Introduction to Achievements of the National Key Laboratory of Powder Metallurgy at Central South University – Issue 1

Research Background: Piezoelectric actuators have advantages such as fast response speed, high resolution, anti-interference, small volume, simple structure, and easy installation, achieving successful applications in many fields including national defense, biomedical, and optoelectronics. Piezoelectric actuators require piezoelectric ceramics to exhibit high electrostrictive strain and low strain hysteresis under relatively low electric fields, as well as good temperature stability and excellent fatigue characteristics. Among lead-free piezoelectric ceramic systems, BNT-based relaxor ferroelectric ceramics have garnered widespread attention due to their ability to achieve high electrostrictive strain through phase structure regulation. However, the key issue of achieving high strain and low hysteresis while maintaining good stability remains unresolved.

Achievement Introduction: Using bismuth sodium titanate-strontium titanate ((Bi_0.5Na_0.5)_1-x/100S_x/100TiO₃) ceramics as the research object, this study breaks through the bottleneck of low strain and high driving electric field based on their inherent advantages of low strain hysteresis and excellent stability, combined with grain core-shell structure and lattice defect structure regulation. The ferroelectric “core” in the ceramic grains can promote the field-induced relaxor-ferroelectric transition; the defect dipoles within the ceramic induce a biased electric field, resulting in asymmetric bipolar strain curves and high unipolar strain. When x = 30, the ceramics exhibit a huge unipolar strain of 1.03% with a strain hysteresis of 27%; when x = 35, the ceramics maintain a stable large signal piezoelectric strain coefficient of around 1000 pm/V under a low electric field of 3~5 kV/mm, with strain hysteresis <10%. This low hysteresis high strain also shows near-zero residual strain and good temperature and cycling stability. Through in-situ transmission electron microscopy and piezoelectric force microscopy characterization, this study reveals the interaction between defect dipoles and polar nanoregions at the domain and lattice scales, as well as the structural origins of low hysteresis and large strain in this system.

The first unit of the paper is the National Key Laboratory of Powder Metallurgy at Central South University, with the first author being Associate Researcher Zhou Xuefan from Central South University, and the corresponding authors being Professor Zhang Dou, Professor Song Miao from Central South University, and Associate Professor Qi He from the University of Science and Technology Beijing. This work was also strongly supported by Professor Huang Houbing and Dr. Tang Shiyu from the School of Materials at Beijing Institute of Technology.

Figure1 Unipolar Strain Performance of BNST Ceramics: Under the influence of domain engineering and defect engineering, BNST-30 reached S_uni at 8 kV/mm of 1.03%, with hysteresis of 27%; BNST-35 reached S_uni at 10 kV/mm of 0.78%, with hysteresis of 4.4%; BNST-35‘s d₃₃^* maintained around 1000 pm/V under low field of 3~5 kV/mm, with hysteresis less than 10%.

Research Background: Piezoelectric ceramics possess electromechanical coupling characteristics and have wide applications in sensing, driving, and energy harvesting. Among lead-free piezoelectric ceramic systems, KNN has achieved piezoelectric performance comparable to commercial lead-based piezoelectric ceramics through multi-element doping to construct polycrystalline phase boundaries (PPB), showing tremendous potential. However, there are still two challenges regarding the theoretical framework and actual performance of KNN ceramics. Firstly, the theoretical framework understands multi-element doping as a change in the average phase structure, often neglecting the influence of the properties of each doped element on the structure, leading to KNN ceramics with similar PPB structures exhibiting vastly different performances. Secondly, the PPB achieved through multi-element doping in KNN-based piezoelectric ceramics is highly temperature-sensitive, with piezoelectric performance (d₃₃) rapidly deteriorating as the temperature deviates from room temperature, significantly limiting its practical applications.

Achievement Introduction: This study first theoretically proves the existence of two different atomic-scale local ferroelectric distortions (LFD) hidden within the average PPB structure. Furthermore, it elucidates how these LFD interact with PPB, proposing an atomic-scale physical picture. Based on this atomic-scale physical understanding, a strategy to address the practical performance challenges of lead-free piezoelectric ceramics is further constructed. This strategy involves regulating the LFD of the doping sites, flattening the polarization reversal barrier while constructing a diffuse phase transition. This method achieves an excellent d₃₃ of ~430 pC/N and demonstrates excellent temperature stability (△d₃₃~7%) in the range from room temperature to 100°C, showing application potential in piezoelectric sensing. Further optimization through annealing can enhance the temperature stability range to 150°C (△d₃₃~8%), while maintaining a high d₃₃ of ~380 pC/N, comparable to classic temperature-stable lead-based piezoelectric ceramics. This research establishes a framework for addressing the dilemma of insufficient high piezoelectric coefficient and temperature stability in lead-free piezoelectric ceramics.

The National Key Laboratory of Powder Metallurgy at Central South University is the first unit, with Professor Zhang Dou from Central South University and Professor Zhang Shujun from the University of Wollongong in Australia as co-corresponding authors. The first author of the paper is doctoral student Zou Jinzhuzhu from the 2021 cohort of Zhang Dou’s team. This work was also strongly supported by Professor Song Miao and Associate Researcher Zhou Xuefan from the Powder Metallurgy Research Institute.

Figure2 Illustration of the Strategy for Synergistically Improvingd₃₃ and Temperature Stability and Corresponding Performance. Specifically, this strategy reducesA site local ferroelectric distortions (LFD) to achieve (a₁). On one hand, introducing a large number of regions with reducedLFD in the matrix promotes polarization rotation (a₂). On the other hand, moving the orthorhombic-tetragonal (O-T) phase peak above room temperature constructs a diffuse phase transition (PPT) phase boundary (a₃). The red arrows indicate that the reduced local energy barriers can compensate for the loss caused by the destruction of the room temperaturePPT (i.e., moving the O-T temperature above room temperature).KNN-5.5(Bi_0.5K_0.5)_1-xBa_xZrO₃ (x=0, 0.27, 0.36, 0.45)‘s (b) temperature-dependent dielectric constant and (c)d₃₃.