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Synthesis, Magnetic and Microwave Properties of One-dimentional Ni and Co Particles

Author: MaFei
Tutor: LiJianGong
School: Lanzhou University
Course: Materials Physics and Chemistry
Keywords: Wire-like Sea-urchin-like Morphology control Nanoparticles Complex permittivity Complex permeability Microwave absorption
CLC: TB383.1
Type: PhD thesis
Year: 2008
Downloads: 213
Quote: 2
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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.

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