Browsing by Author "Livramento, Vanessa"
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- Blistering of W–Ta composites at different irradiation energiesPublication . Mateus, R.; Dias, Marta; Lopes, J.; Rocha, J.; Catarino, N.; Duarte, P.; Gomes, R. B.; Silva, C.; Fernandes, H.; Livramento, Vanessa; Carvalho, Patricia Almeida; Alves, E.; Hanada, K.; Correia, J.B.Pure tungsten and tantalum plates and tungsten–tantalum composites produced via mechanical alloying and spark plasma sintering were bombarded with He+ and D+ energetic ion beams and deuterium plasmas. The aim of this experiment is to study the effects caused by individual helium and deuterium exposures and to evidence that the modifications induced in the composites at different irradiation energies could be followed by irradiating the pristine constituent elements under the same experimental conditions, which is relevant considering the development of tailored composites for fusion applications. Higher D retentions, especially in tungsten, and superficial blistering are observed in both components after helium exposure. The blistering is magnified in the tantalum phase of composites due to its higher ductility and to water vapour production under deuterium irradiation. At lower irradiation energies the induced effects are minor. After plasma exposure, the presence of tantalum does not increase the D content in the composites.
- Caracterização de fases de carbono nano estruturado em compósitos de cobre/carbonoPublication . Nunes, D.; Livramento, Vanessa; Correia, J.B.; Almeida Carvalho, Patricia; Alves, L. C.; Vilarigues, M.
- Characterization of Cu2ZnSn(SSe)4 monograin powders by FE-SEMPublication . Neves, Filipe; Livramento, Vanessa; Martins, Isabel M.; Esperto, Luís; Santos, Mário J. G.; Correia, J.B.; Muska, K.; Holopainen, T.The design and synthesis of high-efficiency materials to convert solar to electrical energy is an increasingly important research field. Within the photovoltaic technologies, crystalline Si have an 80% share while the remaining 20% are mostly thin film solar cells based on Cu(In,Ga)(S,Se)2 (CIGSSe) and CdTe [1,2].
- Characterization of Cu2ZnSn(SSe)4 monograin powders by FE-SEMPublication . Neves, Filipe; Livramento, Vanessa; Martins, Isabel M.; Esperto, Luís; Santos, Mário J. G.; Correia, J.B.; Muska, K.; Holopainen, T.The design and synthesis of high-efficiency materials to convert solar to electrical energy is an increasingly important research field. Within the photovoltaic technologies, crystalline Si have an 80% share while the remaining 20% are mostly thin film solar cells based on Cu(In,Ga)(S,Se)2 (CIGSSe) and CdTe [1,2]. However,the cost, the abundance and the environmental impact of the elemental components cannot be neglected. For these reasons, Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe)and their solid solutions CZTSSe has attracted much attention recently since they can provide the development of cost competitive solar cells. The CZTS-based solar cells consist of earth abundant and relatively inexpensive elements and represent an environmentally friendly alternative compared to the above mentioned systems [3]. The energy conversion efficiency of the CZTS-based solar cells has increased from 0.66% in 1996 to 11.1% recently [4].
- Effects of helium and deuterium irradiation on SPS sintered W–Ta composites at different temperaturesPublication . Mateus, R.; Dias, Marta; Lopes, J.; Rocha, J.; Catarino, N.; Franco, N.; Livramento, Vanessa; Almeida Carvalho, Patricia; Correia, J.B.; Hanada, K.; Alves, E.Energetic He+ and D+ ions were implanted into different W–Ta composites in order to investigate their stability under helium and deuterium irradiation. The results were compared with morphological and chemical modifications arising from exposure of pure W and Ta. Special attention was given to tantalum hydride (Ta2H)formation due to its implications for tritium inventory. Three W–Ta composites with 10 and 20 at.% Ta were prepared from elemental W powder and Ta fibre or powder through low-energy ball milling in argon atmosphere. Spark plasma sintering (SPS) was used as the consolidation process in the temperature range from 1473 to 1873 K. The results obtained from pure elemental samples and composites are similar. However, Ta2H is easily formed in pure Ta by using a pre-implantation stage of He+, whereas in W–Ta composites the same reaction is clearly reduced, and it can be inhibited by controlling the sintering temperature.
- Elemental interdiffusion in W-Ta composites developed for fusion applicationsPublication . Mateus, R.; Dias, Marta; Livramento, Vanessa; Nunes, D.; Almeida Carvalho, Patricia; Hanada, K.; Correia, J.B.Tungsten (W) was select for an extensive use in nuclear fusion devices due to its low neutron activation, high melting point and sputtering threshold as well as low hydrogen inventory. Nevertheless, W is brittle at low and moderate temperatures, which results in abnormal thermal stress, component fracture and extra erosion under reactor operation due to inherent thermal cycling events. An attractive way to solve these problems involves the addition of other refractory metals in the W matrix and tantalum (Ta) is a natural candidate. It has a high ductility, toughness and radiation resistance relative to those of W and transmutes to W by high-energy neutron irradiation. Recently, IST proposed the production of W-Ta composite by mechanical synthesis.
- Multiscale Copper-uDiamond Nanostructured CompositesPublication . Nunes, D.; Livramento, Vanessa; Fernandes, H.; Silva, C.; Shohoji, Nobumitsu; Correia, J.B.; Carvalho, Patricia AlmeidaNanostructured copper-diamond composites can be tailored for thermal management applications at high temperature. A novel approach based on multiscale diamond dispersions is proposed for the production of this type of materials: a Cu-nDiamond composite produced by high-energy milling is used as a nanostructured matrix for further dispersion of micrometer sized diamond. The former offers strength and microstructural thermal stability while the latter provides high thermal conductivity. A series of Cu-nDiamond mixtures have been milled to define the minimum nanodiamond fraction suitable for matrix refinement and thermal stabilization. A refined matrix with homogenously dispersed nanoparticles could be obtained with 4 at.% nanodiamond for posterior mixture with ƒÝDiamond and subsequent consolidation. In order to define optimal processing parameters, consolidation by hot extrusion has been carried out for a Cu-nDiamond composite and, in parallel, for a mixture of pure copper and ÝDiamond. The materials produced were characterized by X-ray diffraction, scanning and transmission electron microscopy and microhardness measurements.
- Novel approach to plasma facing materials in nuclear fusion reactorsPublication . Livramento, Vanessa; Correia, J.B.; Nunes, D.; Carvalho, Patricia Almeida; Fernandes, H.; Silva, C.; Hanada, K.; Shohoji, Nobumitsu; Osawa, E.A novel material design in nuclear fusion reactors is proposed based on W-nDiamond nanostructured composites. Generally, a microstructure refined to the nanometer scale improves the mechanical strength due to modification of plasticity mechanisms. Moreover, highly specific grainboundary area raises the number of sites for annihilation of radiation induced defects. However, the low thermal stability of fine-grained and nanostructured materials demands the presence of particles at the grain boundaries that can delay coarsening by a pinning effect. As a result, the concept of a composite is promising in the field of nanostructured materials. The hardness of diamond renders nanodiamond dispersions excellent reinforcing and stabilization candidates and, in addition, diamond has extremely high thermal conductivity. Consequently, W-nDiamond nanocomposites are promising candidates for thermally stable first-wall materials. The proposed design involves the production of WAV-nDiamondAV-Cu/Cu layered castellations. The W, W-nDiamond and W-Cu layers are produced by mechanical alloying followed by a consolidation route that combines hot rolling with spark plasma sintering (SPS). Layer welding is achieved by spark plasma sintering. The present work describes the mechanical alloying processsing and consolidation route used to produce W-nDiamond composites, as well as microstructural features and mechanical properties of the material produced Long term plasma exposure experiments are planned at ISTTOK and at FTU (Frascati).
- Synthesis of FeTi hydrogen storage material via ball milling: effect of milling energy and atmosphere.Publication . Livramento, Vanessa; Rangel, C. M.; Correia, J.B.; Shohoji, Nobumitsu; Silva, R. A.Attempts were made earlier to synthesize and activate the FeTi intermetallic during ball milling (BM), for H2 storage using sodium boron tetra-hydride (NaBH4) additive as a process controlling agent. Simple reactive milling starting from Fe and Ti powders resulted in heavy agglomeration of powders, due to the self sustaining nature of the reaction following an incubation period. When NaBH4 was used as the process control agent to avoid agglomeration, this resulted in the production of titanium hydride besides FeTi, and as a consequence unfavorable irreversibility in the subsequent hydrogen charging/discharging cycles [1,2,12]. The present work reports on modifications introduced in the synthesis process by changing two processing parameters, namely the milling energy and atmosphere composition
- Tungsten–nanodiamond composite powders produced by ball millingPublication . Nunes, D.; Livramento, Vanessa; Mardolcar, U. V.; Correia, J.B.; Carvalho, Patricia AlmeidaThe major challenge in producing tungsten–nanodiamond composites by ball milling lies in successfully dispersing carbon nanoparticles in the metallic matrix while keeping carbide formation at a minimum. Processing windows for carbide minimization have been established through systematic variation of the nanodiamond fraction, milling energy and milling time. Materials characterization has been carried out by X-ray diffraction, scanning and transmission electron microscopy and microhardness testing. Nanostructured matrices with homogeneously dispersed particles that preserved the diamond structure have been produced. Differential thermal analysis has been used to evaluate the composites thermal stability.