Table of contents

Reflectance calculations for diamond-like carbon (DLC) antireflection thin-film coatings on porous silicon (PS) have been carried out using the optical matrix approach method. Comparison with the reflectance spectrum obtained for other antireflection coatings shows a much lower reflectance with a larger energy range including the ultraviolet, visible and infrared regions of the solar spectrum for the DLC/PS double layer. This finding is relevant in solar cell applications.

This article reviews a spectroscopic method for extracting plasma electron temperatures and electron energy distributions: trace rare gases optical emission spectroscopy. Specifically, traces of Ne, Ar, Kr, and Xe are added to the plasma and the intensities of emissions from the Paschen 2p levels are recorded. Intensities are also computed from a model that includes direct excitation from the ground state, as well as two-step excitation through the ³P₂, and ³P₀ metastable levels. A Maxwellian electron energy distribution function (EEDF), described by an electron temperature (T_e), is assumed, and T_e is extracted from the best match between the observed and calculated relative emission intensities. By choosing emission from specific sets of levels, the range of electron energies effective in exciting emission can be selected and various portions of the EEDF can be investigated. Accurate measurement of T_e depends critically on accurate cross sections for electron impact excitation, and hence a large portion of this article is devoted to a critical review of this subject. Improving on previous treatments, the model for computing emission intensities and electron temperatures includes a complete analysis of the complex excitation and de-excitation of the metastable levels. Previous measurements of T_e and EEDFs in chlorine and oxygen inductively coupled plasmas are re-evaluated with the current model. In general, the current version of the model yields similar results; specific differences are discussed.

We report on the preparation of high J_c YBCO coated conductors on non-magnetic Ni–W substrates. In order to minimize the deleterious effects of tungsten oxide formation on the epitaxial growth of oxide buffer layers and YBCO films, an electrodeposition approach has been developed to epitaxially grow an about 2 µm Ni layer (Ed–Ni layer) on Ni–3 at% W tapes. With this method, the surface for epitaxy of the first oxide seed layer is not exposed to the Ni–W surface, and high quality CeO₂/yttria-stabilized zirconia/CeO₂ buffer layers and YBCO films have been grown on the Ed–Ni/Ni–3 at% W substrates by pulsed laser deposition. A critical current density, J_c, of 1.49 MA cm⁻² has been achieved at 75.2 K and in a magnetic field of 83 mT.

Magnetocaloric effect (MCE) in low-cost iron-based binary alloys R₂Fe₁₇ (R = Y, Pr) has been investigated by measuring their magnetic properties. The Curie temperature of these alloys is found to be close to room temperature. The specific heat measurement of these materials indicates a second order ferromagnetic/paramagnetic phase transition. The MCE properties in these alloys are comparable to that of gadolinium, which is very expensive for domestic use. Therefore these binary alloys are suitable as room temperature MCE materials. The effect of pulsed magnetic field on the MCE properties has also been studied up to 25 T.

The structure and magnetic properties of (Nd_1−xEr_x)₃Fe₂₅Cr_4.0 compounds with x = 0–0.8 have been investigated using x-ray powder diffraction (XRD) and magnetic measurements. It has been found that all the compounds crystallize in a Nd₃(Fe,Ti)₂₉-type structure. Substitution of Er for Nd leads to a contraction of the unit-cell volume. The Curie temperature, T_c, and the saturation magnetization, M_s, of (Nd_1−xEr_x)₃Fe₂₅Cr_4.0 decrease monotonically with increasing Er content. The easy magnetization direction (EMD) of Nd₃Fe₂₅Cr_4.0 at room temperature is close to the [040] direction but may be a little out of the basal plane. With increasing Er content, the EMD changes closer to the [40−2] direction and the tilt angle increases. Both the XRD patterns and ac susceptibility indicate the appearance of a spin reorientation for x = 0–0.4 as the temperature decreases from room temperature to 77 K. The spin reorientation temperature, T_sr, increases monotonically with increasing Er content from 158 K for x = 0 to 198 K for x = 0.4. A first order magnetization process (FOMP) occurs for all the compounds, and the critical field of the FOMP decreases with increasing Er content from 6.6 T for x = 0 to 2.0 T for x = 0.7.

The condition of the interphase interface in model exchange-spring CoPt/Co bilayers was correlated with the magnetic response. Polycrystalline bilayer samples of 16.7-nm Co and 25-nm L1₀-type CoPt were fabricated and subsequently annealed to vary the condition of the interphase interface. Although the thickness of the soft phase component exceeded that for ideal (i.e. completely reversible) interlayer exchange coupling, this thickness allowed correlations between the interfacial condition and the magnetic quality of the exchange spring components to be observed and quantified via magnetic recoil measurements. Annealing the bilayers at 300°C for 20 min produced a 63.0% increase in the coercivity of the soft component, while at the same time the coercivity of the hard component was decreased by 18.5%.

We report on the magnetic and martensitic transformation properties of Co₃₉Ni₃₃Al₂₈ ferromagnetic shape memory alloy ribbons. We found that the phase formation of Co₃₉Ni₃₃Al₂₈ is strongly dependent on the method of preparation. The conventional as-cast ingot sample contains a large amount of the γ phase embedded in the primary β phase, while melt-spun ribbons contain the pure β phase. Co₃₉Ni₃₃Al₂₈ ribbons exhibit a perfect thermoelastic martensitic transformation from a cubic to a tetragonal structure at 240 K during cooling. The martensite structure can be well described by the L1₀ lattice, similar to that of Ni–Al alloys. The martensitic phase at 5 K exhibits a saturation magnetization of 48.67 emu g⁻¹ and saturates at about 8000 Oe. A large increase in coercive force after ageing at 500°C for 1 h has been found due to the change in atomic chemical ordering. The temperature dependence of the saturation magnetization indicates that magnetization can be well interpreted by spin-wave theory at temperatures lower than 200 K. The material shows a recoverable strain of 500 ppm upon the martensitic transformation.

Non-linear increases in elastic, piezoelectric (direct and reverse) and dielectric coefficients have been measured under a high electrical field or under high mechanical stress. The permittivity and reverse piezoelectric coefficient can be measured by applying a high voltage at a low frequency, while the elastic compliance and direct piezoelectric coefficient can be measured at the first radial resonance frequency in order to apply a high stress. The non-linear behaviour has been analysed at the radial resonance of a disc.

In all the materials tested, the results show that there is a close relation between the non-linear increments of the different coefficients. An empirical model has been proposed in order to describe and understand these relations. It is assumed that either the strain or the electrical displacement is produced by intrinsic and extrinsic processes, but only the latter, which consist mainly in the motion of domain walls, contribute to the non-linearity. The model enables us to find the domain wall contribution to elastic, piezoelectric and dielectric non-linearities, and allows us to compare the amplitudes of the fields and stresses that produce the same displacement of domain walls.

We have studied theoretically the influence of an external electric field on the electronic structure and free charge carrier profile of a GaAs layer with a non-central position of the δ-doped layer. The electron energy structure was calculated by solving Poisson and Schrödinger equations self-consistently. As a result of the calculations the confining potential, sub-band energy levels, their occupation and the total free carrier density were obtained. On moving the δ-doped layer from the centre of the structure towards the surface, another potential well occurs with electron concentration peak near the second surface of the sample. Approaching the semiconductor surface any further leads to alternating population of the energy levels between the two potential wells. A similar effect of the side peak creation has been described in the literature by applying an external electric field to the structure. Our study shows that the population of discrete states may be governed either by the δ-doped layer position relative to the surface or by the magnitude and the orientation of an external electric field. The external electric field, depending on its orientation, deepens this newly created potential well or pushes the electrons from the new potential well out.

Three different inorganic–organic hetero-junctions (A, ITO/SiO₂/Alq₃/Al; B, ITO/Alq₃/SiO₂/Al; and C, ITO/SiO₂/Alq₃/SiO₂/Al) were fabricated. Emission could only be observed under positive bias in devices A and B but under both positive and negative biases in device C, according to their brightness waveforms. The recombination of hot electrons and holes, i.e. electron–hole pairs, produced blue emission in devices B and C, and the recombination of electrons injected from Al with the accumulated holes that are excited by hot electrons produced green emission in device A. Hence, the emission of these devices is attributed to cathodoluminescence-like emission.

Divalent europium doped strontium di-aluminate (SrAl₄O₇ : Eu²⁺) nanocrystals have been synthesized using a facile sol–gel polymer thermolysis method. The photoluminescence characteristics of smaller particles (ϕ ∼ 15 nm) show a significant difference with respect to their bulk counterpart. In this nanocrystalline system, the electronic structure of the Eu²⁺ excited state seems to undergo considerable modification induced by the surface states involved in slow relaxation kinetics that mimic phosphorescence like features.

The influence of the driving frequency on the absolute oxygen atom density in an O₂ radio frequency (RF) capacitively coupled plasma (CCP) was investigated using vacuum ultraviolet absorption spectroscopy with pulse modulation of the main plasma. A low-power operation of a compact inductively coupled plasma light source was enabled to avoid the significant measurement errors caused by self-absorption in the light source. The pulse modulation of the main plasma enabled accurate absorption measurement for high plasma density conditions by eliminating background signals due to light emission from the main plasma. As for the effects of the driving frequency, the effect of VHF (100 MHz) drive on oxygen atom production was small because of the modest increase in plasma density of electronegative O₂ in contrast to the significant increase in electron density previously observed for electropositive Ar. The recombination coefficient of oxygen atoms on the electrode surface was obtained from a decay rate in the afterglow by comparison with a diffusion model, and it showed agreement with previously reported values for several electrode materials.

Measurements of electron density and temperature of helium plasmas in a cw running magnetic multipole plasma source by repetitively laser-pulsed 90° Thomson scattering are reported. This is the first experiment in which this technique has been applied to such plasmas. Measurements are performed at a helium gas pressure of p_g = 5 Pa, the discharge voltage was U_d = 100 V, the discharge current was 5 A ≤ I_d ≤ 30 A, and the cathode heating current was 80 A ≤ I_h ≤ 140 A. Electron energy distribution functions obtained from the Thomson scattering spectra are studied. The obtained plasma parameters are electron temperature 1.5 eV ≤ kT_e ≤ 5 eV and density 10¹² cm⁻³ ≤ n_e ≤ 4 × 10¹² cm⁻³, respectively. The sensitivity of detection of the experiment is in the range of 10⁹ electrons and the accuracy of the electron temperature and electron density are specified to 15% and 20%, respectively. In addition, the neutral density and helium gas temperature are obtained from the Rayleigh component of the scattered spectra. Langmuir probe measurements are performed under the same plasma conditions and a comparison of the results with Thomson scattering shows good agreement between the two diagnostics.

This paper is mainly concerned with Nd:YAG laser induced plasmas in air at different pressures. A modelling of the formation of laser plasmas is presented. The preionization phase is described using time differential equations that are solved in the basis of the Crank–Nicolson second order scheme. Moreover, the following dynamic phase, described as a strong shock wave expanding out of the focal volume, is simulated by using a two-dimensional compressible flow calculation code. The evolution of pressure, temperature, densities and velocities are analysed and compared with experimental measurements. The mean electron density of the plasma measured through an interferometric method leads to ( ), which correlates with computed results.

During an experiment involving coating of dust grains trapped in an RF discharge using a sputtering dc-magnetron source, a rotating dust ring was observed and investigated. After the magnetron was switched on, the dust cloud levitating above the RF electrode formed a ring rotating as a rigid body. Langmuir probe diagnostics were used for the measurement of plasma density and potential. It was discovered that the coupling of the dc-magnetron source to the RF discharge causes steep radial gradients in electron density and plasma potential. The rotation of the dust ring is attributed to the azimuthal component of the ion drag force, which appears due to the azimuthal drift of the ions caused by crossed radial electric and axial magnetic fields. In order to get more insight into the mechanism of dust ring rotation, a Particle-in-Cell simulation of a rotating dust cloud was performed. The results of the experiment and simulation are presented and discussed.

The plasma enhanced hot filament chemical vapour deposition process used for deposition of carbon nanotubes was investigated using Langmuir probes and optical emission spectroscopy. The electron density, electron temperature and plasma potential were measured in relation to the discharge voltage and substrate temperature. The results are discussed in connection with the conditions for the growth of nanotubes. It is demonstrated that the production of reactive heavy particles like hydrocarbon ions takes place predominantly in the sheath region, close to the negative biased substrate. This conclusion is supported by model calculations of the discharge current.

Multi-wall nanotubes (MWNTs) of carbon were produced by pulsed arc discharges between a room temperature sample and a counter-electrode, with peak currents of 7–100 A, and pulse lengths of 0.2–26 µs, in open air at selected locations on the sample. The samples were 10 × 10 mm² graphite plates, carbon-coated 200 mesh copper grids, and Ni-coated glass slides. The counter-electrodes were graphite in the form of 1 × 4 mm² bars or 4 mm diameter rods with a cone tip of 28°, or 0.1 mm diameter steel rods. Randomly oriented MWNTs (typically 5–15 walls) with a diameter of ∼ 10 nm and lengths of up to 3 µm were produced on the samples with a single 0.2 µs pulse, implying linear growth rates of up to 15 m s⁻¹. MWNTs were produced with both polarities and with all types of counter-electrodes used when the substrate contained carbon. Near vertically oriented MWNTs were deposited on the Ni/glass samples using a graphite counter-electrode. The simplicity, rapidity and selectivity of the process may facilitate wider study and practical application.

We have investigated in a systematic way the interface properties of Fe₂O₃ grown on different buffer layers using an electron beam deposition technique. For films deposited directly onto Si(001) substrate and on Al buffer layer, we observed the presence of metallic Fe at the Si(001) and Al interfaces, respectively. We also detected the presence of SiO₂ and Al₂O₃ by x-ray photoelectron spectroscopy. For Fe₂O₃ films deposited on Cu buffer layer, however, no other phases were observed at the interface. We explain our results using the enthalpy of formation. The enthalpy of formation of SiO₂ and Al₂O₃ is much lower than that of Fe₂O₃, thus inhibiting the crystalline growth of Fe₂O₃ at the interface. The enthalpy of formation of CuO is, however, greater than that of Fe₂O₃, and thus promoting the growth of Fe₂O₃ to be crystalline. These results were corroborated by our transmission electron microscopy studies.

Nanoparticles of polycrystalline Ge have been grown in a freshly cleaved single crystal NaCl (100) substrate, starting from Au/Ge bilayer films prepared using the evaporation method during annealing. The experimental results indicate that fat fractal Ge patterns can be formed in Au/Ge bilayer films by annealing at 100°C for 60 and 70 min. Here, we report in detail interdiffusion assessment of nanoparticles in fat fractal patterns. The scaling exponent (or fractal dimension) of polycrystalline Ge clusters in fat fractal patterns is larger than that of the conventional diffusion-limited aggregation. The formation of fractal patterns and the perplexing scaling behaviour may result from the random successive nucleation and growth mechanism.

Nano-sized precipitation in high-dose implanted Si has been investigated using high-resolution transmission electron microscopy of cross-sectional specimens (XHRTEM). Zn and Bi (50 keV) have been implanted in Si at doses of 5 × 10¹⁶ cm⁻² and 10¹⁶ cm⁻², respectively. In spite of the different diffusivities of these species in Si, their low solubility resulted in precipitation of nano-sized metallic inclusions whose inner structures revealed a synthesis of superlattices, composed of the host Si matrix and the implanted species.

The complex permittivity, permeability and electromagnetic (EM) wave absorption properties have been investigated for resin compacts containing 75 wt% composite powders of α-Fe/C(a) and Fe₂B/C(a). The real and imaginary parts ( and ε_r') of relative permittivity for the resin composites remained almost constant in the 0.05–20.05 GHz range. The imaginary part of the relative permeability (μ_r') exhibited a wide peak in the 1–9 GHz range for α-Fe/C(a) and in the 2–18 GHz range for Fe₂B/C(a). Both resin composites exhibited good EM wave absorption properties (RL < −20 dB) in the 4.4–8.3 GHz range with an absorber thickness of 1.9–3.4 mm, and in the 7.5–16 GHz range for an absorber thickness of 1.2–2.2 mm, respectively.

Decreasing the particle size results in a decrease in pyroelectroluminescence (PEL) in LiNaSO₄ : Eu. The nanocrystalline LiNaSO₄ : Eu gives no PEL, making it much more suitable for thermoluminescence dosimetry of low doses of ionizing radiations such as x-rays and gamma rays, compared with the microcrystalline LiNaSO₄ : Eu.

A novel theoretical treatment of linear shift-invariant imaging systems is developed, which allows explicit solution of forward and inverse problems in the case of large values of the generalized Fresnel number. The method will be useful in a variety of domains including optical instrumentation, biomedical imaging and materials science. Our approach is used to integrate two particularly topical x-ray imaging methods, namely propagation-based and analyser-crystal-based x-ray phase-contrast imaging.

Dynamic phenomena during discharge of a silo are not well understood. This study uses a dual approach to quantify the motion of both the particulate solid at the wall of a model silo and the wall itself. The motion of particles was recorded visually using a digital CCD camera. The particles are seen to move with an intermittent motion. Image analysis techniques were then applied to obtain quantitative measurements of the particulate velocity. In relation to the particle motion, the silo walls were observed to vibrate during discharge. These vibrations were measured using a laser vibrometer to obtain independent measurements of the radial displacement and velocity. This enabled comparisons to be made between the particulate motion and the wall vibrations. Results are presented for barley and for polyethylene terephthalate pellets.

More than four years ago Pharr proposed the concept of the virtual rigid indenter to describe the elastic stress part of indentation problems beyond the linear elastic limit (Pharr 1999 Mechanical Properties of Films, Coatings and Interfacial Materials (IL Chiocco, Italy, June 27–July 2, 1999). He suggested substituting the problem of a well defined indenter acting on a predeformed surface with the problem of an effectively shaped indenter and a flat surface. This way he was able to explain the unloading curves of quite a variety of different materials even with sharp indenters like Vickers or Berkovich indenters. The method works well even when for the sake of simplicity the assumed effective indenter is chosen to have symmetry of revolution, which a real indenter does not (e.g. Vickers and Berkovich indenters are four and three sided pyramids, respectively).

To make this concept applicable, the resulting pressure distributions of a wide range of different indenter shapes are needed. The author presents solutions for indenters with symmetry of revolution with a radius (r) dependent shape described as a series of powers of r, up to power 8. In addition, the resulting potential functions are calculated in order to be able to construct the resulting elastic field completely analytically and in closed form.

The effect of the different indenter shape on the resulting elastic indentation depth is discussed, and some practical examples are considered.