Table of contents

The magnetic and transport properties of the Co₂FeGa Heusler alloy have been investigated. The results show that the temperature dependence of the magnetization follows the spin-wave behaviour at low temperature. The electrical resistivity behaves according to a ∼T² power law, which may be mainly attributed to electron–electron scattering, and the contribution of electron–phonon scattering to the resistivity seems to be small. We have not observed remarkable magnetoresistance in our measurements. Point contact Andreev reflection measurements of the spin-polarization yield a polarization of 59%, which is consistent with the theoretical prediction by a first-principles calculation.

Biocompatible magnetic dispersions have been prepared from γ-Fe₂O₃ nanoparticles (5 nm) synthesized by continuous laser pyrolysis of Fe(CO)₅ vapours. The feasibility of using these dispersions as magnetic resonance imaging (MRI) contrast agents has been analysed in terms of chemical structure, magnetic properties, ¹H NMR relaxation times and biokinetics. The magnetic nanoparticles were dispersed in a strong alkaline solution in the presence of dextran, yielding stable colloids in a single step. The dispersions consist of particle-aggregates 25 nm in diameter measured using transmission electron microscope and a hydrodynamic diameter of 42 nm measured using photon correlation spectroscopy. The magnetic and relaxometric properties of the dispersions were of the same order of magnitude as those of commercial contrast agents produced using coprecipitation. However, these dispersions, when injected intravenously in rats at standard doses showed a mono-exponential blood clearance instead of a biexponential one, with a blood half-life of 7 ± 1 min. Furthermore, an important enhancement of the image contrast was observed after the injection, mainly located at the liver and the spleen of the rat. In conclusion, the laser pyrolysis technique seems to be a good alternative to the coprecipitation method for producing MRI contrast agents, with the advantage of being a continuous synthesis method that leads to very uniform particles capable of being dispersed and therefore transformed in a biocompatible magnetic liquid.

AlInGaN quaternary epilayers with varying In mole fraction were investigated using triple-axis x-ray diffraction and photoluminescence measurements. The indium compositional fluctuation is enhanced with increasing In mole fraction, whereas the mosaicity of the AlInGaN epilayers is determined through the GaN template quality. Based on the analysis of the temperature dependence of the PL peak position, it is found that the localization effect strengthens with increasing In mole fraction due to the larger fluctuations of the In distribution. Increasing the influence of the localized state results in increasing the emission intensity and FWHM with the In content.

In this paper, we present an optical study of the state of N₂ produced in an inductively coupled plasma. The operation of the discharge was characterized using ion flux measurements and broadband optical emission, and a clear change from capacitively to inductively coupled behaviour was observed with increasing applied power. The typical ion flux at 100 W and 10 mTorr was found to be 1.8 × 10¹⁸ m² s⁻¹, from which a ion density of ∼1.5 × 10⁹ cm⁻³ was inferred. Diode laser cavity enhanced absorption spectroscopy (CEAS) was used to probe the state via the band at 686 nm. P₃₃ band head spectra were used to determine both the translational (T_tr) and rotational (T_rot) temperatures of the molecules at the v = 0 level. These were found to be in equilibrium but dependent on plasma parameters; in a 10 mTorr discharge, T_rot ≈ T_tr, varying from ∼300 K at 5 W to ∼450 K at 400 W applied power. Absolute number densities in individual spin–rotation states were determined by calibrating the CEAS technique using the cavity ringdown time to measure the mirror reflectivity. The overall population in the v = 0 level was found to be (1.19 ± 0.07) × 10¹⁰ cm⁻³ under typical conditions of 100 W radio frequency power and 10 mTorr pressure, corresponding to a discharge efficiency for the production of this level of ∼10⁻⁵. A kinetic scheme is presented to account for the pressure and power dependence of the A-state concentration in the v = 0 level.

A global (volume averaged) model of oxygen discharges is used to study the transition from a recombination dominated discharge to a detachment dominated discharge. The model includes the metastable oxygen molecules O₂(a ¹Δ_g) and and the three Herzberg states . Dissociative attachment of the oxygen molecule in the ground state and the metastable oxygen molecule O₂(a ¹Δ_g) are the dominating channels for creation of the negative oxygen ion O⁻. At high pressures, dissociative attachment of the Herzberg states contributes significantly to the creation of the negative oxygen ion, O⁻. The detachment by a collision of the metastable oxygen molecule with the oxygen ion, O⁻, is a significant loss process for the O⁻ at pressures above 10 mTorr. Its contribution to the loss is more significant at a lower applied power, but at the higher pressures it is always significant. Detachment by collision with O(³P) is also an important loss mechanism for O⁻. We find that ion–ion recombination is the dominating loss process for negative ions in oxygen discharges at low pressures and calculate the critical pressure where the contributions of recombination reactions and detachment reactions are equal. This critical pressure depends on the applied power, increases with applied power and is in the range 5–14 mTorr in the pressure and power range investigated.

The development of a discharge channel in coplanar dielectric barrier arrangements is investigated numerically. Its behaviour in oxygen, like the spatial and temporal distributions of the field strength, charged and neutral particles and energy density, is described in detail. It is found that the streamer development is mainly determined by photoemission. A cathode layer appears near the position where the cathode directed streamer touches the dielectric surface. Secondary electron emission by ion collisions becomes significant and the parameters of the cathode layer are near those of a normal glow discharge. The charge transfer and energy release happen in the conductive channel of the discharge, which appears on the dielectric surface as a result of the cathode streamer development. The field strength in the conductive channel is nearly constant and about 70–100 Td in oxygen and air.

Barrier discharges in coplanar arrangements consists of a multitude of discharge channels (microdischarges) on the dielectric surface. The parameters of such an ensemble of channels are investigated numerically using a two-dimensional model. The channel width and density along extended pairs of electrodes are introduced in the model in order to obtain a more accurate description of the behaviour of the whole device. Using this model voltage–charge curves (Lissajous figures) can be calculated. The simulated voltage–charge curves, the device capacitances for the conditions discharge 'on' and 'off', inception voltages and current densities are obtained. They correspond well with experimental results.

External magnetic fields are used extensively to steer the cathode spot of arc discharges in order to improve target utilization and minimize droplet generation. Optical emission spectroscopy (OES) and electrostatic probe measurements in a Cr arc discharge were used to characterize the effect of the external magnetic field on the ion flux to the substrates and on the composition and time evolution of the plasma. A combination of a permanent magnet array and an electromagnetic coil was used to vary the shape and strength of the magnetic field on the cathode surface. Finite element modelling of the magnetic field distribution identified two types of geometry—through-field, with lines normal to the cathode surface, and arched-field, with lines forming a magnetic 'tunnel'. The magnetic flux densities measured with a Hall probe were in the range from −15 to +15 mT. The particular shape and strength of the magnetic field determined the specific confinement regions and diffusion pathways for the plasma. The total ion saturation current density at the substrate position was in the range between 2 and 11.5 mA cm⁻² depending on the magnetic field shape. The magnetic field strongly influenced the relative optical emission from Cr⁰, Cr¹⁺ and Cr²⁺ metal species, and the resulting charge state distribution. Time-resolved OES and probe measurements of a particular position on the arc cathode revealed that an Ar plasma is trapped near the cathode and is sustained even when the cathode spot is a significant distance from the observation volume. The importance of this 'residual' Ar plasma for the charge state distribution of metal ions is discussed.

The operation of pseudosparks (thyratrons with cold cathodes) of the TDI1-150k/25 type was investigated in the nanosecond time range of control and switching. Their most important characteristics are their reliability of firing—practically in 100% of cases, a jitter time—less than 4 ns, and the possibility of them switching off (interruption of a current) at a particular moment (after the first half of a discharge cycle). Investigations have shown that the TDI1-150k/25 device has all the above characteristics (which meet the current industrial demands) if the proper operational regimes are chosen. We have proven its parameters with a set of four switches working in parallel at the current level of a few hundred kiloamperes at our installations of the Dense Plasma Focus type, having an energy of several kilojoules.

The atmospheric pressure glow discharge burning in nitrogen with small admixture of organosilicon compounds such as hexamethyldisilazane or hexamethyldisiloxane was used for the deposition of thin organosilicon polymer films. The properties of the discharge were studied by means of optical emission spectroscopy and electrical measurements. The deposited films were characterized by atomic force microscopy, x-ray photoelectron spectroscopy, infrared transmission measurements, ellipsometry, depth sensing indentation technique and contact angle measurements. The films were polymer-like, transparent in the visible range, with uniform thickness and without pinholes. The film hardness varied from 0.3 to 0.6 GPa depending on deposition conditions, the elastic modulus was in the range 15–28 GPa and the surface free energy was in the range 26–45 mJ m⁻². The studied films exhibited good adhesion to the substrate.

Carbon nanotubes (CNTs) are produced using a 100 kW dc non-transferred plasma torch and C₂Cl₄ as the carbon precursor. Catalytic metallic nanoparticles are generated in situ using the tungsten metal vapours emitted by the electrode erosion process. Large quantities of multi-walled carbon nanotubes (MWNTs) and spherical (onion-like) carbon structures are observed under FE-SEM and TEM. Preliminary results are given here on the effect of the type and pressure of the gas in the reactor on the zone of CNT nucleation. A large amount of CNTs, over 50 µm in length, is observed to grow within the torch nozzle at 0.26 atm pressure in helium and argon. Increasing the pressure to 0.66 atm in both gases has the effect of pushing the CNT production downstream in the gas phase within the main reactor.

Carbon nitride films have been fabricated by a dc facing-target reactive sputtering system for various N₂ fractions (P_N) in the gas mixture. Complementary measurement techniques, including atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and high-resolution transmission electron microscopy (HRTEM), were used to systematically study the morphology and microstructure of the films. AFM images show that the average surface roughness increases with increasing P_N. XPS analyses indicate that the concentration of N is not directly proportional to P_N, and it rises quickly to a saturated value of ∼33 at% at a P_N of 20%, which can be attributed to the chemical sputtering effect. The ratio N–C(sp²)/N–C(sp³) increases with increase in P_N from 0% to 20%, and then decreases with further increase in P_N. However, the number of sp²-hybridized C atoms continues to increase over the whole range of P_N, as evidenced by Raman and FTIR measurements. The growth of a disordered sp² C structure at P_N below and above 20% can be attributed to the incorporation of N and the compressive stress relaxing, respectively. Raman scattering and HRTEM analyses reveal an incomplete ordering process of the sp² C structure with increase in P_N.

The nano-scratch behaviour of diamond-like carbon films on a Ti alloy and Si substrate was evaluated. For both samples, three processes—fully elastic recovery, plastic deformation, and delamination and pulling-off of the films, occur successively with increasing load during scratching. The loads (Lc_L) corresponding to the peeling-off of the films during the up-loading were 75 and 70 mN for Ti alloy and Si. However, the films on Si were delaminated during unloading, and the relevant load (Lc_U) was only 45 mN. This probably originates from the distribution status of the plastic deformation both in the films and the substrates. Therefore, the nano-scratch test can be applied not only to obtain the cracking resistance (Lc_L) characterizing the cohesion strength of films during up-loading but also to determine the delamination resistance (Lc_U) related to the adhesion strength of the film–substrate during unloading.

Three models of surface stress on a rectangular cantilever beam are presented. The surface stress is modelled as a corresponding concentrated moment at the beam free end, a corresponding concentrated moment plus a corresponding concentrated axial load at the beam free end, and a corresponding uniformly distributed axial stress plus bending moment per unit length along the beam span, respectively. The results of the three models are compared under three different loading scenarios. We also present an analysis of the error source, when using Stoney's formula to predict the surface stress, by comparing the kinematic and loading assumptions of the three models. The surface stress effects on structure deflection are usually modelled as bending moments applied at structure free edge(s)/end(s). Modelling the surface stress effect along the beam neutral axis is presented and compared with modelling its effect at free edge(s)/end(s). The stiffening effect of tensile surface stress is also studied.

Expansion of cellular polypropylene films through an increase in gas pressure and subsequent pressure release at elevated temperatures prior to charging is known to enhance the piezoelectric d₃₃-coefficient of the material. By means of a second pressure expansion the piezoelectric activity can be further increased by more than 40% in comparison with samples subjected to only a single expansion. The effectiveness of the double-expansion process must be attributed to the gain in thickness through the second expansion, following the charging and metallization processes. This thickness change causes a decrease in Young's modulus and thus an increase in d₃₃. Typical d₃₃-coefficients of 1400 pC N⁻¹ at 0.01 Hz and about 500 pC N⁻¹ at 25 kHz have been achieved.

The main objective of this investigation is to examine the effects of a new technique, double-ageing, on the microstructure and mechanical properties of the ultrahigh strength steel Aermet 100. Under the condition of double-ageing, there is no apparent decrease in the steel strength. However, the impact fatigue life can be prolonged by 35.5% and the dynamic fracture toughness raised by 22.6% by this technique compared with normal ageing. Based on the observation of the microscopic structure, the physical mechanism of prolongation of fatigue life and enhancement of the stability of the reverted austenite is analysed further. The results show that this new technique is a breakthrough in optimally combining the strength and toughness of Aermet 100 steel. In light of the current understanding of this subject, the toughness of the reverted austenite formed at the time of heat preservation at the higher temperature of the double-ageing process increases drastically. Moreover, during the treatment at the lower temperature of the double-ageing process, the carbon separating from the martensitic ferrite of Aermet 100 steel will diffuse into the reverted austenite, resulting in a decrease of the brittleness of the martensite and an increase of the stability of the reverted austenite.

This paper presents a theoretical study of the phenomenon of acoustic imaging using sonic crystals, which are made of two-dimensional regular arrays of rigid cylinders placed in parallel in air. The scattering of acoustic waves is computed using the standard multiple scattering theory, and the band structures are computed using the plane-wave expansion method. It is shown that not only can properly arranged arrays behave as acoustic lenses but the focusing effect can also be well described by the lensmaker's formula. Possible applications are also discussed.

Local variations in filler particle concentration and/or shape and orientation in static filler/polymer composites are modelled as distributions of percolation thresholds. The concentration variations can be due to insufficient mixing, formation of semicrystalline voids during cooling from the melt, shrinkage during polymer curing, flow during physical compression or the like. Irregular filler shapes, especially elongated shapes, reduce the percolation threshold; thus, natural variations in the shapes and orientations of filler particle aggregates lead to locally varying percolation thresholds. A distribution of percolation thresholds leads to an apparent average percolation threshold based on the conductivity below the mean percolation threshold. For filler concentrations above the apparent percolation threshold, the dielectric constant continues to increase before reaching a lowered peak value at the mean percolation threshold and then decreasing. This can explain some 'anomalous' published experimental results concerning the dielectric constant just above the percolation threshold. In the frequency plane, the percolation threshold distribution can lead to a slight reduction of the apparent critical exponents x and y of the frequency dependencies of the conductivity and relative dielectric constant, respectively. Our experimental results on ethylene butylacrylate copolymer/carbon black composites support the theory.

In industrial applications, the controlled adjustment (trimming) of resistive elements via the application of high voltage pulses is a promising technique, with several advantages with respect to more classical approaches such as the laser cutting method. The microscopic processes governing the response to high voltage pulses depend on the nature of the resistor and on the interaction with the local environment. Here we provide a theoretical statistical description of voltage discharge effects on disordered composites by considering random resistor network models with different properties and processes due to the voltage discharge. We compare standard percolation results with biased percolation effects and provide a tentative explanation of the different scenarios observed during trimming processes.

Triboluminescence (TrL) arises on the fracture of many solids. Spectral evidence suggests that the TrL of the majority of materials is due to electrical discharge. Gas discharge is also invoked to explain the TrL of certain materials in which there is no spectral evidence for it. A technique for determining if the TrL of a material is due to discharge, independent of spectral information, is described here. Electrostatic probes are used to monitor the potential of the fracture surfaces of a crystal during cleavage. The signal from a photomultiplier reveals if the TrL is due to discharge between the fracture surfaces. The use of this technique is illustrated through studies of sucrose, sodium chloride, and a highly TrL (and highly photoluminescent) terbium complex. Oppositely charged fracture surfaces are found in sucrose; however, the sense of the electrification is opposite to that predicted by the existing theory of piezoelectric charging. It was confirmed that the TrL of both sucrose and sodium chloride is associated with electrical discharge. In contrast, the TrL of the terbium complex commenced prior to electrical discharge. This demonstrates that the TrL of this material is not due to the fluorescent re-emission of the ultraviolet component of the discharge spectrum, as had been suggested previously.

The aim of this work is to study the electron irradiation behaviour of an insulating material surface using a scanning electron microscope (SEM). The charging phenomena caused in two kinds of insulating materials (quartz and glass) by continuous electron irradiation have been observed. The discharging phenomena following switching off of irradiation have also been studied. The trapped charge density is determined by using the so-called electrostatic influence method based on the measurement, during and after the irradiation, of the influence and leakage currents using an arrangement adapted to the SEM. The experimental results reveal that the behaviour under irradiation of glass is entirely different from that of quartz. The trapped charges are found to be different, and the dependence of charging on the primary beam energy is discussed.

The charging and discharging time constants have been determined accurately, and their evolution versus the mean electron penetration depth is qualitatively explained. Moreover, the role of secondary electron emission in the regulation mechanism of charging is underlined.

The importance of topological features of grains in the evolution of grain structure is well recognized in isothermal systems. However, during fusion welding, strong spatial gradients of temperature exist in the heat-affected zone (HAZ), and this region undergoes rapid heating and cooling. The effects of spatial and temporal variations of temperature on the topological class distribution, relationship between size and topology of grains and the interdependence between grain topology and its neighbours are not known. Topological features of grains in the HAZ of Ti–6Al–4V alloy welds were measured for various heat inputs in the range 0.55–4.33 MJ m⁻¹. The topological class distributions were also calculated using a three-dimensional Monte Carlo model utilizing thermal cycles computed from a well tested numerical heat transfer and fluid flow model. The computed results showed that the topological class distributions were unaffected by the spatial and temporal variations of temperature. Experimental investigations of a few sections confirmed the simulation results. The average grain size for each edge class varied linearly with the edge class number. The local topological environment, i.e. the average number of sides of neighbours, n_n, varied linearly with the inverse of the number of sides of grains, 1/n_r, at a given location in the HAZ. Locations with the same topological environment showed the same grain size, indicating the significant influence of grain topology on grain growth in the HAZ.

In this paper we propose an electrode design and a switching pattern of the applied DC electrode potentials for a microfluidic device to be used in size separation of DNA molecules. Estimates on the separation resolution, which are based on numerical solutions of a Newton-type equation on time-averaged quantities, are presented for an input batch sample of DNA fragments with sizes up to 220 base pairs (bp). The active area of the device (which can be microfabricated by standard photolitographic techniques) is a channel 6 µm wide, 8 µm deep and 150 µm in length, flanked by 23 plane parallel integrated electrodes, individually addressed with low DC voltages, up to ± 25 V. In the active area a time-dependent non-uniform electric field, or a travelling dielectrophoretic wave (TDW) is being produced. In order to enhance the separation resolution, the polarization DC potentials are switched with a relatively high frequency (≈ 10⁻⁷ s), which is chosen accordingly with the buffer conductivity and dielectric constants of the fluid and particles. Since the external field is of DC type, we put forward an explanatory model of the dielectric response of the DNA to the time-dependent applied field. We then numerically investigate the size-dependent response of the DNA in a low conductivity buffer (≈0.01 Ω⁻¹ m⁻¹) under the influence of the electric field, which is calculated by means of the method of moments. The results of the computer modelling indicate the existence of a threshold value for the size of the successfully transported molecules, which can be adjusted by varying the velocity of the dielectrophoretic wave produced by the system. The estimated error in selecting a chosen group of molecules with sizes above a specified value is about 5 bp, while the processing times are of the order of hundred of seconds.

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