Synthesis,Structure And Mangetic Property Studies Of Catbom-Encapsulated Magnetic Nanoparticles

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.