Synthesis Of BaFe₁₂O₁₉ Nano-powders Obtained By Glycinate Combustion Method And Their Properties

Microwave absorbing materials have been widely in stealth technology and in electro-magnetic compatibility (EMC) technology, and they are researching hotspot in these technology field. In most of absorbers, the ferrite is the main component because of good performance and low cost. But the defects of large crystal size and high density are not benefit for the preparation of light absorber. In response to the need for high performance absorber, in this paper, the composite-coating absorbers with hollow cenosphere as nuclear are prepared by sol-gel glycinate combustion technology.This thesis presents a new technology to synthesize barium ferrite based on sol-gel method by using the glycine as the chelator. 1. The barium ferrites have been successfully synthesized by using the glycine as the chelator. Different factors and treatment were selected to obtain better magnetic properties.(1) The optimal stoichiometry of metal ion (Fe/Ba) is 11-12. Antiferromagnetic BaFe2O4 can be found in the sample with less iron, the coercive force of barium ferrite are enhanced, which can be explained by the pinning effect of BaFe2O4.(2) The optimal ratio of glycine matter (glycine:metal ions) is 3:1. When the quantity of organic material becomes less, the metal ions were too agglomerated in the chain of glycine to form the pure barium ferrite; On the contrary, the metal ions can be dispersed in the chain of organic matter when more organic materials were used in the experimental process, thus pure barium ferrite can only be obtained after higher temperature treatment.(3) The aggregation of the particles can be significantly decreased by adding glycol to the precursor solution. And particles with smaller grain size could be finally obtained.(4) By pre-calcining the precurser at 500℃before high-temperature calcination, the formation ofα-Fe2O3 phase can be avoided, and the crystallization temperature can be lowered.(5) The optimal calcined temperature is 800℃. The resultant magnetic parameters of barium ferrite is Hc=4960.3Oe,Ms=45.45emu/g, and the grain size is about 50~70nm; When the calcined temperature is 900℃, the grain size is above 200nm, and the coercive force decrease.2. Thin films of barium hexaferrite are prepared on fly ash cenpsphere particles by sol-gel glycinate combustion method.(1) Pretreatment process is very important in the preparation of the BaFe12O19-coating hollow glass microspheres. The research showed that coupled microspheres had an enhanced of the combination with gel, resulting in more active points on the surface of micro-spheres.(2) Repetitious coatings or adding some ultrafine BaFe12O19 powder to the precursor solution are the two ways to improve the thickness of film. The saturation magnetization and the remanent magnetization of Barium hexaferrite coated cenospheres are smaller than those of ultrafine BaFe12O19 powder. The sol particles of M-BaFe12O19 are absorbed on cenospheres to form films by chemisorption. The formation mechanism is hetergeneous. Adding ultrafine BaFe12O19 powder into the sol can accelerate nucleation, and then thick films can be prepared.(3) The real part and imaginary part in the complex dielectric permittivity of the BaFe12O19-coated cenospheres is smaller than those of BaFe12O19 ultrafine powder. The microwave absorbance curves of BaFe12O19-coated cenospheres and BaFe12O19 ultrafine powder are similar, which both have the largest value at 12.6GHz.