Table of contents

Extreme ultraviolet interference lithography (EUV-IL) is a newly developed technique for the production of periodic nano-structures with resolution below 20 nm. The technique is based on coherent radiation that is obtained from undulators at synchrotron radiation laboratories. The high resolution is afforded by small wavelength and practical absence of the proximity effect at this energy. The throughput of this parallel exposing method is much higher than that of the serial electron-beam lithography. Interference schemes based on both reflection (mirrors) and diffraction (gratings) optics have been realized. Both one-dimensional and two-dimensional patterns such as arrays of dots have been achieved. Achromatic interference schemes have been developed to make efficient use of the beam power available from the wideband sources in the extreme ultraviolet region. EUV-IL is used in a growing number of applications; examples include fabrication of self-assembly templates, magnetic nanodot arrays and nano-optical components.

Magnetic materials exhibit strikingly different performances at different length scales, especially when their sizes reach nanometer scale, such as ultra-thin films, at which their magnetic properties vary dramatically with the change in material length scale. In order to demonstrate such peculiar behaviour, a numerical simulation was carried out using a carefully devised model, in which the Landau–Lifshitz–Gilbert equation governs the evolution of magnetization. The simulation results clearly showed that there was a critical length at which the coercivity reached a maximum value. In addition, when the length scale was sufficiently small, for example, when it was comparable to or smaller than the exchange length, the phenomenon of coercivity almost vanished and the material was in the so-called superparamagnetic state. The effect of an external stress field on magnetic domain pattern was also taken into account in the present study. The former can affect the latter due to the coupling of the magnetization and elastic fields.

Magnetic properties of nano-crystalline soft magnetic alloys have usually been correlated to structural evolution with heat treatment. However, literature reports pertaining to the study of nano-crystalline thin films are less abundant. Thin films of Fe₄₀Ni₃₈B₁₈Mo₄ were deposited on glass substrates under a high vacuum of ≈ 10⁻⁶ Torr by employing resistive heating. They were annealed at various temperatures ranging from 373 to 773 K based on differential scanning calorimetric studies carried out on the ribbons. The magnetic characteristics were investigated using vibrating sample magnetometry. Morphological characterizations were carried out using atomic force microscopy (AFM), and magnetic force microscopy (MFM) imaging is used to study the domain characteristics. The variation of magnetic properties with thermal annealing is also investigated. From AFM and MFM images it can be inferred that the crystallization temperature of the as-prepared films are lower than their bulk counterparts. Also there is a progressive evolution of coercivity up to 573 K, which is an indication of the lowering of nano-crystallization temperature in thin films. The variation of coercivity with the structural evolution of the thin films with annealing is discussed and a plausible explanation is provided using the modified random anisotropy model.

Thermal electrical resistivity, magnetic hysteresis and magneto-impedance behaviour of melt spun and annealed Co_71−XFe_XCr₇Si₈B₁₄ (X = 0, 2, 3.2, 4, 6, 8 and 12 at.%) were investigated. The addition of Fe in the system changed crystallization as well as the magnetic properties of the materials. The alloy containing 6 at.% Fe showed an increase in resistivity during the first crystallization process. A TEM micrograph indicated the formation of nanostructure during the crystallization process. The GMI properties of the alloys are evaluated at a driving current amplitude of 5 mA and a frequency of 4 MHz. The two-peak behaviour in the GMI profile was observed for all the samples. It is found that the alloy with 4 at.% Fe has the maximum GMI ratio because of the nearly zero magnetostriction value of the sample. About 62% change in the GMI ratio was observed in the alloy with 4 at.% Fe when annealed at 673 K. The anisotropy field was also minimum for the annealed alloy. The results were explained by the formation of directional ordering and the reduction of the magnetostriction constant of the alloy due to nanocrystallization during the annealing process.

The solid–solid interaction between Fe₂O₃ and CdO-yielding CdFe₂O₄ has been monitored using an XRD and a vibrating sample magnetometer. The effect of sintering temperature and dopant concentration on the bulk density of CdO/Fe₂O₃ was investigated using the usual ceramic method. The results showed that the solid–solid interaction between the CdO and Fe₂O₃ took place at a temperature ⩾ 600 °C yielding CdFe₂O₄ of moderate degree of crystallinity. This reaction was much stimulated in presence of ZnO and also by increasing the calcination temperature within 700–900 °C. On the basis of magnetic measurements the possibility of formation of CdFe₂O₄ was evidenced at a lower temperature (500 °C). These measurements also showed the stimulation effect of ZnO towards ferrite formation. The activation energy of the formation of CdFe₂O₄ was calculated from XRD measurements and found to be 102, 75, 66, 60 and 53 kJ mol⁻¹ for pure samples and those doped with 0.75, 1.5, 3 and 6 mol/% ZnO, respectively. The addition of ZnO (0–1.5 mol%) increased the bulk density of the investigated system which suffered a significant decrease upon increasing the dopant concentration above 1.5 mol% ZnO due to the formation of cadmium ferrite even for solids calcined at 500 °C.

The dc magnetic field and frequency dependences of the transmission coefficient of the four-pole network containing a core made of La_0.6Pb_0.4MnO₃ manganite were studied. It is shown that the frequency dispersion of the magnetic permeability of the core material is large at frequencies of about 100 kHz. The factors which influence the value of the transmission coefficient are analysed. Large low-frequency variations of parameters of the four-pole network containing the magnetic circuit made of manganite can be promising for possible device applications.

Investigation of BiFeO₃ is being conducted intensively. However, preparing perfect samples with high resistance is still a prerequisite for the clarification of the properties of BiFeO₃. Here we report electrical and magnetic properties and phase transitions of BiFeO₃ ceramic synthesized by a rapid sintering processing. The dependence of the sintering products on the sintering temperatures revealed that single-phased BiFeO₃ ceramic was synthesized at 880 °C using this rapid sintering technique. The as-prepared BiFeO₃ sample shows a high resistivity of 6.7 × 10¹⁰Ω cm at an external electric field of 100 kVcm⁻¹. Vibrating sample magnetometer (VSM) measurement verified that the high resistance BiFeO₃ ceramic presented an antiferromagnetic characterization.

The thermally activated magnetization reversals in magnetic nanodevices have been theoretically investigated. This kind of thermal fluctuations is found to be the dominant cause of intrinsic write error in magnetoresistive random access memory elements, on which our study is aimed. Calculations are performed in the Stoner–Wohlfarth model assuming a single-domain free layer magnetization. The thermal fluctuations as a form of random telegraph noise in output voltage level are systematically characterized in both the time and frequency domains. The relationship of the thermal fluctuations with the free layer magnetic property, temperature and external magnetic field has been established.

A systematic investigation of heavy charged particle-to-gamma thermoluminescent (TL) efficiencies has been performed for the total signal and peak 5 of LiF : Mg,Ti (TLD-100) dosemeters. Experimental and theoretical efficiencies are presented for protons as well as helium, carbon, nitrogen and oxygen ions. Proton, helium and carbon irradiations were performed at incident energies between 0.7 and 11 MeV. For nitrogen and oxygen, two energies, corresponding to 'mirror' values below and above the Bragg peak energy, were used to measure TL efficiencies for the same linear energy transfer (LET) entrance value. The energies chosen were 4.83 and 9.95 MeV for nitrogen ions and 6.02 and 12.95 MeV for oxygen ions. Distinct curves are found for each ion species. Data for energies higher than the Bragg peak energy follow the well-known tendency, efficiency values decrease with increasing LET. Efficiencies measured below the Bragg peak display the opposite effect, efficiency increases with increasing LET. Presenting results as a function of incident energy shows a monotonic and single valued behaviour, efficiency decreases with decreasing energy. The measured efficiencies of the higher-energy data (above the Bragg peak) were found to be 1.45 times greater than their low-energy counterparts. Modified track structure theory efficiency calculations were performed for all the ions investigated using radial dose distributions obtained via Monte Carlo simulations in solid-state LiF and 8.1 keV x-rays as test radiation. The theoretical values show agreement with data within 40% for both peak 5 and the total signal. The calculations predict the observed behaviour, higher efficiency for higher ion energy.

Key characteristics of a newly prepared tissue-equivalent, highly sensitive thermoluminescence dosimeter, Li₂B₄O₇:Cu,Ag,P, are presented. The material was developed at the Institute of Nuclear Sciences, Belgrade, in the form of sintered pellets. A new preparation procedure has greatly increased the sensitivity of the basic copper activated lithium borate and the glow curve of Li₂B₄O₇ : Cu,Ag,P consists of a well-defined main dosimetric peak situated at about 460–465 K with a sensitivity which is about four to five times higher than that of LiF : Mg,Ti (TLD-100). The exceptionally good response features of Li₂B₄O₇ : Cu,Ag,P are attributed to the incorporation of Cu as a dopant. Both low and high temperature emission spectra are presented and the origins of the various emission bands are considered. Additional data are provided from electron paramagnetic resonance measurements.

The effect of the concentration of 4-(dicyanomethylene)-2-t-butyl-6- (1, 1, 7, 7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as dopant in tris(8-hydroxyquinoline) aluminium (Alq₃) on the charge carrier recombination was studied by transient electroluminescence (EL). The electron-hole recombination coefficient (γ) was determined from the long-time component of the temporal decay of the EL intensity after a rectangular voltage pulse was turned off. It was found that the coefficient monotonically decreased with an increase in the DCJTB-doping concentration. The monotonic decrease is attributed to concentration quenching on the excitons and coincided well with the reduction of the EL efficiency.

By blending monochlorocyclohexyl- polyhedral oligomeric silsesquioxanes (MCC-POSS) into poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1.4-phenylenevinylene (MEH-PPV), enhanced luminous efficiency (LE, cd A⁻¹) and brightness were observed in polymer light-emitting diodes (PLEDs) using Al as the cathode. PLEDs made from MEH-PPV with 0.5 wt.% of MCC-POSS exhibit LE of 1 cd A⁻¹, four times higher than that observed in simple MEH-PPV devices. x-ray diffraction studies, measurements of photovoltaic response and measurements of photoconductivity from the films of pure MEH-PPV and MEH-PPV with MCC-POSS demonstrated that the improved device performance results from mixing of MCC-POSS with MEH-PPV. The enhanced electron-injection into the emissive layer results from the effect of the ionic conductivity introduced by the addition of MCC-POSS.

B-Sb films were synthesized from B/Sb/.../B multilayer films deposited by electron gun evaporation onto a fused silica substrate and subjecting the above multilayer to rapid thermal annealing at 773 K for 3 min. The films thus prepared were characterized by x-ray diffraction (XRD), transmission electron micrographs (TEM), photoelectron spectroscopy (XPS) and optical studies. XPS studies indicated the ratio of B : Sb ∼ 1 : 1. XRD and electron diffraction patterns indicated the reflections from (100), (111), (102) and (112) planes of zinc blende BSb. The band gap evaluated from optical studies (∼0.51 eV) was found to be an indirect one and tallied well with that obtained form ab initio calculation for BSb near X (Γ_15 V → Δ_min) ∼ 0.527 eV. The refractive index of the films varied between 1.65 and 2.18 with increasing energy of the incident photon and the plasma frequency (ω_p) was estimated to be ∼2.378 × 10¹⁴ s⁻¹. The effective mass was computed to be ∼0.085 m_e.

The ac and dc conductivities (σ_ac and σ_dc) of amorphous semiconductor Sm doped Se (namely, SeSm_0.005) films, prepared by thermal evaporation, were measured under vacuum in a wide range of frequency and temperature. The ac conductivity versus frequency plots were analysed by considering a power law: σ_ac ∝ ω^s (s ⩽ 1). A comparison between values of the index s with those numerically calculated from different conduction models reveals that correlated barrier hopping (CBH) is a fairly good model to describe the dominant ac conduction mechanism. The concept of the Meyer–Neldel (MN) rule in the expression of the relaxation time is considered for both ac and dc experimental data. The validity of the CBH model based on the MN (normal and inverted) rule is studied and discussed. Besides, results of the real dielectric constant (ε'), loss factor (ε') and loss tangent (tan δ) together with the Cole–Cole diagrams and the optical (ε_∞) and static (ε_s) dielectric constants for a-SeSm_0.005 films are given and discussed.

The phononic band structures of three-dimensional solid phononic crystals consisting of three types of lattices (face centred cubic, body centred cubic and simple cubic) with four different shapes (spherical, cubic, rhombic dodecahedral and truncated octahedral) of scatterers are studied numerically. The results show that reorienting of non-spherical scatterers can change the size of the band gap. For all kinds of shaped scatterers, the face centred cubic lattice is the optimal array structure. For each lattice the shape of scatterers that can generate the largest gaps is also presented.

Carbon nanoparticle-polyimide (BTDA-ODA) composite thin films were shown to have semiconducting behaviour with a pronounced negative temperature coefficient of resistance (NTC) over a temperature range 300–425 K, unlike conventional polymer-based composite films that show predominantly positive temperature coefficients of resistance. Electrical resistivity of the nanocomposite thin films decreased progressively by six orders of magnitude as the carbon volume fraction was increased from 1% to 10% and showed a characteristic composite behaviour in the smearing region prior to the percolation threshold. The dominant conduction mechanism was shown to be fluctuation induced tunnelling in the Coulomb gap with an optimum hop energy (W_opt) and a hop distance (R_opt) of about 23.1 ± 0.05 meV and 15.8 ± 0.04 nm, respectively for a 1 vol% nanocomposite, consistent with the T^−1/2 behaviour in the model of Adkins (1989 J. Phys.: Condens. Matter.1 1253) for cermets. The NTC parameters, characteristic of thermistor performance of these thin films, indicated that polymer based nanocomposites can potentially be considered for NTC thermistor applications. The materials presently studied, when compared with commercially available ceramic thermistors, were shown to exhibit smaller thermal sensitivity parameters, which originate from their smaller activation energy for charge transport. Polymer based semiconductive composites show promise for use in flexible and light-weight electronics, particularly since they can be easily mass manufactured and imprinted on microelectronic substrates.

Praseodymium doped zirconia powder (ZrO₂: (0.53 at%) Pr³⁺) was prepared by a co-precipitation technique and annealed in air at a temperature T_a = 950 °C. The x-ray diffraction pattern shows a nanocrystalline structure composed of 29.6% monoclinic and 70.4% cubic-tetragonal phases. Medium infrared and Raman analysis confirms the monoclinic/cubic-tetragonal crystalline structure and proves the absence of praseodymium aggregates in the material. Photoluminescent spectroscopy over excitations of 457.9 and 514.9 nm (at 20 K), shows two emission spectra composed of many narrow peaks in the visible–near infrared region (VIS-NIR) of the electromagnetic spectrum, associated with 4f inter-level electronic transitions in praseodymium ions incorporated in the zirconia. Excitation and emission spectra show the different mechanisms of the direct and non-direct excitation of the dopant ion (Pr³⁺), and the preferential relaxation of the material by charge transfer from the host (zirconia) to the 4f5d band and the 4f inter-level of the dopant ion (Pr³⁺). No evidence of energy transfer from the host to the dopant was observed.

A study of wavelength and bandwidth fluctuations of a rhodamine 6G dye laser, transversely pumped by high repetition rate copper vapour laser, using a near 360° curved and a straight duct dye cell, is presented. The axial modes, which are indistinguishable in the straight duct cell, are clearly distinguishable in the curved duct cell dye laser. The curved duct dye cell has smaller fluctuations in wavelength and bandwidth as compared with the straight dye cell.

By using Nd:YVO₄ as the gain medium and the Raman medium simultaneously, the actively Q-switched operation of the self-Raman Nd:YVO₄ laser at 1176 nm was realized. The output characteristics including the average power, pulse energy and pulse width versus the incident pump power and pulse repetition rate were investigated. At a pulse repetition rate of 20 kHz an average power up to 0.57 W was obtained with the incident pump power of 10.2 W, corresponding to a conversion efficiency of 5.6% with respect to the diode laser input power. Meanwhile, an analysis of the self-Raman Nd:YVO₄ laser was carried out by using the rate equations. The obtained theoretical results were in agreement with the experimental results on the whole.

Optical parameters of NaY(WO₄)₂ crystals were calculated from absorption spectra with Judd–Ofelt theory. Intensive infrared-to-visible frequency upconversion has been discovered and investigated under a 974 nm CW laser diode excitation at room temperature. The bright blue upconversion fluorescence emission is analysed in detail. The optimal concentration of Tm³⁺, which is about 1 at% for a Yb³⁺ concentration of 10 at% for the blue emission in the crystals, is much higher than that in other hosts. The experimental results and theoretical analysis suggest that NaY(WO₄)₂ with high phonon energy is most probably an even more promising crystal than fluoride.

By vertical sedimentation, silica micro-spheres were grown in different shapes of concave micro-zones which were etched on a (100) p-silicon substrate. The following were found: this method can effectively raise the quality of films by avoiding cracks; the geometry of the micro-zones affects the sediment of the film; regular hexagons and triangles best facilitate the growth of photonic crystals. This method is practical for its ability to fabricate self-assembly photonic crystals in previously designed small areas.

Regular ZnO tetrapods with different morphologies have been obtained on Si(100) substrate via the chemical vapour deposition approach. Varying the growth temperature and gas rate, we have obtained different structured ZnO materials: tetrapods with a large hexagonal crown, a flat top and a small hexagonal crown. The results suggest that these tetrapods are all single crystals with a wurtzite structure that grow along the (0001) direction. However, photoluminescence spectra shows that their optical properties are quite different: for those with large hexagonal crown, the green emission overwhelms that of the near band-edge (NBE) ultraviolet (UV) peak, while others have only a strong NBE UV peak at ∼386 nm.

Plasma nitridation of high speed M2 steel is studied using a microwave assisted electron cyclotron resonance plasma source. A mixture of hydrogen and nitrogen with a proportion of 7 : 3 was used as the nitriding gas. The density of plasma for this forming gas was measured to be ∼10¹⁸ m⁻³ as estimated by a double probe method. The M2 steel was nitrided at various temperatures of 400, 450, 500 and 550 °C under similar reactor conditions. The nitrided layer was characterized by using various analytical techniques which included structural, morphological, chemical and microhardness measurement. The maximum thickness of the nitrided layer was found to be 48 µm for samples nitrided at 550 °C. No layer corresponding to white/compound layer is seen in our analysis. The hardness is seen to increase from ∼580 HV to ∼1190 HV for samples nitrided at 550 °C. Electron cyclotron resonance plasma nitridation is observed to be much faster and a cleaner process as compared with the conventional nitriding process.

In circuit breakers, high temperature arcing may lead to material erosion at the contacts. In this paper, numerical investigations have been performed in order to study the arc behaviours under the influence of copper vapours contamination in a simple Laval nozzle. The arc is assumed to be in local thermodynamic equilibrium. The erosion rate is estimated by considering the energy balance processes at the contact surface. Computations have been performed by a commercial computational fluids dynamics package (PHOENICS). The effects of contact polarity have also been investigated. It has been found that the presence of copper vapours cools down the arc temperature at the region close to the contacts. However, at current zero, the copper vapours concentration is very low. Post arc simulation has been performed in order to determine the critical rate of rise of recovery voltage (RRRV). Good agreement has been achieved with the experimental measurement of RRRV. It has been predicted that the contact erosion has a negligible effect on the interruption capability of the breaker.

Numerical investigation of steady-state interaction of a high-pressure argon plasma with a cylindrical tungsten cathode is reported. A whole 'zoo' of very diverse modes of current transfer is revealed. Detailed results are given for the first five (three-dimensional) 3D spot modes, four of them branching off from the diffuse mode and one from the first axially symmetric spot mode. Divergences in the general pattern of solutions, which have been present in preceding works, are resolved. Hypotheses on stability of steady-state solutions, available in the literature, are analysed. It is found that these hypotheses provide an explanation of the fact that the transition between diffuse and spot modes is difficult to reproduce in the experiment but they do not explain the indication that it is the low-voltage branch of the first 3D spot mode that seems to occur in the experiment. Thus, the question of stability of steady-state solutions remains open: an accurate stability analysis, as well as additional experimental information is required.

A model is presented to explain the mechanism of striation formation in ac plasma display panel discharges. A simple Monte-Carlo simulation code was used to follow electron motion in such discharges. The obtained features of striations, such as a striation pitch, striated accumulation of charged particles on a dielectric surface above an anode and phase oppositions of electron density and average energy are consistent with previous experiments and numerical simulations. Comparison of the experimental and theoretical space pitch of the striations allows an indirect determination of the electric field.

In this paper a point-to-plane, low-pressure discharge in nitrogen is numerically studied. A classical 1.5D discharge model is used to describe the charged species evolution. This model has already permitted the description the behaviour of the main discharge parameters, as for instance the electron current density, which influences the production of active species. The evolution of neutral species is now obtained by solving the kinetic equations at each grid point.

After summarizing some of the main discharge aspects, an insight is given into the kinetics of the vibrationally resolved electronic states of molecular nitrogen. The latter play a central role in most surface treatment applications of such discharges. The dominant gain and loss mechanisms for the excited states' densities are identified, emphasizing on the metastable state , whose importance in surface treatment applications has already been shown by previous studies.

The spatio-temporal description of the N₂( ) and N₂( ) vibrationally excited states shows that when the glow regime is established, the former attains for v = 1 a density of 10¹⁴ cm⁻³, whereas the latter attains for v = 8 the value 10¹¹ cm⁻³, which are their maximum values. Both can efficiently supply their potential energy to inflict surface modification of materials. Vibrational distribution functions (VDFs) are given for the electronic states during the successive discharge phases. The VDFs are compared with Boltzmann distributions and deviations from the Boltzmann regime are discussed.

Predictions of the model concerning the luminous activity of such a discharge are presented and compared with experimental measurements yielding satisfactory qualitative agreement.

The BH A ¹Π → X ¹Σ⁺ (0–0) transition was studied, in the emission mode, for gas temperature determination. Two typical conditions were used: doped diamond deposition conditions (H₂/CH₄/B₂H₆ 99%:1%:0–100 ppm, from 2500 to 7550 Pa) and the vessel cleaning condition (H₂, 85 Pa). Among the P, Q and R branches of the A ¹Π → X ¹Σ⁺ (0–0) transition, only the unresolved Q branch is clearly observed in emission mode under the doped diamond deposition conditions, whereas the full rotational structure is observed under the vessel cleaning condition. We developed a simple simulation model based on calculation of the position of the rotational lines and the line intensities as a function of the temperature and profile width. After validation of the model using the R resolved structure from our data and from other authors' data, we applied this attractive method to a quick temperature determination in the unresolved Q branch simulation. The calculated temperatures were compared to other well established results obtained under the same conditions. Kinetics considerations led to the conclusion that the rotational temperature of the A excited states accounts for the gas temperature in both the above-mentioned conditions. It is also shown that two distinct rotational populations are present, suggesting the existence of two excitation mechanisms.

Anodes for high intensity discharge lamps made of cylindrical tungsten rods and the plasma in front of them are investigated in a special lamp filled with argon and other noble gases at pressures of 0.1–1 MPa. The arc attachment on these anodes takes place in a constricted mode. The temperature is measured pyrometrically along the electrode axis and the anode fall electrically. The electron temperature, T_e, and the electron density, n_e, within the anodic boundary layer are determined spectroscopically with high spatial resolution. It is found that the power input into the anode increases nearly linearly with the arc current. The proportionality constant is mainly determined by the work function of the electrode material and T_e but is independent of the electrically measured anode fall and scarcely dependent on the electrode dimensions. The constriction is more pronounced in cold anodes, with maxima of T_e and n_e in front of the electrode surface, than on hot anodes with thermionic electron emission and vaporization of the electrode material. The distances of the T_e- and n_e-maxima from the anode surface are increased and T_e is reduced in front of the anode with increasing anode temperature. The experimental findings may be explained by a model of the anodic boundary layer consisting of a thin sheath in front of the surface and a more extended constriction zone. The current and voltage are anti-parallel within the sheath. The power which is needed to sustain the sheath is supplied by an enhanced electrical power input into the constriction zone.

A simple integral arc model for ablation arcs in tubes is presented. The model predicts the arc temperature in the axis, the average electrical field strength, the pressure generation and the mass ablation rate at the tube wall. The model is entirely based on first principles and does not require fit parameters. This is achieved with the help of detailed numerical arc simulations which use a combination of a radiative energy transfer code and computational fluid dynamics. This allows a physically correct and consistent averaging over the arc when deriving the integral model. New experiments for validation of the model were done. The results of the model are in satisfactory agreement with the older and new experimental data. The dependences of the predicted arc parameters are discussed for tube radii and lengths in the parameter range R = 2–8 mm and L = 40–80 mm, respectively.

A compact, repetitive pulsed power system has many applications, but some basic aspects of the breakdown characteristics need further investigation. The purpose of this paper is to investigate the breakdown characteristics of dry air with repetitive nanosecond pulses. The variables affecting the discharge conditions, including the applied pulse voltage, pulse repetition frequency, gap distance and gas pressure, are investigated. The relationship between the breakdown time lag, the repetitive stressing time, and the number of applied pulses to breakdown with pulse repetition rate, E-field strength and pressure are obtained, and the breakdown characteristics and mechanism are also discussed. It is suggested that activated neutral species and residual ions should be taken into account, and repetitive nanosecond-pulse breakdown is characterized by the accumulation effect.

A two-phase mixture (TPM) is a mixture of gas and macroparticles of high concentration. Based on Townsend's theory, a new cell-iterative model in analytical form for the breakdown mechanism in TPM is presented. Compared with the original cell-iterative model in our previous paper, the obstructive factor of the macroparticles that influences the electron avalanche propagation is considered, except for the macroparticles distorting the electrical field and capture of the electrons. The cell attractive parameter k is presented according to the classical continuum theory for field charging. The modified Paschen law for a TPM is presented to calculate the breakdown voltage. The breakdown voltage of the TPM, U_TPM, increases gradually with an increase in the macroparticle number density (m). The voltage U_TPM is lower than that of the pure gas at low m values and larger at high m values. With a decrease of the macroparticle volume fraction and the dielectric mismatch, the voltage U_TPM increases gradually at low m values and decreases gradually at high m values. The voltage U_TPM at pd = 200 cm Torr is lower than that at pd = 760 cm Torr for low m values and larger for high m values. This kind of abnormal breakdown characteristic in the TPM occurs in the case of high macroparticle volume fraction. On the other hand, the minimum of the TPM's Paschen curve increases with increase in m. It provides the possibility and the conditions of greatly increasing the breakdown voltage in a nearly uniform field.

In order to evaluate the effect of corner contour evolution stability on the erosion rate of exit edge and the contour evolution of Hall thruster channel wall corners, the erosion rate of exit edge and the contour evolution of initial right-angle corner have been investigated using the wave approach based on Huygens' principle for both uniform and non-uniform ion sputtering conditions and the effect of corner contour evolution stability on the eroded contour predicted by reconstructing Huygens' wavelet at the channel edge. Geometric analyses show that the stability of corner contour evolution, which is determined by the ion incident angle only, has a significant effect on the erosion rate of exit edge and the contour evolution of channel wall corner while the initial right-angle of channel wall corner cannot be kept intact under non-uniform ion bombardment. The similarity between simulation and experimental results indicates that the corner contour evolution stability is likely to be one of the major causes for the formation of corner eroded contour which has a significant effect on the edge erosion rate of the Hall thruster channel wall.

In our recent investigation, a 3.3 kJ Mather type plasma focus was used to synthesize FeCo nano-particles. The tops of the anodes normally used were replaced by the materials to be synthesized using different numbers of focus shots. It is observed that the characteristics of the nano-particles depend not only on the numbers of focus shots but also on the angular position of the mounted sample. SEM results show that the size of the nano-particles is smaller when the sample is mounted at a bigger angular position. The size of the particles increases linearly with the increase in the number of focus shots. EDX shows that the FeCo nano-particles are stoichiometric. The magnetic properties measured using a vibrating sample magnetometer identify the soft magnetic nature of the FeCo nano-particles. The saturation magnetization has been found to increase in samples prepared with a higher number of focus shots using repetitive plasma focus NX2.

Vanadium oxide VO_x films were deposited by reactive RF magnetron sputtering by applying a substrate bias, in which the Ar ions in plasma impacted the growing film surface. The vanadium valence of the VO_x film decreased when the substrate negative bias voltage was increased. The VO₂ film was successfully deposited at a substrate temperature of 400 °C and with a bias voltage of −50 to −80 V. The transition temperatures of the VO₂ films with a substrate bias of −50 and −80 V were about 56 °C and 44 °C, respectively.

Surface properties of polyvinylidene fluoride (PVDF) films have been modified by treatment in a radio frequency (RF) nitrogen plasma, at different treatment powers (25, 50 and 75 W). The surface properties of the PVDF films before and after RF plasma treatment, as well as the quality of the deposited copper layer, were investigated by various investigation methods (Scanning electron microscopy, atomic force microscopy, ATR-FTIR and contact angle measurements). RF plasma treatment significantly improved the adherence and the quality of the deposited metal layer. The results obtained for PVDF RF plasma treated were compared with those corresponding to a microwave plasma treatment.

The influence of substrate temperature and film thickness on the nanostructure of titanium (HCP) and silver (FCC) thin films deposited on glass substrates under UHV conditions by electron beam evaporation is investigated. The preferred orientation, nanostrain and stacking and twin fault probabilities in Ag and Ti films are determined as a function of film thickness and substrate temperature. A (111) preferred orientation is observed for silver films, which is dependent on both the film thickness and substrate temperature, with the highest value at a substrate temperature of 500 K. Ti/glass films showed a (002) preferred orientation. Nanostructural parameters such as the crystallite size (size of coherently diffracting domains) and nanostrain are evaluated using the Scherrer and Stocks–Wilson relations, the Williamson–Hall plot, and the single-Voigt (SV), double-Voigt (DV) and Warren–Averbach (WA) methods. Analysis of the results obtained using these methods showed that the most suitable approaches to x-ray diffraction line broadening analysis, applicable to both FCC and HCP polycrystalline thin film structures, are SV, DV and WA. The results show that the crystallite sizes increase with substrate temperature and film thickness, while the nanostrain and lattice constants decrease with thickness. The crystallite size distribution function was obtained from the size broadened part of the DV function, and the results show a shift in the maximum to larger sizes with increasing temperature and thickness.

The chemical vapour synthesis (CVS) route is a versatile process that can be used for the synthesis of nanocrystalline ceramics with very small crystallite sizes having a narrow particle size distribution. In this study, a CVS technique was used to prepare nanocrystalline titania from tetraisopropyl orthotitanate at a processing temperature of 1273 K (1000 °C). High resolution transmission electron microscopy, x-ray diffraction and nitrogen adsorption techniques were used for the characterization of the as-synthesized powders. Green bodies were produced by a combination of uniaxial and cold isostatic pressing, which were then sintered. A simple pressureless sintering route was established that led to the production of a dense titania ceramic with a uniform microstructure and an average grain size well in the nanophase regime.

A new equation was developed to describe quantitatively the dielectric loss of materials that is a key parameter in the fields of microwave chemistry and microwave processing of materials under microwave irradiation. This equation can be applied to explain the complicated thermal runaway phenomenon that has relations with dielectric relaxation and to simulate the dielectric loss under microwave irradiation. It was also found that the simulation results based on this equation are in good agreement with the Volge–Fulcher law. The new equation has potential application in the study of microwave interaction with solid materials, especially in materials' processing which involves microwave chemistry, synthesis, sintering, melting, joining, surface-modifications, quality improvements, etc and it is hoped that the dynamic process of material under microwave irradiation could be explained.

Nanocrystalline powders of La-modified lead zirconate titanate (PLZT: 7/60/40) solid solution have been synthesized via mechanochemical processing. No excess PbO was used in the starting composition. Scanning and transmission electron microscopy revealed the formation of powders on the nano-scale. Sintering the powders resulted in dense compacts, at reduced sintering temperatures. Well-saturated ferroelectric hysteresis loops with a P_r on the order of ∼48 µC cm⁻² and coercive fields of ∼7 kV cm⁻¹ were recorded on the samples. Strain levels up to > 0.25% with limited hysteresis (<0.05% in the low field region) indicate the tremendous potential of these ceramics for high-performance actuators, especially in cases where a high degree of positioning accuracy is required.

The percolation behaviour of conductive composites containing particles of different sizes was analysed. A composite was simulated as the media containing small conductive particles distributed in the channels between large insulative particles, where each large particle is covered by n monolayers of the filler particles. The simulations were done for the cases of two-dimensional (2D) and three-dimensional (3D) lattices. It was shown that the percolation filler concentration x_* versus the particle size ratio λ = R/r and the number of monolayers n may be approximated as , where d is the space dimensionality; is the site random percolation threshold; n_eff is the effective number of monolayers, which decreases with increase in n and n_eff → n in the limit of n → ∞. The scaling behaviour of the percolation threshold inside the layers confined by the large particles was analysed. The data obtained at different values of λ and n gave the same correlation length exponent values as for the classical random percolation both for 2D and 3D cases. Analysis of the electrical conductivity behaviour near the percolation threshold in 2D systems showed the existence of the obvious differences at different values of λ and n, though the conductivity exponents s and t retained their universal values typical for the random percolation. The accuracy of the developed theoretical approach was experimentally tested for the polyvinyl chloride–copper (PVC–Cu) and polycarbonate–copper (PC–Cu) composites.

Using the plane wave expansion method we have studied the effect of symmetry on the complete acoustic band gaps in two-dimensional binary phononic crystals including five types of straight rod arranged in hexagonal lattices. For each type of rod, two cases are considered: water rods in mercury (low-density scatters in high-density background) and mercury rods in water (high-density scatters in low-density background). The crystals' symmetry can be changed by rotating the noncircular rods, and the results show that the extreme (both the maximum and minimum) gaps at any given filling factor appear under high symmetry. So the above maximum gaps can be obtained by adjusting the rods' orientation. And the peaks of those maximum gaps are relative to the rods' rotational symmetry, the relations of which are different for the two cases with different density contrast.

(1 − x)Na_0.5Bi_0.5TiO₃ − xK_0.5Bi_0.5TiO₃ (NKBT100x; x being the mole ratio) piezoelectric ceramics were prepared by a conventional solid reaction method. The microstructure and ferroelectric properties of NKBT100x solid solution ceramics with compositions of Na_0.5Bi_0.5TiO₃ (NBT)-rich and K_0.5Bi_0.5TiO₃ (KBT)-rich were investigated by SEM, XRD, temperature dependence of dielectric constant, and hysteresis loop, respectively. XRD studies showed that the pure perovskite structure existed in NKBT100x ceramics with x ⩽ 0.50, while a small amount of a second phase, K₄Ti₃O₈, appeared in the ceramics with x ⩾ 0.70 due to the structural instability of KBT-rich ceramics. The grain size decreased with increasing KBT content, showing that the KBT-rich solid solutions were very difficult to densify fully. The temperature dependence of the dielectric constant of NKBT100x ceramics showed that the depolarization temperature (T_d) reached a minimum value at the composition of x = 0.16 (NKBT16) near the morphotropic phase boundary, which suggested that the ferroelectric state of co-existence between the rhombohedral phase of NBT and tetragonal phase of KBT were not as stable as the single phase of NBT and KBT, respectively. The poled density NKBT50 (x = 0.50) showed high piezoelectric properties, high T_d and low dielectric loss, which was a good candidate for lead free piezoelectric ceramics working at relatively high temperatures.

Highly oriented Ba_0.5Sr_0.5TiO₃ (BST) thin films were grown on MgO (001) single-crystal substrates by pulsed-laser deposition at 800 °C in oxygen pressure ranging from 1.2 × 10⁻³ to 40 Pa. A strong correlation was observed between the growth process, structure and dielectric properties for the BST films. The dielectric properties in the low frequency range 10 k–1 MHz were measured in the interdigital capacitor configuration. The tetragonal distortion (ratio of in-plane and surface normal lattice parameters, D = a/c), surface morphology and dielectric response of the films are strongly dependent on the oxygen deposition pressure. With increase in oxygen pressure, the in-plane strain for the BST films changes from compressive to tensile. The BST film grown at 25 Pa exhibits the best overall dielectric properties. This corresponds to the film with a slight in-plane tensile strain (D = 1.0012). It is believed that a reasonable tensile strain, which increases the ionic displacement and thus promotes the in-plane polarization in the field direction, could enhance the tunability and dielectric constant. Based on the BST film grown at 25 Pa, distributed phase shifters were successfully fabricated with promising performance. The phase shifter shows a relatively low insertion loss of about 3.5 dB at zero bias and 10 GHz, a good return loss better than −15 dB for all phase states from dc to 16 GHz and a differential phase shift of about 43° with 120 V dc bias at 10 GHz.

The space charge region Ω₀ around a conductor at potential V₀, as calculated in the depletion approximation, has a minimum property, which can be expressed in terms of the functional F = W − V₀Q, where W is the system energy and Q the charge on the conductor. It is shown that F attains its minimum value for the actual region Ω₀, relative to other regions with the same uniform charge density but different shape and extent. The identification of Ω₀ through the minimum principle is illustrated in two cases, where the spherical symmetry allows an analytical treatment.

A numerical model is proposed to describe the arc and its interaction with a composite material in an anodic configuration. After a validation step with experimental results in two dimensions (2D) from the literature, the model is used to quantify the degradation level of the material versus the pulse duration and the current intensity value. The flux components show the importance of the Joule effect in the composite. Then, in order to simulate the aircraft displacement, an external convective force is applied. A three-dimensional (3D) model is thus developed and used to evaluate the degradation of the composite material. This model shows the behaviour of the plasma column representing the lightning strike and quantifies the power transferred to the anode.

In this paper we propose a constructal approach to understanding and predicting the morphology of agglomerates of aerosol particles and also to the design of air-cleaning devices. We show that particle agglomeration begins in spherical shape and switches to needle-shaped agglomeration, which performs best as a particle collector at long times. The shape of the needle depends on the dipolar moment of the particles, while the critical number of particles prior to switching to needle shape does not depend on the particle properties.We also show how constructal theory leads to the design of air-cleaning devices that achieve maximum performance per unit volume, under the imposed global constrains. The optimal geometry (internal spacing) of devices composed of parallel-plate channels or tubes and porous filters is shown to depend on known non-dimensional numbers that characterize particle deposition. The work reported in this paper adds to many studies that show how constructal theory provides a deterministic basis for the generation of flow configuration everywhere.

Some of the units in table 1, on page 1759, are incorrect. The values 0.16 ns, 0.39 ns and 1.80 ns should read as 0.16 ms, 0.39 ms and 1.80 ms. The values 29.08 μs and 28.07 μs should read as 29.08 ns and 28.07 ns. The corresponding values stated in the text are correct. The corrected table 1 is included in the attached PDF file.