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Introduction:
(K, Na)NbO3 (KNN) based piezoelectric ceramics are the most promising lead-free system to replace lead zirconate titanate (PZT) in the piezoelectric field due to their suitable piezoelectric coefficient and high Curie temperature. Currently, the research on the performance regulation of KNN has matured, and more related studies are targeting practical application fields, but there is still a long way to go before mature products can be incubated. Based on the unclear relationship between the microstructure of KNN and its piezoelectric performance, dielectric performance, and temperature stability in practical applications, Professor Zhang Dou’s team from Central South University took KNN energy harvesters (PEHs) as an example, combining performance testing, atomic-scale scanning transmission electron microscopy (STEM), and phase field simulations to explore the relationship between KNN phase structure, polarization configuration, and its macroscopic performance, which is expected to inspire the design of lead-free ferroelectric materials for piezoelectric applications. This work was published on February 2, 2025, in Acta Materialia (https://doi.org/10.1016/j.actamat.2025.120801), with Master’s student Zeng Shangren from Central South University as the first author, and PhD student Zou Jinzhuzhu and Professor Zhang Dou as the corresponding authors.
Research Content:
This work is based on the classic KNN system (1-x)K0.45Na0.55NbO3–xBi0.5K0.5ZrO3 (abbreviated as KNN-xBKZ). Testing found that in the R-O phase boundary and O-T phase boundary, the KNN-0.05BKZ sample, despite having the highest piezoelectric coefficient and dielectric constant at room temperature, exhibited inferior energy harvesting performance and temperature stability compared to the KNN-0.045BKZ sample, which had lower piezoelectric coefficient and dielectric constant. The KNN-0.045BKZ with a higher O-T phase boundary showed the best energy harvesting performance, with a power density of up to 13 μW/mm3, and excellent temperature stability in the range of 25-60 °C.
Figure 1: Phase boundaries under KNN-xBKZ modulation
Figure 2: Energy harvesting performance of KNN-xBKZ
Figure 3: Comparison of temperature stability between KNN-0.045BKZ and KNN-0.05BKZ
Through atomic-scale scanning transmission electron microscopy, it can be observed that for the KNN-0.045BKZ sample with a higher O-T peak, its T phase content is lower, and the polarization angle region size of the R/O phase can reach 4-10 unit cells, with a significant polarization deviation angle. In contrast, the KNN-0.05BKZ sample displays a dispersed nanoscale R/O phase structure (less than 3 unit cells), with a lower deviation angle. This local heterogeneous structure can further flatten the Gibbs free energy, promote polarization rotation, and enhance the piezoelectric performance and dielectric constant of KNN-0.05BKZ, while the increased dielectric constant causes the energy harvesting performance of KNN-0.05BKZ to decrease.
Figure 4: Atomic-scale images of KNN-0.045BKZ and KNN-0.05BKZ
This work further analyzed the impact of different polarization configurations on performance through phase field simulations. For KNN-0.045BKZ and KNN-0.05BKZ with different sizes of R/O phase PNR regions, a large-angle polarization region, which can stably exist dominated by volume energy, and a small-angle polarization region, which is more influenced by interface energy, were set. From their response to the external field, the large-angle polarization region responds less to the external field, while the small-angle polarization vector shows a significant deflection. This polarization rotation significantly improves its dielectric performance, and this small-angle polarization region embedded in the T phase can significantly enhance its dielectric constant. The results show that the higher O-T phase boundary of KNN-0.045BKZ expands the size of the R/O phase PNR, is less influenced by interface energy, and is difficult to polarize under an external field, resulting in a moderate dielectric constant; while the nanoscale PNR in KNN-0.05BKZ responds more sensitively to the external field, leading to a higher dielectric constant, which in turn causes its energy harvesting performance to decrease.
Figure 5: Phase field simulation of PNR with different sizes under external field response
Conclusion:
This work thoroughly discusses the relationship between the structure, piezoelectricity, dielectric performance, and thermal stability of KNN ceramics and proposes a new strategy for optimizing energy harvesting performance. The research results indicate that when the polarization configuration of KNN ceramics achieves a balance between enhancing the piezoelectric coefficient and dielectric coefficient, optimal energy collection performance (including piezoelectric coefficient, energy quality factor, output power density, etc.) and more stable temperature stability can be achieved, which is crucial for the consistency of device performance in practical applications. This work not only provides theoretical guidance for the performance optimization of KNN ceramics but also offers new ideas for the design and application of lead-free piezoelectric ceramics.
Figure 6: Impact of KNN structure on piezoelectric performance, dielectric performance, and temperature stability
Paper Link:
https://doi.org/10.1016/j.actamat.2025.120801
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