| With the development and extensive application of electronic systems, the problems of electromagnetic interference and radiation have become more serious, which resulted in a growing and intense interest in electromagnetic-absorber technology. Among the various materials with the capability of absorbing electromagnetic radiation, magnetic metal powder exhibit excellent properties of microwave absorption. However, the conventional absorptive materials such as metal powder and ferrites are quite heavy, which restricts their use in application requiring lightweight. As one of ways to overcome this problem, microwave absorptive materials with low density have been developed. Magnetic hollow powder is one of materials with low density used as microwave absorber. Among the methods of fabricate hollow spheres, the template-synthetic route is often employed. In our group, the hollow nickel spheres have been prepared by the autocatalytic reduction method and colloidal particle (Ni(OH)2) was used as the autocatalytic agent and a sacrificial core in the reactions. Compared with traditional template-synthetic route, the autocatalytic reduction method is simple and low-cost, which causes that it has potential application in preparing other materials with hollow structure in the future.The main purpose of this thesis is the preparation and improvement of electromagnetic-absorber with low density. Magnetic hollow powders with different component and particle size have been prepared and characterized. And their magnetic, microwave and optical properties were investigated. By numeric computation, the application of magnetic hollow powders on microwave and solar thermal absorber was predicted. The main content and conclusions of this thesis are described as follows:(1) Using the autocatalytic reduction method, nickel, Ni-Co and Ni-Fe3O4 composite hollow spheres (HSs) were prepared. During the preparation process, the increasing concentration of NaOH reduces the particle size of final products. For the Ni-Co or Ni-Fe3O4 composite spheres, the content of Co or Fe3O4 increases with an increase of concentration of Co2+ or Fe2+ in the solution. (2) Ni-Co double-layered HSs were synthesized by the electroless plating technology. The phosphorus content in the cobalt coatings increases with the increasing reductant concentration in the plating solution. As the concentration of complexing agent increases during the preparation process, the element of coatings varies from flake to virgulate particle. During electroless plating process, the increasing load reduces the mass of cobalt in the double-layered HSs. And the decrease of particle size of nickel hollow spheres improves the catalytic activity and enhances the cobalt mass during the deposition process.(3) The influence of morphology and component content on the magnetic properties of magnetic HSs was investigated. When particle size of Ni HSs becomes smaller, the magnetic properties, viz. saturation magnetization (Ms), remanence (Mr) and coercivity (Hc), decreases. And the reducing temperature results in a widening of the hysteresis loop and an increase of magnetic properties. The grain size grows bigger after heat treatment process, which enhances the magnetic properties. Because of the strong anisotropy of cobalt, the magnetic properties of Co-coated Ni HSs increase with the increasing Co content. And the increasing phosphorus content in the cobalt coatings reduces the grain size and results in the decrease of coercivity. The structure of coatings varies from flake to virgulate particle cause the reducing magnetic properties. The magnetic properties of Ni-Co composite HSs increase with Co content. When particle size becomes smaller, the coercivity and remanence decrease. An increase of magnetite content in Ni-Fe3O4 composite HSs results in the increase of magnetic properties. And the reducing particle size leads to the decrease of coercivity of Ni-Fe3O4 composite HSs.(4) For spheres-wax composite with volume ratio of 1:1, when the particle size of Ni HSs decreases, the complex permittivity increases and permeability becomes smaller. A resonance peak induced by orientation polarization appears around 15GHz on the dielectric spectra and a natural resonance peak presents around 4.8GHz on the permeability spectrum.(5) Coating Co film on the surface of Ni HSs enhances the permeability and cause the decrease of permittivity because of the net-structure of coatings. Both permittivity and permeability of Ni-Co double-layered HSs increase with an increase of phosphorus content. When the shape of the element, that forms the cobalt coating, varies from flake to virgulate particle, the real part of permittivity decreases while imaginary part increase. And both real and imaginary part of permeability decrease because of the reducing cobalt content. For Ni-Co double-layered HSs, the minimal RL can be obtained for the product with lower cobalt content, higher phosphorus content and virgulate particle structure.(6) Both permittivity and permeability of Ni-Co composite HSs increase with the cobalt content. However, after the cobalt content is bigger than that of nickel, the electromagnetic parameters descend. When the particle size becomes smaller, permittivity increases while permeability decreases. The minimal RL (-45.3dB) was predicted at 5.3GHz for the composite HSs with the Ni:Co of 6:1.(7) For Ni-Fe3O4 HSs-wax composite with the ratio of 4:6, with the increase of magnetite content, the permittivity decrease at the whole measurement range and the permeability becomes bigger at higher frequency. The reducing particle size results in the increase of permittivity. But the influence of particle size on permeability is different in different frequency range. The permeability of Ni-Fe3O4 HSs has an increasing trend at low frequency and becomes smaller at higher frequency. For composite layer containing Ni-Fe3O4 HSs, a minimal RL (-42.2 dB) was observed at 14.8 GHz with a thickness of 1.5 mm.(8) The influence of particle size on optical properties of Ni HSs was investigated. The optical absorption intensity increases obviously in ultraviolet-near infrared region with a decrease of particle size. While in infrared region, Ni hollow spheres almost have no absorption. After the heat treatment process, the grain size of samples becomes bigger and the absorptance decreases in UV-Vis-NIR region. For smaller particles, the absorption peak in ultraviolet range moves from 375 to 440nm because of the increase of grain size. Solar absorber coatings containing Ni HSs have been prepared on aluminum plate substrate by dip-coating method. The absorptance of coatings was about 0.98.In this paper, a cobalt film was coated on Ni HSs by electroless plating technology, which enhances the magnetic and microwave absorption properties of Ni HSs. By the autocatalytic reduction method, the Ni-Co composite HSs with various particle sizes and component contents were prepared. And the power-wax composite layer exhibite excellent absorbing capability (RL=-45.3dB) at 5.3GHz. The Ni-Fe3O4 composite HSs with different particle size and component content were synthesized. And for a composite layer with the thickness of 1.5mm, a minimal RL (-42.2 dB) was observed at 14.8 GHz was predicted. |
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