Table of contents

This paper describes the recent progress in understanding the nature of striations in rare gas plasmas. Striations are ionization waves with unique properties determined by transport phenomena, ionization processes and electron kinetics in current-carrying plasmas. Recent progress in understanding the physics of striations is mainly associated with the advances of non-local electron kinetics in spatially inhomogeneous plasmas and the development of computational models of gas discharges. It has been proven that kinetic analysis of electrons is necessary for nearly all types of striations. Most strikingly, the non-hydrodynamic behaviour of electrons reveals itself in kinetic resonances of the electron distribution function in spatially periodic electric fields, resulting in a variety of striations at low pressures. Moving striations in classical dc discharges have been obtained in computer experiments. Renewed interest in striations has been recently generated by the observation of striations inside pixels of plasma display panels and other micro-plasma devices. High sensitivity of striations to the state of electron gas and ionization kinetics makes them an ideal tool for testing discharge models and advanced plasma diagnostics. Further studies of striations in glow discharges could shed light into the physics of different plasma sources used in modern technologies.

There are more things between cathode and anode than are dreamt in your philosophy.

H Raether

We investigate composite Fe–SrF₂ films to explore new materials showing the granular-type tunnelling magnetoresistance (TMR) effect. The films are composed of Fe nano-particles embedded in a SrF₂ matrix. The electromagnetic properties of the ferromagnetic metal–insulator films are confirmed, including super-paramagnetism, tunnel conduction and TMR. We obtain a TMR of 3% at RT and 5% at 77 K. The film resistivity is much higher than that of oxide-based granular film. We also fabricate the nanostructures of granular films and aim at the application of the single-electron transistor. As a result, we observed a Coulomb blockade (CB) region and CB oscillation at low temperature.

Polycrystalline iron oxide films were fabricated using the reactive sputtering method without substrate heating. Structure characterization indicates that the dominant phases in the films evolve from α-Fe to pure Fe₃O₄ with the increasing O₂ flow rate. In polycrystalline Fe₃O₄ films, disordered atoms exist at the grain boundaries. Magnetic properties analyses reveal that the room-temperature magnetization first decreases and later increases due to the variation of the volume fraction of the paramagnetic FeO phase with a Néel temperature of 198 K. The magnetoresistance MR (= [R(H) − R(0)]/R(0)) of the films increases from 0.1% for pure Fe film to 10.6% for the Fe₃O₄ film at 80 K under a 90 kOe field. The transport mechanism of FeO–Fe₃O₄ and Fe₃O₄ films is suggested to be the tunnelling process, which satisfies the log ρ ∼ T^−1/2 relation. The Hall resistivity of the Fe₃O₄ film decreases with increasing temperature. The ordinary and extraordinary Hall coefficients of the Fe₃O₄ film at 300 K are about 100 and 420 times larger than those of bulk Fe.

Fully epitaxial SmCo₅(25 nm)/Fe/SmCo₅(25 nm) trilayer films were prepared, in which the easy magnetization axis of both hard magnetic SmCo₅ layers is uniquely aligned along one orientation within the film plane. The interlayer exchange coupling between the hard and soft magnetic phase is studied by hysteresis measurements in the magnetic hard and magnetic easy orientation and by dc-demagnetizing loops for different thicknesses of the soft Fe layer. The good coupling together with the optimum texture provided by the epitaxial growth lead to excellent magnetic properties. Fully coupled trilayers are achieved for 6 nm thin Fe layers, which maintain a square hysteresis with high coercivity of μ₀H_c = 1.5 T and a 30% remanence enhancement over a single hard magnetic layer. For thicker Fe layers (16 nm) the exchange coupling leads to reversible rotation processes within the soft phase before the trilayer switches irreversibly at a field of 0.9 T. The still acceptable coercive field and the further improved remanence lead to a maximum energy density of 224 kJ m⁻³ (28 MG Oe). The specific epitaxial relation between SmCo₅ and Fe with its unique orientation of the SmCo₅ easy axis throughout the whole layer stack presents the essential step in the realization of thick, textured, exchange coupled multilayers with high energy density.

We have proposed a new type of double exchange-biased Cu/FeMn/NiFe/Cu/NiFe/FeMn/Ta spin valve with two ferromagnetic layers exchange biased in opposite directions by two antiferromagnetic layers. Sputtering the bottom and top NiFe/FeMn bilayer in magnetic fields of opposite directions, the hysteresis loop, which consists of two loops shifted in opposite directions from the zero magnetic field, was obtained. The NiFe layers are in the antiparallel state in zero magnetic field, and the switching to parallel state can be tuned by selecting the proper exchange bias fields. Moreover, due to the modified spin valve expanding the switching window of the antiparallel state, it makes fabricating the spin valve more flexible, especially for spin valves in which enough large exchange coupling cannot be realized at some fabricating conditions.

CuIn_1−xAl_xSe₂ thin films (x = 0–1.0) were prepared by the four-source co-evaporation technique onto soda lime glass substrates held at 673 K. The films are found to be nearly stoichiometric as determined from Rutherford back scattering (RBS) analysis. Surface analysis of the films was carried out by x-ray photoelectron spectroscopy. X-ray diffraction and scanning electron microscopy are used to examine the structure of the films. The films are found to be single phase and chalcopyrite in structure. The lattice parameters are found to vary nonlinearly with x. Optical absorption studies reveal a three-fold optical band structure and the band gaps are found to increase nonlinearly with the increase in Al content. Crystal field and spin–orbit parameters are determined from the band gaps using a quasi-cubic model. The deformation potential and the percentage of d-orbital contribution to p–d hybridization are determined using the deduced crystal field and spin-orbit parameters. All the films are p-type conducting and the resistivity is found to increase with the increase in Al content. Room temperature Hall mobility and the carrier concentration of the films are determined.

The near-field gap effects are investigated in planar dielectric microdisc and waveguide coupling structures, emphasizing miniaturization of integrated sensor systems. The simulation results show that the resonance frequency is not obviously affected by the gap dimension when the gap between a microcavity and its coupler is larger than 300 nm. However, the resonance frequency shifts observably with a further decreasing gap to the nanometre level. This shift is generally larger than the cavity resonance linewidth in the 10 µm diameter microdisc system, but is comparable to the cavity resonance linewidth in the 2 µm diameter microdisc system. With increasing gap, the cavity Q increases exponentially until it is saturated at a limit Q factor. An optimal gap dimension exists for maximum light energy transfer and storage. The concept of optimum gap is introduced and defined at the gap dimension where half-maximum energy storage capability is achieved; meanwhile, the cavity Q is high and the resonance frequency remains stable.

The thermoluminescence (TL) studies of CaS : Bi nanocrystalline phosphors prepared by the wet chemical co-precipitation method and exposed to γ-rays have been discussed. The average grain size of the samples was estimated as 35 nm using Scherrer's equation. The samples exhibit a complex TL glow curve with multiple peaks. Sample exposed to γ-rays showed that the TL intensity increases linearly as the radiation doses increased in the 1.012–40.48 mGy range. The trap parameters namely, activation energy (E), order of kinetics (b) and frequency factor (s) of the main peaks of the CaS : Bi(0.08 mole%) sample have been determined using Chen's method. The effect of different dopant concentrations and different heating rates has also been discussed.

In this paper we propose an optical configuration of a horizontal-switching liquid crystal (LC) cell with two positive C-plates and a single A-plate in order to improve the optical property in the diagonal direction, which is a weakness of horizontal-switching LC cells. The proposed configuration's optical design was performed on a Poincaré sphere with the trigonometric method. From calculations, we show that the proposed structure can increase the contrast ratio in the diagonal direction by about 10 times.

We have investigated the nanostructure and microstructure resulting from simultaneous ns laser irradiation with deposition of Co films on Si(001) substrates. The spatial order and length scales of the resulting nanopatterns and their crystalline microstructure were investigated as a function of film thickness h and laser energy density E using a combination of atomic force, scanning electron and transmission electron microscopies. The results could be classified into two distinct categories based on the laser energy density used. It was observed that the thickness-dependent E required to melt the Co film (E_Co) was lower than for Si (E_Si) primarily because of the higher reflectivity of Si. Consequently, for energy densities E_Co < E₁ < E_Si that preferentially melted the Co film, spatially ordered nanoparticles were formed and were attributed to capillary-driven transport in the liquid phase. The ordering length scale corresponded to the interference fringe spacing Λ and the microstructure was primarily Co metal and a metal-rich silicide phase. The native oxide layer played an important role in minimizing the Co–Si reaction. For laser energy densities E₂ ⩾ E_Si, spatially ordered patterns with periodic length scales L < Λ were observed and resulted from interference of an incident laser beam with a beam transmitted into the Si substrate. These nanopatterns showed one- as well as two-dimensional spatial ordering of the nanostructures. The microstructure in this laser energy regime was dominated by silicide formation. These results suggest that various nanostructures and microstructures, ranging from nearly pure metal to a Si-rich silicide phase, can be formed by an appropriate choice of the laser energy simultaneously with film growth.

An organic capping layer has been used to increase the light output of the top-emitting organic light-emitting device (TEOLED) employing highly reflective Ag as anode and semitransparent Ag as cathode. The transmittance of the cathode with a 50 nm thick optimized top-capping layer for a wavelength of 520 nm is nearly twice that of the controlled device. Accordingly, an efficient TEOLED capped with a 50 nm thick refractive index-matching layer had been fabricated. Especially, together with the independence of the view angle for the emission peak, the luminance and efficiency for the TEOLED at low voltage (4 V) are 677 cd m⁻² and 15 cd A⁻¹, respectively, as was desired for the displays.

The Bloch mode expansion method is used to numerically study the propagation properties of the hetero-structure phononic waveguide. The numerical results show that when two or more than two different lattice constant phononic waveguides are combined together, there exist two or more than two perfect reflection frequency regions, and they appear independently without interfering with each other, The numerical results also show that the interface between two semi-infinite phononic waveguides affects the resonant peak property of the wave propagation.

This paper is aimed at investigating the inception voltage of the positive corona in point-to-plane gaps in atmospheric air as influenced by a surrounding dielectric enclosure. First, the method of calculation of the inception voltage is explained. It is based on the criterion of self-sustained growth of onset streamers. This calls for calculating the electric field in the vicinity of the stressed point with and without the surrounding enclosure. Then, the calculated inception voltages are compared with those measured experimentally.

In this paper we study the effects of photoionization processes on the propagation of both negative and positive ionization fronts in streamer discharge. We show that negative fronts accelerate in the presence of photoionization events. The appearance and propagation of positive ionization fronts travelling with constant velocity is explained as the result of the combined effects of photoionization and electron diffusion. The photoionization range plays an important role in the selection of the velocity of the ionization front as we show in this work.

The scaling on pressure for a discharge singlet oxygen generator based on the rf discharge excitation of O₂ flow is studied in the context of the problem of oxygen–iodine laser pumping. With this aim, the evolution of O₂(a ¹ Δ_g) and molecules as well as O(³P) atoms in 13.56 and 81 MHz discharges at pressures up to 15 Torr has been investigated in detail. It is shown that fast quenching of O₂(a¹Δ_g) by atomic oxygen with increasing pressure and energy input causes rapid saturation of the O₂(a¹Δ_g) density in the discharge and limits the O₂(a ¹ Δ_g) yield on a rather low level of a few per cent. Covering of the discharge tube walls with mercury oxide for fast catalytic removal of oxygen atoms allows us to greatly increase the O₂(a ¹ Δ_g) yield as well as to avoid fast quenching of O₂(a ¹ Δ_g) in the early afterglow. It enables us to succeed in obtaining the rather high O₂(a ¹ Δ_g) yields at such high pressure as 10–15 Torr. So the singlet oxygen yield is ∼10–12% at ∼10 Torr. The transition to the higher frequency of 81 MHz even increases greatly the O₂(a ¹ Δ_g) yield up to ∼16%.

A 2D self-consistent simulation of an rf discharge in a gas flow in pure oxygen over a wide range of discharge parameters was carried out. The simulation was made at the experimental conditions of Part I of this paper for the discharge tube with an HgO coating where the most effective production of O₂(a ¹Δ_g) was experimentally observed. The simulation goal is to study the features of transversal rf discharge self-organization at different rf frequencies, 13.56 and 81 MHz, to reveal the most optimal conditions for both the O₂(a ¹Δ_g) yield and its energy efficiency. It was shown that the energy part absorbed by electrons increases with the frequency. The spatial discharge structure was studied at both frequencies. It was revealed that at the higher rf of 81 MHz the discharge operates in a mode of the normal current density even at high input powers. The kinetic processes which determined both O₂(a ¹Δ_g) loss and production at experimental conditions were studied and discussed. The increase in rf frequency from 13.56 to 81 MHz reveals an increase in the SO yield and efficiency. It also connects with the decreasing of input energy losses in the sheaths.

The diameter and branching structure of positive streamers in ambient air are investigated with a fast iCCD camera. We use different pulsed power circuits and find that they generate different spatial streamer structures. The electrodes have a point-plane geometry and a distance of 40 or 80 mm, and the peak voltages over the discharge gap are up to 60 kV. Depending on circuit and peak voltage, we observe streamers with diameters varying gradually between 0.2 and 2.5 mm. The streamer velocity increases with the diameter, ranging from 0.07 to 1.5 mm ns⁻¹, while the current density within the streamers stays almost constant. The thicker streamers extend much further before they branch than the thinner ones. The pulsed power supplies are a switched capacitor supply with an internal resistance of 1 kΩ and a transmission line transformer supply with an impedance of 200 Ω; additional resistors change the impedance as well as the voltage rise time in the case of the capacitor supply. We observe that short rise times and low impedance create thick streamers close to the pointed electrode, while a longer rise time as well as a higher impedance create thinner streamers at the same peak voltage over the discharge.

A novel atmospheric pressure plasma fluidized bed (APPFB) with one liquid electrode was designed, and its preliminary discharge characteristics were studied. The glow discharge in the APPFB was generated by applying a low power with helium (He) gas, and the plasma gas temperature was no higher than 320 K when the applied power was lower than 11 W. The plasma optical emission spectrum (OES) of the gas mixture consisting of He and hexamethyldisiloxane (HMDSO) was recorded by a UV-visible monochromator. The calcium carbonate powders were modified by APPFB using HMDSO in the He plasma. The powder surface energy was decreased greatly by coating an organosilicon polymer onto the powder surface. This surface modification process changed the wettability of the powder from super-hydrophilicity to super-hydrophobicity, and the contact angle of water on the modified powders surface was greater than 160°.

We have studied the Stark effect for Rydberg p states of noble gas atoms using laser optogalvanic spectroscopy. The measurements were performed in the sheath region of noble gas glow discharges. Laser excitation from metastable states to np states was monitored by optogalvanic spectroscopy. Clear Stark shifts were observed and the experimental results were compared with a theoretical calculation based on solving the Schrödinger equation of noble gas atoms in an electric field. There was good agreement between the calculation results and the experimental results. The analysis shows that the Stark shift of np states of noble gas atoms can be used for electric field measurements. This method can be a useful alternative to electric field methods based on excitation to nf states because of the higher transition probability of np states.

A hyperthermal hydrogen/deuterium atom beam source with a defined energy distribution has been employed to investigate the kinetically induced electron emission from noble metal surfaces. A monotonous increase in the emission yield was found for energies between 15 and 200 eV. This, along with an observed isotope effect, is described in terms of a model based on Boltzmann type electron energy distributions.

The Bohm sheath criterion in single- and two-ion species plasma is studied with laser-induced fluorescence using a diode laser. Xenon is added to a low pressure unmagnetized dc hot filament argon discharge confined by surface multidipole magnetic fields. The Ar II transition at 668.614 nm is adopted for optical pumping to detect the fluorescence from the plasma and to measure the argon ion velocity distribution functions with respect to positions relative to a negatively biased boundary plate. The structures of the plasma sheath and presheath are measured by an emissive probe. The ion concentrations of the two-species in the bulk plasma are calculated from ion acoustic wave experiments. Results are compared with previous experiments of Ar–He plasmas in which the argon ions were the heavier ion species. Unlike the previous results, the argon speed is slower than its own Bohm velocity near the sheath–presheath boundary in the Ar–Xe plasma where argon ions are the lighter ion species. We argue that this result is consistent with the behaviour of the helium ion required by the generalized Bohm criterion in the previous experiments with Ar–He plasmas. Further, our results suggest that the measured argon ion speed approaches the ion sound speed of the system.

This paper first presents an experimental electrical and optical study of the development of an electrical discharge in water. The point–plane water gap is subjected to a 0.02 µs/350 µs impulse voltage. A Schlieren device associated with an image converter or a photomultiplier demonstrates that the discharge phenomenon requires heating of the water located around the extremity of the point. This thermal process leads to the formation of gas bubbles in which an electrical discharge propagates. In the experimental conditions a threshold value of 80 J is necessary to create bubbles. No UV or IR light emission is recorded before the presence of bubbles is detected. When the energy conditions are sufficient (⩾200 J), the volume of bubbles grows until the whole inter-electrode space is filled; then a breakdown of the gap occurs. When this happens, a high amplitude pressure shock wave is generated. In the second phase of this work the shock wave created by the gap breakdown was studied for energy levels up to 100 kJ. It is clearly pointed out that the pressure shock wave peak value depends on the energy remaining at breakdown time. For a constant remaining energy, the peak pressure value increases with increasing gap length.

ZnO–SnO₂ thin films were deposited by filtered vacuum arc deposition system and characterized using x-ray diffraction (XRD), energy dispersive spectroscopy, atomic force microscopy (AFM), spectrophotometer and ex situ variable angle spectroscopic ellipsometry. According to the XRD analysis the films were amorphous, independent of the deposition conditions. The root-mean-squares (RMS) of surface roughness and the average grain size obtained from the AFM images were 0.2–0.8 nm and 15–20 nm, respectively. Averaged optical transmission was 85%, and the refractive index and extinction coefficient of the films were in the range 2.05–2.28 and 0.001–0.044 at 500 nm wavelength, respectively. The range of the optical band gap of the films was 3.43–3.70 eV, depending on deposition conditions. The lowest resistivity was of the order of 10⁻² Ω cm for films deposited on 400 °C heated substrates, while films deposited on substrates at room temperature were non-conducting, and films on 200 °C heated substrates were weakly conducting(∼10^1–2 Ω cm). The resistivity of films decreased with increasing pressure for 200 and 400 °C heated substrates relative to RT deposited films. The effect of deposition conditions on the optical constants was analysed statistically by single and two sided variance analysis, using the analysis code 'Analysis Of Variance' to determine the significance of the differences between sets.

In this paper we focus on the application of the constructal theory in predicting the dendritic solid structure. First we analyse the marginal stability criterion from the perspective of the constructal principle. Having as a guiding principle the constructal law we have shown that among the whole range of possible dendrite tip radiuses predicted by the stability analysis the dendrite tip will choose the smallest one, that is a radius equal with the smallest perturbation wavelength leading to instabilities. We identify as well the existence of a competition between the diffusion controlled growth and the dendritic growth. Second, we develop a model for the secondary arm spacing. We identify a competition between the lateral diffusion controlled growth of a needle and the dendritic growth of lateral secondary arms. By analysing this competition we are able to characterize the sidebranching mechanism and to finally compute the secondary arm spacing. The result is in good agreement with the experimental results. Finally, the primary arm spacing is analysed from the perspective of the constructal law. The constructal law predicts that the only way the columnar tips can optimize the solidification process is to minimize the spacing between two adjacent tips, namely λ₁. By quantifying the two mechanisms responsible for the selection of λ₁, the dendrite division and the dendrite overgrown mechanisms, we were finally able to obtain a model for the primary arm spacing. This model is also validated against various experimental data.

From the basic equations of thermoelectricity, we model the thermal regimes that develop in multilayered nanopillar elements experiencing continuous charge currents. The energy conservation principle was applied to all layer–layer and layer–electrode junctions. The obtained set of equations was solved to derive the temperature of each junction. The contribution of the Peltier effect is included in an effective resistance. This model gives satisfactory fits to experimental data obtained on a series of reference nanopillar elements.

We propose a new method for studies of laser-induced heating and melting of metallic foils. The method is based on time-integrated measurements of the surface infrared thermal emission. The experimental data are compared with a model where two equations describe the evolution of electron and lattice temperatures and the emissivity is found from the Drude model with the temperature-dependent electron collision frequency. A good agreement between the experimental data and the model is found for the aluminium samples. It is less satisfactory for the copper, but a signature of phase melting can also be pointed out. A multi-pulse laser irradiation study indicates significant changes in the surface emittance, related to preheating, oxidation and/or chemical modification of the copper sample. The proposed method is relatively simple and complementary to the pump–probe technique.

The electron emission from single-wall carbon nanotubes (SWCN) is investigated theoretically by using the tunnelling theory with the tight-binding approach. We compare the electron emission from SWCN in different temperatures and electric fields. In the normal condition, T < 1000 K and 2 V nm⁻¹ < F < 8 V nm⁻¹, the field electron emission (FEE) dominates the electron emission process, which follows approximately the Fowler–Nordheim behaviour. The multi peaks of the FEE energy distribution occur near the Fermi level. At high temperature T ⩾ 1000 K the FEE current is larger than that of the thermionic electron emission (TEE) current in F < 6 V nm⁻¹ but smaller for F > 6 V nm⁻¹. A minimum of the fractional TEE current occurs at the electric field, 5–6 V nm⁻¹ minimum. This is because the strong electric field suppresses both the width and the height of the vacuum potential barrier, which leads to a competition between the FEE and the TEE current.

ZrO₂ films were prepared by nitrogen-assisted radio frequency (RF) reactive sputtering on n-type silicon (100). Optical properties and band alignments of ZrO₂ films on Si have been investigated by using x-ray photoelectron spectroscopy (XPS) and spectroscopy ellipsometry (SE). The improved film quality was obtained after annealing in N₂/O₂ ambient with stable interfacial properties. Optical properties were analysed based on the SE fitting results. According to the valence-band spectrum results, the zero-field energy-band alignments for ZrO₂/SiO₂/Si stacks were also extracted. Our results indicate that the nitrogen-assisted process shows its outstanding ability in the control of interfaces and has potential application in microelectronics.

Computer-controlled apparatus for the measurements of dynamic Young's modulus and internal friction as functions of temperature, using the piezoelectric ultrasonic composite oscillator technique (PUCOT), has been designed and demonstrated. The versatility of the apparatus and its resonance tracking techniques have been proven by detecting subtle structural phase transformations within the Ca_1−xSr_xTiO₃ perovskite system as well as dramatic changes accompanying the martensitic transformation in a Au–Cd shape-memory alloy. The simplicity of the PUCOT technique allows a system such as this to be constructed easily and with low cost.

The underlying domain structures of ferroelastic ceramics have a large influence on their macroscopic electromechanical properties. The profile shape functions of certain pseudo-cubic peaks in diffraction patterns collected from these materials can provide a great deal of information about such domain structures. Using both constant-wavelength neutron and high-energy synchrotron x-ray diffraction, profile shape functions of the pseudo-cubic 002 reflection are evaluated in a soft, tetragonal PZT ceramic. Errors in the integrated intensity ratio, important for measuring the degree of domain boundary movement in these materials, are subject to further scrutiny. It is shown that an asymmetric Pearson VII type distribution, integrated numerically between reasonable limits, gives the most accurate value of relative domain populations in these materials. It is also shown that the diffuse scattering caused by ferroelastic domain walls may be used to estimate the amount of material that is affected by microstrains originating at these walls.

Sr₂Bi₄Ti₅O₁₈ (SBTi) and Sr₂Bi_4−x/3Ti_5−xV_xO₁₈ (x = 0.018, SBTV) films were deposited on Pt/Ti/SiO₂/Si(100) substrates using a sol–gel method. Structure, morphology and electric properties were investigated systematically. These films were randomly oriented and composed of rod-like grains. The remanent polarization (2P_r) and coercive field (E_c) of SBTi films were 23 µC cm⁻² and 71 kV cm⁻¹, respectively. This value of 2P_r was much higher than the reported value of the SBTi thin film prepared by pulse laser deposition. A small content substitution of V⁵⁺ for Ti⁴⁺ was also confirmed to be effective for further improvement in the ferroelectric properties of SBTi films. The V-modified SBTi films showed a very large remanent polarization (2P_r) of 34 µC cm⁻². More importantly, both films showed high fatigue resistance against continuous switching up to 4 × 10¹⁰ cycles and excellent charge-retaining ability up to 4 × 10⁴ s.

The optical properties of PP thin films of radiation-graft copolymerization of N-vinyl-2-pyrrlidone and the modification of grafted polymer by α, β-unsaturated nitrile are investigated in the UV–visible region. The absorption coefficient (α) is calculated as a function of the degree of grafting and modification copolymerization. The values of the optical band gap (E_g) have been obtained from the indirect allowed transitions in k-space. The width of the tails of localized states in the band gap (E_u) was evaluated from Urbach edges. Both the parameters (E_g) and (E_u) vary with the degree of grafting and the modification of grafting copolymers for irradiated and unirradiated samples.

We report the structural and transport properties of RNiO₃ (R: Nd, Pr) thin films prepared by pulsed laser deposition over various substrates without high pressure annealing. An excimer laser KrF with wavelength of 248 nm was used for deposition. Various substrates such as single-crystal SrTiO₃ (100), LaAlO₃ (100) and Si (100) wafer are used for deposition of films to understand the effect of lattice mismatch on transport properties. Various parameters such as substrate temperature, O₂ pressure, laser fluence and deposition time are optimized to get good quality RNiO₃ films. The best quality films are found to be well textured with good crystalline properties. Well-defined first order metal–insulator phase transition (T_MI) was observed in the best quality NdNiO₃ films deposited on LaAlO₃. However, the PrNiO₃ films deposited on LaAlO₃ show completely metallic behaviour throughout the temperature range with no T_MI (transition temperature). Metal–semiconductor transition has been observed in the PrNiO₃ films deposited on SrTiO₃. We try to explain this behaviour due to strain-induced growth of the films.

This paper introduces a new transient double hot-wire technique for the direct measurement of the thermal diffusivity of nanofluids. A correlation to be used with the double hot-wire technique to calculate the effective thermal diffusivity of nanofluids is also developed. Several types of nanofluids were prepared by suspending different volume percentages (1–5%) of titanium dioxide (TiO₂), aluminium oxide (Al₂O₃) and aluminium (Al) nanoparticles in ethylene glycol and engine oil. The thermal diffusivities of these nanofluids determined directly by this technique were found to increase substantially with the increased volume fraction of nanoparticles in base fluids. Based on the calibration results obtained for the base fluids, ethylene glycol and engine oil, the measurement error is estimated to be within 1.2%. The measured thermal diffusivities of nanofluids were found to be significantly higher than those calculated from the thermal diffusivity expression (i.e. α_eff = k_eff/(ρ c_p)_eff) by using the effective thermal conductivities and volumetric specific heats obtained from the conventional hot-wire method and from the volume fraction mixture rule, respectively.

In the 'orthogonal gauge' technique stress gauges mounted to measure both longitudinal and transverse stresses in a shocked matrix allow the shear strength of the matrix material to be determined. A useful measure of the sensitivity of an orthogonal gauge system to changes in the yield strength of the matrix material is provided by the ratio between the resistance change of gauges of a given cross-sectional shape mounted in transverse and longitudinal orientations, (the T/L ratio). A low T/L ratio indicates a more sensitive, and therefore more potentially useful, system. A Eulerian hydrocode has been used to compute the resistance change of manganin wires of rectangular cross-section and infinite length embedded in both longitudinal and transverse orientation in steel targets subjected to nominally one-dimensional shocks. Configurations in which the gauges were (a) embedded directly into the steel and (b) mounted within thin strengthless polymer layers were studied. It is shown that in case (a) the T/L ratio reduces as the aspect ratio of the gauge cross-section increases. When the gauges are embedded in a strengthless polymer layer (case (b)), the T/L ratio is lower than when the gauges are embedded directly into the matrix but in this case the aspect ratio of the manganin conductor has little influence on the T/L ratio. The observed resistance changes are explained in terms of the stresses in the gauges. The results give insight into the factors which control resistance change and offer the prospect of improvements to current gauge designs.

Nanocrystal substitutional semiconductor alloys Mg₃(Bi_xSb_1−x)₂ (nano-Mg₃(Bi_xSb_1−x)₂) with a mean grain size of ∼30 nm were prepared by mechanical alloying plus hot-pressing, and their dc electrical and thermoelectric properties were investigated from room temperature down to 20 K. The results indicated that lattice parameters a and c of nano-Mg₃(Bi_xSb_1−x)₂ increased linearly with increasing Bi content x, in agreement with Vegard's law. The dc resistivity ρ of nano-Mg₃(Bi_xSb_1−x)₂ decreased monotonically with increasing x, and a drop of over five orders of magnitude was reached at 300 K when x increased from 0 to 1. Moreover, the temperature behaviour of the resistivity of nano-Mg₃(Bi_xSb_1−x)₂ changed sensitively with x, and a transition from the semiconducting state (i.e. dρ/dT < 0) to the metallic state (dρ/dT > 0) occurred between x = 0.7 and 0.8. Meanwhile, this transition was verified by the measurements of the temperature behaviour of the Seebeck coefficient S of nano-Mg₃(Bi_1−xSb_x)₂ with different x. In addition, Mott's ρ ∝ T^−1/4 law was observed at lower temperature regimes for the nano-Mg₃(Bi_xSb_1−x)₂ (x ≠ 0), suggesting the occurrence of hopping conduction. Although experiments showed that the Seebeck coefficient of nano-Mg₃(Bi_1−xSb_x)₂ decreased monotonically with x, their thermoelectric power factors PF changed non-monotonically, and a maximum PF of 1.4 µW cm⁻¹ K⁻² was achieved at room temperature for x = ∼0.8, which was more than three orders magnitude greater than that of monolithic Mg₃Sb₂.

Lasers are being used to weld zinc-coated steels due to high welding speed, high aspect ratio, and narrow heat affected zone. However, escape of high-pressure zinc vapour in the welding process can damage the weld pool continuity and cause large voids and serious undercuts in the final welds. In this paper, a mathematical model and the associated numerical techniques have been developed to study the transport phenomena and defect formation mechanisms in pulsed laser keyhole welding of zinc-coated steels. The volume-of-fluid (VOF) method is employed to track free surfaces. The continuum model is used to handle the liquid phase, the solid phase and the mushy zone of the metal. The enthalpy method is employed to account for the latent heat during melting and solidification. The transient heat transfer and melt flow in the weld pool during the keyhole formation and collapse processes are calculated. The escape of zinc vapour through the keyhole and the interaction between zinc vapour and weld pool are studied. Voids in the welds are found to be caused by the combined effects of zinc vapour–melt interactions, keyhole collapse and solidification process. By controlling the laser pulse profile, it is found that the keyhole collapse and solidification process can be delayed, allowing the zinc vapour to escape, which results in the reduction or elimination of voids. The good agreement between the model predictions and the experimental observations indicates that the proposed model lays a solid foundation for future study of laser welding of zinc-coated steels.

The present work aims to theoretically establish that the employment of an axial electric field can substantially augment the rate of microfluidic transport occurring in peristaltic microtubes. For theoretical analysis, shape evolution of the tube is taken to be arbitrary, except for the fact that the characteristic wavelength is assumed to be significantly greater than the average radius of cross section. First, expressions for the velocity profile within the tube are derived and are subsequently utilized to obtain variations in the net flow rate across the same, as a function of the pertinent system parameters. Subsequently, the modes of interaction between the electro-osmotic and peristaltic mechanisms are established through the variations in the time-averaged flow rates for zero pressure rise and the pressure rise for zero time-averaged flow rates, as expressed in terms of the occlusion number, characteristic electro-osmotic velocity and the peristaltic wave speed. From the simulation predictions, it is suggested that a judicious combination of peristalsis and an axial electrokinetic body force can drastically enhance the time-averaged flow rate, provided that the occlusion number is relatively small.

A mathematical model is developed to study the combined influences of electromagnetohydrodynamic forces in controlling the fluid flow through parallel plate rectangular microchannels. The electric double layer (EDL) effects are modelled by employing the classical Poisson–Boltzmann equation. The governing fluid flow equations are subsequently solved, in an effort to obtain closed form expressions depicting the variations in the overall flow rate as a function of various influencing system parameters. It is revealed that, with the aid of a relatively low-magnitude magnetic field, a substantial augmentation in the volumetric flow rates can be achieved. However, with magnetic fields of higher strengths, strongly opposing volumetric forces might offset any further possibilities of flow rate augmentation. Certain critical non-dimensional parameters are also identified, which can play significant roles in the overall flow augmentation mechanism.

A computational analysis of laser keyhole welding is achieved. The main driving force to make the molten pool as a narrow and deep keyhole is the recoil pressure induced by evaporation of the material. Also, the multiple reflection effect on the keyhole wall plays an important role in making the keyhole deeper and raising its total energy absorption rate. Multiple reflection and Fresnel absorption are implemented simultaneously with the proposed ray tracing technique in a discrete grid cell system during the simulation for every single time step. In particular, the Fresnel absorption model is chosen as an energy transfer mechanism from laser beam to workpiece. With all the governing equations including continuity, momentum and energy equation, the VOF method is adopted to trace the free surface of the molten pool. Simulation results are compared with the experimental ones to verify its validity. A pulsed Nd : YAG laser was used for keyhole welding experiments on mild steel plates of 7 mm thickness. It was observed that the generated keyhole maintains its solidified shape without any closing phenomenon both in the experiments and in the simulations.

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