| Piezoelectric ceramics are widely used in medical,industrial,and aerospace fields such as sensors and ultrasonic devices due to their ability to convert electrical energy to mechanical energy.The widely used piezoelectric ceramics are lead zirconate titanate(Pb(Zr,Ti)O3,PZT)-based ceramics due to their high working temperature range and excellent electrical properties.However,the presence of lead in PZT-based ceramics can lead to obvious environmental problems during production and use,so with the increasing attention to environmental protection the lead-free piezoelectric ceramics are becoming a major research hotspot nowadays.Among the lead-free candidates,sodium potassium niobate((K,Na)NbO3,KNN)-based ceramics are widely studied due to their good electrical properties and high Curie temperature.However,it has been found that KNNbased piezoelectric ceramics have the disadvantage of narrow sintering interval,which is usually caused by the volatilization of alkali metal elements and the bimodal distribution of ceramics grain size.While the volatilization of alkali metal elements can be solved by improving the preparation process,but the bimodal distribution of grain size has rarely been reported.Therefore,this work discusses the anomalous growth behavior of KNNbased ceramics and designs a two-step sintering method to adjust the microscopic morphology of KNN-based ceramics and enhance the electrical properties of ceramics.The details are as follows:0.96K0.5Na0.5NbO3-0.04(Ba0.5Na0.5)ZrO3 and 0.98K0.5Na0.5NbO30.02(Ba0.5Na0.5)TiO3 ceramics near the phase boundary at different sintering temperatures were prepared by the conventional sintering method,and it was found that the T-phase content in the samples increased with the increase of the sintering temperature,while and an anomalous grain growth behavior was found by Arrhenius images of grain size versus temperature:the grain growth of both ceramics displays three stages with increasing sintering temperature.In the first stage,increasing the sintering temperature in a relatively low temperature range leads to an increase in average grain size.As the sintering temperature increases to a higher temperature range,grain growth enters the second stage,where increasing the sintering temperature leads to a decrease in grain size,showing antithermal grain growth behavior.In the third stage,the ceramic resumes the thermal activation behavior with increasing grain size with increasing sintering temperature.The reasons for this non-Arrhenius grain growth behavior are discussed in detail.In addition,the effect of the non-Arrhenius grain growth behavior on the piezoelectric properties was investigated.Subsequently,0.97(K0.5Na0.5)NbO3-0.03CaTiO3,0.95(K0.5Na0.5)NbO3-0.05SrTiO3 and 0.95(K0.5Na0.5)NbO3-0.05BaTiO3 ceramics were prepared using the conventional sintering method to investigate the effect of radii of doping elements and sintering temperature on grain growth and electrical properties.All ceramics were sintered at different temperatures.Based on the average grain growth constants of the ceramics plotted with different sintering temperatures,it was found that all ceramics have a significant non-Arrhenius grain growth behavior.In addition,the poor reproducibility and narrow sintering interval of KNN-based ceramics were further explained by investigating the influence of the non-Arrhenius behavior and the radius of doping elements on the electrical properties as the non-Arrhenius behavior.From the above study,it was found that the bimodal distribution of grain size in KNN ceramics is caused by the different growth rates of two different growth boundaries,so a new two-step sintering method was designed by and used to prepared 0.96K0.5Na0.5NbO3-0.04(Ba0.5Na0.5)ZrO3 and 0.98K0.5Na0.5NbO3-0.02(Ba0.5Na0.5)TiO3 ceramics,it was found that the phase structures of the two ceramics did not change significantly with the change of sintering conditions.Meanwhile,due to the presence of two different growth boundaries,both ceramics exhibited some microstructural changes that violated the thermal activation characteristics,and the introduction of the two-step method could also significantly change the grain size distribution of the ceramics.And during the sintering process,a liquid phase appears in the(Ba0.5Na0.5)TiO3-modified ceramics,which makes it exhibit some different sintering behaviors from the(Ba0.5Na0.5)ZrO3-modified ceramics.In addition,the influence of the preparation conditions on the electrical properties was investigated,providing a good reference for the application of two-step sintering in(K,Na)NbO3-based ceramics. |
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