SrBi₄Ti₄O₁₅及其共生结构掺杂改性研究

【作者】 朱骏；

【导师】 陈小兵；

【作者基本信息】 扬州大学 ， 凝聚态物理， 2003， 硕士

【摘要】 本论文着重于层状钙钛矿结构铁电材料SrBi₄Ti₄O₁₅（SBTi）和共生结构铁电材料Bi₄Ti₃O₁₂-SrBi₄Ti₄O₁₅（BIT-SBTi）的掺杂改性研究。对它们陶瓷样品的微结构、铁电和介电性能的研究，有助于了解掺杂对层状钙钛矿结构铁电材料的性能，特别是剩余极化影响的机理，并指导设计有实用性能的用于非易失性铁电随机存储器的新型铁电材料。 采用固相烧结工艺，制备了不同La掺杂量的SBTi和BIT-SBTi和V掺杂SBTi铁电陶瓷样品。用X射线衍射对它们的结构进行了分析，发现这些样品都是随机取向，且掺杂基本未改变材料原来的晶体结构。 对不同掺杂量的样品的铁电性能测试结果显示，适当的掺杂量都能有效改善两种材料的铁电性能。掺杂后，剩余极化（2P_r）都呈现出先增大，后减小的规律。La掺杂SrBi₄Ti₄O₁₅中，掺杂量为0.25时，2P_r达到极大值：24.2μC·cm^-2，比未掺杂时增大近50％。La掺杂BIT-SBTi系列材料的2P_r在掺杂量为0.50时出现极大值，为25.6μC·cm^-2，相对未掺杂时提高近60％。并且La掺杂还能降低SBTi的矫顽场E_c，SrBi_3.75La_0.25Ti₄O₁₅的E_c为60.8kV·cm^-1，比SBTi下降了近50％。钒对SBTi的B位离子的掺杂，在不损害材料的热稳定性的同时，改善了材料的耐压性能，并使2P_r大大提高，增至50.0μC·cm^-2以上。 La掺杂SBTi时，一方面，掺杂引起的晶格畸变减小使2P_r下降，另一方面，较稳定的La³⁺取代稳定性较差的Bi³⁺后，氧空位的减小导致空间电荷浓度降低，使2P_r增大。La掺杂SBTi的2P_r先增大，后减小正是这二者相互竞争的结果。而2P_r最大时的掺杂量与La掺杂Bi₄Ti₃O₁₂不同，这是由于SBTi的A位已有较稳定Sr²⁺，起着与掺杂离子La³⁺相似的作用。La掺杂Sr₂Bi₄Ti₅O₁₈的2P_r的变化规律验证了这种观点。 SBTi和BIT-SBTi经A位掺杂后，居里温度（T_c）都随掺杂量的增加而降低，在掺杂量分别为0.75以上和1.50时，二者都出现了弛豫铁电体的典型特征。 由于La掺杂未改变BIT-SBTi的晶体结构，可以认为，掺杂后BIT-SBTi的T_c扬州大学硕士学位论文是其组元居里温度的平均值的关系没有改变。基于此，可以估算La在组元中的分布。结果显示，当掺杂量不超过1.25时，较多的份十取代Brr的Bi3+，而在掺杂量为1.50时，SBTi中的La含量超过BIT单元。 今后的工作要点主要为:（l）用La系其它元素对sB五掺杂，通过改变离子半径，研究2Pr的规律，验证我们提出的A位掺杂对材料ZPr影响机理;（2）B位掺杂对Brr-sB五的性能影响研究;（3）这些材料的薄膜制备及性能测试。更多还原

【Abstract】 The effect of doping in A or B sites on the structure, ferroelectricity and dielectricity of layer-structured ferroelectrics SrBi4Ti4O15 (SBTi) and intergrowth ferroelectrics Bi4Ti3O12-SrBi4Ti4O15 (BIT-SBTi) has been studied. These studies will be helpful to understand the mechanism of the property variations of layer-structured ferroelectrics caused by doping and to guide the design of new useful materials for no-volatile ferroelectric random access memories.The ferroelectric ceramics of SBTi and BIT-SBTi doped by lanthanum (SBLT and BLT-SBLT) and SBTi doped by vanadium have been prepared by solid-state reaction method. The structure was analyzed by X-ray diffraction. It is found that the samples are randomly origined and the structure has been changed little by doping.The ferroelectricity of the two materials has been improved by doping. Their remnant polarization (2Pr) increases at first, then decreases with the increase of La or V content. As for SBTi doped by La, when La content is 0.25, the 2Pr reaches a maximum value of 24.2 #C$cm-2, increased by near 50% compared with that of SBTi. The 2Pr of BIT-SBTi doped by La with a content of 0.25 has a maximum value of 25.6 #C$cm-2, increased by near 60% in comparison with that of BIT-SBTi. The coercive field of SBTi is decreased by La doping, and the Ec of SrBi3.75La0.25Ti4O15 is 60.8 kV$cm-1 and decreases about 25% in comparison with that of SBTi. The substitution of the ions at B site of SBTi by V can improve the electric breakdown voltage without changing the Curie temperature, and the 2P, is enlarged to over 50.0 #C$cm-2.As La doped SBTi, on the one hand, it should bring about less structural distortion which causes a decrease of polarization. On the other hand, as Bi3+, which is less stable, is substituted by La3+, the decrease of the concentration of space charge due to the oxygen vacancies should lead to the increase of 2Pr. Therefore, the variation of 2Pr is determined by the competition of these two mechanism. Because there is an Sr2+ ion in the A site of SBTi, which is more stable and acts as La3+, the 2Pr of SBLT reaches amaximum value when La content is small, compared with La doped BIT. This viewpoint is concided with the 2Pr of Sr2Bi4Ti5O18 ferroelectrics doped by La.The Curie temperatures of SBLT and BLT-SBLT decrease with the increase of La content. For SBLT, when La content excesses 0.75, the samples exhibit typical characteristics of relaxor type ferroelectrics. BLT-SBLT with 1.50 La exhibits characteristics of relaxor type ferroelectrics as well.Based on the fact that the crystal structure of BIT-SBTi is not affected by doping and by assuming that the Curie temperature of BLT-SBLT is about the middle of its two constituents, La content in both BLT and SBLT has be estimated. The simulation result shows that when La content is less than 1.25, La content in BLT constituent is higher than that in SBLT, but when the content is 1.50, La content in SBTi is higher.The future work will be concentrated as following: (1) The mechanism of the effect of doping in A site on 2Pr should be clarified by the research on the rule of 2Pr of SBTi doped by the lanthanide serials having different ions radii; (2) The properties BIT-SBTi doped in the B site should be studied; (3) The thin films of these materials should be prepared and their properties should be studied.更多还原