Study On Low Temperature And Rapid Sintering Of Nickel Zinc Ferrite

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.