32 research outputs found
Synthesis and characterization of group IV and V metal diboride nanocrystals via borothermal reduction of metal oxide with NaBH4
Group IV and V metal diborides (MB2) have a unique combination of properties such as a very high melting point (\u3e3000°C), high hardness, good solid-state phase stability, high thermal and electrical conductivity. Metal diboride-based ceramics are expected to be potential candidate materials for ultra-high-temperature applications in the aerospace industry [1]. Due to the poor sinterability of commercial powders, the availability of nanometric boride particles has indeed the potential to improve several stages of ceramic processing [2], or for instance to facilitate the sintering of bulk ceramics due to enhanced particle reactivity [3]. Several synthesis have been developed to achieve nanoborides: chemical route from inorganic precursors, mechanical alloying and self-propagating high-temperature synthesis [4–6].
In this work we proposed the synthesis of group IV and V metal diboride (MB2, M= Ti, Zr, Hf, Nb, Ta) nanocrystals by a thermal treatment of the metal oxide and sodium borohydride (NaBH4) at 700°C under atmospheric pressure [7]. The reaction occurs first via decomposition of NaBH4, followed by the formation of amorphous boron and crystalline ternary species with general formula NaxMyOz and NaxByOz. Finally all of the intermediary species yield metal diboride (MB2) and sodium meta-borate (NaBO2).
Synthesized TiB2 nanocrystals have an average size of 11 nm and the powder has a specific surface area (s.s.a) of 33.45 m2/g. ZrB2 grains have a platelet morphology with an aspect ratio of 10, average size of 22.5 nm and s.s.a of 24.97 m2/g; HfB2 has a similar morphology with a crystals size of 28 nm, while the s.s.a is even higher, 36.36 m2/g. As far as we know, the latter is the finest powder obtained via borothermal reduction of metal oxides ever reported. Synthesized NbB2 powder consists of crystallites around 12 nm and has a s.s.a of 21.09 m2/g. TaB2 powder has a s.s.a of 11.38 m2/g and consists of 200 nm agglomerates of spherical and needle-shaped nanocrystals with average size of 11 nm
Key issues in the manufacturing of solid oxide fuel cells with nanometric powders
The climate change, the decreasing of petroleum supplies and the abrupt increase of the energy demand due to the emerging countries and to an energy-hungry society, has driven the interest towards new energy and more efficient devices of energy production. Only a strong acceleration of alternative devices of energy production and an increase of renewables, can succeed in reducing pollution, improving the climate and at the same time assuring the energetic autonomy and competitiveness. In this scenario, electrochemical cells show several economic and environmental advantages compared to the conventional industrial processes. Fuel cells are an excellent alternative to the conventional systems of energy production in terms of CO2 emissions, low noise and flexibility of fuels and generated power. Solid oxide fuel cells (SOFC) in particular, are one of the most promising energy devices for their high efficiency, modularity, low emissions and the possibility to be directly fuelled with natural gas, GPL and alcohols. Lot of efforts are however necessary to develop commercially available generators and to increase their stability lowering at the same time their costs. These hurdles can be partially overcome lowering the operating temperature but also using more economic and easily scalable manufacturing techniques. These objectives can be reach deepened the knowledge on the relationships between SOFC materials and the main industrial production processes (tape casting and screen printing) necessary to obtain cell of dimensions close to the commercial ones with easily scalable processes. In this work the main issues related to tape casting and screen printing of nanopowders for SOFC ceramic devices is presented. Nano-powders represent the forefront of materials for SOFC. Nano-structured powders exhibit in fact important size-dependant properties such as high catalytic activity, low sintering temperatures and therefore high performances. Aim of this study is to find the correlation that link the process parameters with the nano-materials properties in order to enhance the performances and the durability both of the materials and of the final device. One of the most critical issue is to produce homogeneous and stable ceramic suspensions of nanopowders. The process optimization can be obtained merging the surface and morphological properties of the nanopowders considered (shape, dimensions\u27 distribution, surface area, etc.) to its behavior in suspension (viscosity, zeta potential, etc.) either organic of water-based, in order to obtain a well dispersed and homogenous system. With this kind of control, it is possible to produce large area reliable devices with the necessary reproducibility and reliabilit
One step production process for large area supporting cathode
Tape casting is a cheap and easily scalable shaping technique used to produce large-area, flat ceramic substrate for SOFC applications. To obtain elements with the desired porosity it is necessary adding a pore forming agent in the tape casting slurry. In this work, the possibility to obtain porous La0.8Sr0.2MnO3-Ce0.8Gd0.2O2 (LSM-GDC) supporting cathode without the use of pore formers was evaluated. The reactive sintering approach was therefore considered to exploit the porosity induced by the precursor decomposition during a single thermal treatment of calcining-debonding-sintering. Through this approach the La0.8Sr0.2MnO3 phase was formed directly during the sintering step. This process allowed to obtain 10x10 cm2 LSM-GDC tapes with values of mechanical strength, porosity and permeability suitable for fuel cells applications without pore former addition and in a single thermal step. To the author knowledge this is the first time that a large area supporting cathode has been produced by tape casting using the reactive sintering approac
Key issues in the manufacturing of solid oxide fuel cells with nanometric powders
The climate change, the decreasing of petroleum supplies and the abrupt increase of the energy demand due to the emerging countries and to an energy-hungry society, has driven the interest towards new energy and more efficient devices of energy production. Only a strong acceleration of alternative devices of energy production and an increase of renewables, can succeed in reducing pollution, improving the climate and at the same time assuring the energetic autonomy and competitiveness. In this scenario, electrochemical cells show several economic and environmental advantages compared to the conventional industrial processes. Fuel cells are an excellent alternative to the conventional systems of energy production in terms of CO2 emissions, low noise and flexibility of fuels and generated power. Solid oxide fuel cells (SOFC) in particular, are one of the most promising energy devices for their high efficiency, modularity, low emissions and the possibility to be directly fuelled with natural gas, GPL and alcohols. Lot of efforts are however necessary to develop commercially available generators and to increase their stability lowering at the same time their costs. These hurdles can be partially overcome lowering the operating temperature but also using more economic and easily scalable manufacturing techniques. These objectives can be reach deepened the knowledge on the relationships between SOFC materials and the main industrial production processes (tape casting and screen printing) necessary to obtain cell of dimensions close to the commercial ones with easily scalable processes. In this work the main issues related to tape casting and screen printing of nanopowders for SOFC ceramic devices is presented. Nano-powders represent the forefront of materials for SOFC. Nano-structured powders exhibit in fact important size-dependant properties such as high catalytic activity, low sintering temperatures and therefore high performances. Aim of this study is to find the correlation that link the process parameters with the nano-materials properties in order to enhance the performances and the durability both of the materials and of the final device. One of the most critical issue is to produce homogeneous and stable ceramic suspensions of nanopowders. The process optimization can be obtained merging the surface and morphological properties of the nanopowders considered (shape, dimensions\u27s distribution, surface area, etc.) to its behavior in suspension(viscosity, zeta potential, etc.) either organic of water-based, in order to obtain a well dispersed and homogenous system. With this kind of control, it is possible to produce large area reliable devices with the necessary reproducibility and reliabilit
Key issues in processing metal-supported proton conducting anodes for SOFCs applications
BaCe0.65Zr0.2Y0.15O3-δ(BCZY) have been recently proposed for IT-SOFCs due to its high proton conductivity. A the same time considerable efforts are directed towards the development of metal-supported cells. The combination of the potential advantages offered by either proton conductors based cells and metal supported configuration has never been explored before. In this work the technological issues to produce proton conducting BCZY-Ni anodes stainless steel-supported were carefully investigated. A tailored porous metal support was produced by tape casting. Afterwards the anode was deposited by screen printing and the resulting bilayer sintered in reducing atmosphere. Each step of the production process was throughly investigated. A cations interdiffusion between the metallic support and the anodic layer was observed in all the range of temperatures considered. The influence of a CeO2 barrier layer and anode thickness on the cations diffusion and a successful production of planar crack-free anode was deeply analyze
Investigation of the corrosion behaviour of AISI 316L stainless steel sintered in different conditions
Corrosion resistance in different environments and structural features of dental casting alloys for resin bonding
The influence of different electrolytes on the corrosion behaviour of three dental casting alloys for resin bonding with different no- bility was investigated. The alloys were studied in the metallurgical state representing that in the oral cavity. Structural and morpholo- gical characterisation was performed by means of optical and elec- tronic microscopy and EDS microanalysis. The corrosion resistance was evaluated by means of potentiodynamic anodic polarization curves recorded in three different aggressive environments: artifi- cial saliva (pH � 6.7), Ringer's solution and 0.1 N NaCl solution added with lactic acid (pH � 2). The latter solution is recom- mended by UNI EN ISO 8891 for the static immersion corrosion test. For comparison purposes the electrochemical behaviour of the pure alloying metals in this solution was also investigated. Results showed a satisfactory corrosion resistance of the alloys examined. The synthetic saliva was the least aggressive electrolyte for all of them but it did not allow to distinguish the corrosion behaviour of these alloys although their differences in nobility and structure. The chloride-containing solutions showed a better distinction in the corrosion behaviour, as matter of facts, as the chloride content increased, the higher (Cu � Ag) amount was in the alloy, the higher was its current density in the immunity region of the anodic polarisation curves. The environment recommended by the normative UNI EN ISO 8891 resulted the most sensitive and effective for the discrimination of the corrosion resistance of precious metal alloys for resin bonding with different nobility by means of the potentiodynamic test
Cementi vetro-ionomerici per applicazioni odontoiatriche ed influenza dell'introduzione di polveri metalliche sulle loro proprieta'
Relazione tra microstruttura, resistenza alla corrosione ed al tarnish di leghe auree utilizzate in odontoiatria protesica
Structural characterisation and corrosion resistance of Ga-precious metal alloys formed by liquid-solid reaction at room temperature
An attempt to eliminate Hg from dental amalgams was made by substituting it with low melting Ga-based alloys, liquid at room temperature. However more information is needed on the influence of alloy composition and their questioned corrosion resistance. In this paper the reaction of some liquid Ga alloys and some solid precious metal alloys with different nobility was studied. Structural features, hardness and corrosion resistance of the obtained composite materials were investigated as a function of composition of the starting alloys, liquid/solid ratio and different mixing methods. Every combination of the solid precious metal powders and the liquid Ga-alloys gives rise to similar composite materials. The structure always consists of unreacted solid alloy particles embedded in a complex matrix composed of many reaction phases. The formation of some phases depends on the composition of the solid alloy, others originate all the time; however their topography and morphology may be different as well as their compositional range. The high porosity generally present in the composite materials markedly affects hardness values; nevertheless some prepared materials reach hardness comparable with the one of commercial amalgams. The nobility of the solid alloy and porosity percentage play a determinant role on the corrosion behaviour. In all cases the low corrosion resistance of the experimented materials may be attributed to a galvanic coupling between the reaction intermediate phases and the unreacted liquid alloy remained inside the pores