【作者】 胡大为；

【导师】 王燕民；

【作者基本信息】 华南理工大学 ， 材料学， 2010， 博士

【摘要】 四氧化三铁(Fe3O4)纳米粒子形貌对其性质和应用具有重要的影响。本论文采用弱外磁场(磁场强度小于500Gs)辅助氧化共沉淀法合成制备具有不同形貌的纳米Fe3O4颗粒;通过表面改性的方法将制备得到的Fe3O4纳米粒子分散于基载油中制备磁流体,并通过磁流体稳定性测试仪分析最佳的制备工艺,应用流变仪研究制备得到的磁流体的流变学特征;采用仿真和数值计算方法设计、优化磁流体传动装置,研究磁流体的传动性能及其与磁性粒子形貌的关系。采用自行设计的弱外磁场辅助氧化共沉淀法合成纳米Fe3O4颗粒的装置,以NaOH、FeCl2和H2O2为原料,通过改变反应物浓度和辅助磁场大小制备纳米Fe3O4颗粒;经SEM、XRD、FTIR、VSM等技术检测分析表明,该方法可合成制备具有无规则、正八面体、六方片状等不同形貌的纳米Fe3O4颗粒;弱外磁场在合成过程中能够加速诱导α-FeOOH向Fe3O4的相转变,缩短反应的时间,提高产物的纯度;弱外磁场的引入,并不会改变产物的晶格点阵形状,只对产物晶体的形貌产生作用;不同形貌的Fe3O4颗粒具有不同的磁性能,由于六方片状Fe3O4纳米晶形状的各向异性和粒度大小的影响,其饱和磁化强度小于正八面体型Fe3O4纳米晶的饱和磁化强度。论文通过磁流体稳定性理论分析计算,确定了磁流体的稳定分散条件;并通过设计采用L-C振荡电路原理的磁流体稳定性测试仪,确定制备稳定磁流体的条件。通过采用流变仪测试和研究利用不同形貌磁性颗粒制备得到的磁流体的磁流变学性能。结果表明:在未加外磁场的情况下,其流变学特性表现为Newtonian流体特性;在外加不同强度的水平方向磁场下,磁流体的粘度和剪切应力基本上不变,表现出Newtonian流体特性;外加不同强度的竖直方向磁场下,磁流体的粘度和剪切应力随外加磁场强度的增大而增大,表现出Bingham流体特性。正八面体形貌纳米Fe3O4颗粒由于其特殊的形貌,其制备得到的磁流体的粘度、剪切应力随外加磁场强度的变化较大。论文通过圆盘式磁流体传动器件的工作原理建立了数学计算模型,根据磁流体传动的基本原理,设计了用于磁流体传动的实验装置,并利用仿真和数值分析的方法对传动装置的关键部位材料选择、磁路进行了优化设计。依据设计的磁流体传动装置,建立不同传动盘间隙下的有限元分析模型,分析得出了不同间隙下传递转矩和电流强度的关系。通过建立磁流体传动性能测试平台,对磁流体传递扭矩过程中的各主要因素进行了研究。结果表明:在传动盘之间间隙一定时,磁流体传递扭矩的大小和磁感应强度的大小有关,磁流体传递的扭矩在磁性粒子未达到其饱和磁化强度时,传递扭矩大小随磁感应强度增大而迅速增大,但随着磁感应强度的进一步加大,磁性粒子逐步达到其饱和磁化强度,磁流体传递扭矩大小的增长减缓,最后几乎不再增大;传动盘之间的间隙对磁流体传递扭矩的大小影响较大,间隙越大,传递的扭矩越小;传动盘之间的转速差对磁流体传递扭矩的大小影响较小,在低转速差下传递的扭矩随转速差的增加而有所增加,但超过一定的转速差后,由于磁流体的剪切稀化效应,传递的扭矩将有所减小;磁流体中磁性纳米粒子的形貌对磁流体传递扭矩的大小有一定的影响,正八面体形貌的磁性纳米粒子相对于无规则和六方片状形貌的磁性纳米粒子,其磁流体能够传递更大的扭矩。更多还原

【Abstract】 The morphology of nano-particles of magnetite (Fe3O4) has an important impact on the relative properties and application in engineering due to the specifications. In this dissertation, a weakly magnetic field (<500Gs) assisted oxidative co-precipitation method was proposed to synthesize the nano-particles of Fe3O4 with different morphologies. The as-synthesized nano-particles, which were modified via a surface modification method, could be dispersed in a base oil to prepare the magnetic fluids. A magnetic fluid stability testing device was manufactured to evaluate the stability of the magnetic fluids with the nano-particles using different chemicals. The rheological characteristics of the magnetic fluids with and without an external magnetic field were also investigated. The effect of the morphology of the nano-particles on the transmission properties of magnetic fluids was analyzed by a designed magnetic fluid transmission device, which was optimized via the simulation software and the numerical analysis technique.The nano-particles of Fe3O4 with different morphologies (i.e., irregular, octahedron and six-party flake) were synthesized using NaOH, FeCl2 and H2O2 as raw materials in a designed oxidation co-precipitation synthesis device with weakly magnetic field assistance. The synthesis was carried out at various reactant concentrations and magnetic inductions. The analysis by scanning electron microscopy (SEM), X-ray diffraction (XRD), fourier transform infrared spectrometer (FTIR) and vibrating sample magnetometer (VSM) shows that the morphologies of nano-particles of Fe3O4 synthesized appear irregular, octahedron and six-party flake as well. The application of weakly magnetic field to the synthesis could accelerate the phase transformation fromα-FeOOH to Fe3O4, shorten the reaction time and improve the purity of the products. The magnetic field could not have an effect on the lattice shape of the product, but could affect the morphology of the crystalline grain. The nano-particles with different morphologies possessed different magnetic properties due to the shape anisotropy and the crystalline (or particle) size. The saturation magnetization of the nano-particles with the shape of six-party flake was lower than that of the nano-particles with the shape of octahedron.In this dissertation, the stability of the magnetic fluid and the particle dispersion in the magnetic fluid were analyzed via the theoretical calculation. A test device for the magnetic fluid stability, which was designed and manufactured based on the principle of L-C oscillator circuit, was used to determine the parameters (i.e., surfactant concentration and assistant surfactant concentration) for the preparation of the magnetic fluids. The magneto-rheological properties of the magnetic fluids with the nano-particles with different morphologies were examined by a rheometer in the absence and presence of a vertical or horizontal magnetic field. The results show that the magneto-rheological properties in the absence of a magnetic field follow a Newtonian fluid behavior. The viscosity of magnetic fluid at various shear rates in the presence of a horizontal magnetic field could not be varied. The viscosity of the magnetic fluid in a vertical magnetic field increased with increasing the magnetic induction. The rheologic behaviour of the magnetic fluid became a Bingham plastic fluid in the presence of a vertical magnetic induction. The viscosity of the magnetic fluids containing the nano-particles with the octahedron shape changed with the applied magnetic field.A mathematical model for the magnetic fluid transmission was established according to the working principle of the disk-type magnetic fluid transmission device. The experimental device for the magnetic fluid transmission was designed. The materials used and the magnetic circuit in the device were determined by the simulation method and the numerical analysis technique. The models at different disk drive spaces in the device were established via the finite element method (FEM). The relation between the transmission torque and the current intensity at different disk spaces was analyzed. The main parameters (i.e., current intensity, disk space, speed difference and particle morphology) in the measurement of the magnetic fluid transfer torque were investigated in the magnetic fluid transmission device. The results show that the transfer torque was correlated to the magnetic field intensity in a certain gap between the drive plates when the magnetization of magnetic particles does not reach a saturation value. The transmission torque increased with increasing the magnetic field intensity. However, the magnetic particles gradually reached the saturation magnetization when the magnetic field intensity was further increased, leading to the constant transmission torque. The gap between the drive disks had an impact on the transmission torque. The greater gap could give the smaller transmission torque. The speed difference between the transmission disks had a little effect on the transmission torque at a lower speed. The transmission torque increased with increasing the speed difference. However, the transferred torque would decrease when the speed difference exceeded a certain limit due to the shear-thinning effect of the magnetic fluid. The morphology of the magnetic particles in the magnetic fluid could affect the transfer torque. The nano-particles of Fe3O4 with octahedral morphology could transfer a greater torque rather than the nano-particles with irregular or six-party flake-like morphologies.更多还原