To provide a complete picture of the energy landscape of Al 2 O 3 at the nanoscale, we directed this study toward understanding the energetics of amorphous alumina (a-Al 2 O 3 ). a-Al 2 O 3 nanoparticles were obtained by condensation from gas phase generated through laser evaporation of α-Al 2 O 3 targets in pure oxygen at25 Pa. As-deposited nanopowders were heat-treated at different temperatures up to 600 °C to provide powders with surface areas of 670−340 m 2 /g. The structure of the samples was characterized by powder X-ray diffraction, transmission electron microscopy, and solid-state nuclear magnetic resonance spectroscopy. The results indicate that the microstructure consists of aggregated 3−5 nm nanoparticles that remain amorphous to temperatures as high as 600 °C. The structure consists of a network of AlO 4 , AlO 5 , and AlO 6 polyhedra, with AlO 5 being the most abundant species. The presence of water molecules on the surfaces was confirmed by mass spectrometry of the gases evolved on heating the samples under vacuum. A combination of BET surface-area measurements, water adsorption calorimetry, and high-temperature oxide melt solution calorimetry was employed for thermodynamic analysis. By linear fit of the measured excess enthalpy of the nanoparticles as a function of surface area, the surface energy of a-Al 2 O 3 was determined to be 0.97 ± 0.04 J/m 2 . We conclude that the lower surface energy of a-Al 2 O 3 compared with crystalline polymorphs γand α-Al 2 O 3 makes this phase the most energetically stable phase at surface areas greater than 370 m 2 /g.

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This article reports a comparative characterization of ultrafine MgAl2O4 spinel nanoparticles synthesized by polymeric precursor (Pechini) and coprecipitation methods. The nanoparticles were evaluated in terms of purity and surface cleanliness, size distribution, state of agglomeration, and sintering behavior. Powders synthesized by the Pechini technique were highly agglomerated and revealed a bimodal particle size distribution centered around 12 and 27 nm. Thermal analysis and infrared spectroscopy measurements indicated that carbon species remained on the surface of the powders only to be released when temperatures exceeded 1000°C. Isothermal sintering of such nanopowders at 1300°C showed a maximum relative density of only 54%. MgAl2O4 synthesized via coprecipitation created small nanoparticles, around 5–6 nm after calcination at 800°C, with significantly less agglomeration. Compared with the precursor‐derived powders, excellent sinterability of the coprecipitated powders was obtained under the same sintering conditions. Relative densities above 90% were obtained after only 10 min, which further increased to greater than 95% after 20 min with no sintering aids or dopants. The results highlight the importance of purity and processing control to exploit the beneficial high sinterability of nanoparticles.

Using Field Assisted Sintering Technique/Spark Plasma Sintering the effect of heating rate on the sintering of zinc oxide at a temperature of 400°C has been investigated. For the highest heating rate of 100°C/min, relative density larger than 95% was achieved whereas at low heating rates only little shrinkage occurred. Hardness measurements, Transmission Electron Microscopy, and impedance spectroscopy revealed clear differences between heating rates. It was found that residual water is responsible for this behavior, enhancing particle rearrangement and diffusion kinetics.

Controlling sintering is a critical aspect for the processing of dense parts and to improve stability of nanoparticles. Dopants are typically used for this purpose, but the extension of the role of dopants in the phenomena is still not completely understood. In this work we demonstrate the possibility of inducing desintering in a ceramic system by programming a dopant redistribution during heat treatment. Tin dioxide doped with manganese was sintered up to intermediate densities and density was decreased afterward by exposing the sample to a lower temperature. A change in the oxidation state and ionic radius of manganese caused it to segregate at high temperatures and to partially redissolve in the crystal at a lower one. This interfacial chemistry change caused a decrease of the dihedral angle at lower temperature, creating a driving force for porosity volume increase (dedensification) by mass flow against curvature potential. The result is predictable from extrapolations of pore stability theories but never directly demonstrated, and may explain why some doped systems do not follow regular sintering predictions, i.e. interfacial chemistry and energetics can change during processing, affecting driving forces.