【作者】 马飞；

【导师】 李建功；

【作者基本信息】 兰州大学 ， 材料物理与化学， 2008， 博士

【摘要】 线状纳米和亚微米磁性金属颗粒,具有高的饱和磁化强度和强的形状各向异性,其磁性和微波特性与各向同性的磁性材料有所不同。传统的制备线状磁性金属的方法一般借助模板和磁场诱导。而这些方法普遍存在产率低,产物后处理困难,实验装置复杂等诸多缺点。本论文从制备线状的磁性金属颗粒出发,分别采用两步还原法制备了线状Ni纳米颗粒,采用常温液相还原法制备了海胆状、球形Ni纳米颗粒,采用水热法制备了亚微米线状Co颗粒。分析了实验条件对最终产物形貌、尺寸的影响,并在此基础上探讨了颗粒形貌的控制机理。研究了它们的微结构、磁性与微波特性,并探讨了结构与性能之间的关系。（1）两步法合成线状Ni纳米颗粒:采用液相法制备了棒状的Ni（N₂H₄）₃Cl₂配合物,考察了反应温度对棒状配合物形貌的影响,发现反应温度越高得到的配合物的长径比越大。然后,以该配合物为先驱体利用H₂还原,成功制备了线状Ni纳米颗粒,Ni纳米线的直径为50-100nm,长度达到微米量级。（2）常温液相法制备海胆状、球形Ni纳米颗粒:在乙醇溶液内通过改变水合肼和氢氧化钠滴加次序和反应温度,分别制备了海胆状和球形Ni纳米颗粒,实现了对产物的形状控制。海胆状颗粒由许多锥形和棒状的结构单元组成,这些锥形和棒状单元的直径为30-50 nm,长度达几百个纳米,锥形单元的长径比随反应温度的提高而提高。液相法制得的球形颗粒尺寸分布较窄,在70℃和80℃制备温度下分别为50nm和30nm。（3）水热法制备亚微米线状Co团簇颗粒:成功制备了亚微米线状Co团簇颗粒,线状单元的长度在5-6μm,直径在200-500nm。XRD谱显示制得的样品为六方密堆（HCP）相。（4）对纳米Ni颗粒和亚微米Co颗粒的饱和磁化强度、矫顽力、剩磁比进行了研究。Ni纳米颗粒和Co微米颗粒饱和磁化强度值均较块体低,导致饱和磁化强度降低可能的原因是金属颗粒表面的氧化层和表面、界面磁矩的无序化。Ni纳米颗粒的矫顽力和剩磁比受颗粒形貌和尺寸的影响:海胆状Ni纳米颗粒因具有较强的形状各向异性和较小的颗粒尺寸,其矫顽力和剩磁比均最大;球形Ni颗粒因为具有较强的小尺寸效应和表面效应,其矫顽力和剩磁比次之。线状颗粒因尺寸较大,颗粒内部出现多畴,其反磁化过程由畴壁的移动决定,因此具有最小的矫顽力和剩磁比。亚微米线状Co颗粒的矫顽力远高于块体和相同尺寸球形颗粒的值。其可能原因是线状颗粒具有较强的形状各向异性,使颗粒内部的磁矩取向被钉扎在沿线的轴向上。（5）对海胆状、球形Ni纳米粉体/石蜡复合材料,亚微米线状Co/石蜡复合材料的复数介电常数进行了研究。在0.1-18GHz频率范围内,复数介电常数的实部和虚部数值均较低,说明复合体系均具有较高的电阻率,有利于阻抗匹配和微波吸收。不同海胆状Ni粉体体积浓度（11.5%,18.2%,23.3%）复数介电常数的值均基本为常数,并随着Ni粉体体积浓度的增加而增加。（6）对海胆状、球形Ni纳米粉体/石蜡复合材料,亚微米线状Co/石蜡复合材料的复数磁导率进行了研究。海胆状Ni纳米粉体复合材料磁导率虚部的共振峰出现在4.3-4.7GHz频率范围,共振峰峰值随Ni体积浓度的增加而增加,共振峰频率随Ni体积浓度的增加向低频移动;低体积浓度（11.5%）海胆状Ni/石蜡复合样品中出现两个共振峰,第一个共振峰可能为自然共振峰,第二个共振峰则可能为交换共振峰。球形Ni纳米粉体复合材料共振峰出现在2.8GHz,共振峰位相比海胆状样品较低,这归因于海胆状Ni颗粒具有较大的形状各向异性。亚微米线状Co/石蜡复合体系的磁导率虚部在4.4GHz左右出现一个宽化的共振峰,该共振峰为复合材料内自然共振和涡流效应共同作用的结果。（7）对海胆状和球形Ni纳米粉体复合材料的微波吸收特性进行了研究。复合材料在较宽的频率范围内有良好的微波吸收性能（RL＜-10 dB）。随着样品厚度的增加,吸收峰位置向低频移动,且峰值和吸收峰的数目也随样品厚度而变化。海胆状比球形Ni复合样品的吸收带宽和吸收峰值均更大,说明其在0.1-18GHz频率范围内具有更加优异的电磁波吸收性能。对亚微米线状Co颗粒/石蜡复合样品的微波吸收性能进行了研究。发现在样品厚度为6-9mm时反射系数RL在8.0-15.0GHz频率范围内小于-10dB。微米级金属Co颗粒内存在涡流效应,相比纳米级的Ni颗粒其吸波性能较差。（8）采用磁各向异性场随机取向模型对海胆状和球形Ni纳米粉体复合材料的复数磁导率进行了理论计算。发现对于海胆状和球形复合样品,磁各向异性场分别取0.7kOe和0.187kOe,饱和磁化强度取实验测得值4.898kGs时得到的理论计算图谱和实验所得图谱基本一致。更多还原

【Abstract】 One-dimentional ferromagnetic metal nanoparticles or sub-micron particles have high saturation magnetisation and large shape anisotropy. Their magnetic and microwave properties are different from isotropic material. Traditional methods to synthesize anisotropic ferromagnetic metal nanoparticles usually depend on templates or the inducement of magnetic field. These methods have many disadvantages such as low yield, post-processing difficulties, complexity of experimental devices and so on. In this work, a couple of chemical methods were reported to synthesize Ni and Co nanoparticles or sub-micron particles. Wire-like Ni nanoparticles were successfully obtained via a two-step method. Sea-urchin-like, sphere-like and chain-like Ni nanoparticles were synthesized by wet-chemical methods. Wire-like Co sub-micron particles were synthesized by hydrothermal methods. The influence on the morphology and size of the products by the reaction condition was analyzed. The mechanism of the morphology control was discussed. Furthermore, their microstructure, magnetism and microwave properties were studied and the relationship between microstructure and magnetic properties were discussed.（1）Ni nanowires were synthesized by a two-step method. First, rod-like coordination compound Ni（N₂H₄）₃Cl₂ was synthesized in alcoholic solution. The aspect ratio of the rods increased with the increase of reaction temperature. Second, Ni nanowires with diameters of 50-200 nm and lengths up to micrometers have been synthesized by using hydrogen as the reducing reagent based on the rod-like coordination compound Ni（N₂H₄）₃Cl₂ as the precursor.（2）Wet-chemical methods to synthesize sea-urchin-like and spherical Ni nanoparticles were studied. In alcoholic solution, the morphology of final products was controlled by varying the adding sequence of H₂H₄·H₂O, NiCl₂ and NaOH solutions. The sea-urchin-like Ni particles were composed of rods and cones with the diameter of 30-50 nm and length up to hundreds of nanometers. The aspect ratio of the rods and cones increased with the increase of reaction temperature. The size of spherical Ni nanoparticles varies in little range with the mean diameter of 50 nm and 30 nm for the reaction temperature at 70℃and 80℃respectively.（3） Synthesis of sub-micron wire-like Co clusters by hydrothermal methods was studied. The products were hexagonal close packed （HCP） structure. The clusters were composed of sub-micron wires with diameter of 200-500 nm and length of 5-6μm.（4） The saturation magnetisation, coercivity and remanence ratio （M_r/M_s） of the Ni nanoparticles and sub-micron Co particles were investigated. The saturation magnetisation for all the particles decreased compared to the bulk state because of the oxidation layer of the metal and the magnetic disorder in the surface or interface of particles. The coercivity and remanence ration of the Ni nanoparticles were influenced by the particle morphology and size. Sea-urchin-like Ni nanoparticles have the largest value duo to the contribution of small size effect and the large shape anisotropy, spherical Ni nanoparticles have large vale duo to their size close to the critical size of magnetic domain. The wire-like Ni nanoparticles have the lowest value duo to the particle size far beyond the magnetic single domain size, which means the magnetic reversal mechanism is controlled by the domain wall motion. The coercivity and remanence ratio of wire-like sub-micron Co particles is much larger than the bulk Co and spherical sub-micron Co particles. This is because the magnetic moment was pined up along the axis of the wires resulted from the large shape anisotropy of the wire-like structure.（5） The complex permittivity was investigated respectively for sea-urchin-like and sphere-like Ni nanoparticle-paraffin wax composites and Co sub-micron particle-paraffin wax composites. For all the composites the complex permittivity shows almost constant and low value indicating high resistivity of the composites which is needed for impedance matching condition. The complex permittivity for different volume fraction of sea-urchin-like Ni nanoparticles was constant and increased with the volume fraction.（6） The complex permeability was investigated respectively for sea-urchin-like, sphere-like Ni nanoparticle-paraffin wax composites and Co sub-micron particle-paraffin wax composites. The resonance peak for sea-urchin-like particles appears at 4.3-4.7 GHz and shifts to lower frequency with the increasing volume fraction of the Ni nanoparticles. Two resonance peaks appear for the sample with the Ni volume fraction of 11.5%. The first one attributes to the natural resonance and the second one is duo to the exchange resonance. The resonance peak of the spherical Ni particle appears at 2.8 GHz which is lower than the sea-urchin-like particles duo to absence of shape anisotropy. The resonance peak of the imaginary part of complex permeability for Co sub-micron particle-paraffin wax composite appears at 4.4 GHz which attributes to both the natural resonance and eddy current effect.（7） The microwave absorbing properties of the sea-urchin-like and sphere-like Ni nanoparticle composites were investigated. The reflection loss （RL） of the composites is less than -10 dB in a broad frequency range indicating excellent microwave absorbing properties. The RL peak shifts to low frequency with the increase of the composite thickness and the peak value and number also change with the composite thickness. The microwave absorbing properties of sub-micron Co composites were investigated. The numerical simulations show that the reflection loss values of the composites are less than -10 dB in the 8.0-15.0 GHz frequency range with the composite thickness of 6-9 mm. The microwave absorbing efficiency of sub-micron Co composites was lower compared to the Ni nanoparticle composites because of eddy current effect.（8） The frequency dependent permeability for the sea-urchin-like and sphere-like Ni nanoparticles has been calculated in the model with the random distribution of anisotropic magnetic field taken into account. When the saturation magnetisation is 4.898 kGs and the anisotropic field is 0.700 kOe for sea-urchin-like nanoparticles and 0.187 kOe for spherical nanoparticles, the calculated complex permeability is consistent with experimental values.更多还原