The stability of nanoparticles is strongly dependent on the thermodynamics of interfaces. Providing reliable data on surface and grain boundary energies is therefore of key importance for predicting and improving nanostability. In this work, we used a combination of high-temperature oxide melt drop solution calorimetry and water adsorption microcalorimetry to demonstrate the effect of a dopant (manganese) on both surface and grain boundary energies of SnO 2 , and discussed the impacts on the average particle size at a given temperature. The results show a significant decrease in the grain boundary energy with increasing manganese content and a concomitant moderate decrease in the surface energy, consistently with segregation enthalpy values acquired from an analytical fitting model. The results explain the measured increase in stability with increasing dopant content (smaller sizes) and suggest the grain boundary energy has a much more important role in defining particle stability than previously supposed.

Nanocrystalline ceramics offer interesting and useful physical properties attributed to their inherent large volume fraction of grain boundaries. At the same time, these materials are highly unstable, being subjected to severe coarsening when exposed at moderate to high temperatures, limiting operating temperatures and disabling processing conditions. In this work, we designed highly stable nanocrystalline yttria stabilized zirconia (YSZ) by targeting a decrease of average grain boundary (GB) energy, affecting both driving force for growth and mobility of the boundaries. The design was based on fundamental equations governing thermodynamics of nanocrystals, and enabled the selection of lanthanum as an effective dopant which segregates to grain boundaries and lowers the average energy of YSZ boundaries to half. While this would be already responsible for significant coarsening reduction, we further experimentally demonstrate that the GB energy decreases continuously during grain growth caused by the enrichment of boundaries with dopant, enhancing further the stability of the boundaries. The designed composition showed impressive resistance to grain growth at 1100°C as compared to the undoped YSZ and opens the perspective for similar design in other ceramics.

This work presents experimental data on the surface and grain boundary energies of tin dioxide nanoparticles at room temperature and high temperature conditions (quenched from 1300°C), and a discussion of impacts on the fundamental understanding of the nondensification mechanism of SnO 2 during sintering. The results were obtained using a combination of water adsorption microcalorimetry, high-temperature oxide melt drop solution calorimetry, and scanning electron transmission microscopy. At room temperature, the average surface and grain boundary energies of anhydrous SnO 2 were 1.20 6 0.02 and 0.71 6 0.08 J m À2 , respectively. At high temperature, SnO 2 showed a surface energy of 0.94 6 0.03 J m À2 . This remarkable decrease was attributed to the lower oxygen pressure and was associated with a decrease in contact angle during sintering. This observation indicates a moderate but significant thermodynamic reason behind nondensification behavior of SnO 2 in addition to common kinetic descriptions: high sintering temperatures and atmospheres cause smaller dihedral angles that decrease sintering stresses.

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.

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