M. Kimura, A. Ando and Y. Sakabe,     Murata Manufacturing Co., Ltd, Japan

Abstract:

Lead zirconate titanate (PZT) is the most common piezoelectric ceramic and exhibits excellent properties as is well known. In this chapter, basic knowledge of PZT, including its crystal structure and phase relations, is presented at the beginning. Compositional design and shaping approach to highlight the excellent performance characteristics are also described, including recent advances; in particular, multilayer structure and low temperature sintering have become important in PZT applications. Future trends are discussed briefly at the end.

Key words:

lead zirconate titanate (PZT)

perovskite structure

morphotropic phase boundary (MPB)

multilayer ceramics

low temperature sintering

2.1 Introduction

Piezoelectric ceramics were first put to practical use around 1950,1 and have been actively used in a wide range of industries since then. Various applications have been devised using their electromechanical transducer ability, including communication circuit components, ultrasonic transducers, sensors and actuators. Such a wide application area is one characteristic of piezoelectric ceramics compared to other electro-ceramics. Many materials have been studied as possible candidate as practical piezoelectric ceramics over the years. In particular, lead-free materials are being actively surveyed because of increasing awareness of environmental issues at present, and several materials of interest have been discovered.2 However, most practical materials on the market are still based on lead zirconate titanate (Pb(Zr,Ti) O3: PZT), which is a solid solution of PbZrO3 and PbTiO3. The solid solutions were first studied by G. Shirane and co-workers in 1952,3–5 and the phase diagram was reported by E. Sawaguchi in 1953.6 Furthermore, the piezoelectricity was revealed by B. Jaffe et al. in 1955.7 Since then, this material has seen enormously developments by many research teams as a representative electronic material to date. PZT-based ceramics realize the necessary high performance characteristics for various usages at a relatively low cost, and it is the so-called ‘king of piezoelectric materials’.

A distinctive feature of PZT is its large piezoelectricity. PZT has a perovskite-type crystalline structure, which is represented by the compositional formula ABO3, and the structure is suitable to achieve large piezoelectricity, especially when the A site is filled by Pb. Moreover, the feature can be enhanced by compositional optimization. The piezoelectricity intensifies on a composition of phase boundary between rhombohedral and tetragonal phases in the solid solution. The phase boundary is better known as morphotropic phase boundary (MPB). In Section 2.2, the relationship between PZT’s crystalline structure and the excellent piezoelectric properties, containing MPB, are discussed.

On the other hand, some electrical properties required for practical usages are not necessarily highest on the MPB. For example, tetragonal phase PZT generally indicates higher heat-proof characteristics, and it is often chosen for applications requiring high temperature reliability. Tetragonal, rhombohedral and the MPB compositions are appropriately chosen to suit the demands of each application in this way. The phases of PZT are easily controlled by the compositional change of the zirconate and titanate ratio in several situations. Also, various doping elements change PZT’s properties drastically by substituting on the cation sites. Doping with low-valent elements generally improves the mechanical quality factor Qm of PZT; while high-valent elements often increase the piezoelectric d constant. These kinds of compositional modifications are commonly used on the material design of PZT. A wide variety of electrical properties is often more important than large piezoelectricity for practical applications, and this can easily be realized by compositional modifications. In Section 2.3, these compositional modification techniques are described.

Moreover, PZT has the advantage of good shaping flexibility. Piezoelectric devices utilize the mechanical displacement or vibration, and therefore the performance is greatly influenced by a device shape also in the case of non-resonant devices. Structural innovations have been extensively studied to obtain higher performance. Ceramics have good forming flexibility, and in addition PZT has good workability. As is well known, there are other piezoelectric materials with superior piezoelectricity, for example, piezoelectric single crystals. However, their shaping flexibility is inferior to that of PZT ceramics, and unlike PZT they do not have a wide range of applications.

One of the most important shaping technologies is the multilayer technique. Ceramic components co-fired with multiple inner electrodes are often used in PZT applications, because of their high displacement, high reliability and low production cost. Noble metals with high melting point were generally used initially as an inner electrode material for the co-fired multilayer component. However, base metals or base metal alloys, whose melting points are generally lower than those of noble metals, are preferable for the inner electrode material on the cost front. Therefore, sintering temperature reduction of piezoelectric ceramics is decisively important, and has been actively studied. In Sections 2.4 and 2.5, recent trends in piezoelectric ceramic applications are briefly introduced, and the shaping techniques of PZT are discussed with a main focus on the multilayer technique. Then, sintering temperature reduction techniques in PZT are also described.

PZT-based ceramics are expected to be used in a wide range of applications at least for the forseeable future, because of their high performance and good industrial usability. Creation of new markets is also anticipated for PZT. At the end of the chapter, remaining research challenges and future trends in applications are briefly described.

2.2 Crystalline structure and phase relations

PZT is a solid solution of lead zirconate (PbZrO3) and lead titanate (PbTiO3), and has perovskite crystalline structure, the same as the two compositional end members. Perovskite-structured materials indicate excellent properties as functional materials on dielectric, piezoelectric, electro-optic, semi-conducting or superconducting fields.8 They play major role in modern electronics, and are indispensable for the forseeable future.

The perovskite ABO3 structure of PZT is shown in Fig. 2.1(a). A-site cations build a cuboid box, and an oxygen octahedron falls within the box. A B-site cation is placed around the center of the oxygen octahedron. In the case of PZT, the A-site is filled by Pb ions, and the B-site is randomly filled by Zr or Ti ions. The oxygen octahedron is sifted off the center of the cuboid box formed with Pb ions, and the B-site cations are also sifted from the center of the oxygen octahedron in the ferroelectric state of PZT as shown in Fig. 2.1(b). The spontaneous polarization is formed between those cations and the oxygen octahedron.

Cohen and Krakauer calculated the electron density distribution of perovskite barium titanate (BaTiO3) using first principle computer simulation in 1990,9 and suggested that the large spontaneous polarization of BaTiO3 is strongly affected by the covalent bonding between the Ti and O ions. BaTiO3 is essentially an ionic binding crystal, however their calculation results indicated that covalent bonding certainly existed between the Ti 3d and O 2p states. The covalent bonding probably induces large crystalline distortion and an electric dipole. Therefore, the large spontaneous polarization of BaTiO3 is possibly due to the covalent bounding. Piezoelectric strain of ferroelectric materials S is roughly expressed by Eq. (2.1) with electrostriction coefficient Q, spontaneous polarization Ps, dielectric permittivity e and induced electric field E, and large spontaneous polarization leads to large piezoelectricity

2.1

From these findings, the perovskite structure is suitable to achieve large piezoelectricity.

On the other hand, the electronic states belonging to the Ba ion did not contribute to the covalent bonding and only played the role of a ‘spacer grid’ in the BaTiO3 crystal in these studies. However, it was reported in another paper by Cohen10 that the Pb 6s state contributed to the covalent bonding between the Ti and O ions in the case of PbTiO3, and the contribution possibly strongly enhanced the crystal distortion. Spontaneous polarization of PbTiO3 is three times larger than that of BaTiO3. Cohen proposed that this large spontaneous polarization is due to the Pb 6s electrons hybridized with the covalent bonding between the Ti and O ions, and this is the reason why PbTiO3 indicates larger piezoelectricity than BaTiO3. This Pb 6s contribution was confirmed also by X-ray diffraction MEM (maximum entropy method)...