Rhombohedral-orthorhombic-tetragonal Research Articles

Coexistence of three ferroelectric phases and enhanced piezoelectric properties in BaTiO3–CaHfO3 lead-free ceramics

Perovskite ferroelectrics possess the fascinating piezoelectric properties near a morphotropic phase boundary, attributing to a low energy barrier that the results in structural instability and easy polarization rotation. In this work, a new lead-free system of (1-x)BaTiO3-xCaHfO3 was designed, and characterized by a coexistence of ferroelectric rhombohedral-orthorhombic-tetragonal (R-O-T) phases. With the increase amount of CaHfO3 (x), a stable coexistence region of three ferroelectric phases (R-O-T) exists at 0.06≤x≤0.08. Both large piezoelectric coefficient (d33∼400pC/N), inverse piezoelectric coefficient (d33*∼547pm/V) and planar electromechanical coupling factor (kp∼58.2%) can be achieved for the composition with x=0.08 near the coexistence of three ferroelectric phases. Our results show that the materials with the composition located at a region where the three ferroelectric R-O-T phases coexist would have the lowest energy barrier and thus greatly promote the polarization rotation, resulting in a strong piezoelectric response.

Identification of Phase Boundaries and Electrical Properties in Ternary Potassium-Sodium Niobate-Based Ceramics

A large piezoelectric constant (d33) of ∼480 pC/N was attained in new ternary (1-x-y)K0.5Na0.5Nb0.96Sb0.04O3-xBaSnO3-yBi0.5Na0.5ZrO3 ceramics by forming rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundary using the variations of x and y, and such a phase boundary was successfully confirmed by the convergent beam electron diffraction (CBED) patterns. For (1-x)K0.5Na0.5Nb0.96Sb0.04O3-xBaSnO3, the orthorhombic (O) phase is well-maintained for 0 ≤ x ≤ 0.015, and both the R and T phases can be introduced to (0.99-y)K0.5Na0.5Nb0.96Sb0.04O3-0.01BaSnO3-yBi0.5Na0.5ZrO3 with y = 0.025-0.04 by simultaneously tailoring their compositions (x and y); then, R-O-T multiphases can be well-established. The CBED patterns strongly support the existence of R-O-T multiphases in the ceramics with y = 0.035. When the phase transitions endure from O to R-O-T, their piezoelectric activity endures a leapfrog development from ∼165 to ∼480 pC/N. In the region of the R-O-T phase boundary, a large d33 of ∼480 pC/N was attained in the ceramics with x = 0.01 and y = 0.035. In addition, the ceramics with x = 0.01 and y = 0.04 possess a high strain of ∼0.274% due to the multiphases coexistence. According to the variations of dielectric and ferroelectric properties, the enhancement in εr and Pr plays a part in the improved d33 except for the R-O-T phase boundary. We believe that the (K, Na)NbO3 ternary systems can be used to promote piezoelectric activity by forming new phase boundaries.

Enhanced piezoelectric properties in potassium-sodium niobate-based ternary ceramics

(1−x)K0.5Na0.5Nb1−ySbyO3-zSrZrO3-xBi0.5Na0.5HfO3 (KNNS-SZ-BNH) lead-free ceramics were developed by the conventional solid-state reaction method. Effects of the additives (Bi0.5Na0.5HfO3, SrZrO3 and Sb5+) on their phase structure, microstructure, and electrical properties were investigated. The rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundary can be established in the ceramics with 0.03≤x≤0.05, 0.04≤y≤0.06, and 0.01≤z≤0.025, and then their piezoelectric properties were improved. The ceramics with x=0.03, y=0.04 and z=0.01 possess the optimum piezoelectric properties (d33=470±5pC/N, kp=0.51±0.02, and TC=244°C). We believe that R-O-T multiphase coexistence is mainly responsible for the enhancement of piezoelectric properties.

Strong piezoelectricity in (1 - x)(K0.4Na0.6)(Nb0.96Sb0.04)O3-xBi0.5K0.5Zr1-ySnyO3 lead-free binary system: identification and role of multiphase coexistence.

Here we report a strong piezoelectric activity in (1 - x)(K0.4Na0.6)(Nb0.96Sb0.04)O3-xBi0.5K0.5Zr1-ySnyO3 lead-free ceramics by designing different phase boundaries. The phase boundaries concerning rhombohedral-orthorhombic-tetragonal (R-O-T) and rhombohedral-tetragonal (R-T) multiphase coexistence were attained by changing BKZS and Sn contents and then were identified by the X-ray diffraction patterns as well as temperature-dependent permittivity and ν1 Raman modes associated with BO6 perovskite octahedron. A high strain (strain = 0.21-0.28% and d33* = 707-880 pm/V) and a strong piezoelectric coefficient (d33 = 415-460 pC/N) were shown in the ceramics located at the multiphase coexistence region. The reported results of this work are superior to that (d33* ∼ 570 pm/V and d33 ∼ 416 pC/N) of the textured (K,Na,Li)(Nb,Ta,Sb)O3 ceramics [Nature 2004, 432, 84]. We believe that the material system of this work will become one of the most promising candidates for piezoelectric actuators.

Modification of both d33 and TC in a potassium-sodium niobate ternary system.

In this work, we simultaneously achieved a giant d33 and a high TC in a lead-free piezoelectric ternary system of (1-x-y)K0.48Na0.52NbO3-xBiFeO3-yBi0.5Na0.5ZrO3 {(1-x-y)KNN-xBF-yBNZ}. Owing to the rhombohedral-orthorhombic-tetragonal (R-O-T) phase coexistence and the enhanced dielectric and ferroelectric properties, the ceramics with a composition of (x = 0.006, y = 0.04) show a giant d33 of ∼428 pC N(-1) together with a TC of ∼318 °C, thereby proving that the design of ternary systems is an effective way to achieve both high d33 and high TC in KNN-based materials. In addition, a good thermal stability for piezoelectricity was also observed in these ceramics (e.g., d33 > 390 pC N(-1), T ≤ 300 °C). This is the first time such a good comprehensive performance in potassium-sodium niobate materials has been obtained. As a result, we believe that this type of material system with both giant d33 and high TC is a promising candidate for high-temperature piezoelectric devices.