Surface energies have postulated importance to catalysis, crystal growth, sintering, polymorphism, and many other fields. This importance is even more critical when dealing with nanostructured materials, where the surface-to-volume ratio is considerably higher and the surface term accounts for a much larger fraction of the total free energy. Here we present a novel approach to experimentally assess the average anhydrous and hydrated surface energies of oxides, and used the method to determine the surface energy of gamma-alumina. The method uses a water adsorption setup combined with a microcalorimeter where the heat of adsorption can be monitored as a function of the adsorbed amount. Maintaining a closed system, the approach enables the correlation of the molecular configuration of absorbed water with the thermodynamic data, and hence the definition of the point where a liquid water configuration exists (at high relative humidity). This information allows the calculation of the surface energy at room temperature for any coverage state by using the adsorption calorimetric data. For gamma-alumina, a close relationship between the water adsorption behavior and the surface energy was observed, evidencing that higher surface energies are associated with highly energetic dissociative behaviors of water and a continuous surface energy decrease upon water adsorption. Three adsorption stages were clearly observed from the combination of adsorption isotherm and microcalorimetric data, consistently with presented models.

Retaining large surface areas in alumina powders during high-temperature annealing is a major challenge in applications as catalyst supports and ceramic precursors. This is because the alumina surface area drastically decreases with transformation from the γ modification (defect spinel structure) into the α modification (corundum structure). The objective of this work is to show the thermodynamic basis of using additives, such as Zr and Mg, to control the γ-Al2O3 surface and bulk energetics and to manipulate the transformation temperature and surface area. These additives are observed to change the pattern of phase transformation and densification. Direct measurements of heats of solution in a lead borate melt of pure and doped alumina as a function of surface area enabled us to experimentally derive trends in the surface energies of hydroxylated surfaces. Accounting for heats of water adsorption measured on pure and doped alumina surfaces allowed us to delineate the thermodynamic effects of hydration on surface energies. Zr-doped γ-alumina showed a higher energy of the hydroxylated surface than did pure γ-alumina but showed a lower energy of the anhydrous surface. Mg addition does not change surface energies significantly but decreases the energetic instability of the bulk γ phase.

Sintering and nanostability (defined as the stability against sintering) are critical phenomena present in the processing and application of nanoparticles. With important implications in obtaining high-quality dense ceramics with fine grains or in enabling high surface areas in nanoparticles for catalytic applications, the control of these interrelated phenomena has been the focuses of several studies. From a thermodynamic perspective, it is recognized that surface energy is a fundamental parameter in both cases, since it is the main driving force for sintering and also the reason that nanoparticles are thermodynamically unstable and have the tendency to coarsen at elevated temperatures. The role of grain-boundary energies is less recognized as relevant, but is also connected to densification, grain growth, and nanoparticle stability. In this paper, we review the critical aspects of the role of interfacial energies in the microstructure evolution, in particular addressing them as parameters to allow better control in addition to more conventional kinetic parameters. The concept is based on the nonsingularity of interfacial energies in a given system, which varies with temperature, atmosphere, and most importantly, chemical composition-this last offering a method to induce particular microstructural evolutions. While the model assumes isotropic grain boundaries but consequences to anisotropy are also discussed. The paper presents examples of the role of dopants on interfacial energies, how this is quantitatively related to their segregation at the interfaces, and the impact in sintering and nanostability. Given the importance of interface energetics to these phenomena, we also present a short review on the current methods used to obtain reliable interface thermodynamic data.

The surface energy of a nanomaterial is a primary characterization parameter that has important implications on the material’s properties and applications including: catalysis, prediction of polymorphic stability, optimization of consolidation processes, and general colloidal science issues. However, currently, there is a dearth of techniques and theory to accurately measure surface energies for nanocrystalline solids and oxides. Therefore, we present a water adsorption microcalorimetry model using a novel second order surface energy differential equation to calculate the surface energy of anhydrous and hydrated (at any given level) nanocrystalline oxides. The experimental setup, used to feed data to the model, uses a water adsorption apparatus to measure the adsorbed water content on the specimen’s surface as a function of pressure (∼2 μmol dose) coupled with a microcalorimeter that can measure changes in the differential heat of water adsorption during the dosing. Using the experimental data and derived model, we can regression-fit the adsorbed water and differential heat of water adsorption data over the entire pressure range, vacuum to saturation, to calculate the surface energy via numerical integration of the second-order differential equation with the appropriate boundary conditions. We have tested the method using two technologically relevant nanoparticles, yttria doped zirconia (10YSZ) and γ-alumina. As key results, the calculated anhydrous surface energies for 10YSZ and γ-alumina are thus 1.54 ± 0.08 and 1.57 ± 0.04 J·m–2, respectively, and correlate well with other theoretical and experimental data. The model also provides relevant information on the changes of interaction energies between water molecules and the oxide surface.

The surface and interface enthalpies of cubic stabilized zirconia solid solutions containing 8, 10, and 12 mol % Y 2 O 3 were determined by a combination of calorimetric, morphological, and structural analyses techniques. Nanocrystalline samples with several surface areas and degrees of agglomeration were synthesized by simultaneous precipitation and annealing at temperatures of 470-900 °C. Samples were characterized by X-ray diffraction and Raman spectroscopy. Surface areas were measured by N 2 adsorption, and interface areas were estimated by comparing surface areas from N 2 adsorption to those derived from an analysis of the crystallite sizes refined from X-ray diffraction data. Calorimetric measurements of heat of solution in a sodium molybdate melt, as a function of surface and interface areas, enabled us to experimentally derive trends in the surface and interface enthalpies of hydroxylated surfaces. Accounting for heats of water adsorption measured by microcalorimetry allowed us to obtain the surface enthalpies (energies) of the anhydrous surfaces at each composition. Average surface enthalpies were determined to increase with yttria content, from 0.85 ( 0.07 J/m 2 (for 8 mol % yttria) to 1.27 ( 0.08 J/m 2 (for 12 mol % yttria) for the hydrous surface and from 1.16 ( 0.08 J/m 2 to 1.80 ( 0.10 J/m 2 for the anhydrous surface. Interface enthalpies were determined to be in the range of 0.9 ( 0.5 J/m 2 for all studied compositions. Comparisons with measured surface energies for pure ZrO 2 , and Y 2 O 3 nanopowders and grain-boundary energies for YSZ dense nanoceramics are presented.

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.

Highly stable ceria nanoparticles (< 11 nm) with different manganese contents were prepared by a co-precipitation method. The powders were studied by x-ray diffraction, transmission electron microscopy, electron energy loss spectroscopy, and water adsorption microcalorimetry. The data show that only a small fraction of the manganese ions dissolved into ceria fluorite structure as solid solution, and most segregated on the particles' surface, causing decrease of the average surface energy of the particles with increasing dopant concentration. This was confirmed by direct surface energy measurements using water adsorption microcalorimetry, and has consequences on particle coarsening behavior. That is, the results explain why manganese doped ceria nanoparticles show stronger resistance to coarsening as compared to undoped ceria. The enthalpy of surface segregation of manganese was calculated and discussed as an important parameter to design highly metastable ceria nanoparticles on a thermodynamic basis.

Materials for applications in hostile environments, such as nuclear reactors or radioactive waste immobilization, require extremely high resistance to radiation damage, such as resistance to amorphization or volume swelling. Nanocrystalline materials have been reported to present exceptionally high radiation-tolerance to amorphization. In principle, grain boundaries that are prevalent in nanomaterials could act as sinks for point-defects, enhancing defect recombination. In this paper we present evidence for this mechanism in nanograined Yttria Stabilized Zirconia (YSZ), associated with the observation that the concentration of defects after irradiation using heavy ions (Kr+, 400 keV) is inversely proportional to the grain size. HAADF images suggest the short migration distances in nanograined YSZ allow radiation induced interstitials to reach the grain boundaries on the irradiation time scale, leaving behind only vacancy clusters distributed within the grain. Because of the relatively low temperature of the irradiations and the fact that interstitials diffuse thermally more slowly than vacancies, this result indicates that the interstitials must reach the boundaries directly in the collision cascade, consistent with previous simulation results. Concomitant radiation-induced grain growth was observed which, as a consequence of the non-uniform implantation, caused cracking of the nano-samples induced by local stresses at the irradiated/non-irradiated interfaces.