The correlation between the microstructure and macroscopic properties of (K,Na,Li)(Nb,Ta)O₃ ceramic via rare earth oxide doping

Abstract

Li_0.05(Na_0.51K_0.49)_0.95(Nb_0.95Ta_0.05)O₃ (abbreviated as KNNLT) with addition of La₂O₃, CeO₂, respectively, were prepared by a conventional ceramic fabrication technique. X-ray diffraction pattern shows that the coexistence of orthorhombic and tetragonal phase structure for KNNLT was not changed by adding La₂O₃ or CeO₂. SEM observation indicates that these two kind dopants have a remarkable effect on the microstructure of KNNLT ceramics. The T_o–t (temperature for the orthorhombic to tetragonal phase) was shifted toward lower temperature by the addition of rare earth oxide and typical relaxor ferroelectric is demonstrated especially for KNNLT with high content of La₂O₃. An addition of 0.5% CeO₂ could strengthen the piezoelectric and dielectric constant of KNNLT ceramics to some extent. KNNLT ceramics with 0.5% CeO₂ obtain optimum electrical properties: d₃₃=272 pC/N, k_p=45%, ε_r=784 and tan δ=1.4%.

Introduction

Among the well-known lead-free piezoelectric ceramics systems, K_xNa_1−xNbO₃ (KNN)-based systems have become a hot area of focus and are recognized as one of the most promising candidates for application. Because they possess good piezoelectric and dielectric properties especially after A-site and/or B-site being partially replaced [1], [2]. However, the major drawbacks of KNN ceramics are (i) the need for special treatment of the starting powders due to volatility of alkaline elements, (ii) high sensitivity of the properties to stoichiometry, and (iii) complex densification processes [3]. In order to solve the densification problems, different advanced processes such as spark plasma sintering [4] and hot pressing [5] were developed to promote the sintering, which lead to the enhancement of both the density and the related properties. Although the high piezoelectric constant is induced in KNN-based ceramics by the above mentioned preparation techniques, their complicated preparation process still hinders the practical application.

The doping of metal oxide such as MnO₂, Fe₂O₃, NiO, CuO is a feasible approach to improve the sintering of the piezoceramics [6], [7], [8], [9], [10], [11], [12]. The densification of KNN-based materials was evidently improved by the forming of the liquid phase induced by these sintering aids and hence the piezoelectricity of the ceramics could be promoted. These doping mechanisms could be generally divided into two categories and lots of literature have been reported in these two kinds doping mechanisms. One is donor doping and the other is acceptor doping. The doping mechanism is closely related with the ionic radius and electrovalence of doping metal element. The effect of La₂O₃ and CeO₂ on the microstructure and electrical properties of lead-free system such as BNT, BNT–BKT–BT and BT have been investigated [13], [14], [15]. However, there are few literature on KNN-based ceramics doped with La₂O₃ or CeO₂ [16]. Due to the similarity of ionic radius between Na⁺, K⁺ and La³⁺, Ce³⁺, it is believed that the introduction of rare earth oxide might bring about considerable influence on the microstructure and performance. Taking the above into consideration, the research concerning the influence of doping of rare earth oxide on KNN-based materials would be meaningful.

In this work, Li_0.05(Na_0.51K_0.49)_0.95(Nb_0.95Ta_0.05)O₃ (abbreviated as KNNLT) was used as matrix and rare earth oxide (La₂O₃, CeO₂) were selected as dopants. The influences of dopant on the phase structure, the microstructure, the dielectric, ferroelectric and piezoelectric properties of perovskite-type KNN-based ceramics were investigated. The doping mechanism of the rear earth oxide was also discussed. In doing so, we could fully understand that piezoelectric response is dependent on multiple variables such as grain size, and compact degree. These factors have been thoroughly studied and can be used to further modify the magnitude of piezoelectric response once a proper formulation has been identified.