvinyl chloride films. The piezoelectric properties of films were found to be affected greatly by the presence of the plasticizer. The effects of annealing, poling conditions and poling modes were studied to optimize the piezoelectric properties. Due to the higher dipolar orientation, the simultaneous stretching and corona poling (SSCP) at an elevated temperature improves greatly the piezo- and pyroelectric properties of PVC film in comparison to stretched and corona poling (STCP). The maximum value of the piezoelectric coefficient (d₃₁)~7.2±1.1 pC N^-1 was found to be for SSCP-PVC films. The pyroelectric coefficient was found to be (p)~0.30 and 0.16 nC K^-1 cm^-2 at 35 °C for SSCP and STCP films, respectively. The observations of polarization hysteresis film loops suggest the existence of the cooperative dipolar motion in the PVC film. The maximum value of polarization was found to be ~7.0 and 6.0 nC m^-2 under 78 MV m^-1 electric field for SSCP and STCP films, respectively bending piezoelectric coefficient (β₃₃₁) of ~15 nC m^-1 was found for SSCP films.

A spray pyrolysis technique has been employed to prepare Ag added La_0.67Ca_0.33MnO₃ (Ag-LCMO) polycrystalline film and its properties are compared with those of the La_0.67Ca_0.33MnO₃ (LCMO) film prepared under similar conditions. The addition of silver in LCMO films improves surface morphology and reduces the lattice constant. During the synthesis process, the silver in the Ag-LCMO film melts and segregates at the grain boundaries. The semiconductor-metal transition temperature (T_p) increases by 10 K and normalized voltage noise reduces by more than one order of magnitude. Improvement in surface morphology and better oxygenation of the film due to the presence of silver leads to improvement in the properties of polycrystalline LCMO film.

The surface and bulk properties of GaAs(111)B with Se surface layers of different thicknesses have been studied. Insight into the mechanism of this type of surface treatment has been provided by surface-sensitive photoemission spectroscopy. It has been found that Se deposited at room temperature on an As rich GaAs(111)B surface initially reacts with As followed by the formation of a Se overlayer. A relatively large spectral component due to Se-As bond emission was observed. The experimental results show that the Ga and As core levels shift around 0.27 eV after an initial Se deposition, and no additional shift was observed after further Se deposition.

The Johnson, Kendall and Roberts (JKR) adhesion theory is frequently applied to study the adhesion mechanics of thin compliant layers coated on to glass substrate in an atomic force microscope (AFM). These compliant layers are subjected to micro indentation in the AFM using a flat or spherical indenter. But, JKR theory is based on the contact of homogeneous, isotropic solids. The finite element analysis used by Sridhar et al (1997) (J. Phys. D: Appl. Phys.30 1710-19) to study the adhesion mechanics of the three-layered crossed-cylinder arrangement of the surface force apparatus (SFA) has been extended to this AFM geometry. The effect of indenter elasticity is included for both spherical and flat punch probes. Computations of contact size and contact compliance each as a function of load are presented for a range of values of adhesion energy and elastic modulus ratio of layer and substrate.

Nanostructured silicon thin films were deposited by a pulsed plasma enhanced chemical vapour deposition process using an argon-silane mixture. For each deposition temperature (T_S), the plasma modulation was chosen in such a way that the discharge was switched off just before the onset of powder formation. The optical and electrical properties of the films were investigated as a function of T_S and compared to those of films deposited under the same plasma conditions but with a continuous wave (CW) plasma. While the CW films show a gradual improvement in their properties with increasing T_S, a drastic change appears in the optical and electrical properties of hydrogenated nanostructured silicon (ns-Si:H) films for T_S>50 °C. In particular, for a deposition temperature of 150 °C a more compact material with a small surface roughness and a high conductivity was obtained. This improvement is correlated to the discharge conditions and particularly to the small size of the clusters embedded in the deposited film.

Polycrystalline samples of SnBi₄Te₇ and PbBi₄Te₇ with deviations from stoichiometry in different sublattices were synthesized by melting and quenching followed by prolonged annealing and hot-pressing. At room temperature SnBi₄Te₇ exhibits both n-type conductivity (Te deficiency and Sn or Bi excess) and p-type conductivity (Te excess and Sn or Bi deficiency) whereas only n-type conductivity samples were obtained for PbBi₄Te₇ regardless of the type of deviation from stoichiometry. Both compounds possess high carrier concentrations in the range 4×10¹⁹-4×10²⁰ cm^-3 and a relatively low carrier mobility of 35-70 cm² V^-1 s^-1. The electrical resistivity and Seebeck coefficient have been measured over the temperature range 100-800 K on samples with different deviations from stoichiometry. The temperature dependence of the electrical transport properties of the compounds is typical of heavily-doped semiconductors. The potential for thermoelectric applications using these compounds is discussed.

The effective electromagnetic, thermal and mechanical properties of heterogeneous media are commonly related to composition. Stratified composites are entities that are important in physics and engineering and are a class of compounds actively investigated for their dielectric properties. Such systems consist of composite inclusions of multicomponent materials embedded in a matrix of a distinct material. The effects of the volume fraction, permittivity and thickness of the various constituents on the complex effective permittivity of the heterostructures, in the quasistatic approximation, are reported. We use an exact numerical technique for the evaluation of the dielectric characteristics of three-component composites, arranged in a regular simple cubic lattice, which is based on the field calculation package PHI3D and a boundary integral equations solution scheme of Laplace's equation. `Exact' means that the correct properties are computed for the given microstructure and choice of individual component properties because all internal electric multipole interactions contributing to the polarization of the material medium are taken into account. The predictions of the Steeman and Maurer (dipolar) theory (Steeman P A M and Maurer F H J 1992 Colloid Polym. Sci.270 1069) are compared with our numerical results where appropriate. An important feature, suggested by the results presented for stratified spheres and spheroids, is that any approach based solely on the dipole approximation must fail to predict the effective permittivity of dielectric heterostructures over the entire range of volume fraction of inclusions. The results discussed give motivation for the development of careful experiments which can be used to test the numerical simulations and to fabricate synthetic composite materials in which new electromagnetic properties can be obtained from the combination of several materials.

Experimental data on the dielectric hysteresis and the capacitance-voltage characteristics are analysed for polycrystalline Pb(Zr_xTi_1-x)O₃ thin films. An analytical description of the effect of the domain structure, 90° rotation and 180° switching processes on the dielectric permittivity is proposed by taking into account the poling field characteristics of different domain types. A distinct correlation between these processes and an asymmetric behaviour of the dielectric permittivity of polydomain grains at E>0 and E<0 is theoretically established and compared with the experimental results.

Thermostimulated luminescence (TSL) phosphors based on CaSO₄:Dy, Zn and CaSO₄:Tm, Cd have been studied. These phosphors show an increase in the 220 °C TSL peak intensity by a factor of about 50% compared to CaSO₄:Dy, as there is no change in the position of the peak temperature. Their dose response characteristics to ⁶⁰Co gamma radiation is similar to that of CaSO₄:Dy. However, there are very minor changes in their photoluminescence intensity and excitation spectra compared with those of CaSO₄:Dy.

The luminescence sensitivity of natural quartz is significantly enhanced if heated beyond the first and second crystal phase transitions, in the temperature range 500-1100 °C. This work employs a number of complementary spectroscopic methods to obtain a fuller understanding of the mechanisms of this sensitization process. Spectrally resolved radioluminescence is used to study the role played by hole trapping recombination sites. Linearly-modulated optically stimulated luminescence is used to probe a number of different optically active electron donor defects. Spectrally resolved thermoluminescence simultaneously monitors the processes of electron eviction from donor centres, and recombination at acceptor sites. Finally, high-frequency electron paramagnetic resonance is used to monitor changes in population and structure of defects present within the lattice, some of which partake (either directly or indirectly) in the emission processes of the samples. The work leads to the conclusion that luminescence is generally sensitized in quartz by the removal of oxygen vacancy E' centres (which act either as non-radiative recombination centres, or as luminescence centres emitting in the deep UV, beyond the range of our instrumentation). In some samples, further luminescence enhancement is accompanied by heat induced creation of donor defects. The spectral results show that, not only is there competition between the acceptors for recombining electrons, but also that the emission undergoes significant thermal quenching at elevated temperatures. Comparison of the radioluminescence and thermoluminescence measurements enables the quenching energies of two emission bands (360, 470 nm) to be obtained directly.

Y-Ba-Cu-O superconducting thin films with high-quality parameters were deposited routinely by the YAG double-laser system with the four-lens beam shaping system. The properties of the plasma plume generated by the laser beam with a specific intensity distribution were investigated. Power dissipated by the substrate heater was decreased during the deposition process to compensate energy in-flow from the plasma plume. A correlation was found between the superconducting film properties and the thermal protocol of the deposition process, the geometry of the set-up, the laser beam intensity distribution, the plasma plume shape and oxygen pressure.

The encapsulation effect of thermal chemical-vapour-deposition polymer films (TCVDPF) has been studied for organic light-emitting diodes (OLED). We used poly-p-xylylene (PPX) and/or poly-2-chloro-p-xylylene (PCPX) films as TCVDPF. For the encapsulation, TCVDPF was deposited on the OLED during a dry process at room temperature. The lifetime of an encapsulated OLED with 0.6 µm thick TCVDPF was about four times longer than that of a non-encapsulated OLED in air. The lifetime of the OLED increased with increasing thickness of the TCVDPF. We conclude that the effect of TCVDPF encapsulation on the long-term stability of the OLED is due to the prevention of oxygen and moisture in air and several other factors.

A modified Williamson-Hall analysis was applied to synchrotron transmission x-ray diffraction data obtained for a series of three melt-spun magnetic nanocomposite `exchange-spring' materials comprised of Nd₂Fe₁₄B and varying amounts of α-Fe ranging from 0-32 vol%. Analysis of the data demonstrates that there is no measureable contribution from microstrains or other lattice distortions in the Nd₂Fe₁₄B phase. This result indicates that the Nd₂Fe₁₄B Bragg peak broadening arises from particle size only (on the order of 440 Å) and that the shape of the melt-quenched Nd₂Fe₁₄B grains is approximately spherical. In contrast, the α-Fe phase exhibits Bragg peak broadening that may be attributed to a combination of both anisotropic strain and particle-size broadening effects. The average particle size of the α-Fe phase is 360 Å, significantly larger than that determined from the Scherrer formula, and the average stress, deduced to be tensile, of the α-Fe particles is approximately 1 GPa, which translates to an rms strain = ⟨e²⟩^1/2 of approximately 0.1%. These results provide a physical basis to explain both the marginal exchange-derived remanence enhancement and the anomalous elevated-temperature coercivities found in these alloys.

The magnetization reversal of ferromagnetic Co-based amorphous microwires carrying ac current is studied. The frequency spectra of the voltage V_φ induced in the pick-up coil wound around the microwires were analysed and were found to depend on the ac current amplitude I₀ and external longitudinal dc magnetic field H_e. The even harmonics are shown to dominate in the frequency spectra of V_φ if the ac current amplitude exceeds some threshold value, which strongly depends on H_e. A high sensitivity of the even harmonic amplitudes to the longitudinal dc field is demonstrated. The experimental data are described in terms of the Faraday law and the quasi-static Stoner-Wohlfarth model. The results obtained may be of importance for the design of weak magnetic field sensors.

An experimental study has been carried out on the dynamical process taking place in the plasma generated by a TEA CO₂ laser (400 mJ, 100 ns) on a zinc target when surrounded by helium gas of pressure ranging from 2 Torr to 1 atm. Plasma characteristics were examined in detail on the emission lines of Zn I 481.0 nm and He I 587.6 nm by means of an unique time-resolved spatial distribution technique in addition to an ordinary time-resolved emission measurement technique. The results reveal, for the first time, persistent shock wave characteristics in all cases throughout the entire pressure range considered. Further analysis of the data has clarified the distinct characteristics of laser plasmas generated in different ranges of gas pressure. It is concluded that three types of shock wave plasma can be identified; namely, a target shock wave plasma in the pressure range from 2 Torr to around 50 Torr; a coupling shock wave plasma in the pressure range from around 50 Torr to 200 Torr and a gas breakdown shock wave plasma in the pressure range from around 200 Torr to 1 atm. These distinct characteristics are found to be ascribable to the different extents of the gas breakdown process taking place at the different gas pressures. These results, obtained for a TEA CO₂ laser, will provide a useful basis for the analyses of plasmas induced by other lasers.

In this paper, the Ohmic power input to the discharge capillary is recovered and used to analyse the basic processes involved in electrothermal (ET) plasma devices operated in an ablation-controlled-arc regime. Such an interplay between theory and experiment is necessary to reduce the number of reasons which might be responsible for the reported discrepancies between theory and experiment, as well as to illuminate the subject of ablation controlled arcs. A consistent methodology for determining detailed composition and thermodynamic functions of the non-ideal plasma generated in such devices is presented and used in the present computations. Different non-ideality effects, due to Debye-Hückel corrections in the Gibbs free energy, which have been ignored in prior publications, have been taken into account. A semi-analytical model for an ET plasma source with non-ideal effects is described and incorporated into a comprehensive computer code to simulate plasma evolution and flow in the discharge capillary. The model is one-dimensional, time dependent and uses the recovered Ohmic power input in the source term of the energy equation. The developed code has been used to investigate the ablation process and has shown the inappropriateness of a widely used ablation model. Code predictions for different plasma parameters are presented, discussed, and compared to available experimental data.

Cathode erosion is a problem in electric arcs. Hollow cathodes have been found to erode less due to more diffuse current attachment. A theoretical model explains several observed properties of hollow cathodes. This model is usually integrated numerically but an analytical solution is possible in some circumstances. The model allows the prediction of the cathode temperature and plasma properties. Results of the theory are compared to experimental data.

This work is devoted to the modelling of branching streamers, propagating in liquid dielectrics using an equivalent electrical network, physical laws and energetic considerations. This model enables the prediction of the characteristics of the streamer such as the current, the electrical charge, the energy and the power injected in the liquid medium, the local electric field, the streamer pattern and its velocity. The shape and amplitude of the different characteristics so simulated are in quite good accordance with those reported in the literature.

It appears from our modelling that the shape and the propagation mode of the streamer depend not only on the test conditions but also on the local electric field. The values of the mobility of charge carriers within the streamer channels, the local viscosity and temperature deduced from this model indicate that both electronic and gaseous mechanisms are implicated in the streamer development.

Dielectric breakdown in insulators is a commonly observed phenomenon in gases, fluids and solids at large electric field amplitudes. The resulting discharge patterns generally exhibit a non-Euclidean, fractal dimension, representing breakdown channels in the insulating material. In this study dielectric breakdown is investigated for anisotropic fluids in Hele-Shaw cells, i.e. liquid crystals in a plate capacitor. The observed burn patterns are inscribed into the substrates, suggesting that they are the result of the surface degradation, due to the breakdown current at the electrode surface. They are the same on both substrate plates with an equal fractal dimension and of radial symmetry, even though the insulating material is anisotropic. Investigations were carried out as the electric field was varied from dc to ac conditions, for instant field application versus slow ramping, for materials with positive and negative dielectric anisotropy, for a variation of the alignment coating, in several different liquid crystal phases and as a function of cell gap, applied field amplitude, frequency and temperature. In all cases the fractal dimension of the burn pattern increases as the breakdown voltage increases.

The carburization of ethylene pyrolysis tubes is characterized using hardness, x-ray diffraction (XRD), scanning electron microscopy (SEM) and optical microscopy. The through-wall changes in magnetic properties due to carburization are quantified using vibrating sample magnetometer (VSM) measurements. The thickness and strength of the carburized layers are correlated with non-destructive measurements of the impedance of a C-core electromagnet, placed on the outside of the tubes, as a function of driving frequency. A two-dimensional finite-element mathematical (FEM) model is used to interpret the experimental impedance measurements. Good agreement was attained between the fitted model, the optical micrographs, hardness and VSM measurements.

Analytical models are developed which describe the K⁺ ion concentration profiles after single and double reversals of the electric field during K⁺-Na⁺ field assisted ion exchanges in glass. The models assume that drift transport is the dominant transport process and that, to first order, diffusion effects can be ignored. Concentration dependent, mobility ratio effects are included in the model. The expressions are compared with computer simulations based on the full drift-diffusion equation with nonlinear self-diffusion coefficients. The work provides a greater understanding of certain experimental results that were recently obtained by the same authors.

The performance of an externally and internally irreversible regenerated Brayton refrigerator is analysed. Analytical relationships between cooling load and pressure ratio, as well as between coefficient of performance (COP) and pressure ratio of a real closed irreversible regenerated Brayton refrigeration cycle coupled to constant- or variable-temperature heat reservoirs are derived. The irreversibilities considered in the analysis include the heat transfer losses in the hot- and cold-side heat exchangers and the regenerator, the non-isentropic expansion and compression losses in the compressor and expander, and the pressure drop losses in the piping system. The optimal performance characteristics of the cycle may be obtained by optimizing the distribution of the heat conductance or heat transfer surface areas among the two heat exchangers and the regenerator. The influences of the effectiveness of the regenerator as well as the hot- and cold-side heat exchangers, the efficiencies of the expander and the compressor, the pressure recovery coefficient, and the inlet temperature ratio of the heat reservoirs on the cooling load and the COP are examined and shown by numerical examples.} \setlength\abswidth{32pc

The end of the caption of figure 3 is corrected to `...ion beam. i = 10µA.'

The end of the caption of figure 4 is corrected to `...experimental points; curve, Gaussian fit.'

On p L15 the last line on the page: [12] is corrected to [12a].

The end of the caption of figure 5 is corrected to `...figure 3. i = 10 µA.'

On p L16, the third line from the top of the second paragraph on the left-hand side: `distributions' is corrected to `distribution'.

The fourth line from the bottom of the second paragraph on p L16: [15, 16] is corrected to [15].

The last line before the References on p L16: [12] is corrected to [13].

In reference [10] the page number is corrected to 699.

An extra reference is inserted in the Bibliography: [12a] Hesse E, Mair G L R, Bischoff L and Teichert J 1996 J. Phys. D: Appl. Phys.29 2193

On the right-hand side of p L11 of the article, the units of brightness in the eleventh line from the top are corrected to A cm^-2 sr^-1.

The caption of figure 1 is corrected to `The energy distribution of the Ge⁺⁺ ion for a current of 3 µA and a source temperature of 485°C: full square, experimental points; solid curve: Gaussian fit; y-direction, arbitrary units; x-direction, eV (scale shown in the figure).'

On the left-hand side of p L12 of the article, the second and third lines from the top are corrected to `Figure 1 shows the energy distribution of the Ge⁺⁺ ion, emitted....'.