High Curie temperature bismuth layer-structured ferroelectric calcium bismuth niobate (CaBi<sub>2</sub>Nb<sub>2</sub>O<sub>9</sub>) piezoelectric ceramics

Piezoelectric materials play a key role in various kinds of modern electronic devices. High Curie temperature piezoelectric materials are required for piezoelectric sensors under critical conditions for nuclear power plants, automotive, aeronautic, and aerospace applications. These piezoelectric sensors usually withstand high temperature of 500°C or higher. To meet the requirements of these applications, piezoelectric materials must possess high piezoelectric performance and high electrical resistivity as well as high Curie temperature. The Aurivillius-type bismuth layer-structured ferroelectrics (BLSFs) are the most promising high-temperature piezoelectric materials because of their very high Curie temperature ( T c). Structurally, the BLSF compounds are composed of fluorite-like (Bi2O2)2+ layers and perovskite-like (A m −1B m O3 m +1)2− blocks, which arrange along the c axis alternately and regularly. The general formula is (Bi2O2)2+(A m −1B m O3 m +1)2−, where A is a mono-, di-, or trivalent metallic ion (or their combination) suited to dodecahedral coordination; B is a transition metallic ion suited to octahedral coordination, and m is the number of BO6 octahedral in the perovskite-like blocks and varies from 1 to 4. In this paper, the crystal structure, piezoelectric properties (herein piezoelectric constant d 33 value), and Curie temperature ( T c) of prototype BLSF ceramics have been summarized, and the summarized results indicate that the piezoelectric properties of high T c BLSF compounds are very low. The prototype BLSF ceramics with T c of >700°C exhibit low piezoelectric d 33 value of no more than 10 pC/N, and the prototype BLSF ceramics with T c of >900°C exhibit low piezoelectric d 33 value of no more than 5 pC/N. Calcium bismuth niobate, CaBi2Nb2O9 (CBN), was firstly found and synthesized by Sweden scientist Aurivillius B, in 1949. But until 2005, the investigations on the piezoelectric properties of CBN were firstly reported by Yan H X and Reece M J. After that, the piezoelectric properties of CBN have attracted much attention. The compound CBN has a very high Curie temperature T c of ~940°C, and it is considered to be potential material as piezoelectric sensors and other electronic components for high-temperature applications. However, its particular bismuth layer-structured crystal structure restricts the rotation of the spontaneous polarization ( P s) in the two-dimensional orientation ( a - b plane), and large coercive field ( E c) makes the material difficult to be poled sufficiently. The prototype CBN ceramic exhibits very low piezoelectric performance, with low piezoelectric d 33 value of 5 pC/N. In this paper, the piezoelectric properties of bismuth layer-structure ferroelectric calcium bismuth niobate (CaBi2Nb2O9) piezoelectric ceramics have been summarized. The results indicate that A-site ions substitution is more effective than B-site ions substitution to enhance the piezoelectric properties of CaBi2Nb2O9 ceramics, and A-site and B-site ions co-substitution is more desirable to optimize the piezoelectric performance. The piezoelectric d 33 can be promoted to >16 pC/N by compositions adjusting, while the Curie temperature remains higher than 900°C. Texture technologies, such as templated grain growth (TGG) and spark plasma sintering, have been introduced, and they are good methods to further enhance the piezoelectric performance of CaBi2Nb2O9 ceramics. The piezoelectric d 33 can be promoted to higher than 20 pC/N by texture technologies. Overall, the CaBi2Nb2O9 ceramics are promising materials for high-temperature piezoelectric sensor applications. Finally, perspectives on how to further enhance the piezoelectric performance of CaBi2Nb2O9 ceramics are presented.