Study On The Preparation, Characterization And Properties Of Silica Matrix Magnetic Nanocomposites

The interest in nanocomposites consisting of magnetic ultra-fine or nanosized particles embedded in an immiscible insulating non-magnetic matrix has grown considerably in recent years. The sol-gel derived amorphous silica matrix is an excellent host for supporting different types of guest nanoparticles, the porous nature of the amorphous silica matrix provides nucleation sites for nanomagnetic particles, and minimizes the aggregation phenomena imposing an upper limit to the size of the particles. A series of nanocomposites, such as γ -Fe₂O₃/SiO₂, CoFe₂O₄/SiO₂ and NiFe₂O₄/SiO₂ were investigated. The growth mechanism of magnetic particles was studied in detail. Besides, the evolution of the interaction between particles and matrix in the process of the particle growth, as well as its effects on the nanocomposites' structure and magnetic properties were assessed. The effect of different parameters on magnetic particles and particle size distribution were also discussed. The main results are as follows: Nanocomposites with γ-Fe₂O₃ nanoparticles uniformly dispersed in silica matrix were successfully synthesized using tetraethylorthosilicate (TEOS) as a silica precursor and iron nitrate as ferric oxide precursor. However, when iron chloride was used as precursor, hematite was obtained in the final composite. It was found that the dried gels obtained at low temperatures (T<400℃) were amorphous, when the dried gels were heat treated at 600℃, a lot of γ-Fe₂O₃ nanoparticles were formed, while a further increase of temperature resulted in the γ to α-Fe₂O₃ transformation. Salt concentration and calcining temperature are consistently used to control the particle size and particle size distribution, which tend to result in small crystalline size or broad particle size distribution. Hydrochloric acid was used in this paper instead. The addition of acids to the sols resulted in a way to increase particle size (5.2～17.5nm) and to narrow the particle size distribution of γ-Fe₂O₃ in the final composites. On increasing the amount of hydrochloric acid, transformation from superparamagnetism to ferromagnetism would happen. Coercivity of γ-Fe₂O₃/SiO₂ nanocomposites obtained in this paper was as high as 1.99×104 A?m-1. Magnetic nanocomposites formed by cobalt ferrite particles dispersed in silica matrix were prepared by sol-gel process, starting from tetraethylorthosilicate and metallic nitrates. The xerogels were amorphous. Heat treatment at 400℃ resulted in CoFe2O4 clusters being partially formed and CoFe2O4 clusters were formed in large quantities,when the heat treatment temperature was increased to 600℃. The formation reaction of CoFe2O4 clusters was accompanied by a rearrangement of the silica matrix network. Further increase the heat treatment temperature to 800℃, materials with CoFe2O4 nanocrystals well crystalline dispersed in the silica matrix could be obtained. The formation of CoFe2O4 clusters was accompanied with the rearrangement of silica network and with the enhancement of the Si-O-Fe bonds between the CoFe2O4 clusters and the surrounding silica network. While breakage of the Fe-O-Si bonds in the interface between the particles and matrix would happen when annealing temperature was raised to as high as 800℃. The effect of three parameters: (a) the thermal treatment temperature, (b) the ageing temperature and (c) salt concentration on the structural and magnetic properties of the composites were examined. By controlling these parameters, CoFe2O4/SiO2 nanocomposites with different crystallite size and magnetic properties could be obtained. It was remarkable that the addition of acids to the sols resulted in a way to increase particle size and to narrow the particle size distribution of cobalt ferrite in the composites. Furthermore, studies of the influence of the initial drying temperature on particle size of cobalt ferrite suggested that a well-established silica network provide more confinement to the growth of CoFe2O4 particles. Therefore, increasing the drying temperature before heat treatment favored the formation of a complete silica network, which in turn hindered the growth of CoFe2O4 nanoparticles and resulted in an effective refinement and improved the uniformity of CoFe2O4 particles. Changing of the salt concentration would result in the transformation from superparamagnetism to ferromagnetism. The magnetic properties of nanocomposites prepared exhibit a strong dependence on the particle size. By the increase of particle size, composites with a progressive increase of the density of magnetization were produced. In particular, nanocomposites could get coercivity values as high as 1.59×105A?m-1, which is more than two times higher than the coercivity of bulk cobalt ferrite (0.78×105A?m-1). Nanocomposites with nickel ferrite nanoparticles uniformly dispersed in silica matrix were successfully synthesized using tetraethoxysilane as a precursor of silica and metallic nitrates as precursors of ferrite. NiFe2O4/SiO2 nanocomposites dried at 150℃ and then calculated at 800℃ with [Fe]/[Si] lower than 13.5% showed superparamgnetism at RT. Changing of the salt concentration would result in thetransformation from superparamagnetism to ferromagnetism. The magnetic properties exhibited a strong dependence on the particle size. By the increase of particle size, the density of magnetization varying from 2.7 to 13.5 A?m2?kg-1 were produced. In particular, nanocomposites could get coercivity values as high as 2.01×104 A?m-1, which is much higher than the coercivity of bulk cobalt ferrite((～0.47×103 A?m-1). Asymmetric hysteresis loops resulting from the exchange-spring coupling were found in nanocomposites prepared in this paper, which, As far as authors know, has never been reported before in this kind of materials.