Synergistically enhancing the piezoelectric activity and Curie temperature of CaBi4Ti4O15 ceramics via co-doping Gd/Mn at A/B-site

In recent years, there is growing demand in using piezoelectric materials in harsh environments and at high temperatures, such as for structural health monitoring of hot components in aircraft engines, gas turbines, nuclear power plants, and petroleum refineries, etc. Unfortunately, the conventional piezoelectric materials such as lead zirconate titanate (PZT) are limited to the operating temperatures below 300 ℃ due to its limited Curie temperatures (T_C≤380 ℃). There is an urgent demand for developing the piezoelectric materials with high Curie temperature.

Due to a high T_C (≥500 ℃), a low dielectric permittivity and aging rate, as well as a good resistance to fatigue, bismuth layer structured ferroelectrics (BLSFs, also called the Aurivillius-type compounds) are suitable for the high-temperature piezoelectric applications. These compounds have a general chemical formula of (Bi₂O₂)²⁺(A_m-1B_mO_3m+1)^2-, where m represents the number of BO₆ octahedra in the perovskite-like layer (A_m-1B_mO_3m+1)^2-. As a member among the BLSFs family, CaBi₄Ti₄O₁₅ (CBT, m=4) is considered as a strong competitor among the high-temperature piezoelectric materials. It possesses a high Curie temperature (T_C=790 ℃), but its piezoelectric coefficient is very small (d₃₃≤10 pC/N). The poor piezoelectric activity of BLSFs is attributed to the limited switching of spontaneous polarization in the a-b plane, as well as the difficult poling process subjected to the defects-pinning related high coercive field and the high conductivity at elevated temperature. Ions substitution occurring at different crystallographic sites could introduce special lattice defects and then modify the electrical properties of ferroelectric materials. Many materials scientists have studied the improvement of piezoelectric activity of CBT through various cations substituting for A-site Ca²⁺ and/or B-site Ti⁴⁺ in the perovskite-like layer, but the improvement of piezoelectric activity is always accompanied by the decline of Curie temperature. It is well known that it is hard to achieve both high piezoelectric activity and high Curie temperature in BLSFs, especially harder in those non-textured CBT ceramics that were prepared by the conventional sintering process.

Recently, a research team for advanced functional materials led by Qingyuan Wang and Yu Chen from Chengdu University, China, reported a kind of Gd/Mn co-doped CBT ceramics that were prepared by the conventional sintering process. The doping concentration effects of Gd³⁺ on the structures and properties of CBT were studied, the underlying mechanisms of Gd³⁺/Mn³⁺ substituting at A/B-site for synergistically enhancing the piezoelectric activity and Curie temperature of CBT were also revealed. The proposed approach can also provide a guidance for the future composition design and performance optimization of BLSFs used for high-temperature piezoelectric devices.

The team published their work in Journal of Advanced Ceramics on August 5, 2024.

“In this work, the method of double-ions co-substituting at different crystalline sites was tried to modify the electrical properties of CBT. A kind of Gd/Mn co-doped CBT ceramics with chemical formula of Ca_1-xGd_xBi₄Ti₄O₁₅ + 0.2 wt.%MnO₂ (CBT-100xGM, x=0~0.11) were prepared by the conventional sintering process. The optimized composition with x=0.08 (CBT-8GM) showed a high T_C~809 ℃ and a high d₃₃~23 pC/N and, as well as a g₃₃ value up to 21.5×10^-3 Vm/N. Also, it can keep a residual d₃₃~80% after being annealed at 700 ℃. These excellent properties provides this material with great application potentials in the high-temperature piezoelectric devices such as ultrasound transducer and accelerometer, etc, with operating temperature exceeding 500 ℃.” said by Yu Chen, a professor at the School of Mechanical Engineering at Chengdu University, whose research interests focus on the field of high-temperature piezoelectric materials and devices.

“Single-phase of four-layered Aurivillius CaBi₄Ti₄O₁₅ ceramics had been successfully synthesized by a conventional sintering process, at high Gd doping concentration (x=0.11), the unit cell was transformed from orthorhombic to pseduo-tetragonal (prototype) symmetry.” said by Yu Chen.

“T_C values of the CBT-100xGM ceramics increased from 793 ℃ to 809 ℃ with increasing x from 0 to 0.08. All the samples investigated were found with the first-order phase transition behavior and identified as the mixed-type ferroelectrics with both displacement and order-disordered characteristics”. “The introduction of MnO₂ greatly promoted the ferroelectric domain switching of CBT, in addition of the donor substitution effect of Gd³⁺, an increased remanent polarization associated with a reduced coercive field were achieved in the CBT-100xGM ceramics when x≤0.08”. said by Yu Chen.

“Above 400 ℃, the electrical conduction of CBT-100xGM ceramics can be attributed to the intrinsic behavior that is predominated by the long-distance migration of oxygen vacancies. The corresponding activation energy (E_a) were found to increase from 1.59 eV~1.78 eV with increase in x from 0 to 0.08”. “Three overtone modes were observed to follow the fundamental radial mode in the CBT-100xGM ceramics. The resonance-antiresonance pair peaks of the sample with x=0.11 tend to disappear at 700 ℃, indicating its thermal depoling behavior beginning from this temperature”. said by Yu Chen.

“The optimized composition with x=0.08 showed both a high T_C~809 ℃ and a high d₃₃~23 pC/N and, as well as a good anti-thermal depoling ability. The synergistic enhancement in the piezoelectric activity and Curie temperature of CBT can be mainly attributed to the donor-substituting effect of Gd³⁺ at A-site, as well as the decreased elastic compliance contributed by MnO₂ as the B-site dopant”. said by Yu Chen.

Other contributors include Zhi Zhou, Lingfeng Li, Daowen Wu from the School of Mechanical Engineering at Chengdu University.

This work was supported by the National Natural Science Foundation of China (Grant No. 12372179), as well as supported by the State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and astronautics (Grant No. MCMS-E-0522G01).

About Author

Dr. Yu Chen achieved both a Bachelor’s degree (2006) and Master’s degree (2009) in Materials Science at Sichuan University, China, and achieved a Ph.D. degree (2016) in Solid Mechanics after a joint educational project between Sichuan University and Liverpool University (U.K). After he got his doctor’s degree, he joined in the Chengdu University. Dr. Chen is currently a Full Professor and the Deputy Director of Human Resource Department at this university. His main research interests focus on the ferroelectric/piezoelectric materials and their applications in smart sensing devices and energy harvest systems. He led two research projects supported from Chinese National Natural Science Foundation and published 51 papers in SCI-indexed journals including ACS Energy Lett., J. Adv. Ceram., J. Materiomics, Mater. Design, J. Am. Ceram. Soc. Ceram Int., etc. In 2014, he won the Outstanding Paper Award from the Joint Conference of 9th Asian Meeting on Ferroelectrics & 9th Asian Meeting on Electroceramics (AMF-AMEC-2014). In 2018, he received the Innovation Talent Award of Ceramic Technology from The Chinese Ceramic Society.

Dr. Qingyuan Wang has been a Professor at Sichuan University (China) since 2001 and served as the President at Chengdu University between 2014 and 2024. Now, he has been employed as a part-time Professor at Chengdu University. His research primarily focuses on Material Science & Solid Mechanics, encompassing areas such as Very High Cycle Fatigue (VHCF), Green Constructional Materials, Mechanical Behavior of Structural Materials and Structures, and Composite Repairs of damaged Structures. Dr. Wang has published over 200 papers in SCI-indexed journals and has been recognized as one of the Most Cited Chinese Researchers by Elsevier from 2014 to 2022. Moreover, he has been honored with several prestigious fellowships and awards, including the JSPS Fellowship, the Excellent Scientist award under the "100 Talents Program" of the Chinese Academy of Sciences, the Sichuan “TianFu” Outstanding Scientists award, the National Distinguished Young Scholars, the First-class Natural Science Research Award from the Chinese Ministry of Education (2006), the First-class Science & Technology Research Award of Sichuan Province (2014 and 2019), and the Chinese Natural Science Award (2018).

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