Preparation Of High Performance KNN And BT Lead-free PiezoelectricCeramics And Corresponding Mechanism

In this thesis, thesegregation phenomenonas an important factor causing the narrowsintering temperature window for （K, Na）NbO₃-based ceramics was reported. It wasfound that two-step sintering and low temperature sintering could successfullyovercame the problem. Moreover, ultrafine-grained （K, Na）NbO₃-based ceramicswereprepared by using of nano-powders from water-based sol-gel method. In addition, thepiezoelectric properties ofBaTiO₃ceramics were grain size effect. Based on themechanism, high performance BaTiO₃ceramics could be obtained by controlling thegrain size properly.According to the analysis of KNbO₃-NaNbO₃phase diagram, the segregationphenomenon was easy to occur in the sintering process of （K, Na）NbO₃-based ceramics.It leaded to the appearance of liquid phase and compositionalinhomogeneity, resultingin nonuniform microstructuresand degraded electrical properties accordingly.Thecorresponding experimental results verified the above mechanism.Mechanism analysis and experimental results showed thattwo-stepsintering could effectively suppress the volatilization of alkali metal elements,and couldalso effectively inhibit the occurrence of segregation phenomenon. As aresult, two-step sintering was aneffective way to broaden the sinteringtemperature range of （K, Na）NbO₃-based ceramic, and high-performance （K,Na）NbO₃-based ceramicscould be prepared over a wide sinteringtemperaturerange. Low temperature sintering by using of nano-powder couldalso achieve the effect oftwo-step sintering. By using nano-powder, dense （K,Na）NbO₃-based ceramics could be obtained over a low and wide temperature range.Besides, low temperature sintering using nano-powder could also effectivelysuppress the volatilization of alkali metal elements, inhibit the occurrence ofsegregation phenomenon and hardly bring any impurity phases. Therefore,high-performance （K, Na）NbO₃-based ceramicscould be prepared over a wide andlow sintering temperaturerange.By using the nano-powder prepared from a water-based sol-gel method, theK_0.5Na_0.5NbO₃ultrafine-grained ceramic with the grain size of200-600nmweresuccessfully prepared by the conventional sintering. Its d33was148pC/N and k_pwas38.9%. Furthermore, by using the nano-powder prepared from awater-based sol-gel method, the(Na_0.52K_0.44Li_0.04)(Nb_0.86Ta_0.06Sb_0.08)O₃ultrafine-grained ceramic with the grainsize of200-400nmwere successfully prepared by the spark plasma sintering.Itsd33was296pC/N and kpwas49.7%.The results showed that thepiezoelectric properties of BaTiO₃ceramicswere highly grain sizedependent. In the grain size range of0.6-12.8μm, the2.4μmBaTiO₃ceramic exhibited the best performancewithd33of519pC/N.