Study On The Structure And Dielectric Properties Of Aurivillius Ferroelectric Oxide

Aurivillius ferroelectrics,also named as bismuth layer-structured ferroelectric materials.It is a kind of lead-free and environment-friendly material which has high Curie temperature,good fatigue resistance,low dielectric loss and excellent stability.Therefore,it's widely used in manufacturing of high temperature and pressure equipment,sensors and ferroelectric memory.The Curie temperature is the temperature corresponding to the maximum value of the dielectric constant,the Curie temperature is an important parameter to measure whether bismuth layer-structured material suitable for high temperature and pressure equipment,In order to improve its deficiency that single bismuth layer-structured material is difficult to be practical,doping modification becomes a hot topic.Many researchers are dedicated to prove that ion substitution can change the tolerance factor,thus affecting the change of Curie temperature.In this thesis,in order to explore whether other factors can affect the Curie temperatures,well as the influence of different substitution on bismuth layer-structured materials' structure and dielectric properties,iron ions with different valences and different radius were used to substitute different location in aurivillius ferroelectrics.Seven kinds of Aurivillius compounds were synthesized by using a solid state sintering method.They are SrBi_4-xLnxTi₄O₁₅ ceramics whose A-site was substituted by yttrium or holmium,Sr_1-x/2?LnK?_x/4Bi₄Ti₄O₁₅ ceramics whose A-site was substituted by potassium and yttrium or potassium and holmium.Sr_1-xBaxBi₄Ti_4-xZrxO₁₅ ceramics,whose A-site was substituted by barium and B-site was substituted by zirconium,Sr_1-xK_xBi₄Ti_4-xNb_xO₁₅ ceramics,whose A-site was substituted by potassium and B-site was substituted by niobium,Sr_1-xK_xBi₄Ti_4-3x/4Nb_x/2W_x/4O₁₅ ceramics,whose A-site was substituted by potassium and B-site was substituted by niobium and tungsten as well.These compounds' main phase is four-layer Aurivillius compounds SrBi₄Ti₄O₁₅.Another two Ba-based bismuth layer-structured materials are four-layer BaBi_4-xLn_xTi₄O₁₅ ceramics and three-layer BaBi_3-xLnxTi₂NbO₁₂ ceramics,whose A-site was substituted by yttrium orholmium.Experimental results revealed that the Curie temperature of SrBi₄Ti₄O₁₅ ceramic was516.9?.The solid solution quantity in A-site was bigger than B-site.When the exchange share was 30%,A single bismuth layered perovskite phase could still be synthesized in A-site substitution.However,another phase was synthesized in AB-site substitution.In the four-layer structure SrBi_4-xLn_xTi₄O₁₅ ceramics and Ba Bi_4-xLn_xTi₄O₁₅ ceramics whose A-site was substituted by Ln³⁺,tolerance factor is the dominant factor which affect the Curie temperature.While in the three-layer structure BaBi_3-xLn_xTi₂NbO₁₂ ceramics whose A-site was substituted by Ln³⁺,the dominant factor that affect the Curie temperature is the relative displacement along the a-axis direction by Bi ions in A-site and octahedralWhen A-site was substituted by Y³⁺/K⁺ and Ho³⁺/K⁺,the tolerance factor tends to be stable,the Curie temperature rose with the increase of substitution amount.In the three series of samples whose AB-site was substituted by by Ba²⁺/Zr⁴⁺?K⁺/Nb⁵⁺?K⁺/Nb⁵⁺/W⁶⁺,tolerance factor rised with the doping,whereas Curie temperature and the dielectric constant both vary reversely with the amount of substitution.Their change rates of Curie temperature per tolerance factor?35?Tc/?35?t are 2.7×104?,1.5×104?and 0.9×104?.Therefore conclusion can be made that ions with different valences or different radius can intensify the disformation of octahedron,therefore promote the rise of the Curie temperature.When tolerance factor increased,the higher the fluctuations in valence and size of the compound are,the slower the Curie temperature drops.Additional,the substitution of AB-site by Ba²⁺/Zr⁴⁺ is favorable for the growth of crystal.What's more,the exchange of AB-site and K⁺/Nb⁵⁺ is benefit for the growth rate of c-axis part of crystal.Suitable amount of different ions replacement can enhance the remnant polarization.