Study On The Structure Control Of Ferrite / Surface Carbon Layer And Its Electromagnetic Characteristics

With rapid development of information technology, various microwave and millimeter wave components have been widely used, which also brings the increasing demands of techniques such as anti-electromagnetic interference, electromagnetic wave absorption and shielding. Therefore, it is of great interest to develop electromagnetic wave absorbing materials with broadband, high efficiency, light weight and multi-function.M-type hexaferrite is believed to be one of the common microwave absorbers due to its excellent electromagnetic characteristics in high frequency range, however its narrow absorbing band, relatively large density and insufficient impedance matching confine its further applications. To improve its performances further, the methods including element doping, making nano-materials, surface modification are supposed to be the good options.In this thesis, taking rare-earth doped hexaferrite nano powders as the starting materials, their particle surfaces is to be modified based on polymerization and further carbonization. The influences of the processing conditions on the formation of surface microstructures are to be investigated. Moreover, the effects of surface microstructures on the electromagnetic characteristics are clarified. Based on the above designed technical route, the following works are involved:1) Ferrite with polyaniline (PANI) surface layers have been fabricated by in-situ polymerization method by using lanthanum doping barium hexaferrite nano powders as the starting materials, which have prepared by sol-gel auto-combustion method. The influences of polymerization processing conditions and starting compositions on the micro-structures of polyaniline surface layers have been studied; 2) The surface carbonized layers on ferrite nano particles have developed based on the thermal carbonization of PANI. The relationships of the thermally carbonizing conditions including temperatures, atmospheres, holding times and ramp rates with the formation and micro-structures have been illustrated.3) The compositions and structures of surface carbonized layers on the electromagnetic wave characteristic parameters of ferrite nano powders have been investigated and the technique based on the design of surface layers to change the electromagnetic wave characteristic parameters has been discovered; 4) The facile one-step method to construct carbonized layers on ferrite nano particles has been explored based on glucose participated hydrothermal method and the influences of carbonized layers on the electromagnetic properties have been clarified.Based on the systematical experiments, characterizations and discussions, the mainly results and conclusions have been made as follows:1. Single phase lanthanum doped barium hexaferrite nano powders with average particle size of 130nm have been successfully synthesized via sol-gel auto-combustion method. The PANI layers on surfaces of ferrite nano particles have been constructed based on in-situ polymerization route and the thickness of PANI layers increased from 53nm to 133nm as the weight ratio of aniline monomer increased from 10:3 to 10:7.2. After thermal treatment of ferrite nano particles with PANI surface layers, the surface carbonized layers have formed and their microstructures greatly relied on the thermal treatment. As the temperature increased from 200℃ to 500℃, the PANI surface layers experience the phase conversion and final transformed to carbon layers. The optimized parameters for thermal carbonization were at 500℃ for 2 hours with ramp rate of 2℃/min in the vacuum conditions. By using ferrite nano powders with different surface PANI for carbonization, the thickness of carbonized surface layers could be controlled in the range of 50 to 120nm according to different PANI thickness.3. Ferrite nano powders with PANI surface layers approached a higher permittivity and the dielectric loss increased due to interfacial polarization. After thermal carbonization, the formation of carbonized layers on the surface of ferrite nano particles also made the dielectric loss increase and it achieved to the maximum as the thickness of carbonized layer was 50 nm.4. The carbonized layers on the surfaces of ferrite nano particles have been successfully constructed by a facile one-step method based on glucose participated hydrothermal technique. The thickness of carbonized layers were homogenous and it increased from 10nm to 50nm by increasing the glucose content. The exsitence of carbonized layers on the surface of ferrite nano particles made the dielectric loss increase due to the enhancement of interfacial polarizations.