【作者】 彭晓领；

【导师】 严密；

【作者基本信息】 浙江大学 ， 材料学， 2008， 博士

【摘要】 在综述了梯度功能材料制备方法和研究进展的基础上,本论文提出了一种基于组元磁导率差异制备梯度材料的新方法。具体如下:将梯度磁场作用于由磁性粒子和非磁性粒子组成的复合浆料,磁性粒子将向强磁场区域移动,沿磁场方向形成成分梯度,通过后续固化、干燥和烧结,最终制得梯度材料。采用这种新方法成功制备出ZrO₂/Ni梯度材料。根据实验需要设计了麦克斯韦线圈和亥姆霍兹线圈;采用旋转粘度仪分析了ZrO₂/Ni复合浆料的粘度特性;通过金相显微镜、扫描电镜和能谱分析仪等手段,研究了固相分数、磁场梯度、成型时间、磁性粒子浓度等因素对ZrO₂/Ni梯度材料组织和成分分布的影响;通过Monte Carlo模拟的方法,研究了磁场强度、磁场梯度、磁性粒子相互作用、磁性粒子浓度和非磁性粒子浓度等因素对体系中粒子分布的影响。本文的主要研究结果如下:获得了不同磁场中Ni和ZrO₂复合浆料粘度的变化规律。无外加磁场时,Ni和ZrO₂复合浆料的粘度随PVP分散剂含量的增加先增大后减小,在PVP分散剂含量为1.8 wt.%时,浆料粘度值最低。在磁场中,浆料的相对粘度随着PVP分散剂含量的增加而逐渐降低。浆料粘度随固相体积分数的增加而迅速增大。当磁场强度增加时,浆料粘度先迅速增大,然后逐渐趋于稳定。此外,浆料粘度随Ni粒子浓度增加而增大。这是因为Ni粒子会在磁场中团聚形成链状团簇,随着磁场强度和Ni含量的增加,Ni团簇逐渐增大、增多,导致粘度增大。掌握了Ni在ZrO₂/Ni梯度材料厚度方向的成分梯度随固相分数、磁场梯度、磁场作用时间、磁性粒子浓度的变化规律。随固相体积分数的增加,样品致密度逐渐增大,而成分梯度逐渐降低。随着磁场梯度增加至3.0 T/m,Ni成分梯度逐渐增大,而沿厚度方向显微硬度逐渐降低。粒子在浆料中的移动不是瞬时过程,与磁场作用时间有关。随着磁场作用时间的延长,成分梯度逐渐增大,在作用时间超过15 s后,浆料结构保持稳定。此外,随浆料中Ni浓度的增加,Ni成分梯度逐渐降低,当Ni含量达到15 wt.%时,成分梯度已经消失。提出了磁性粒子在梯度磁场中的分布模型,合理解释实验结果。采用Monte Carlo方法分别模拟了匀强磁场和梯度磁场中磁性粒子的运动规律。结果表明,在匀强磁场中磁性粒子团聚形成平行磁场方向的链状团簇。团簇随计算时间的增加而逐渐长大,最后趋于稳定。随着磁性粒子间相互作用的增加,团簇尺寸逐渐增大。但是,体系中粒子分布却不受磁场强度的影响。随着磁性粒子浓度的增加,相邻磁性粒子间距变小,彼此间磁相互作用增强,导致团簇尺寸逐渐增大。体系中的非磁性粒子对磁性粒子的移动存在阻碍作用,并且随着非磁性粒子浓度的增加,阻碍作用逐渐增强。在梯度磁场中,随着磁场梯度的增加,体系中磁性粒子的成分梯度逐渐增大;随着磁性粒子间磁相互作用的增强,团簇尺寸逐渐增大,而成分梯度逐渐减小;此外,非磁性粒子对磁性粒子在梯度磁场中的移动存在阻碍作用,随着非磁性粒子浓度的增加,磁性粒子沿磁场方向的成分梯度逐渐降低。更多还原

【Abstract】 Functionally graded materials（FGMs）are a class of special composites with continuously changed mechanical,physical and chemical properties,which have normally been fabricated by means of vapor deposition,powder metallurgy,plasma spraying,self-propagation high temperature synthesis,electro deposition,etc.Based on the distinct difference in magnetic susceptibility between components,a new approach was developed to prepare FGMs.When a gradient magnetic field was applied on the suspension comprised of both magnetic and nonmagnetic particles,the magnetic ones were driven towards high field regions,thus a composition gradient along the field direction was formed.After subsequent drying and sintering process,FGMs were obtained.ZrO₂/Ni FGM with a continuously changing composition has been successfully fabricated using this method.Uniform and gradient magnetic fields were designed to meet the need of experiments.The viscosity variations of suspension composed of both Ni and ZrO₂ particles within and without magnetic fields were investigated.The effects of influencing factors on microstructures of ZrO₂/Ni FGMs were studied systematically.The distributions of particles under both uniform and gradient magnetic fields were studied using two-dimensional Monte Carlo simulations.The effects of field strength,field gradient,magnetic interaction and concentration of magnetic and nonmagnetic particles on distribution of particles were investigated.Flux meter,rotational viscometer,scanning electron microscope（SEM）,energy dispersive spectrometer（EDS）and optical microscope（OM）were employed.The results are summarized as follows:The field dependence of viscosity of suspensions composed of both Ni and ZrO₂ particles was obtained.With increasing PVP content,the viscosity increased,then reached the maximum value at 1.8 wt.%,and finally decreased.However,the relative viscosity decreased as PVP content increased in the presence of magnetic field.Moreover,the viscosity increased rapidly as the solid content increased.When a magnetic field was applied on the suspension,the viscosity of suspension increased dramatically.Furthermore,the viscosity increased as a function of field strength and Ni content,which can be attributed mainly to the magnetization of Ni particles.Ni particles aggregated to form chain-like clusters under magnetic fields.The size and number of clusters increased as field strength and Ni content increased,thus the viscosity of suspension increased.A composition gradient of Ni was formed along the field direction in ZrO₂/Ni FGMs prepared under gradient magnetic fields.As the solid content in suspension increased,the density of FGMs increased,but the composition gradient decreased.With increasing field gradient from 0.5 to 3.0 T/m,composition gradient of Ni increased,but the microhardness along thickness direction decreased.Moreover,the migration of magnetic particles under gradient magnetic fields was not an instantaneous process but a time-dependent one.The composition gradient increased with the prolongation of holding time,and remained nearly unchanged after 15 s.However,the composition gradient decreased as Ni concentration increased from 4 to 15 wt.%,which can be attributed mainly to the formation of Ni clusters.A schematic model was proposed to explain the distribution of particles in gradient magnetic fields.Through the two-dimensional Monte Carlo simulations,the distributions of particles under uniform and gradient magnetic fields in the suspension comprised of both magnetic particles （MPs）and nonmagnetic particles（NPs）were studied.Chain-like MP clusters were formed along the field direction.With the prolongation of computing time,clusters grew gradually, and finally kept stable.The size of clusters increased with increasing interaction between MPs. However,the distribution of particles remained unchanged as the field strength varied.As the concentration of MPs increased,the distance between MPs decreased and the interaction increased,resulting in the growth of clusters.A gradient distribution of MPs was formed along the field gradient direction under gradient magnetic fields.The composition gradient increased with increasing the field gradient,however,decreased as the interaction between MPs increased,accompanied by the formation of chain-like MP clusters.Moreover,NPs were found to hinder the translation of MPs along the field direction both in uniform and gradient magnetic field.As the NP concentration increased,the translation of MPs became difficult.更多还原