【作者】 杨曦；

【导师】 陈乾旺；

【作者基本信息】 中国科学技术大学 ， 材料物理与化学， 2009， 博士

【摘要】 外加磁场诱导是一种可以影响产物成核和生长的过程,进而调控产物形貌、结构和性质的有效手段。六方相金属钴纳米材料又因为其强烈的磁各向异性、高的饱和磁化强度和矫顽力,成为磁性材料研究领域的一个热点。本论文旨在研究磁场下化学反应的特点,深入分析磁场对化学反应的作用机制,另外制备了不同形貌的钴微纳米磁性材料,并对其形成机理和相关磁性能作了较为系统的研究。详细内容概括如下:1.选用金属配合物异烟酸钴作为前驱体钴源,以NaOH溶液辅助,强碱性条件下水合肼还原。利用水热反应合成了超薄钴纳米片自组装而成的微米球结构。钴纳米片的厚度只有10 nm,其片平面法线方向与六方相钴的易磁化轴方向[001]平行。配合物异烟酸钴分子在这个自组装结构的形成过程中起了软模板的作用。通过改变反应条件,发现反应体系中前驱体钴源浓度低和NaOH浓度高时,对均一的微米球结构形成有利。研究了自组装结构的磁性能,其磁滞回线显示材料在室温下呈现软磁性,矫顽力很低,只有24.5 Oe,这可能是由钴纳米片本身各向异性导致的磁矩结构和纳米片各向同性的自组装方式造成的。反应中施加外磁场,发现磁场对产物形貌有一定影响,但杂乱而不规则。这可能由于本反应体系的反应速率过快而致。2.以异烟酸钴作为反应物,氨水辅助下,弱碱性环境中次亚磷酸钠还原。合成了特殊的六方相金属钴的杉树状三维枝晶。枝晶具有特别的分级结构,主轴(类似树干)沿着[001]方向生长,叶片一层层平行于主轴生长,生长方向平行于(001)面。同一层的叶片呈六方对称排列。前驱体异烟酸钴在产物形成过程中起了关键的作用。这种三维枝晶结构具有特殊的磁性质,矫顽力只有184.9 Oe,比以前报道的钴枝晶的矫顽力要低,其原因可能是我们的三维枝晶的整体各向异性比那些枝晶低。在反应中引入外加磁场,发现磁场对产物影响不大,可能是因为在非平衡态下枝晶生长过程很快,磁场力几乎可以忽略。这种特殊的具有分级超结构的三维枝晶在对物理研究可能有新的应用,另外在微纳米器件方面也可能有潜在应用。3.通过实验,选择了合适的化学反应体系,探索磁场下化学反应的特点,磁场力和其他作用力的综合作用机制。以异烟酸钴作为前驱物,氨水辅助,弱碱性环境中水合肼还原,详细研究了磁场力与反应体系中其他力的作用过程。通过水热还原合成了钴纳米片自组装而成的土豆形态的微米球结构。相同实验条件下,引入0.35 T的外加磁场诱导,制备了由土豆微球串成的一维多晶链状结构。系统地研究了制备条件对样品形貌、结构以及磁性能的影响。根据实验结果提出可能的生长机制:异烟酸钴分子在溶液中聚集成球形团簇,球形簇在磁场作用下会迁移到磁力线附近,反应更容易沿着磁力线发生,从而形成了土豆微球长成的一维直链。直链的的基本组装单元钴纳米片,在0.35 T的外加磁场作用下,没有直接组装成一维结构,而是先组装成土豆微球,微球再串成直链。这表明,由于配合物分子间(或钴纳米片间)的相互作用形成的球形簇和土豆微球组装结构在外磁场作用下是稳定的,磁场力不能破坏它们。由此可以看出,实验中施加的外磁场力弱于配合物分子(或钴纳米片)间的相互作用(范德华力)。在300 K下测量了两个样品的磁滞回线,显示室温下样品呈软铁磁性。而磁场下合成的样品的矫顽力略比零磁场下制备的样品高,这可能是因为土豆形微米球组装方式的各向异性比一维直链低。更多还原

【Abstract】 In recent years,magnetic field has been introduced as a new tool to control chemical reactions and materials synthesis.Anisotropic hexagonal Co nanocrystals are important topics in magnetic nanomaterials research for their high saturation magnetization and magnetic coercivity.The objective of this dissertation is to search the special features of the chemical reactions occurring under magnetic fields and synthesize micro-and nanostructured cobalt magnetic materials.The formation mechanisms and magnetic properties of these materials have also been systematically studied.The main parts of the results are summarized below:1.Cobalt microspheres were prepared through the self-assembly of nanoflakelets formed by reduction of a special metal organic complex cobalt bis（4-pyridine carboxylate） tetrahydrate（CoL₂（H₂O）₄,L=4-pyridine carboxylate） with hydrazine at 160℃for 36 h.The Co fiakelet is ultrathin with the thickness of about 10 nm,whose normal direction（[001]） is parallel to the easy magnetization axis.The groups of isonicotinic acid of CoL₂（H₂O）₄ molecule and the agglomerate morphology of the molecules in solution play important roles in the formation of microspheres.Low concentration of CoL₂（H₂O）₄ and high concentration of NaOH in the system are beneficial to the fabrication of the microspheres.The hysteresis loop shows that the assembly exhibits peculiar soft magnetic properties at room temperature,which may result from both the anisotropy and the self-assembly manner of the nanoflakes.The self-assembly manner is isotropic,while the ultrathin nanoflake itself is anisotropic.The sample formed via the same route with an external magnetic field applied irregular.This may be attributed to the fast reaction rate.2.Ordered 3D firtree-like hexagonal cobalt dendritic superstructures have been prepared with a metal complex precursor CoL₂（H₂O）₄ by sodium hypophosphite reduction.The dendrite has a main axis along the[001]direction and the leaves, which grow parallel to the（001） plane,arrange layer by layer along the axis.The complex ligands are proposed to be responsible for the formation of 3D dendrite. The hysteresis loops measured at 300 K reveals a ferromagnetic behaviour with saturation magnetization of 134.0 emu/g and coercivity of 184.9 Oe.The coercivity is lower than that of other cobalt dendritic crystallites,Which may result form the lower total morphology anisotropy of our sample.The products formed via the same route under an external magnetic field are a majority of dendritic microcrystals.This means that the influence of the variation of the morphology of the sample.There are two possible reasons.One is that the growth rate of the dendrites is too fast and the other is the interactions,inducing the growth of dendrites,are much stronger than the magnetic force.These 3D dendritic microcrystals may bring about new opportunities in physical research and have potential applications in micro-/nanodevices.3.The special features of the chemical reactions occurring under magnetic fields are studied.And the details of the effects induced by the combined action of the magnetic field force and these factors in the reaction system are discussed. Particular hexagonal cobalt potato-like microspheres consisting of nanoflakes have been obtained with a metal complex precursor CoL₂（H₂O）₄ by hydrazine reduction. And one-dimensional polycrystalline cobalt chains and wires composed of potato-like particles were formed via the same route with external magnetic fields applied.We systematically investigated the influence of synthetic conditions on the morphology,structures and magnetic properties and it is suggested that the complex molecules tend to agglomerate into spherical clusters that gather along the magnetic lines of force due to magnetic attraction.Then the reduction may occur easily along the lines of magnetic force,as a result the linear chains are formed.The Co nanoflakes,which are the basic units of cobalt spheres,do not assemble into wires or chains directly under a 0.35 T magnetic field.This means that the spherical clusters and the potato-like spheres are stable under the external magnetic field owing to the interaction between complex molecules（or nanoflakes） and that the magnetic field force is weaker than the interactions among complex molecules（or nanoflakes）.The hysteresis loops measured at 300 K reveal soft magnetic behaviors of the samples. The coercivity of the AF sample is higher than that of the ZF sample,which may be attributed to the reason that the assembly manner of a potato-like sphere has lower shape anisotropy as a total than that of the one-dimensional chain.更多还原