Repositório Institucional da Universidade de Aveiro
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Optical doping of ZnO with Tm by ion implantation
No full textZnO [0 0 0 1] single-crystals were implanted at room temperature with 150 keVTm+ ions at a fluence of 5 × 1015cm -2. Each sample was then subjected to one single 30min air annealing at 800°C, 950°C and 1050°C. The Tm lattice site location and defect recovery were investigated with Rutherford Backscattering/Channeling Spectroscopy. Detailed angular scans along the [0 0 0 1] direction show that 94% of the Tm ions occupy substitutional Zn sites (SZn) in the as-implanted sample. All the annealing temperatures lead to a reduction of this fraction to 30%. Also, progressive damage recovery and Tm segregation to the surface were observed, being enhanced at 1050°C. Photoluminescence (PL) studies with above band gap excitation performed on these samples revealed no luminescence on the as-implanted state. The 800°C air annealing promotes the Tm3+ optical activation and a well-defined near-infrared intraionic emission is observed. For higher annealing temperatures, in spite of no change of the Tm fraction at SZn sites, a decrease of the Tm intraionic emission was observed. These results suggest that optical activation of Tm ions is related with the defect density in their environment. © 2003 Elsevier B.V. All rights reserved.CERN/FIS/43725/200
Multiuser precoding techniques for a distributed broadband wireless system
No full textIn this paper we propose and asses multiuser linear precoding techniques for the downlink of distributed MIMO OFDM systems. We consider a distributed broadband wireless system where the base stations are transparently linked by optical fiber to a central unit. We further assume that both the distributed base stations and the user terminals are equipped with an antenna array. This architecture provides a high speed backhaul channel allowing an efficient joint multiuser multicell processing. The precoder is designed in two phases: first the intercell interference is removed by applying a block diagonalization algorithm. Then the system is further optimized by using a new power allocation algorithm, based on minimization of the sum of inverse signal-to-noise ratio (SNR−1) on each user terminal over the available subcarriers. The motivation to minimize the sum of SNR−1 instead of bit error rate is the fact that the first criterion achieves a closed-form solution, which is more interesting from practical point of view. The aim is to propose a practical distributed precoding technique to remove the intercell interference and improve the user’s fairness at the cell-edges. The performance of the proposed scheme is evaluated and compared with an iterative precoding scheme designed to minimize the bit error rate, extended to the proposed multiuser distributed scenario, considering typical pedestrian scenarios based on LTE specifications.Portuguese CADWIN - PTDC/EEA-TEL/099241/200
Integral graphs and (k, τ)-regular sets
No full textA subset of the vertex set of a graph G, W ⊆ V (G), is a (k, τ)-regular set if it induces a k-regular subgraph of G and every vertex not in the subset has τ neighbors in it. In this paper we deal with the existence of (k, τ)-regular sets associated with all distinct eigenvalues. We show some families that have this property and we give some results concerning the existence of such sets considering restrictions on the symbol of circulant graphs. © 2009.FCTcofinanced by the European Community Fund FEDER/POC
Optical filters and resonant cavities based on di-ureasil organic-inorganic hybrids
No full textSol-gel derived poly(oxyethylene)/siloxane organic-inorganic di-ureasil hybrids containing different amounts (20-60% mol) of methacrylic acid (McOH) modified zirconium oxo-clusters (Zr-OMc) were processed as thin films and transparent and shape controlled monoliths. Laser direct writing was used to create channel waveguides, Bragg gratings, Fabry-Perot cavities and optical filters. The resulting Fabry-Perot optical cavity displays a free spectral range of 16.55 GHz and a fringe intensity contrast of 5.35 dB. Optical rejection values between 6.7 and 10.4 dB were obtained by varying the amount of the Zr-OMc oxo-clusters
Enhanced photoluminescence features of Eu(3+)-modified di-ureasil-zirconium oxocluster organic-inorganic hybrids
No full textIn this work, a series of transparent di-ureasil hybrids containing different amounts of methacrylic acid modified zirconium tetrapropoxide (ZrMcOH) nanoclusters (5-85 mol%) and incorporating EuCl(3) and [Eu(tta)(3)(H(2)O)(2)](tta = thenoyltrifluoroacetonate) complex were prepared. These hybrids are multi-wave-length emitters due to the convolution of the host intrinsic emission (electron-hole recombinations occurring in siliceous and urea cross-linkages) Eu(3+) intra-4f(6) transitions. The ZrMcOH incorporation deviates the maximum excitation wavelength of the hybrid host intrinsic emission from the UV (365 nm) to the blue (420 nm) and enhances the absolute emission quantum yield from 6.0 +/- 0.6% to 9.0 +/- 0.9%, and contributes to an increase in the (5)D(0) lifetime values, quantum efficiency due to a decrease in the non-radiative transition probability and OH groups coordinated to the Eu(3+) ions. (C) 2010 Elsevier B.V. All rights reserved
Análise matemática: unidades teórico-práticas
No full textEste livro está organizado em torno das unidades de trabalho que constituem a(s) disciplina(s) de Análise Matemática, na sua vertente teórico-prática.
A par de um conjunto de exercícios representativos, são apresentadas algumas resoluções com a finalidade de ajudar o aluno a melhor se enquadrar na matéria proposta e, simultaneamente, oferecer-lhe a possibilidade de tomar contacto com o elaborar correcto e preciso de uma resposta
Use of interferometric techniques for measuring the displacement field in the plane of a part-through crack existing in a plate
No full textAn experimental procedure based on holographic interferometry coupled with computer-aided image processing techniques was used to estimate the stress intensity factor (SIF) of a part-through crack in a plate. The loading consisted of a uniform remote tensile stress, leading to an opening and edge rotation displacement field along the crack plane. The evaluation of the crack opening displacements and edge rotations, carried out after the numerical interpretation of the fringe pattern, made possible an estimation of the SIF along the crack line, following the theoretical basis of the Line-Spring Model developed by Rice and Levy.(nao teve
The modal analysis of a pipe elbow with realistic boundary conditions
No full textA vibration analysis for the determination of the natural frequencies and the associated eigenmodes of a pipe elbow with end-flanges or tangent terminations was performed. A numerical investigation of this problem was achieved with a semi-analytic definition finite ring element and a commercial finite element code. To assess the accuracy of the numerical solution for the elbow vibration, an experimental modal analysis was performed on a curved and on a straight pipe. The responses were processed by a data acquisition system which performs a fast Fourier transform on the time histories to convert them from a time to frequency domain, these leading to the extraction of natural frequencies and mode shapes associated with the test-specimen. The results were compared with the corresponding ones from the numerical approach and discussion about the results completes the paper.(Not applicable
Java environment for a JUMPtec WebToNet embedded system
No full textCurrent automation applications are often based in embedded
systems with limited resources. However, even with these constraints,
they often require Internet connectivity and fast programming
processes. This paper presents a demonstrator based on a Jumptec
system built to show the possibility of using Java as the programming
language for small embedded systems. The steps to integrate the Java
virtual machine in the system are described and the simplicity of
developing programs for the target system is shown. Also, a small
application example with real world I/O is briefly presented
Precise solutions of the one-dimensional Monge-Kantorovich problem
No full textThe Monge-Kantorovich problem on finding a measure realizing the transportation of mass from ℝ to ℝ at minimum cost is considered. The initial and resulting distributions of mass are assumed to be the same and the cost of the transportation of the unit mass from a point x to y is expressed by an odd function f(x - y) that is strictly concave on ℝ+. It is shown that under certain assumptions about the distribution of the mass the optimal measure belongs to a certain family of measures depending on countably many parameters. This family is explicitly described: it depends only on the distribution of the mass, but not on f. Under an additional constraint on the distribution of the mass the number of the parameters is finite and the problem reduces to the minimization of a function of several variables. Examples of various distributions of the mass are considered