Structural evolution of the R‐T phase boundary in KNN‐based ceramics

Abstract

Although a rhombohedral‐tetragonal (R‐T) phase boundary is known to substantially enhance the piezoelectric properties of potassium‐sodium niobate ceramics, the structural evolution of the R‐T phase boundary itself is still unclear. In this work, the structural evolution of R‐T phase boundary from −150°C to 200°C is investigated in (0.99−x)K0.5Na0.5Nb1−ySbyO3–0.01CaSnO3–xBi0.5K0.5HfO3 (where x = 0‐0.05 with y = 0.035, and y = 0‐0.07 with x = 0.03) ceramics. Through temperature‐dependent powder X‐ray diffraction (XRD) patterns and Raman spectra, the structural evolution was determined to be Rhombohedral (R, <−125°C)→Rhombohedral + Orthorhombic (R + O, −125°C to 0°C)→Rhombohedral + Tetragonal (R + T, 0 °C to 150°C)→dominating Tetragonal (T, 200°C to Curie temperature (TC)) → Cubic (C, >TC). In addition, the enhanced electrical properties (e.g., a direct piezoelectric coefficient (d33) of ~450 ± 5 pC/N, a conversion piezoelectric coefficient (d33∗) of ~580 ± 5 pm/V, an electromechanical coupling factor (kp) of ~0.50 ± 0.02, and TC~250°C), fatigue‐free behavior, and good thermal stability were exhibited by the ceramics possessing the R‐T phase boundary. This work improves understanding of the physical mechanism behind the R‐T phase boundary in KNN‐based ceramics and is an important step toward their adoption in practical applications.

Enhanced piezoelectricity and temperature stability in LaFeO3‐modified KNN‐based lead‐free ceramics

“…It is worth noting that the LaFeO 3 content has an obvious impact on the phase structure of the KNN‐based ceramics; that is, with an increasing amount of LaFeO 3 , the ratio of I {002}L θ (intensity for the peak with a low diffraction angle) to I {002}H θ (intensity for the peak with a high diffraction angle) decreased, and only one single peak occurred when x ≥ 0.008. Generally, the ratio of I {002}L θ to I {002}H θ with a value of 2 corresponded to the orthorhombic (O) phase, while 1/2 corresponded to the tetragonal (T) phase . Therefore, the decrease in I {002}L θ / I {002}H θ indicated an increased ratio of the tetragonal (T) phase with increasing x value.…”

“…Generally, the ratio of I {002}Lθ to I {002}Hθ with a value of 2 corresponded to the orthorhombic (O) phase, while 1/2 corresponded to the tetragonal (T) phase. 15,19 Therefore, the decrease in I {002} Lθ /I {002}Hθ indicated an increased ratio of the tetragonal (T) phase with increasing x value. In previous reports, it was widely observed that the (00l) peaks in the KNN-based ceramics with the T phase greatly shifted to the low 2θ direction when compared with that of the O phase, 20,21 and this behavior was attributed to the larger value of the lattice parameter c in the T phase compared to that of the O phase.…”

Defect suppression in CaZrO3‐modified (K, Na)NbO3‐based lead‐free piezoceramic by sintering atmosphere control

“…Since then, thousands of resources have been devoted into the research of these materials. Lately, with tremendous effort contributed by scientists, impressive results have been achieved, especially in the field of lead‐free piezoelectrics . With the capability of interconverting mechanical and electrical energy, piezoelectric materials are widely exploited in industries .…”

Sliding Wear Behavior of TaC‐Containing Ti(CN)‐WC‐Ni/Co Cermets

“…The formation of such mechanically mixed layer is often reported as the consequence of material transfer in the sliding wear of ceramic composites. 28,29 Thus, the extent of material transfer differs for the investigated cermets. On the other hand, EDS analysis obtained from the steel ball surface worn against C4 cermet at 20 N reveals negligible presence of Ti (see Fig.…”