Carbon Combustion Synthesis of Rare Earth Ferrites

We report the synthesis of submicron powders of rare earth ferrites based on LnFe2O4, where Ln is Y, Er, Yb by using Carbon Combustion Synthesis of Oxides (CCSO).  The CCSO is a modified form of self-propagating high temperature synthesis that uses carbon as a fuel instead of a pure metal.  The reactant mixture contains in addition to oxides and/or carbonates about 6-12 wt. % a carbon powder. Following local ignition, the exothermic reaction between carbon and oxygen, (=-393.5 kJ/mol), provides the heat for the solid reactions and heats the adjacent reactant layer. This generates a self-sustaining temperature (up to 1000° C) wave that propagates through the reactant mixture at a velocity of up to 2.5 mm/s. The major parameters affecting the process are the carbon concentration in the reactant mixture and the ability of the oxygen infiltration to the reaction zone. A stable self-propagating reaction front can be obtained only at carbon and oxygen concentrations exceeding a critical value. The reaction front temperature and average velocity increased as the carbon concentration in the reaction mixture increased approaching eventually asymptotic values. The concentration of the carbon in the reactant mixture enabled control of the moving front temperature and average velocity as well as the particle size and surface area.