Direct measurements ofquasi-zero grain boundary energies in ceramics

Abstract

Nanocrystalline bulk materials (also called nanograined materials) are intrinsically unstable due to the excess grain boundary (GB) free energies. Dopants designed to segregate to boundaries have been proposed to lower excess GB energies, increasing stability against coarsening and enabling nanostructure features to survive high temperature processing and operational environments. It has been theoretically proposed that the GB energy of a material can eventually become zero as a function of dopant concentration, signifying negligible driving force for growth-an infinitely stable nanomaterial. In this work we use ultrasensitive microcalorimetry to experimentally measure the absolute GB energy of gadolinium-doped nanocrystalline zirconia as a function of grain size and show that the energy can indeed reach a quasi-zero energy state (;0.05 J/m 2 ) when a critical GB dopant enrichment is achieved. This thermodynamic condition leads to unprecedented coarsening resistance, but is a temperature dependent function; since increasing temperatures deplete the GB as the dopant dissolves back in the crystalline bulk.

Unraveling the Role of Sn Segregation in the Electronic Transport of Polycrystalline Hematite: Raising the Electronic Conductivity by Lowering the Grain‐Boundary Blocking Effect

“…In ionic solids, the segregation phenomenon was found to have a substantial effect on the electronic and catalytic properties, as well as on the sintering process. At certain level of impurity or on adding atoms, the segregation at the grain boundary may lower the surface or interfacial energy which may modify the defect concentration at the grain boundary and consequently modify the back‐to‐back Schottky barrier . Therefore, it seems reasonable to have a preferential electron path through the polycrystalline hematite sample.…”

Controlling sintering and grain growth of nanoceramics

“…grain boundary energies have shown that impurities may significantly lower the grain boundary energy of ceramics down to values below 0.025 J/m 2 [7]. While this may seem irrelevant front the exponential dependence of the grain boundary mobility, some impurities may have only minor effects on the activation energy and mobility, and thus the system will be mostly controlled by the thermodynamic aspect, as observed in Gd-doped yttria-stabilized zirconia discussed in the next session.…”

“…However, until recently, the grain boundary energy has not been considered a variable to design (meta)stable nanocrystalline ceramics. Following theoretical framework developed for the control of nanocrystalline metal alloys [25][26][27][28][29][30], a few papers have reported virtual stopping of grain growth of ceramics by introducing dopants targeting a reduction of the grain boundary energy [7,[31][32][33]. The grain boundary energy was lowered to essentially zero [7], signifying a negligible driving force for coarsening.…”

Review of thermo‐kinetic correlation during grain growth in nanocrystalline materials

“…To solve the poor thermal stability of NC materials, many ways, including the introduction of foreign solute atoms, the precipitation of second particles, etc., have been attempted through using the methods of theoretical models, experimental techniques and computer simulations. These investigations can be commonly classified into two categories, that is, the thermodynamic stabilization arising from reduced GB energy [20][21][22][23][24][25][26] and the kinetic stabilization due to the decreased GB mobility [27][28][29][30] (or the increased activation energy for GB migration). Table 1 summarizes the representative review articles and their foci.…”