Study Of The Structure And Properties Of BiFeO₃-based High Temperature Piezoelectric Ceramics Near The Morphotropic Phase Boundaries

The Curie temperature（TC）is the ferroelectric-paraelectric phase transition temperature in piezoelectric materials.Generally,the higher Curie temperature Tc,the higher the use or working temperature.Therefore,the high TC is required for the high-temperature piezoelectric material.The working temperature of piezoelectric materials is usually limited to less than half of their Tc to work well and be used normally at high temperature in general.Some literature defines piezoelectric materials with TC>400 ℃ as high-temperature piezoelectric materials.Even the piezoelectric sensors used in the nuclear energy industry and aerospace vehicles require the Curie temperature Tc over 500℃.In addition,due to simple synthesis process,.low cost and high piezoelectricity for piezoelectric ceramics,dominating the market of piezoelectric materials.So far the common high-temperature piezoelectric ceramics mainly include lead titanate PbTiO₃-based,potassium sodium niobate（K,Na）NbO₃-based,bismuth aluminate BiAlO₃-based and ferrite bismuth BiFeO₃-based perovskite structure,as well as bismuth layer and tungsten bronze structure system and so on.BiFeO₃ with rhombohedral structure shows large FE polarization（～90-100 μg/cm2）at room temperature.BiFeO₃ with high TC（～823℃）presents potential candidates for the application of high-temperature piezoelectric device and also attracted increasing attention.However,improved piezoelectric property has been achieved through various methods such as chemical doping and quenching at the cost of their Tc values.Moreover,some shortcomings such as the low resistivity and the high dielectric loss at high temperature still remain in this system,which is a longstanding major obstacle for the practical applications of high-temperature piezoelectric materials at present.In this thesis,the rhombohedral BiFeO₃ was selected as the research object,and we construsted and studied MPB in（1-x）BiFeO₃-xBa1-ySryTiO₃,（1-x）BiFeO₃-xBa0.5Sr0.5MnO₃ and（1-y）BiFe1-xCrxO₃-yBaTi1-xMnxO₃ solid solutions between BiFeO₃ and another the tetragonal perovskite with ferroelectricity.By systematically studying the crystal structure,dielectricity,resistivity,ferroelectricity and piezoelectricity of these high-temperature piezoelectric ceramics,the possibility of the coexistence of high piezoelectricity and high Curie temperature in BiFeO₃-baesd system was analyzed,and whose micro-mechanism were also discussed.In addition,the resistivity at high temperature and the origin in these systems are also discussed.The main results are as follows:（1）（1-x）BiFeO₃-xBa1-ySryTiO₃（BFO-BST）ceramics were prepared with the conventional solid-state reaction process.A series of MPB can be constructed by varying both x and y.And the structure and physical properties（ferroelectricity,dielectricity,resistivity,piezoelectricity and so on）between these MPB samples were compared and studied in the BFO-BST system.All the MPB samples show enhanced piezoelectric properties and relatively high Curie temperature Tc.The d33,Tc and ferroelectric polarization decrease with increasing Sr content y in all the MPB samples.Detailed structural analysis shows that the lattice distortion of rhombohedral and tetragonal phases increases overall and decreases,respectively,with increasing y in the MPB samples.Higher tetragonal distortion and lower rhombohedral distortion are revealed to play an important role in the piezoelectric performance near MPB in the lead-free BFO-BTO based solid solutions.These results suggest a possible new strategy for designing high-performance piezoelectric ceramics.（2）（1-x）BiFeO₃-xBa0.5Sr0.5MnO₃（BFO-BSMO）（0≤x≤1）ceramics were prepared with the sol-gel method.The structure,ferroelectricity,magnetism,dielectricity,resistivity and piezoelectricity of BFO-based ceramics were studied via incorporating BSMO.Some electric properties such as piezoelectricity of were studied near the MPB.And the enhanced piezoelectric properties and high Tc were observed in lead-free BFO-based system.The analysis of room-temperature X-ray diffraction（XRD）and Raman spectra suggests that the crystal structures of rhombohedral R3c,cubic Pm3m and hexagonal P63/mmc for the samples with 0≤x≤0.1,0.16<x<0.5 and 0.7<x≤1.0,respectively.It is found that the samples with x=0.16 and 0.7.which lie in the structure phase boundary,contain mixed phases of rhombohedral and cubic,and cubic and hexagonal,respectively.Detailed studies suggest that the samples with x<0.16 show ferroelectric properties,and the x=0.1 sample exhibits high ferroelectric Curie temperature Tc～687℃ and the piezoelectric constant d33～1 3 pC/N.The piezoelectric constant d33 reaches a maximum with increasing BSMO content x to 0.16.Detailed analysis shows that the relatively high concentration of low-valence Mn3+ions is found to influence significantly the structure,ferroelectricity.piezoelectricity and resistivity in these samples.Although the high Curie temperature was observed in this system,the piezoelectric coefficient is low and the low resistivity at high temperature still needs to be resolved.（3）We investigated the properties of BFO-BTO ceramics via incorporating simultaneously Cr and Mn ions.（1-y）BiFe1-xCrxO₃-yBaTi1-xMnxO₃（BFC-BTM）（0≤x≤0.04,0.2≤y≤0.27）ceramics were fabricated using the conventional solid-state method.We study systematically the structure,dielectricity.ferroelectricity.resistivity and piezoelectricity of Cr-Mn co-doped BFC-BTM（0≤x≤0.04,0.2≤1≤0.27）ceramics,and the origin of improved Tc in the Cr-Mn modified samples was discussed.With increasing Cr-Mn content x.the y=0.2-0.27 samples show the structure transition from rhombohedral to tetragonal phase with the MPB region at x～0.01-0.02.The Cr-Mn co-doping samples show coexistence of relatively high Td/TC（～580-645℃）and relatively large d33（～96-135 pC/N）.Especially,coexistence of high depolarization temperature Td（～551℃）and relatively large piezoelectric constant d33～116 pC/N,superior to most of those of BFO-based high-temperature piezoceramics reported previously,were observed in the sample y=0.24,x=0.02.In addition,the resistivity of these Cr-Mn co-doped samples are higher than BFO.And for the y0.24x0.01 and y0.24x0.02 samples the resistivity is higher than～1 MΩ·cm below～315 and 270℃,respectively.Our results demonstrate that constructing MPB via considering simultaneously the role of lattice distortion might be important in achieving both high piezoelectricity and high Tc in BFO-based piezoelctric creamics.