Optical behaviour of Er doped rutile by ion implantation

Rutile single crystals were implanted at room temperature with fluences of 5 × 10¹⁵ Er⁺/cm² ions with 150 keV energy. Rutherford backscattering/channeling along the 0 0 1 axis reveals complete amorphization of the implanted region. Photoluminescence reveals the presence of an optical centre close to the intra-ionic emission of Er³⁺ in the as-implanted samples. After annealing at 800 °C in air no changes were observed in the aligned RBS spectrum. On the contrary, annealing in reducing atmosphere (vacuum) induces the epitaxy of the damage layer. These results are unexpected, since for implantations of other ions under the same conditions, epitaxial recrystallization of the damage region occurs at this temperature. On the other hand, photoluminescence studies show the presence of new Er-related optical centres with high thermal stability in the samples annealed under oxidizing conditions. Annealing at 1000 °C in vacuum leads to the complete recrystallization of the damaged region. At this temperature a large fraction of Er segregates to the surface.     

Fouling tendency of ash resulting from burning mixtures of biofuels. Part 1: Deposition rates

Specified mixtures of peat with bark and peat with straw were burned in a lab-scale entrained flow reactor that simulates conditions in the superheater region of a conventional biomass-fired boiler. The deposition rates were recorded on an air-cooled probe that was inserted into the reactor at the outlet. For both mixtures, the deposition behaviour followed a non-linear pattern, which suggests that physico-chemical interaction between the types of ash has taken place. Peat seems to act as a cleansing agent in all mixtures with straw, while it acts as a cleansing agent in mixtures with bark only up to a share of 50 wt% bark. Between 50 and 100 wt% bark, it seems that peat adds to the deposition. The results indicate that it is possible to burn up to 30 wt% bark (renewable biofuel and pulp mill waste) and up to 70 wt% straw (renewable biofuel and agricultural waste) in mixtures with peat (CO₂-neutral fossil fuel) without encountering increased deposition rates.     

Dielectric and Piezoelectric Properties of Pb(Mg1/3Nb2/3)O3–PbTiO3–Pb(Mg1/2W1/2)O3 Ceramics

Data are presented on the phase composition, crystal structure, microstructure, and dielectric and piezoelectric properties of (1 – y)[(1 – x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃]–yPb(Mg_1/2W_1/2)O₃ (x = 0.30–0.36; y = 0, 0.05, 0.10) ceramics. It is shown that the use of fine-particle magnesia as a starting reagent ensures the formation of single-phase materials. The ceramics with a rhombohedral structure are found to exhibit relaxor behavior. Increasing the content of the Pb(Mg_1/2W_1/2)O₃ perovskite leads to ordering of the domain structure of poled ceramics and increases their piezoelectric charge coefficient d ₃₁ and the width of their phase transitions.     

An enhanced strain 3D element for large deformation elastoplastic thin-shell applications

In this work a previously proposed solid-shell finite element, entirely based on the Enhanced Assumed Strain (EAS) formulation, is extended in order to account for large deformation elastoplastic thin-shell problems. An optimal number of 12 enhanced (internal) variables is employed, leading to a computationally efficient performance when compared to other 3D or solid-shell enhanced elements. This low number of enhanced variables is sufficient to (directly) eliminate either volumetric and transverse shear lockings, the first one arising, for instance, in the fully plastic range, whilst the last appears for small thickness values. The enhanced formulation comprises an additive split of the Green-Lagrange material strain tensor, turning the inclusion of nonlinear kinematics a straightforward task. Finally, some shell-type numerical benchmarks are carried out with the present formulation, and good results are obtained, compared to well-established formulations in the literature.Funding by Ministério da Ciência e do Ensino Superior (FCT and FSE) (Portugal) under grant PRAXIS XXI/ BD/21662/99; as well as the funding by FEDER, under grant POCTI/EME/47289/2002, are gratefully acknowledged.     

Volatile organic compounds in rural atmospheres of central Portugal

Atmospheric concentrations of volatile organic compounds were measured at two rural sites in central Portugal. The sites were chosen to be in line with the summer northwesterly sea breezes in order to study the evolution of the chemical composition of air masses during transport to inland areas. The most abundant non-oxygenated hydrocarbon in the ambient air was isoprene and the monoterpenes alpha-pinene, beta-pinene and 1,8 cineol. The maximum isoprene levels (6-7 ppb) were recorded at the most inland site, suggesting an enrichment of coastal air masses with biogenic emissions during transport over eucalyptus forests. Formaldehyde was the most prominent carbonyl compound in the atmosphere but acetaldehyde and acrolein were also abundant. Concentrations of carbonyl compounds had a tendency to be higher inland, particularly for glyoxal, methyl glyoxal, methyl vinyl ketone, metacrolein and pentanal. The observed increases indicate that carbonyls were produced by photochemical oxidation of biogenic hydrocarbons in aged air masses with coastal origin. Isoprene, monoterpenes and various carbonyls exhibited pronounced diurnal variations, which are explained on the basis of emissions from vegetation, oxidation pathways of biogenic hydrocarbons and meteorological conditions.     

Evaluation of the infrared absorption in nm-thick heavily boron-doped Si1-xGex layers on silicon

Infrared absorption due to free carriers was measured in the spectral range of 1.5–25 m at 77 and 300 K on a large variety of heavily p-doped, p⁺⁺=(2-7)×10²⁰cm^-3 strained Si_1-xGe_x quantum wells on Si grown by molecular beam epitaxy (MBE). The validity of the commonly applied theoretical approaches to describe the spectra has been thoroughly analyzed. The well-known dependence yielded by the Drude theory is not observed. Moreover, it is shown that the Boltzmann kinetic equation is invalid under our experimental conditions. The necessity of a new theory, where the interaction energy of carriers with scattering centers would be included into the energy spectrum of the system, is shown.     

Mechanically-Activated Synthesis and Mixed Conductivity of TbMO4−δ (M = Zr, Hf) Ceramics

Terbium hafnate and zirconate ceramics with submicron grain sizes were prepared via mechanically-activated synthesis. X-ray and electron diffraction and infrared (IR) absorption spectroscopy showed that TbZrO_4– has a disordered fluorite-type structure, while TbHfO_4– is partially ordered, containing pyrochlore microdomains. The oxygen ion transference numbers determined by the modified e.m.f. technique under oxygen/air gradient, vary in the range 0.08–0.26 at 873–1123 K, increasing with temperature. The activation energies for ionic and p-type electronic transport are 82–83 and 29–40 kJ/mol, respectively. The ionic conduction becomes dominant in reducing atmospheres, but tends to decrease at low p(O₂). Oxygen partial pressure dependencies of Seebeck coefficient can be described by a model common for oxide phases with mixed ionic and electron-hole conductivity. Due to partial cation ordering, terbium hafnate exhibits lower ionic and hole transport as compared to TbZrO_4–. The average thermal expansion coefficients of TbMO_4– (M = Zr, Hf) ceramics in air, calculated from dilatometric data, are (11.5–12.4) × 10^–6 K^–1 at 600–1200 K and (18.4–20.3) × 10^–6 K^–1 at 1200–1420 K.     

Microstructural Design and Properties of LaCoO3/La2(Zr,Y)2O7−δ Composites

LaCoO_3–/La₂(Zr,Y)₂O₇-based composites were designed in view of three main properties: electrical conductivity, thermal expansion and resistance to thermal cycling. Composition and processing conditions were investigated on the basis of an experimental plan according to the Taguchi method. The phase distribution was estimated from image analysis and related to electrical and thermo-mechanical behavior. Results indicate that an homogeneous and fine grained phase distribution is essential in order to obtain materials with the desired thermal expansion, electrical properties and thermomechanical properties.     

Dynamic Resource Allocation for Beyond 3G Cellular Networks

The Multicarrier CDMA Transmission Techniques for Integrated Broadband Cellular Systems (MATRICE) project addresses a candidate solution for a Beyond 3G (B3G) air-interface based on Multi-Carrier Code Division Multiple Access (MC-CDMA). It investigates dynamic resource allocation strategies at the Medium Access Control (MAC) layer to support the transport of Internet Protocol (IP) packets over the air-interface in a cost effective manner and maximise the cell capacity with a target QoS. A candidate Dynamic Resource Allocation (DRA) protocol architecture is proposed that is based on cross-layer signalling to provide reactive resource allocation according to the fast channel and traffic variations. In-line with B3G expectations, the proposed DRA handles a very large number of users with inherent flexibility and granularity necessary to support heterogeneous traffic, and still with limited complexity. Thanks to the modular architecture of the DRA, various scheduling policies are investigated and compared in terms of capacity and reactivity to the system environment. Simulation results have shown that the MATRICE system has the potential to deliver broadband heterogenous services in a cost-effective manner, and emerge as a propespective candidate air-interface for B3G cellular networks.     

6H hexagonal structure of low loss Ba3MTiWO9 (M = Mg, Zn) dielectrics

The crystal structure of Ba₃MTiWO₉ (M = Mg, Zn) oxides has been found to be 6H hexagonal, space group P6₃/mmc, parameters of unit cell: a = 5.7943(1) Å, c = 14.1642(1) Å and a = 5.7993(1) Å, c = 14.1626(1) Å for M = Mg and Zn, respectively. The tungsten and titanium atoms are randomly distributed in pairs of face-sharing octahedra separated by octahedral layers containing magnesium or zinc. It was revealed that the structures are well ordered which is believed to result in a low dielectric loss at a microwave frequency reported for these materials.