Table of contents

Praseodymium-doped ZrO₂ photoluminescent (PL) powders have been prepared by a co-precipitation process. Different doping concentrations in the start mixture and annealing temperatures for the precipitated were studied. It is observed that the crystalline structure and the crystallinity of the powders depend on the annealing temperature. For temperatures higher than 750°C, the material presents the monoclinic phase with an average crystalline grain size of 100 nm. Measurements on photoluminescence show excitation and emission spectra. For an excitation wavelength of 294 nm, all the PL emission spectra show the peaks located at 490, 505, 536, 567, 589, 616, 645, 725 and 745 nm associated with ³P₀ → ³H₄, ³P₀ → ³H₄, ³P₁ → ³H₅, ³P₀ → ³H₅, ³P₀ → ³H₅, ³P₀ → ³H₆, ³P₀ → ³F₂, ³P₀ → ³F₃ and ³P₀ → ³F₄ transitions in the Pr^{3 +} ion, respectively.

An overview is given of the state-of-the-art in optical attosecond pulse generation and measurements. The emission of ultrashort bursts of XUV radiation from a laser driven plasma is described and analysed in the framework of a semiclassical model that explains essential features of the emitted spectrum. While under most conditions, trains of XUV bursts, separated by the half-cycle time of the driving laser field are emitted, few-cycle laser pulses of a well-defined carrier–envelope phase can yield isolated XUV pulses of sub-femtosecond duration. A time resolving correlation technique that relies on the interaction of electrons with a strong laser light field allows the measurement of attosecond electron dynamics from systems excited by these ultrashort XUV pulses.

The decomposition behaviour and the effect of Ge on the microstructural evolution in Fe_89−xZr₇B₃Cu₁Ge_x (x = 0 and 3 at%) alloys were examined using differential scanning calorimetry, x-ray diffraction (XRD), transmission electron microscopy (TEM) and field emission scanning TEM with nanoprobe energy-dispersive x-ray spectroscopy (NanoEDXS). The onset temperatures of primary crystallization for amorphous Fe_89−xZr₇B₃Cu₁Ge_x alloys were found to be independent of the Ge content. The XRD and TEM results showed that the crystallization takes place through two stages of amorphous→bcc Fe+residual amorphous → α−Fe+Fe₃Zr, regardless of Ge content. The addition of Ge has an effect on reducing the grain size of the Fe₈₉Zr₇B₃Cu₁ alloy by 10–15%. The NanoEDXS results confirmed that Ge is enriched in the residual amorphous matrix together with Zr, resulting in enhanced Curie temperature and the exchange stiffness of the intergranular region. The refined grain size and the partitioning of Ge into the intergranular region together resulted in improved soft magnetic properties. Further, the NanoEDXS results implied that unlike Cu, Ge does not assist in the heterogeneous nucleation of the bcc Fe phase during the primary crystallization reaction.

In this paper, fifth-order geometric aberrations of round electron lenses have been restudied. Improved analytical formulae of the aberration coefficients were derived by the computer algebra software—Mathematica—and their correctness verified by cross-checking with numerical integration and differential algebraic techniques for a given electromagnetic lens.

The effects of applied electric fields on donor and exciton states confined in GaAs–(Ga,Al)As quantum wells are studied in the effective mass approximation and within the variational and fractional-dimensional space approaches. In the fractional-dimensional scheme, an effective isotropic medium is used to model the anisotropic Coulomb-bound state + quantum well + electric-field system, whereas a hydrogenic-like wave function is used in the variational procedure. Results are obtained for the binding energies, virial-theorem values, and scaling properties of the Coulomb-bound states under growth-direction applied electric fields. We show that a ground-state wave function assumed as a D-dimensional hydrogenic wave function leads to a virial-theorem value of 2 and a hyperbolic scaling for binding energies of Coulomb-bound states versus quantum-confined Bohr radii, in contrast with results using the variational approach. Both methods result in Stark shifts for the exciton-peak energies in good agreement with electroabsorption experiments.

Charge transport and light emission properties of an array of silicon nanoclusters (NCs), sandwiched between a p-type and an n-type doped silicon crystal, are studied theoretically by assuming that electrons and holes enter from the opposite sides of the array in response to an applied electric field. The size of the NCs considered ranges from 16 nm down to 3.6 nm and their spatial distribution is optimized so that light emission, resulting from radiative recombinations, is peaked in the visible red around 1.8 eV. The light emission efficiency is limited by the carrier hopping times and is found to be in the range 2–0.5%, for fields ranging from 100 kV cm⁻¹ to 500 kV cm⁻¹, respectively.

Using a variational approach, we have calculated the impurity position dependence of the photoionization cross-section and the binding energy of a hydrogenic donor impurity in a quantum well wire in the presence of the magnetic field as a function of the photon energy. Our calculations have revealed the dependence of the photoionization cross-section and the impurity binding on the applied magnetic field, the size of the wire and the impurity position.

The optical properties of five metal phthalocyanines (MPcs) thin films (cobalt phthalocyanine (Pc), copper Pc, iron Pc (FePc), nickel Pc, and zinc Pc), were studied by spectroscopic ellipsometry. Thin films of these MPcs were evaporated in high vacuum onto glass substrates, quartz substrates, and silicon substrates. Spectroscopic ellipsometry measurements were performed in the wavelength range 250–800 nm. The absorption spectra were measured, and the film thickness and surface roughness were studied by atomic force microscopy. Determination of optical functions was performed by simultaneous fitting of the experimental data for samples on glass and silicon substrates. Fitting results using point-to-point fitting, the Lorentz model, the modified Lorentz model, the relaxed Lorentz model, and the dual Lorentz model were compared. Models with six oscillators were used to fit the optical functions of FePc, while five oscillators were used for the other four Pcs. It was found that modifications of the Lorentz model are more suitable for description of the optical functions of the MPcs compared with the conventional Lorentz model.

The interaction between a metallic plasma produced by laser ablation and an ambient gas (argon, air and nitrogen) at atmospheric pressure is studied. The experimental results are compared with theoretical ones given by numerical simulation. Aluminium and copper targets are used. The uniform repartition of iron impurities included in the target (1.6% for Al and 2% for Cu) is warranted by the manufacturer. The Nd : YAG laser delivers pulses of 8 ns FWHM duration with an energy ranging from 70 to 100 mJ at a rate of 10 Hz. The temperature measurements have been performed by the Boltzmann diagram method using iron lines. The influence of ambient gas and target material is studied. After an Abel transform, we observe a maximum of the line intensities off the plasma axis. This shows the formation of gas plasma around the metallic one before complete diffusion of the ambient gas in the metallic plasma. The hydrodynamic model built in one dimension uses the continuity equations. This code is based on the control volume method of Patankar. A temperature and mass fraction profiles are needed as initial conditions. This model is able to describe the temporal evolution of the temperature and of the diffusion in the plasma. It allows us to study the mixture of the metallic plasma and the ambient gas as soon as the plasma is thermalized and local thermodynamic equilibrium is established. There is good agreement between the results of the simulation and the experimental results.

Dual frequency capacitive discharges are designed to offer independent control of the flux and energy of ions impacting on an object immersed in a plasma. This is desirable in applications such as the processing of silicon wafers for microelectronics manufacturing. In such discharges, a low frequency component couples predominantly to the ions, while a high frequency component couples predominantly to electrons. Thus, the low frequency component controls the ion energy, while the high frequency component controls the plasma density. Clearly, this desired behaviour is not achieved for arbitrary configurations of the discharge, and in general one expects some unwanted coupling of ion flux and energy. In this paper we use computer simulations with the particle-in-cell method to show that the most important governing parameter is the ratio of the driving frequencies. If the ratio of the high and low frequencies is great enough, essentially independent control of the ion energy and flux is possible by manipulation of the high and low frequency power sources. Other operating parameters, such as pressure, discharge geometry, and absolute power, are of much less significance.

Based on quasipotential analysis, the Sagdeev potential equation has been derived to study the sheath characteristics in magnetized plasma. Theoretical studies have tried to determine if there are new features in the sheath region. Because of the ubiquitous presence of dust in every plasma environment, our motivation is to estimate the interaction of charged dust grains levitated into the magnetized plasma sheath. We have calculated the size of dust grains, the generation of surface potential and the net forces acting on it inside the sheath, for some chosen laboratory plasma parameters. The estimation of dust dynamics in the sheath should take into account that it is controlled by its properties like size, surface potential and the force acting on it, besides the ambient plasma properties. The overall observations of the forces on different dust sizes enable us to obtain information about the formation of dust atmosphere.

I–V characteristics of a helium corona discharge are discussed for both positive and negative modes. Experimental measurements confirm Townsend's theory for only a part of the I–V characteristic. The flow effect has been correlated to a pressure variation inside the discharge chamber, which in turn gives rise to a corona current change. Corona discharges produce UV radiation through excitation and recombination processes. Currents produced by electron extraction from a stainless steel surface located 74 mm away from the discharge have been measured. Corona currents in the range of 1–7 µA induce currents on the metallic target of 10–30 pA (positive corona), whereas currents of 15–50 µA give rise to 60–140 pA (negative corona).

In recent work, we found that the stability of the negative corona charge in radio frequency plasma treated polytetrafluoroethylene (PTFE) films (18 µm thickness) strongly depends on the plasma sources, the exposure time and the condition of the film in the plasma, i.e. the film orientation on the holder and whether the film is one-sided metallized or non-metallized, as well as the film side for corona charged. Using Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy, we conclude that two factors affect the negative charge stability: oxide formed on the surface and positive charges trapped in the film. The oxides serve to retain the negative corona charges and the plasma-generated positive charges recombine with the negative corona charges and cause the corona charge discharge after heating.

The objective of this investigation is to understand the inception of ice surface corona streamers. A validated onset criterion that is useful for practical electrode geometries is presented. High-speed streak photography techniques were used to observe the streamers propagating over an ice surface. The effects of several experimental parameters such as freezing water conductivity and HV electrode radius on the streamer inception parameters of an ice surface have been examined. Empirical models have been proposed, based on elements derived from the inception parameters to account for corona streamer propagation velocity and inception voltage/field on an ice surface. Since the propagation velocity is a power function of the electric field, its value has been expressed in terms of the inception voltage.

The spatial distribution of ozone density is measured in pulsed corona discharges with a 40 µm spatial resolution using a two-dimensional laser absorption method. Discharge occurs in a 13 mm point-to-plane gap in dry air with a pulse duration of 100 ns. The result shows that the ozone density increases for about 100 µs after the discharge pulse. The rate coefficient of the ozone-producing reaction, O + O₂ + M → O₃ + M, is estimated to be 3.5 × 10⁻³⁴ cm⁶ s⁻¹. It is observed that ozone is mostly distributed in the secondary-streamer channel. This suggests that most of the ozone is produced by the secondary streamer, not the primary streamer. After the discharge pulse, ozone diffuses into the background from the secondary-streamer channel. The diffusion coefficient of ozone is estimated to be approximately 0.1 to 0.2 cm² s⁻¹.

Thomson scattering (TS) experiments have been performed in the region near the electrodes of a dc powered model lamp filled with 1–2 bar argon gas. In order to suppress the false stray light and Rayleigh scattered photons, a triple grating spectrograph was used. In this way the electron density and electron temperature could be measured in the near-electrode region for different arc currents. In order to get the electron density and electron temperature out of the TS spectrum, both collective and non-collective TS fitting procedures were used for the data processing. It was found that the radial profile of the electron temperature is more or less flat in the anode region with a value of about 1 eV. The electron density increases with arc current and is of the order of 10²¹ m⁻³. From the result, we can see that the plasma regions investigated are not in local thermal equilibrium in the sense that, locally, the rate of ionization is much larger than that of recombination.

We report a study of residual stress in PbTiO₃ (PT) thin films prepared on Si substrates by a polymeric chemical method. The E(1TO) frequency was used to evaluate the residual stress through an empirical equation available for bulk PT. We find that the residual stress in PT films increases as the film thickness decreases and conclude that it originates essentially from the contributions of extrinsic and intrinsic factors. Polarized Raman experiments showed that the PT films prepared by a polymeric chemical method are somewhat a-domain (polar axis c parallel to the substrate) oriented.

The Ba(Zr_xTi_1−x)O₃ (BZT) thin films were prepared by deposition of relevant materials on LaNiO₃-coated silicon substrates by the sol–gel process. The films thus prepared show a (100) preferred orientation in structure and vary with zirconium content. With the addition of Zr, the grain sizes of films were found to decrease, whereas their microstructure became dense. The changes in the films' grain sizes and microstructure were attributed to the addition of Zr to the BaTiO₃ lattice. Dielectric constant measurements revealed that the thin films show a relaxor behaviour and have a diffuse phase transition characteristic when x is 0.35. The hysteresis loops of the dielectric properties of films measured at room temperature were considered to be the result of the polarization state of the domains in these films in both the ferroelectric or paraelectric state. This leads to a distribution of Curie temperatures. The highest figure of merit value, K, was found for the BZT thin film with a very low dielectric constant. This indicates that BZT films have the potential to be used as microwave tunable devices with superior performance.

Micro/nanomechanical characterization of a decagonal single-grain quasicrystal, Al_74.7Co_16.9Ni_8.4, has been carried out. Hardness and elastic modulus were measured by nanoindentation using a nanoindenter. Fracture toughness was measured by microindentation using a microindenter. It was found that the Al_74.7Co_16.9Ni_8.4 quasicrystal shows a high hardness of 11.4 GPa and an elastic modulus of 177 GPa but a low fracture toughness of 0.81 MPa m^1/2. Nanoindentation resulted in a pile-up around the indent. A small hysteresis was found in the multiple indentation of the quasicrystal. Indentation deformation, creep and fracture mechanisms were discussed in conjunction with the destruction of the quasicrystalline structure and the formation of the crystalline structure in indentation.

This paper deals with the analysis of the influence of moisture on the dielectric properties of a composite insulator core made of epoxy resin and fibre glass, through measurements of the effective permittivity. The experiments are carried out on different core samples—dry and boiled in distilled water over different time intervals. The measured values of the complex permittivity of the core samples are discussed in the light of the results obtained by a numerical approach based on the boundary integral equation method and the PHI3D package. The comparison of the experimental and simulated results aims at finding the water content in the fibre glass.

Electrical properties of rare earth elements (REEs) in some geological materials were characterized during the upgrading of monazite from Egyptian black sand. It has been found that there was a significant relationship between concentrations of REEs and dc conductivity. Also, dielectric constant, dielectric loss, polarization, relaxation time and resonance frequency of samples containing REEs, were measured at a frequency range up to 1 MHz. From these measurements, it has been found that the values of electrical conductivity, resonance frequency and dielectric polarization are inversely proportional to the concentration of REEs. For most relations, the correlation coefficients were found to be better than 99%.

Phase-sensitive high-speed imaging of terahertz (THz) radiation was achieved by introducing the optical heterodyne detection method to high-speed electro-optic sampling of THz images. Intense THz pulses obtained from a large-aperture photoconductive antenna were used as the source. The electric field distribution on the focal plane of focused half-cycle THz pulses was observed using the apparatus. Annular spatial profiles were observed in the time-dependent images. The frequency-dependent distribution calculated by means of Fourier transformation showed almost diffraction-limited focusing.

In the calendering process a tape or sheet of deformable material passes through the nip between hard cylindrical rollers. Usually the rolls are driven at the same peripheral speed, but small differences in speed, often referred to as 'creep', can occur if one of the rolls is externally driven and the other is driven by the friction in the contact. In these circumstances it has been observed that a tape that enters the nip adhering to the driven (slower) surface may transfer at exit to the driving (faster) surface but not the other way round. The mechanics of this transfer process is examined theoretically and experimentally in this paper for the case of double sided adhesive tape. It is argued that on emerging from the nip the tape will separate from the surface at which the shear strain in the adhesive is greater and that for transfer to occur the contact load must be sufficient to cause plastic extension of the tape.

Flexural vibrations of thin metal wires owing to a high, pulsed electric current have been investigated. The current is sufficiently low to inhibit melting but sufficiently high to induce stresses, leading to the wire fragmentation. The problem is treated numerically on the basis of the theory of three-dimensional linear elasticity. The model has been verified on the well-known exact, eigenmode solution for the flexural vibrations of an infinite wire. The agreement is excellent. Further, the model has been used to study vibrations owing to two sources. The first one is perturbations of wires owing to the Lorentz force leading to a kink-type instability similar to that in plasmas. As the main cause of the wire fragmentation has been previously found to be the thermal expansion of material owing to Joule heating, this problem mainly serves to compare results between the three-dimensional and the one-dimensional, thin-rod models. Comparison of the growth rate of the instability obtained by the two models has shown an excellent agreement. The second source of vibrations is the magnetic fields induced in the external electric circuit. The results show that depending on the shape of the circuit, the induced stresses may exceed 20 MPa for the aluminium wires used in the low-current experiments. Although the external fields are not the main source of the wire fragmentation, these values alone may cause the fracture process at elevated temperatures.

This paper reports the first step in the development of a tactile array suitable for the presentation of haptic information in virtual reality. The system is based on the electric field dependence of the viscosity of electrorheological fluids. The simulation, as well as the experimental realization of single tactels is described. The mathematical approach is based on the Eckart model (Eckart W 2000 Continuum Mech. Thermodyn.12 341–62) and its validity is demonstrated by comparing the resulting yield stress with the experimental results from Wunderlich (2000 Dissertation Universität Erlangen-Nürnberg). Two different tactel designs are realized and the experimental results are compared with numerical simulation. The design of modification B is shown to be applicable for the realization of an actuator array with high spatial resolution.

The processing–microstructure-property relations have been studied in order to understand the role of the addition of CeO₂ (up to 0.9 mole%) in the ZnO–Bi₂O₃ based varistor recipe. The microstructural investigation suggests that CeO₂ is segregated at the corners of the ZnO grains in addition to the existence of the Zn₇Sb₂O₁₂ spinel phase. However, the α -spinel phase was observed instead of the β -spinel phase that is usually found in most commercial and laboratory ZnO–Bi₂O₃ based varistors. The α -spinel phase is more stable than the β -spinel phase and does not transform to the pyrochlore phase during the cooling process. The most significant effect of the CeO₂ particles is the ZnO grain refinement owing to the pinning effect of the grain growth. The average grain size decreases from 7.8 to 5.7 µm when compared to the 0.9 mole% CeO₂-added sample against the CeO₂-free sample. This grain refinement results in a significantly enhanced breakdown field when compared to the CeO₂-free sample. The coefficient of nonlinearity of the current–voltage (I–V) characteristics is found to be nearly identical for the CeO₂ added varistor materials. However, when a slower cooling cycle (1°C min⁻¹ instead of 4°C min⁻¹) is used in the sintering process, these varistor materials exhibited a high nonlinear coefficient (α = 29 ± 5) as extracted from the I–V behaviour.

Iridescence in the bird Bostrychia hagedash is interesting in the sense that instead of the usual single, fairly narrow, reflection peak seen for many birds and insects, up to six strong peaks were found covering the spectral range from the near ultraviolet (UV) to the near infrared (IR). By recording reflectance spectra over this range and comparing it to model calculations based on a structural analysis using electron microscopy, the mechanism for iridescence as well as the optical constants of the materials comprising the coloured feathers, could be established. It was discovered that iridescence is mainly based on a relatively thick but uniform keratin outer layer on the feather barbules. In contrast to iridescence in most other birds, the melanin-containing platelets inside the barbules only serve to define the layer thickness of the keratin without contributing to the iridescence in any other way. Unlike previous work where keratin was assumed to be transparent, it was established that both keratin and melanin are absorbing and that keratin exhibits an anomalous dispersion in the green, blue and near UV.