Ferroelectric Phases and Relaxor States in the Novel Lead-Free ( 1 – X ) Bi1/2k1/2tio3 - X Bisco3 System ( 0 ≤ X ≤ 0.3 )

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  • Topic: Piezoelectricity, Condensed matter physics, Lead zirconate titanate
  • Pages : 14 (4458 words )
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  • Published : April 11, 2013
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Ferroelectric phases and relaxor states

in the novel lead-free

( 1 – x ) Bi1/2K1/2TiO3 - x BiScO3 system ( 0 ≤ x ≤ 0.3 ) Leticia Martín-Arias, Miguel Algueró and Alicia Castro*. Instituto de Ciencia de Materiales de Madrid, CSIC. Cantoblanco, 28049 Madrid, Spain. acastro@icmm.csic.es * acastro@icmm.csic.es Tel: +34 91 334 9000 Fax: +34 91 372 0623

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Abstract

The novel lead-free Bi-based ferroelectric system with perovskite structure (1 − x) Bi1/2K1/2TiO3 – x BiScO3 was synthesized by a conventional solid-state reaction method for compositions with 0 ≤ x ≤ 0.3. The phases obtained were characterized by X-ray powder diffraction and scanning electron

microscopy. Dense ceramics were prepared by a conventional sintering technique at 950-1000°C depending on composition, and their structure and electrical properties were systematically investigated. The XRD analysis shows that the samples possess a pure perovskite structure for x < 0.3 and reveals a phase evolution in the symmetry from tetragonal for x < 0.1 to pseudocubic for 0.1 ≤ x ≤ 0.3. Electrical properties clearly indicate that a transition from conventional ferroelectric to relaxor ferroelectric behavior occurs when increasing the (Bi, Sc) content between 5% and 10%. Implications in the design of novel lead-free piezoelectric materials are discussed.

Keywords: powders-solid state reaction, dielectric properties, ferroelectric properties, piezoelectric properties, perovskites.

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1 Introduction

Environmental pollution is one of the main causes of concern nowadays, which has led to harsh environmental regulations being taken. In the case of piezoceramics, they require the search for lead-free materials with comparable properties to those of the well-known lead-based ferroelectric perovskites like Pb(Zr,Ti)O3 (PZT)1. In PZT ceramics with compositions lying near the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases, high dielectric and piezoelectric properties are found. This is due to the presence of a monoclinic Cm phase at the MPB, which provides a path for the polarization to rotate between its directions in the rhombohedral and tetragonal phases, with the positive outcome of an enhanced electromechanical response2. Several solid solutions have been targeted in the quest for lead-free or lead-less substitutes for PZT, which were designed to obtain analogous MPBs and equally high piezoelectric properties3-6. K1-xNaxNbO3 is one such system that has been thoroughly investigated during the last years because of its high Curie temperature and good piezoelectric properties7. High values of piezoelectric coefficients, comparable to those of PZT, are obtained by the downward shift of the temperature of the orthorhombic to tetragonal phase transition TO-T, by doping. High coercive field and low remnant polarization, with some exceptions, and their difficult processing are, however, cause of concern. Yet their main problem is the temperature dependence of their properties and degradation caused by thermal cycling between the different ferroelectric states. This is a common problem for materials that have been engineered to be at polymorphic phase transitions (PPT), those for which the phase transition is controlled with temperature, so materials with MPBs, which involve a phase evolution with composition, are thus preferred. Examples of actual MPB systems, are BiScO3 - BaTiO38,9 and the widely studied Bi1/2Na1/2TiO3 - BaTiO310 for which enhanced properties have been reported. However, in the latter case, Tc shifts to lower temperatures with BaTiO3 addition as compared with that of the Bicontaining perovskite. Also, there is a ferro-antiferroelectric transition that occurs at temperatures much 3

lower than Tc, which further decreases the operation temperature range. Therefore, a suitable candidate to replace PZT has not been found yet. A lead less material with interesting properties is BiScO3-PbTiO311. The...
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