| The piezoelectric materials, can achieve the conversion between electrical energy and mechanical energy, are highly skilled functional materials. Currently, the major piezoelectric materials have been dominated by lead-based materials owing to their outstanding piezoelectric properites. However, these materials are facing great challenge because the toxic "lead" threatens on environment and human healthy. Thus there is an urgent need to develop lead-free piezoelectric materials. Then the lead-free piezoelectric materials have attracted much attention in recent years. In2009, it has been reported that the Ba(Zr0.2Ti0.8)O3-(Ba0.7Ca0.3)Ti03(abbreviated as BZT-BCT) ceramics displayed amazing high-performance at the morphotropic phase boundary (MPB) regions, even higher than high-end PZT ceramics. Many researchers then have focused attention on this level of piezoelectricity, and the origin of large piezoelectricity is of interest again. Therefore, by reason of these interests of investigation, the investigation issue was drafted out. Firstly, the (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3(i.e.,0.5Ba(Zr0.2Ti0.8)03-0.5(Bao.7Cao.3)Ti03) lead-free piezoelectric ceramics were prepared by using conventional solid-state reaction method. Secondly, to explore and investigate the origin of large piezoelectricity near the MPB that strongly depended on temperature, the (Ba1-xCax)(Zr0.1Ti0.9)03and (Ba0.85Ca0.15)(ZryTi1-y)03lead-free ceramics were designed and successfully prepared. Thirdly, the effects of Zr and Ca contents on diffuse phase transition, microstructure and ferroelectric properties of (Ba1-xCax)(Zr0.2Ti0.9)O3and (Ba0.85Ca0.15)(ZryTi1-y)O3also were investigated. Finally, in order to looking for the more vertical MPB in the temperature and composition diagrams of Ba(Zr,Ti)O3-(Ba,Ca)TiO3lead-free ceramics the (Ba1-xCax)(ZryTi1-Y)O3ceramics were prepared, and the phase transition principle of (Ba1-xCax)(ZryTi1-y)O3lead-free ceramics were investigated.Some prominent conclusions drawn out of the investigations, which can be summarized as follows:1. The effects of calcined and sintered temperature on microstructure and electric properties of (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3lead-free ceramics were investigated. The results showed that the optimal electric properties were obtained with calcined temperature and sintered temperature at1200℃and1400～1440℃, respectively. The piezoelectric properties are as follows:d33=543～572pC/N, kp=0.55～0.57, Qm=125.2. To approach the MPB region from rhombohedral(R) and tetragonal (T) field by the designed and prepared (Ba1-xCax)(Zr0.1Ti0.9)03and (Ba0.85Ca0.1)(ZryTi1-y)03ceramics, the phase transition behavior and piezoelectric properties were investigated. The results indicated that the MPB of0.5Ba(Zr0.2Ti0.8)03-0.5(Bao.7Cao.3)TiO3(i.e.,(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3) ceramic actually attributed to the promixity of O(orthorhombic)â†'T and Oâ†'R phase transitions, driven compositions and temperatures. Therefore, the MPB strongly depend on temperature. These results also indicated that the O phase was unstable at narrow region and served as the structural bridge connecting R and T phases, which might be related to large piezoelectric response(d33=572pC/N,kp=0.57) in the MPB region closed to the T sides.3. The effects of Zr and Ca content on diffuse phase transition, microstructure and ferroelectric properties of (Ba1-xCax)(ZryT1-y)O3and (Ba0.85Ca0.15)(ZryTi1-y)03lead-free ceramics were investigated, respectively. The results indicated that enhanced diffuse phase transition behavior (DPT) is intimately related with the small grain of (Ba1-xCax)(ZryTi1-y)O3and (Ba0.85Ca0.15)(ZryTi1-y)O3ceramics, which significantly impact the ferroelectric properties at MPB region of (Ba0.85Ca0.15)(Zr0.1Ti0.9)03ceramics. As a result, for the (Ba1-xCax)(ZryTi1-y)O3ceramics, the optimal ferroelectric properties were observed in the R phase field at x=0.05(the ferroelectric properties were Pr=13.20μC/cm2, Ps=7.11μC/cm2and Ec=239v/mm), and weren’t obtained near the MPB region(with the composition at x=0.15, the ferroelectric properties are Pr=11.36μC/cm2, Ps=17.19μC/cm2and EC=238v/mm.). Whereas, for (Ba0.85Ca0.15)(ZryTi1-y)03ceramics, it is different from the (Ba1-xCax)(ZryTi1-y)O3ceramics that the optimal ferroelectric properties were obtained near MPB region (with the composition at y=0.10). This evidence indicated that the optimal ferroelectric properties which might be caused by the proper Tc.4. The phase transition principle of (Ba1-xCax)(ZryTi1-y)O3were investigated, the phase diagrams of (Ba1-xCax)(ZryTi1-y)O3were produced. According to the phase transition principles, the formulae of the TO-R(Oâ†'R phase transition temperature) and TO-T (Oâ†'T phase transition temperature) as a function of Ca contents and Zr contents were established, respectively, and the results revealed that the O phase should exist between R and T phases at narrow region in the MPB region of Ba(Zr,Ti)O3-(Ba,Ca)TiO3pseudo-binary systems with various Zr content in Ba(Zr,Ti)O3and Ca content in (Ba,Ca)TiO3. Moreover, it was found that triple-phases-point shift to the higher (Ba,Ca)TiO3contents and the temperature dependence of the MPB turn weak with the increase of the Zr contents in the Ba(Zr,Ti)O3. The MPB, with weak temperature dependence, was observed on the phase diagram of Ba(Zr0.3Ti0.7)03-(Ba0.8Ca0.2)TiO3pseudo-binary systems, indicating the MPB composition of Ba(Zr0,3Ti0.7)03-(Ba0.8Ca0.2)TiO3 system will display good temperature stability of piezoelectric properties. |
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