Preparation, Characterization And Formation Mechanism Of Iron(Manganese) Silicide Nanostructures

Transition metal silicides, a class of refractory intermetallic compounds between transition metals and slicon, have unique physical and chemical properties and have successfully been applied in the fields of complementary metal-oxide-semiconductor devices, thin-film coatings, bulk structural components, electrical heating elements, thermoelectric materials and photovoltaics. Particularly, transition metal silicide nanomaterials show special electrical, optical, magnetic and thermoelectric properties, and even possess the potential application in catalysis. However, conventional preparation methods including metallurgical method and physical method can’t meet the requirements of preparing transition metal silicide nanomaterials. Therefore, for the wide application of transition metal silicide nanomaterials, looking for simple and controllable methods with general applicability is of great significance. Based on above purposes, metal-organic chemical vapor deposition (MOCVD), pyrolysis and ball milling methods have been used to prepare iron silicide nanoparticles on silica, manganese silicide nanoparticles on silica and iron silicide-aluminium oxide nanocomposites in this paper. In addition, formation mechanism and magnetic properties of the as-prepared iron silicide nanostructures have also been preliminarily investigated. The main research contents and results are listed as follow:(1) FeSi nanoparticles on silica have successfully been prepared by MOCVD of Fe(CO)4(SiCl3)2as a single source precursor at400℃and atmospheric pressure. Fe(CO)4(SiCl3)2with the majority of the trans-configuration was synthesized from Fe3(CO)12and SiHCl3in the absence of water and air. Powder XRD patterns, TEM and high resolution TEM images revealed that FeSi nanoparticles had an average particle size of about10.8nm. The formation mechanism of FeSi nanoparticles on silica was investigated by in-situ FTIR spectroscopy, and it was presumed that Fe(CO)4(SiCl3)2adsorbed on silica tended to lose one SiCl4molecule firstly and transformed into Fe=SiCl2(CO)4, and FeSi nanoparticles formed finally through the elimination of carbonyl groups and dissociation of Si-Cl bonds with the promotion of H2.(2) MnSi nanoparticles on silica have also been successfully prepared by MOCVD of Mn(CO)5(SiCl3) as a single-source precursor at400℃and atmospheric pressure. Mn(CO)5(SiCl3) was synthesized from Mn2(CO)10and SiHCl3in the absence of water and air. Charaterization results showed that MnSi nanoparticles, with a size of about5-6nm, were uniformly dispersed on the silica support. The formation mechanism of MnSi nanoparticles on silica was investigated by in-situ FTIR spectroscopy. The results demonstrated the formation details of MnSi nanoparticles from Mn(CO)5(SiCl3) adsorbed on silica through the elimination of carbonyl groups and dissociation of Si-Cl bonds with the promotion of H2. (3) Fe3Si nanoparticles on silica have successfully been prepared by pyrolysis of polydimethylsilane as the silicon source and ferrocene as the iron source. Powder XRD patterns, TEM and high resolution TEM images, and XPS spectra revealed that Fe3Si nanoparticles obtained at600℃with an average particle size of about7.6nm were highly dispersed on the silica. With the pyrolysis temperature increasing to800℃, the average particle size increased to16.2nm and a new phase Fe5Si3appeared. Moreover, a thin oxidation layer composed of Fe2O3and SiO2formed around the Fe3Si and Fe5Si3nanoparticles when they were exposed to air.57Fe Mossbauer spectra, M-H curves and FC and ZFC curves demonstrated that the as-prepared nanoparticles presented superparamagnetic behavior at room temperature and ferromagnetic behavior at low temperature. Especially, the reduced particle size had a great impact on the magnetic properties of the as-prepared nanoparticles, including saturation magnetizations, Curie temperatures and blocking temperatures.(4) Fe3Si-Al2O3nanocomposites have successfully been prepared by ball milling the powder mixture of Si, A1and Fe3O4followed by calcining in H2at900℃. Characterization results revealed that the phase of the as-prepared nanoparticles transformed from Fe to Fe5Si and to Fe3Si, FeSi and Fe5Si3mixed phases with Si addition increasing, which were further confirmed by57Fe Mossbauer spectra. The average particle size of the smaller nanoparticles was about20nm, and the average particle size of the larger nanoparticles was about60nm due to the aggregation effect. Moreover, a small amount of amorphous Si and SiO2formed on the surface of the nanoperticles. M-H curves demonstrated that the as-prepared nanoparticles all presented ferromagnetic behavior at room temerature.