【作者】 刘桂香；

【导师】 代波；

【作者基本信息】 西南科技大学 ， 材料科学与工程， 2022， 博士

【摘要】 电子信息技术的发展要求电子元器件朝小型化、集成化和多功能化方向发展,多种功能材料在内电极材料的导联作用下叠层式共烧是实现这一目标的关键技术。为了避免叠层共烧元器件中常用内电极材料Ag（熔点为961℃）的扩散而恶化元器件的性能,集成元器件中作为电感、抗电磁干扰等常用NiZn铁氧体材料必须兼备低温烧结和高性能的特点。传统方法制备NiZn铁氧体陶瓷的温度～1200℃,保温时间～4h,这种烧结制度不能满足NiZn铁氧体的低温共烧工艺。本文围绕NiZn铁氧体材料的低温快速烧结展开工作,首先通过对比传统固相反应法和水热法制备的NiZn铁氧体纳米粉体的可烧结性能发现,尽管两者尺寸均为纳米级,但由于传统固相反应的高温环境使粉体产物结晶更完整、缺陷少,所以其烧结活性低于低温水热环境下制备的纳米粉体。本论文第四章、第五章和第六章的烧结实验所用原料均为水热法制备的平均晶粒尺寸为15.9 nm的NiZn铁氧体粉体。然后探索了具有“大电场、小电流”特点的电场/电流辅助烧结NiZn铁氧体的烧结机理,得出NiZn铁氧体烧结过程中施加的电场对其低温快速烧结起主导作用:烧结过程中电场的存在促进带电粒子迁移,并促进氧空位产生而降低烧结活化能,其共同作用促进NiZn铁氧体的烧结。采用电场/电流辅助烧结法,在烧结温度为950℃时保温20 min制备出致密度>95.34%的亚微米级晶粒尺寸的NiZn铁氧体陶瓷,其饱和磁化强度≥74.6 emu/g、矫顽力≤15.0 Oe。接着分析了具有“大电流、小电场”工作特点的放电等离子体烧结法制备NiZn铁氧体的烧结机理,得知在直流模式或者近直流模式下,具有半导体导电特性的NiZn铁氧体的快速烧结主要是基于电流的焦耳热效应。随着NiZn铁氧体样品温度的升高,其电导率增大,将会有部分电流通过烧结样品而使样品自身产生焦耳热,该热量主要集中在NiZn铁氧体颗粒接触位置,此处产生局部高温,导致样品颗粒瞬间熔融产生颈部,在颗粒间颈部持续产生的热量以及颈部位置处拉普拉斯应力的共同作用下,使得NiZn铁氧体的晶粒生长和致密化在极短时间内完成。采用放电等离子体烧结法,在直流或近似直流模式下,当烧结温度处于850-900℃、升温速率为60-150℃/min,保温时间为3-9 min时,可制备出致密度>98.59%的亚微米级晶粒尺寸的NiZn铁氧体陶瓷,其饱和磁化强度≥71.2 emu/g、矫顽力≤15.8 Oe,剩磁≤0.9 emu/g。最后,通过分析CuO、Bi₂O₃、Y₂O₃和TiO₂掺杂对NiZn铁氧体性能的影响机理得知:低熔点Bi₂O₃最明显的作用是极大促进NiZn铁氧体的烧结,它可为烧结体系引入液相,促进颗粒重排、溶解-沉淀传质和提高离子扩散速率,进而促进NiZn铁氧体晶粒的长大和提高样品致密度;添加剂TiO₂的最明显作用是降低NiZn铁氧体的饱和磁化强度。具有强烈占据尖晶石B位的非磁性Ti⁴⁺在取代NiZn铁氧体晶格B位Fe³⁺的同时,会将部分B位的Fe³⁺还原成较低磁矩的Fe²⁺,从而大幅度降低B次晶格的磁矩,继而使NiZn铁氧体的磁矩大幅下降;添加剂Y₂O₃最明显的作用是抑制NiZn铁氧体晶粒的长大。Y₂O₃掺入NiZn铁氧体后产生的次晶相-正交晶体结构的Fe YO₃包裹住NiZn铁氧体晶粒,阻碍传质的进行,从而抑制NiZn铁氧体晶粒生长和样品致密化。采用传统无压烧结法,在900℃就可制备出致密度≥95.16的CuO-Bi₂O₃共掺杂和TiO₂-Bi₂O₃共掺杂NiZn铁氧体陶瓷,在950℃可制备出致密度为≥95.13%的单组份掺杂Bi₂O₃和Y₂O₃-Bi₂O₃共掺杂的NiZn铁氧体陶瓷。在这些NiZn铁氧体陶瓷中,950℃烧结的CuO-Bi₂O₃共掺杂陶瓷样品的静态磁性能最好,其饱和磁化强度为72.1 emu/g,矫顽力为4.4 Oe。更多还原

【Abstract】 The development of electronic information technology requires the miniaturization,integration and multi-function of electronic components.The multi-layer co-firing of various functional materials with internal electrode materials is the key technology to achieve this goal.In order to avoid the the performance deterioration of the integrated components due to the diffusion of the commonly used internal electrode material Ag（melting point:961°C）,NiZn ferrite material,as one of the basic materials of electronic components,must have the characteristics of low temperature sintering and high performance.NiZn ferrite ceramics were prepared by traditional sintering method with sintering temperature of～1200°C and holding time of～4 h,which doesn’t satisfy co-firing requirement of NiZn ferrite with Ag.This dissertation focused on the low-temperature rapid sintering of NiZn ferrite materials.Firstly,the sinterability of NiZn ferrite nano-powders prepared by traditional solid-phase reaction method and hydrothermal method was compared.It was found that the sintering activity of the latter was higher than that of the former,although both sizes were nanoscale,which was due to the fact that the former has more complete crystallization and fewer defects in the high temperature environment of the traditional solid-state reaction.The raw material used in the subsequent sintering experiments of the dissertation was the NiZn ferrite powder with an average grain size of 15.9 nm prepared by hydrothermal method.Secondly,the sintering mechanism of NiZn ferrite prepared by electric field/current assisted method with the characteristics of"large electric field and small current"was explored.The results showed that the applied electric field played a dominant role during the rapid sintering of NiZn ferrite.The applied electric field accelerated the charged particles migration and promoted the generation of oxygen vacancies,thereby reducing the sintering activation energy,which jointly promoted the sintering of NiZn ferrite.When the sintering temperature was 950°C and the duration time was 20 min,the NiZn ferrite ceramics with submicron grain size,a relative density>95.34%,a saturation magnetization≥74.6 emu/g and a the coercivity of≤15.0 Oe were prepared by electric field/current assisted sintering method.Then,the sintering mechanism of NiZn ferrite prepared by spark plasma sintering method with the characteristics of"large current and small electric field"was analyzed.It was found that the rapid sintering of NiZn ferrites with semiconducting properties under DC or near-DC mode was mainly based on the Joule heating effect of current.As the temperature of NiZn ferrite increased,some current flowed through the sintering sample due to its increased electrical conductivity,causing Joule heat by itself.The Joule heat was mainly concentrated at the contact position between the NiZn ferrite particles,where local high temperature was generated,resulting in the instantaneous melting of the sample particles and the formation of necks.The grain growth and densification of NiZn ferrite are completed in a very short time under the combined action of the continuous Joule heat generated and the Laplace stress at the neck.Using the spark plasma sintering method with DC or approximate DC mode,the NiZn ferrite ceramics with submicron grain size,a relative density>98.59%,a saturation magnetization≥71.2 emu/g,a coercivity≤15.8 Oe and a remanence≤0.9 emu/g were prepared when the sintering temperature is in the range of 850°C-900°C,the heating rate of 60-150°C/min and the holding time of 3-9min.Finally,the influence mechanism of CuO,Bi₂O₃,Y₂O₃ and TiO₂ doping on the properties of NiZn ferrite was analyzed.The most obvious effect of Bi₂O₃ with low melting point was to greatly promote the sintering of NiZn ferrite.It could introduce liquid phase into the sintering system,promoted particle rearrangement,dissolution-precipitation mass transfer and increased ion diffusion rate,thereby promoting NiZn ferrite grains growth and increased sample density.The most obvious effect of TiO₂ is to drastically reduce the saturation magnetization of NiZn ferrite.The non-magnetic Ti⁴⁺that strongly occupies the spinel B site replaced the Fe³⁺at B site of the NiZn ferrite lattice,and at the same time reduced part of the Fe³⁺to Fe²⁺with lower magnetic moment at B site,which greatly reduced the magnetic moment of the B sublattice,and then greatly reducing the magnetic moment of the of NiZn ferrite.The additive Y₂O₃ can significantly inhibit the grain growth of NiZn ferrite.When Y₂O₃ was doped into NiZn ferrite,the produced secondary phase-Fe YO₃ with orthorhombic crystal structure would wrap the NiZn ferrite grains and hindered mass transfer,which inhibited the grain growth of NiZn ferrite and the densification of the sample.Using the traditional pressureless sintering method,CuO-Bi₂O₃ co-doped and TiO₂-Bi₂O₃ co-doped NiZn ferrite ceramics with a density of≥95.16%were prepared at 900°C,and Bi₂O₃doped and Y₂O₃-Bi₂O₃ co-doped NiZn ferrite ceramics with a density of≥95.13%were prepared at 950°C.Among these NiZn ferrite ceramics,the CuO-Bi₂O₃ co-doped ceramic sintered at950°C exhibits the best static magnetic properties with a saturation magnetization of 72.1emu/g and a coercivity of 4.4 Oe.更多还原