【作者】 王克强；

【导师】 赫晓东；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2010， 博士

【摘要】 Ni-Zn铁氧体具有优异的软磁性能,应用广泛,但传统的制备工艺周期长、能耗大,严重浪费资源。高温自蔓延合成（SHS）技术制备铁氧体磁粉所需时间短,能耗低,污染小,可以取代固相法中的预烧工艺,弥补铁氧体传统制备工艺的不足。本文选用NiO-ZnO-Fe₂O₃-Fe-O₂-NiCO₃为原料,研究了SHS工艺参数和原料中NiCO₃含量对SHS制备铁氧体磁粉的影响机理,并系统研究了磁粉制备工艺、烧结工艺和掺杂对烧结铁氧体磁环微观结构和磁性能的影响,从中探索实现低成本SHS制备铁氧体的有效途径。放热系数k和氧气压力决定SHS能否进行,原料中NiCO₃含量对产物的颗粒均匀性和铁氧体化率有明显的影响。利用热力学公式计算体系的绝热温度,确定放热系数k的范围,研究不同放热系数k、氧气压力和NiCO₃含量与产物相组成、微观结构和磁性能的关系。由于SHS反应的温度非常高,在反应过程中,NiCO₃会发生分解,生成的NiO参与SHS反应,CO₂则从原料中溢出,疏松原料,利于氧气的渗透及反应完全;CO₂的溢出还对产物的颗粒均匀性有影响。结果显示,在放热系数k为0.5,氧气压力0.5 MPa及NiCO₃含量为3at%时,SHS制备的Ni-Zn铁氧体磁粉颗粒均匀性较好,平均粒径约为0.7-0.8μm,磁粉中铁氧体尖晶石相含量较高,适合后续处理及制备烧结磁环。将SHS制备的铁氧体磁粉造粒、成型并进行烧结。采用适当参数制备的SHS磁粉,具有较高的铁氧体化率,含有一定量的微细颗粒,微细颗粒在烧结时可以起到助熔剂的作用;颗粒具有较规则的多面体形状,利于后续处理和烧结,制备的磁环具有较优的磁性能,其初始磁导率μi和磁损耗分别为147和532 mW·cm-3。通过与固相法比较,SHS制备的铁氧体磁环具有较高的初始磁导率μi,但是损耗较高,因此降低损耗是SHS制备烧结铁氧体磁环的研究重点。烧结温度升高,保温时间延长,都会促进晶粒长大,孔隙率降低,提高磁环的初始磁导率μi和降低损耗,但过高的烧结温度和过长的保温时间,会造成ZnO的挥发和孔隙率的增加,减缓初始磁导率μi的增幅和损耗的降幅。为了更好的改善铁氧体的磁性能,特别是降低磁环的损耗,研究Bi₂O₃、SiO₂及Nb₂O₅等添加剂的掺杂对铁氧体磁环的影响。单独添加某一添加剂时,Bi₂O₃和SiO₂会不同程度促进晶粒长大,但不利于孔隙率的降低,而Nb₂O₅的添加对晶粒生长影响不大,但明显降低孔隙率。1wt% Bi₂O₃复合较少含量的SiO₂及Nb₂O₅掺杂时,磁环的晶粒长大,孔隙率降低,磁环磁性能改善效果显著优于单独添加任一添加剂。之后,随着SiO₂含量的增加,磁环晶粒长大,孔隙率降低,但是饱和磁化强度Ms降低过快,导致磁环的初始磁导率μi和损耗性能恶化;Nb₂O₅含量继续增加,晶粒长大的同时,孔隙率也增加,磁性能迅速恶化。1wt% Bi₂O₃复合掺杂0.2wt% Nb₂O₅时,磁环具有优异的初始磁导率μi和损耗性能,分别为289和213 mW·cm^-3,与未掺杂的磁环相比,初始磁导率μi的增幅和损耗的降幅分别为100%和60%。研究晶粒尺寸对铁氧体损耗的影响,然后选择微观结构相近的样品,并对损耗进行分离,研究掺杂Nb₂O₅对不同损耗的影响。在低磁通密度Bm和低频下,Ni-Zn铁氧体的损耗主要是磁滞损耗,受微观结构的影响要大于掺杂,晶粒越小,损耗越大,并且随着磁通密度Bm和频率增加,损耗的增加越明显。通过损耗分离可知,掺杂主要是影响铁氧体的磁滞损耗,对铁氧体涡流损耗和剩余损耗之和的影响较小,几乎可以忽略。更多还原

【Abstract】 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.更多还原