Table of contents

For the first time nanocrystalline diamond (NCD) films were deposited by the pulsed microwave plasma assisted chemical vapour deposition process starting from an Ar/H₂/CH₄ gas mixture. Comparisons with continuous mode deposition gave evidence for the improvement in film quality when the microwave power was modulated with a pulse repetition rate in the range 50–1000 Hz. A reduction in grain size and surface roughness, especially at low pulse repetition rate, accompanied by a decrease in soot particle formation was observed. A thermo-chemical plasma model, developed for pulsed Ar/H₂/CH₄ microwave discharges, provides evidence for the fact that the pulsed mode permits the enhancement of the mole fraction of the C₂ dimer assumed to be the growth precursor of NCD. This may be responsible for a high secondary nucleation rate improving the nanostructure of the film in pulsed discharges.

This topical review discusses magnetic microelectromechanical systems (MagMEMS). A number of exemplars are drawn from the authors' own work, and a review is given of other significant literature in the area. The case is made that MagMEMS offer unique features over conventional MEMS, such as remote interrogation and self-test. A review of the key magnetic principles and magnetic materials is included by way of a guide to the general reader. Finally, a roadmap for the future development of MagMEMS is set out.

The microstructure transformation and the magnetic entropy change of LaFe_11.5Si_1.5 melt-spun ribbons were investigated. The melt-spun ribbons show a homogeneous distribution of elements, and the homogeneity develops further after 5 h annealing. XRD results show that the ribbons are composed of NaZn₁₃-type LaFe_11.5Si_1.5 compounds and an α-Fe phase. After annealing, the ribbons crystallize well in the NaZn₁₃-type structure and exhibit a very large magnetic entropy change. A first-order magnetic transition is observed in the annealed ribbons, and this is believed to be the origin of the large magnetic entropy. These results suggest that NaZn₁₃-type LaFe_11.5Si_1.5 compounds with a large magnetic entropy change can be produced by melt-spinning, the annealing time can be shortened significantly and the production cost of the magnetic refrigerant will be cut down accordingly.

The dependence of the magnetization loops, coercivity, anisotropy constant, ΔE and magnetoimpedance (MI) effects in positively magnetostrictive Fe₇₁Cr₇Si₉B₁₃ amorphous wires on annealing conditions were studied. Samples were annealed at temperatures of 440°C and 460°C for durations between 0.3 and 300 min. The results indicate that progressive annealing first leads to relief of internal stresses and annealing of wires at 460°C for 60 min produces the minimum coercivity of about 1 A m⁻¹. Further annealing increases the coercivity and anisotropy, due to partial crystallization at the surface. It was found that the anisotropy changes its direction to the circumferential direction with the surface crystallization. The maximum change in Young's modulus was measured to be about 75% in a partly surface crystalline sample. It was observed that the magnitude of the MI effect of the stress relieved sample could exceed 200% at 1 MHz.

This paper reports results of a study on the acoustic properties of a ferrofluid, APG-513A. The anisotropy of the ultrasonic wave absorption coefficient was determined in a sample subjected to an external magnetic field. The measurements were performed in a strong magnetic field H_DC = 125 kA m⁻¹, for temperatures of 1–60°C. The dependence of the absorption coefficient of the ultrasonic wave (f = 3.6 MHz) on the angle between the direction of measurement and that of the magnetic field provided important information on the ferrofluid structure in a magnetic field. A comparison of the experimental results with the predictions of the Taketomi theory allowed a determination of the cluster radius and the number density of the colloidal particles. These data allowed us to draw conclusions about the changes in the parameters describing the structure of the ferrofluid at different temperatures. The per cent contribution of the magnetic particles in the cluster structures was estimated. On the basis of the measurements performed, it was possible to establish the proportion of the ultrasonic wave used for excitation of the translational and rotational degrees of freedom. The results revealed a significant effect of temperature on the translational and rotational components of the absorption coefficient. Moreover, the magnetic susceptibility was measured versus the external magnetic field intensity.

The correct description of the macroscopic (overall) physical properties of micro-inhomogeneous (composite) materials by micro-mechanics methods (the self-consistent schemes, for example) requires information about the microstructure and texture of such materials. Often, such information (of shapes inhomogeneities and peculiarities of their spatial distribution) is not available. In this paper, we suggest a method of calculation of the characteristics of micro-inhomogeneous materials that could be applied to a wide class of isotropic materials. This method assumes the absence of a closed order in the material microstructure (a 'perfectly disordered' (PD) medium). We used the PD approximation to predict the effective thermo- and poroelastic, electric and thermal properties of micro-inhomogeneous media.

The expressions for the effective characteristics obtained by this method are always inside of the Hashin–Shtrikman universal bounds. For a two-phase material with fluid component, the effective bulk module satisfies Gassmann's relation for fluid-filled porous media and generalizes such a relation to inhomogeneous thermoelastic and poroelastic media. A comparison of the theoretical results with available experimental data shows a satisfactory coincidence even in the case of high contrast of the component properties.

The simplicity of the numerical realization of the method makes it attractive for applications in the absence of detailed information about the material's microstructure.

The pulsed Townsend technique has been used to measure the electron drift velocity, the density-normalized effective ionization coefficient (α − η)/N (α and η are the ionization and attachment coefficients, respectively), the density-normalized longitudinal diffusion coefficient ND_L, and the ratio between the longitudinal diffusion coefficient and the electron mobility D_L/K, in CHF₃ mixed with Ar and N₂, over a wide range of the density-normalized electric field strength E/N, from 0.2 to 400 Td (1 Td = 10⁻¹⁷ V cm²). The CHF₃ content in the mixtures was varied between 1% and 50%. Regions of negative differential conductivity (NDC) appear in the plots of the electron drift velocity as a function of E/N. This effect is more pronounced for the CHF₃–Ar mixtures than for the CHF₃–N₂ ones, since it results from the presence of a Ramsauer–Townsend minimum in the momentum transfer cross section for Ar, which is absent in N₂. For the CHF₃–N₂ case, a shallow region of NDC is observed, and it is thought to be due to various inelastic collision processes between the electrons and the buffer gas, and also to the steep fall of the scattering cross sections for CHF₃ at low electron energies. Additionally, the dependence of ND_L on E/N displays well-defined minima at low E/N, which are a result of the strong inelastic energy loss of the electrons. The effective ionization coefficients were found to be weakly dependent on the concentration mixture for CHF₃–N₂, while a strong dependence on this parameter was found for the CHF₃–Ar mixture. In all cases, the attaching character of the mixtures was found to be very small. For low E/N (< 20%) in the CHF₃–N₂ mixture it was observed that the values of (α − η)/N are higher than those expected due to electron impact. We believe that Penning ionization may be responsible for this effect.

A synchrotron radiation absorption experiment was used to measure the absolute density of ground level Hg atoms and Hg⁺ ions in Hg–Ar discharge plasmas at high power densities. Ion densities from this experiment were tested using microwave interferometry and compared with Langmuir probe measurements of the electron density. Radial cataphoresis was observed in this experiment and compared with model predictions. The agreement was satisfactory between the experiment and model. The role of cataphoresis in the power balance of Hg–Ar plasmas was further explored in the research described in the companion paper (Lister et al. 2004 J. Phys. D: Appl. Phys.37: 3099–3106).

Discrepancies reported in the literature between numerical predictions and experimental measurements in low-pressure Hg discharges at high current densities are considered. Elements of a one-dimensional fluid model and recent spectroscopic and Langmuir probe measurements are combined in a semi-empirical way to individually examine components of the positive column power balance and the discharge conductivity. At a Hg vapour pressure of 0.81 Pa (6.1 mTorr) and a current density of 300 mA cm⁻², previous discrepancies in the power balance and discharge conductivity are simultaneously resolved by assuming a higher electron density than that obtained from the Langmuir probe measurements. This conclusion is supported by independent measurements of ion density reported in a companion paper. The importance of radial cataphoresis under these conditions, particularly with regard to radiation transport, is highlighted. This work is of particular interest for the design of fluorescent lamps operating at high current densities.

The ionization layer (presheath) separating a fully ionized low-temperature thermal plasma from the space-charge sheath adjacent to a solid surface is described by means of a (multi)fluid model. The character of the solution is governed by α, the ratio of the ionization length to the mean free path for ion–atom collisions. If α ⩾ 1, the solution is determined by physically transparent boundary conditions, namely, by the Bohm criterion at the sheath edge and the condition of full ionization on the plasma side of the ionization layer. If α < 1, the latter condition becomes ineffective. An alternative boundary condition is found for a certain range of α below unity, α_cr ⩽ α < 1. An approximate approach which spans the whole range of α is suggested. While being incomplete theoretically, this approach is sufficient for practical purposes and gives results that are in agreement with experiment. On the other hand, the question of what is the lacking boundary condition in the range 0 < α < α_cr remains open and challenging.

Recently, simulations have been carried out to investigate a dual frequency capacitive plasma in a system consisting of a pair of coaxial cylindrical electrodes. Considering the same asymmetric configuration, it is shown here that a sharp plasma–boundary model may be used to capture the essential dynamical features of the simulations.

Experimental and theoretical studies of singlet oxygen excitation in travelling microwave (TMW) discharges are presented. Singlet oxygen O₂(a¹Δ_g) concentrations and atomic oxygen mole fraction have been measured for different pressures, input powers and distances from the MW resonator. It was shown that a steady-state TMW discharge with a coaxial cavity resonator could provide a maximal O₂(a¹Δ_g) yield of 22% for 2 Torr of pure oxygen and 27–30% for He : O₂ = 1 : 1 mixture. The two-dimensional (r, z) model developed for calculations of plasma-chemical kinetics, heat and mass transfer was used for simulation of processes in the TMW discharge under study. Effects of gas pressure, gas flow rate and input power are studied and compared with experimental measurements of O₂(a¹Δ_g) concentrations and atomic oxygen mole fractions.

We study the angular dependence of the exchange field and the coercivity of an in-plane exchange biased system in hysteresis loop measurements with the field applied out of the film plane by a Stoner–Wohlfarth model where the coherent rotation of the magnetization is assumed. The model shows an overall 1/cosψ dependence of both the exchange field and the coercivity, where ψ is the angle the applied field makes with the film plane. However, when ψ is close to 90°, the exchange field and the coercivity deviate from the 1/cosψ function. The coercivity in particular goes to zero when ψ approaches 90°. The principle and methodology of the model can also apply to other magnetic systems where the coercivity displays similar angular behaviour.

Electronic interband transitions influence the surface plasmon resonance in metallic nanoparticles significantly. We derive expressions for the resonance frequency, the bandwidth and the maximum of the light absorption cross section of noble metal nanoparticles, taking into account the interband transitions in the dielectric function. We propose a simple method for determining the width of the plasmon resonance based on an analysis of the dielectric permittivity of the constituent metal.

The transformation of Pd/Si to Pd₂Si/Si is studied using Auger electron spectroscopy over a wide temperature range of 370–1020 K. The Pd film gets totally converted to Pd₂Si upon annealing at 520 K, and beyond 570 K, Si starts segregating on the surface of silicide. It is found that the presence of surface oxygen influences the segregation of Si. The time evolution study of Si segregation reveals that segregation kinetics is very fast and the segregated Si concentration increases as the temperature is increased. Scanning electron microscopy measurements show that Pd₂Si is formed in the form of islands, which grow as the annealing temperature is increased.

Transverse resistance measurements are reported on patterned epitaxial thin films of optimally doped La_2/3Ca_1/3MnO₃ oxides. We show that the voltage drop measured across a micron-size patterned track displays a temperature dependence that, particularly in a temperature region around the Curie temperature, cannot be explained solely by the contribution of a longitudinal resistivity component. We argue that this response reflects inhomogeneous current flow, which sets in dramatically around the Curie temperature, likely related to charge trapping at microstructural defects. Indeed, we show that appropriate thermal annealing allows reducing to some extent their contribution. Thus we conclude that this experimental set-up allows for the monitoring of how electronic phase separation develops in optimally doped manganites. The impact of this effect on the determination of Hall coefficients is discussed.

An experimental determination technique for simultaneous measurements of the thermal optical and linear thermal expansion coefficients of tantalum pentoxide thin films in the infrared wavelength region is described. A tantalum pentoxide thin film deposited directly onto the end face of a single mode optical fibre is illuminated by a super-luminescent diode source and its spectrum on reflection, between 1460 and 1620 nm, is monitored at various temperatures using an optical spectrum analyser. Temperature induced change in the index of refraction and the film thickness are determined from the reflection spectrum to calculate the thermal optical and linear thermal expansion coefficients simultaneously.

It is a well-known fact that the breakdown voltage of dielectrics in a divergent field may be increased by using an additional insulating layer (a barrier) placed at the optimal position between the electrodes. This paper is devoted to the investigation of the barrier effect in solid dielectrics in a quasi-uniform electric field, i.e. the study of the relationship between partial discharge (PD) activity and the barrier position in the insulating gap. It is established that the PD parameters depend on the barrier position in the gap. In particular, the ignition voltage of critical PDs is increased by more than 50% at the optimal barrier position compared to that for other barrier positions.

The ferroelectric and optical properties of Bi_3.25Nd_0.75Ti₃O₁₂ thin films deposited on (111)Pt/Ti/SiO₂/Si substrates using a chemical solution method were investigated. Bi_3.25Nd_0.75Ti₃O₁₂ thin films were polycrystalline and had (117) random orientation with large rod-like grains. They showed good ferroelectricity with a remnant polarization of 13.8 µC cm⁻² and a coercive field of 120 kV cm⁻¹. The dielectric constant and the dissipation factor were about 200 and 0.040, respectively, at a frequency of 10 kHz. No polarization fatigue was observed for the Pt/Bi_3.25Nd_0.75Ti₃O₁₂/Pt capacitor after 10⁹ switching cycles. The optical constants (refractive index and extinction coefficient) in the wavelength range of 200–1700 nm were obtained by spectroscopic ellipsometry measurements. The optical band gap was estimated to be about 3.61 eV. The dispersion of the refractive index in the interband transition region followed the single electronic oscillator model.

Thermoluminescence and phototransfer thermoluminescence glow curves, and the corresponding emission spectra of LiF (TLD-100) crystals were studied over the temperature range 100 to 300 K after pre-irradiation heat treatment at 373, 423 and 673 K for 1 h. The TL glow curve displays six glow peaks over this temperature range. The intensity and peak temperatures of glow peaks above 200 K are highly affected by the pre-irradiation heat treatment while the glow peaks below 200 K are not affected by this treatment. The various emission spectra include 'ultraviolet' bands at 5.42, 4.42 and 3.57 eV and 'visible' bands at 3.07, 2.65 and 2.22 eV. The first glow peak, seen at 138 K, emits in both the ultraviolet and visible with similar intensity. The next glow peak, seen at 156 K with reduced intensity, also emits in both regions but with relatively more in the ultraviolet than the visible. The peak intensity and energy of the individual visible emission bands are strongly affected by pre-irradiation heat treatment, while the ultraviolet bands are weakly affected. The data are explained with a band-gap model that extends Mayhugh–Christy's model for thermoluminescence above room temperature.

Polycrystalline samples of the modified Pb(Zr_1−x Ti_x)O₃ (PZT) composition, with representative formula Pb_0.92(La_zBi_1−z)_0.08(Zr_0.65Ti_0.35)_0.98O₃ (PLBZT), a family of relaxor ferroelectrics, were prepared via the chemical route with z = 0.3, 0.6 and 0.9. Crystalline phases of powders calcined at different temperatures and the microstructure of the sintered pellets were investigated by x-ray diffraction (XRD) analysis and scanning electron microscopy, respectively. XRD confirms the result obtained by differential scanning calorimetry. The XRD profile shows that the samples having z = 0.9 and 0.6 do not exhibit a pyrochlore phase, whereas the samples with z = 0.3, have 3% of the pyrochlore phase. Microstructural analysis suggests that the shape of grains and intergranular residual pores are modified upon La doping. The dielectric constant and dielectric losses were measured as a function of frequency at room temperature for different frequencies starting from 0.1 kHz to 1 MHz. The dielectric constant was found to be strongly influenced by frequency whereas the Curie temperature remained almost the same. Finally, we conclude that the dielectric constant, loss and activation energy of PLBZT strongly suggest that these compounds are suitable for the preparation of high value capacitors and may be good candidates for device applications.

One of the key criteria in measuring the effectiveness of position sensitive detectors (PSDs) is that of sensitivity—how large an electrical signal can be produced for a given change in the incident light position. The larger the signal produced, then the less additional electronics for amplification required. In this paper we report on optimizing device response by altering substrate thickness and substrate resistivity. We have found that thinner substrates result in greatly increased sensitivities with equal if not improved linearities compared with the equivalent thicker substrate devices. In this work we compare three Schottky metals, Al, Ti and Ta. The improvements in sensitivities for Al devices were greatest at more than 100%, while with Ti and Ta, improvements were better than 50%. Another aspect of the work currently under investigation is the effect of substrate resistivity on the sensitivity and linearity of devices. Preliminary results so far show that high resistivity devices produce markedly higher sensitivities but also respond more to levels of background light, to which low resistivity devices are essentially impervious.

In this paper we present details of the following:

1. ab initio calculations of a set of electron impact cross sections for atomic Zn; and

2. transport coefficients and distribution functions for an electron swarm in Zn vapour, obtained from a multiterm solution of Boltzmann's equation using these cross sections, over a range of reduced fields,E/N, and gas temperatures of practical interest.

Our work has been motivated, in part, by recent suggestions that zinc could be an attractive replacement for mercury in making high-pressure gas discharge lamps more environment-friendly (Born M 2001J. Phys. D: Appl. Phys.34 909; Born M 2002 Plasma Sources Sci. Technol.11 A55). Current models of such lamps require a knowledge of the plasma electrical conductivity, which can be calculated from the (e, Zn) cross sections and mobility coefficients presented in this paper.

Curves of the crack growth resistance parameters K_R, J_R and R as a function of crack length were obtained for IM1-24 nuclear grade graphite subject to oxidation in CO₂ at 900°C up to 20% burn-off. For the unoxidized graphite, the curves show three regions: (i) an initial rise attributed to the development of bridging in the crack wake zone, (ii) a plateau region where the process zone ahead of the crack tip and the crack bridging zone reach steady states and (iii) falling R and J_R curves (or a rising K_R curve) when the crack tip approaches the back of the edge specimen. For oxidation up to ∼11% burn-off, the values of R, J_R and K_R decrease progressively and the plateaux become shorter. At higher burn-off values, the plateau is not found. These trends indicate that the process zone size increases progressively with oxidation, but the length of the crack bridging zone remains unaffected by oxidation at least up to 10% burn-off. The initial values of R and K_R, are close to the values of the linear elastic fracture parameters K_Ic and G_Ic, respectively.

Diamond is a very attractive material to realize radiation detectors due to its exceptional properties (thermal, mechanical, optical, electronic, etc). But, the detector performances are highly dependent on the film quality. In this work, two chemical vapour deposition (CVD) diamond films, differing in film quality, obtained by a hot-filament CVD technique were used to fabricate radiation detectors. The measurements of photocurrents and pulse height distributions (PHDs) of the two detectors were carried out using 5.9 keV x-rays from a ⁵⁵Fe source. For the detector with a better film quality, the electrical contact is fine Ohmic for bias voltages up to 150 V, and a dark current of 23.3 nA and the photocurrent of 16.8 nA are obtained at an electrical field of 50 kV cm⁻¹. The peak of the PHD is well separated from the noise, indicating a high counting efficiency and a high signal-to-noise ratio. After several minutes' stabilization, the time-dependent photocurrent reveals that the photocurrent increases and then levels off due to the polarization effect.

The production of iodine atoms by means of glow discharge in gas flow, with a view to using them in chemical oxygen–iodine lasers, was studied. A dc glow discharge was sustained between coaxial electrodes in the vortex flow of argon, used as the carrier gas, with the addition of iodine-containing molecular precursors CH₃I and HI.

In the experiments a high atomic iodine concentration of more than 10¹⁶ cm⁻³ was achieved at a pressure of the Ar carrier of up to 30 Torr. The dissociation degree of the precursor molecules was 80–100% for CH₃I and 100% for HI.

The loss of iodine atoms in volumetric processes and due to diffusion to the walls of the transporting duct from the discharge region to the active medium was estimated. For the existing designs of iodine injectors of oxygen–iodine lasers, the loss of iodine atoms during transportation can be reduced.