Table of contents

SnO₂ nanobelts have been fabricated by a simple thermal evaporation using a halide source as starting material. The size and morphology of the SnO₂ nanobelts were measured by scanning electron microscopy and transmission electron microscopy. The products consist of a large quantity of beltlike nanostructures with typical lengths in range of several tens to several hundreds of micrometres and the typical width of SnO₂ nanobelts is in the range of 20–100 nm. X-ray diffraction pattern and Raman spectrum from the sample show that the as-synthesized nanobelts are rutile type structure.

The effects of nitridation on the permeability-frequency spectra in Nanoperm alloys were studied. In the initial stage of nitridation, the nitrogen atom diffusion into Nanoperm alloys may make the extent of increase of the relaxation frequency f₀ exceed the extent of decrease of initial permeability μ_i. However, as the materials are nitrided for 200 min in a nitrogen atmosphere of 0.03 MPa at 400°C, μ_if₀ shows a tendency to be constant and the increase of the strength of domain wall pinning with nitriding time becomes the main factor affecting the dynamic magnetization process. Nitridation can improve the soft magnetic properties of nanocrystalline soft magnetic materials in the higher frequency range.

This paper continues the development of a new approach for the design of shim and gradient coils, used in magnetic resonance imaging (MRI) applications. A cylindrical primary coil of radius a and length 2L is placed inside a co-axial shield cylinder of radius b. An active shielding strategy is used to create a desired target field at an arbitrarily specified (cylindrical) location within the primary coil, and to annul the field at a certain radius outside the shield. The form of the interior target field may be chosen arbitrarily by the designer, although zonal and tesseral harmonics are typically used in MRI applications. The method presented here designs coil windings on both the primary and shielding cylinders, to produce fields that conform to the specified interior target field and the annulled field exterior to the shield. An additional feature of the method presented here is that the target field inside the primary coil is matched at two different radii, to improve overall accuracy. The method is illustrated by designing several shielded shim coils, for creating higher order tesseral fields located asymmetrically within the coil. The simpler case of pure zonal fields is discussed separately and applied to the design of some higher order shielded coils.

A silicon-based spin transistor of novel operating principle has been demonstrated in which the current gain at room temperature is 1.4 (n-type) and 0.97 (p-type). This high current gain was obtained from a hybrid metal/semiconductor analogue to the bipolar junction transistor which functions by tunnel-injecting carriers from a ferromagnetic emitter into a diffusion driven silicon base and then tunnel-collecting them via a ferromagnetic collector. The switching of the magnetic state of the collector ferromagnet controls the collector efficiency and the current gain. Furthermore, the magnetocurrent, which is determined to be 98% (140%) for p-type (n-type) in −110 Oe, is attributable to the spin-polarized base diffusion current.

We study how the finite dimensions of a charge moving uniformly in a medium affect spectral distributions of the radiation intensity and the photon number. The integral radiation intensity and the total number of emitted photons are also evaluated, both for a charge of finite dimensions moving in a dispersion-free medium and for a point-like charge moving in a dispersive medium. Equating the radiated energy to the kinetic energy loss, we find how a charge should move if all its energy losses are due to the Vavilov–Cherenkov radiation. For the medium with dispersion and damping we use a simple formula describing frequency distribution of the radiation intensity. Applying it to two substances with known parametrizations of dielectric permittivity, we find that, in the presence of damping, both the value (this is not surprising) and position of the frequency distribution of the radiated energy crucially depend on the observation distance and the damping parameter. This raises uneasy questions on the interpretation of experimental data. We also analyse complications arising when the charge moves in one medium, while observations are made in another, optically less dense medium (this is a typical experimental situation). It is shown that the radiation spectrum of a charge moving in a dielectric sample is discrete.

The influence of calcium co-doping on the optical properties of the x-ray storage phosphor BaFBr : Eu²⁺ is determined by photo-stimulated luminescence techniques. It is found that the incorporation of calcium into the lattice results in a broadening of the photo-stimulation peak due to a calcium induced F_A(Br⁻, Ca²⁺)-centre with stimulation maxima at 540 and 680 nm. The optical cross-sections for the photo-stimulated process are determined by utilizing stimulation light with linearly increasing intensity. Furthermore, it is shown that the sensitivity for x-rays, i.e. the number of storage centres formed during x-ray exposure, increases up to a doping level of 1 mol% while it drops rapidly at higher calcium concentrations.

The photoconductivity of TlGaSe₂ layered single crystals are investigated in the temperature range 78–300 K. Both the ac-photoconductivity (ac-PC) and the spectral distribution of the photocurrent were studied at different values of light intensity, applied voltage and temperature. Dependences of carrier lifetime on light intensity, applied voltage and temperature have been investigated as results of the ac-PC and dc-photoconductivty (dc-PC) measurements. The temperature dependence of the energy gap width was described as a result of studying the dc-PC.

In order to improve the realism of our Plasimo computer model [1, 2] in modelling multi-ion mixtures, we developed and implemented a self-consistent multicomponent diffusion model based on frictions. As presented, this diffusion model includes effects of the ambipolar electric field as well as any external electric fields. Moreover, it is comparatively easy to include other diffusion contributions. This model was shown to produce good and consistent results for both single and multi-ion mixtures.

The possibility of the achievement of a high concentration, N_H⁻ of negative hydrogen ions in low-voltage cesium–hydrogen discharge plasma is investigated. The H⁻ concentration is determined from the experimentally measured absorption of laser radiation in the discharge plasma. It is shown that the H⁻ concentration N_H⁻∼10¹²–10¹³ cm⁻³ may be obtained in the discharge plasma if the plasma electron temperature T_e≈0.5–1 eV. A comparison between the discharge theory and the experiments was fulfilled. It is shown that experimental results coincide well with theoretical calculations.

Electrical treeing is of interest to the electrical generation, transmission and distribution industries as it is one of the causes of insulation failure in electrical machines, switchgear and transformer bushings. In this paper a deterministic electrical tree growth model is described. The model is based on electrostatics and local electron avalanches to model partial discharge activity within the growing tree structure. Damage to the resin surrounding the tree structure is dependent on the local electrostatic energy dissipation by partial discharges within the tree structure and weighted by the magnitudes of the local electric fields in the resin surrounding the tree structure. The model is successful in simulating the formation of branched structures without the need of a random variable, a requirement of previous stochastic models. Instability in the spatial development of partial discharges within the tree structure takes the role of the stochastic element as used in previous models to produce branched tree structures. The simulated electrical trees conform to the experimentally observed behaviour; tree length versus time and electrical tree growth rate as a function of applied voltage for non-conducting electrical trees. The phase synchronous partial discharge activity and the spatial distribution of emitted light from the tree structure are also in agreement with experimental data for non-conducting trees as grown in a flexible epoxy resin and in polyethylene. The fact that similar tree growth behaviour is found using pure amorphous (epoxy resin) and semicrystalline (polyethylene) materials demonstrate that neither annealed or quenched noise, representing material inhomogeneity, is required for the formation of irregular branched structures (electrical trees). Instead, as shown in this paper, branched growth can occur due to the instability of individual discharges within the tree structure.

Reflectance anisotropy spectroscopy (RAS) is used to study anisotropic polymer thin films. The measured spectra are interpreted in terms of molecular orientation at the polymer surface. It is shown that RAS spectra of thin transparent films resemble the logarithmic derivative of the corresponding polarization-averaged reflection spectra. The amplitudes of the spectra are found to be highly sensitive to the optical retardation in the anisotropic layer.

ZnO–SiO₂ composite films consisting of ZnO nanoparticles embedded in a SiO₂ matrix (ZnO : SiO₂ = 20 : 80 molar ratio) were prepared by the sol–gel technique. Annealing in the range 473–773 K in air, hydrogen and oxygen atmospheres showed high stability of the particle radius due to effective capping within the silica matrix. The optical band gap in the composite films varied in the range 4.23–4.29 eV with variation of particle radius from 1.42 to 1.47 nm due to quantum confinement. Photoluminescence (PL) spectra of ZnO nanoparticles recorded in the range 80–300 K showed two defect level emission bands within 2.38–2.66 eV and the excitonic emission band centred at ∼4.22 eV. The effect of atmosphere controlled annealing was also studied with a view to understanding and controlling the visible defect-related luminescence in comparison to the UV excitonic luminescence. It was observed that the intensity of the two defect-related peaks observed in the PL spectrum could be decreased substantially by annealing the films (at ∼773 K) in oxygen atmosphere. The intensity ratio of the excitonic peak to the defect level peak could be increased up to 5 : 1 at room temperature (∼300 K) and 7 : 1 at low temperature (∼80 K).

Al doped zinc oxide thin films with different electrical and optical properties have been developed by both non-reactive and reactive rf-magnetron sputtering in Ar and Ar+H₂ atmospheres, respectively. The thin films prepared under Ar+H₂ gas ambient at substrate temperatures of 100°C and 300°C show high conductivity and improved IR-reflectivity. The lowest resistivity obtained is 4.5×10⁻⁴ Ω cm at 300°C. The transmission of the ZnO : Al film in the visible range is above 90% and that at 1400 nm is only 3.2%. Most of the IR-region is thus reflected. The carrier concentration of this transparent, conducting ZnO film prepared under Ar+H₂ atmosphere is 2.3×10²¹ cm⁻³. Tranmission electron micrographs reveal that the average crystallite of the sample deposited under Ar+H₂ ambient is smaller compared to those prepared under Ar ambient. The ⟨100⟩, ⟨002⟩ orientations of ZnO with wurtzite structure are observed from transmission electron diffraction pattern.

Highly conductive BaRuO₃ thin films have been grown heteroepitaxially on (100) LaAlO₃ substrate using pulsed laser deposition technique over temperature range varying from 600°C to 775°C. Atomic force microscopy, scanning tunneling microscopy and x-ray diffraction (XRD) reveal that the films are well textured and are c-axis oriented with an in-plane epitaxial relationship of ⟨010⟩⟨100⟩BaRuO₃//⟨110⟩LaAlO₃. The resistivity of the films has been found to be a strong function of substrate temperature during film deposition. Both metallic and semiconducting behaviour has been observed in films deposited at 700°C or above. However, films deposited at lower temperature show only metallic behaviour as seen in bulk BaRuO₃. XRD studies showed that the films deposited at lower temperature (600°C), exhibit poor crystallinity, while films deposited at 700–775°C are much better crystallized, and have enlarged unit cells but have a conductivity that decreases with increasing temperature (at least a low temperatures). We believe that the substitution of the small Ru cations by larger Ba cations and grain boundary scattering could be the main possible causes of these interesting conductive properties. This unique phenomenon, which is not observed in bulk, may provide new features useful in the fabrication of novel electronic devices.

The thermal expansion coefficients of LaFe_11.7Si_1.3 from 85 to 413 K are studied. The volume expands significantly at the Curie temperature 188 K when it changes from the paramagnetic to the ferromagnetic state. The crystallographic structures at 85 and 298 K were determined, and no evidence of a structural transition was found. The thermal expansion coefficients are near constants when the temperature is about 40 K higher or lower than the Curie temperature. From 85 to 143 K, the thermal expansion coefficient is about 3.61(±0.26)×10⁻⁶ K⁻¹, and from 233 to 413 K, it is about 1.17(±0.04)×10⁻⁵ K⁻¹. From 153 to 213 K, the negative nonlinear expansion coefficients are related to the metallic magnetism.

Magnetic force microscopy has been used to characterize the paramagnetic to ferromagnetic transition caused by carbide precipitation in a HPM alloy used in ethylene production. The MFM images are compared to conventional atomic force microscope micrographs. The element segregation in the alloy is quantified using scanning electron microscope energy dispersive x-ray analysis linescans, and compared to Curie temperature data for iron, nickel and chromium alloys.

A common interpretation of the results of thermally stimulated current experiments is in terms of a fixed set of states present in the system with a range of activation energies, of which specific subsets are selected and identified by thermal slicing (TS) (or fractional polarization) experiments at different polarization temperatures T_p. On the other hand, such an interpretation is not consistent with many current theories of supercooled liquids and of the glass transition. The results are presented of our calculations on thermally activated Fredrickson–Andersen model systems, which indicate that the TS technique does identify the isothermal response of the system at T_p. However, this response need not be associated with specific states present in the system, but can arise for instance from a competition between different relaxation paths for the dipole moments of the individual particles. In addition, the compensation law is found to be obeyed for systems with and without cooperative effects, so that it seems to have no fundamental significance.

An iterative, automated method for characterization of piezoelectric specimens of a rectangular bar in the longitudinal extensional mode has been developed. The complex materials constants (dielectric constants, elastic compliance coefficients and piezoelectric constants) of piezoelectric specimens with various Q were accurately determined. A new method to estimate the appropriate initial values of the complex materials constants for a nonlinear iteration process has been presented. This nonlinear iterative fitting procedure was carried out for the experimental admittance spectra Y = G+jB for various specimens with high (4600) or low (50) Q values and the validity of the method was verified.

The variable dose (VD), T_m–T_stop, initial rise, variable heating rate, peak shape and computerized glow curve deconvolution (CGCD) methods were used to determine the number of peaks, the order of kinetics (b), the activation energy (E_a) and attempt-to-escape frequency (s) associated with the dosimetric glow peak of α-Al₂O₃ : C after β-irradiation at a dose level between 0.02 and 288 Gy. The E_a–T_stop and CGCD methods indicated that the dosimetric peak of this crystal is the superposition of at least two components (peak 2a and 2b). When the dose level reaches and exceeds saturation point, a third peak (peak 2c) is also evident on the low temperature side of the dosimetric peak. The VD and CGCD methods indicated that both components of the dosimetric peak have general-order kinetics that change with doses. The dose responses of peak 2a follow linear, supralinear, saturation and sublinear patterns, whereas peak 2b does not exhibit supralinear behaviour. They are saturated at different dose levels, peak 2a at D = 10 Gy and peak 2b at D = 5 Gy. E_a–T_stop plots obtained at two different dose levels (D = 10 and 1 Gy) indicated that the efficiency of thermal quenching effect on this sample is highly dependent on the dose level and decreases with increasing dose level, which also has pronounced effects on the evaluated kinetic and thermal quenching parameters of the dosimetric peak.

Deep penetration laser welding is associated with the deposition of very high irradiation of a laser beam to a workpiece to form a slender capillary into the workpiece, denoted as a keyhole, as well as violent plasma generation which consists of metal vapour, ionized atoms, and free electrons. Plasma resides both outside and inside the keyhole, known as plasma plume and keyhole plasma, respectively. In this paper, we quantitatively characterize the plasma absorption of a laser beam inside the keyhole based on photodiode measurements and spectroscopic measurements in 20 kW-class CO₂ laser welding processes. The results show that the plasma absorption of a laser beam cannot be neglected in certain welding conditions. Furthermore, this plasma absorption mainly occurs in the top portion of the keyhole, resulting in a wider weld shape in the top portion of the weld bead.

Thermal local micro- and nano-probes are currently used in scanning thermal microscopy (SThM) to characterize nano-structured media. In the active mode of SThM, the very small contact between the heated probe and the sample surface defines the region of the sample excitation. In such a case, the depth profiling is limited by the dimension of the contact zone and by the ac frequency of the heat source modulations. This paper furnishes the links between the mean temperature of the probe apex and the heat flow going through this apex. The response of the probe depends on the radius of the probe–sample contact and the heated zone appears, in the case of a mesososcopic source, only weakly sensitive to the thermal characteristics of the sample. The resolution of the scanning apparatus is enhanced in the case of a small buried default analysed in the frame of the first Born approximation of the heat diffusion equation.

Capacitance–voltage (C–V) and transient capacitance characteristics of devices based on a thiophene-based conjugated polymer have been studied. C–V characteristics showed a hysteresis behaviour similar to one previously observed in current–voltage (I–V) measurements. The hysteresis behaviour has been explained by considering accumulation of space charges in the polymer layer near the electrodes of the device. The amount of space charges has been found to increase with voltage amplitude. An asymmetry has been observed in charge distribution under forward and reverse bias directions. A voltage pulse was applied to `write' a state, which has been `read' by measuring the capacitance of the device. The polymer has been found to retain the stored data for more than 2 h. The slow relaxation of the stored charges, as evident from capacitance transients, has been identified to be responsible for memory applications of the polymer-based devices. We have established that the displacement current due to capacitance variation can generate the observed hysteresis in the I–V characteristics of the devices and hence memory effect in polymer-based devices.