Real‐Time Capturing of Microscale Events Controlling the Sintering of Lead‐Free Piezoelectric Potassium‐Sodium Niobate

Abstract

are used for a widespread of technological applications such as sensors, transducers, actuators and energy harvesting. [1][2][3] The implementation of KNN ceramics to the full spectrum of piezoelectric techno logies imposes to narrow their performance gap with regard to lead-based piezoelectric ceramics. In this context, a great deal of effort was devoted to the development and properties optimization of KNN ceramics through morphotropic and/or polymorphic phase boundaries engineering and sintering process optimiza tion. [4][5][6][7] Fundamentally, beyond the composition, the physical properties of ceramics are subjected to the influence of grain boundaries, grain size and grain size distribution. [8][9][10] Whereas point defects are often accumulated at the grain-boundary during the sintering process, the ferroelectric domains size, domain configurations and domain walls density are grain size dependent. [11][12][13] Therefore, an optimization of the KNN properties requires a good understanding and control of the grain growth rate and the elementary events determining the microstructural changes during sintering process.It is well known that the conventional sintering of stoichiometric KNN ceramics is a challenging task due to the hygroscopic nature of the alkali carbonates, the volatility of alkali elements, cuboidal grains that slow down the densification, and Sintering is a very important process in materials science and technological applications. Despite breakthroughs in achieving optimized piezoelectric properties, fundamentals of K 0.5 Na 0.5 NbO 3 (KNN) sintering are not yet fully understood, facing densification versus grain growth competition. At present, microscale events during KNN sintering under reducing atmospheres are real-time monitored using a High Temperature-Environmental Scanning Electron Microscope. A two contacting KNN particles model satisfying the Kingery and Berg's bulk diffusion model is reported. Dynamic events like individual grain growth and grain elimination process are explored through a postanalysis of recorded image series. The diffusion coefficient for oxygen vacancies of 10 −8 cm 2 s −1 and average boundary mobility of 10 −9 cm 4 J −1 s −1 are reported for the KNN ceramics. Moreover, the local pore shrinkage is consistent with the Kingery and François's concept of pore stability except that pore curvatures are not all concave, convex or flat due to anisotropic grain-boundary energies. The global grain growth kinetics are described using parabolic and/or cubic laws. The effect of atmospheres and microstructure evolution on the intrinsic and extrinsic contributions to the dielectric response using Rayleigh's law is also explored. These results bring a new breath for KNN sintering studies in order to adapt the sintering process.

High-Temperature Piezoelectric Response and Thermal Stability of BiGaO3 Modified BiFeO3–BaTiO3 Lead-Free Piezoelectric Ceramics

“…The other one is that more Bi 2 O 3 is added with the addition of BiGaO 3 , resulting in the generated impurity phase Bi 25 FeO 40 leading to the generation of a more liquid phase and promoting the growth of ceramic grains [36,37]. On the other hand, in pure dense ceramics, the parabolic law indicates that the grain boundary mobility controls grain growth [39], and the doping of BiGaO 3 may promote grain boundary migration. The relative density of BF-BT-xBG ceramics is shown in Figure 3.…”

Enhanced Piezoelectric Properties in a Single-Phase Region of Sm-Modified Lead-Free (Ba,Ca)(Zr,Ti)O3 Ceramics

“…With the introduction of Sm, the grain size gradually decreased from 17.7 μm ( x = 0) to 1.7 μm ( x = 1.5). The decreasing trend of the grain sizes is reported to be related to the decrease of the grain boundary mobility after doping [ 29 ].…”

Field‐Induced Multiscale Polarization Configuration Transitions of Mesentropic Lead‐Free Piezoceramics Achieving Giant Energy Harvesting Performance

“…For example, when the sintering temperature is below 1145 °C, the grain growth in 4BHT ceramics is extremely slow, showing fine grains. Once the temperature is above 1145 °C, driven by the temperature as well as the subcrystalline boundary energy (Figure S4, Supporting Information), [16,17] 4BKH ceramics start to form the recrystallized seeds (named as eutectic liquid). Under capillary forces, these seeds gradually nucleate and grow into coarse grains.…”