Sintering and Nanostability: The Thermodynamic Perspective

Abstract

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.

Probing the densification mechanisms during flash sintering of ZnO

“…5(aef) show the SEM images of sintered CuO-doped TiO 2 specimens prepared by the wet chemistry method. Low-magnification images show uniform structures and the average grain size increased substantially from 800 C to 900 C. Interestingly, a secondary phase (presumably CuO) appeared to wet some triple-grain junctions (with low dihedral angles that, in addition to the nanoscale IGFs at GBs, can also promote sintering and affect microstructural development, as reviewed and discussed by Castro and Gouvêa [20]) of the (aec) SEM images and (def) EDS elemental maps of titanium, copper, and oxygen of an 80 mol. % CuO þ 20 mol.…”

Li2O‐doped MgAl2O4 nanopowders: Energetics of interface segregation

“…One strategy that has been successfully used to obtain ceramic nanopowders is the use of additives that are segregated at the interfaces (GBs and surfaces) and reduce the interfaces energy, reducing the driving force for particle growth . If additive segregation at the interfaces occurs in nanoscale crystalline domains, the thermodynamic effects of segregation are more evident than those in coarser materials owing to the higher interface density …”

“…The interface energy decrease due to segregation (Equation ) reduces the driving force for coarsening, increasing the stability of the particles against growth and enhancing the nanostability. These concepts were described in detail in a previous report …”

Irradiation-induced grain growth and defect evolution in nanocrystalline zirconia with doped grain boundaries

“…24,25 This equation is based on Gibbs adsorption isotherm, and is valid for dilute systems and when dopants have positive tendency to segregation. 24,25 This equation is based on Gibbs adsorption isotherm, and is valid for dilute systems and when dopants have positive tendency to segregation.…”