【作者】 刘冬；

【导师】 严密； 吴琛；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2018， 博士

【摘要】 软磁复合材料是磁粉与绝缘介质通过粉末冶金工艺混合压制而成,广泛应用在变压器线圈,谐振电感以及电机磁芯中。尽管电子电力领域对软磁材料的需求日益增长,其较高的损耗尤其是涡流损耗严重限制了高频应用。对于软磁复合材料,制备厚度薄且均匀、电阻率高以及与磁粉结合性强的绝缘包覆层是降低涡流损耗的关键。传统的有机包覆如环氧树脂、酚醛树脂、硅树脂等,尽管与磁粉基体有很好的结合性,但是树脂的热稳定性能差,一般高于200 ℃以上退火时就会发生分解,限制了后续热处理温度,不能有效去除内应力。利用磷酸对磁粉表面进行钝化生成磷酸盐包覆层,是工业中最常用到的工艺。然而高温退火过程中,磷酸盐由于晶化或者分解造成电阻率急剧降低致使涡流损耗迅速增加。为了降低损耗,具有高电阻率、高耐热温度的氧化物包覆材料成为了研究热点。本文选取高磁导率、低磁晶各向异性以及较高电阻率的FeSiAl合金作为软磁复合材料的主体。采用溶胶凝胶工艺制备了新型磷酸盐-氧化铝复合包覆膜、硝酸钝化生长致密氧化膜以及调整硝酸钠钝化液浓度和pH值制备不同成分结构的绝缘膜,主要创新点如下:(1)采用溶胶-凝胶工艺制备了 A1203绝缘膜,充分利用磷酸盐与磁粉基体的高结合强度以及金属氧化物的高电阻率高耐热温度,制备了新型磷酸盐-氧化铝复合绝缘包覆层,并对不同绝缘包覆膜的化学成分以及高温退火过程中的演变进行了详细的研究探讨以解释磁性能的变化。(2)采用“原位”钝化方法,利用硝酸对FeSiAl磁粉进行表面氧化,利用球差校正高分辨透射电镜以及能谱仪详细表征分析了钝化膜的厚度、显微结构与成分分布,揭示了钝化膜随浓度变化的生长机制与演变规律。利用点缺陷模型分析了点蚀反应的机理。最后,揭示了不同钝化生长条件下钝化膜的演变与软磁复合材料磁性能的关系。(3)利用氧化性盐硝酸钠对FeSiAl磁粉进行原位钝化包覆,通过改变钝化液浓度以及pH值,系统研究了不同钝化条件下绝缘膜的厚度,显微结构和组成;结合绝缘膜的生长机制与高温退火过程中的演变规律,深入分析了软磁复合材料的磁性能变化规律;利用热力学分析探讨了在不同腐蚀钝化条件下绝缘层的生长机制。更多还原

【Abstract】 Soft magnetic composites(SMCs)containing metallic magnetic powders surrounded by insulation coatings are fabricated by powder metallurgy process and are widely used in applications such as transformer coil,resonant inductor and electric motor core.Despite their rapid development urged by the ever-rising requirements from the electric and electronic industry,the inevitable loss especially the eddy current loss refrains their applications at high frequencies.For the SMCs,the key to tackle such issue is to fabricate uniform insulation layers with small thickness,high electrical resistivity and strong adhesion to the magnetic powders.Although traditional organic coatings provide satisfactory adhesion and flexibility,most of them including the epoxy,phenolic and silicon resins exhibit poor resistance against high temperature annealing and will decompose after annealing above 200 ℃.This limits the temperature used for the post-annealing process and hinders complete relaxation of the internal stress.Surface passivation by phosphoric acid in the formation of phosphate coatings is the most commonly used method.It has,however,been reported that heat treatment leads to significantly reduced electrical resistivity of the phosphate due to its crystallization and/or decomposition at relatively high annealing temperatures.Oxide insulation coating with high resistivity and thermal stability serves as an excellent alternative to reduce the core loss.In this study,FeSiAl soft magnetic alloy with low magnetocrystalline anisotropy,high permeability and low electrical resistivity has been selected to prepare the SMCs.New phosphate-alumina coating has been prepared by sol-gel method.Nitric acid in-situ passivation has been used to fabricate compact insulation coatings.Besides,FeSiAl powders have been surface oxidized by NaNO3 solution with varied concentration and pH values.The main results of the work are as follows:(1)Al2O3 insulation coating has been prepared by sol-gel process.New phosphate-alumina hybrid coating has been fabricated due to its satisfactory adhesion to the powders combined with high electrical resistivity and thermal stability.Composition and evolution of different insulation coatings after high temperature annealing has been studied to explain the changes of magnetic performance for the FeSiAl SMCs.(2)Nitric acid in-situ oxidation has been used to fabricate insulation coatings of the FeSiAl SMCs.Detailed high-resolution transmission electron microscopy(TEM)and energy dispersive X-ray spectroscopy(EDS)mapping analysis provide direct and comprehensive evidence of the thickness,microstructure and elemental distribution of the oxidation layers,based on which the evolution and growth mechanism of the coatings with varied concentrations have been revealed.The origin of pitting corrosion has been explained based on the point defect model.Correlating the magnetic performance and evolution of the coating layers under different conditions has been revealed.(3)FeSiAl powders have been surface passivated by NaNO3 solution with varied concentration and pH values.The thickness,microstructure and composition of the passivation layer have been investigated.Correlating the growth mechanism and evolution during the annealing process of the passive films reveals the enhanced magnetic properties of FeSiAl SMCs.Thermodynamic analysis displays the growth mechanism of the insulation coating under different passivation conditions.更多还原