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- Aerogel cathodes for electrochemical CO2 reduction [Comunicação oral]Publication . Messias, Sofia; Fialho, Maria T.; Paninho, A. B.; Branco, Luis C; Nunes, A. V. M.; Martins, Rodrigo; Mendes, Manuel Joao; Nunes, D.; Rangel, C. M.; Machado, AnaABSTRACT: Electrochemical reduction of carbon dioxide powered by renewable energy to produce fuels and chemicals is a technology with potential to contribute to an economy based on a carbon neutral cycle. The development of cost effective, highly active and stable catalysts for CO2 electroreduction is being intensively researched. This work addresses the development of aerogel supported copper-zinc bimetallic catalysts[1]. Aerogels are substances with exceptional properties with many current and potential applications [2-3]. Due to their high surface area, stability in corresponding gaseous or liquid phases, transport through large meso and macropores they are especially suited as catalysts and carrier materials for catalysis and, when electric conductive for electro-catalysis. Aerogels prepared by the sol gel method and impregnated with metallic particles will be tested as cathodes for the co-electrolysis of CO2 and water to produce syngas at temperatures near room temperature and high-pressure. In this way this process can be directly coupled to other high pressure processes, such as Fischer-Tropsch that use high pressure syngas as raw material. Productivities and faradaic efficiencies will be evaluated. The characterization of the aerogel-based cathodes will be undertaken by surface analysis techniques. BET surface areas will be determined. The catalytic cathodes will be tested in an ionic liquid-based electrolyte as a way to increase current densities, due to the high CO2 solubilities exhibited by some ionic liquid families.
- Nanodiamond dispersions in nanostructured metalsPublication . Nunes, D.; Correia, J.B.; Carvalho, Patricia Almeida
- Tungsten microstructural changes induced by ISTTOK plasma dischargesPublication . Mateus, R.; Carvalho, Patricia Almeida; Correia, J.B.; Nunes, D.; Gomes, R. B.; Duarte, P.; Fernandes, H.; Silva, C.; Alves, E.
- 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.
- 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.
- Nickel–carbon nanocomposites: Synthesis, structural changes and strengthening mechanismsPublication . Nunes, D.; Vilarigues, M.; Correia, J.B.; Carvalho, Patricia AlmeidaThe present work investigates Ni–nanodiamond and Ni–graphite composites produced by mechanical synthesis and subsequent heat treatments. Processing of nickel–carbon nanocomposites by this powder metallurgy route poses specific challenges, as carbon phases are prone to carbide conversion and amorphization. The processing window for carbide prevention has been established through X-ray diffraction by a systematic variation of the milling parameters. Transmission electron microscopy confirmed the absence of carbide and showed homogeneous particle distributions, as well as intimate bonding between the metallic matrix and the carbon phases. Ring diffraction patterns of chemically extracted carbon phases demonstrated that milled nanodiamond preserved crystallinity, while an essentially amorphous nature could be inferred for milled graphite. Raman spectra confirmed that nanodiamond particles remained largely unaffected by mechanical synthesis, whereas the bands of milled graphite were significantly changed into the typical amorphous carbon fingerprint. The results on the annealed nanocomposites showed that milling with Ni accelerated graphitization of the carbon phases during heat treatments at 973 and 1073 K in both composites. At the finer scales, the nanocomposites exhibited a remarkable microhardness enhancement (∼70%) compared with pure nanostructured nickel. The Hall–Petch relation and the Orowan–Ashby equation are used to discuss strengthening mechanisms and the load transfer ability to the reinforcing particles.
- Tuning cathode porosity for electrochemical reduction of CO2 at high pressure [Resumo]Publication . Messias, Sofia; Fialho, Maria T.; Paninho, A. B.; Nunes, A. V. M.; Branco, Luis C; Nunes, D.; Martins, Rodrigo; Mendes, Manuel Joao; Rangel, C. M.; Machado, AnaABSTRACT: The development of active and stable catalytic cathodes is critical for advancing electrochemical carbon dioxide reduction into fuels and chemicals from Lab to market. This is a technology with a high potential to contribute to combat climate changes by using captured CO2, water and renewable energy [1]. The use of pressures higher than atmospheric pressure to carry out the co-electrolysis of CO2 and water has been recognized as an important process intensification parameter to increase productivities and energy efficiency [2]. Ongoing work addresses the preparation of aerogels by the sol gel method and impregnation with zinc and copper metallic particles to be used as cathodes for the co-electrolysis of CO2 and water to produce syngas at temperatures near room temperature and high-pressure. Ionic liquid-based electrolytes are used to increase CO2 concentration at the surface of the electrode and consequently productivities, as some ionic liquid families are known to solubilize high amounts of CO2. Aerogels have been investigated for many different applications including as catalyst supports, due to their high surface area, stability in gaseous or liquid phases, and efficient transport through large meso and macropores. The present work reports a strategy to tune the pore sizes of the catalytic electrodes by the use of reticulating agents and supercritical CO2 drying. Productivities and faradaic efficiencies of the porous materials with the different reticulating agents are compared and interpreted in respect to their surface characterization e.g. BET surface areas and morphologies determined by SEM. The potential of new aerogel-based catalytic cathodes on the efficiency of the electrochemical CO2 reduction will be discussed and its impact in fostering supercritical fluids technology through its use in processes for the mitigation of climate changes.
- Microstructure characterization of ODS-RAFM steelsPublication . Mateus, R.; Carvalho, P.; Nunes, D.; Alves, L. C.; Franco, N.; Correia, J.B.; Fernandes, H.; Silva, C.; Alves, E.; Lindau, R.Results of the microstructural characterization of four different RAFM ODS Eurofer 97 batches are presented and discussed. Analyses and observations were performed by nuclear microprobe and scanning and transmission electron microscopy. X-ray elemental distribution maps obtained with proton beam scans showed homogeneous composition within the proton beam spatial resolution and, in particular, pointed to a uniform distribution of ODS (yttria) nanoparticles in the Eurofer 97 matrix. This was confirmed by transmission electron microscopy. Scanning electron microscopy coupled with energy dispersive spectroscopy made evident the presence of chromium carbide precipitation. Precipitates occurred preferentially along grain boundaries (GB) in three of the batches and presented a discrete distribution in the other, as a result of different thermo-mechanical routes. Additional electron backscattered diffraction experiments revealed the crystalline textures in the ferritic polycrystalline structure of the ODS steel samples.
- Microstructural characterization of the ODS Eurofer 97 EU-batchPublication . Mateus, R.; Carvalho, Patricia Almeida; Nunes, D.; Alves, L. C.; Franco, N.; Correia, J.B.; Alves, E.Four as-processed forms (Plate 16, Plate 6, Rod 20 and Rod 12.5) of the ODS Eurofer 97 EU-batch produced under different thermomechanical conditions have been investigated by scanning nuclear microprobe, scanning and transmission electron microscopy, energy dispersive X-ray spectroscopy, electron backscattered diffraction, hightemperature X-ray diffraction and microhardness measurements. The materials presented a ferritic microstructure with a homogeneous distribution of Y. The thicker plate presented a fine carbide dispersion while the other forms showed carbide morphologies corresponding to pseudo-pearlitic and pseudo-bainitic transformations with wellmatched hardness values. Hot rolling induced crystallographic textures of the {101}<101> type, rotary swaging resulted in a complex texture, and extrusion produced a strong <101> fiber texture. X-ray diffraction experiments at high temperature showed that at a cooling rate of 5 ºC/min the complete austenite-to-ferrite transformation occurs between 760 and 750 ºC compromising the material quenchability.
- Production of Cu/Diamond composites for first-wall heat sinksPublication . Nunes, D.; Correia, J.B.; Carvalho, Patricia Almeida; Shohoji, Nobumitsu; Silva, C.; Fernandes, H.; Alves, L. C.; Hanada, K.; Osawa, E.Due to their suitable thermal conductivity and strength copper-based materials have been considered appropriate heat sinks for first wall panels in nuclear fusion devices. However, increased thermal conductivity and mechanical strength are demanded and the concept of property tailoring involved in the design of metal matrix composites advocates for the potential of nanodiamond dispersions in copper. Copper-nanodiamond composite materials can be produced by mechanical alloying followed by a consolidation operation. Yet, this powder metallurgy route poses several challenges: nanodiamond presents intrinsically difficult bonding with copper; contamination by milling media must be closely monitored; and full densification and microstructural homogeneity should be obtained with consolidation. The present line of work is aimed at an optimization of the processing conditions of Cu-nanodiamond composites. The challenges mentioned above have been addressed, respectively, by incorporating chromium in the matrix to form a stable carbide interlayer binding the two components; by assessing the contamination originating from the milling operation through particle-induced X-ray emission spectroscopy; and by comparing the densification obtained by spark plasma sintering with hot-extrusion data from previous studies.