Abnormal grain growth in (K, Na)NbO₃‐based lead‐free piezoceramic powders

Abstract

Abnormal grain growth (AGG) is frequently observed in sintered (K, Na)NbO 3 (KNN)-based piezoceramics. However, in the present study, abnormal grain growth was unexpectedly discovered in calcined KNN-based powders. To explain the phenomenon, three well-established models that account for the AGG in sintered ceramics were discussed, including (a) liquid-phase-assisted grain growth, (b) two-dimensional nucleation grain growth, and (c) complexion coexistence.However, the AGG in calcined powders was concluded to be none of them, but a consequence of the A-site compositional inhomogeneity in the K 2 CO 3 -Na 2 CO 3 -Nb 2 O 5 ternary system. Since repeated calcination and ball milling have low efficiency on solving AGG and the accompanied compositional inhomogeneity, abnormal grains were found to coexist with normal grains at a very high calcination temperature, that is, 1000°C. The compositional inhomogeneity is believed to be remaining even after sintering and consequently deteriorate the comprehensive performances, which might be a determinant for the unstable reproduction of KNN-based piezoceramics.

Hybrid Atmosphere Processing of Lead-Free Piezoelectric Sodium Potassium Niobate-Based Ceramics

“…In addition to abnormal grain growth at high temperatures, abnormal growth and coarsening of KNN-based powders can also be a problem at lower temperatures, hindering further densification [26,28,29]. The reactivity of the alkali elements towards ambient CO 2 and H 2 O can be a cause of this coarsening: Alkali carbonates or hydroxides have low melting points and can form a transient liquid phase at low temperatures (~800 • C) that promotes particle coarsening [26].…”

Dense KNN Polycrystals Doped by Er2O3 Obtained by Hot Pressing with Hexagonal Boron Nitride Protective Layer

“…It suggests that h-BN deposited on sample surface reacted with powders during sintering. As a result, abnormal grain growth, typical for KNN ceramics [ 25 , 26 ], was observed in close proximity to the surface. This phenomenon could be enhanced by the presence of boron, which has a positive effect both on densification and grain growth during thermal processing [ 24 ].…”

Influence of chemically synthesized powder addition on K0.5Na0.5NbO3 ceramic’s properties

“…KNN-based piezoceramics show, in fact, interesting characteristics such as high T C , good temperature stability of their piezoelectric properties, good mechanical quality factor and fatigue resistance, low density, and biocompatibility [2]. However, pure KNN and KNN-based materials still suffer from reproducibility issues such as volatilization of lowmelting Na 2 O and K 2 O species during calcination or sintering steps, which can detrimentally affect their final stoichiometry [3], chemical inhomogeneities in the calcined powders and sintered samples [4], a very narrow sintering temperature range, immediately below its solidus temperature [5], and abnormal grain growth phenomena reported to occur during sintering [6] or even during calcination [7]. Moreover, it was demonstrated in previous works [8,9] that in undoped K 0.5 Na 0.5 NbO 3 , the final piezoelectric properties are strongly affected by each step of the production process; among these, insufficient milling treatment of the starting powders can lead to chemical composition inhomogeneities in the ceramics, resulting in poor piezoelectric properties.…”