Synthesis And Magnetic Properties Of Barium Ferrite By Saccharide Sol-gel Method

Barium ferrite has been widely used as permanent magnets, magnetic recording material, magnetic tapes and magnetic disks because of high magnetic anisotropy, great coercivity and excellent chemical stability. Previous studies show that their magnetic properties strongly depend on their microstructure. When the diameter of barium ferrite particle is close to the critical single-domain size, it possesses better magnetic properties. In order to obtain uniform-sized and homogeneous nanoparticles of barium ferrite, various techniques have been developed, such as chemical coprecipitation, sol-gel method, colloidal synthesis, glass crystallization, hydrothermal method, micro emulsion mediated process, organic resin method, aerosol synthesis method, and so on. One of the most important features of these methods is to obtain the nano-sized hexagonal ferrite at relatively low calcined temperature.This thesis presents a new technology to synthesize barium ferrite based on sol-gel method by using the saccharide (glucose, sucrose and starch) as the chelator.1. The barium ferrites have been successfully synthesized by using the glucose as the chelator. Different factors and treatment were selected to obtain better magnetic properties.(1) The optimal stoichiometry of metal ion (Fe/Ba) is 11.8. 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 organic matter (M/P) is 12.8:20. When the quantity of organic material becomes less, the metal ions were too agglomerated in the chain of organic matter 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 optimal reaction temperature is 900℃. The resultant magnetic parameters of barium ferrite is H_c=5300Oe, M_s=53emu/g, and the grain size is about 60～100nm; When the calcined temperature is 1100℃, the grain size is about 600～900nm, and the coercive force decrease due to the formation of multidomain.(4) The optimal holding time of heat treatment is three hours. Intermediate phases were produced when the holding time is less than three hours; More holding time will give an increase of the diameter and a decline of the coercivity.(5) Acid treatment. The coercive force of barium ferrite remains the same value, and 4.4% increases of saturation magnetization were found after the treatment.2. Pure barium ferrite can be prepared at 900℃by sol-gel method using the sucrose as the clelator. The magnetic parameters of barium ferrite is H_c=5310Oe, M_s=53.3emu/g, and the grain size is about 100²00nm; When the calcined temperature is 1100℃, the grain size is about 600-1500nm, the formation of bigger grains results in the decline of the coercive force of barium ferrite.3. Pure barium ferrite can be prepared at 900℃by sol-gel method using the starch as the clelator. The magnetic parameters of barium ferrite is H_c=5400Oe, M_s=54.9emu/g, and the grain size is about 150～300nm; When the calcined temperature is 1100℃, the grain size is about 2000～3000nm, the coercive force of barium ferrite is about 2000 Oe.4. The microstructure and magnetic properties of barium ferrite synthesized by three different organic materials are compared.(1) The particles size will increase and become flakily by using polysaccharides as delator.(2) Coercivity will decrease while the precursors were annealing at higher temperature, especially for polysaccharide series.(3) When the polysaccharide was used as the delator, better magnetic properties can be obtained at a lower temperature.