Recursos de colección

Estudo Geral - Universidade de Coimbra (19.630 recursos)

ESTUDO GERAL é a designação do repositório digital da produção científica da Universidade de Coimbra, com o objectivo de divulgar conteúdos digitais de natureza científica de autores ligados à Universidade de Coimbra. A sua criação insere-se no movimento de Acesso Livre à Literatura científica (Open Access), ao qual o Conselho de Reitores das Universidades Portuguesas aderiu em 2006 e que a Universidade de Coimbra subscreveu. À semelhança de outras grandes universidades nacionais e internacionais, a UC tem o maior interesse em aumentar a sua presença na rede informática mundial, sendo cada vez mais - e também por essa via - um emissor de conhecimento e cultura.

FCTUC Física - Artigos em Revistas Internacionais

Mostrando recursos 1 - 20 de 363

  1. Nonempirical density functionals investigated for jellium: Spin polarized surfaces, spherical clusters, and bulk linear response

    Tao, Jianmin; Perdew, John P.; Almeida, Luís Miguel; Fiolhais, Carlos; Kümmel, Stephan
    Jellium, a simple model of metals, is a standard testing ground for density functionals both for bulk and for surface properties. Earlier tests show that the Tao–Perdew–Staroverov–Scuseria (TPSS) nonempirical metageneralized gradient approximation (meta-GGA) for the exchange-correlation energy yields more accurate surface energies than the local spin density (LSD) approximation for spin-unpolarized jellium. In this study, work functions and surface energies of a jellium metal in the presence of “internal” and external magnetic fields are calculated with LSD, Perdew–Burke–Ernzerhof (PBE) GGA, and TPSS meta-GGA and its predecessor, the nearly nonempirical Perdew–Kurth–Zupan–Blaha meta-GGA, using self-consistent LSD orbitals and densities. The results show...

  2. Density functional theory study of the oxoperoxo vanadium(V) complexes of glycolic acid. Structural correlations with NMR chemical shifts

    Justino, Licínia L. G.; Ramos, M. Luísa; Kaupp, Martin; Burrows, Hugh D.; Fiolhais, Carlos; Gil, Victor M. S.
    The DFT B3LYP/SBKJC method has been used to calculate the gas-phase optimized geometries of the glycolate oxoperoxo vanadium(V) complexes [V2O2(OO)2(gly)2]2-, [V2O3(OO)(gly)2]2- and [VO(OO)(gly)(H2O)]-. The 51V, 17O, 13C and 1H chemical shifts have been calculated for the theoretical geometries in all-electron DFT calculations at the UDFT-IGLO-PW91 level and have been subsequently compared with the experimental chemical shifts in solution. In spite of being applied to the isolated molecules, the calculations allowed satisfactory reproduction of the multinuclear NMR solution chemical shifts of the complexes, suggesting that the theoretical structures are probably close to those in solution. The effects of structural changes on...

  3. Characterization of an energy dispersive X-ray fluorescence imaging system based on a Micropattern Gaseous Detector

    Silva, A. L. M.; Azevedo, C. D. R.; Oliveira, C. A. B.; Santos, J. M. F. dos; Carvalho, M. L.; Veloso, J. F. C. A.
    An Energy Dispersive X-ray Fluorescence (EDXRF) imaging system based on a Micropattern Gas Detector has already shown good results for di erent applications. An X-ray tube, a pinhole camera and a Micro-Hole and Strip Plate (MHSP) based detector are the main components of the experimental system. The detector uses an MHSP in a Xe atmosphere at 1 bar, acting as a photon counting device, i.e., it is capable to record each single event retaining the energy and the interaction position (2D-sensitive detector) information of the incident photon, demonstrating to be a promising device for EDXRF imaging applications. This work presents studies of energy resolution, energy linearity and spatial resolution/elemental...

  4. X-ray fluorescence (conventional and 3D) and scanning electron microscopy for the investigation of Portuguese polychrome glazed ceramics: Advances in the knowledge of the manufacturing techniques

    Guilherme, A.; Coroado, J.; Santos, J. M. F. dos; Lühl, L.; Wolff, T.; Kanngießee, B.; Carvalho, M. L.
    This work shows the first analytical results obtained by X-Ray Fluorescence (XRF) (conventional and 3D) and Scanning Electron Microscopy with Energy Dispersive System (SEM-EDS) on original Portuguese ceramic pieces produced between the 16th and 18th centuries in Coimbra and Lisbon. Experts distinguished these productions based only on the color, texture and brightness, which originates mislabeling in some cases. Thanks to lateral and spatial resolution in the micrometer regime, the results obtained with μ-XRF were essential in determining the glaze and pigment thicknesses by monitoring the profile of the most abundant element in each “layer”. Furthermore, the dissemination of these elements throughout...

  5. Relativistic particle in a three-dimensional box

    Alberto, Pedro; Das, Saurya; Vagenas, Elias
    We generalize the work of Alberto, Fiolhais and Gil and solve the problem of a Dirac particle confined in a 3-dimensional box. The non-relativistic and ultra-relativistic limits are considered and it is shown that the size of the box determines how relativistic the low-lying states are. The consequences for the density of states of a relativistic fermion gas are briefly discussed.

  6. Mean-filed theories with mixed states and associated boson expansions

    Providência, J. da; Fiolhais, C.
    A variational derivation of the Liouville-von Neumann equation of quantum-statistical mechanics is presented, in order to formulate mean-field approximations appropriate to mixed states. The Hartree-Fock and the RPA at finite temperatures are particular cases of the general formalism. A thermal boson expansion is defined, which allows us to describe anharmonic motion around a thermal excited state. In a numerical application on the basis of the Lipkin model, temperature-dependent phase transitions are observed.

  7. The meteorological observations in Coimbra and the portuguese participation in weather forecast in europe

    Leonardo, António José F.; Martins, Décio R.; Fiolhais, Carlos
    In the early nineteenth century, regular meteorological observations started at the Faculty of Natural Philosophy of the University of Coimbra (FPUC). From 1854 to 1856 these observations were published in O Instituto, a journal of an academic society of the same name, founded in Coimbra in 1852. This new area of science aroused great interest, offering itself as unexplored territory waiting for scientific investigation. In reaction to the pioneering work at the Polytechnic School of Lisbon of Guilherme Pegado, who founded the first meteorological observatory in Portugal in 1854, the FPUC established a Meteorological and Magnetic Observatory in Coimbra. The main actor was, from 1863, the physicist Jacinto António...

  8. Thermodynamics at work : on the pressure derivative of the specific heat

    Güémez, J.; Fiolhais, C.; Fiolhais, M.
    Thermodynamics relates measurable quantities such as thermal coefficients and specific heats. The first law, which implies that the enthalpy is a function of state, yields a relation for the pressure derivative of the specific heat cP . The second law gives a simpler and well-known relation for this pressure derivative. We compare the values of the pressure derivative of cP obtained from the first and second laws to the values obtained from measurements for water at different pressures. The comparison illustrates the scope and methodology of thermodynamics.

  9. The Cartesian diver and the fold catastrophe

    Güémez, J.; Fiolhais, C.; Fiolhais, M.
    The motion of the Cartesian diver is studied, both theoretically and experimentally, and interpreted as an example of a fold catastrophe, where the control parameter is the external pressure.

  10. Science learning in virtual environments : a descriptive study

    Trindade, Jorge; Fiolhais, Carlos; Almeida, Leandro
    Usually, students learn more if the method of instruction matches their learning style. Since Physics and Chemistry deal with three-dimensional (3-D) objects, the ability to visualize and mentally manipulate shapes is very helpful in their learning. In fact, much of what Physics and Chemistry students know takes the form of images. However, little attention has been given to the pedagogical effectiveness of visual stimuli in those disciplines. Computers are being increasingly used as teaching tools. The new approaches include simulations, multimedia presentations and, more recently, virtual environments. Computer-based worlds are useful to visualize physical and chemical processes allowing for better conceptual understanding. Since 3-D virtual environments need to be explored...

  11. Revisiting Black’s Experiments on the Latent Heats of Water

    Güémez, J.; Fiolhais, C.; Fiolhais, M.

  12. Sadi Carnot on Carnot’s theorem

    Güémez, J.; Fiolhais, C.; Fiolhais, M.

  13. Visualizing molecules : on-line simulations and virtual reality

    Trindade, J.; Paiva, J. C.; Fiolhais, C.

  14. Effects of Lattice Structure in the Dynamics of Coupled Elements

    Ribeiro, André S.; Lind, Pedro G.
    We investigate the influence of lattice geometry in network dynamics, using a cellular automaton with nearest-neighbor interactions and two admissible local states. We show that there are significant geometric effects in the distribution of local states and in the distribution of clusters, even when the connection topology is kept constant. Moreover, we show that some geometric structures are more cohesive than others, tending to keep a given initial configuration. To characterize the dynamics, we determine the distributions of local states and introduce a cluster coefficient. The lattice geometry is defined from the number of nearest neighbors and their disposition in...

  15. A study of the magnetic structures of chromium-silicon solid solutions

    Costa, M. M. R.; Ziebeck, K. R. A.; Brown, P. J.
    The antiferromagnetic structures of Cr-Si solid solutions in the range 1.5-6.0 at.% Si have been studied by neutron powder diffraction. The results confirm that the commensurate antiferromagnetic structure is stabilised at around 1.8 at.% Si. Both the magnetic moment and the Neel temperature are found to decrease with increasing silicon content and tend to zero at about 7.2 at.% Si. There is some evidence that the magnetic electrons are of Eg symme-ry and it is suggested that the reduction of magnetic moment is due to the stabilisation of the I2g relative to the Eg electrons in the chromium band brought...

  16. The Secondary Scintillation of Rare Gases under the Influence of Magnetic Fields

    Ferreira, M. Fátima A.; Conde, C. A. N.; Campos, N. Ayres de; Gil, J. M. S. C.
    The influence of magnetic fields on the secondary scintillation of xenon and argon is studied. A small gas cell is used with excitation by alpha-particles and magnetic fields perpendicular to the direction of the electric fields. For reduced electric fields normally used in gas proportional scintillation counters the intensity of the secondary scintillation remains constant, within an accuracy of about ± 1 % for xenon and ± 2% for argon, when the magnetic field intensity varied from 0 to 0.4 Tesla. A discussion and interpretation of the experimental data obtained is presented. It is concluded that gas proportional scintillation counters...

  17. Einstein e o Prazer da Física: Passados Cem Anos, a Física Continua Divertida

    Fiolhais, Carlos

  18. A 1 GSPS VME data acquisition module

    Loureiro, C. F. M.; Combo, A. M. C. F.; Correia, C. M. B. A.; Varela, P.; Manso, M. E.; Varandas, C.; Serra, F.
    In the following a new data acquisition architecture is proposed allowing high sampling rates along with a large memory data buffer. The modular design allows up to four 250 MHz, 8-bit acquisition channels to operate in an interleaved way, achieving 1 GSPS. Each channel can acquire continuously up to 3 MBytes of data (or 12 MBytes when interleaved). Since several modules can coexist in an acquisition system, provision was made for several parallel operations, including trigger distribution and download of digital signal processing programs. The module will be used in a dedicated acquisition system for the reflectometry diagnostic on the...

  19. 3-Oxoandrosta-4,6-dien-17[beta]-yl 2-methyl-1H-imidazole-1-carboxylate and 3-oxo-5[alpha]-androst-17[beta]-yl 2-methyl-1H-imidazole-1-carboxylate: C-H...[pi] and [pi]-[pi] intermolecular interactions

    Silva, Manuela Ramos; Moreira, Vânia M.; Cardoso, Cláudia; Beja, Ana Matos; Salvador, Jorge A. R.
    The title compounds, C24H30N2O3, (I), and C24H34N2O3, (II), both contain an androstane backbone and a 2-methylimidazole-1-carboxylate moiety at the 17-position. Compound (I) contains two symmetry-independent molecules (denoted 1 and 2), while compound (II) contains just one molecule in the asymmetric unit. The C-C-O-C torsion angle that reflects the twisting of the 2-methylimidazole-1-carboxylate moiety from the mean steroid plane is 143.1 (2)° for molecule 1 of (I), 73.1 (3)° for molecule 2 of (I) and 86.63 (17)° for (II). The significance of this study lies in its observation of significant differences in both molecular conformation and supramolecular aggregation between the molecules...

  20. Conformation of 17-chloro-16-formylandrosta-5,16-dien-3[beta]-yl acetate and 17-chloroandrosta-5,16-dien-3[beta]-yl acetate

    Silva, Manuela Ramos; Moreira, Vânia M.; Cardoso, Cláudia; Beja, Ana Matos; Salvador, Jorge A. R.
    In the title compounds, C22H29ClO3, (I), and C21H29ClO2, (II), respectively, the B rings adopt a half-chair conformation and the D rings adopt an envelope conformation. A twist of the steroid skeleton of both compounds is observed. There is a positional disorder of the acetoxy group of (II), with the terminal atoms disordered over two positions with near equal occupancy. Quantum-mechanical ab initio calculations using a molecular orbital Hartree–Fock method were performed for the isolated molecules, thus allowing the distinction within the structural features of these two androstane derivatives of which characteristics are intrinsic to the molecules and which are due to packing effects. The skeletal twisting was found to be innate to...

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