Synthesis of Barium-Ferrite Ultrafine Particles by a Hydrothermal Method

点击翻译按钮获取中文摘要

Barium-ferrite ultrafine particles were synthesized from two kinds of starting materials; either ferric nitrate or goethite and barium hydroxide, by means of a hydrothermal method. The effects of alkali (NaOH) concentration, stirring speed in the autoclave, hydrothermal temperature and time on the particle shape and size were studied, and the mechanism of particle formation was deduced. The reaction conditions were clarified for which hexagonal plate-like particles of less than 0.1mum in size, suitable for coated perpendicular magnetic recording media, are synthesized.In the hydrothermal synthesis from an aqueous suspension containing ferric nitrate and barium hydroxide, the nucleation and the particle growth occur simultaneously in the early stage of the reaction. After about lh has passed, the particle growth due to Ostwald ripening becomes dominating. As the temperature and hence the pressure in the autoclave rises, the rate of particle growth increases and particles grow up to larg … More e sizes. The particle size decreases with increasing molar ratio (R) defined by (OH^-]/[NO^-_] and increasing stirring speed (n). When R <greater than or equal> 6 and n <greater than or equal> 3OOrpm at 300ﾟC, hexagonal plate-like Ba-ferrite parti- cles of less than 0.1mum in size and 0.02mum in thickness can be producedHexagonal plate-like Ba-ferrite particles of submicron size are also prepared when goethite is employed as a reactant. At 300ﾟC, Ba-ferrite was formed after a short hydrothermal treatment. As the treatment proceeded, the particle size decreased. This may be due to the fact that the crystallinity is enhanced by the treatment. The particle size decreases with rising temperature and increasing stirring speed and alkali concentration.The uncertainty of size distribution of goethite particles is apt to cause a wide size distribution of prepared Ba-ferrite particles. Therefore, the operating conditions in which extremely fine goethite particles of a narrow size distribution are produced, were searched for. By shirting the oxygen concentration in the feed stream fed to the bubble column with a draft tube to a lower level at a low conversion, the size distribution of the prepared particles becomes narrow (geometric standard deviation less than 1.18). Less