Studies On Piezoelectricity And Mechanism In Bismuth Based Composite Polar Ceramics

Traditional inorganic piezoelectric materials can be grouped into the following two groups:a)ferroelectric single crystals and ferroelectric polycrystals(such as PbTiO3,BaTiO3,Pb(Ti,Zr)O3,K0.5Na0.5NbO3,Na0.5Bi0.5TiO3,Pb(Ti,Zr)O3 piezoelectric ceramics,single crystals and thin films);b)nonferroelectric piezoelectric single crystals and piezoelectric oriented oriented thin films(such as SiO2,La3Ga5SiO14 single crystal,ZnO,AIN oriented films).Since piezoelectricity cannot exist in centric materials,all of the above materials are noncentrosymmetric.However,the discovery of flexoelectricity(the coupling between strain gradient and poling)expands piezoelectric materials to all insulators.It was recently reported by Lubomirsky et al.that some amorphous films(including BaTiO3,BaZrO3 and SrTiO3)also exhibit piezoelectricity if they are pulled through temperature gradient.This polar amorphous phase was termed quasi-amorphous and the appearance of polarity was proposed to be due to the plastic strain gradient induced partial alignment of the local bonding units.This polar amorphous phase provided a new direction for the design of polar inorganic material and the effect was then named plastic flexoelectricity by Yudin and Tagantsev in their latest review paper.Recently,we discovered a special class of bulk non-ferroelectric composite ceramics by traditional preparation technology.These materials exhibit both direct and inverse piezoelectric effects without undergoing an electrical poling process.It has great significance to study the origin of the anomalous macroscopic polarization for their lower dielectric constants,lower dielectric losses,higher depoling temperature and more stable thermal behaviors.According to the previous study,these sintered polar composites including Bi12TiO20-Na0.5Bi0.5TiO3,Bi12TiO20-SrTiO3,Bii2TiO20-Na0.5Bi4.5Ti4o15 do not possess obvious grain-oriented microstructures,and their piezoelectricity does not depend on ferroelectricity.Therefore,the macroscopic polarization cannot simply originate from grain orientation and have no relation with ferroelectric phase.Amorphous phase and distorted local bonding units may exist in these systems.Inspired by the random network of local bonding units theory of quasi-amorphous films,we proposed that this poling effect might also be classified as plastic flexoelectricity and that macroscopic polarization should be mainly contributed by the plastic strain gradient-induced partial alignment of grain boundary amorphous phases.The relevant data is very limited for this viewpoint.Moreover,the symbol is different and the magnitude of the piezoelectric strain constant is not constant for these materials.One needs to further enhance and improve their piezoelectricity to meet the practical application.Non-ferroelectric Bi12TiO20-BaSnO3 and Bi12TiO20-CaTiO3 composite were fabricated via an interfacial interaction between the presynthesized Bi12TiO20 and BaSnO3,Bi12TiO20 and CaTiO3.Although these composite ceramics contain no ferroelectric phases or oriented grains,they can still exhibit detectable piezoelectricity.This work could effectively eliminate the polar contribution of self-assembled impurity phases.Here,the relative content of amorphous and crystalline phases were estimated using XRD and DSC techniques.The main role of the crystalline BaSnO3 and CaTiO3 may be to suppress recrystallization and volatilization of the melting Bi12TiO20.The larger lattice constant might be more effectively to suppress recrystallization of Bi12TiO20,which may be the main reason that nearly amorphous Bi12TiO20 is formed in the Bi12TiO20-BaSnO3 system..The piezoelectricity in this system indicates that this polarization could exist in low crystallinity bulk materials.For the Bi12TiO20-CaTi03 composite ceramics,the piezoelectricity has relationship with the sintering temperature.The highest d33 value of 8 pC/N was obtained in the sample sintered at 860? which were found to contain a large amount of amorphous Bi12TiO20 and to possess the lowest density.According to the Raman spectra,the BiOs polyhedra of amorphous Bi12TiO20 phase distorted seriously in polarized samples.The present work confirms that Bi12TiO20 is a very interesting material that can form a macroscopic polar amorphous structure under certain conditions.Temperature gradient-driven plastic flexoelectricity of the grain boundary amorphous phases may be the main poling mechanism.The excellent temperature stability of the resonance frequency makes them promising candidates for high-temperature piezoelectric applications.In addition,composite ceramics with the nominal formula BaTiO3-xBi2O3(x=0.15,0.2,0.25,0.5,0.667 and 1)were sintered under a small thermal gradient.For the nonpiezoelectric x=0.15 composition,only the original BaTiO3 and aBi2O3 could be detected in the sintered composites.While the monoclinic a Bi2O3 changed into cubic Bi12Ti20 and a very small amount of rhombohedral Bi8.11Ba0.89O13.05 in the x=0.2-1 samples;both direct and converse piezoelectric effects were observed for these samples without undergoing the electrical poling process.We attempted to study the compositions with x>1;however,we failed to obtain dense samples with these compositions probably due to excessive volatilization of the Bi2O3 during the sintering process.In this study,the largest value of the d33 piezoelectric constant of approximately 12 pC/N was found in the x=0.667 sample,and this sample also exhibited the highest amorphous content.Because these composites do not contain obviously oriented grains and their piezoelectricity does not depend on ferroelectricity,temperature gradient-driven plastic flexoelectricity of the grain boundary amorphous phases is likely to be the main poling mechanism.Thus,the depoling temperature of this type of polar materials is determined solely by the melting temperature of Bi12TiO20.In addition,these composite materials can have a zero temperature coefficient of the resonance frequency in the high temperature region,and their piezoelectric constants are also comparable to those of the widely studied bismuth-layer structure ferroelectric ceramics.In order to provide' further evidence to support the relationship between interface interaction and volume ratio,the piezoelectricity in the Bi12TiO20-BaTiO3 system was also present.Since the macroscopic polarization have great relationship with the interface interaction,the piezoelectricity in this system could be enhanced by increasing the contact area.Nanoparticles exhibit high surface-to-volume ratios,which could increase the interfacial interaction and thus inhibit the recrystallization in the composites.Here,bulk Bi12TiO20-BaTiO3 nanocomposites are fabricated through the high-temperature interfacial reaction between nanometer-sized BaTiO3 particles and melting Bi12TiO20.Although the obtained nanocomposites are nearly amorphous and display very weak ferroelectricity,they exhibit relatively strong piezoelectricity without undergoing the electrical poling process.According to XRD,SEM and Raman techniques,the variation of crystallinity and structure that developed along the temperature gradient was confirmed.The magnitude of the piezoelectric constant was found to have great relationship with the crystallinity and distortion of BiO5 polyhedra of amorphous Bi12TiO20.The highest piezoelectric constant of 13pC/N was obtained together with the lowest crystallinity and highest degree of distortion of BiO5 polyhedra.Combining the results with thermal annealing experiment,it can be confirmed that the formation of macroscopic polar amorphous phases is closely related to the inhomogeneous plastic deformation of the amorphous Bi12TiO20 during the sintering process.These results highlight the key role of plastically deformed amorphous Bi12TiO20 in the Bi12TiO20-based polar composites,and the temperature gradient driven coupling between the plastic strain gradient and polarization in amorphous phases is the main poling mechanism for this special type of bulk polar material.According to Lubomirsky's studies,substrate could effectively suppress the crystallization in quasi-amorphous films.Here,Bi12TiO20-BaTiO3 composite ceramics were sintered on Al2O3 single crystal substrate.Our experiments show that the Al2O3 single crystal substrate could effectively suppress the infiltration and volatilization of Bi12TiO20.Because the sintering temperature is higher than the melting temperature of Bi12TiO20,the denser molten Bi12TiO20 would move down slowly during the sintering process.Thus,the compositon and amorphous content were inhomgenous along thickness direction.This further increased the strain gradient,which.would contribute to the macroscopic polarization.In this system,the symbol and magnitude of piezoelectric constant was relative homogeneity,the highest d33 was up to 20pC/N.Using the Al2O3 single crystal substrate sintering method,the high bismuth content sample of BaTiO3-xBi2O3(x>1)was obtained.According to SEM pictures,layer structures with thickness of several dozens to several hundreds were obtained.Amorphous phase and grain orientation were present according to XRD technique.The highest piezoelectric constant d33 was up to 22pC/N when x=50.In this system,both the orientation of amorphous phase and grain may contribute to the macroscopic polarization.The enhanced piezoelectric constant and decreased dielectric constant give a high piezoelectric voltage coefficient in the high bismuth content sample.Here,the Mg TiO3 and CaTiO3 with lower dielectric constant were introduced to fabricate Bi12TiO20-MgTiO3 and Bi12TiO20-CaTiO3 system.The piezoelectric voltage coefficient was enhanced to 47x10-3Vm/N.The Bi12TiO20-0.5MgTiO3 exhibit relative strong piezoelectricity(d33=18pC/N),very low dielectric loss(tg?=0.6%),low dielectric constant(?r=50),high electromechanical coupling coefficient(k=13%),high piezoelectric voltage coefficient(g33=41×10-3Vm/N)and excellent thermal stability over the entire temperature range from room temperature to 750?,and making them promising candidates for high-temperature piezoelectric applications.