A Study on the Relationship Between Grain Size and Electrical Properties in (K,Na)NbO₃‐Based Lead‐Free Piezoelectric Ceramics

Abstract

The 0.5 wt% MnO2‐doped 0.95(K0.5Na0.5)(Nb0.965Sb0.035)O3‐0.01CaZrO3‐0.04(Bi0.5K0.5)(Hf0.98Ti0.02)O3 (95KNNS‐1CZ‐4BKHT) lead‐free ceramics with grain size from 0.31 to 3.83 µm are fabricated by hot‐press sintering (HPS), microwave sintering (MS), conventional sintering (CS), two‐step sintering (TSS), and a combination of two of the above‐mentioned methods. Then, the grain size effects on the phase transition, electrical properties, domain structure, and temperature stability of the ceramics are investigated. The phase structure evolves from single rhombohedral (R) phase to coexisted rhombohedral‐tetragonal (R‐T) phases around 1 µm. The electromechanical properties are strongly grain size dependent. The values of piezoelectric coefficient (d33) and planar electromechanical coupling factor (kp) first rise sharply when the grain size is less than 1 µm, and then increase mildly in the grain size range from 2.55 to 3.04 µm and finally almost remain a constant (≈450 pC N−1 and 54%) when the grain size is larger than 3.31 µm. The R‐T phase coexistence and strip‐like nanodomain structure characterized by an easier switching feature contribute to the high piezoelectric and electromechanical properties of coarse‐grained samples. This work not only clarifies the relationship between grain size and electrical properties in KNN‐based ceramics, but also provides a novel strategy for developing high‐performance lead‐free piezoceramics.

Reduced degree of phase coexistence in KNN-Based ceramics by competing additives

“…Generally, the composition-dependent phase structure can also be identified by εr-T curves [32]. With an increasing x, TO-T gradually increases from 44 oC (at x=0) to 118 oC (at x=0.03) and TR-O slightly increases from -50 oC to -36 oC (Figures 4(a-e)).…”

“…despite the reduced AGS and the reduced degree of phase coexistence. It was reported that the piezoelectricity was enhanced by the increasing grain size and the construction of multi-phase 14 coexistence [32,39]. As compared in Table 2, the ceramics with x=0.03 show the superior electrical properties to those of other KNN-based ceramics with an AGS of 1-2 µm [32,[40][41][42][43].…”

Second‐order‐transition like characteristic contributes to strain temperature stability in (K, Na)NbO3‐based materials

“…Due to Pb toxicity in commercial lead‐based piezo‐materials, lead‐free piezoelectrics have received much attention in recent years . Potassium‐sodium niobate [(K, Na)NbO 3 , KNN] is one of the most promising lead‐free candidates because of its high Curie temperature ( T C =415 °C) and relatively good piezoelectric constant ( d 33 ) . For small signal applications, such as sensors and microphones, d 33 is a research emphasis.…”

“…[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] Potassium-sodium niobate [(K, Na)NbO 3 , KNN] is one of the most promising lead-free candidates because of its high Curie temperature (T C =415 °C) and relatively good piezoelectric constant (d 33 ). 8,[25][26][27][28] For small signal applications, such as sensors and microphones, d 33 is a research emphasis. d 33 in KNN-based materials has been promoted greatly, which is even comparable to that obtained in lead-based materials, via technology improvement (reactive templated grain growth) and the establishment of phase boundaries.…”

“…d 33 in KNN-based materials has been promoted greatly, which is even comparable to that obtained in lead-based materials, via technology improvement (reactive templated grain growth) and the establishment of phase boundaries. 8,12,18,[26][27][28][29] However, for displacement-actuator applications, comparable normalized strain (d 33 *, strain normalized by a given electric field) is another hot topic in the field of lead-free materials. 26,[30][31][32] Nevertheless, a high d 33 * and good temperature stability for KNN-based ceramics is a contradiction because the phase transition in such materials is sensitive to the surrounding temperature, in which phase compositions deviate and performance declines with the variation in temperatures.…”