Special phase transformation and crystal growth pathways observed in nanoparticles†

Phase transformation and crystal growth in nanoparticles may happen via mechanisms distinct from those in bulk materials. We combine experimental studies of as-synthesized and hydrothermally coarsened titania (TiO(2)) and zinc sulfide (ZnS) with thermodynamic analysis, kinetic modeling and molecular dynamics (MD) simulations. The samples were characterized by transmission electron microscopy, X-ray diffraction, synchrotron X-ray absorption and scattering, and UV-vis spectroscopy. At low temperatures, phase transformation in titania nanoparticles occurs predominantly via interface nucleation at particle–particle contacts. Coarsening and crystal growth of titania nanoparticles can be described using the Smoluchowski equation. Oriented attachment-based crystal growth was common in both hydrothermal solutions and under dry conditions. MD simulations predict large structural perturbations within very fine particles, and are consistent with experimental results showing that ligand binding and change in aggregation state can cause phase transformation without particle coarsening. Such phenomena affect surface reactivity, thus may have important roles in geochemical cycling

An efficient hierarchical nanostructured Pr 6 O 11 electrode for solid oxide fuel cells

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Microstructural influence of electrosprayed Pt thin films on the catalytic performances and electrochemical promotion of NO reduction by C3H6

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Crystallite Size Effect on the Tetragonal-Monoclinic Transition of Undoped Nanocrystalline Zirconia Studied by XRD and Raman Spectrometry

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Pressure-less spark plasma sintering effect on non-conventional necking process during the initial stage of sintering of copper and alumina

cited By 14International audienceIn the present study, we focus on the characterization of the necking mechanisms during the early stages of pressure-less spark plasma sintering (PL-SPS) compared to conventional sintering (CS) of two different types of powdered materials (Cu and α-Al 2O 3). SEM observations of the evolution of particle morphology and necks from the as-received powders to sintered ones show the nature of the neck between particles which were either in contact or not. For alumina, no particular necking process (melt or viscous bridge) was observed regardless of the sintering conditions (PL-SPS and CS), even for a very high heating rate 455 °C/min. For copper, this neck morphology is unequivocally not typical of conventional ones, thus, suggesting mass transport by an ejection mechanism. This particular morphology was seen occasionally. In comparison, the conventionally sintered Cu particles presented a smoother surface, with conventional curved necks suggesting the contribution of surface diffusion mechanisms. Based on partial pressure calculations, a direct thermal effect might not explain the observed non-conventional neck for copper. On the other hand, local field enhancement effect and local favourable thermal breakdown voltage conditions are described and discussed in order to support the experimental results. © Springer Science+Business Media, LLC 2012

Spark plasma sintering kinetics of pure α-alumina

cited By 36International audienceThe microstructure development of sintered alumina at different stages in the spark plasma sintering process has been investigated. Based on classical kinetics laws and adapted Coble creep models of hot pressing, the densification and grain growth kinetics were analyzed as a function of various parameters such as heating rate, sintering temperature, and dwell duration. It is found that sintering kinetics are greatly influenced by heating rates. The local temperature gradients at interparticle contacts during high heating rate can be from three to 10 times higher than in low heating rate cycles. Plastic yield might lead to instantaneous densification at an early stage of sintering. However, grain-boundary diffusion probably dominates at low heating rate whereas grain coarsening, in respect of thermal equilibrium establishment, is unavoidable with high heating rate during the initial-stage sintering at low temperatures. During the final-stage sintering, fast grain growth mechanisms such as surface diffusion and pore-controlled grain-boundary migration may dominate over densification controlled by grain-boundary sliding or lattice diffusion. The grain size-density trajectory corroborates that low heating rate is much favorable to achieve near full density with fine grain size at low sintering temperatures. © 2011 The American Ceramic Society

A screening design approach for the understanding of spark plasma sintering parameters: A case of translucent polycrystalline undoped alumina

cited By 13International audienceAn experimental screening design was used to evaluate the effects of spark plasma sintering (SPS) parameters such as heating rate, sintering temperature, dwell duration, and green-shaping processing on the relative density, grain size, and the optical properties of polycrystalline alumina (PCA). It is shown that heating rate and sintering temperature are the most critical factors for the densification of PCA during SPS. Green-shaping processing could prevent grain growth at low SPS sintering temperatures. No predominant SPS parameters are observed on the optical properties. Hence, the optical properties of PCA are controlled by microstructural evolution during the SPS process. © 2009 The American Ceramic Society

La 4 Ni 3 O 10−δ as an efficient solid oxide fuel cell cathode: electrochemical properties versus microstructure

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