Study On The Propreties And Preparation Of Bismuth Ferrite Based High Temperature Lead-free Piezoelectric Ceramics

Piezoelectric materials are a kind of important functional materials that realize the conversion between mechanical energy and electrical energy and thus are popularly utilized to fabricate sensors,buzzer,drives and other electronic devices.Currently,the widely used piezoelectric materials in the market is mainly lead zirconate titanate?Pb?Zr,Ti?O₃,PZT?based piezoelectric ceramics material.But because of the PZT piezoelectric ceramics with more lead raw materials in the preparation process,will cause serious pollution and damage to the ecological environment,and the Curie temperature is less than 250 oC,there is great depolarization and the aging problem in high temperature environment.In recent years,with the rapidly development of atomic energy technology,aerospace,automotive,metallurgy and petroleum chemical industry,it is urgent need to work at high temperatures without failure of piezoelectric devices,and puts forward higher requirements for high temperature piezoelectric properties,high temperature lead-free piezoelectric materials have also been more and more attention.Perovskite structure BiFeO₃ of lead-free piezoelectric ceramics has good piezoelectric properties,and have high Curie temperature,is expected to be widely used in high temperature piezoelectric sensors.In this paper,BiFeO₃ based ceramics were prepared by solid state reaction method.By improving the sintering process,compounding with BaTiO₃,and adding excess Bi2O₃,the effects for the structure,piezoelectric and ferroelectric properties of BiFeO₃ based ceramic materials were systematic studied,and the conductive mechanism of inhibition is also discussed.1.The two step sintering process was used to prepare the pure phase bismuth ferrite ceramics,and the results were compared with the traditional solid phase sintering process.The experimental results show that the BiFeO₃ ceramics prepared by the conventional solid-state reaction system is usually had Bi24Fe2O₃9 and Bi2Fe4O9 impure phases,and the resistivity is relatively low.However,through two step sintering process,the impure phases gradually reduce with the increase of the high temperature T1,and then the pure phase BiFeO₃ ceramics can be obtained when T1 = 850 oC and T2 = 750 oC.At the same times,the ceramics get better performance of piezoelectric at 42 pC/N,the resistivity is also have a certain enhancement,arrived at 1.57×108 ?.cm.However,to further improve the T1,it will produce a large number of impure phases,and the resistivity will fall sharply.Through the analysis,we believe that this is due to the synthesis temperature of ferrite bismuth is narrow,when the temperature is too high will result in more volatilization of Bi,making the stoichiometric ratio imbalance,resulting in a more complex phase.2.The study found that the two step sintering process can fabricate the pure phase BiFeO₃ ceramics,but the larger leakage current makes it difficult to reach the saturation polarization in the polarization process,greatly reduce the piezoelectric activity,seriously hindered the application of BiFeO₃ ceramics as a high temperature lead-free piezoelectric material with strong piezoelectric activity.By different proportion BaTiO₃ compound with BiFeO₃,the influence of the proportion BaTiO₃ for the phase,crystal structure,the ferroelectric and piezoelectric properties of the BiFeO_3-xBaTiO₃?x = 0.20,0.25,0.33,0.25?ceramics were studied.The study found that the resistivity of the ceramic has improved greatly,and the samples have reached saturation polarization.When x = 0.33,the phase of ceramics is morphotropic phase boundary?MPB?that coexisting with the rhombohedral and pseudocubic.Meanwhile,the piezoelectric properties of the ceramics significantly increased up to 163 pC/N,the Curie temperature is also have 437 oC,the remnant polarization up to 21.45 ?C/cm2,the maximum strain is 0.082%,especially,the resistivity is greatly improved to 7.97×109 ?.cm.The results of these studies indicated that the leak current of the BiFeO₃ based ceramics can be effectively suppressed by compound with BaTiO₃,so as to improve the electrical properties of the Bi FeO₃ based ceramics.3.Choose the system of near the MPB composition 0.67BiFeO₃-0.33 BaTi O₃ ceramics to study by the two-step sintering method.The results revealed that the resistivity of the ceramics is further improve to 2.78×1010 ?.cm,correspodingly,the ferroelectric and piezoelectric properties has also improved,and the piezoelectric coefficient reached 171 pC/N,and the aging properties of the ceramic were also studied.The study through comparing two kinds of sintering process found that the piezoelectric property of ceramics prepared by two-step sintering method is better than the conventional solid-state reaction's.In addition,In addition,the Bi element of BT-BF ceramics is easy to volatile in the high temperature sintering process,which makes the component deviation from stoichiometric ratio.In order to make up the volatilization of Bi elements,the excess Bi was added in the proportion of 0.67BiFeO₃-0.33BaTiO₃ near MPB,and the effects of Bi excess on the crystal structure,ferroelectric and piezoelectric properties of the ceramics were systematically studied.It was found that the excess Bi had no significant effect on the crystal structure of the ceramics,but it was helpful to the sintering of the ceramics,which made up the volatilization of Bi,promoted the growth of the grains and improved the density of the ceramics.The proper amount of Bi can effectively improve the resistivity of ceramics,so as to increase the degree of polarization of the ceramics,and effectively improve the ferroelectric and piezoelectric properties of the ceramics.But if the excessive Bi is too much,in the ceramic grain boundary will be formation liquid phase in the sintering process,and if the grain boundary precipitation material is too much,it will hinder the lattice domain wall motion,make the piezoelectric property of ceramics decreased.When the excessive Bi arrived at x = 1.5%,the piezoelectric properties of ceramics reached a maximum of d33 = 207 pC/N,the Curie temperature is 423 oC,the maximum strain Smax reached 0.136%,the saturation polarization Pmax reached 53.91 ?C/cm2,the remnant polarization Pr reached 37.67 ?C/cm2,and the resistivity increased to 3.03×1010 ?.cm.