【作者】 闫涛；

【导师】 陈建峰； 刘晓林；

【作者基本信息】 北京化工大学 ， 化学工艺， 2006， 博士

【摘要】 本文系统地研究了超重力反应沉淀法制备的纳米钛酸钡粉体介电性能与烧结成瓷后陶瓷体微观结构特性；考察纳米钛酸钡粉体铁电相变的“尺寸效应”,并探索钛酸钡铁电相变的影响因素；采用“超重力反应沉淀结合水热晶型转化”的新方法,制备出颗粒尺寸在45-125nm范围内可控、结构缺陷少、晶体结构趋于四方相和符合化学计量比的单分散性纳米钛酸钡粉体；并考察了水热晶型转化过程中钛酸钡粉体的生长机理。首先以氯化钡、四氯化钛和氢氧化钠为原料,采用超重力反应沉淀法,在中试装置上制备出35nm的立方相钛酸钡粉体,粉体比表面积(53m2/g)较大。虽然该粉体具有很高的烧结活性,在1120℃烧结成瓷后陶瓷体的晶粒尺寸为0.5μm左右,但介电性能较低(室温介电常数约为2600,介电损耗达20%)。当粉体分别在500℃和900℃煅烧后,颗粒尺寸仍在100nm以下,粉体的比表面积减小到25m2/g和13m2/g左右。900℃煅烧后的粉体的晶体结构由原来的立方相向四方相转变。两种粉体在1140-1180℃烧结成瓷后陶瓷体晶粒尺寸增大到0.7-1.2μm左右,陶瓷体的介电性能升高(室温介电常数分别约为3600和4500,介电损耗为2%)。实验结果说明四方相钛酸钡粉体可以获得较高介电性能的陶瓷材料。利用XRD、Raman、TEM和TG手段分析400-1200℃温度范围煅烧后的钛酸钡粉体的铁电相变与颗粒尺寸之间的关系,得到钛酸钡粉体的“临界粒径”(铁电相转变)为70μm左右。实验发现纳米钛酸钡粉体中存在的羟基缺陷与钡离子空位是影响纳米钛酸钡粉体铁电相变的关键因素。热煅烧方法得到的钛酸钡粉体存在团聚现象严重和颗粒尺寸分布不均匀的缺点。其次,采用超重力反应沉淀结合水热晶型转化方法制备纳米钛酸钡粉体。首先进行小试实验,以立方相钛酸钡悬浮液为原料,在180-240℃水热环境下,制备出分散性好、结构缺陷少、晶体结构向四方相转变、粉体钡钛摩尔比符合化学计量比和颗粒尺寸为70-90nm的钛酸钡粉体。然后将水热晶型转化方法移植到超重力技术平台,得到颗粒尺寸在45nm-125nm范围内可控、规则形貌、单分散性和结构缺陷少的纳米钛酸钡粉体。粉体在1200℃烧结成瓷后表现出较好的介电性能(介电常数约4500,介电损耗2%左右)。对水热晶型转化过程的水热晶型转化机理进行了考察,得出组分扩散到钛酸钡晶核表面的扩散过程是水热晶型转化控制步骤的结论,实验结果符合Ostwald ripening模型和LSW理论。更多还原

【Abstract】 The dielectric and ceramic microstructural properties of sintered ceramics using nanosized BaTiO3powders prepared by high gravity reactive precipitation method (HGRP) were systematically studied. The critical size and lattice expansions of nanosized BaTiO3particles in the size ranging from30nm to250nm, as well as the influencing factors of ferroelectric phase transformation were investigated. In addition, a novel high gravity reactive precipitation plus hydrothermal recrystallization method was developed to synthesize nanosized BaTiO3powders with characteristics of low concentrations structural defects, stoichimetric composition, tailored particle size in the range of45-125nm, and pseudo-cubic structure. At last, the particle growth mechanism during the hydrothermal recrystallization process was also investigated. As a result, the following results had been obtained.Although nanosized HGRP BaTiO3powders with particle size of35nm had high sinterability and the grain size of sintered ceramics was about0.5μm, the dielectric constants of nanosized BaTiO3powders were very low (about2600) and the dielectric dissipations were about20%. If the powders calcined at500℃and900℃to increase the particle size (below100nm) and promote the crystal structure changing from cubic to tetragonal, the dielectric constant increased to3600and4500and the dielectric dissipation downed to2%-3%, respectively. These results were in accordance with the conclusion that better dielectric properties were obtained by tetragonal powders, as opposed to cubic ones.Critical size and anomalous lattice expansion in nanosized BaTiO3powders were investigated. The lattice constants of BaTiO3powders were obtained from the XRD patterns of the particles in the size range of30-250nm prepared by calcinations method at the temperature range of400-1200℃. The present results indicated that the structural change from a tetragonal (ferroelectric) phase to a cubic (paraelectric) one around70nm in diameter, which was in good agreement with a critical diameter recently reported. Large lattice expansions of more than1.7%were detected in the particles down to30nm in diameter. The origin of the expansion was discussed on the basis of elastic constraints. The crystal structure of samples with higher concentrations of structural defects displayed to cubic paraelectric phase, while the ones with smaller particle size and lower concentrations of structural defects changed to tetragonal. These results suggested that the widely cited’size effect’ model was inappropriate and the distortion of Ti-O bonds within the BaTiO3lattice was possible, even for particles as small as40nm and having less concentrations of hydroxyl lattice defects and barium and titanium vacancies.Regarding the structural defects existed in the nanosized BaTiO3powders, the author had attempted to develop a novel synthesis method, high gravity reactive precipitation plus hydrothermal recrystallization method, to synthesize BaTiO3powders characterized by less structural defects, precise stoichiometric composition, and particle size below100nm. Based on LaMer model for monodispersed particle formation, we realized that one of the extreme measures to separate growth from nucleation may be the’seeding’method and a quick change of the temperature immediately after a limited nucleation, which leaded to the higher supersaturation within the nucleation range to plunge into the lower supersaturation below maximum concentration for nucleation (’supersaturation quenching’). The experiment was first conducted at beaker trials, and the experimental results of beaker trials can achieve those goals. The experimental parameters obtained from beaker trials were applied to the HGRP platform. Nanosized BaTiO3powders with tailored particle size in the range of45-125nm, spherical morphology, and fewer concentrations of structural defects were synthesized by controlling synthesis parameters during the hydrothermal recrystallization process. The hydrothermal recrystallization growth kinetics was determined by monitoring the particle size as a function of seeding time and at different seeding temperatures. The results showed that the growth kinetics of the BaTiO3particles follows the Lifshitz-Slyozov-Wagner theory for Ostwald ripening. In this model, the higher curvature and hence chemical potential of smaller particles provides a driving force for dissolution, and the larger particles continue to grow by diffusion-limited transport of species dissolved in solution.更多还原