RESUMEN

This paper discusses the role of nitrogen (N2) gas flow conditions on the formation of silicon nitride (Si3N4) nano-felts from polysiloxane-impregnated polyurethane (PU) foams. The polymeric foam was converted into an amorphous silicon oxycarbide (SiOC) artefact during pyrolysis, which was then transformed, at a higher temperature, into a Si3N4 felt through a reaction between the decomposition products of SiOC with N2. The study identified that a N2 flux of ~2.60 cm.min-1 at the cross-section of the furnace (controlled to 100 cm3.min-1 at the inlet of the furnace using a flowmeter) substantially favored the transformation of the parent SiOC foam to Si3N4 felts. This process intensification step significantly reduced the wastage and the energy requirement while considering the material production on a bulk scale. The study also inferred that the cell sizes of the initial PU templates influenced the foam to felt transformation.

RESUMEN

We report on an ultrarapid (6 s) consolidation of binder-less WC using a novel Ultrahigh temperature Flash Sintering (UFS) approach. The UFS technique bridges the gap between electric resistance sintering (≪1 s) and flash spark plasma sintering (20-60 s). Compared to the well-established spark plasma sintering, the proposed approach results in improved energy efficiency with massive energy and time savings while maintaining a comparable relative density (94.6%) and Vickers hardness of 2124 HV. The novelty of this work relies on (i) multiple steps current discharge profile to suit the rapid change of electrical conductivity experienced by the sintering powder, (ii) upgraded low thermal inertia CFC dies and (iii) ultra-high consolidation temperature approaching 2750 °C. Compared to SPS process, the UFS process is highly energy efficient (≈200 times faster and it consumes ≈95% less energy) and it holds the promise of energy efficient and ultrafast consolidation of several conductive refractory compounds.

RESUMEN

Due to unique osteogenic properties, tricalcium phosphate (TCP) has gained relevance in the field of bone repair. The development of novel and rapid sintering routes is of particular interest since TCP undergoes to high-temperature phase transitions and is widely employed in osteoconductive coatings on thermally-sensitive metal substrates. In the present work, TCP bioceramics was innovatively obtained by Ultrafast High-temperature Sintering (UHS). Ca-deficient hydroxyapatite nano-powder produced by mechanochemical synthesis of mussel shell-derived calcium carbonate was used to prepare the green samples by uniaxial pressing. These were introduced within a graphite felt which was rapidly heated by an electrical current flow, reaching heating rates exceeding 1200 °C min-1. Dense (> 93%) ceramics were manufactured in less than 3 min using currents between 25 and 30 A. Both ß and α-TCP were detected in the sintered components with proportions depending on the applied current. Preliminary tests confirmed that the artifacts do not possess cytotoxic effects and possess mechanical properties similar to conventionally sintered materials. The overall results prove the applicability of UHS to bioceramics paving the way to new rapid processing routes for biomedical components.

Asunto(s)

Materiales Biocompatibles , Fosfatos de Calcio , Cerámica , Calor , Ensayo de Materiales , Temperatura

RESUMEN

The flash sintering behavior of yttria-stabilized zirconia/alumina composites was investigated to understand the role of the fundamental processing and testing parameters (electric field intensity, electric current limit, thermal insulation, homogeneity and dispersion of the two phases) on densification. A strong relation between the composite compositions and the electric parameters needed to promote flash sintering is revealed. Interestingly, the composite preparation method, which affects the two-phases dispersion homogeneity, was shown to have a relevant effect on the flash onset conditions, where the more homogeneous material is more difficult to be flashed. Moreover, the use of a simple thermal insulation system around the green body allowed to improve the final density of the composites under constant electric current.

RESUMEN

In this work, ceria-based ceramics with the composition Gd0.14Pr0.06Ce0.8O2-δ and Sm0.14Pr0.06Ce0.8O2-δ, were synthesized by a simple co-precipitation process using either ammonium carbonate or ammonia solution as a precipitating agent. After the calcination, all of the produced samples were constituted by fluorite-structured ceria only, thus showing that both dopant and co-dopant cations were dissolved in the fluorite lattice. The ceria-based nanopowders were uniaxially compacted and consequently flash-sintered using different electrical cycles (including current-ramps). Different results were obtained as a function of both the adopted precipitating agent and the applied electrical cycle. In particular, highly densified products were obtained using current-ramps instead of "traditional" flash treatments (with the power source switching from voltage to current control at the flash event). Moreover, the powders that were synthesized using ammonia solution exhibited a low tendency to hotspot formation, whereas the materials obtained using carbonates as the precipitating agent were highly inhomogeneous. This points out for the first time the unexpected relevance of the precipitating agent (and of the powder shape/degree of agglomeration) for the flash sintering behavior.

RESUMEN

This paper reports the electrochemical, optical and thermal effects occurring during flash sintering of 8 mol % yttria-stabilized zirconia (8YSZ). In-situ observations of polycrystalline and single crystal specimens revealed electrochemical blackening/darkening during an incubation period prior to flash sintering. The phenomenon is induced by cathodic partial reduction under DC fields. When using a low frequency AC field (0.1⁻10 Hz) the blackening is reversible, following the imposed polarity switching. Thermal imaging combined with sample colour changes and electrical conductivity mapping give a complete picture of the multi-physical phenomena occurring during each stage of the flash sintering event. The partial reduction at the cathode causes a modification of the electrical properties in the sample and the blackened regions, which are close to the cathode, are more conductive than the remainder of the sample. The asymmetrical nature of the electrochemical reactions follows the field polarity and causes an asymmetry in the temperature between the anode and cathode, with the positive electrode tending to overheat. It is also observed that the phenomena are influenced by the quality of the electrical contacts and by the atmosphere used.