【作者】 杨鑫；

【导师】 韩高荣； 徐刚；

【作者基本信息】 浙江大学 ， 材料学， 2014， 博士

【摘要】 钙钛矿结构氧化物材料因其具有压电性、铁电性、高温超导等物理性质被广泛的应用于各个领域。开展钙钛矿结构氧化物微纳结构可控制备的研究,探索其微结构与性能之间的关联,为其实际应用提供理论指导,具有重要的科学价值。本文首先简要概述了钙钛矿结构氧化物的晶体结构特点及其性能,并分别对BiFeO3、BaTiO3、CaTiO3三种材料的特征进行了概述,总结和评述了其微纳结构的制备方法。针对BiFeO3难以获得纯相的科学问题,本文采用水热法成功制备了纯相BiFeO3;针对二维单分散单晶钙钛矿铁电材料难以合成等难题,本文首次采用水热法成功制备了单分散的二维单晶BiFeO3微米片,首次采用溶胶—凝胶水热法成功制备了单分散的中空状单晶BaTiO3纳米颗粒；针对高介电纳米陶瓷难以获得等问题,本文采用放电等离子烧结技术成功获得了高介电BaTiO3纳米陶瓷。利用X射线衍射(XRD)、场发射扫描电子显微镜(SEM)、高分辨透射电镜(HRTEM)、傅里叶变换红外光谱(FT-IR)等手段系统研究了这些材料的物相、微结构及生长过程,揭示了其生长机制；获得了制备这些材料的工艺条件,并对其性能进行了研究。本文主要研究内容和结果如下：(1)采用水热法,通过对实验条件的优化,获得了制备纯相BiFeO3的工艺参数。并对BiFeO3进行了A位Ca掺杂。研究表明,产物为纯相BiFeO3多面体结构,尺寸为150-800nm。通过Ca的掺杂,有利于提高产物的光催化性能和磁学性能。其中,Ca设计掺杂量为20%的产物降解率相对最高,光照4.5h对RhB的降解率达到100%。同时,该产物在室温下具有明显的铁磁性,其剩余磁化强度Mr为3.8×10-3emu/g,矫顽场H。为2880e。(2)设计并选用C6H10BiNO8在水热反应中不仅作为反应原料还作为表面改性剂,首次采用水热法成功制备了单分散的单晶BiFeO3微米片,其形成的最佳条件为Bi/Fe比为1：1,KOH浓度为0.4M,反应前驱物搅拌时间为12h,反应前驱物的超声波分散时间为20min,水热反应温度为200℃,水热反应时间为12h。所制备的微米片为钙钛矿型斜方相BiFeO3(JCPDS86-1518)结构,具有规则的矩形形貌,其暴露面为{012}面,横向尺寸约为8μm,厚度为510-550nm。通过水热时间系列实验研究结果表明,在形成过程中,由原料C6H10BiNO8引入的有机配体在Bi25FeO40纳米颗粒的表面和BiFeO3片状结构上的选择性吸附,导致了二维取向聚集过程和自组装过程发生。Bi25FeO40纳米颗粒在碱溶液中快速形成并二维取向聚集形成BiFeO3纳米片,BiFeO3纳米片沿薄片的垂直方向自组装,形成表面粗糙的圆形BiFeO3微米片,经过表面重构和Ostwald熟化过程,最终形成表面光滑的BiFeO3微米片。(3)光学带隙的测试结果显示BiFeO3微米片的带隙宽度为1.88eV,该值小于其它文献的报道值(2.2-2.8eV)。测试了产物的磁学性能,结果表明BiFeO3微米片显示出弱铁磁性。以BiFeO3微米片和聚偏氟乙烯(PVDF)为原料,BiFeO3与PVDF的质量百分比为30%,以ITO玻璃为基板,通过旋涂法获得复合薄膜,其介电性能结果显示,在频率范围在1000Hz～1MHz内,复合薄膜的介电常数-210,这个值大于其它文献报道的纯相BiFeO3的介电常数值(BiFeO3陶瓷εr<130,BiFeO3薄膜εr<1-10)。(4)采用溶胶—凝胶水热法,制备了单分散的BaTiO3纳米颗粒。以晶粒尺寸为10-40nm的BaTiO3纳米颗粒为烧结原料,采用放电等离子烧结技术获得了BaTiO3纳米陶瓷,其物相为立方相BaTiO3(JCPDF31-0174)和四方相BaTiO3(JCPDF05-0626)共存。陶瓷的致密度均大于90%,其颗粒粒径范围在10-500nm。介电性能研究表明,陶瓷的低频介电常数均达到105数量级,弛豫频率则大于105Hz。其介电损耗最小值均小于0.06。随着样品致密度的增大,弛豫频率和相应的介电损耗峰往高频移动。样品的介电温谱显示,同一温度下,随着频率的增加,样品介电常数整体呈下降的趋势。(5)以Ti02溶胶为钛源,Ba(CH3COO)2为钡源,钡钛摩尔比选用3：1,KOH浓度为4M时,200℃水热反应6h时制备了单分散的中空状单晶BaTiO3纳米颗粒,颗粒分散性好,尺寸为50-90nm。产物的N2吸附—脱附的实验结果显示,粉体的BET比表面积为27.01m2g-1。系统研究了BaTiO3纳米颗粒的微结构和生长过程,研究结果表明,中空状BaTiO3纳米颗粒的形成可能经历了一个梯度晶化的过程。(6)采用溶胶—凝胶法成功合成了介孔CaTiO3材料,产物为小颗粒自组装而形成,其主相为钙钛矿型正交相CaTiO3(JCPDS22-0153)。样品比表面积的范围在2-40m2/g之间,平均孔径的大小在18-34nm之间。介孔CaTiO3材料的形成机理可能是：高分子P123起到了包裹在晶核表面并使其规则排布的作用,而反应原料中过量的CaCO3的残余,与后处理酸洗过程中的HN03反应,生成Ca(NO3)2和C02,原先CaCO3占有的位置形成孔洞,从而获得介孔CaTiO3材料。更多还原

【Abstract】 Perovskite oxides have found wide applications in various fields because of the piezoelectric, ferroelectric and high-temperature superconductive properties. Therefore, exporing the controllable synthesis of perovskite oxides and the correlation between their microstructure and their property is of significant importance for providing theoretic guidance to practical applications, which is fundamental scientific values.In this dissertation, the crystal structure characteristics and the property of the perovskite oxides were reviewed in brief firstly. And then the characteristics of BiFeO3, BaTiO3and CaTiO3and their preparation methods have been summarized and commented in detail. However, the difficulty in obtaining pure phase BiFeO3remains a challenge in scientific research. Focusing on this problem, we report on the successfully synthesis of pure phased BiFeO3by a hydrothermal method. Moreover, the preparation of monodispersed anisotropic single-crystal perovskite ferroelectric materials remains a problem. Focusing on this problem, it was reported that for the first time, we successfully synthesized monodispersed2D single-crystal BiFeO3microplates and monodispersed hollow-structured single-crystal BaTiO3nanoparticles. Thirdly, the difficulty in the preparation of nanoceramics with high dielectric constant inspired us to successfully prepare nanoceramics with high dielectric constant by using spark plasma sintering method. By employing a variety of characterization approaches, such as XRD, SEM, HRTEM and FT-IR, comprehensive and in-depth investigations on the phase, microstructure, growing process and the formation mechanism of crystal growth has been performed. The main contents and results are listed as follows:(1) The optimal hydrothermal experimental parameters to synthesized pure phase BiFeO3have been achieved. Ca-doped BiFeO3has been successfully synthesized by the hydrothermal method. The resultant product is pure-phased BiFeO3polyhedrons with a size distribution in a range of150～180nm. The Ca doping is favorable for the improved photocatalytic and magnetic property of the product. In particular, the most efficient photocatalytic activity was discovered when the Ca doping concentration was20%, which100%degradation of RhB can be achieved after4.5h of illumination. Moreover, the product exhibited a remarkable ferromagnetic property at ambient temperature, which the remnant magnetization was determined to be Mr=3.8×10-3emu/g, and the coercive field was288Oe.(2) Monodispersed single-crystal BiFeO3microplates have been successfully synthesized by using C6H10BiNO8as both starting material and surfactant. The optimal condition was determined to be Bi/Fe molar ratio of1:1, the concentration of KOH of0.4M, the stirring time for precursor of12h, ultra sonic dispersion time of20min, the hydrothermal time and temperature of12h and200℃. The as-synthesized BiFeO3microplates adopt perovskite orthorhombic structure (JCPDS86-1518) with a regular rectangle shape. The exposed facet of the as-synthesized BiFeO3microplates was determined to be{012} with a lateral size of about8μm and a thickness of510-550nm. Investigations on time serial hydrothermal experiments reveal that the organic ligand in the starting material of CeH10BiNO8was selectively absorbed on the surface of Bi25FeO40nanoparticles as well as BiFeO3nanoplates, resulting in the2D oriented aggregation procedure and a subsequent self-assembly process.The BiFeO3nanoplates were firstly formed by the fast formation of Bi25FeO40nanoparticles and their2D oriented aggregation. Secondly, a self-assembly process occurred in the direction vertical to the BiFeO3thin plates, leading to the formation of the sphereical-shaped BiFeO3microplates with a coarse surface. Thus in the final process, BiFeO3microplates with smooth surface were obtained by a surface structure reconstruction and Ostawald ripening process of the above as-synthesized BiFeO3microplates with coarse surface.(3) The measurement on the optical adsorption reveals that the BiFeO3microplates have a band gap value of1.88eV, smaller than that of the reported value (2.2-2.8eV). The magnetic measurement shows that the as-synthesized BiFeO3microplates have weak ferromagnetism. Then, BiFeO3microplate/PVDF composited film (the mass load of BiFeO3microplates was set as30%) on ITO substrate was prepared by a spin-coating method. The as-prepared BiFeO3microplates/PVDF composite film exhibited a permittivity of～210in the frequency range of1000Hz～1MHz, which is far larger than that of pure phase BiFeO3(BiFeO3ceramics, εr<130; BiFeO3thin films, εr<1～10) in the previous reports.(4) Monodispersed BaTiO3nanoparticles have been synthesized via a sol-gel combined with a hydrothermal method. And then BaTiO3nanoceramics were prepared by using the above BaTiO3nanoparticles with a size of10～40nm as feedstock via a spark plasma sintering method. The as-sintered BaTiO3nanoceramics consists of both cubic and tetragonal perovskite BaTiO3(JCPDF31-0174and05-0626, respectively). The density of the BaTiO3nanoceramics is above90%and the particle size distribution is in the range of10～500nm. The results from the dielectric measurement reveal that the dielectric constant of BaTiO3nanoceramics in the low frequency range is up to105, and the relaxation frequency was above105Hz with a dielectric loss lower than0.06. Furthermore, the relaxation frequency and corresponding peak dielectric loss both shift towards high frequency as the density of the BaTiO3nanoceramics increased. The temperature-dependence of dielectric measurement reveals that the dielectric permittivity of the BaTiO3nanoceramics decreased as the frequency increased at the same testing temperature.(5) Monodispersed and hollow-structured single-crystal BaTiO3nanoparticles have been synthesized via a hydrothermal treatment at200℃for6h when the molar ratio of starting materials of TiO2and Ba(CH3COO)2was set as1:3and the concentration of KOH was4M. The particle size of the as-synthesized BaTiO3nanoparticles was in the range of50-90nm and are well dispersed. the BET surface area of the powder BaTiO3nanoparticles has been deteremined to be27.01m2g-1on the basis of N2adsorption-desorption measurement. Comprehensive investigations on the microstructure and growing process of the BaTiO3nanoparticles have been performed, indicating that the hollow-structured BaTiO3nanoparticles should experience a gradient crystallization process.(6) Mesoporous orthorhombic perovskite CaTiO3(JCPDS22-0153) has been successfully synthesized via a sol-gel method by the self-assembly of nanoparticles. The BET surface area was anlysized to be in the range of2-40m2/g with the average pore size of18～34nm. The formation mechanism of the as-synthesized CaTiO3was proposed. Firstly, the molecules of P123were coated on the surface of the nucleates that enabled a regular alignment of these nucleates; then the excess CaCO3reacted with HNO3solution to form Ca(NO3)2and CO2, thus leading to the pore formation within the previous sites occupied by CaCO3to form porous CaTiO3.更多还原