In this study, varying amounts of MnO 2 up to 5 wt.% were added to magnesium aluminate spinel (MA) bodies using a solid-state sintering method at 1200–1600°C. The effect of MnO 2 addition on the phase composition, microstructure, distribution of elements, and ionic valence of MA was investigated via X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy, respectively. The results showed that Mg 2+ ions in MA crystals were replaced by Mn 2+ ions, resulting in the formation of the (Mg 1- x Mn x )Al 2 O 4 solid solution. The distorted crystal structures promoted the sintering reactions, and the mechanical characteristics of MA were greatly improved by the solid solution strengthening process. When the additive amount of MnO 2 was 5 wt.% and the sintered temperature reached at 1600°C, excess manganese ions hardly dissolved into the lattice of MA. And these ions were only distributed at the grain boundaries of MgAl 2 O 4 , forming a “barrier” that hindered the migration and diffusion of particles, thereby suppressing the sintering process and weakening the mechanical strength of MA.
Yang Yan,Li Jie,Zhang Huaiwu,Wang Gang,Gan Gongwen,Rao Yiheng
The development of miniaturized high frequency ferrite ceramics is attracting widely interest for next generation MLCI (multi-layer chip inductors). However, there exist short comings in the microstructure adversely affecting magnetic properties. In this study, 0.5 wt% MnO 2 - x wt% Bi 2 O 3 ( x = 0.1, 0.5, 1.0, 1.5, 2.0, and 3.0) were introduced into NiCuZn ferrites to adjust the microstructure at low temperatures. Also, the microstructure changes and high frequency magnetic properties of NiCuZn ferrites were investigated via controlling the addition of MnO 2 –Bi 2 O 3 and sintering temperatures. SEM results indicated that quantitative MnO 2 can suppress abnormal grain growth when optimized amount of Bi 2 O 3 was added. In addition, various amounts of Bi 2 O 3 were added to adjust the magnetic properties by controlling grain growth. Results revealed that 0.5 wt% MnO 2 -1.5 wt% Bi 2 O 3 composite addition is a critical point to enhance the homogeneity of the samples. Also, interestingly, the dominant contribution to dynamic magnetization mechanism changes from spin rotation to domain wall motion when the content of Bi 2 O 3 is greater than 1.0 wt%. Finally, fine microstructure of NiCuZn ferrites with high permeability ( μ ′ ≈ 291), high saturation magnetization ( M s ≈ 58.83 emu/g), high saturation flux density ( B s ≈ 290.04), high Q factor (~ 75) and enhanced cutoff frequency (~ 60 MHz) can be obtained sintered at 925 °C. Thus, this work may provide new guidance for developing RF frequency LTCC NiCuZn ferrite ceramics.
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