【作者】 吴爱兵；

【导师】 杨桦；

【作者基本信息】 吉林大学 ， 物理化学， 2011， 博士

【摘要】 碳包覆磁性纳米颗粒作为一类新型的磁性纳米复合材料受到了研究者们的广泛关注。碳包覆层不仅能够对纳米磁性颗粒起到保护作用使之免疫于外界环境的侵蚀而保持其固有的性质,而且能够赋予磁性纳米颗粒以新颖的性质,如导电性和生物相容性等。碳包覆磁性纳米颗粒不仅在磁性记录介质、铁磁流体等方面具有重要的应用价值,而且在生物传感器、靶向药物载体等方面具有巨大的应用潜力。目前为止合成碳包覆磁性金属纳米颗粒的方法包括电弧放电法、化学气相沉积法、热解法、聚能法和爆炸法等,尽管每种方法都有优缺点,但是这些方法大多需要较高的能量和复杂的实验装置,导致了繁琐的实验操作和较高的成本,因而在实际应用中受到很大限制。发明一种简单而高效的合成碳包覆纳米金属颗粒的方法仍然是一个巨大的挑战。本论文综述了碳包覆磁性纳米颗粒的国内外研究现状,系统介绍了这类纳米复合材料的合成方法和应用领域。三聚氰胺是一种重要的有机化工原料,是生产三聚氰胺甲醛树脂(MF)的原料,还可应用于具有阻燃性的热塑性塑料和耐火材料的合成等。据相关文献报道,三聚氰胺既可以作为氮化剂也可以作为碳化剂。在此基础上,我们发明了一种新颖而简单的以三聚氰胺为碳源的热解法来合成碳包覆磁性纳米颗粒。这个方法的实质是三聚氰胺和金属氧化物之间发生的固态复分解反应。此方法中我们以水热法制备的纳米氧化物或复合物纳米颗粒作为金属或合金的前驱体,通过与三聚氰胺在高纯氮气氛中高温条件下的固态复分解反应成功合成出一系列碳包覆纳米磁性颗粒。通过X射线衍射(XRD)、透射电子显微镜(TEM)、高倍透射电子显微镜(HRTEM)、拉曼(Raman)光谱和振动样品磁强计(VSM)等手段对样品进行了表征和测试。主要工作如下：1、我们以一步水热法制备的Fe/Fe3O4纳米复合物为金属前驱体,用三聚氰胺作碳源在600-700℃的温度范围内合成出了碳包覆的Fe/Fe3C纳米复合物。TEM和HRTEM表征结果说明：600℃产物颗粒分散于无定形碳基质中；650℃产物为接近球形的石墨包覆的纳米颗粒；700℃产物为石墨包覆的纳米棒。我们根据相关文献报道提出了具有不同微观结构和形态的产物的可能的形成机理,同时对合成过程中所涉及的反应进行了归纳和解释。另外在利用XRD对产物相组成分析的基础上,对产物的磁性能的变化进行了研究。2、在上一步合成碳包覆的Fe/Fe3C纳米复合物的基础上,我们通过改变反应条件成功合成出了碳包覆金属Fe单质纳米颗粒。另外我们分别以水热法制备的纳米复合物Fe/Fe3O4、氧化物Co3O4和NiO(经过进一步煅烧得到)为各自金属前驱体,用三聚氰胺作还原剂和碳源合成出了碳包覆金属Co、Ni纳米颗粒。根据XRD分析结果我们将在不同温度条件(650-750℃)下合成的碳包覆金属Co、Ni纳米颗粒样品进行对比发现：没有金属碳化物Co3C、Ni3C的生成。根据文献我们对此进行了解释。除了利用TEM和HRTEM观察确认碳包覆纳米颗粒的石墨壳的存在外,我们还用拉曼(Raman)光谱测试进一步研究了石墨的结晶性。利用VSM在室温条件下测试样品的磁性能,分别探讨了不同温度条件所得的碳包覆金属Co、Ni纳米颗粒样品磁性能的变化。3、用一步水热法分别制备的掺杂不同量的Co和Ni的铁钴合金/钴铁氧体、铁镍合金/镍铁氧体纳米复合物为合金前驱体,用三聚氰胺作还原剂和碳源在较低的温度(650℃)条件下合成出了具有不同Co/Fe、Ni/Fe比例的碳包覆合金Fe-Co、Fe-Ni纳米颗粒。另外在合成碳包覆的Fe/Fe3C纳米复合物的基础上,改变反应条件探讨是否有Co或Ni取代的碳化铁(Fe, M)3C (M=Co或Ni)相的生成。XRD表征结果说明产物依然为合金(无碳化物等杂相存在)。从热力学观点我们对此进行了分析和证明。根据由VSM对样品的室温磁性能测试结果,我们分别研究了碳包覆合金Fe-Co、Fe-Ni纳米颗粒样品的磁性能随Co/Fe比和Ni/Fe比的变化规律。4、用一步水热法分别制备的掺杂不同量(0.1-0.4)的La或Nd的铁钴合金/钴铁氧体纳米复合物为合金前驱体,用三聚氰胺作还原剂和碳源在高温(700或800℃)条件下合成出了碳包覆的掺杂稀土的Fe-Co合金纳米颗粒。XRD表征结果显示La与Fe、Co形成体心立方(bcc)结构的合金,而Nd以氧化物Nd2O3的形式存在。根据由VSM对样品的室温磁性能测试结果,我们探讨了碳包覆的掺杂稀土La和Nd的Fe-Co合金纳米颗粒的磁性能随稀土La或Nd掺杂量的变化规律。更多还原

【Abstract】 Carbon-encapsulated magnetic nanoparticles (CEMNPs) have attracted great attention as one kind of novel magnetic nanomaterials. The carbon encapsulation can immunize the nanoparticles against environmental degradation and therefore retain their intrinsic nanocrystalline properties. Moreover, carbon encapsulation can endow these magnetic nanoparticles with better electrical conductivity and higher biocompatibility. CEMNPs not only possess wide applications in magnetic recording media and ferrofluids but also have very promising applications in biosensors and targeted drug delivery.Up to date, CEMNPs have been synthesized by various methods, such as arc discharge, chemical vapor dposition (CVD), explosion, thermal decomposition method and cumulative method. Most of these methods, however, involve high energy consumption and require sophisticated apparatus, which typically leads to complicated operation and high costs in terms of practical applications. Therefore, it is still a big challenge to develop a simple and cost-effective method to synthesize CEMNPs.In this thesis, we review relevant studies of CEMNPs at home and abroad. Especially we summarize the various synthetic methods of CEMNPs and their several applications. The organic compound melamine (C3N3(NH2)3) is an important chemical raw material, which is extensively used in the syntheses of melamine-formaldehyde resins, flame-retardant thermoplastic polyester and of fireproof materials. It has been reported that melamine can be used both as nitridation reagent and carburization reagent. Here we describe a simple method to synthesize CEMNPs using melamine as the carbon source. The essence of this method is considered to be a modified solid-state metathesis reaction between melamine and metal oxide. In this method, the hydrothermally prepared nanoparticles of metal oxides or alloy/oxide composites are utilized as metal or alloy precursors. A series of CEMNPs have been successfully fabricated through solid-state reactions between melamine and the precursors in ultra pure nitrogen atmosphere. X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman spectrometry and vibrating sample magnetometry (VSM) have been used to characterize the materials. The main work of the thesis is as follows:1. Fe/Fe3O4 nanocomposite was firstly prepared by the hydrothermal one-process method and then used as the metal precursor. Fe/Fe3C nanocomposites were synthesized using melamine as the carbon source. The morphology and structure of products were examined by TEM and HRTEM characterization. The nanocrystalline Fe/Fe3C composite particles formed at 600℃are embedded in an amorphous carbon matrix, while the products obtained at 650℃and 700℃are composed of carbon-encapsulated near-sphere and rod-like nanoparticles respectively. The possible formation mechanisms of as-prepared nanostructures with different morphologies are discussed. Meanwhile the involved reactions in the method are summarized. The effects of reaction temperature, reaction time and the amount of melamine on the magnetic properties of products are studied according to XRD and VSM characterizations.2. On the basis of the synthesis of carbon-encapsulated Fe/Fe3C nanoparticles, we successfully synthesized carbon-encapsulated Fe nanoparticles by changing reaction conditions. Furthermore, metal oxides Co3O4 and NiO nanoparticles were prepared by hydrothermal method and subsequent calcination and used as metal precursors. Carbon-encapsulated Co and Ni nanoparticles were also successfully fabricated using melamine as reduction reagent and carbon source. The XRD characterization results show that no carbide (Co3C and Ni3C) are formed in the syntheses of carbon-encapsulated Co and Ni nanoparticles repectively, which is different from that of carbon-encapsulated Fe nanoparticles. The reasons for this result are discussed based on relevant literature. Besides XRD, TEM and HRTEM characterizations, Raman spectrometry was employed to examine the crystallinity of the carbon shell of CEMNPs. VSM was used to measure the magnetic properties of materials as a function of magnetic field at room temperature. The effects of temperature on the morphology and magnetic properties of CEMNPs are mainly investigated.3. Different amounts of Co and Ni substituted Fe/Fe3O4 nanocomposites were prepared by the one-step hydrothermal method and used as precursors for Fe-Co and Fe-Ni alloy respectively. Carbon-encapsulated Fe-Co and Fe-Ni alloy nanoparticles were formed through solid-state reactions between their respective precursor and melamine. Additionally on the basis of synthesis of Fe/Fe3C nanocomposites, we investigated if the Co or Ni substituted cementite (Fe, M)3C (M=Co or Ni) could be formed simultaneously with alloys by changing reaction conditions. The XRD characterization results reveal that the as-prepared products are still alloys with no carbide impurities. The formation of alloy has been interpreted from the thermodynamic point of view. The magnetic properties of carbon-encapsulted Fe-Co and Fe-Ni alloy nanoparticles with different Co/Fe and Ni/Fe proportion, respectively, have been discussed according to the room temperature VSM measurement results.4. Different amounts of La or Nd substituted cobalt-iron alloy/cobalt ferrite nanocomposites were prepared by one-step hydrothermal method. La or Nd subsititued Fe-Co alloy nanoparticles encapsulated in carbon were synthesized using melamine as reduction regent and carbon source. The XRD characterization results reveal that body-centered cubic (bcc) La-Fe-Co alloys are formed while Nd exists in the form of oxide Nd2O3. The effects of the amount of La and Nd substitution on the magnetic properties carbon-encapsulated Fe-Co alloy nanoparticles have been investigated respectively based on VSM measurement results.更多还原