| Ferroelectric with tungsten bronze structure is an important kind of functional materials from the viewpoint of diverse properties(dielectric,ferroelectric,photoelectric,piezoelectric,pyroelectric,and non-linear optic properties)as a result of flexible structure and adjustability.This thesis mainly focus on the correlation of the composition-structure-property in tetragonal tungsten bronze(TTB)compounds,SrxBa1-xNb2O6(SBN),Sr1-xCaxNaNb5O15(SCNN)and Sr2NaNb5O15(SNN)ceramic systems were employed in this thesis to discuss the influence of A-site filled degree on phase structure,microstructure morphology and electrical properties by introducing Na+in A-site.The main conclusions were as follows:1.The(Sr0.53Ba0.47)2 5-0.5xNaxNb5O15(SBNN)ceramics with high-density weresuccessfully synthesized by the conventional solid-state reaction method.The influences of A-site filled degree on phase structure,microstructure morphology and electrical properties were studied by introducing alkalis Na+ in A-site to modulate structure from unfilled to filled tungsten bronze.The results showed that with the increase of Na+content in A-site,A-site filled degree increased,along with the changing of the structural type from 'unfilled' to 'filled' state,and the pure tungsten bronze structure could be obtained in all compositions.The anisometric morphology was particularly obvious in the 'filled' tungsten bronze structure ceramics.With further increasing x above 1.0,Na+ ions cannot enter into the extra smaller C sites to form the 'stuffed'structure due to bigger radius,which resulting in the existence of perovskite structure NN-based phase.This conclusion was verified by the results of XRD and EDX.It can be concluded that for SBNN ceramics,by introducing Na+ contents to increase A-sites filled degree,the distortion degree of NbO6 polar unit and interaction strength inside NbO6 octahedron were increased,which was beneficial for the dielectric and ferroelectric properties.Thus,better comprehensive dielectric and ferroelectric properties were obtained for filled SBNN ceramics(Tc = 257 ?,?m? 2730,Pr ? 4.8?C/cm2 and Ec = 10.7 kV/cm).2.Effects of different Sr2+ concentration in A-sites on the microstructure,dielectric and ferroelectric properties were investigated in detail for(SrxBa1-x)2NaNb5O15 ceramics by changing the contents of Sr2+/Ba2+.When x<1.0,all filled SBNN ceramics showed pure tungsten bronze structure,SNN(Ccm21)and BNN(Bbm2)showed different space group symmetry,and the MPB existed in filled SBNN ceramic with x = 0.6,which markedly improved dielectric and iron electronic properties(em = 3622,tan8 = 0.024,?r=875,Pr=5.0 ?C/cm2 and Ec = 11.3 kV/cm).When x = 1.0,the second phase Na0.5Sr0.25NbO3 appeared for SNN ceramics due to solid solution limit,decreasing the electrical properties of SNN ceramics.In addition,the low temperature dielectric anomaly peak occurred due to the polarization perturbation and the local structural fluctuation,and the anomaly peak strengthened and shifted to higher temperatures with increasing Sr2+ content.3.Structural modulations were controlled by the Na+ concentration for(Sr0.925Ca0.075)2 5-0.5xNaxNb5O15(0.4 ? x ? 1.0)ceramics,and the influences on the phase structure,microstructure,transparent properties,dielectric properties and ferroelectric properties were studied to further verificate the influences of A-site filled degree on electric properties.The TTB pure phase could be obained for all ceramic samples as x<1.0.Introduction of Na+ increased A-site filled degree,which was beneficial to the dielectric and ferroelectric properties of the SCNN ceramics because it enhanced the distortion degree of the NbO6 octahedron in the unit cell and increased the interaction strength inside NbO6 octahedron.The electric properties of the filled SCNN ceramics(x=1.0)deteriorated because of the presence of a second cubic NSN phase in the sintering process.Overall,the best dielectric and ferroelectric properties were obtained at x = 0.8(Tc = 290 ?,?r = 1394,?m= 1320,tan? = 0.006,Pr = 5.5 ?C/cm2 and Ec = 19 kV/cm).Apart from excellent electrical performance,SCNN ceramic possessed a certain optical properties(35-45%).4.For(Sr1-xCax)2.1Na0.8NbsO1(SCN0.8N,0.0 ? x ? 0.15)ceramic materials system,the influences of different Ca2+ concentration on phase structure,microstructure,electrical properties and optical properties were studied by changing the Sr2+/Ca2+content.All SCN0.8N ceramic samples obtained TTB pure phase.With increasing x,the anisometric pillar-type grains decreased,while the equiaxed shape grains increased for SCN0.8N ceramics.Also,the abnormal grain growth was facilitated by the formation of little liquid phase in ceramics with higher x.The SCN0.8N ceramics with x = 0.075 showed a smaller grain size as well as uniform dense microstructure morphology,at the same time showed the excellent electrical performance(?r= 1420,?m = 1326,tan? =0.006,Pr= 5.1 ?C/cm2 and Ec= 14 kV/cm).In addition,a low-temperature dielectric anomaly peak associated with the ferroelastic transition was enhanced and shifted to higher temperature as x increased.5.Filled Sr2NaNb5O15(SNN)ceramics were easy to form NSN second phase in the sintering process,thus largely worsening electrical properties of SNN ceramics.In order to eliminate NSN second phase,the effects of introducing Bi3+ concentration in Sr2BixNa1-3xNb5O15 ceramics on the microstructural,dielectric,ferroelectric and transparent properties were studied in detail.The results showed that the NSN phase was effectively eliminated by introducing Bi3+ to the SNN based ceramic in sintering process.When x = 0.04,the grains were relatively.uniform and showed clear grain boundaries,thus obtaining better transparent properties and better comprehensive electric properties:?m= 1768,Pr= 7.1 ?C/cm2 and Ec= 13.9 kV/cm. |
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