Co-Doped Strategy Enhances Electrical Properties of High-Temp Ceramics

In the field of aerospace, a high-temperature piezoelectric vibration sensor is one of the few key devices that can be monitored in a high-temperature and harsh environment, so it is particularly urgent to develop high performance high-temperature piezoelectric ceramics as the core component of this kind of sensor. Bi₄Ti₃O₁₂ (BIT), as one vital type of bismuth layered structure ferroelectrics (BLSFs), has great application prospects in high-temperature environments due to its excellent T_C of 675 ℃. However, the volatilization of Bi during the sintering process in BIT-based ceramics leads to the generation of oxygen vacancy defects, resulting in relatively low piezoelectric activity. The proposed B-site non-equivalent co-doped strategy has been proven to be a useful way to effectively reduce the concentration of oxygen vacancies and to improve the comprehensive electrical properties of BIT-based ceramics.

A research group led by Professor Yejing Dai from Sun Yat-sen University in Shenzhen, China, recently reported a new non-equivalent co-doped strategy for BIT-based high-temperature piezoelectric ceramics to solve the above mentioned problems. By the B-site modification for the BIT-based ceramics, it is usually hard to obtain both a high piezoelectric coefficient and a high Curie temperature, as well as large resistivity at high temperatures. There seems to be a mutual restriction between d₃₃ and T_C due to the difficulty of simultaneously achieving excellent electrical properties and good structural stability. This research is aimed at synergistically optimizing the two parameters by using the B-site non-equivalent co-doped strategy of combining high-valence Ta⁵⁺ and low-valence Cr³⁺.

Dai et al. published their research in the Journal of Advanced Ceramics on February 21, 2024 (2024, 13(3): 263-271).

"In this research, we chose high-valence Ta⁵⁺ and low-valence Cr³⁺ non-equivalent co-doped BIT ceramics to solve the problem that high piezoelectric performance, high Curie temperature, and high-temperature resistivity could not be achieved simultaneously in BIT-based ceramics. A series of Bi₄Ti_3−x(Cr_1/3Ta_2/3)_xO₁₂ ceramics were synthesized by the solid-state reaction method. The phase structure, microstructure, piezoelectric performance, and conductive mechanism of the samples were systematically investigated. The B-site non-equivalent co-doping strategy combining high-valence Ta⁵⁺ and low-valence Cr³⁺ significantly enhances electrical properties due to a decrease in oxygen vacancy concentration. When the doping content is 0.03 mol, ceramics exhibit a high piezoelectric coefficient of 26 pC·N⁻¹ and a high Curie temperature of 687 ℃. Moreover, a significantly increased resistivity of 2.8×10⁶ Ω·cm at 500 ℃ and good piezoelectric stability up to 600 ℃ are also obtained for this composition. All the results demonstrate that Cr/Ta co-doped BIT-based ceramics have great potential to be applied in high-temperature piezoelectric applications." said Ms. Xuanyu Chen, the first author of the paper and a doctoral student in the School of Materials at Sun Yat-sen University.

The non-equivalent co-doping strategy is an effective method to enhance the electric performance of BIT-based ceramics. Through the introduction of non-equivalent ion pairs, the concentration of oxygen vacancy defects in the BIT ceramics was effectively reduced, and the anisotropy of the grain growth decreased. This provides a new idea for further improving the piezoelectric properties of BIT-based ceramics and promoting their application in the field of high-temperature sensing.

The next step of the research group is to induce A-site ions such as La³⁺ on a B-site non-equivalent co-doped basis. "We expect that A/B site co-doping will further increase the piezoelectric activity of the BIT-based ceramic, and then we will reveal the effect of A/B site co-doping on the domain structure of the sample compared to the B-site non-equivalent co-doping," Ms. Xuanyu Chen said. The aim of the research team is to fabricate bismuth-layered piezoelectric ceramic devices with excellent electrical properties suitable for working at high temperatures.

Other contributors include Ms. Ziqi Ma, Professor Bin Li, and Professor YeJing Dai from the School of Materials at Sun Yat-sen University in Shenzhen, China.

This work is financially supported by the National Natural Science Foundation of China (No. 52172135), the Youth Top Talent Project of the National Special Support Program (No. 2021-527-07), the Leading Talent Project of the National Special Support Program (No. 2022WRLJ003), and the Guangdong Basic and Applied Basic Research Foundation for Distinguished Young Scholars (Nos. 2022B1515020070 and 2021B1515020083).