【作者】 吕欣；

【导师】 陈湘明；

【作者基本信息】 浙江大学 ， 材料学， 2014， 博士

【摘要】 Fe基复合钙钛矿陶瓷作为一类非常重要的电子陶瓷类材料,关于其独特的介电特性的物理起源长期以来一直备受科研工作者的关注。本论文对几种Fe基复合钙钛矿陶瓷的晶体结构、磁学性能、介电性能和输运特性进行了系统研究。通过对陶瓷样品微结构的表征和分析,深入地探讨了各物理性能与其结构之间的关系,揭示了该类材料介电弛豫的物理本质,同时对Fe基复合钙钛矿固溶体从弥散铁电体向弛豫铁电体演化的规律做了深入探讨。得到如下主要结论：Ca(Fe1/2Ta1/2)O3陶瓷具有B位无序的正交钙钛矿结构,空间群为Pbnm(62)。晶体结构中同时存在氧八面体的同向和反向扭转,并且氧八面体中心的Ca离子在赝立方<110>方向存在位移。Ca(Fe1/2Ta1/2)O3陶瓷在150-400K的温度范围内在介电损耗随着温度的变化曲线图谱中可以观察到典型的介电弛豫现象,属于热激活过程。该介电弛豫的激活能与Ba(Fe1/2Nb1/2)O3陶瓷的激活能比较接近,其物理起源可以认为是Fe离子的混价结构形成了偶极子之间电荷的跃迁。经过氧气氛退火处理以后低温的介电弛豫没有发生明显的变化,高温介电常数有略微下降。氧气氛退火处理对样品的氧空位数目以及Fe离子混价结构没有明显的影响。Sr(Fe1/2Ta1/2)O3陶瓷具有B位无序的正交钙钛矿结构,空间群为Pbnm(62), Sr(Fe1/2Ta1/2)O3陶瓷在300-600K的温度范围内存在明显的介电弛豫,属于热激活过程。Sr(Fe1/2Ta1/2)O3陶瓷的介电常数虚部随着频率的升高单调递减,说明该弛豫现象为非本征行为。随着温度的升高,除了晶粒以外,晶界电导对于介电弛豫的贡献逐渐增大。常温时,低频的介电常数随着样品的电极材料以及厚度的不同有明显变化,说明低频下Sr(Fe1/2Ta1/2)O3陶瓷较高的介电常数主要来源于样品/电极耗尽层的界面极化效应。Sr(Fe1/2Ta1/2)O3陶瓷的载流子跃迁方式符合三维可变程跳跃模型。由于晶体内存在局域化学成分起伏而产生的Fe-O-Fe团簇使得该陶瓷材料在20K以下呈反铁磁性；其内部的氧空位造成的Fe2+/Fe3+混价结构使该材料在较低温度下可测得磁滞回线,存在弱铁磁性。二步固相反应法制得的B位无序Pb1-xBax(Fe1/2Nb1/2)O3(x=0,0.05,0.1,0.15,0.2,0.3)固溶体陶瓷具有立方结构,其空间群为Pm3m。除了x=0组分外,其余成分的固溶体陶瓷在300K左右都出现了类似Ba(Fe1/2Nb1/2)O3中的德拜弛豫现象,这一现象主要是由Fe2+/Fe3+点缺陷和氧空位引起的。固溶体陶瓷随着Ba含量的增加逐渐由弥散铁电体向弛豫铁电体转变,并且铁电性逐渐减弱。在介电性能上表现为铁电相变介电峰出现频率色散,峰强逐渐减弱,峰位向低温方向移动,峰宽逐渐增大。造成体系由弥散向弛豫铁电体转变的原因是由于Ba的加入打破了Pb离子的网络体系,使系统向无序不均的方向发展,导致铁电畴的进一步减小。二步固相反应法制得的B位无序Fb1-xBax(Fe1/2Nb1/2)O3(x=0,0.05,0.1,0.15,0.2,0.3)固溶体陶瓷具有立方结构,其空间群为Pm3m。随着Ba含量的增加其逐渐由弥散铁电体向弛豫铁电体转变,并且铁电性逐渐减弱。x=0,0.05的固溶体样品的铁电转变温度在室温以上,其余成分的铁电转变温度在室温以下。由于Pb1-xBax(Fe1/2Nb1/2)O3固溶体陶瓷存在局域对称性破缺,从而在室温下具有较强的拉曼响应。与Pb-O键有关的F2u。模在Pb1-xBax(Fe1/2Nb1/2)O3固溶体陶瓷样品的拉曼图谱中缺失,说明该体系固溶体陶瓷样品的铁电性主要是由B位离子的位移贡献的。Ba1-xBix(Fe(1+x)/2Nb(1-x)/2)O3(x=0.1,0.3,0.5,0.7,0.9)固溶体陶瓷均为单相,其中x=0.9的成分点样品为菱方扭转钙钛矿结构,空间群为R3c。其余组分固溶体陶瓷样品均为立方结构,空间群为Pm3m。Ba1-xBix(Fe(1+x)/2Nb(1-x)/2)O3(x=0.1,0.3,0.5,0.7,0.9)固溶体陶瓷的介电常数和损耗均随着x增大而减小。当x=0.1,0.3时其与Ba(Fe1/2Nb1/2)O3介电性能类似,随x的增大介电性能逐渐向类似BiFeO3的介电性能转变。由于Ba0.1Bi0.9(Fe0.95Nb0.05)O3固溶体陶瓷相比于整个固溶体陶瓷体系而言介电损耗较小,性能较优,因此对其进行等离子放电烧结并进行氧气氛退火处理。随着退火时间的增加,Ba0.1Bi0.9(Fe0.95Nb0.05)O3固溶体陶瓷的介电性能持续改善。在氧气氛下退火30小时的Ba0.1Bi0.9(Fe0.95Nb0.05)O3固溶体陶瓷在613.1K发生反铁磁-顺磁相变,并在该温度点存在明显的介电异常。在室温下,该固溶体陶瓷的磁介电系数为-0.4%。更多还原

【Abstract】 As an important group of dielectric materials, Fe-based complex perovskites ceramics have drawn losts increasing scientific attention, while the physical nature of their dielectric behavior has always been very controversial. In the present thesis, the structure, magnetic, dielectric and transport properties of the Fe-based complex perovskite ceramics were investigated systematically. Meanwhile, the conversion between the diffuse ferroelectrics and relaxor of their solid solutions was demonstrated. The relationship between the physical properties and the structure has been discussed, and the physical origin of dielectric relaxation and the relaxor ferroelectric behavior was revealed.XRD data suggested that Ca(Fe1/2Ta1/2)O3ceramic had orthorhombic structure with B-site disorder. There was only one dielectric relaxation observed in the low temperature ranges, respectively. Compared with Ba(Fe1/2Ta1/2)O3, Sr(Fe1/2Ta1/2)O3and Ba(Fe1/2Nb1/2)O3, the much lower dielectric constant and lower dielectric loss were determined in Ca(Fe1/2Ta1/2)O3because of the obviously weaker Fe2+/Fe3+mixed-valent structure. The oxygen annealing had little influence on oxygen vacancies and Fe2+/Fe3+mixed-valent structure.The X-ray powder diffraction analysis confirmed that Sr(Fe1/2Ta1/2)O3has a B-site disordered orthorhombic structure in space group Pbnm(62). Only one broadened dielectric peak with strong frequency dispersion was observed in present ceramic, which was significantly different from that for the analogue Ba(Fe1/2Nb1/2)O3and Ba(Fe1/2Ta1/2)O3. The strong dependence of sample thickness and electrode material indicated that the dielectric relaxation behavior at lower frequency was due to the interface effects. The present ceramic was spin glass state with slight ferromagnetic behavior below the Neel temperature (20K). The co-presence of Fe2+and Fe3+was comfirmed by the μff value.XRD analysis confirmed that the structures of Pb1-xBax(Fe1/2Nb1/2)O3solid solutions were cubic, and the dielectric nature changed from diffuse ferroelectric to relaxor ferroelectric with increasing x, while the phase transition temperature Tc (or Tm) decreased monotonously. The diffuse ferroelectric phase transition was observed in the ceramics with0≤x≤0.05. For Pb1-xBax(Fe1/2Nb1/2)O3with0.1≤x≤0.2, relaxor ferroelectric behavior was determined, and Vogel-Fulcher equation and new glass model could be used to describe the relaxor behavior.The crystal structures of all Pb1-xBax(Fe1/2Nb1/2)O3compositions were cubic and the cell volume indicated a sudden change at x=0.075. Pb1-xBax(Fe1/2Nb1/2)O3ceramics with x>0.075were paraelectric, while those for x<0.075were ferroelectric at room temperature. The phonon modes revealed that the differences of local structures of the ceramics might mostly caused by the disappearing of off-center BO6octahedron. The ferroelectric related distortion still could be discovered in paraelectric solid solutions with x>0.075. The modes related to Fe-O-Fe were stable with increasing Ba-content, while the phonon mode corresponded to Nb-O-Nb changed intensively. Meanwhile, the other three Nb-O related modes:the B-O asymmetric stretching mode near700cm-1, the Alg mode of the rigid B’-O-B"(Fe-O...Nb) bonds near780cm-1, the B-O bond related mode near850cm-1also presented the V-types inflection near x=0.075. These behaviors revealed that the ferroelectricity in the present system was due to the displacement of Nb cation. Moreover, the Nb-rich areas should exhibit the stronger ferroelectricity than other areas.According to XRD analysis, the structure of Ba1-xBix(Fe(1+x)/2Nb(1-x)/2)O3(x=0.1,0.3,0.5,0.7,0.9) solid solution ceramics varied from original cubic symmetry to rhombohedral distorted perovskite. With increasing x, the two dielectric abnormities caused by Fe2+/Fe3+mixed-valent structure tapered off, the dielectric loss decreased evidently. Oxygen annealed Ba0.1Bi0.9(Fe.9sNb0.05)O3ceramics prepared by SPS showed dielectric abnormity at613.1K. This temperature was the antiferromagnetic transition temperature confirmed by DSC. The phenomenon indicated the strong magnetodielectric effect at Neel temperature. Even at room temperature the magnetodielectric coefficient was as high as-0.4%.更多还原