Study On The Synthesis Of Barium Ferrite With Self-Propagating Combustion Technology

Magnetic lead-type (M type) barium ferrite (BaFe₁₂O₁₉) is widely used as millimeter-band microwave materials, microwave absorbing materials and high-density perpendicular magnetic recording medias for its superior advantage in terms of lower prices of raw material, excellent chemical stability, higher coercivity and energy product as well as its uniaxial magnetic anisotropy etc.In this study, Magnetic lead-type barium ferrite was synthesized with two different approaches respectively, namely self-propagating high-temperature synthesis (SHS) and self-propagating low temperature synthesis (LCS). In terms of SHS, the influence of the iron content, pressing density, replacement of the barium, doped lanthanum as well as magnetic field inspiration on SHS were investigated. In terms of LCS, the influence of sintering temperature, sol pH, molar ratio of citric acid to metal ions as well as surfactants on LCS were investigated, respectively. The barium ferrite phase composition, microstructure and magnetic properties were also characterized with the help of XRD, SEM, VSM. Our study showed that both of the two approaches could be used in preparing M-type barium ferrite effectively.In the study of SHS, the iron powder content had a relatively closer connection with the magnetic properties of samples. As an example, magnetic properties of barium ferrite reached the optimal point when iron mass fraction was 25.74%, in which condition the coercivity was 644.8Oe, saturated magnetization (Ms) was 26.26emu/g, residual magnetization (Mr) was 7.292emu / g, respectively. However, once the iron mass fraction exceeded 25.74%, coercivity, Ms and Mr would drop significantly. In addition, the replacement of BaO2 with BaO was better than with BaCO3, as the replaceing effect of the former was superior to the latter. To replace BaO2, extra oxygen was needed. The pressing density in barium ferrite also had an optimal value of 1.888 g/cm³. Doped lanthanum could obviously change the magnetic properties of barium ferrite and barium ferrite exhibited the best comprehensive magnetic properties when the replacing capacity of La³⁺ was 0.3.It was possible to synthesize BaFe₁₂O₁₉ with the help of SHS under external magnetic field conditions, and the increase of magnetic field strength would be helpful for the improvement of magnetic properties. Degree of magnetization would exert certain influence on the composition and magnetic properties of the final products. Moreover, the crucial period that magnetic field played its role in SHS synthesis was during the course of self-propagating combustion. When magnetic field strength was 20T, the relationship between magnetic properties of BaFe₁₂O₁₉ and magnetic field direction was shown as follows: positive magnetic field> vertical magnetic field> reverse magnetic field.In the study of LCS, sintering temperature had a major impact on the magnetic properties of samples. And with the increase of sintering temperature, magnetic properties were improved accordingly. Coercivity reached its maximum of 5.509kOe at 900℃, while Ms and Mr both reached their maximum of 59.84emu/g and 31.99emu/g at 1000℃, respectively. When the sintering temperature exceeded 1000℃, coercivity, Ms and Mr dropped significantly. The optimal conditions of the synthesis of barium ferrite with LCS approach were shown as follows: pH of former solution was 7.00; ratio of citric acid to metal ions was 2:1; sintering temperature was 1180℃. Addition of appropriate surfactants could also help enhance the magnetic properties.In comparison with SHS, barium ferrite powder characterized by a smaller particle size, uniform distribution and better magnetic properties could be prepared with LCS. To sum up, synthesis of barium ferrite with LCS technology had a broad prospect.