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Characterization of Cu2ZnSn(SSe)4 monograin powders by FE-SEM
- F. Neves, V. Livramento, I. Martins, L. Esperto, M. Santos, J.B. Correia, K. Muska, T. Holopainen
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- Journal:
- Microscopy and Microanalysis / Volume 19 / Issue S4 / August 2013
- Published online by Cambridge University Press:
- 06 August 2013, pp. 101-102
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- August 2013
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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. 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. The energy conversion efficiency of the CZTS-based solar cells has increased from 0.66% in 1996 to 11.1% recently.
The present work shows preliminary results that are related to the characterization of CZTSSe monograin powders by scanning electron microscopy. High purity metal compounds, S and Se powders were used as precursors for the synthesis of the Cu2ZnSn(SSe)4 monograin powders. The precursor powders were mixed to the desired composition and, additionally, KI was added as a flux material. Afterwards, the powders were blended in a mixer and encapsulated in quartz ampoules. The blended powders were degassed under dynamic vacuum at room temperature, sealed and annealed isothermally between 700 ºC and 780 ºC for a time ranging between 44 h and 136 h. After synthesis the flux material was removed with deionized water and the powders were sieved into several fractions. The morphology, microstructure and chemical composition of the synthesized powders was obtained with a Philips XL30 field-emission scanning electron microscope (FE-SEM) equipped with a backscattered electron (BSE) detector and an integrated EDAX energy dispersive X-ray spectroscopy (EDS) microanalysis system.
The typical morphology obtained for the CZTSSe powders can be seen in Figure 1. Basically, the particles show a polyhedral morphology with some of them showing a needle shape, i.e. a large shape factor (L/D>>1). Moreover, it was also observed a slight increase of the median particle size with the increase of the synthesis temperature. Due to the complexity of the synthesis of CZTSSe monograins, the formation of binary or ternary phases is a common feature. A very good control over the synthesis parameters is then required not only to obtain the desired phase but also to have a tight control over the stoichiometry of the material. Taking this into account, SEM/BSE observations and EDS analysis are two powerful techniques for evaluate the degree of the compositional homogeneity of the CZTSSe monograins. Figure 2 puts in evidence the degree of the homogeneity of the CZTSSe monograins in relation to their size. Overall, and independently of the synthesis conditions, the powder particles consisted predominantly of CZTSSe monograins and, in a much smaller extent, of particles having a ZnS or ZnS(Se) core coated by a CZTSSe layer. The presence of undesired phases occurred more frequently in the larger powder particles (> 100 µm) and the degree of homogeneity was lower for the powders synthesized at lower temperature and for a shorter time. However, when the EDS results obtained for CZTSSe monograins belonging to different size fractions are compared no major variations in the content of the five elements can be inferred.
Elemental interdiffusion in W-Ta composites developed for fusion applications
- R. Mateus, M. Dias, V. Livramento, D. Nunes, P.A. Carvalho, K. Hanada, J.B. Correia
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- Journal:
- Microscopy and Microanalysis / Volume 19 / Issue S4 / August 2013
- Published online by Cambridge University Press:
- 06 August 2013, pp. 123-124
- Print publication:
- August 2013
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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.
The composite should reveal the individual properties of the pristine phases as long as the interdiffusion between the components is significantly avoided during the consolidation/sintering route of the final material. Sintering operations at temperatures higher than 1300ºC lead to significant improvements in the final densification and thermal conductivity of the composites, which is crucial for fusion applications. However, W and Ta interdiffusion can be relevant above 1300ºC, mainly due to diffusion of W into Ta, and the aim of the present work is to control the mechanism.
W-Taf composites presenting 10 and 20 at.% of Ta where produced by alloying W powders and Ta fibres with a planetary ball milling route (MA) and by consolidating the mixture with spark plasma sintering (SPS) in the 1300-1600ºC temperature range. The final densifications remain fairly constant in both composites after sintering at different temperatures (83 to 87%) and the elemental interdiffusion remained low at 1300ºC. Nevertheless, the diffusivity of W in Ta became significant at 1600ºC, leading to the formation of a solid solution zone with a stoichiometry close to W16Ta84. The mechanism was followed by scanning electron and energy-dispersive X-ray spectroscopies (SEM/EDS; Figures 1 and 2, Table 1). Fabrication routes yielding high densifications and low interdiffusion are currently under investigation.
The work has been supported by the Contract of Association between Euratom and IST and by the Fund. Ciência e a Tecnologia contracts PTDC/CTM/100163/2008, Pest-OE/SADG/LA0010/2011 and PEST-OE/CTM-UI0084/2011.
Studies on deuterium retention in W-Ta based materials
- M. Dias, R. Mateus, N. Catarino, V. Livramento, J.B. Correia, P.A. Carvalho, K. Hanada, N. Pinhão, P. Barquinha, E. Alves
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- Journal:
- Microscopy and Microanalysis / Volume 19 / Issue S4 / August 2013
- Published online by Cambridge University Press:
- 06 August 2013, pp. 125-126
- Print publication:
- August 2013
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The high melting point, high sputtering threshold and low tritium inventory rendered W as a potentially suitable material in fusion devices. The major problem associated with presently available tungsten grades as structural materials is its brittleness at lower temperatures. This is further worsened by irradiation embrittlement. A strategy for ductility improvement is producing a composite, with the brittle W matrix being reinforced by short fibres of tantalum. As Ta is more ductile than W it can therefore divert or stop cracks propagating in the W matrix. In the present research Ta short fibres and powder were used as reinforcement component for W by alloying Ta short fibres or powder in a W powder matrix. The composites were subsequently irradiated with deuterium to assess the retention of this hydrogenic species in the materials.
The irradiated composites, with Ta contents of 10 or 20 at%, were produced from pure elemental powders (W-Ta powder composites), and pure W powder and Ta fibre (W-Ta fibre composites) with 100 μm in diameter by low energy ball milling in argon atmosphere. These materials were consolidated via spark plasma sintering (SPS) in the temperature 1200 to 1600 ºC range. Pure W and Ta plates (controls) and W-Ta composites were irradiated with He+ ions (optional pre-implantation step) and D+ ion beams at room temperature with fluences in the 1020-1021 at/m2 range. Blistering and deuterium retention in W and Ta plates and in W-Ta composites were studied with scanning electron microscopy (SEM), X-ray diffraction and thermal desorption analysis. The deuterium concentration was evaluated trough nuclear reaction analysis (NRA) using 0.75 to 2.1 MeV 3He+ beam and the D(3He,p)α reaction.
The investigations showed that deuterium irradiation induced microstructural modifications producing blistering in Ta plates as well as in W-Ta composites (Figure 1). These effects increased substantially in the Ta and W-Ta materials with the He pre-implantation step, while blisters have not been observed in the W plates either for D+ or He+ plus D+ implantation. Higher deuterium retention was observed by thermal desorption for the composites than for the W plates. Moreover the present study revealed that D+ trapping in the composites is dependent on the microstructure with higher retention observed for fibers than for powder.
This work has been performed under the Contract of Association between EURATOM and Instituto Superior Tecnico. Financial support was also received from the Fundação para a Ciência Tecnologia (FCT) grants with references PTDC/CTM/100163/2008, Pest-OE/SADG/LA0010/2011 and PEST-OE/CTM-UI0084/2011. The authors wish to acknowledge Jorge Rocha the implantation in W-Ta samples.
W-Diamond/Cu-Diamond nanostructured composites for fusion devices
- D. Nunes, V. Livramento, J.B. Correia, R. Mateus, P.A. Carvalho, N. Shohoji, H. Fernandes, C. Silva, E. Alves, K. Hanada, E. Osawa
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- Journal:
- MRS Online Proceedings Library Archive / Volume 1125 / 2008
- Published online by Cambridge University Press:
- 15 March 2011, 1125-R07-08
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- 2008
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A novel material design for nuclear fusion reactors is proposed based on Cu-Diamond and W-Diamond nanocomposites. The proposed design involves the production of W/W-Diamond/CuDiamond/Cu functionally graded material. W, W-Diamond, Cu-Diamond and Cu nanostructured composite powders were produced independently by mechanical alloying and subsequently consolidated/welded through spark plasma sintering. Modulation of processing parameters allowed controlling the extent of unfavourable conversion of Diamond into tungsten carbide, as well as to overcome the Diamond intrinsically difficult bonding to copper. Microstructural features and microhardness of the as-produced materials are presented.