Study On Preparation And Property Of Hexagonal Ferrite Hollow Microspheres

Ferrites are conventional wave absorbing materials, which have such advantages as strong absorption, simple techniques and cheap raw materials, but they are quite heavy and have difficulties in increasing the permeability in GHz region. Hollow ferrites particles are the alternative to conventional solid ferrites particles, which have lower density, larger specific surface area and stability. Moreover, it is expected electromagnetic wave can be reflected and absorbed again and again in the cavum of hollow particles which become its black hole. In addition, their electromagnetic parameters can be adjusted in a certain range by changing the thickness of the shell and the diameter of hollow microspheres, and the ability of absorbing microwave can be expected to improve. Wave absorbing materials of hollow structure are the research trend.In this thesis, the structure and basic characteristic of hexagonal ferrite absorbent were summarized, and popular methods to prepare ultrafine ferrite particles and hollow microspheres were introduced. Hexagonal ferrite hollow microspheres were prepared by flame powders spraying technique and flame suspending solution spraying technique. The phase component, microstructure and electromagnetic properties of the synthesized hollow microspheres were studied as functions of heat-treated techniques (like temperature, time, the speed of heating) in detail.W-type BaCo2Fe16O27 and M-type BaFe12O19 hexagonal ferrite hollow microspheres were fabricated by flame powders spraying technique using oxygen and acetylene as fuel. The method resulted in abroad particle size distribution (mainly from 4 m to 20 m ), and the density was about 2.50 g/cm3. The process of heat-treatment showed M-type ferrite was straightly formed from metal oxide without the transition of spinel phase. The formation process of W-type ferrite underwent from oxide of metal, the M-type transition phase to final W-phase. When the heating rate was 5 ℃/min, at 900 ℃ for 3 hrs and 1200 ℃ for 4 hrs, pure M-type and W-type ferrites were formed in airrespectively. With heat treatment temperature increasing, crystal structure was more intact, the saturation magnetization increased and the coercive force decreased. With heat treatment temperature increasing, the complex permittivity of W-type ferrite hollow microspheres decreased; ' firstly increased and then decreased, increased and there was an obvious resonance peak; the complex permittivity ?of M-type ferrite hollow microspheres increased; ' increased, " was basically unchangeable.W-type BaCo2Fe16O27 hexagonal ferrite hollow microspheres were also fabricated by flame suspending solution spraying technique. The results showed hollow microspheres had a narrow particle size distribution (mainly from 2 m to 10 m ), and the density was about 1.50 g/cm3. When the speed of heating was 1.5 /min and heat-treatment temperature was 1250 , pure W-phase ferrite was formed. In this condition, the saturation magnetization was 58 emu/g and the coercive force was 45 Oe.