| Magnetoelectric(ME)materials have drawn increasing interest due to its potential applications in memory,filter,sensor and other fields as the ability of transforming electrical signals into magnetic signals and vice versa.However,single phase magnetoelectric materials are rare,and most of them are also not technologically applicable because of the weak ME response and low operating temperature.So more attentions are put into ME composites rather than single phase ME materials,and the high ME coefficients,which are up to several V/(cm"Oe),are often reported in ME composites for the past few years.Although giant ME responses have been reported in composites,there are still some problems that restrict their applications.Firstly,the distribution of magnetic phase in composites has great effects on electrical properties,especia'lly for particulate composites.For example,the aggregation of magnetic particles in piezoelectric matrix phase will aggravate electrical leakage and weaken the ME response in composites.So well-dispersed magnetic phase in composites are important for obtaining high ME response.Secondly,as it's strain modulated ME effect in composites,the strength of the interface between piezoelectric and magnetic phase has a significant impact on ME response.However,the strength of the interface always attenuate during cycling or operating times and the ME responses become unstable in composites.From another point of view,single phase ME materials,without phase boundary,would have more stable ME responses.In addition,traditional ME materials base on the coupling of piezoelectric and magnetostrictive responses are limited to crystal symmetry of the piezoelectric phase.The only way to broaden the operating temperature is to use piezoelectric materials with high Curie temperature,which imposes a restriction on other materials.To solve the problems above,my research started from fabricating the(1-x)BiFeO3-xBaTiO3 ceramics,and then the phase structure,microstructure,electrical and magnetic properties were also discussed.Furthermore,we researched the effects of reduction,bismuth deficiency and quenching on electrical,magnetic and magnetoelectric properties of(1-x)BiFeO3-xBaTiO3 ceramics,respectively.The dissertation is divided into six chapters.Excluding chapter 6,each chapter is summarized as follows:In Chapter 1,we first provide an overview on the microstructure,electrical and magnetic properties of single phase ME materials,such as BiFeO3,BiFeO3-BaTiO3,TbMnO3,Mn3B7O13I and BaMnF4.Then we concluded some common piezoelectric materials,magnetostrictive materials and ME composites.In addition,ME composites with various connectivities between piezoelectric and magnetostrictive materials was presented.The mechanism and influence factors of flexoelectric effect were discussed and the testing methods for flexoelectric coefficient were also introduced.Finally,the application of ME effect in memory and filter are introduced.In Chapter 2,the structural and physical properties of(1-x)BiFeO3-xBaTiO3 ceramics fabricated by solid state sintering method are discussed.By comparing the difference in phase structure and abnormal increase in ferroelectric and piezoelectric responses,we proved the existence of morphotropic phase boundary near x=0.3.Similarly,we also compared the difference in magnetic hysteresis loop of(1-x)BiFeO3-xBaTiO3 ceramics,and researched the effects of Ti4+ replacing Fe3+ on ferromagnetism.Finally,we summarized the test equipment and principles for ME coefficient.In Chapter 3,we introduced a ME coulping effect between flexoelectric response and magnetostrictive respons.An enormous apparent flexoelectric coefficient of 157pC/m in 0.75BiFeO3-0.25BaTiO3 ceramics was obtained with asymmetric reduction process at 800? for 10min.At the same time,we also found that reduction can enhance the ferromagnetism of 0.75BiFe?3-0.25BaTi?3 ceramics,and the asymmetric reduction process induced a magnetic gradient through thickness.So we assumed that strain gradient can be induced by magnetic gradient under a bias field,and the electric signal from flexoelectric response can be detected.Finally,we detected the ME coefficient peak emerged at the bending resonance frequency of wafer,which proved our previous assumption.In Chapter 4,we fabricated particle ME composites with uniform dispersion through non-stoichiometric chemical constitution in 0.75Bi(1-x)FeO(3-1.5X)-0.25BaTiO3 ceramics.With BaFe12O19 emerged in matrix phase with bismuth deficiency,we found it turn out to be particle composites.An enhanced ferromagnetism was also observed in composites for the reason that BaFe12O19 is a traditional ferromagnetic material.The piezoelectric responses were stable at 31-35pC/N with x<0.12.Futher increasing the x value make the composites more leaky and decrease the piezoelectric coefficients.A similar trend was also found in ME responses,the ME coefficient first increased and then decreased with x,the peak value was 119mV/(cm·Oe)in x=0.12.In Chapter 5,we proposed a method to enhance the ME responses in(1-x)BiFeO3-xBaTiO3 ceramics through quenching process.The piezoelectric responses were greatly enhanced after quenching and the peak value was 218 pC/N at x=0.3.In addition,the ferromagnetism and ME responses were also enhanced after quenching,the peak value of ME coefficient was 1.298 V/(cm·Oe)at x=0.25. |
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