Table of contents

Semiconductor nanowires (NWs) represent a unique system for exploring phenomena at the nanoscale and are also expected to play a critical role in future electronic and optoelectronic devices. Here we review recent advances in growth, characterization, assembly and integration of chemically synthesized, atomic scale semiconductor NWs. We first introduce a general scheme based on a metal-cluster catalyzed vapour–liquid–solid growth mechanism for the synthesis of a broad range of NWs and nanowire heterostructures with precisely controlled chemical composition and physical dimension. Such controlled growth in turn results in controlled electrical and optical properties. Subsequently, we discuss novel properties associated with these one-dimensional (1D) structures such as discrete 1D subbands formation and Coulomb blockade effects as well as ballistic transport and many-body phenomena. Room-temperature high-performance electrical and optical devices will then be discussed at the single- or few-nanowire level. We will then explore methods to assemble and integrate NWs into large-scale functional circuits and real-world applications, examples including high-performance DC/RF circuits and flexible electronics. Prospects of a fundamentally different 'bottom-up' paradigm, in which functionalities are coded during growth and circuits are formed via self-assembly, will also be briefly discussed.

The effects of magnetostatic interactions between monodisperse single-domain nanoparticles are described in terms of the particle concentration and structural ordering within the assembly. The resulting energy level distribution, time-dependent behaviour and spatial correlations are discussed in the context of the properties of bulk dipolar ferromagnets, bulk spin glasses and non-interacting monodomain particles. Both standard measurements and novel characterization techniques are described. The results from research groups studying monodisperse magnetic nanoparticles are compared to illustrate the distinguishing characteristics of interacting assemblies and to highlight important questions that remain.

CoPt and FePt nanowire arrays were successfully prepared by electrodeposition into nanochannels of porous alumina membranes. The as-deposited CoPt alloy has a face centred cubic structure and displays soft magnetic properties. Heat treatment at 700 °C for different durations, under vacuum condition was carried out in order to obtain the ordered face centred tetragonal phase L1₀, exhibiting hard magnetic properties. After annealing, arrays of nanowires actually show hard magnetic properties with coercive fields up to 1.1 T at room temperature. Phase transformation, structural and magnetic properties were analysed and necessary conditions to obtain optimum magnetic properties are concluded.

The first results obtained with FePt nanowires denote a more complicated system since the as-deposited material shows no magnetization. Magnetism appears only after annealing at 700 °C minimum. Coercivity up to 1.1 T has been obtained at room temperature but with inhomogeneous phase composition.

This paper describes the successful development of a light and compact 0.5 MV spiral-strip transformer, with the secondary winding contained in vacuum and based on magnetic self-insulation. Ensuring trouble-free operation required the use of conductive elastomers in electric field grading techniques and the adoption in the secondary winding of glass/ceramic conductor spacers. It is demonstrated that the primary-current/secondary breakdown-voltage characteristic is a function of the vacuum pressure, with only 52 kA being necessary to produce 0.5 MV at 10⁻⁶ Torr. The difficult task of modelling the transformer required 3D electric and magnetic field computation, together with state-of-the-art calculation of the electron flow in the vacuum. Based on the results obtained to date, scaling up to multi-megavolt transformers can readily be envisaged.

The influence of an imperfect interface on exchange bias (EB) properties is investigated. Within the framework of the domain state model, the EB field H_EB and the coercive field H_C are determined using computer simulations, and they are found to depend strongly on the details of the interface structure. This dependence is sensitive to the dilution of the antiferromagnet (AFM) with non-magnetic defects in the bulk. For the optimal interface structure, giving greatest EB, the optimal dilution is found to be much less than that for an ideal-interface system, taking a value in better agreement with experimental results. Even without any defects in the bulk of the AFM the interface roughness leads to EB for thin antiferromagnetic layers, in accordance with the model by Malozemoff. Finally, the thickness dependence of rough-interface systems is found to differ significantly from that of ideal-interface systems.

SnO₂ nanocrystalline thin films were deposited on glass substrates by the spray pyrolysis technique in air atmosphere at 375, 400, 425, 450 and 500 °C substrate temperatures. The obtained films were characterized by using XRD. The room temperature photoluminescence (PL) spectra of these films have near band edge (NBE) and deep level emission under the excitation of 325 nm radiation. NBE PL peak intensity decreased consistently with temperatures for samples prepared at 400, 450 and 500 °C, while a sudden reduction in intensity is observed for the sample prepared at 425 °C. A similar effect was observed for the optical transmittance spectra. These effects can be explained on the basis of the change in population of oxygen vacancies as indicated by the change in Δa values.

Microstructural changes and fractal Ge nanocrystallites in polycrystalline Au/amorphous Ge thin bilayer films upon annealing have been investigated by scanning electron microscopy, transmission electron microscopy observations and x-ray energy-dispersive spectroscopy (EDS). Experimental results indicated that the microstructure of the metal Au film plays an important role in metal-induced crystallization for Au/Ge thin bilayer films upon annealing. Interestingly, we found the position exchange of Au and Ge films and the formation of the fractal Ge nanocrystallites induced by annealing. EDS microanalysis indicated that although there is lateral interdiffusion of Au and Ge atoms, the thickness of the fractal region and the matrix remain nearly the same. At the same time, EDS shows that there are also Au aggregates extending out of the films. It is suggested that, besides the preferred nucleation at the Au/Ge interface, the breaking of Ge–Ge bonds may stimulate the crystallization of amorphous Ge, so that the crystallization temperature of Au/Ge system is much lower than that of the isolated amorphous Ge system.

Samarium (dibenzoylmethanato)₃ bathophenanthroline (Sm(DBM)₃ bath) was employed as an emitting and electron transport layer in organic light emitting diodes (OLEDs), and narrow electroluminescent (EL) emissions of a Sm³⁺ ion were observed in the visible and near infrared (NIR) region, differing from those of the same devices with Eu³⁺- or Tb³⁺-complex EL devices with the same structure. The EL emissions of the Sm³⁺-devices originate from transitions from ⁴G_5/2 to the lower respective levels of Sm³⁺ ions. A maximum luminance of 490 cd m⁻² at 15 V and an EL efficiency of 0.6% at 0.17 mA cm⁻² were obtained in the visible region, and the improved efficiency should be attributed to introducing a transitional layer between the N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-diphenyl-4,4'-diamine (TPD) film and the Sm(DBM)₃ bath film and the avoidance of interfacial exciplex emission in devices. Sharp emissions of Sm³⁺ ions in the NIR region were also observed under a lower threshold value less than 4.5 V.

The effect of oxygen on the electrical characteristics of organic thin film transistors with pentacene as the active layer has been investigated. The saturation currents and mobilities of the transistors increase as the ambient oxygen concentration decreases, which is ascribed to the formation of a charge transfer complex between pentacene and O₂. The deposition rate of the pentacene layer affects this phenomenon. The transistor with the pentacene layer deposited at a rate of 15 nm min⁻¹ shows higher sensitivity to oxygen concentration than the device with the pentacene layer deposited at 30 nm min⁻¹. We suggest that when deposited at a lower rate the pentacene film is less compact, leading to easier entrance of oxygen into the charge accumulation region.

A simple model of thermoluminescence (TL), in which the trap and the recombination centre exist in the same defect complex, is assumed. It is called the localized recombination model (LRM). The analytical expression derived for this model turns out to be of first order irrespective of whether the system is recombination dominant or retrapping dominant. In this paper TL glow curves are calculated for this model using analytical as well as numerical techniques. The results show the following: (1) the glow curves calculated by the two techniques coincide with each other. However in retrapping dominant cases, the initial assumptions made by the original proponents of the model in deriving the analytical expression are not found to be valid, which apparently invalidates the analytical expression. This contradiction is reconciled in the paper and a new criterion is found for the validity of the analytical expression. (2) Even though the glow peaks of LRM always obey first order kinetics, their shapes may be typically of non-first order particularly when the value of s'/β is below about 10⁶, where s' is the pre-exponential factor and β is the linear heating rate. In view of these findings it is emphasized that kinetic analysis of TL glow curves of real materials carried out purely on the basis of shape factor may lead to dubious results. It is recommended that a glow peak may be assigned non-first order only when dose dependence of peak temperature is observed.

A low average power pulsed Nd : YAG laser is used for bead-on-plate welding of low carbon steel plate. The criterion for welding in the conduction and keyhole regime together with the effect of laser process parameters on weld quality is studied. It is established that for a weld with full penetration and velocities viable for industrial applications, welding in the conduction mode would not be suitable. The formation of a keyhole is found to be highly dependent on laser pulse parameters. With an average power in the range 220–240 W and a peak power of 1.8 kW, full penetration through keyhole generation of a 0.7 mm thick st14 sheet metal is achieved.

A lithium (Li) and nitrogen (N) dual-doped p-type ZnO film (ZnO : (Li,N)) was deposited on c-plane sapphire by RF-magnetron sputtering of Zn–2 at.% Li alloy using mixed gases of oxygen and nitrogen and then annealing in N₂ flow. It has a carrier concentration of 3.07 × 10¹⁶ cm⁻³ and Hall mobility of 1.74 cm² V⁻¹ s⁻¹. XPS measurement shows that there are Li_Zn–N complexes in the p-type ZnO : (Li,N), which are demonstrated by photoluminescence measured at various temperatures and different excitation powers to be acceptors and responsible for p-type conductivity of the ZnO : (Li,N). The optical level of the Li_Zn–N complex acceptor is estimated to be about 126 meV by measurement of emission energy of free electron to the acceptor level.

The formation of light-emitting porous silicon (PSi) in a HF solution by adding an oxidizing agent KIO₃ is presented. The PSi layers are formed by photoetching under Xe lamp illumination. The use of a Xe lamp enables the formation of large PSi layers on n-Si substrates. Yellow photoluminescence peaking at ∼2.0 eV is observed for the sample prepared in a HF/KIO₃ solution for 6 min, which is about 80 times stronger than that formed in a pure HF solution for 105 min. The surface topography as characterized by atomic force microscopy reveals features of the order of 20–100 nm. The PSi formation mechanism can be successfully explained with the aid of a surface energy-band diagram of n-Si in the HF/KIO₃ electrolyte.

A simple method for estimating the frequency responses of directly modulated lasers from optical spectra is presented. The frequency- modulation index and intensity-modulation index of a distributed feedback laser can be obtained through the optical spectrum analyses. The main advantage is that the measurement setup is very simple. Only a microwave source and an optical spectrum analyser are needed and there is no need to use a calibrated broadband photodetector. Experiment shows that the proposed method is as accurate as the swept frequency method using a network analyzer and is applicable to a wide range of modulation powers.

Measurements are described of electron number density in the near-field of a 200 W Hall plasma accelerator using a 90 GHz microwave interferometer. The system is of a phase-bridge design, utilizing two signal arms from a fixed frequency source (one passing through the plasma) that recombine at two balanced mixers. A line-integrated electron density is obtained by comparing the in-phase and quadrature signals from the mixers. Measurements across chords through a 'symmetric' plasma plume are Abel inverted to give the radial variation in the plasma density, which is found to be toroidal in shape, reaching values as high as 4 × 10¹⁷ m⁻³. The first direct non-intrusive measurements of the electron density in the near-field of this source reveal structure not previously resolved with electrical probes. This interferometer is suitable for measuring time-dependent plasma density fluctuations offering non-intrusive information about plasma oscillations and instabilities in the near exit region where electrical probes are known to interfere with the discharge operation.

We perform a numerical investigation of the supersonic expansion of a laboratory jet produced in Pilot-PSI. Pilot is a linear device consisting of a cascaded arc plasma source, a vacuum vessel for plasma expansion and magnetic coils for jet collimation. We concentrate here on (1) the neutral gas dynamics and the detailed steady jet morphologies resulting from experimentally realized nozzle configurations; and (2) the plasma collimation due to the applied magnetic fields. The former approach analyses numerical simulations modelled by means of the compressible hydrodynamic equations, comparing 1D spherical and 2D axially symmetric results with related experimental efforts. An in-depth analysis of the characteristics of the stationary flow pattern allows a study of the properties of the supersonic expansion: a well defined free-jet flow shock structure is formed in the first region of the expansion and fully described. Also, a predicted correlation between the variation of the pressure in the vessel and the position of the Mach disc is confirmed. We investigate changes in the supersonic jet structures due to a geometrical variation of the nozzle source diameter, revealing a more complicated shock structure which we analyse using the characteristic lines. A comparison with the free-jet flow is made and the influence of the nozzle geometry on the flow is discussed. The magnetic collimation of the plasma beam is demonstrated by means of single fluid magnetohydrodynamic simulations, which clearly show the transition from compressible gas to fully ionized plasma regimes. A qualitative agreement with measured Pilot-PSI density profiles indicates to which extent such single fluid descriptions aid in the interpretation of observed plasma jet dynamics.

Experimental and theoretical studies have been performed on the diffuse and spot operation modes of a low-pressure dc discharge in argon. Two coiled-coil cathodes of different sizes, each of them uncoated and coated with an emitter mix of alkali oxides, are used in measurements of the coil temperatures at increasing and decreasing heater power and different discharge currents. The model of the cathodes is based on the solution of heat balance equations for the cathode and the surrounding gas and equations for the particle fluxes of electrons and ions. The experimentally observed dependences of the spatially resolved cathode temperature on the coil coating, discharge current and heating power are compared with the calculated results. Good agreement between the measured and calculated cathode temperature profiles is obtained, which validates the model. The investigations reveal the impact of discharge and emission currents on the temperature profiles of uncoated and coated cathodes and provide an insight into the main features of spot formation.

Xe gas in low pressure discharge tubes (20 mm diameter, 800 mm length, commercial hollow electrodes at 650 mm distance, 50 Hz, 60 mA) was quantitatively determined using quadrupole mass spectrometry (QMS). The temporal Xe gas evolution over 5 to 20 h of gas discharge revealed the consumption and recovery of Xe gas to be dependent on the prevailing gas composition. The net consumption in the standard Xe/He(2.0/98) gas mixture was much faster than that in Xe/Kr/He(1.9/93/5). Xe recovery during discharges in 'preloaded' tubes refilled with pure gas revealed an increasing efficiency in the sequence He < Ne < Ar < Kr, being most efficient with the first refilling. The temporal evolution of Xe gas was simulated by a kinetic model based on two equilibria between Xe gas and Xe incorporated in the glass tube walls (1) and the electrodes (2). It is concluded that the experimental setup, the elaborated measurement procedures and the kinetic model provide a good basis for further quantitative investigations. The net Xe gas consumption during discharge, which is presently limiting the tube lifetime, may be reduced particularly by improving the discharge gas composition and the electrode materials.

The growth of AlN on c-plane sapphire substrates is reported for a growth temperature of 1100 °C in a radio frequency plasma assisted molecular beam epitaxy system. The high growth temperature in combination with Al-rich growth conditions resulted in a high crystalline quality (FWHM of the 0002 x-ray reflection of 650'') and low O incorporation. Furthermore, the incorporation of Si and Mg as n- and p-type dopants, respectively, has been studied for these growth conditions. For Si doping the corresponding cell temperature was varied between 1300 and 1350 °C. Secondary ion mass spectrometry (SIMS) showed a homogeneous Si depth profile up to a concentration of 7 × 10²⁰ cm⁻³, but the Si doped layers remained highly resistive. Incorporation of Mg was observed only at a low growth temperature of 830 °C; at higher growth temperatures SIMS revealed a strong surface segregation effect while the amount of Mg incorporated into the AlN layer remained below the detection limit. The build-up of a Mg accumulation layer at the growth surface due to segregation was found to cause a significant reduction in growth rate.

Using molecular-dynamics simulation, we study the excitation of stress waves in crystalline Ar overlayer films adsorbed on a metal substrate. When the metal is suddenly heated, e.g. by a short-pulse laser, the thermal expansion excites stress waves in the overlayer film. The oscillations induced by the stress wave in the film are long-lived and are damped only when the film starts melting; they vanish upon ablation of the entire film. The heating induced by the stress wave in the overlayer film and the role of the Kapitza effect—i.e. the thermal interface resistance of the metal-Ar boundary—are discussed. The simulated peak stresses agree well with the prediction of thermoelastic theory, if the Kapitza effect is taken into account. For stronger heating, the stress wave is asymmetric in that the compressive part is stronger than the tensile part.

Multi-element (AlCrTaTiZr)N coatings are prepared by reactive RF magnetron sputtering. The influence of substrate bias (0 to −200 V) on the deposition rate, composition, structure and mechanical properties of these coatings is investigated. A reduction in both deposition rate and Al concentration is observed with increasing substrate bias. The grounded substrate is found to possess a columnar structure. The columnar structure is not so apparent in the denser coatings deposited at an applied substrate bias of −150 V. Furthermore, a minimum in coating roughness is found to occur at the intermediate substrate bias of −100 V. All the (AlCrTaTiZr)N coatings appear to have a single FCC structure from XRD analysis. Furthermore, it is determined from XRD that there is an increase in both (111) peak intensity and grain size, and a decrease in the lattice strain, for increasing substrate bias. The residual stress, hardness, elastic modulus and toughness of the coatings are found to be greatly enhanced by the substrate bias. The highest hardness and elastic modulus of approximately 36 Gpa and 360 GPa, respectively, were obtained at a substrate bias of −150 V.

The surface evolution of an elastic layer subjected to a far-field uniform tensile stress in an electric field is analysed by using the linear perturbation theory. The driving force controlling the surface growth is the gradient of the chemical potential associated with surface energy, elastic strain energy and electric energy, and surface diffusion is the dominant mechanism of mass transport. An analytical expression of the dispersion relation is obtained, which describes the morphological evolution of the layer in an electric field. It is found that there exists a region, in which any surface perturbation will not lead to the morphological instability of the elastic layer due to the confinement of electromechanical interaction. The electromechanical interaction can either enhance or suppress the growth of the surface perturbation.

A calculation approach for acoustic impedance and time-of-flight of an elastic thin layer embedded between a thin substrate and a substrate of infinite extent is described utilizing the reflection coefficients for the normally–incident ultrasonic reflected signals. As the difference of the phase-shift in the embedded layer at two neighbouring angular frequencies is not greater than π, analytical relational expressions for acoustic impedance and time-of-flight of the embedded layer with the measured data are derived, respectively. Utilizing two analytical relational expressions, acoustic impedance and time-of-flight can be calculated without considering acoustic impedance of the substrate of infinite extent. The influences of the changes in the parameters of the known layers and the experimental noises on calculating acoustic impedance and time-of-flight of the embedded layer are numerically investigated.

Laser-induced backside wet etching (LIBWE) is an indirect etching technique that was first demonstrated for fused silica applying a pyrene doped hydrocarbon solution and was recently demonstrated for gallium as a highly absorbing liquid. In this study the etching of fused silica with mercury as liquid absorber is demonstrated using a nanosecond UV laser (excimer laser λ = 248 nm, 25 ns pulses) radiation. The high absorption coefficient of mercury enforces the absorption of the UV laser radiation within a 100 nm region near the solid interface so that, despite the high reflectivity, the threshold for etching was measured to be 0.76 J cm⁻². With rising laser fluence the etch rate increases nearly linearly in the whole laser fluence range and reaches a value of 650 nm/pulse at a laser fluence of about 10 J cm⁻². The etched surfaces were investigated by scanning electron microscopy, atomic force microscopy and interference microscopy and show well defined etch groove edges and a flat, smooth etched bottom that features an rms roughness of about 2.2 nm. The laser-induced backside etching of fused silica with mercury combines a low etching threshold, high etch rates and a still smooth etching of well-defined patterns into material surfaces.

Disruption of the plasma membrane causes serious changes in the dielectric properties of biological cells. The changes have been simulated with spherical cell models having holes in the plasma membrane. The complex permittivity of a cubic system including a cell model was calculated by a numerical technique based on the three-dimensional finite difference method. For a cell without hole, the complex permittivity showed dielectric relaxation (β-dispersion) as predicted from interfacial polarization theories for the single-shell model. When there is one hole in the membrane, the cell has anisotropic dielectric properties depending on whether the axis through the centres of the hole and the sphere is parallel (the parallel orientation) or perpendicular (the perpendicular orientation) to the electric field direction. In the parallel orientation, dielectric relaxation (called 'α-dispersion') appeared at lower frequencies in addition to the β-dispersion, whereas only the β-dispersion was found in the perpendicular orientation. When there were two holes at the opposite poles of the cell, the 'α-dispersion' did not appear and the intensity of the β-dispersion decreased with increasing size of the holes. When two holes located in the hemisphere of the cell, however, the 'α-dispersion' appeared again. These results suggest that the occurrence of the 'α-dispersion' requires either the presence of one hole or the localization of holes.

Charge transfer of polycrystallines and mononocrystals of K₃H(SO₄)₂ superprotonic conductor was investigated. We have shown that in an external dc electric field the protonic charge transport assisted by electrons behaves nonlinearly below the temperature of fast proton phase transition. This can be considered as a soliton-like behaviour. The current fluctuations, which are proportional to the basic current, are most probably evoked by the formation of charge density waves which are pinned/depinned in the crystal lattice. ac investigations also indicate nonlinear character of conductivity.

This paper introduces an alternative optical method to measuring liquid penetration into porous highly scattering media. Using pure glycerol, the method was tested by measuring glycerol sorption into cellulose fibre tissue with a grammage of 115 g m⁻². During the wetting process, dynamical changes in the scattering properties of the fibre tissue were detected by optical coherence tomography. Measurements were made from a single point on the front and back surface of a sample. Although the effect of penetration on the optical properties of a porous structure can be seen independent of measurement direction, the border between the dry and wetted area is detectable only in front surface measurements. In addition, the paper experimentally investigates the temporally and spatially dependent swelling behaviour of paper.

In this paper, a piezoelectric ceramic thin ring with an inner metal disc is studied. The radial vibrations of a metal thin disc and a piezoelectric ceramic thin ring are analysed. Their electro-mechanical equivalent circuits in radial vibration are obtained. Based on the electro-mechanical equivalent circuits and the radial boundary conditions, the composite electro-mechanical equivalent circuit of the combination of a piezoelectric ceramic thin ring and a metal disc is obtained and the resonance and anti-resonance frequency equations are derived. The relationship between the resonance frequency, the anti-resonance frequency, the effective electro-mechanical coupling coefficient and the geometrical dimensions is analysed. The resonance and anti-resonance frequencies are measured using the Agilent Precision Impedance Analyzer. It is illustrated that the measured radial resonance and anti-resonance frequencies are in good agreement with the theoretical results.

This paper reports the results from experimental studies of the effect of applied stress on the propagation of broadband ultrasonic pulses in selected samples of aluminium alloy. For one particular 5086 alloy of aluminium used, results have shown changes in the spectral characteristics of the detected ultrasonic signals with applied stress. In this case, measurements of the relative transit times for various bulk wave modes exhibit two distinct dependences on stress, for stresses below and above approximately 40–50 MPa. These regions must be related to changes in the microstructural parameters of the material and we propose a possible mechanism for this that is consistent with the earlier published literature. It is also shown that by taking into account the influence of changes in the spectral characteristics of the signals in the process of loading in the elastic region, one is able to increase the accuracy of stress measurement by the ultrasonic method.

Nanocrystalline Ni_0.5Zn_0.5Fe₂O₄ spinel ferrite with a grain size of 50 nm was prepared by using the ceramic method. The grain size was further reduced to 14 nm by milling the as-prepared ferrite particles in a high-energy ball mill. From the impedance spectroscopy studies we have observed that the dc electrical conductivity increases upon milling. Furthermore, the cation distribution data, as obtained from the in-field Mössbauer and extended x-ray absorption fine structure measurements, suggested a decrease in the conductivity for the milled sample. The increase in conductivity of the milled sample is, therefore, attributed to conduction by the oxygen vacancies created by mechanical milling. The higher values obtained for the activation energy for conduction are also evidence for the oxygen vacancy conduction. The increase in Néel temperature from 573 to 611 K on reducing the grain size from 50 to 14 nm has been explained based on the changes in the cation distribution. The observed increase in the coercivity of the milled sample has been attributed to surface anisotropy of increasing number of ions on the surface. The Mössbauer spectra show canted spin structure for the milled samples.

The evaluation of the upward connecting leader inception from a grounded structure has generally been performed neglecting the effect of the propagation of the downward stepped leader. Nevertheless, field observations suggest that the space charge produced by streamer corona and aborted upward leaders during the approach of the downward lightning leader can influence significantly the initiation of stable upward positive leaders. Thus, a physical leader inception model is developed, which takes into account the electric field variations produced by the descending leader during the process of inception. Also, it accounts for the shielding effect produced by streamer corona and unstable leaders formed before the stable leader inception takes place. The model is validated by comparing its predictions with the results obtained in long gap experiments and in an altitude triggered lightning experiment. The model is then used to estimate the leader inception conditions for free standing rods as a function of tip radius and height. It is found that the rod radius slightly affects the height of the downward leader tip necessary to initiate upward leaders. Only an improvement of about 10% on the lightning attractiveness can be reached by using lightning rods with an optimum radius. Based on the obtained results, the field observations of competing lightning rods are explained. Furthermore, the influence of the average stepped leader velocity on the inception of positive upward leaders is evaluated. The results obtained show that the rate of change of the background electric field produced by a downward leader descent largely influences the conditions necessary for upward leader initiation. Estimations of the leader inception conditions for the upper and lower limit of the measured values of the average downward lightning leader velocity differ by more than 80%. In addition, the striking distances calculated taking into account the temporal change of the background field are significantly larger than the ones obtained assuming a static downward leader field. The estimations of the present model are also compared with the existing leader inception models and discussed.

In laser welding experiments of glass, keyhole shapes are observed by two high-speed cameras from two perpendicular directions. From the obtained keyhole pictures, it can be seen that in medium- and low-speed laser penetration welding, the main distortion of the keyhole is not the section metamorphosis from rotational symmetry, but the bending of its centre line. Based on such a keyhole photograph, the keyhole profiles and its centre line are determined by the method of polynomial fitting. Then, under the assumption of a circular cross section at each depth of the keyhole, the behaviour of the laser beam in the keyhole is analysed by tracing a ray of light using geometrical optics theory; the Fresnel absorption and multiple reflections in the keyhole are systematically studied, and the laser intensities absorbed on the keyhole walls are calculated. The absorbed laser intensity is not distributed uniformly on the keyhole wall. The keyhole wall absorbs laser intensity mainly on the half-part of the keyhole wall near the front wall. Because of the high absorptivity of the glass, Fresnel absorption from the first incidence of a laser beam plays a dominant role in the final laser intensity distribution on the keyhole wall, multiple reflections have some minor effects on the intensity distribution on the bottom part of the keyhole.

A thermo-ecological performance analysis of an irreversible intercooled and regenerated closed Brayton heat engine exchanging heat with variable-temperature thermal reservoirs is presented. The effects of intercooling and regeneration are given special emphasis and investigated in detail. A comparative performance analysis considering the objective functions of an ecological coefficient of performance, an ecological function proposed by Angulo-Brown and power output is also carried out. The results indicate that the optimal total isentropic temperature ratio and intercooling isentropic temperature ratio at the maximum ecological coefficient of performance conditions (ECOP_max) are always less than those of at the maximum ecological function ( ) and the maximum power output conditions ( ) leading to a design that requires less investment cost. It is also concluded that a design at ECOP_max conditions has the advantage of higher thermal efficiency and a lesser entropy generation rate, but at the cost of a slight power loss.

The control volume method (CVM) is the basic technique of computational fluid dynamics, a mathematical model for the simulation of keyhole and pool formation during deep penetration laser beam welding. Using CVM is different from using the finite element method. A three-dimensional volumetric heat source model consisting of a rotary Gaussian volumetric heat source and a double ellipsoid heat source is developed. Taking into account buoyancy and gravity, SIMPLE solution and first order upwind discretization under-relaxation iteration are used. The modelling is carried out for Ti–6Al–4V in laser keyhole welding using CVM. The local temperature distribution in the fusion zone is simulated separately with regard to dependence on laser power inputs and welding speeds. The results indicate that eddies occur near the top and bottom surfaces and help heat transfer and bulbous weld pool formation. To assess the accuracy of the developed model, the computed and experimentally determined pool shapes of laser keyhole welding are compared for welds on titanium alloys.

The purpose of this paper is to investigate the effect of quantum properties of the working medium on the performance of an irreversible quantum Ericsson cooler with spin-1/2. The cooler is studied with the losses of heat resistance, heat leakage and internal irreversibility. The optimal relationship between the dimensionless cooling load R^* versus the coefficient of performance ε for the irreversible quantum Ericsson cooler is derived. In particular, the performance characteristics of the cooler at the low temperature limit are discussed.

In the second paragraph of the Discussion (section 4, on page 4439) the sentence

θ_R increases monotonically with q, whereas I_R1/I₀ decreases with q.

should be removed. The preceding sentence

θ_R decreases monotonically with q, whereas I_R/I₀ increases with θ_R.

is correct and is retained.

The caption for figure 4 on page 1897 should be changed to read as:

Figure 4. Left: Ion probe TOF signals at 4.5 cm from the target and at various angles for laser ablation of Ag in 0.12 mbar of argon. Right: Angular distribution of the TOF values for the second peak maximum recorded at 7.5 cm from the target in various pressures of argon compared with the theoretical model.

Equations (3.5), (4.1) and (4.3) are corrected as in the attached PDF file. These equations were correct in our analysis and simulation codes. The discussions, figures and conclusions of our paper are unaffected.