Mobility transition at grain boundaries in two‐step sintered 8 mol% yttria‐stabilized zirconia

Abstract

Stagnation of grain growth is often attributed to impurity segregation, which becomes more severe as the grain size grows. In this respect, there is no evidence for segregation-induced slowdown in the grain growth of yttria-stabilized cubic zirconia, which obeys the parabolic law when the size increases by more than ten times. However, lowering the temperature below 1300°C triggers an abrupt slowdown, constraining the average grains to grow by less than 0.5 lm in 1000 hours despite a relatively large driving force imparted in the fine grains of~0.5 lm.Yet isolated pockets of abnormally large grains, and even most remarkably, pockets of abnormally small grains, emerge in the same latter sample. Such extreme bifurcation of microstructure has never been observed before, and can be explained by an inhomogeneous distribution of immobile four-grain junctions.The implications of these findings for two-step sintering are discussed.

On the Origin of the Large Remanent Polarization in La:HfO2

“…In addition, La is known to reduce leakage current in HfO 2 ‐based thin film capacitor by compensating oxygen vacancies . Accordingly, a lower amount of oxygen vacancies La‐doped films can impact atom diffusion as shown for Y stabilized ZrO 2 films . Transferring these ideas to HfO 2 thin films (knowing that this is not 100% fair), dopants might change the grain growth rate and phase formation of HfO 2 nuclei and the ratio of homogeneous (nuclei in the film volume) and heterogeneous (nuclei at the interfaces) nucleation.…”

“…[7] It should further be noted that La is one of the dopants that increase the crystallization temperature of HfO 2 the most. [57,58] Transferring these ideas to HfO 2 thin films (knowing that this is not 100% fair), dopants might change the grain growth rate and phase formation of HfO 2 nuclei and the ratio of homogeneous (nuclei in the film volume) and heterogeneous (nuclei at the interfaces) nucleation. From a ceramics perspective, dopants were utilized to alter diffusion to promote the densification during sintering.…”

Ultra‐Uniform Nanocrystalline Materials via Two‐Step Sintering

“…However, in 8YSZ we have recently identified, for the first time, a transition temperature T tr : 2‐grain‐controlled boundary migration above T tr , and 3/4‐grain‐junction‐controlled boundary migration below T tr . [ 45 ] Moreover, there is an unphysically high activation energy (10.8 eV) for 3/4‐grain‐junction‐controlled boundary migration that suggests more and more grain junctions become frozen at lower and lower temperature, so the few remaining mobile boundaries will also become immobile if there are pores to pin them. Therefore, it is advantageous to choose a T 2 as low as feasible, which is 100 °C below T tr (1300 °C) in our 8YSZ study.…”

“…According to Hillert's solution that follows the mean‐field approach of Lifshitz, Slyozov, and Wagner (LSW), [ 6–9 ] the theoretical ratio σ, which is the ratio of the standard deviation Σ of the grain‐size distribution to the average grain size G avg , should be 0.354 in normal grain growth. Since real materials do not have grain boundaries of the same energy and mobility, they should have a larger σ during grain growth, which is true for a large number of dense ceramics sintered in various ways [ 4,10–45 ] as shown by the blue data points in Figure . Meanwhile, Figure 1 also reveals that smaller σ tends to come with a smaller G avg .…”

Modeling of flash sintering of ionic ceramics

“…The resultant bimodal grain size microstructure is in qualitative agreement with abnormal grain growth of 8YSZ experimental results, as reported by Dong and Chen. 75 The macroscopic scale A set of coarse grain numerical descriptions have been developed to predict the onset of flash sintering, which is macroscopically determined by the electroluminescence produced by the sample. 4,7,8,76 Raj and co-authors proposed a lumpedparameter model based on blackbody radiation to determine the average temperature of a sample as a function of physical dimensions, furnace temperature, and applied electrical power.…”