Material demonstrating a huge piezoelectric response developed by Kumamoto University

A research group including Assistant Professor Yukana Terasawa of the Faculty of Advanced Science and Technology at Kumamoto University, Distinguished Professor Yusuke Yamauchi of the Graduate School of Engineering at Nagoya University, and the University of Queensland (Australia), announced that they have successfully synthesized a nanoporous Ba_0.85Ca_0.15(Ti_0.9Zr_0.1)O₃ thin film of barium titanate co-doped with calcium (Ca) and zircon (Zr), that exhibits a giant piezoelectric response. This material can be utilized as a renewable energy source, thereby providing an alternative to fossil fuels. The results were published in the international journal Chemical Science on May 20th.

An insulator is a material through which an electric current hardly flows; however, when an external voltage is applied to it, dielectric polarization occurs. Among dielectrics, materials that have the property of generating an electric charge when pressure is applied (positive piezoelectric effect) or strain (deformation) when voltage is applied (reverse piezoelectric effect) are called piezoelectric materials. These materials are commonly used in sensors, actuators, oscillators, and ultrasonic diagnostic equipment.

Lead zirconate titanate (PZT), which exhibits piezoelectricity, has been used as a major piezoelectric material. However, it has a high environmental impact. Therefore, barium titanate (BaTiO₃) is expected to serve as an alternative owing to its optical permittivity and low dielectric loss. However, its piezoelectric properties do not match those of lead-based materials.

Yamauchi and his team have been investigating the fabrication of metals and metal oxides in porous form to develop new energy materials. They have established a method for fabricating porous metal materials with controlled pore size using polystyrene (PS) and polyethylene oxide (PEO) block copolymers (PS-b-PEO) as templates. In this study, the research group synthesized a nanoporous BCZT thin film with a nano-sized pore (porous) structure and investigated its piezoelectric properties (BCZT = (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃). The piezoelectric strain constant (d₃₃), which indicates piezoelectric properties, was about 7500 pm^V-1 for the nanoporous BCZT thin film, which is more than 10 times higher than that of the nonporous BCZT thin film (approximately 600 m^V-1). This value is more than one order of magnitude larger than that of lead zirconate titanate (PZT) ceramics, a typical conventional lead-based piezoelectric material. Quantitative analysis of lattice distortion revealed approximately 30% distortion in the nanoporous BCZT thin film. However, the strain in the nonporous BCZT thin film was approximately 0.3%. The introduction of the nanoporous structures may have introduced substantial lattice strain, resulting in enhanced piezoelectric properties, and the piezoelectric properties were successfully improved by controlling the structure at the nano-level.

Terasawa said, "In this research, we have introduced nanoporous structures (nanoscale pores) into lead-free piezoelectric materials and succeeded in synthesizing a material with high piezoelectricity. The surface area and pore volume of a material can be increased with an increase in the porosity. Porous materials serve a wide variety of industrial applications, such as adsorption, catalysts, and sensor materials. We believe that applying porosity to any material brings numerous application potential."

Journal Information
Publication: Chemical Science
Title: Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O₃ thin film
DOI: 10.1039/d3sc06712b