Preparation And Sintering Of Ni-Zn Ferrites Using Self-propagating High Temperature Synthesis

Nickel-Zinc ferrites possess outstanding soft magnetic properties and are widely used in electronics and communication field. Conventionally, ferrite powders are made by solid-state reaction method which requires high energy consumption at elevated temperatures for long time. Fortunately, the Self-propagating High-temperature Synthesis (SHS) route for the ferrite formation can partially eliminated these drawbacks. SHS has many potential advantages, such as low processing cost, simplicity of process and energy efficiency. The raw materials used to synthesize Ni-Zn ferrite powders by SHS method were NiO-ZnO-Fe₂O₃-Fe-O₂-NiCO₃. The effects of process parameters and different NiCO₃ content in the raw materials on the microstructure and magnetic properties of Ni-Zn ferrite powders were systematically studied. The influence of SHS process, sintering process and doping on the microstructure and magnetic properties of Ni-Zn ferrites were investigated respectively, and optimum technical conditions were obtained.The SHS process was controlled by the exothermic coefficient k and oxygen pressure. The increase of NiCO₃ content in the raw materials can significantly enhance the percent conversion and result in uniform particle of the product. The adiabatic combustion temperature was calculated and the exothermic coefficient of the system was determined. The effcts of exothermic coefficients, oxygen pressures and NiCO₃ content on the phase composition, microstructure and magnetic properties of products were studied. NiCO₃ decomposed into NiO and CO₂ during SHS reaction and CO₂ escaped from the powders and left open pores. Thus the O₂ gas was easy to infiltrate through the powders to guarantee the reaction taking place continuously. As a result, the reaction proceeded more completely. The escaping of CO₂ also influenced the particle uniform of products simultaneously. When k, O₂ pressure and the NiCO₃ content were 0.5, 0.5 MPa and 3at%, respectively, the powder was able to prepare sintered ferrites for its mean size was 0.7-0.8μm and has the relatively high purity of spinel phase.The SHS ferrite powders were prilled, pressed to cores and sintered. The composition, morphology, structure and magnetic properties of toroid were analyzed and compared with production obtained by solid reaction. The powder prepared by SHS contained a few minuteness particles, which can act as fluxing agent during sintering process. The Ni-Zn toroid prepared by SHS method had excellent magnetic properties, with theμi value of 147 and power loss was 532 mW·cm^-3. Compared with the ferrites produced by conventional solid reaction, theμi of ferrites prepared by SHS method was better, but the power loss was worse.Increasing sintering temperature and holding time helps the toroid form homogenous grain with big grain size and less pores, which had better magnetic properties. However, increasing sintering temperature and holding time too much will cause the increase of porosity and volatilization of ZnO, which will deteriorate the improvement of magnetic properties of toroid.A small amount of additives can greatly affect the properties of ferrites. The effects of Bi₂O₃, SiO₂ and Nb₂O₅ addition on the microstructure and magnetic properties of Ni-Zn ferrite prepared by SHS method were systematically studied. Doping of Bi₂O₃ and SiO₂ accelerated the grain growth, but increase the porosity. Addition of Nb₂O₅ reduced the porosity in the toroid. When co-doping with the 1wt% Bi₂O₃, a small amount addition of SiO₂ and Nb₂O₅ largely improved magnetic properties of toroid since the grain growth and low porosity. With more doping of SiO₂ and Nb₂O₅, the saturated magnetization (Ms) was rapidly decreased and resulted in the deterioration of toroid. The Ni-Zn ferrite had excellent magnetic properties when co-doping 0.2wt% of Nb₂O₅ and 1wt% of Bi₂O₃ wih theμi value of 289 and power loss value of 213 mW·cm-3. The decrease of power loss range and increase amplitude ofμi was 60% and 100% respectively compared to the toroid without doping.The effects of microstructure and doping of Nb₂O₅ on the power loss were analyzed. To further study this phenomenon, frequency responses of the divided hysteresis losses (Ph) and the total of eddy current losses and residual losses (Pe+Pr) for ferrites with doping of Nb₂O₅ were studied. The effect of microstructure on the power loss was greater than the doping when the samples were excited at low frequency and low magnetic flux density (Bm). With the increasing of testing frequency and Bm, power loss of Ni-Zn ferrites with smaller grain size increased more observably than the one with big grain size. The doping obviously influenced the Ph, and almost impacted the Pe+Pr.