Design, Fabrication And Properties Of BiFeO₃-PbTiO₃Based High Power Piezoelectric Ceramics And Devices

BiFeO₃PbTiO₃based piezoelectric ceramics have drawn much attention owing to theirrelatively high Curie temperature, high dielectric strength, good ferroelectric and piezoelectricproperties, which have potential application in high-temperature piezoelectric field. However,development of BF–PT materials has been hampered due to their high coercive field, lowmechanical quality factor and high dielectric loss. In this work，（1-x）Bi(Ga_0.05Fe_0.95)O₃-xPbTiO₃（BGF-PT） ceramics for x=0.25,0.3,0.35and0.45have been fabricated by the solid state reactionprocess. The MPB effects of BGF-PT have been studed. Piezoelectric ceramics of0.6(Bi_0.9La_0.1)FeO₃-0.4Pb(Ti_1-xMn)x)O₃（BLF-PTM） for x=0,0.01,0.02and0.03, with greatelectomechanical properties and temperature stability, were fabricated by sol-gel process combinedwith a solid state reaction method. The dielectric, piezoelectric and elastic nonlinearities forBLF-PTM ceramics have been discussed. Unipole radial high power piezoelectric transformer wasprepared using BLF-PTM ceramics, and the equavilent circuit model has been built，and itselectrical properties have been characterized at room and elevated temperature.BGF-PT at MPB is coexistence of rhombohedral and tetragonal phases, exhibting enhanceddielectric constant, remanent polarization and decreased dielectric loss. XPS analysis indicates thatMPB components have less concentration of Fe²⁺and oxide impurities. The impedance analysisshows that only grain contributed to the electrical properties of x=0.3and0.35, while both grain andgrain boundary contributed to that of x=0.25and0.45. The motion of oxygen vacancies contributeto the conductivity in the high temperature region, and MPB compositions shows high activationenergy.BLF-PTM ceramics have perovskite structure, and are coexistence of rhombohedral andtetragonal. Mn modification promotes grain growth. Upon using Mn substituents, BLF-PTMceramics exhibites the characteristics of hard piezoelectrics, such as （1） low polarization,（2） lowdielectric constant and loss,（3） high mechanical quality factor Qm,（4） low internal friction,（5） lowelastic modulus,（6） asymmetrical P-E loops. The dielectric loss, piezoelectric constant d33,piezoelectric voltage coefficient g33, planar coupling coefficient kp, mechanical quality factor Qmand Curie temperature Tcof0.006,124pC/N,37×10-3Vm/N,0.338,418and473oC, respectively,are achieved for BLF-PTM of x=0.01. kp，d33and Qmof Mn modified BLF-PTM show greattemperature stability, the depoled temperature Tdis370oC, and Qmis still more than200at250oC.A large vibration velocity of1.1m/s is achieved for this composition, with only6oC temperaturerise.Under higher applied fields, the nonlinear dielectric, elastic and piezoelectric effects areobserved in BLF-PTM. The dielectric and piezoelectric properties of BLF-PTM without Mnmodification obey the Rayleigh law under high electric field, whereas Mn modified BLF-PTM donot. The third order nonlinear elastic effect results in the shift of resonance frequency. However, thenonlinear dielectric constant, piezoelectric coefficient, nonlinear elastic compliance andelectromechanical coupling coefficient decrease by Mn substituents, attributed to its stable domainstructure, leading to low nonlinearities.According to equivalent circuit model, the match load and efficiency of unipole radialpiezoelectric transformer is found to be8.1k and2.1k,91.2%and84.3%at resonancefrequency and third order resonance frequency. Under resonance frequency and third order resonance frequency, the power density and efficiency of piezoelectric is30.1W/cm³and23.5W/cm³,83.4%and80.2%. With temperature rising, the voltage gain increases initially and thendecreases; the maximum voltage gain is35.6at200oC. The power density of piezoelectrictransformer is20W/cm³at250oC, and exhibits large power density ever at300oC. Our resutsshows that the transformer prepared by BLF-PTM is a kind of high temperature and high powerpiezoelectric transformer.