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Piezoelectric materials are functional materials that can convert electrical energy into mechanical energy and vice versa. Piezoelectric ceramics are widely used in electronic devices such as transducers, sensors, actuators, and energy harvesters due to their positive piezoelectric effect (i.e., piezoelectric ceramics generate charges on their surface when subjected to external forces) and inverse piezoelectric effect (i.e., piezoelectric ceramics deform under an external electric field). The application of piezoelectric materials permeates every corner of modern society, as people may encounter applications of piezoelectric materials almost every day. Little do we know that when lighting a cigarette, gas stove, or water heater, a type of piezoelectric ceramic has quietly served you. Piezoelectric ceramics have extensive applications not only in industrial and civilian products but also in military applications. Over the past few decades, lead-based piezoelectric materials have dominated the market due to their excellent piezoelectric properties. However, to protect the environment and meet the sustainable development goals of humanity, exploring high-performance lead-free piezoelectric materials has become an urgent issue in this field.
Recently, Professor Zhai Jiwei from Tongji University, Associate Professor Shen Bo, and Professor Zhang Shujun from the University of Wollongong, Australia collaborated to prepare KNN-based piezoelectric ceramics with high texture degree by adopting texturing processes and optimizing the composition design and sintering process of potassium sodium niobate (KNN). The piezoelectric coefficient d33 reaches ~700 pC/N, the electromechanical coupling coefficient kp is 76%, and the electric field-induced strain S is ~0.3%. Additionally, the material has a high Curie temperature and relatively excellent electric/thermal performance stability. The study found that the high piezoelectric performance mainly comes from three aspects: (I) The anisotropic design of piezoelectric crystals fully utilizes the <001> oriented orthorhombic (O) and rhombohedral (R) phases coexisting crystal structure, which has the most suitable engineering domain structure, i.e., a larger number of energy-equivalent polarization vectors (Ps) and a smaller angle θ between the polarization vector and the direction of the external electric field. (II) The high lattice distortion caused by the textured ceramics under external voltage is an intrinsic contribution to the improvement of piezoelectric performance. In addition, the new intermediate phase generated during the polarization process facilitates the polarization process. (III) TEM and PFM characterization results show that the reduction in domain size (nano-domains) due to the introduction of template grains in textured ceramics is beneficial for the movement of domain walls under the action of an external electric field. This research provides guidance for the design of high-performance lead-free piezoelectric ceramics. The related research results were published in Advanced Materials under the title “Ultrahigh Piezoelectric Properties in Textured (K,Na)NbO3-Based Lead-Free Ceramics.”
Figure 1. Phase structure and performance of KNN-based ceramics with different template contents
(a) XRD patterns of KNN-based piezoelectric ceramics with different NN template contents at room temperature;
(b) Comparison of texture degree F, piezoelectric coefficient d33, and planar mode electromechanical coupling coefficient kp of KNN-based piezoelectric ceramics with different NN template contents;
(c) Comparison of important parameters d33, kp, Tc, and other typical lead-free piezoelectric ceramic systems and lead-based piezoelectric ceramics for KNN-3T piezoelectric ceramics;
Figure 2. Ferroelectric and electric field-induced strain performance of ceramics with different template contents
(a) P-E hysteresis loop of KNN-based ceramics with different NN template contents;
(b) Unidirectional electric field-induced strain curves of KNN-based ceramics with different NN template contents;
(c) Relationship between piezoelectric coefficient d33 and direct current polarization electric field strength;
(d) Relationship between low alternating current electric field strength and piezoelectric coefficient;
Figure 3. In situ synchrotron radiation XRD analysis of randomly oriented and textured ceramics
(a, b) In situ synchrotron radiation XRD patterns of randomly oriented KNN-0T and textured KNN-3T ceramics for (001)/(100) and (002)/(200) crystal planes;
(c) Changes in interplanar spacing and lattice distortion degree of KNN-0T and KNN-3T samples with electric field strength;
(d) Schematic diagram of possible polarization reversal path from the spontaneous polarization vector [111] of rhombohedral phase to the spontaneous polarization vector [101] of orthorhombic phase under external electric field;
Figure 4. TEM analysis of textured ceramics
(a-c) Phase diagram of randomly oriented KNN-0T ceramics and flipping diagram of electric domains under direct current voltage;
(d-f) Phase diagram of textured KNN-3T ceramics and flipping diagram of electric domains under direct current voltage;
(g-h) PFM phase-voltage and amplitude-voltage diagrams of KNN-0T and KNN-3T ceramics;
This study utilized the two-step molten salt method to prepare sheet sodium niobate NN as a template, optimizing the template content and sintering process to produce highly <00l>c oriented textured KNN-based piezoelectric ceramics. High piezoelectric performance was achieved in KNN-3T piezoelectric ceramics, with a piezoelectric coefficient d33 of ~700 pC/N, electric field-induced strain S of ~0.3%, and Curie temperature Tc of ~242 °C. The high piezoelectric performance mainly originates from the suitable engineering domain structure of the <00l> oriented crystals of the R-O phase component. Additionally, in situ synchrotron radiation XRD studies indicate that a large degree of lattice distortion is an intrinsic contribution to the enhancement of piezoelectric performance, while the new phase generated during polarization serves as a “bridge” for polarization reversal. TEM and PFM analyses show that compared to randomly oriented ceramics, the large number of nano-domains in textured ceramics with reduced domain wall energy is also a source of high piezoelectric performance, facilitating the movement of domain walls under the action of an external electric field. This study demonstrates that high-performance textured KNN-based piezoelectric ceramics are an excellent choice for applications in lead-free piezoelectric ceramics in electromechanical conversion functional devices.
Reference Link: Ultrahigh Piezoelectric Properties in Textured (K,Na) NbO3‐Based Lead‐Free Ceramics (Adv. Mater., 2018, DOI: 10.1002/adma.201705171)
This article is provided by Professor Zhai Jiwei’s research team from Tongji University.
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