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- Structural and Optical Characterization of Mechanochemically Synthesized CuSbS2† [Abstract]Publication . Esperto, Luís; Figueira, Isabel; Mascarenhas, João; Silva, Teresa; Correia, J.B.; Neves, FilipeABSTRACT: The present work describes experimental studies related to the characterization of CuSbS2 directly synthesized after 2 h of mechanochemical synthesis (MCS) at 340 rpm, starting from mixtures of elemental powders. X-ray diffraction (XRD) and UV-VIS-NIR spectroscopy were carried out to analyze the crystal structure, degree of crystallinity, crystallite size and optical properties of the mechanochemically synthesized CuSbS2 powders. Rietveld refinement was carried out using Diffrac. TOPAS (Bruker AXS). Thermal stability of the synthesized materials was evaluated by the vacuum thermal heat treatment of the mechanochemically synthesized CuSbS2 powders at 350 °C for 24 h. Furthermore, the CuSbS2 powders were also analyzed by field-emission scanning electron microscopy (FE-SEM), laser diffraction, and differential thermal analysis.
- Microwave versus Conventional Sintering of NiTi Alloys Processed by Mechanical AlloyingPublication . Teixeira, Rodolfo da Silva; Oliveira, R. V.; Rodrigues, Patricia Freitas; Mascarenhas, João; Neves, Filipe; Paula, Andersan dos SantosABSTRACT: The present study shows a comparison between two sintering processes, microwave and conventional sintering, for the manufacture of NiTi porous specimens starting from powder mixtures of nickel and titanium hydrogenation-dehydrogenation (HDH) milled by mechanical alloying for a short time (25 min). The samples were sintered at 850 degrees C for 15 min and 120 min, respectively. Both samples exhibited porosity, and the pore size results are within the range of the human bone. The NiTi intermetallic compound (B2, R-phase, and B19 ') was detected in both sintered samples through X-ray diffraction (XRD) and electron backscattering diffraction (EBSD) on scanning electron microscopic (SEM). Two-step phase transformation occurred in both sintering processes with cooling and heating, the latter occurring with an overlap of the peaks, according to the differential scanning calorimetry (DSC) results. From scanning electron microscopy/electron backscatter diffraction, the R-phase and B2/B19 ' were detected in microwave and conventional sintering, respectively. The instrumented ultramicrohardness results show the highest elastic work values for the conventionally sintered sample. It was observed throughout this investigation that using mechanical alloying (MA) powders enabled, in both sintering processes, good results, such as intermetallic formation and densification in the range for biomedical applications.
- Novel two-step processing route combining mechanical alloying and microwave hybrid sintering to fabricate dense La9.33Si2Ge4O26 for SOFCsPublication . Santos, Mário João dos; Alves, Cátia; Oliveira, Fernando Almeida Costa; Marcelo, Teresa; Mascarenhas, João; Cavaleiro, A.; Trindade, B.In this work, microwave hybrid sintering at 1300 and 1350 °C was carried out for densification of La9.33Si2Ge4O26 mechanically alloyed powder with apatite structure. The pellets sintered at these two temperatures present the same structure (apatite) with relative densities of 92 and 96%, respectively. Mechanical analysis performed on sintered materials revealed the following results: hardness of 7.1 and 8.0 GPa, Young's modulus of 122 and 133 GPa, yield strength of 1807 and 2073 MPa and fracture toughness of 1.5 and 1.0 MPa m1/2, respectively.
- Processamento de materiais termoelétricos baseados na tetraedrite usando minério de tetraedrite-tenantitePublication . Esperto, Luís; Figueira, Isabel; Mascarenhas, João; Salgueiro, Rute; Silva, Teresa; Lopes, E.B.; Gonçalves, António Pereira; Correia, J.B.; de Oliveira, Daniel Pipa Soares; Almeida, P.; Neves, FilipeRESUMO: O desenvolvimento e implementação de tecnologias com baixo potencial de aquecimento global e energeticamente eficientes requer uma contínua inovação na área dos materiais para a energia. Nesta estratégia insere-se a pesquisa sobre o processamento de materiais termoelétricos baseados na tetraedrite. No presente trabalho procede-se à avaliação da viabilidade da incorporação direta, sem qualquer pré-tratamento, de minério da série tetraedrite-tenantite no processamento de materiais à base de tetraedrite por síntese mecanoquímica. Para tal prepararam-se misturas de pós contendo diferentes rácios de amostras de tetraedrite sintética, também obtida por síntese mecanoquímica, e de amostras de minério, recolhidas na mina de Neves-Corvo e na escombreira da mina do Barrigão. A caraterização estrutural e microestrutural dos materiais processados após 1 h e 2 h de síntese mecanoquímica permitiu verificar a formação de um composto constituído por tetraedrite-tenantite-(Fe) como fase principal, independentemente do minério de origem. Estes resultados são uma indicação do sucesso da abordagem seguida.
- Scale up of microwave annealed FA0.83Cs0.17PbI1.8Br1.2 perovskite towards an industrial scale [Comunicação oral]Publication . Mascarenhas, João; Barreiros, M. Alexandra; Brites, Maria JoãoABSTRACT: Perovskite solar cells (PSCs) efficiency has rapidly increased from the initial 2009's 3.8 to recent 22.7%. This high efficiency has attracted serious attention of the researchers and industry worldwide also due to their low material cost, and simple soluction-based fabrication process.
- Charge transport and recombination of dye sensitized 1D nanostructured-TiO2 films prepared by reactive sputteringPublication . Sequeira, S.; Lobato, K.; Torres, Erica; Brites, Maria João; Barreiros, M. Alexandra; Mascarenhas, JoãoDye sensitized solar cells (DSCs) are governed by light absorption, charge injection, electron transport and recombination and electrolyte diffusion. One way to improve the efficiency of these devices is by the design of highly ordered nanostructured semiconductor materials.The advantages can be two-fold: Firstly charge transport within the metal-oxide can be enhanced and hence thicker films can be employed and secondly, the complete permeation with a solid-state hole-transport medium of the sensitized metal-oxide can be facilitated. Nanostructured materials should promote vectorial electron diffusion and have as few recombination sights as possible so as to further enhance electron lifetimes and electron collection efficiencies. These materials should also have a high surface area so as to allow for efficient dye-loading and hence light absorption. Highly ordered TiO2 nanostructured films were prepared by reactive sputtering and their charge transport characteristics evaluated in DSCs. These were compared to DSCs employing mesoporous TiO2 films prepared by doctor blade technique using commercial paste. Charge transport characteristics were evaluated by impedance spectroscopy (IS), incident photon to current conversion efficiencies (IPCE) and current-voltage (iV) curves under simulated AM1.5G irradiation. Film morphology and structural properties were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively.
- Structural and optical characterization of mechanochemically synthesized CuSbS2 compoundsPublication . Esperto, Luís; Figueira, Isabel; Mascarenhas, João; Silva, Teresa; Correia, J.B.; Neves, FilipeABSTRACT: One of the areas of research on materials for thin-film solar cells focuses on replacing In and Ga with more earth-abundant elements. In that respect, chalcostibite (CuSbS2) is being considered as a promising environmentally friendly and cost-effective photovoltaic absorber material. In the present work, single CuSbS2 phase was synthesized directly by a short-duration (2 h) mechanochemical-synthesis step starting from mixtures of elemental powders. X-ray diffraction analysis of the synthesized CuSbS2 powders revealed a good agreement with the orthorhombic chalcostibite phase, space group Pnma, and a crystallite size of 26 nm. Particle-size characterization revealed a multimodal distribution with a median diameter ranging from of 2.93 mu m to 3.10 mu m. The thermal stability of the synthesized CuSbS2 powders was evaluated by thermogravimetry and differential thermal analysis. No phase change was observed by heat-treating the mechanochemically synthesized powders at 350 degrees C for 24 h. By UV-VIS-NIR spectroscopy the optical band gap was determined to be 1.41 eV, suggesting that the mechanochemically synthesized CuSbS2 can be considered suitable to be used as absorber materials. Overall, the results show that the mechanochemical process is a viable route for the synthesis of materials for photovoltaic applications.
- Effect of particle size of starting oxide powders on the performance of doped-lanthanum oxyapatite produced by mechanical alloying followed by microwave sinteringPublication . Oliveira, Fernando Almeida Costa; Marcelo, Teresa; Alves, Cátia; Santos, Mário João dos; Mascarenhas, João; Trindade, B.La9.33Si2Ge4O26 oxyapatite powders were synthesized at room temperature through mechanical alloying of La2O3, GeO2 and SiO2 precursor powders with different particle sizes as well as crystal structure in the case of silica powder (crystalline/amorphous). The mechanical alloyed mixtures were subsequently sintered by microwave heating at 1350 C for 1 h in order to obtain dense and homogeneous materials. All sintered materials consisted of the target apatite phase although minor amounts of secondary phases (e.g.La4GeO8) were also present only in samples obtained from micrometric SiO2 powders with a crystalline structure. The microstructure of the materials obtained from nanometric SiO2 with an amorphous structure was found to be more homogenous than the ones obtained from micrometric/crystalline silica. The mechanical behavior of the samples was slightly dependent on the particle size of the precursors and on the SiO2 crystallinity.
- Dye assessment in nanostructured TiO2 sensitized films by microprobe techniques [Poster]Publication . Barreiros, M. Alexandra; Mascarenhas, João; Corregidor, V.; Alves, L. C.; Guimarães, Fernanda; Torres, Erica; Brites, Maria JoãoDye sensitized solar cells (DSCs) have received considerable attention once this technology offers economic and environmental advantages over conventional photovoltaic (PV) devices. The PV performance of a DSC relies on the characteristics of its photoanode, which typically consists of a nanocrystalline porous TiO2 film, enabled with a large adsorptive surface area. Dye molecules that capture photons from light during device operation are attached to the film nanoparticles. The effective loading of the dye in the TiO2 electrode is of utmost importance for controlling and optimizing solar cell parameters. Relatively few methods are known today for quantitative evaluation of the total dye adsorbed on the film. In this work, a new approach combining microprobe techniques namely, Ion Beam Analytical (IBA) techniques using a micro-ion beam (Rutherford Backscattering Spectrometry (RBS) and Particle Induced X-ray Emission (PIXE)) and Electron Probe Micro-Analysis (EPMA) was carried out to assess dye distribution and depth profile in TiO2 films and the dye load based on Ru/Ti mass ratio. Different 1D nanostructured TiO2 films were prepared, morphologically characterised by SEM, sensitized and analysed by the referred techniques. Dye load evaluation in different TiO2 films by three different techniques (PIXE, RBS and EPMA/ wavelength dispersive spectrometry (WDS)) provided similar results of Ru/Ti mass fraction ratio. Moreover, it was possible to assess dye surface distribution and its depth profile, by means of Ru signal, and to visualise the dye distribution in sample cross-section through X-ray mapping by EPMA/ energy dispersive spectrometry (EDS). PIXE maps of Ru and Ti indicated an homogeneous surface distribution. The assessment of ruthenium depth profile by RBS showed that some films have homogeneous Ru depth distribution while others present different Ru concentration in the top layer (2 ìm thickness). These results are consistent with the EPMA/EDS maps obtained. EPMA (WDS and EDS) together with IBA techniques proved to be powerful tools for functional materials characterisation and provided very promising results in the study of nanostructured TiO2 sensitized films.
- Effect of vanadium and carbon content on the sinterability of water atomised high speed steel powdersPublication . Mascarenhas, João; Oliveira, M. Manuela; Wright, C. S.This paper presents the results of an investigation into the sintering behaviour of three vanadium enriched variants of T42 high-speed steel. Powders were prepared by water atomization with vanadium and carbon contents of 6-8 wt% and 2.2 -2.7 wt%, respectively. These were annealed, die pressed and sintered in vacuum. All three alloys were sintered to full density giving "as-sintered" microstructures comprising globular MC carbides dispersed in a martensitic matrix. Optimum sintering temperatures were in the range 1240-1250 degrees C with lower optimum temperatures associated with higher carbon levels. Sintering characteristics are correlated with phase diagrams calculated using ThermoCalc (TM) software and TCFe2000 database. The implications for the design of sinterable vanadium containing high-speed steels are discussed.