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Materials (Basel). 2015 Dec 1;8(12):8117-8146. doi: 10.3390/ma8125449.

Sintering of Lead-Free Piezoelectric Sodium Potassium Niobate Ceramics.

Author information

1
Jožef Stefan Institute, Jamova cesta 39, Ljubljana 1000, Slovenia. barbara.malic@ijs.si.
2
Jožef Stefan International Postgraduate School, Jamova cesta 39, Ljubljana 1000, Slovenia. barbara.malic@ijs.si.
3
Technische Universität Darmstadt, Alarich-Weiss-Str. 2, Darmstadt 64287, Germany. koruza@ceramics.tu-darmstadt.de.
4
Jožef Stefan Institute, Jamova cesta 39, Ljubljana 1000, Slovenia. jitka.hrescak@ijs.si.
5
Jožef Stefan International Postgraduate School, Jamova cesta 39, Ljubljana 1000, Slovenia. jitka.hrescak@ijs.si.
6
Slovenian National Building and Civil Engineering Institute, Dimičeva 12, Ljubljana 1000, Slovenia. janez.bernard@zag.si.
7
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China. wang-ke@tsinghua.edu.cn.
8
School of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, Korea. johnfisher@jnu.ac.kr.
9
Jožef Stefan Institute, Jamova cesta 39, Ljubljana 1000, Slovenia. andreja.bencan@ijs.si.

Abstract

The potassium sodium niobate, K0.5Na0.5NbO₃, solid solution (KNN) is considered as one of the most promising, environment-friendly, lead-free candidates to replace highly efficient, lead-based piezoelectrics. Since the first reports of KNN, it has been recognized that obtaining phase-pure materials with a high density and a uniform, fine-grained microstructure is a major challenge. For this reason the present paper reviews the different methods for consolidating KNN ceramics. The difficulties involved in the solid-state synthesis of KNN powder, i.e., obtaining phase purity, the stoichiometry of the perovskite phase, and the chemical homogeneity, are discussed. The solid-state sintering of stoichiometric KNN is characterized by poor densification and an extremely narrow sintering-temperature range, which is close to the solidus temperature. A study of the initial sintering stage revealed that coarsening of the microstructure without densification contributes to a reduction of the driving force for sintering. The influences of the (K + Na)/Nb molar ratio, the presence of a liquid phase, chemical modifications (doping, complex solid solutions) and different atmospheres (i.e., defect chemistry) on the sintering are discussed. Special sintering techniques, such as pressure-assisted sintering and spark-plasma sintering, can be effective methods for enhancing the density of KNN ceramics. The sintering behavior of KNN is compared to that of a representative piezoelectric lead zirconate titanate (PZT).

KEYWORDS:

KNN; lead-free piezoelectric; microstructure; sintering; sodium potassium niobate

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