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First Author (or Co-First Author): Zhu Lifeng, Liu Dong, Shi Xiaoming
Corresponding Authors (or Co-Corresponding Authors): Deng Shiqing, Zhang Boping, Zhang Shujun, Li Jingfeng
Affiliated Institutions: Beijing University of Science and Technology; Tsinghua University; University of Wollongong, Australia
Paper DOI: https://doi.org/10.1038/s41467-025-56074-8
Potassium Sodium Niobate((K,Na)NbO3) based ceramics are considered one of the most promising candidates to replace commercial lead-based piezoelectric ceramics due to their environmental friendliness, excellent piezoelectric properties, and high Curie temperature. In this paper, we achieved ultra-high piezoelectric performanced33 ~ 807 pC·N−1 and excellent longitudinal electromechanical coupling coefficient(k33 ~ 88%) along with a high Curie temperature(Tc ~ 245°C) in(K,Na)(Nb1-xSbx)O3-Bi0.5Na0.5ZrO3-BiFeO3 ceramics based on lattice softening and grain orientation strategies.
Piezoelectric materials are functional materials that can convert mechanical energy into electrical energy and vice versa, and they are irreplaceable key materials in fields such as artificial intelligence, healthcare, and electronic information. For example, piezoelectric ultrasonic transducers, one of the important applications of piezoelectric materials, can be used in transportation, communication sensing, manufacturing processing, and medical diagnosis. According to statistics, in 2019, the global piezoelectric ceramics market size reached 48.1 billion yuan, and it continues to grow rapidly, expected to reach 63.8 billion yuan by 2026; currently, lead-based piezoelectric ceramics account for 99% of the market share. However, lead is a toxic element, and in lead-based piezoelectric ceramics, the content of PbO (or Pb3O4) accounts for more than 60%. Due to the volatility of PbO and the solubility of Pb2+, traditional lead-based piezoelectric ceramics pose serious threats to the ecological environment and sustainable development of human society during production, use, and disposal. The EU issued the RoHS 2.0 directive in August 2017, stating that certain lead-based piezoelectric products in the EU market will no longer be exempted by July 21, 2026. Therefore, environmentally friendly lead-free piezoelectric materials will gradually replace lead-based piezoelectric materials in some fields. According to statistics, in 2019, the global market size for environmentally friendly lead-free piezoelectric ceramics reached 172 million US dollars, and the average annual compound growth rate is expected to reach 20.8% over the next five years, with the global market size expected to reach 443 million US dollars by 2024. As a global benchmark for new materials research, the Materials Research Society (MRS) established a subcommittee for lead-free piezoelectric materials for the first time in 2020. The development of environmentally friendly lead-free piezoelectric materials has become one of the important scientific frontiers and technological competition focuses in the field of functional materials internationally.
In order to seize the technological high ground and develop high-performance lead-free piezoelectric ceramics, Chinese scholars proposed the concept of “environmentally compatible ferroelectric piezoelectric ceramics” internationally at the end of the last century. With continuous national funding, researchers have achieved a series of internationally recognized innovative research results in recent years, such as significant achievements in the piezoelectric performance and temperature stability of KNN-based lead-free piezoelectric ceramic systems. However, compared with lead-based piezoelectric ceramics, the comprehensive performance of KNN-based lead-free piezoelectric ceramics still needs further improvement.
3. Research Motivation: In order to enhance the comprehensive performance of KNN-based ceramics, this study proposed a strategy combining lattice softening and grain orientation, designing and preparing(K,Na)(Nb1-xSbx)O3-Bi0.5Na0.5ZrO3-BiFeO3 based lead-free piezoelectric ceramics. The doping of Sb5+ ions not only expands the unit cell volume but also promotes lattice “softening”. In addition, grain orientation will promote the spontaneous polarization direction of its dipoles to align along the <001> direction. Due to lattice softening and grain orientation,(K,Na)(Nb1-xSbx)O3-Bi0.5Na0.5ZrO3-BiFeO3 based lead-free piezoelectric ceramics achieved excellent piezoelectric comprehensive performance atx=0.05, whered33~807pC/N, k33~88% and Tc ~ 245°C.
4. Figure and Text Analysis:
Figure 1. Design principle diagram of high-performanceKNN based lead-free piezoelectric ceramics
Figure 1 shows the design principle diagram of high-performanceKNN based lead-free piezoelectric ceramics. Figure 1a shows the 2D planar diagram of the perovskite structure of KNN-based ceramics. Figure 1b illustrates the relationship between the interaction energy between atoms and the distance between atoms, that is, the distance betweenK+/Na+ ions and the highly electronegativeB5+ ions will be greater than the distance betweenK+/Na+ ions andNb5+ ions. Thus, the introduction ofB5+ ions will lead to an increase in the unit cell volume and a shift in the off-center position, resulting in lattice softening. Figure d describes the spontaneous polarization direction of the lattice in a typical KNN system. Figure e represents the KNN system doped withB5+ ions; due to lattice softening, non-<001> or non-<011> spontaneous polarization directions appear. Figure e shows the texturedB5+ ion-doped KNN system. Due to the influence of interfacial energy and grain boundary energy, its spontaneous polarization direction is nearly aligned along the<001> direction. Based on the above principles, this paper designs the(K,Na)(Nb1-xSbx)O3-Bi0.5Na0.5ZrO3-BiFeO3 system, achieving excellent comprehensive piezoelectric performance.
Figure2. Comparison of atomic-scale structures of untextured and texturedKNN-5Sb ceramics
Figure 2a shows the spherical aberration results of untexturedKNN-5Sb ceramics. Due to the introduction ofSb5+ ions leading to lattice softening, a large number of non-<001> and non-<011> polarization directions appear inKNN-5Sb ceramics. In texturedKNN-5Sb ceramics, the B site ion shifts nearly align along the<001> polarization direction, as shown in Figure 2b.
Figure3. Comparison of electrical performance of untextured and texturedKNN-xSb ceramics
Due to lattice softening and grain orientation, texturedKNN-5Sb ceramics achieved excellent comprehensive performance, whered33 ~ 807pC/N and d33*g33∼64.7 × 10-12 m2·N−1, these results are currently the highest reported values in KNN.
Figure 4. Comparison of untextured and texturedKNN-xSb ceramics andPZT-5 1-3 composite piezoelectric materials
Figure 4a shows the preparation process and physical image of 1-3 type piezoelectric composite materials. Figures 4b-4e show the echo curves of untexturedKNN-0Sb, untexturedKNN-5Sb, texturedKNN-5Sb, andPZT-5 ceramics in 1-3 composite piezoelectric materials. From the figures, it can be seen that texturedKNN-5Sb 1-3 composite piezoelectric materials exhibit high voltage signals and large bandwidth, even surpassing the 1-3 composite piezoelectric materials ofPZT-5 ceramics. These results indicate thatKNN-5Sb based ceramics have great potential to replace lead-based piezoelectric ceramics.
5. Conclusion and Outlook:
This paper designs and prepares (K,Na)(Nb1-xSbx)O3-Bi0.5Na0.5ZrO3-BiFeO3 lead-free piezoelectric ceramics based on the strategies of “lattice softening” and “grain orientation”. Due to lattice softening and grain orientation, texturedKNN-5Sb based ceramics achieved excellent comprehensive piezoelectric performance, where the piezoelectric coefficientd33=807 pC/N , d33*g33 ~64.7 * 10-12m2/N andk33=88% along with a highTc~245 oC at x=0.05. Furthermore, texturedKNN-5Sb 1-3 piezoelectric composite devices also demonstrated high voltage signals and large bandwidth, outperforming the 1-3 composite piezoelectric materials ofPZT-5 ceramics. These results indicate that texturedKNN-5Sb ceramics have the potential to replace lead-based piezoelectric ceramics in transducer applications.
Paper link:
https://doi.org/10.1038/s41467-025-56074-8
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