Study On The Low-temperature Sintering Of High-permeability NiZn Ferrites

This dissertation focuses on low-temperature sintering of high-permeability NiZn ferrites. The effects of compositions, additions and various preparation parameters on the magnetic property were investigated systematically. As a result, high-permeability NiZn ferrites were synthesized successfully at low sintering temperatures.The conventional oxides method has been applied to prepare high-permeability NiZn ferrites with special attention paid to lower the sintering temperature. The combined effects of CuO and V₂O₅ additions, calcinating temperatures, the first-milling time and the second-milling time on the magnetic properties of samples have been studied systematically. It is found that CuO reduces the forming temperature of the spinel phase and induces liquid phase during sintering, which favors the densification and grain growth. The addition of V₂O₅ also induces liquid phase during sintering, leading to the decrease in sintering temperatures. In addition, the reactive ability of calcined powders can be enhanced by optimizing some preparation parameters, such as calcining temperature, the first-milling time and the second-milling time, etc, which promotes the solid reaction during sintering and results in the improvement of magnetic properties. Through adding 0.20 mol% V₂O₅, calcining at 750 ℃ and grounding for six hours during both the first milling and the second milling, for the sample with the composition of 49.5 mol% Fe₂O₃;32 mol% ZnO;8.5 mol% NiO;10 mol% CuO, a Ni-Zn ferrite with the followed properties has been synthesized at a low sintering temperature of 930 ℃: initial permeability μ, of 1614, relative loss coefficient tanδ/μ₁ of 8.9×10^-6 (100 kHz), relative temperature coefficient a_uir of 2.27×l0^-6(-40 ℃ -60 ℃), curie temperature of 103 ℃, and saturation flux density Bs of 254 mT.The microwave sintering technique has been used to further reduce the sintering temperature of high-permeability NiZn ferrites instead of the conventional sintering. The reason why microwave electromagnetic field promotes the sintering solid reactionand the effect of microwave sintering on the magnetic properties of ferrites have been studied. It is found that microwave electromagnetic field effectively promotes the diffusion of ions and thus accelerates the densification and grain growth. In addition, the difference between the dependence of the densification rate and the grain growth rate on sintering temperature is reduced. The sintering duration is also greatly reduced. As a result, the magnetic properties of ferrites have been greatly improved. When other processes are the same as the oxides method above-mentioned, only through replacing the conventional sintering with microwave sintering, Ferrite with the relative initial permeability of 1700 and the relative loss factor of 8.7><10"6 (100 kHz), relative temperature coefficient of 2.0x10"6(-40 °C⁶0 °C) are achieved at only 930 °C sintering temperature.The sol-gel self-propagated method was also applied to prepare high-permeability NiZn ferrites at low sintering temperatures. The effect of the calcination on the magnetic properties of ferrites has been studied. It is found that by using the sol-gel self-propagated method, in case that the sintering temperature is lower than 1000 °C, for the ferrite with the same composition above-mentioned, the compact microstructure and uniform grains can be obtained without any sintering additives. A pre-calcination is effective to remove the residuals and other phases arising from the inadequate combustion of the gel, which decreases the energy consumption and shortens the reactive time during sintering, promotes the densification of matrix and the grain growth in the sintering process and finally improves the magnetic properties of samples. Using the sol-gel self-propagated technique, for the sample with the same composition above-mentioned, a final ferrite with initial permeability of 1620, relative loss coefficient of lOxlO'6 (100 kHz), relative temperature coefficient of 2.5x10"6(-40 °C -60 °C) is achieved at only 910 °C without adding any sintering aids.