Ba基充满型钨青铜陶瓷的结构、介电性能与铁电相变

发布时间：2018-06-12 17:23

本文选题：充满型钨青铜 + 弛豫铁电体　；参考：《浙江大学》2015年博士论文

【摘要】：本论文系统地研究了Ba基充满型钨青铜钛铌酸盐陶瓷的晶体结构、介电铁电性能以及相变特征,重点讨论了四边形空隙A1位填充离子对其结构与性能的影响规律。揭示了钨青铜结构中弛豫铁电性产生的结构根源、以及铁电相变特征随成分变化的规律。在Ba5RTi3Nb7O30 (R=La, Nd, Sm)陶瓷中,室温下均形成了四方钨青铜结构,当R=La时空间群为P4/mbm;而R=Nd和Sm时,空间群为P4bm。Ba5YTi3Nb7O30室温下形成的四方钨青铜结构稳定性较差。随着五边形(A2)与四边形(A1)填充离子半径差AR增大,Ba5RTi3Nb7O30弛豫铁电相变特征逐渐减弱：Ba5LaTi3Nb7O30为典型的弛豫型铁电体,介电峰出现的温度和频率的关系符合Vogel-Fulcher公式；而在Ba5NdTi3Nb7O30、Ba5SmTi3Nb7O30的介电温谱中频率色散的现象逐渐减弱,同时介电峰出现的温度逐渐向高温方向移动。在所有成分中,除了相变温度附近,低温处都存在至少一个介电异常,这与低温处微区结构的调制、尤其是氧八面体的扭曲或变形有关。在弛豫铁电体Ba5SmTi3Nb7O30中,当一个Ba离子被Ca替换,其介电温谱中频率色散现象将消失,取而代之的是较为尖锐的介电突变峰,表现出正常铁电体相变的特征。同时,Ba4CaSmTi3Nb7030介电温谱中相变峰出现的温度比Ba5SmTi3Nb7O30提高了很多。根据全谱拟合的结果,发现在Ba4CaSmTi3Nb7O30晶胞中,ab平面内[Ti/Nb(2)-O(5)]键长减小很多,说明小离子Ca2+的填充减少了氧八面体在ab平面内的畸变,有利于长程铁电有序的建立。Ba5SmTi3Nb7O30中,四变形空隙A1无序填充了两种不同价态与半径的离子,相比于不同电荷形成的微区电场,晶格中微区内应力的不均分布引起周围氧八面体畸变是弛豫铁电性更为重要的结构起源。在钨青铜结构的弛豫铁电体Ba5RTi3Nb7O30 (R=La、Nd、Sm)中,其低温电滞回线表明,剩余极化值Pr和最大极化值Pmax均逐渐下降。该现象类似于钙钛矿结构的弛豫铁电体中的“弛豫再入”。其出现的原因可能是由于钨青铜较为复杂的结构和离子填充所导致低温处结构的微扰,进而阻碍了极化方向长程铁电有序的建立。另一方面,随着温度降低,铁电畴翻转形核长大速率降低,微区结构成分的不均匀在极化翻转过程中起到钉扎的作用,导致低温处矫顽场逐渐增大,这也是测试过程中出现极化值逐渐下降的一个重要原因。为了进一步研究充满型钨青铜结构中A1位四边形空隙中填充离子对低温处极化响应的影响,制备了Ba5LaxSm1-xTi3Nb7O30 (x=0.1,0.25,0.5,0.8,0.9,0.95)钨青铜铁电陶瓷,并表征了其低温介电弛豫和铁电特性。随着La元素的增加,四面体空隙周围氧八面体晶格畸变增大,导致弛豫铁电性更为明显。同时,随着温度的降低,极化值下降更为显著。通过极化值随着温度的变化曲线可以发现,有序的离子填充和微观结构有利于长程铁电有序的建立,进而减弱低温处“弛豫再入”的特征。Ba4R2Ti4Nb6O30(R= La、Nd、Sm、Eu)陶瓷介电性能及铁电相变随A1和A2位离子半径差的变化规律与Sr基有类似之处。Ba4La2Ti4Nb6O30为典型的弛豫型铁电体。而随着离子半径差△R增大,Ba4Nd2Ti4Nb6O30, Ba4Sm2Ti4Nb6O30和Ba4Eu2Ti4Nb6O30都表现出正常铁电体一级相变的特征。而通过Curie-Weiss拟合发现,Ba4Nd2Ti4Nb6O30中T0为100K,而Ba4Sm2Ti4Nb6O30和Ba4Eu2Ti4Nb6O30中T0分别为-522.36K和-317.9K,都为负值,说明后两者在相变过程中存在与Ba4Nd2Ti4Nb6O30不同的结构变化。通过电滞回线测试发现,Ba4La2Ti4Nb6O30室温为顺电相,冷却到193K出现了较小的剩余极化值。Ba4Nd2Ti4Nb6O30高温时电滞回线为线性,降温至393K时突然出现较大的剩余极化值,符合正常铁电体相变的特征。然而,Ba4Sm2Ti4Nb6O30和Ba4Eu2Ti4Nb6O30中,高温在453K附近出现破折状电滞回线,表现出结构中反铁电有序的特征。随着温度降低,剩余极化值开始出现,铁电相趋于稳定而整体表现出亚铁电性。室温通过压电力显微镜观察,发现微区出现铁电畴,证实其微区非中心对称的结构。在Ba5RTi3Nb7O30和Ba4R2Ti4Nb6O30 (R= Nd、Sm)钨青铜陶瓷变温拉曼光谱分析表明,当测试温度跨越相变温度区间时,对于Ba5RTi3Nb7O30 (R= Nd、Sm)弛豫铁电体,在低温处拉曼峰的强度增大同时部分振动模波数发生变化,而并未突然消失或出现新的拉曼活性峰,说明弛豫弛豫铁电体相变过程中结构的逐渐变化。在Ba4R2Ti4Nb6O30 (R= Nd、Sm)中具有正常铁电体一级相变特征,相变温度以下发现低波数处拉曼振动模的分裂,证实了一级相变中晶体结构的突变与对称性的降低。
[Abstract]:In this paper, the crystal structure, dielectric ferroelectric properties and phase transition characteristics of Ba based tungsten bronze titanium niobate ceramics are systematically studied. The influence of the quadrangular void A1 bit filled ions on its structure and properties is discussed emphatically. The structural origin of the relaxation ferroelectric production in the structure of tungsten bronze and the characteristics of ferroelectric phase transition are revealed. In Ba5RTi3Nb7O30 (R=La, Nd, Sm) ceramics, the structure of the tetragonal tungsten bronze was formed at room temperature. When R=La, the space group was P4/mbm; while R=Nd and Sm, the space group formed at P4bm.Ba5YTi3Nb7O30 room temperature of the tetragonal tungsten bronze structure was less stable. With the difference between the pentagonal (A2) and the quadrangular (A1) ion radius difference, AR increased. The characteristics of a5RTi3Nb7O30 relaxor ferroelectric phase transition gradually weaken: Ba5LaTi3Nb7O30 is a typical relaxor ferroelectrics, the relationship between the temperature and frequency of the dielectric peaks conforms to the Vogel-Fulcher formula, while the frequency dispersion in the dielectric temperature spectrum of Ba5NdTi3Nb7O30, Ba5SmTi3Nb7O30 decreases gradually, and the temperature of the dielectric peak appears gradually to high temperature. In all components, there is at least one dielectric anomaly at low temperature in addition to the phase transition temperature, which is related to the modulation of the microstructure at low temperature, especially the distortion or deformation of the oxygen eight surface. In the relaxor ferroelectric Ba5SmTi3Nb7O30, when a Ba ion is replaced by Ca, the frequency dispersion in the dielectric temperature spectrum will disappear. At the same time, the temperature of the phase transition peak in the Ba4CaSmTi3Nb7030 dielectric temperature spectrum is much higher than that of Ba5SmTi3Nb7O30. According to the results of the total spectrum fitting, it is found that the [Ti/Nb (2) -O (5)] bond length in the ab plane decreases a lot, indicating that the bond length of the [Ti/Nb (2) -O (5)) decreases a lot. The filling of the ion Ca2+ reduces the distortion in the oxygen eight plane in the ab plane, and is beneficial to the long range ferroelectric order in the establishment of.Ba5SmTi3Nb7O30. The four deformed void A1 is disorderly filled with two different valence and radii ions. Compared to the microelectric field formed by different electric charges, the uneven distribution of the stress in the microsphere in the lattice causes the peripheral oxygen eight surface malformation. In the relaxor ferroelectrics Ba5RTi3Nb7O30 (R=La, Nd, Sm) of the tungsten bronze structure, the low temperature hysteresis loop shows that the residual polarization value Pr and the maximum polarization value Pmax decrease gradually. This phenomenon is similar to the "relaxation reentry" in the relaxor ferroelectrics of the perovskite structure. The reasons for its occurrence may be possible. It is due to the complicated structure and ion filling of tungsten bronze which leads to the perturbation of the structure at low temperature, which hinders the establishment of the ferroelectric order in the polarization direction. On the other hand, with the decrease of the temperature, the rate of the ferroelectric domain turn nucleation growth is reduced and the inhomogeneity of the microstructure component plays the role of pinning in the polarization reversal process. At low temperature, the coercive field increases gradually, which is also an important reason for the gradual decline of polarization in the testing process. In order to further study the influence of the filled ions in the A1 bit space of the full tungsten bronze on the polarization response at low temperature, the Ba5LaxSm1-xTi3Nb7O30 (x=0.1,0.25,0.5,0.8,0.9,0.95) tungsten bronze ferroelectric pottery was prepared. The dielectric relaxation and ferroelectric properties at low temperature are characterized. With the increase of the La element, the lattice distortion of the oxygen eight surfaces around the tetrahedron increases and the relaxation ferroelectric property is more obvious. At the same time, the polarization decreases more significantly with the decrease of the temperature. The microstructure is beneficial to the establishment of long range ferroelectric order, and then weaken the dielectric properties of.Ba4R2Ti4Nb6O30 (R= La, Nd, Sm, Eu) ceramics at low temperature. The variation of the change of the ferroelectric phase transition with the difference of the A1 and A2 bit radius is similar to that of the Sr base..Ba4La2Ti4Nb6O30 is a typical relaxor type ferroelectrics, and with the difference of the ionic radius. With the increase of delta R, Ba4Nd2Ti4Nb6O30, Ba4Sm2Ti4Nb6O30 and Ba4Eu2Ti4Nb6O30 all show the characteristics of the first order phase transition of normal ferroelectric. The Curie-Weiss fitting shows that T0 in Ba4Nd2Ti4Nb6O30 is 100K, while Ba4Sm2Ti4Nb6O30 and Ba4Eu2Ti4Nb6O30 T0 are -522.36K and negative. D2Ti4Nb6O30 has different structural changes. Through the hysteresis loop test, it is found that the Ba4La2Ti4Nb6O30 room temperature is a paraelectric phase, and the cooling to the 193K has a small residual polarization value.Ba4Nd2Ti4Nb6O30 at high temperature, the hysteresis loop is linear, and when the temperature drops to 393K, a large residual polarization is suddenly appeared, which is the characteristic of the phase transition of the normal ferroelectrics. However, Ba4Sm2Ti4Nb In 6O30 and Ba4Eu2Ti4Nb6O30, there is a broken wire hysteresis loop near 453K, which shows the characteristics of antiferroelectric order in the structure. With the temperature decreasing, the residual polarization begins to appear, the ferroelectric phase tends to be stable and shows the ferroelectric properties as a whole. The central symmetric structure. In the Ba5RTi3Nb7O30 and Ba4R2Ti4Nb6O30 (R= Nd, Sm) tungsten bronze ceramics, the TM Raman spectrum analysis shows that when the test temperature crosses the phase transition temperature range, for Ba5RTi3Nb7O30 (R= Nd, Sm) relaxor ferroelectrics, the intensity of the Raman peaks at low temperature increases and the number of vibrational modes changes, but does not suddenly disappear. The new Raman active peak shows the gradual change in the structure during the phase transition of relaxor ferroelectrics. In Ba4R2Ti4Nb6O30 (R= Nd, Sm), the first order phase transition of normal ferroelectrics is characterized. The splitting of the Raman vibrational modes at low wave numbers is found below the phase transition temperature, which confirms the decrease of the crystal structure in the first order phase transition.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TQ174.1

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