| With the increase of human environmental protection consciousness and the needs of the social and ecological sustainable development, the research of lead-free ceramics becomes much more important. Tungsten bronze (TB) ferroelectrics are widely used in many fields because of their excellent electrical, electro-optic and nonlinear optical properties. In addition, it is relatively easy to obtain large-size-crystal TB materials with high quality. CaxBa1-xNb2O6(abbreviated to CBN) is considered as one of the most promising lead-free ceramics to substitute for lead based ceramics.It is known that the tungsten bronze niobate ceramics are difficult in obtaining high density and good electrical properties by traditional sintering methods. In this work, the conventional solid state reaction method and traditional sintering process were used to prepare lead-free CBN-based ceramics. The effects of preparing conditions, doping and sintering aids on the phase structure, microstructure and electrical properties were studied. The main results are as follows:Firstly, effects of the preparing conditions and the Ca/Ba ratio on phase structure, microstructure and electrical properties were studied in detail. The XRD results showed that pure CBN phase with tungsten bronze structure could be obtained from the solid solutions of BaNb2O6and CaNb2O6in all ceramics. The CBN phase formation was easiest at x=0.28suggested that the Ca0.28Ba0.72Nb2O6sample might be the most stable. Higher Ca contents could assist in the occurrence of grains with anisometric morphology, but no abnormal grain growth could be found in all compositions. The electrical properties of CaxBa1-xNb2O6ceramics were greatly dependent on x. With increasing x, εm increased firstly from1630to2998and then decreased, and the transition temperature Tc decreased gradually from338℃to190℃. In addition, both remnant polarization (Pr) and coercive field (Ec) decreased initially and then increased with increasing x. The ceramics with most homogeneous microstructure and highest density were obtained at x=0.28, resulting in optimized properties:Tc=234℃, εm=2998, Pr=3.98μC·cm-2, and Ec=14.03kV·cm-1.In order to restrain the abnormal grain growth in Ca0.28Ba0.72Nb2O6ceramics, La2O3and CeO2was selected as dopants. The effects of La2O3and CeO2contents on the phase structure, microstructure, dielectric and ferroelectric properties of Ca0.28Ba0.72Nb2O6ceramics were also studied. When doping La2O3, all ceramics exhibited single TB phase as La2O3content ranged from0.1to0.6wt%. Doping La2O3could inhibit the grain growth, and then prevented the abnormal grain growth behavior. But the electrical properties were deteriorated drastically. When doping CeO2, all ceramics exhibited single TB phase as CeO2content ranged from0.1to0.6wt%. With increasing the content of CeO2, the grain size of CBN-CeO2ceramics became smaller when prepared at same conditions. It could be concluded that the introduction of CeO2played an important role in inhibiting grain growth, so as to avoid the occurrence of abnormal grain growth. In addition, the Curie temperature Tc decreased from234℃to226℃with increasing La2O3content. When x=0.1wt%, the CBN-CeO2ceramics showed better electrical properties:Tc=234℃, εm=2776, Pr=3.4μC/cm2, which were slightly higher than those of Ca0.28Ba0.72Nb2O6basic ceramics.Finally, in order to restrain the abnormal grain growth and further improve the electrical properties Ca0.28Ba0.72Nb2O6ceramics simultaneously. The sintering aids of MnCO3and CuO was added in Ca0.28Ba0.72Nb2O6, respectively. The effects of MnCO3and CuO contents were studied to decrease the sintering temperature and improve the dielectric properties of the ceramics. The results showed that:all samples exhibited single tetragonal tungsten bronze structure. The addition of MnCO3could decrease the calcing and sintering temperatures from1225℃and1340℃to1180℃and1300℃, respectively. But the electrical properties were not improved. Proper amount of CuO aid could inhibit the grain growth and also promote the densification of ceramics. The addition of CuO could also decrease the calcing and sintering temperatures from1225℃and1340℃to1150℃and1300℃, respectively. When CuO amount was0.2wt%, the electrical properties of the ceramics were optimized:Tc=225℃, εm=3400, Pr=4.72μC/cm2, Ec=17.86kV/cm, and which were much higher than those of Ca0.28Ba0.72Nb2O6basic ceramics. |
PDF Full Text Request