Ferroelectric nanocomposite with high dielectric constants
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Ferroelectric nanocomposite with high dielectric constants Mai T.N. Pham 1, B.A. Boukamp, H.J.M. Bouwmeester and D.H.A. Blank MESA+ Research Institute, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands. 1
Corresponding author: [email protected]
ABSTRACT Composites between ferroelectric material and a dispersed metal phase are of great interest due to the improvement in dielectric properties for such applications as high capacitance capacitors, non-volatile memory, ect. Using a colloidal method, Pt particles with a size of 3-5 nm were dispersed homogeneously in a PZT (PbZr0.53Ti0.43O3) matrix. No unwanted reaction phase between PZT and Pt during sintering at 1150 0C could be detected by X-ray diffraction. Electrical properties were investigated by impedance spectroscopy measurement. The effective dielectric constant increased remarkably as a power function of Pt volume content and can be described by the percolation theory. At 25 vol.% of Pt the dielectric constant of the composite is 4 times larger than that of pure PZT. The temperature dependence of the electrical properties is also influenced by the metallic phase fraction. INTRODUCTION Ferroelectric materials are widely used in industry as high-k materials for capacitors and memory devices such as DRAM (Dynamic Random Access Memory) or NVRAM (Non-Volatile Random Access Memory). Among these materials, PbZrxTi1-xO3, abbreviated PZT, is a wellknown material due to its high dielectric constant, high polarisation and low coercive field [1]. Recently, Duan et al. [2] observed an improvement in the dielectric constant of PZT-based composites through addition of Pt, which can be explained by the percolation theory. According to this theory, when a conducting phase is added to an insulating phase, the effective dielectric constant will increase with the volume content of the conducting phase and will increase very sharply near the critical volume fraction. Effective Medium Theory predicts a value of 0.33 for the critical volume fraction and a universal value for the critical exponent. Actually, those values vary, however, with the structural characteristics of materials [3]. Especially, the particle size ratio of conducting phase and insulating phase strongly influences the critical volume fraction. The smaller the ratio, the smaller the critical volume fraction, which can be as low as 0.09 for a ratio of 0.1 [4]. Our purpose is to prepare dual-phase ferroelectric (insulator) - metal (conductor) composites between PZT and Pt. The conventional solid-state reaction is the simplest preparative route. This method, however, results in poor homogeneity and large aggregations of Pt particles, which is mainly due to the large difference in density between PZT (7.8 g/cm3) and Pt (22.4 g/cm3) [2]. A significant improvement in homogeneity can be obtained by first preparing a stable Pt sol, through reduction of a Pt-salt, followed by direct precipitation of nano-sized Pt particles onto PZT particles.
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