Low temperature consolidated lead-free ferroelectric niobate ceramics with improved electrical properties
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Haixue Yan School of Engineering and Materials Science, Queen Mary University of London, London, E1 4NS, United Kingdom; and Nanoforce Technology Ltd., London, E1 4NS, United Kingdom
Mats Nygren Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, SE-10691 Stockholm, Sweden
Mike J. Reece School of Engineering and Materials Science, Queen Mary University of London, London, E1 4NS, United Kingdom; and Nanoforce Technology Ltd., London, E1 4NS, United Kingdom
Zhijian Shena) Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, SE-10691 Stockholm, Sweden (Received 18 June 2009; accepted 3 November 2009)
There is a concerted effort to develop lead-free piezoelectric ceramics. (Na0.5K0.5)NbO3based ceramics have good electrical properties, and are a potential replacement material for lead zirconate titanate piezoelectric ceramics. In this work a commercial powder based on (Na0.5K0.5)NbO3 with an initial particle size of 260 nm was consolidated by spark plasma sintering (SPS). To avoid volatilization, high mechanical pressures were used to minimize the densification temperature. It was found that under a uniaxial pressure of 100 MPa, fully densified compacts can be prepared at 850 C. Ceramics densified at such a low temperature demonstrate an unusually high remanent polarization (30 mC/cm2) and high d33 (146 pC/N). The improved ferroelectric properties are ascribed to the homogeneous, dense, and submicron grained microstructure achieved.
I. INTRODUCTION
Sodium potassium niobate-based ceramics, (Na,K) NbO3 (NKN), have been recognized as an environmentally friendly lead-free piezoelectric material that demonstrates potential for replacing the commonly used lead zirconate titanate (PZT).1,2 In the KNbO3–NaNbO3 solid solution system, the highest piezoelectric constant has been observed for ceramics with morphotropic phase boundary (MPB) composition. There is a MPB located between two different orthorhombic phases at about 50 mol% NaNbO3.3,4 Based on calorimetric studies, Tennery et al.5 found that there are two other MPBs in this system (at 25 C at 82.5 and 67.5 mol% NaNbO3). However, they do not exhibit good piezoelectric properties. Recently, Zhang et al.6 observed a discontinuous change a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0034
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http://journals.cambridge.org
J. Mater. Res., Vol. 25, No. 2, Feb 2010 Downloaded: 17 Mar 2015
in lattice parameter close to the composition of 60 mol% NaNbO3, indicating the presence of another MPB. Since Jaeger et al.7 reported that Na0.5K0.5NbO3 ceramics hot-pressed (HP) at 1100 C have high Curie temperature, TC, (420 C), and piezoelectric constant, d33 (160 pC/N), much attention has been paid to this MPB composition. However, so far no one has been able to repeat that experiment, i.e., prepared NKN ceramics with such a high d33 value, even using a reactive template grain growth method to facilitate the formation of textured ceramics.8 Sintering of NKN to fully dense
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