Effects of the nature of the doping salt and of the thermal pre-treatment and sintering temperature on spark plasma sintering of transparent alumina

A slurry of a-Al2O3 was doped with Mg, Zr and La nitrates or chlorides, in various amounts in the range 150-500 wt ppm and then freeze-dried to produce nanosized doped powder (~150 nm). The powder was sintered by SPS to yield transparent polycrystalline alpha alumina. The influence of the nature of the doping element and the starting salt, the thermal treatment before sintering and the sintering emperature on the transparency of the ceramics were investigated. The transparency of the ceramics of nanosized Al2O3 was shown to depend mainly on the way the powder was prepared, the nature of the doping salt also had an effect. Finally, a high real inline transmittance, reaching 48.1% was achieved after optimization

Effect of amount of doping agent on sintering, microstructure and optical properties of Zr- and La-doped alumina sintered by SPS

SPS-produced α-alumina samples are prepared from powders doped with different amounts of Zr4+ and La3+ cations. Zr4+ cations segregate at grain boundaries. m-ZrO2 particles are formed at 570 but not at 280 cat ppm. A β-alumina LaAl11O18 structure is found at 310 cat ppm when the lanthanum grain boundary solubility limit is exceeded (∼200 cat ppm). 100 cat ppm La is sufficient to block the diffusion path across grain boundaries and inhibit grain growth. Both doping cations disturb the grain boundary diffusion whatever their amount. They delay the densification at higher temperatures while limiting grain growth. The real in-line transmittance (RIT) of α-alumina is improved due to the reduced grain size. Nevertheless, increasing the cation amount leads to an increase in porosity or even the formation of secondary phase particles, both detrimental for optical properties. Finally, optimised amounts of cation of 200 and 150 cat ppm are found for La- and Zr-doped alumina, respectively

Comparison of densification kinetics of a TiAl powder by spark plasma sintering and hot pressing

International audienceDensification kinetics by spark plasma sintering (SPS) and hot pressing (HP) have been compared, under isothermal conditions and with heating rates of 20°C/min. Careful calibration of sample temperature has been carried out to obtain comparable results. In all cases, densification kinetics did not exhibit significant differences, ruling out any influence of the SPS current. The stress exponent n and the activation energy Q of the Norton law describing deformation at high temperature of the powder particles have been determined by isothermal experiments at different stresses and temperatures, respectively. The values obtained, n = 1.9 ± 0.3 and Q = 308 ± 20 kJ/mol for SPS, n = 1.5 ± 0.3 and Q = 276 ± 40 kJ/mol for HP, come close in both techniques. Using these values, anisothermal densification kinetics at heating rates of 20°C/ min and 100°C/min, typical of the SPS, could be analytically reproduced, using literature models. The activation parameters suggest that *Text only Click here to download Text only: monchoux_article_version2.docx Click here to view linked References SPS densification kinetics occurs by dislocation climb controlled by Al bulk diffusion, that is, by classical metallurgical mechanisms

Borohydride oxidation reaction mechanisms and poisoning effects on Au, Pt and Pd bulk electrodes: From model (low) to direct borohydride fuel cell operating (high) concentrations

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Corrélation entre microstructure et propriétés optiques de céramiques transparentes par nanotomographie – FIB

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Recent advances in the understanding of Ni-based catalysts for the oxidation of hydrogen-containing fuels in alkaline media

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Consolidation of mechanically alloyed Cu-Ni-Fe material by spark plasma sintering and evaluation as inert anode for aluminum electrolysis

International audienceA Cu65Ni20Fe15 powder was prepared by mechanical alloying at semi-pilot scale to form a face-centeredcubic (fcc) phase (gamma-phase). This powder was then consolidated by spark plasma sintering (SPS) to form a pellet. The crystallographic structure of the material is not affected by the consolidation process, although there is a slight increase of the crystallite size from 20 to 32 nm, and a slight decrease of the lattice strain from 0.5% to 0.2%. The relative density of the SPS consolidated sample is 95%. Thermogravimetric analysis confirms the good oxidation resistance of the SPS sample with a mass gain of only 0.4% after 20 h of oxidation at 700 degrees C, which is attributed to the rapid formation of a protective NiFe2O4 layer. The SPS sample was then evaluated as inert anode for Al electrolysis in low-temperature (700 degrees C) KF-AlF3 electrolyte. After 20 h of electrolysis at an anode current density of 0.5 A cm(-2), the cell voltage reaches an unstable value of 5.0 V. The purity of the produced aluminum is 99.4% and the wear rate of the electrode is estimated at 1.8 cm year(-1). (C) 2013 Elsevier B.V. All rights reserved

A novel processing route for carbon nanotube reinforced glass-ceramic matrix composites

Conference on Smart Sensor Phenomena, Technology, Networks, and Systems Integration, San Diego, CA, MAR 09-10, 2015International audienceThe current study reports the establishment of a novel feasible way for processing glass-and ceramic-matrix composites reinforced with carbon nanotubes (CNTs). The technique is based on high shear compaction of glass/ceramic and CNT blends in the presence of polymeric binders for the production of flexible green bodies which are subsequently sintered and densified by spark plasma sintering. The method was successfully applied on a borosilicate glass/multi-wall CNT composite with final density identical to that of the full-dense ceramic. Preliminary non-destructive evaluation of dynamic mechanical properties such as Young's and shear modulus and Poisson's ratio by ultrasonics show that property improvement maximizes up to a certain CNT loading; after this threshold is exceeded, properties degrade with further loading increase

Processing of yttria stabilized zirconia reinforced with multi-walled carbon nanotubes with attractive mechanical properties

The improvement of the mechanical properties of carbon nanotube reinforced polycrystalline yttria-stabilized zirconia (CNT-YSZ) was questionable in earlier investigations due to several difficulties for processing of these composites. In the present article, the authors are proposing a successful technique for mixing pre-dispersed CNTs within YSZ particles followed by a fast spark plasma sintering at relatively low temperature, resulting in near full-dense structure with well-distributed CNTs. Composites with CNT quantities ranging within 0.5-5 wt% have been analyzed and a significant improvement in mechanical properties, i.e. Young's modulus, indentation hardness and fracture toughness with respect to monolithic YSZ could be observed. To support these interesting mechanical properties, high-resolution electron microscopy and Raman spectroscopy measurements have been carried out. The analysis of densification shows that the lower densification rate of CNT reinforced composites with respect to the pure YSZ could be attributed to a slower grain boundary sliding or migration during sintering. (C) 2010 Elsevier Ltd. All rights reserved