| As a class of functional materials, piezoelectric materials are able to realize the conversion between electrical energy and mechanical energy, and are thus widely used to fabricate resonators, filters, actuators, sensors, transducers and other electronic devices. Currently, Pb(Zr,Ti)O3-based piezoelectric ceramics take the major position in the practical piezoelectric materials market because of their excellent electrical properties. However, due to the toxicity of lead oxide which is largely used in the production process, there is an increasing demand to develop the environment-benign lead-free alternatives.Historically, BaTiO3 ceramic is the first polycrystalline piezoelectric material that had been widely used as a piezoelectric material. Nowadays, however, it is no longer used as a piezoelectric material since the discovery of PZT. This is mainly because of the poor piezoelectric properties (d33≤190pC/N) in comparison with PZT. BaTiO3 is presently applied either as a dielectric material or as a material for making positive temperature coefficient resistors. Nevertheless, in the past few years, BaTiO3 ceramics that were prepared from hydrothermally synthesized BaTiO3 powders through some special fabrication techniques like microwave sintering, two-step sintering and templated grain growth (TGG) were reported to show high d33 values of 360,460 and 788 pC/N, respectively. More importantly, we have been recently succeeded in obtaining BaTiO3 ceramics with high piezoelectric properties of d33= 419 pC/N by conventional solid-state reaction route with starting raw materials of ordinary BaCO3 and TiO2 powders. These results indicate that BaTiO3-based ceramics have a large potential to be used as a good lead-free piezoelectric material.Various kinds of BaTiO3 ceramics were prepared by the conventional solid-state reaction technique with raw materials of ordinary BaCO3 and TiO2 powders in this study. Their physical properties were investigated in detail.1. BaTiO3 ceramics with different amounts of CuO additive were prepared, and the influence of CuO additive on dielectric and piezoelectric properties was investigated. It has been found that CuO additive acts like a sintering aid in the CuO-modified BaTiO3 ceramics to lower the sintering temperature, promotes the ceramic densification and suppresses the abnormal grain growth during sintering process. From the results of the temperature dependences ofε'and d33, it is apparent that CuO additive has the effect to decrease the orthorhombic-tetragonal phase transition temperature (To-T) and d33 shows the highest value at To-T·For BaTiO3 ceramic, kp is more stable in the orthorhombic phase than in the tetragonal phase, and a proper amount of CuO additive has the effect of enhancing the kp value in the orthorhombic phase. For the modified BaTiO3 ceramic with 1 mol% of CuO additive, it has the good piezoelectric properties of d33=300 pC/N, kp=0.49, k33=0.62 and Qm=425 at room temperature and shows a lower To-T in comparison with the pure BaTiO3 ceramic, and exhibits anε'peak at-5℃and a d33 maximum of 353 pC/N at 0℃, respectively.2. Various BaTiO3 ceramics were prepared from three different kinds of TiO2 raw materials. The differences of TiO2 raw materials and the influence of these TiO2 raw materials on microstructures and piezoelectric properties were investigated. It has been found that TiO2 raw materials show different microstructures and crystalline structures but there is no large difference in those obtained BaTiO3 powders. The average particle sizes of the three kinds of BaTiO3 powders are all about 500 nm and the crystalline structures are tetragonal at room temperature. The optimum sintering temperatures for those BaTiO3 ceramics prepared from the two kinds of TiO2 raw materials with rutile phase are lower, and the d33 and kp values of the corresponding BaTiO3 ceramic samples that were sintered at 1200℃are 350 pC/N,39.2% and 340 pC/N,38.6%, respectively. The grain sizes of the two forementioned samples are about 1-2μm. The optimum sintering temperature for the BaTiO3 ceramic prepared from the TiO2 raw material with anatase phase is higher, and the d33 and kp values of the corresponding BaTiO3 ceramic sample that was sintered at 1330℃are 290 pC/N and 44.5%, respectively. The grain size of this sample is above 10μm. In general, it is desirable to choose the TiO2 raw materials with rutile phase in the viewpoints of piezoelectric properties, sintering temperature and grain sizes.3. The influence of ceramic preparation conditions on the physical properties of BaTiO3 powders and BaTiO3 ceramics was studied in the following ways:(1) two kinds of calcination methods are compared, in which the milled mixture of raw materials is calcined either in the form of powder or in the form of pressed block; (2) different second ball-milling time conditions (of 6,12,18 and 24 hours, respectively) are examined on an ordinary planetary ball-milling machine; (3) a different second ball-milling method is investigated on a high-energy planetary ball-milling machine, in which ZrO2-jars and yttrium-stabilized ZrO2-balls with the diameters of either 6.5 mm or 5 mm are used and the milling time condition is either 2 h or 4 h. In the comparison experiment of different calcination methods, it has been found that the obtained best BaTiO3 ceramic prepared by the powder calcination shows slightly higher piezoelectric properties than the corresponding one prepared by the pressed-block calcination. In the experiment of second ball-milling time, a milling time condition of 12 h is found to be proper for getting BaTiO3 ceramics with high piezoelectric properties and a longer milling-time condition will cause the heavy abrasion contamination. In the high-energy ball-milling experiment, the obtained BaTiO3 powders show larger average particle sizes than those BaTiO3 powders prepared on an ordinary planetary ball-milling machine, and their optimum sintering temperatures are as high as 1310℃or further higher. However, no much difference of influences on both the BaTiO3 powders and the BaTiO3 ceramics is observed between the milling balls of different sizes. Nevertheless, milling time conditions have been found to show an important influence, perhaps largely due to the abrasion contamination. Particularly, an unusual but very interesting phenomenon that a BaTiO3 ceramic with the average grain size of larger than 100μm could show its d33 and kp values as high as 310 pC/N and 44.7% has been found for the first time, which contrasts sharply with the phenomenon of piezoelectric grain-size effect that was recently confirmed in other experimental investigations of BaTiO3 piezoelectric ceramics. |
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