Continuous hydrothermal synthesis and crystallization of magnetic oxide nanoparticles

A continuous hydrothermal synthesis technique was used to produce nanoparticles of two metal oxides (α-Fe₂O₃ and Co₃O₄) and a mixed-metal oxide (CoFe₂O ₄) in this work. Two variations of the technique were investigated. These involved carrying out a precipitation reaction between a metal salt solution and a hydroxide solution at ambient conditions, and at elevated temperatures. The effect of processing variables such as temperature, overall concentration, hydroxide concentration, and relative ratio of iron to cobalt on product characteristics such as composition, particle size, size distribution and morphology was studied.; The single-oxide experiments showed that precipitation at elevated temperature resulted in more uniform particles of α-Fe₂O₃ and Co₃O₄ than precipitation at ambient conditions. The precipitation temperature had no effect on particle size in the case of the mixed-oxides. However, the product from the mixed-oxide experiments at high temperature contained higher amounts of α-Fe₂O ₃.; The precipitation behavior seen in the experiments was examined in terms of the relative solubilities of the cation(s). The relative solubilities were calculated using a thermodynamic model based on the Helgeson-Kirkham-Flowers-Tanger equation of state, Bromley-Meissner activity coefficient model for ions, and the Pitzer activity coefficient model for neutral species. The model showed that simultaneous precipitation of the two metal cations only occurs over a very narrow range of pH and temperature. While manipulating the temperature affects the solubility of iron, decreasing cobalt solubility requires a high pH. Hence, the relative solubilities of iron and cobalt limit the range of processing variables in producing the mixed oxide CoFe₂O₄ . Furthermore, differences in the hydrolysis and dehydration of cobalt and iron affect the particle size, size distribution, and morphology of the product.; A mechanism of particle formation was postulated involving the precipitation of metal hydroxides at ambient conditions, dissolution of the hydroxides as temperature is increased followed by rapid precipitation of metal oxides at elevated temperatures. When the reactants are mixed at elevated temperatures, the mechanism is simply precipitation of metal oxides due to the addition of the hot hydroxide solution.