309 resultados para Sheffield


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Introduction. Besides technical and economic-organisational problems, digital preservation also faces legal issues, especially regarding copyright legislation, since all digital preservation strategies involve copying materials and/or using software which is typically copyrighted. The purpose of this paper is to ascertain the extent to which current copyright laws meet the preservation requirements of library materials..Methodology. A cross-sectional analysis of recently updated national copyright laws as well as the impact of the other two protection methods: contractual and technological.Results. Even after the latest updates current copyright legislation is almost useless for digital preservation activities since the opportunities provided by WIPO Copyright Treaty to adapt and extend copyright exceptions and limitations have been used to the full.Conclusion. We need a legislation reform that will make it possible to carry out all required copying and communication activities and software use, even if circumventing technological protection is needed. But that is not enough for licensed works. The best solution for this kind of work is to include specific clauses in the licences that facilitate preservation activities. Thus, cooperation between both parties, libraries and rights' holders, is essential.

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Antimony based glasses have been investigated for the first time regarding the possibility of holographic data storage using visible lasers sources. Changes in both refractive index and the absorption coefficient were measured using a holographic setup. The modulation of the optical constants is reversible by heat treatment. Bragg gratings were written under visible light of an Ar laser and erased thermally.

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The crystal nucleation rates of a metastable phase (chi) on the surface of a near stoichiometric cordierite glass were determined for temperatures between 839 and 910 degrees C (T-g similar to 800 degrees C). The surface nucleation kinetics of that phase on our glass, as well as on a stoichiometric glass (2 MgO-2Al(2)O(3)-5SiO(2)) studied by other authors, were analysed in terms of the classical nucleation theory; for the first time. It was shown that the effective interfacial energy for surface nucleation is substantially lower than that for homogeneous volume nucleation in silicate glasses, vindicating the assumption of heterogeneous nucleation on free glass surfaces. The average wetting angle between the nucleating crystals and the active solid particles was estimated to be around 46 degrees C. The pre-exponential constant was several orders of magnitude higher than the theoretical values as found for volume homogeneous nucleation in oxide glasses.

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The solar events that occurred at the end of October 2003 gave rise to very strong geomagnetic disturbances that peaked twice with Dst values reaching -345 nT around 0000 UT on 30 October and -400 nT around 2300 UT, on the same day. Disturbances in several ionospheric parameters were observed over Brazil. This work will focus on the ionospheric response to the initial westward prompt penetration electric field and on the strong intensification of the equatorial ionization anomaly that occurred because of the electric field polarity reversal that followed in the early morning hours of 29 October. The F layer peak height over the equator first decreased under the strong prompt penetration westward electric field, which was followed by significant height increase under eastward electric field. We have used Sheffield University Plasmasphere Ionosphere Model (SUPIM) with an intensified westward disturbed electric field in the presunrise hours, presumably due to prompt penetration from the magnetosphere, in order to study the effect of such a field in the ionosphere. The simulation results showed that prompt penetration of magnetospheric electric fields of westward polarity to the nightside equatorial region seems to be the most probable cause of the initial F layer height decreases. The intensification of the equatorial ionization anomaly and the unusual enhancement on F layer peak density, which was not modeled by the SUPIM, are explained as caused by the strong eastward electric field that followed the initial phase in combination with a highly variable disturbed meridional/transequatorial wind system as inferred from the F2 layer peak height variations. The highly dynamic wind pattern, with a short-term response (2-4 hours), is compatible with the predictions of some previous theoretical model calculations reported in the literature.

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The development of Te containing nanoscaled, compositionally modulated Te/As2S3 multilayer structures enables the realisation of optical recording in real time with a CW He-Ne laser illumination (lambda=633 nm) with improved recording parameters in comparison with known Se/As2S3 structures. It is established that the efficiency of the recording process depends on the light stimulated interdiffusion and thickness of sub-layers.

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Verifies the applicability to research on indexers' reading strategies of the process observing technique known as Verbal Protocol or Thinking Aloud. This interpretative-qualitative data collecting technique allows the observation of different kinds of process during the progress of different kinds of tasks. Presents a theoretical investigation into reading and into formal methodological procedures to observe reading processes. Describes details of the methodological procedures adopted in five case studies with analysis of samples of data. The project adopted three kinds of parameters for data analysis: theoretical, normative, empirical (derived from observations made in the first case study). The results are compared, and important conclusions regarding documentary reading are drawn.

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Cooperative energy transfer upconversion luminescence is investigated in Tb(3+)/Yb(3+)-codoped PbGeO(3)-PbF(2)-CdF(2) glass-ceramic and its precursor glass under resonant and off resonance infrared excitation. Bright UV-visible emission signals around 384, 415, 438 nm, and 473-490, 545, 587, and 623 nm are identified as due to the (5)D(3)((5)G(6))->(7)F(1) (J=6,5,4) and (5)D(4)->(7)F(1) (J=6,5,4,3) transitions, respectively, and readily observed. The results indicate that cooperative energy transfer between ytterbium and terbium. ions followed by excited state absorption are the dominant upconversion excitation mechanisms involved. Comparison of the upconversion process in a glass-ceramic sample and its glassy precursor revealed that the former present much higher upconversion efficiency. The dependence of the upconversion emission upon pump power, temperature, and doping content is also examined.

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Energy transfer excited multiwavelength visible upconversion emission and white light generation is described in a single sample of PbGeO(3)-PbF(2)-CdF(2) glass-ceramic triply doped With Ho/Tm/Yb under single infrared laser excitation. Blue (475 nm), green (540 mn), and red (650 nm), upconversion luminescence signals are generated, and the emissions are assigned, respectively, to thulium ((1)G(4)-(3)H(6)), and holmium ((5)S(2);(5)F(4)) -> (5)I(8), (5)F(5) -> (5)I(8)) ions transitions, both excited via successive energy transfers from ytterbium ions. It is experimentally shown that with a proper combination of the rare earth ions contents, white light may be produced, with the simultaneous generation of fluorescence with controllable intensities at the wavelengths of the three primary colours in a single sample and using a single near-infrared excitation source.

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Based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory, we propose two new models to describe the crystallisation kinetics of glass particles and use them to determine the density of nucleation sites, N(s), on glass powders. We tested these models with sintered compacts of diopside glass particles using sinter-crystallisation treatments at 825 degrees C (T(g)similar to 727 degrees C), that covered from null to almost 100% crystallised volume time fraction. We measured and compared the evolution of the crystallised volume fractions by optical microscopy and x-ray diffraction. Then we fit our expressions to experimental data using Ns and R (the average particle radius) as adjustable parameters. For comparison, we also fit to our data existing expressions that describe the crystallised volume fraction in glass powders. We demonstrate that all the methods allow one to estimate N(s) with reasonable accuracy. For our ground and water washed diopside glass powder, N(s) is between 10(10)-10(11) sites.m(-2). The reasonable agreement between experimental and adjusted R confirms the consistency of all five models tested. However, one of our equations does not require taking into account the change of crystallisation mode from 3-dimensional to 1-dimensional, and this is advantageous.

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)