Table of contents

A magnetic tunnel junction (MTJ), which consists of a thin insulating layer (a tunnel barrier) sandwiched between two ferromagnetic electrode layers, exhibits tunnel magnetoresistance (TMR) due to spin-dependent electron tunnelling. Since the 1995 discovery of room-temperature TMR, MTJs with an amorphous aluminium oxide (Al–O) tunnel barrier have been studied extensively. Al–O-based MTJs exhibit magnetoresistance (MR) ratios up to about 70% at room temperature (RT) and are currently used in magnetoresistive random access memory (MRAM) and the read heads of hard disk drives. MTJs with MR ratios significantly higher than 70% at RT, however, are needed for next-generation spintronic devices. In 2001 first-principle theories predicted that the MR ratios of epitaxial Fe/MgO/Fe MTJs with a crystalline MgO(0 0 1) barrier would be over 1000% because of the coherent tunnelling of fully spin-polarized Δ₁ electrons. In 2004 MR ratios of about 200% were obtained in MTJs with a single-crystal MgO(0 0 1) barrier or a textured MgO(0 0 1) barrier. CoFeB/MgO/CoFeB MTJs for practical applications were also developed and found to have MR ratios up to 500% at RT. MgO-based MTJs are of great importance not only for device applications but also for clarifying the physics of spin-dependent tunnelling. In this article we introduce recent studies on physics and applications of the giant TMR in MgO-based MTJs.

Sintering of ice is driven by the thermodynamic requirement to decrease surface energy. The structural morphology of ice in nature has many forms—from snowflakes to glaciers. These forms and their evolution depend critically on the balance between the thermodynamic and kinetic factors involved. Ice is a crystalline material so scientific understanding and approaches from more conventional materials can be applied to ice. The early models of solid state ice sintering are based on power law models originally developed in metallurgy. For pressure sintering of ice, these are based on work on hot isostatic pressing of metals and ceramics. Recent advances in recognizing the grain boundary groove geometry between sintering ice particles require models that use new approaches in materials science. The newer models of sintering in materials science are beginning to incorporate more realistic processing conditions and microstructural complexity, and so there is much to be gained from applying these to ice in the future. The vapour pressure of ice is high, which causes it to sublime readily. The main mechanism for isothermal sintering of ice particles is by vapour diffusion; however other transport mechanisms certainly contribute. Plastic deformation with power law creep combined with recrystallization become important mechanisms in sintering with external pressure. Modern experimental techniques, low temperature scanning electron microscopy and x-ray tomography, are providing new insights into the evolution of microstructures in ice. Sintering in the presence of a small volume fraction of the liquid phase causes much higher bond growth rates. This may be important in natural snow which contains impurities that form a liquid phase. Knowledge of ice microstructure and sintering is beneficial in understanding mechanical behaviour in ice friction and the stability of snow slopes prone to avalanches.

We describe the effects of magnetostatic interactions and film thickness on the switching properties and reversal mechanisms of periodic arrays of elongated Ni₈₀Fe₂₀ thin film rings. The magnetization reversal process, the switching field distributions and the transition fields between different equilibrium states are strongly affected by the inter-ring spacing. For closely packed ring arrays, sharp transitions were observed from the onion-to-vortex state due to collective magnetization reversal of the rings. In agreement with micromagnetic simulations, the range of stability of the vortex state is smaller for closely packed ring arrays compared with isolated rings of similar lateral dimensions.

Measurements of the frequency- and field-dependent complex susceptibility, χ(ω, H), of a suspension of cobalt particles are presented. The measured data indicate the presence of aggregates in the sample and raise the issue of how these polarized aggregates, subjected to a cyclic polarizing field, influence the isotropic properties of the fluid. Here we present the results of such an investigation which is centred on the application of the Weierstrass hypothesis, which dictates that, for the magnetic fluid to be isotropic, a plot of the graph of χ(ω, H) versus H should be symmetrical about the vertical axis. We demonstrate this to be so and also show that if χ is complex, with χ(ω, H) = χ'(ω, H) − iχ''(ω, H) then plots of χ'(ω, H) and χ''(ω, H) versus H are also symmetrical in the isotropic case.

Spruce sawdust (lignocellulose material) was magnetically modified after contact with a water-based magnetic fluid containing maghemite nanoparticles. This procedure allows one to use the prepared material as a new magnetic affinity adsorbent in magnetic separation techniques. The magnetic sawdust was investigated by means of electron spin resonance spectroscopy and conventional magnetic methods over the temperature range 4–300 K.

The experimental results clearly show that the magnetic behaviour of the sample is determined by two distinct magnetic contributions. One of them is connected to the presence of isolated single domain maghemite nanoparticles of ∼11 nm in diameter, which exhibit superparamagnetic behaviour at T > 280 K. The other one is related to the presence of agglomerates of particles coupled by magnetic dipolar interactions.

Nitrogen embedded ZnO : N films prepared by pulsed laser deposition exhibit significant ferromagnetism. The presence of nitrogen ions contained in ZnO is confirmed by the secondary ion microscopic spectrum and by Raman experiments, and the embedded nitrogen ions can be regarded as defects. According to the experimental results, a mechanism is proposed based on one of the electrons in the completely filled d-orbits of Zn that compensates the dangling bonds of nitrogen ions and leads to a net spin of one-half in the Zn orbits. These one-half spins strongly correlate with localized electrons that are captured by defects to form ferromagnetism. Eventually, the magnetism of nitrogen embedded ZnO : N films could be described by a bound magnetic polaron model.

The purpose of this paper is to investigate experimentally the shift of the eutectoid point in the Fe–C binary system when applying a high magnetic field. The eutectoid carbon content is observed to shift from 0.77 wt% to 0.83 wt% under a 12 T magnetic field. A practical and complete calculation method is proposed—on the basis of the statistical thermodynamic model—to calculate the Gibbs free energy of the related phases and predict the shift of the eutectoid point due to a magnetic field in both composition and temperature coordinates. The composition values are seen to be in fair agreement with the experimental data. The calculation of both shifts shows that the rise in eutectoid temperature because of the 12 T field is 28.97 °C. The impact of the magnetic field on both eutectoid carbon content and eutectoid temperature is not linear. The rate of the shift of both carbon content and temperature decreases as the magnetic field rises.

The work aims at understanding the changes in magnetic properties and the magnetoimpedance (MI) effect for the Fe_84.5Zr₄B_8.5Nb₂Cu₁ (FZBNC) nanocrystalline alloys with the annealing temperature (T_a) from the microstructure point of view. The as-quenched FZBNC ribbons were annealed in a vacuum condition at selected temperatures for 20 min. The microstructures of the specimens were examined by the x-ray diffraction technique. The bcc α-Fe phase is identified from the XRD patterns. The mean grain size of the α-Fe phase (D₁) at different annealing temperatures (T_a) was estimated using Scherrer's equation. The results show that D₁ for the sample annealed at 575 °C is about 20 nm, which is the smallest value among all the samples. Measurements on the MI effect reveal that the FZBNC ribbon annealed at 575 °C exhibits the most drastic MI ratio of 756% at 600 kHz accompanied by the field sensitivity of 108% Oe⁻¹, while a large MI effect is not observed in the sample annealed at other temperatures. The origin of the excellent MI effect is investigated in this paper.

This work discusses Ag/zinc oxide-doped indium oxide (ZIO) and ZIO/Ag ohmic contacts to p-GaN for flip-chip light-emitting diode (LED) applications. Optimal conditions for Ag/ZIO and ZIO/Ag contacts are adopted to minimize the specific contact resistance to 1.74 × 10⁻⁴ Ω cm² and 1.15 × 10⁻³ Ω cm², respectively, as revealed by the transmission line model after heat treatment at alloying temperatures of 500 °C and 400 °C, respectively, for 10 min in air. Ag/ZIO contacts following treatment at 500 °C yield a reflectance of around 87% at a wavelength of 470 nm. GaN-based LEDs with Ag/ZIO contacts produced forward biases of 3.09 V at an injection current of 20 mA.

Transition metal (Mn and Ni) doped ZnO nanocrystals with a diameter of 20 nm were fabricated by a low temperature solution processing technique. The incorporation of Ni and Mn into the ZnO lattice is evidenced by the shrinkage or expansion of the lattice parameters of ZnO in response to the transition metal dopants as determined by structure refinements. The E₂ (High) phonon lines in Raman spectra of all the samples show large redshift of more than 6 cm⁻¹ in comparison with that of the ZnO bulk single crystal. Especially, a further redshift of 3 cm⁻¹ and broadening effect in the Raman spectra of the 5% Ni doped ZnO nanocrystals in comparison with the undoped ZnO nanocrystals indicate that the transition metal introduced defects and host disorder into the ZnO. The obtained nanocrystals are ferromagnetic at room temperature.

It has been observed that the electrical resistance of n-CuIn₃Te₅ single crystal varies with time in the course of measurement. The current intensity and the potential drop across the sample decreased by up to 14% and 0.3%, respectively, after 300 s when a constant potential was applied. The single crystal does not obey Ohm's law. The current intensity plot has been fitted to a decay exponential and linear functions in the 0–45 s and 45–300 s ranges, respectively.

The resistance increase of the CuIn₃Te₅ single crystal can be due to the high concentration of intrinsic vacancies existing in the crystal structure. The experimental measurements have been analysed in terms of a hypothesis of ion migration via structural vacancies. The Cu⁺ and/or In⁺⁺⁺ cations are moved towards the cathode and are deposited in a space charge region, SCR. The SCR/electrode interface is modelled by an ideal capacitor and the arrival of ions produces a potential change in this region that is related to the current intensity and the potential drop across the sample.

Studies have been carried out on gel polymer electrolytes comprising poly methyl methacrylate (PMMA)-ethylene carbonate (EC)–propylene carbonate (PC)–salts, LiClO₄, NaClO₄ and (C₂H₅)₄NClO₄ (TEAClO₄) with a view to using them as electrolytes in electrical double layer capacitors (EDLCs) based on activated charcoal powder electrodes. The optimum composition of gel electrolytes, PMMA (20 wt%)–EC : PC (1 : 1 v/v)–1.0 M salts exhibit high ionic conductivity of the order of ∼10⁻³ S cm⁻¹ at room temperature with good mechanical/dimensional stability, suitable for their application in EDLCs. The EDLCs have been characterized using linear sweep cyclic voltammetry, galvanostatic charge–discharge tests and ac impedance spectroscopy. The values of capacitance of 68–151 mF cm⁻² (equivalent to single electrode specific capacitance of 38–78 Fg⁻¹ of activated charcoal powder) have been observed. These values correspond to a specific energy of 5.3–10.8 Wh kg⁻¹ and a power density of 0.19–0.22 kW kg⁻¹. The capacitance values have been observed to be stable up to 5000 voltammetric cycles or even more. A comparison of studies shows the predominant role of anions of the gel electrolytes in the capacitive behaviour of EDLCs.

The power efficiency of organic light-emitting diodes was significantly improved by introducing a novel n-doping (4'7- diphyenyl-1, 10-phenanthroline: 33 wt% 8-hydroxy-quinolinato lithium) layer as an electron transport layer and a p-doping layer composed of 4, 4', 4''-tris (3-methylphenylphenylamono) triphenylamine (m-MTDATA) and tetrafluro-tetracyano-quinodimethane (F₄-TCNQ) as a hole transport layer. Hole-only and electron-only devices were demonstrated to observe an improvement in the conductivity of the transport layers. With this strategy, we demonstrated that the power efficiency was enhanced by ∼100%, luminous efficiency was enhanced by ∼54% while driving voltage was reduced by 32% as compared with the control device. We obtained a power efficiency of 4.44 Lm W⁻¹, which is the best value so far reported for tris (8- hydroxyquinolinato) aluminium (Alq₃)-based emitters. This improvement was ascribed to the improved conductivity of the transport layers and to the better charge balance in the emission zone.

ZnO/Mg_0.2Zn_0.8O multiple quantum wells (MQWs) were grown on a c-plane sapphire (Al₂O₃) substrate by plasma-assisted molecular beam epitaxy. Both the spontaneous and the stimulated emission properties at room temperature (RT) in MQWs were studied under lower and higher excitation densities. The strong emission at 3.330 eV was observed in the MQWs at RT, which was attributed to the free exciton transition by the temperature-dependent photoluminescence measurement. Significantly, RT stimulated emission caused by inelastic exciton–exciton scattering was observed in the ZnO/Mg_0.2Zn_0.8O MQWs grown on Al₂O₃. Thus, the exciton binding energy was determined to be 122 meV in our MQWs.

This paper reports on the fabrication and characterization of a ridge optical waveguide in an Er³⁺/Yb³⁺ co-doped phosphate glass. The He⁺ ion implantation (at energy of 2.8 MeV) is first applied onto the sample to produce a planar waveguide substrate, and then Ar⁺ ion beam etching (at energy of 500 eV) is carried out to construct rib stripes on the sample surface that has been deposited by a specially designed photoresist mask. According to a reconstructed refractive index profile of the waveguide cross section, the modal distribution of the waveguide is simulated by applying a computer code based on the beam propagation method, which shows reasonable agreement with the experimentally observed waveguide mode by using the end-face coupling method. Simulation of the incident He ions at 2.8 MeV penetrating into the Er³⁺/Yb³⁺ co-doped phosphate glass substrate is also performed to provide helpful information on waveguide formation.

In this paper, we report the photoluminescence (PL) properties of porous ZnO nanotubes prepared by a two-step hydrothermal method. The nanotubes show a nearly homogeneous size of about 250 nm diameter, 50 nm wall thickness and 500 nm length. There are always some scattered nanoholes with sizes ranging from several tens of nanometres to about 200 nm on the side wall of each nanotube. A polychromatic PL band consisting of blue, green and orange emissions was realized by introducing the unwanted native defects into the ZnO lattice. Both the porous structure and the production of the unwanted defects are closely correlated with the non-equilibrium reaction condition during the preparation of porous ZnO nanotubes. The origin of the polychromatic PL was discussed together with the measurements of the PL features of the nanotubes annealed at different atmospheres and the lifetime of the three visible emissions. Porous ZnO nanotubes with polychromatic emission are expected to find much potential application in UV-excited wide-spectrum phosphors.

The propagation of counterpropagating modes in finite line-defect two-dimensional photonic crystal waveguides consisting of square lattice dielectric cylinders has been investigated by a numerical study based on multiple scattering theory. In the presence of forward propagating external light at one given frequency, two counterpropagating guided modes with the same spatial frequency can be simultaneously excited. Determined by the Fourier transform of field distribution inside waveguides, the wave vectors and lateral profiles of modes are demonstrated to coincide with theoretical results from a simple model, which only has the advantage of an energy-confining mechanism in waveguides. In addition, the spectral amplitude ratio of the backward mode to the forward mode is found to decay with a larger waveguide width and higher frequency. With an appropriate choice of waveguide width, both backward mode and higher transversal modes can be inhibited effectively, giving rise to highly efficient single-mode transmission in a wide and flat spectrum.

Streamer front propagation involves steep gradients in charge density and electric field. Since the front has to be meshed with a sufficient number of points, adaptive meshing is essential for fast and accurate numerical simulations. In this paper a one-dimensional (1D) moving mesh method recently developed by Tang and Tang (2003 SIAM J. Numer. Anal.41 487–515) is successfully applied to the simulation of streamer discharge. One-and-half (1.5D) and two-dimensional (2D) simulations of streamer discharges in nitrogen at atmospheric pressure are presented. The moving mesh method is combined with the third order ULTIMATE QUICKEST scheme (Leonard 1991 Comput. Math. Appl. Mech. Eng.88 17–74) to solve the advection part of the plasma continuity equations in a selection of classical problems in streamer simulation: point-to-plane and plane-to-plane electrode systems. The combination of the 1D moving mesh method and the high order scheme increases the accuracy of numerical solutions and reduces the computational time.

This work is devoted to the study of the possibility of obtaining the highest O₂(a ¹Δ_g) yield in ED SOG at the high absolute O₂(a ¹Δ_g) concentration needed for developing a powerful oxygen–iodine laser pumped by electric discharge. A singlet oxygen was produced in a transversal rf discharge in the pressure range 10–30 Torr of pure oxygen in the small-diameter (7 mm) quartz tube with HgO coating of the inner walls for removing atomic oxygen to eliminate fast O₂(a ¹Δ_g) quenching. It is shown that pd scaling (p—pressure, d—tube diameter) of the rf discharge actually allows an increase of the absolute O₂(a ¹Δ_g) density. The increase in the rf frequency from 13.56 to 81 MHz results in the essential increase of the O₂(a ¹Δ_g) yield (beyond 15% at such a high oxygen pressure as 15 Torr), but the subsequent transfer to the higher rf frequency of 160 MHz only slightly influences the maximally obtained O₂(a ¹Δ_g) yield. The effect of the NO admixture on the O₂(a ¹Δ_g) production has been also studied. The rate constant of O₂(a ¹Δ_g) quenching by was directly measured. The NO admixture (up to 20%) resulted in the noticeable increase in the O₂(a ¹Δ_g) yield mainly at low energy inputs. But this gain in the O₂(a ¹Δ_g) concentration drops with increasing energy input. Nevertheless it is shown that by combining the O₂ + NO mixture with the HgO coating of the discharge tube walls one can provide the O₂(a ¹Δ_g) yield on the level of ∼21% at 10 Torr, ∼17% at 20 Torr and ∼13% at 30 Torr of O₂ with the efficiency of ∼4–6%. The analysis of the NO admixture influence on the discharge structure and O₂(a ¹Δ_g) production has been carried out by using the 2D model. It was found that at the low energy input the NO admixture acts as an easily ionized species that enlarges the region occupied by plasma. Thus, in the O₂ + NO discharge the normal current density is lower than in the pure oxygen discharge. As a result a higher energetic efficiency of O₂(a ¹Δ_g) production is also observed in the case of the O₂ + NO mixture and the low energy input. In order to provide the optimal conditions for O₂(a ¹Δ_g) production (with regard to the yield and efficiency) in the continuous wave transversal VHF discharge at such high oxygen pressures as of 10–30 Torr it is necessary to find out the range of energy inputs where the VHF discharge operates in the regime of normal current density on the boundary with the abnormal regime and to remove atomic oxygen produced in the discharge by some volume or surface processes.

In this series of three papers we present results from a combined experimental and theoretical, particle-based study to quantitatively describe capacitively coupled radio-frequency discharges in oxygen. The particle-in-cell Monte Carlo model on which the theoretical description is based is described in this paper. It treats space charge fields and transport processes on an equal footing with the most important plasma–chemical reactions. For given external voltage and pressure, the model determines the electric potential within the discharge and the distribution functions for electrons, negatively charged atomic oxygen and positively charged molecular oxygen. Previously used scattering and reaction cross section data are critically assessed and in some cases modified. To validate our model, we compare the densities in the bulk of the discharge with experimental data and find good agreement, indicating that essential aspects of an oxygen discharge are captured.

Axially and temporally resolved optical emission structures were investigated in the rf sheath region of a parallel plate capacitively coupled rf discharge (13.56 MHz) in pure oxygen and tetrafluoromethane. The rf discharge was driven at total pressures of between 10 and 100 Pa, gas flow rate of 3 sccm and rf power in the range 5–100 W. In particular, the emission of the atomic oxygen at 844.6 nm (3p³P → 3s³S⁰) and the atomic carbon at 193 nm (3s¹P⁰ → 2p¹D) were imaged with a lens onto the entrance slit of a spectrometer and detected by a fast ICCD-camera. The spatio-temporally resolved analysis of the emission intensity during the rf cycle (73.75 ns) provides two significant excitation processes inside the rf sheath: the electron impact excitation at the sheath edge, and heavy particle impact excitation in front of the powered electrode. In oxygen plasma the emission of atomic oxygen was found in both regions whereas in tetrafluoromethane the emission of atomic carbon was observed only in front of the powered electrode. The experimental results reveal characteristic dependence of the emission pattern in front of the powered electrode on plasma process parameters (self-bias voltage, pressure) and allow an estimation of the excitation threshold energy and effective cross section of energetic heavy particle loss.

We present results of 1d3v particle-in-cell Monte Carlo collisions simulations of a capacitive RF discharge in oxygen. Several direct comparisons between experiment and modelling are presented. The calculated ion energy distributions show good agreement with the experimentally measured ones for different discharge parameters. A plausible explanation of a double emissive layer near the powered electrode recently discovered in experiments is suggested. Heavy particle dissociative excitation collisions seem to be responsible for the formation of a second emissive layer close to the electrode. Introducing this process into the simulation a rather good agreement of the simulated axial emission profile with the experimentally observed one can be achieved. This delivers an estimate for the cross section of this collision.

The influence of the spatial dependence of the line profile on the shape of a line with asymmetric self-reversal emitted from inhomogeneous discharge plasmas is investigated by solving the equation of radiative transfer using relative distribution functions for the source function and the line broadening including complex overlapping lines. The theoretical results show that the features of an asymmetric lineshape are related to the broadening mechanisms and enable us to infer from the recorded line the dominant broadening mechanism. These results are experimentally confirmed using self-reversed lines emitted from high-pressure lighting plasmas. Our detailed analysis suggests that the one-parameter-approximation (OPA) model is able to reproduce satisfactorily the spectral emissivity as in the case of a spatially constant profile. An improved technique for deducing the emissivity from line-reversal is developed and applied to plasma temperature determination of metal-halide lamps. The obtained results are optimized taking into account the functional dependence of the line width and shift on the position.

A large area explosive emission cold cathode has been fabricated using carbon nanotube (CNT) emitters. The emission characteristics of the CNT cathode have been measured in two different pulse modes. A high emission current density of 344 A cm⁻² has been realized at an applied electric field of 16.7 V µm⁻¹ and the current density maintained at 160 A cm⁻². The distribution and intensity of the electron beams on the cathode surface were investigated. The emission model of the cathode applied with pulse electric field was established and the pulsed emission mechanism of CNTs was explosive electron emission. Meanwhile, the formation of the cathode plasma layer was found. Such a cathode design can be expected to be utilized as a high-current, large area emitter.

This paper shows that the rf capacitive discharge in NF₃ and SiH₄ can burn in three possible modes: weak-current α-mode, strong-current γ-mode and dissociative δ-mode. This new dissociative δ-mode is characterized by a high dissociation degree of gas molecules (actually up to 100% in NF₃ and up to 70% in SiH₄), higher resistivity and a large discharge current. On increasing rf voltage first we may observe a weak-current α-mode (at low NF₃ pressure the α-mode is absent). At rather high rf voltage when a sufficiently large number of high energy electrons appear in the discharge, an intense dissociation of gas molecules via electron impact begins, and the discharge experiences a transition to the dissociative δ-mode. The dissociation products of NF₃ and SiH₄ molecules possess lower ionization potentials, and they form an easily ionized admixture to the main gas. At higher rf voltages when near-electrode sheaths are broken down, the discharge experiences a transition to the strong-current γ-mode.

Recently some correlative works have been done to investigate the effects of collisions between ion and neutral gas atoms on the characteristics of plasma sheath in an external magnetic field. In general, the momentum transferring cross section (and thus the ion collision frequency) has a power law dependence on the ion flow velocity in the depth direction. Usually in the literature, constant collision frequency and constant mean free path are treated. Here, by using a collisional fluid model, we investigate the characteristics of a magnetized plasma sheath where the ion cross section depends on the ion flow velocity. The numerical calculations show that the effects of collisions on ion characteristics are more powerful when the momentum transferring cross section is constant. However, some parameters such as the electron density distribution are independent of this dependence.

The production of ozone in a negative corona discharge fed by carbon dioxide with embedded traces of oxygen and water has been studied. The presence of traces of oxygen in both pure and dry CO₂ leads to an increase in nascent ozone concentrations. In contrast, traces of water vapour (0–800 ppm) are shown to rapidly suppress ozone concentrations with the largest decreases being observed at lowest gas pressures in the discharge (300 Torr). The presence of water vapour did not considerably affect the electrical properties of negative dc corona suggesting that a chemical process is responsible for the ozone loss. We have shown that the addition of water up to a concentration of 1500 ppm has only a marginal effect on the processes of ozone formation but the catalytic cycle of ozone destruction involving OH radicals can be the reason for observed decrease in the total ozone concentration with increasing water concentration.

Thermal ionization of alkali species emitted from thermally decomposing vegetative matter into the combustion zone of a fire makes the zone a weakly ionized gaseous medium. Collision between the medium electrons and neutral flame particles is a dominant form of particle interaction and incident microwave energy absorption process. Electromagnetic wave absorption properties of vegetation fire have implications for the safety of fire fighters during wildfire suppression where communication blackouts have been experienced. Propagation characteristics of electromagnetic waves in a vegetation fire could be deduced from its relative dielectric permittivity. In the experiment, a controlled fire burner was constructed where various dried natural vegetation could be used as fuel. The burner was equipped with thermocouples and used as a cavity for microwaves with a laboratory quality network analyser to determine effective complex permittivity from scattering parameters. A controlled vegetation fire with a maximum flame temperature of 1050 K was set in the burner and X-band microwaves (8.0–9.6 GHz) were made to propagate through the flame. For the flame, at temperatures of 800 and 1015 K, imaginary and real components of effective complex dielectric permittivity were measured to range from 0.113 to 0.119 and from 0.898 to 0.903, respectively.

Light emission in the UV and visible range generated by a planar semiconductor gas discharge system and the possibility of locally increasing the gas discharge light emission (GDLE) in a wide pressure range (2–700 Torr) has been studied. The use of gas discharge gap with microcapillary multichannels spacer (MCS) and an IR light to excite the GaAs photocathode of the system leads to an increase in the GDLE intensity. Electrons entering the multichannels of the discharge gap are multiplied in the electric field by the avalanche mechanism, so that current increases in the discharge gap with microcapillary MCS. In this system the local intensity of GDLE exceeds the intensity of uniform GDLE in the gas discharge system by the same factor as the working area of the photocathode exceeds the total active area of the MCS. Due to the very small electrode gap width we can describe the behaviour of the charged particles in the electric field of our system with the dc Townsend breakdown theory, depending on the pressure range.

A break of an avalanche and subsequent evolution of streamers are considered based on the earlier developed soliton approach to the problems of electric gas breakdown. Such an approach allows one to give a satisfactory explanation of the experiments on the spatial and temporal development of streamers. The author also gives estimates of a number of parameters introduced within the framework of the model; in particular, he estimates the avalanche and streamer masses, the avalanche capacity and bond energy.

A scanning thermal microscope (SThM) in the dc regime was used to study the thermal conductivity of homogeneous in-depth meso-porous silicon in the form of thin films on a monocrystalline silicon substrate. Measurements for different film porosities (30–80%) and thicknesses (100 nm–8 µm) were performed in order to estimate the influence of both layer porosity and thickness on the thermal conductivity values of porous silicon (PS). An analytical model predicting the SThM measurement in the case of ultra-thin monolayered samples was used to calibrate the technique, to analyse experimental data and to determine the thermal conductivity of meso-porous layers. Effective thermal conductivity of meso-PS films was found to decrease when the porosity increases. The effective thermal conductivities measured for thick porous layers (several µm) are in good accordance with those measured by micro-Raman-spectroscopy on bulk meso-PS samples. For submicrometric thicknesses (<1 µm), the effective thermal conductivity of layers decreases significantly with decreasing layer thickness due to the increased sensitivity of measurements to the thermal resistance of the film/substrate interface. An intrinsic thermal conductivity of PS was calculated independently of the film thickness and the values of interfacial thermal resistances were thus estimated. From the apparatus point of view, the results obtained show that the depth being sensed is of the order of a few micrometres for insulating materials and depends on the thermal conductivity of the films.

A systematic study of structure and thermal stability of a [(Mo/Y/) x 5] soft x-ray multilayer (ML) mirror prepared by electron beam evaporation system has been performed by means of x-ray reflectivity (XRR) and grazing incidence x-ray diffraction (GIXRD). The GIXRD patterns show that the Mo and Y layers are polycrystalline in nature. The obtained XRR patterns confirm the very good quality of the ML stack. Isochronal thermal annealing up to the temperature of 400 °C leads to a decrease in interface roughness which in turn results in sharpening of the ML interfaces. Further elevating the annealing temperature above 400 °C oxidizes the Y layers and the interface roughness increases. Thus increase in interface roughness beyond this temperature can be understood in terms of evolution of yttrium oxide, which significantly affects the ML structural properties. The modulation structure is well maintained even after annealing at 600 °C, which confirms very good thermal stability of the Mo/Y ML mirror. Our results demonstrate that low-temperature annealing (up to 400 °C) is an effective method to improve the quality of an as deposited Mo/Y soft x-ray ML mirror. Diffuse scattering measurements show very good correlation with the XRR fitting parameters.

First, GaSb epilayers were grown on (0 0 1)GaAs substrates by molecular beam epitaxy. We determined that the GaSb layers had very smooth surfaces using atomic force microscopy. Then, very short period InAs/GaSb superlattices (SLs) were grown on the GaSb buffer layer. The optical and crystalline properties of the superlattices were studied by low-temperature photoluminescence spectra and high resolution transition electron microscopy. In order to determine the interface of SLs, the samples were tested by Raman-scattering spectra at room temperature. Results indicated that the peak wavelength of SLs with clear interfaces and integrated periods is between 2.0 and 2.6 µm. The SL interface between InAs and GaSb is InSb-like.

This paper is mainly aimed at investigating the relationship of entropy flow with an atmospheric system (typhoon), based on the observational analyses covering its whole life-cycle. The formula for calculating entropy flow is derived starting with the Gibbs relation with data from the NCEP/NCAR reanalysis. The results show that: (i) entropy flow characteristics at different vertical layers of the system are heterogeneous with predominant negative entropy flow in the large portion of the troposphere and positive ones at upper levels during its development; (ii) changes in the maximum surface wind velocity or the intensity of a typhoon are synchronous with the total entropy flow around the typhoon centre and its neighbourhood, suggesting that the growth of a severe atmospheric system relies greatly upon the negative entropy flow being strong enough, and that entropy flow analysis might provide a particular point of view and a powerful tool to understand the mechanism responsible for the life-cycle of an atmospheric system and associated weather events; and (iii) the horizontal pattern of negative entropy flow near the surface might contain some significant information conducive to the track forecast of typhoons.

The third-order optical nonlinearity of a composite film consisting of periodic triangular gold nanoparticle arrays coated with ZnO was investigated by the z-scan technique. The third-order nonlinear susceptibility, χ⁽³⁾, measured at a wavelength of 532 nm with a pulse duration of 10 ns, was up to 1.3 × 10⁻⁵ esu. The figure of merit, χ³/α (where α is the absorption coefficient) was about 1.1 × 10⁻⁹ esu cm. The large optical nonlinearity is likely due to the strong local electromagnetic field near the triangular gold particles and ZnO matrix. The results show that the composite film has great potential for nonlinear optical devices.

A comparison of the effect of rapid thermal nitridation (RTN) of the Si surface in N₂O and NH₃ ambient at different temperatures (700–850 °C) on the dielectric and electrical characteristics of thin (∼20 nm) Ta₂O₅ stacks has been made. The electrical parameters of capacitors (film permittivity, oxide charge, densities of bulk traps, interface and slow states, leakage current) are discussed in terms of the impact of N incorporation in the interface region. The films on both types of RTN-treated Si exhibit ∼100 times lower leakage current than Ta₂O₅ on bare Si, but among the two RTN processes NH₃ nitridation is more beneficial since only it simultaneously increases also the stack permittivity. This improvement in parameters is suggested to be due to a real nitridation of Si surface which occurs under the NH₃ rapid thermal process. RTN in N₂O does not produce resistance to the oxidation substrate and it could explain the observed lack of stack dielectric constant improvement. The composition of the interfacial layer under NH₃ RTN appears to be TaSi-oxinitride-like, while the interface region at N₂O-nitrided Si seems to be SiO₂-like. Each RTN process, however, modifies the Si surface and constitutes a specific interface layer different from that at the bare Si substrate. The composition of this layer defines parameters of the traps close to the substrate, the barrier height at the Ta₂O₅/interface layer and influences the conduction mechanisms in the stacks.

ZrO₂ : Pr films were synthesized by the ultrasonic spray pyrolysis process. X-ray diffraction studies, as a function of the deposition temperature, indicate a tetragonal crystal structure of zirconia as the substrate temperature was increased. Luminescence (photo- and cathodoluminescence) properties of the films were studied as a function of growth parameters such as the substrate temperature and the praseodymium concentration. For an excitation wavelength of 290 nm, all the photoluminescent emission spectra show peaks located at 490, 510, 566, 615, 642, 695, 718, 740 and 833 nm, associated with the electronic transitions ³ P₀ → ³H₄, ³P₀ → ³ H₄, ³P₁ + ¹I₆ → ³H₅, ¹D₂ → ³H₄, ³ P₀ → ³H₆, ¹D₂ → ³ H₅, ¹D₂ → ³H₅, ³ P₀ → ³F_3,4 and ¹D₂ → ³ F₂ of the Pr³⁺ ion. As the deposition temperature is increased, an increasing intensity of the luminescence emission is observed. Also, quenching of the luminescence, with increasing doping concentration, is observed. The chemical composition of the films as determined by energy dispersive spectroscopy is reported as well. In addition, the surface morphology characteristics of the films, as a function of the deposition temperature, are presented.

A three-dimensional numerical model for multiple-line sintering of loose powders on top of multiple sintered layers under the irradiation of a moving Gaussian laser beam is carried out. The overlaps between vertically deposited layers and adjacent lines which strengthen bonding are taken into account. The energy equation is formulated using the temperature transforming model and solved by the finite volume method. The effects of the number of the existing sintered layers, porosity and initial temperature coupled with the optimal combination laser intensity and scanning velocity are presented. The results show that the liquid pool moves slightly towards the negative scanning direction and the shape of the liquid pool becomes shallower with higher scanning velocity. A higher laser intensity is needed to achieve the required overlaps when the number of the existing sintered layers increases. Increasing porosity or initial temperature enhances the sintering process and thus less intensity is needed for the overlap requirement.

FePt/B₄C multilayer composite films with a Fe/Pt two-layer structure and a Fe/Pt/Fe sandwich structure were prepared by magnetron sputtering and subsequent annealing in vacuum. When the B₄C interlayer thickness of the FePt/B₄C films is bigger than 3 nm, a stable value of grain size, coercivity and hardness is observed. The characterizations of the microstructure demonstrate that the diffusion of B and C atoms in the FePt layer is so finite that the multilayer structure of the FePt/B₄C films is maintained after annealing at 500 °C for 30 min. The finite diffusion of B and C atoms is responsible for the FePt grain growth confinement, grain and layer separation and eventually induces the stable value mentioned above. The single composite films may find application in substituting the traditional three-layer structure commonly used in the present data storage technology.

Laser shock processing is involved with high amplitude pressure wave propagation into the specimen. The magnitude and duration of the pressure generated across the evaporating surface during the laser rapid evaporation defines the depth of the deformed region. In the present study, laser shock processing of aluminium is considered and the material response to the laser-induced pressure wave is modelled. The recoil pressure generated at the irradiated surface is formulated while the stress field in the specimen is predicted using finite element analysis. An experiment is carried out to examine the metallurgical changes in the plastically deformed region using electron scanning microscopy (SEM). It is found that the prediction of the depth of the deformed region agrees well with the experimental results. The deformed region is free from cracks, which is observed from the SEM micrographs.

Al-containing hydrogenated amorphous carbon (Al–C : H) films were deposited on silicon substrates using a mid frequency magnetron sputtering Al target in an argon and methane gas mixture. The composition, surface morphology, hardness and friction coefficient of the films were characterized using x-ray photoelectron spectroscopy, atomic force microscopy, nanoindentation and tribological tester. The Al–C : H films deposited at low CH₄ content show high surface roughness, low hardness and high friction coefficient, similar to metallic Al films; in contrast, the Al–C : H films prepared under high CH₄ content indicate low surface roughness, high hardness and low friction coefficient, close to that of hard a-C : H films as wear-resistance films.

A novel type of lithium bis(oxalato)borate (LiBOB) synthesized by the solid-state reaction method has been presented. LiBOB composite polymer electrolytes (CPE) prepared with dispersions of TIO₂/ZrO₂with various concentrations into the host blend matrices of poly(vinylidenefluoride) (PVdF)-poly(vinylchloride) (PVC) are investigated by scanning electron microscopy, x-ray diffraction (XRD) and ac impedance measurements. The plasticizing agent selected for the present study is a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) for the phase separated morphology of the studied polymers. The impedance studies on CPE membranes identify that membranes, with 2.5 wt% of fillers, have enhanced conductivities of 5.43 × 10⁻⁴ S cm⁻¹ and 4.38 × 10⁻⁴ S cm⁻¹, respectively, for TiO₂ and ZrO₂ at ambient temperature. The XRD investigations confirm that the membranes with filler levels exceeding the limit of 2.5 wt% show a gradual increase in the degree of crystallinity, rendering them less conducting. The activation energy calculations also highlight variations in conductivities of all the membranes.

In this paper, we present a new numerical formulation that we have applied to the model for bipolar charge transport, trapping and recombination in polymeric insulators, such as low density polyethylene. The numerical model is based on the high precision Runge–Kutta method for determining mobile and trapped charge local density within the sample, during the application of a dc voltage. In addition, we have applied the finite element method considered as the most general one, which can be applied to any sample shape and boundary conditions. It is applied to resolve Poisson's equation, thus providing the potential and its gradient within the sample and at the dielectric–electrode interfaces. In a first step, the new formulation is validated for the charge dynamic under low dc applied voltage. In a second step, model outputs are analyzed under high dc applied voltage. The appearance of charge packets is revealed for the first time in modelling works although they have long been reported in experimental studies on polyethylene materials. The conditions for the appearance of such packets are discussed.

This paper describes both an experimental study and a numerical investigation of the transverse mechanical behaviour of unidirectional polypropylene/glass fibre composites. A deterministic numerical simulation of the mechanical damage that accounts for the organization within real composites is proposed. This numerical method is based on classical finite element analysis, with tailored phase arrangement that unequivocally accounts for the real samples. This technique permitted some light to be shed on the origin of the transverse brittleness of these composites. It is shown that the number of damaged zones as a function of the applied strain follows a Weibull distribution. The influence of the volume fraction of filler was also estimated, both through experimental and numerical ways. It can be concluded that the reduction of the average distance between fibres increases the local stress enhancement and favours the degradation mechanisms of composites, especially at the glass fibres/polymer interface. The paper also proposes a novel analytical relationship linking the number of damaged zones to the macroscopic transverse mechanical behaviour of commingled composites.

Lead-free ceramics 0.95K_0.5Na_0.5(Nb_0.94Sb_0.06)O₃–0.05 LiTaO₃ + xmol%BaO (0 ≤ x ≤ 3.00) have been fabricated by an ordinary sintering technique. The results of x-ray diffraction show that the ceramic with a perovskite structure changes from tetragonal phase to pseudocubic phase by increasing the doping level of Ba, and the ceramics with 1.00 < x < 1.50 contain both the tetragonal and the pseudocubic phases at room temperature. After the doping with Ba, the grain growth is inhibited, the ferroelectric–paraelectric phase transition temperature (T_c) is decreased, and a diffuse phase transition is induced. The ferroelectric properties (P_r and E_c) show a slight decrease with increasing x up to 1.25, and then become weakened significantly. Because of the coexistence of the tetragonal and pseudocubic phases, the piezoelectric and dielectric properties of the ceramics with 1.00 < x < 1.50 are enhanced significantly. The ceramic with x = 1.25 exhibits the optimum properties: d₃₃ = 261 pC N⁻¹, k_P = 0.52, k_t = 0.46, ε_r = 1503, tan δ = 2.5% and T_c = 314 °C. Our results have also demonstrated that the ceramics are non-deliquescent.

Volatilization of Sb during the fabrication of CoSb₃ by mechanical alloying and then spark plasma sintering has been successfully compensated by adding excess Sb. The average grain size increases apparently with excess Sb content, abnormally grown by about 100 times as the excess Sb is up to 4 at.%. A liquid-phase-related mechanism is used to explain the abnormal growth. The uncompensated sample shows a negative Seebeck coefficient near room temperature, while the sample compensated with 6 at.% excess Sb shows an intrinsic positive Seebeck coefficient and an enhanced ZT value, which has a maximum of about 0.1 at 350 °C, which is two times higher than the uncompensated one. The transition of electrical conductivity from n- to p-type relative to the Sb compensation is discussed in relation to the point defect. A defect equation is given to show the nature of electron generation due to Sb deficiency. The Sb-vacancy not only provides extrinsic carrier but also generates a significant impact on the band gap and hence on the Seebeck coefficient.

We report a systematic first-principles study of structural properties, electronic structures and magnetic properties of Cr doped ZnTe with the Cr concentration up to 0.5. We fully optimize all the structures concerned, both ferromagnetic and antiferromagnetic, and their internal atomic positions. Our calculations show that they are half-metallic ferromagnets with half-metallic gaps over 0.6 eV. Our calculated results are consistent with the fact that only those bulk samples with very small doping concentrations can be stable thanks to the entropy effect. On the other hand, up to 20% Cr doping concentrations have been achieved experimentally in thin films fabricated with nonequilibrium methods. Our calculated results indicate that half-metallic ferromagnetism can be realized in high-quality thin film samples of Cr_xZn_1−xTe with nonequilibrium methods. We explain the half-metallic ferromagnetism in terms of a pd hybridization and resulting large exchange splitting of d-dominated bands. Compatible with appropriate semiconductors, these materials, at least some of them, could be useful in spintronics.

The dielectric properties and ageing behaviour of (1-y)(Ba_0.6Sr_0.4Nb₂O₆)–y(Ba_0.6Sr_0.4TiO₃) ceramics have been studied. A single phase compound is obtained for y ≤ 0.3 as judged from the x-ray diffraction spectra. With the addition of BST, the maximum dielectric constant is increased and the dielectric loss is reduced. The dielectric curves become broader. The abnormal grain growth that is usually observed in Ba_0.6Sr_0.4Nb₂O₆ has also been suppressed by the addition of B_0.6S_0.4TiO₃. The Curie temperature decreases with increasing amounts of BST added and all samples obey the Curie–Weiss law above the Curie temperature. The ageing behaviour of the ceramics is studied and an oxygen vacancy inducing the dielectric ageing mechanism is used to explain the observed results.

Conditions defining melting fluence thresholds for silicon under pulsed laser irradiation are introduced in this paper. The melting conditions include both thermal and non-thermal contributions. Furthermore, a definition of the non-thermal melting threshold is given as the laser fluence above which the non-thermal contribution dominates over the thermal one and a short-lived non-thermal liquid phase appears in the femtosecond time scale. Numerical values of thresholds obtained through a two-temperature model are in good agreement with experimental data. The dependence of fluence thresholds on both wavelength and pulse width is explored and discussed in detail.

Microwave heating of ceramic materials in a cylindrical wave field is analyzed by numerical simulation. The issues of temperature uniformity and stability inside the sample undergoing microwave heating are addressed. The heating regimes leading to development of global and localized thermal instabilities are illustrated. The role of the temperature dependence of the material's microwave dielectric properties in the formation of instabilities is demonstrated. The tuning and matching of a cylindrical cavity containing the sample is analyzed within a simple model for the microwave and millimetre-wave frequency ranges.

Composites of lead zirconate titanate ceramic particles dispersed in non-ferroelectric polymers of different electrical conductivities (polyurethane and polyethylene) have been prepared and poled under ac fields of different amplitudes E_a and frequencies f. Our experimental results reveal that the poling efficiency of the composites (as indicated by the observed remanent polarization P_CR) is strongly dependent on the frequency of the poling field and the electrical conductivity of the polymer matrix. A low-frequency ac field and a high-conductivity polymer matrix would make the poling become more effective. The phenomena can be understood by the simple theoretical model, in which space charges are allowed to accumulate at the particle–matrix interfaces. The increasing trend of P_CR with decreasing f predicted by the model agrees well with the experimental results. It is suggested that the poling process is mainly governed by a crucial parameter 'relaxation time' τ of the interfacial charge. If the poling time is longer than τ, the interfacial charge can accumulate at the interfaces to stabilize the aligned dipoles in the ceramic phase, and hence the composite becomes poled. Otherwise, the composite cannot be effectively poled even under a high electric field.

Several studies indicate that experimental determination of porous medium permeability by a gas flow method may be influenced by gas properties and ambient temperature. In order to explain these influences, a very simple model is developed based on the idea that the flow in the porous medium may results from both pressure-driven and free molecular flows. Measurements of the permeability of sand and a woven fibrous material performed with air and helium flows seem to corroborate this view. Based on the approach developed, we also study the phenomenon known as slip, and a methodology to quantify the slip length is presented.

Spindle-like ZnO doped with 2–5 at% cobalt powders were prepared by a low temperature aqueous solution method. The structural and magnetic properties of the products were investigated. The obtained products were identified to be of hexagonal wurtzite structure without any impurity phase. Also, the Co was incorporated in the ZnO lattice as Co²⁺ and substituted for the Zn site with no evidence of metallic Co. Ferromagnetic behaviour was clearly observed at room temperature for the Co-doped samples. Photoluminescence intensity due to the vacancies varies with the Co concentration, and the ferromagnetism moment increases with the increment of oxygen vacancies.

The complex dielectric properties of CoTiO₃ ceramics were investigated as functions of temperature (130 K ≤ T ≤ 430 K) and frequency (100 Hz ≤ f ≤ 100 kHz). Two thermally activated dielectric relaxations were observed. TiO₂ addition and annealing treatments were used to modify the behaviour of these relaxations. It was found that the low-temperature relaxation can be greatly enhanced by the addition of TiO₂. Annealing in N₂ at 800 °C for 2 h eliminates the low-temperature relaxation but largely strengthens the high-temperature relaxation, whereas annealing in O₂ at 800 °C for 2 h causes the opposite effects. It was suggested that the low-temperature relaxation can be related to the dipolar effect induced by charge-carrier hopping motions while the high-temperature relaxation might be ascribed to the defect dipolar polarization.

Pb_1.17Te thermoelectric polycrystalline materials were fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). The property measurement and microstructural characterization showed that the present material has special features different from traditional Pb_1+xTe ingots with secondary Pb phase. An attractive enhancement of the thermoelectric figure of merit ZT = 0.64 was obtained at 450 K, with a low thermal conductivity of 1.11 W m⁻¹ K⁻¹ at this temperature. Transmission electron microscopy observation showed the existence of randomly dispersed nano features that are responsible for such enhancement, some of which are similar to the nanostructures reported in the AgPb_mSbTe_m+2 system. The origin of these regions is discussed and their influence on thermal conductivity is revealed. The results confirm the effectiveness of such a kind of nano feature in improving thermoelectric properties, especially in reducing thermal conductivity. They also indicate a new way of obtaining thermoelectric materials with such a kind of nano feature via MA and SPS.

In this paper, sensitized ZnS with visible light driven photocatlytic ability was successfully prepared. The obtained ZnS was characterized by x-ray diffraction, UV–visible diffuse reflectance spectra and Fourier transform infrared spectra. The photocatalytic property of the prepared ZnS was evaluated by decomposing methyl orange (MO). These sensitized ZnS powders with a proper molar ratio showed higher photocatalytic activity than TiO₂(P25) under visible light (λ > 420 nm) irradiation. A possible explanation for the visible light activity of the prepared ZnS was proposed.

Our new type of on-chip micro-pump exploits the ac electro-kinetic forces acting in the volume of a fluid in the presence of a temperature gradient. No mechanically movable parts are used. The velocity of the pump flow observed depends on the frequency and strength of the driving ac field and on the temperature gradient across the pump channel. An integrated heating element allows the temperature gradient to be adjusted. Both ac field electrodes and heating element are platinum structures processed on a glass chip. The pump-channel walls and cover are made from polymer and thin-glass, respectively. In this paper, we present measurements of the fluid velocity as functions of the medium conductivity (0.1–1.3 S m⁻¹) and field frequency (300 kHz–52 MHz), voltage across the field-electrode voltage (0–35 V_rms) and the heating element (1.1–3.6 V). Velocities of up to 120 µm s⁻¹ were observed in the pump channel. The advantage of our new design is an evenly shaped cross-section of the pump channel, which reduces the risk of the channel becoming clogged by debris. Ac-electro-osmosis is not a predominant effect in our structures. Pumping could only be observed when the heating current and ac-pump field were applied simultaneously. The effects observed were simulated with the COMSOL Multiphysics program.