| As an important class of electronic materials, piezoelectric materials are able to realize the conversion between electrical energy and mechanical energy, and are thus widely used to fabricate sensors, resonators, filters, actuators, transducers and other electronic components and devices. Currently, Pb(Zr,Ti)O3-based piezoelectric ceramics occupy the dominant position in practical piezoelectric materials because of their excellent electrical properties. However, due to the toxicity of lead oxide which is largely used as a raw material in the production process, there is an increasing demand to develop the environment-benign lead-free alternatives.As the first polycrystalline piezoelectric material, BaTiO3ceramics had been widely applied as a piezoelectric material prior to PZT. Nowadays, however, it is no longer used as a piezoelectric material, mainly because of the poor piezoelectric properties (d33<190pC/N) compared with PZT. Nevertheless, it is noteworthy that BaTiO3ceramics prepared from hydrothermally synthesized BaTiO3powders through some special fabrication techniques such as microwave sintering, two-step sintering and templated grain growth (TGG) were reported to demonstrate high d33values of350,460and788pC/N, respectively. More importantly, we have been recently succeeded in obtaining BaTiO3ceramics with high piezoelectric properties of d33=419pC/N through conventional solid-state reaction route with starting raw materials of ordinary BaCO3and TiO2powders. These results indicate that BaTiO3-based ceramics have a large potential as a good lead-free piezoelectric material and further research is worth doing.Various kinds of BaTiO3ceramics were prepared with raw materials of ordinary BaCO3and TiO2powders in this study. Their physical properties were investigated in detail.1. Two different kinds of TiO2raw materials were deliberately chosen to prepare the various BaTiO3ceramics through conventional solid-state reaction route. The influence of these TiO2raw materials on piezoelectric properties and microstructures of BaTiO3was systematically studied. Although TiO2raw materials have different microstructures and crystalline structures, no significant difference is found in the the obtained BaTiO3powders. The average particle sizes of the two kinds of BaTiO3powders are all about500nm. Using TiO2with rutile and anatase mixed phase as raw materials, BaTiO3ceramics sintered at1210℃have get d33and kp values of350pC/N,35.9%. Using TiO2with rutile phase as raw materials, BaTiO3ceramics sintered at1230℃have d33and kp values of350pC/N,38.7%, respectively. The grain sizes of the two corresponding samples are about2-3μm. In general, however, no much difference of two kinds of BaTiO3ceramics prepared with different TiO2raw materials is observed in the viewpoints of piezoelectric properties, sintering temperature and grain sizes.2. The influence of sintering methods on the microstructures and physical properties of BaTiO3powders and BaTiO3. The experiment was carried out using two different sintering ways:one is ordinary sintering method, the other one is buried sintering method. Two different TiO2materials using together with BaCO3as starting raw materials were employed for the comparison text of sintering methods and the properties of finally prepared BaTiO3ceramics were discussed. In the comparison experiment of different sintering methods, it has been found that the obtained BaTiO3ceramic prepared by the buried method shows higher piezoelectric properties than the corresponding one prepared by the ordinary method. In case of TiO2-B raw material, the piezoelectric constant d33increases from350pC/N to380pC/N, in the mean time, kp rates from38.7%to45.2%. What is more obvious is that BaTiO3ceramic prepared by the buried method possesses effectively improved density than the corresponding one prepared by the ordinary method and the relative density can be increased from93.1%to98.7%. It can be speculated that free oxygen in ceramic and its enter into the lattice is chiefly responsible for the increase of density. Via SEM pictures, it has been found that BaTiO3ceramics prepared by two kinds of sintering methods show different microstructures:the grain in BaTiO3ceramics samples prepared using ordinary sintering is more evenly distributed and the average grain size is about2μm; accordingly, a phenomenon of abnormal grain growth has emerged in BaTiO3ceramics samples obtained through buried sintering, that is, small grains embedded in the large ones. Particularly, an unusual but very interesting phenomenon that a BaTiO3ceramics with large grain size could also show excellent piezoelectric properties has been found which contrasts sharply with the phenomenon of piezoelectric grain-size effect that was recently confirmed in other experimental investigations of BaTiO3piezoelectric ceramics. Thephenomenon suggests that BaTiO3ceramics with large grain size could also promises excellent piezoelectric properties.3. BaTiO3ceramics with different amounts of Sn4+substitution were prepared, and the influence of Sn4+substitution on dielectric and piezoelectric properties was investigated. Temperature instability is a great obstacle to the application of BaTiO3ceramics. However, by comparing the piezoelectric properties in orthorhombic phase with that in tetragonal phase, it is obvious that BaTiO3ceramics present more stable piezoelectric properties in orthorhombic phase than in tetragonal phase. Partially substituting Ti with Sn4+can shift the orthorhombic-tetragonal phase transition temperature upward and is effective in improving the temperature stability. However, the substitution is accompanied by the decrease of piezoelectric activities. It is found that the decrease could be overcomed by incorporating a small amount of CuO additive. CuO additive acts like a sintering aid in the CuO-modified Ba(Ti,Sn)O3ceramics to lower the sintering temperature, promotes the ceramic densification and suppresses the abnormal grain growth during sintering process. Facts have proved that the CuO-modified Ba(Ti,Sn)O3ceramics exhibit better piezoelectric activities and better temperature stability. The dielectric spectra of two Ba(Ti,Sn)O3ceramics (with Sn4+contents of7%and12%, respectively) were mesured at room temperature and compared to that of a BaTiO3ceramic. The two Ba(Ti,Sn)O3ceramics show that their characteresitic frequencies of the dielectcric relaxations due to domain-wall motion are higher than1GHz, similarly to BaTiO3ceramics. |
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