【作者】 王永刚；

【导师】 韩高荣； 徐刚；

【作者基本信息】 浙江大学 ， 材料学， 2009， 博士

【摘要】 钙钛矿结构氧化物因其丰富的物理化学性质,如铁磁性、铁电性、压电性、和庞磁阻效应等,成为功能氧化物纳米材料的研究热点。此外,由于纳米材料的物理和化学性质依赖于它们的形貌和尺寸,控制纳米材料的形貌和尺寸也成为当前纳米材料研究领域的热点之一。本文首先介绍了钙钛矿氧化物的结构特点及合成方法,详细综述了枝杈晶纳米材料和多铁材料的研究进展。针对BiFeO₃磁性弱、单相多铁材料在自然界中非常稀少及钙钛矿氧化物纳米结构的可控合成等难题,本文系统研究了BiFeO₃、Na_0.5Bi_0.5TiO₃、BaTiO₃和SrTiO₃等钙钛矿氧化物纳米材料的水热合成条件、形貌可控的影响因素及掺杂对磁学性能的影响,主要取得了以下创新成果:1.以硝酸铁和硝酸铋为原料,KOH为矿化剂,系统研究了水热法合成BiFeO₃的影响因素,发现KOH浓度的高低决定着最终产物的结构,当KOH浓度为7M和12M时,能够水热合成出尺寸为100-300nm的BiFeO₃颗粒;当KOH浓度较低时,水热合成产物主相为钙钛矿相BiFeO₃,但同时伴有正交相Bi₂Fe₄O₉;而当在水热条件下加入添加剂KNO₃时,可以得到BiFeO₃纳米颗粒,并且随着水热反应时间的延长,能够得到片状BiFeO₃。当引入KNO₃时,NO₃^-离子吸附在BiFeO₃晶核上,降低了BiFeO₃晶核的生长速率,使得BiFeO₃颗粒的尺寸变小,而BiFeO₃纳米片的生长过程符合Ostward熟化生长规律;2.通过过渡金属元素（Co或Ni）的掺杂取代BiFeO₃晶格中的Fe³⁺,可使BiFeO₃的铁磁性得到较大的提高,当Co和Ni的掺杂浓度分别为5%和0.5%时,BiFeO₃表现出最大的铁磁性,室温下饱和磁化强度分别为2emu/g和0.8emu/g。其磁性增强的机理为,Co和Ni的掺杂破坏了BiFeO₃中的G-型反铁磁结构,从而产生了较大的净磁矩,所以BiFeO₃铁磁性得到增强。另外,BiFeO₃的铁磁性并不是随着Co或者Ni掺杂浓度的提高而一直提高,这是因为Co和Ni的原子磁矩均小于Fe的原子磁矩,所以当较多的Fe被Co或者Ni取代时,BiFeO₃的铁磁性就出现了下降;3.在水热法合成Na_0.5Bi_0.5TiO₃中,采用不同的沉淀剂可以控制合成出具有不同形貌的Na_0.5Bi_0.5TiO₃。当采用NaOH作为沉淀剂时,得到的是球形Na_0.5Bi_0.5TiO₃颗粒;而当采用Na₂CO₃作为沉淀剂时,Na_0.5Bi_0.5TiO₃晶粒的各个晶面的生长速率比较接近,得到的是Na_0.5Bi_0.5TiO₃微米级立方体,与Na_0.5Bi_0.5TiO₃晶胞的形状非常相似;4.通过过渡金属元素（Fe或Co）的掺杂来取代Na_0.5Bi_0.5TiO₃晶格中的Ti⁴⁺,可以使Na_0.5Bi_0.5TiO₃获得弱的铁磁性。随着Fe掺杂浓度的增加,Na_0.5Bi_0.5TiO₃纳米晶的磁学性能发生了由抗磁到铁磁到顺磁的变化。一种基于“Fe³⁺-Vo^2--Fe³⁺”结构的F色心交换机制可以合理的解释材料中铁磁性的产生。同时,独立的Fe³⁺离子可以产生顺磁性,所以在Fe掺杂Na_0.5Bi_0.5TiO₃中存在着磁性的竞争。在低掺杂浓度条件下,铁磁性占主导,在较高掺杂浓度下,材料中的顺磁性占主导;5.采用水热法可控合成出BaTiO₃枝权状单晶,通过调节KOH浓度,可以调控BaTiO₃晶体的形貌。当KOH浓度较低时,BaTiO₃晶体处于远离平衡的生长状态,所以得到的是BaTiO₃枝权状单晶;而当KOH浓度逐渐升高时,BaTiO₃颗粒的形貌从枝权状,变化到六角状,最终变为球形;6.采用水热法可控合成出SrTiO₃枝权状单晶,通过调节KOH浓度,可以控制SrTiO₃晶体的形貌。当KOH浓度较低时,得到的是SrTiO₃枝权状单晶;而当KOH浓度逐渐升高时,SrTiO₃晶体的形貌从枝权状,变化到四角状,最终变为球形。负离子配位多面体生长基元模型可以较好地解释SrTiO₃枝杈状单晶的生长机理,负离子配位八面体Ti-O₆作为SrTiO₃晶体结构的生长基元,相互之间以顶角相联接,在[100]方向,八面体的一个顶角相联接,而在[110]方向,八面体是两个顶角相联接,故稳定性比[100]高。所以[110]方向的生长速率比[100]快,是SrTiO₃枝权晶的生长方向。更多还原

【Abstract】 Perovskite oxides have been the focus of many researchers because of theirfruitful physical properties, such as ferromagnetism, ferroelectricity, piezoelectricity,colossal magnetoresistances, and so on. As we all knew that nanomaterials exhibit awide range of physical and chemical properties that depend sensitively on both sizeand shape, and are of both fundamental and technological interest.In this thesis, the structure and synthesis method of the perovskite oxides werefirst introduced, then the progress of the dendrites and multiferroic materials weresummarized detailedly. In order to resolve the problems, such as the weakferromagnetism of BiFeO₃, the sparsity of single phase multiferroic materials innature and the controllable growth of the perovskite oxides, we made a detail researchon the perovskite oxides, such as BiFeO₃, Na_0.5Bi_0.5TiO₃, BaTiO₃, SrTiO₃, andinvestigated detailedly the hydrothermal condition, factors of controllable growth andthe magnetic properties by doping.The main results include:1. Bismuth nitrate and iron nitrate were used as the starting materials and potassiumhydroxide （KOH） was used as mineralizer. The influence factors were investigatedduring the hydrothermal synthesis of BiFeO₃. It was found that the obtained productswere determined by KOH concentration. When KOH concentration was 7M and 12M,BiFeO₃ particles with a diameter 100-300nm were prepared. As KOH concentrationwas low, the as-prepared products were perovskite BiFeO₃ and Bi₂Fe₄O₉ with anorthorhombic structure. When KNO₃ was introduced during the hydrothermal process,BiFeO₃ nanoparticles were synthesized due to the adsorption of NO₃^- ions on thenucleus. In addition, BiFeO₃ nanoflakes were obtained as the reaction time wasprolonged, which was well assistant with the growth rule of Ostward2. The ferromagnetism of BiFeO₃ was noticeably improved by the doping oftransition metal ions (Co²⁺ ions or Ni²⁺ ions). BiFeO₃ exhibited a strongestferromagnetism when Co and Ni doping concentration was 5% and 0.5%, respectively, and the saturation ferromagnetism was 2emu/g and 0.8emu/g, respectively. Themechanism was also explained, which was that the G-type anti-ferromagnetismstructure in BiFeO₃ was destroyed by the doping, and net ferromagnetism wasobtained. In addition, the ferromagnetism was not increased as the dopingconcentration because of the weakness of Co and Ni atomic magnetization comparedto Fe atomics.3. Controllable growth of Na_0.5Bi_0.5TiO₃ could be realized by the choice of differentprecipitator. Na_0.5Bi_0.5TiO₃ crystallites with a spheric morphology were preparedwhen NaOH was used as precipitator. However, Na_0.5Bi_0.5TiO₃ microcubes wereprepared when Na₂CO₃ was used as precipitator, and its morphology was similar tothe cell morphology, which may be due to the average surface growth speed ofNa_0.5Bi_0.5TiO₃ crystallites.4.Based on the research of diluted magnetic semiconductors （DMSs）.Na_0.5Bi_0.5TiO₃ crystals doping transition metal ions （Fe or Co） in the perovskitestructure showed weak ferromagnetism. As transition metal ions doping contentincreased, the magnetism of Na_0.5Bi_0.5TiO₃ nanocrystals developed fromdiamagnetism to ferromagnetism and paramagnetism. It is suggested that theferromagnetic component was not growing as Fe or Co doping increasing. This couldbe attributed to the competition between ferromagnetic and paramagneticcontributions to the magnetism of the sample. The mechanism of the origin offerromagnetism in this material was explained. Ferromagnetism dominated whendoping concentration was low, while paramagnetism dominated in the case of highdoping concentration.5. Single-crystal BaTiO₃ dendrites were controllably synthesized by adjusting KOHconcentration. When KOH concentration was low, BaTiO₃ dendrites were obtainedbecause the growth of BaTiO₃ nucleus was in the state far away from equilibriumstate. As KOH concentration was increased gradually, the morphology of BaTiO₃varied from dendrite, hexagon to sphericity.6. Single-crystal SrTiO₃ dendrites were controllably synthesized by adjustingKOH concentration. When KOH concentration was low, SrTiO₃ dendrites were obtained. As KOH concentration was increased gradually, the morphology of SrTiO₃varied from dendrite, quadrangle to sphericity. The model of the growth units of anioncoordination polyhedra can explain the growth mechanism of SrTiO₃ dendrites.During dendrite growth, the anion coordination polyhedra as growth units areconnected with the structure units on the crystal surface along the direction of stablecombination. The anion coordination polyhedra （Ti-O₆） as the growth units of SrTiO₃perovskite structure connected along the direction of corner. The coordinationpolyhedra has two corners to connect with other coordination polyhedra along [110],while it has only one corner along [100], therefore, the SrTiO₃ dendrites grew along[110] because of the more stable character along [110] compared to that along [100].更多还原