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Flow Method for Rapidly Producing Barium Hexaferrite Particles in Supercritical Water
Author(s) -
Hakuta Yukiya,
Adschiri Tadafumi,
Suzuki Toshiyuki,
Chida Toshihiro,
Seino Kazuei,
Arai Kunio
Publication year - 1998
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1998.tb02643.x
Subject(s) - supercritical fluid , barium hydroxide , phase (matter) , barium , aqueous solution , materials science , stoichiometry , hydroxide , hydrothermal circulation , analytical chemistry (journal) , chemistry , inorganic chemistry , chemical engineering , chromatography , organic chemistry , engineering
We propose a flow method to produce barium hexaferrite (BaO6Fe 2 O 3 ) particles with hydrothermal crystallization in supercritical water. Aqueous iron(III) and barium nitrate solution at room temperature was pressurized to 30 MPa and then mixed with potassium hydroxide solution (OH:NO 3 = 4) at the same conditions to generate metal hydroxides. This mixture was then rapidly heated to 400°C by mixing with supercritical water and then fed into a tubular reactor. Residence time was ~1 min. The reaction was terminated by cooling at the exit of the reactor. The Ba:Fe mole ratio was varied over a range of 0.1‐2. When the Ba:Fe ratio was ~1/12, which is the stoichiometric ratio for BaO6Fe 2 O 3 , the main products were alpha‐Fe 2 O 3 . However, for the case of Ba:Fe > 0.5, fine particles of single‐phase BaO6Fe 2 O 3 were produced. Batch experiments (380°C, 30 MPa) at Ba:Fe = 0.5 in supercritical water at a reaction time of 10 min produced a mixture of alpha‐Fe 2 O 3 and BaO6Fe 2 O 3 . This product transformed to the equilibrium phase, BaO2Fe 2 O 3 , in 4 h as the reaction time increased, which suggests that the BaO6Fe 2 O 3 that formed in supercritical water with our proposed flow method under nonstoichiometric conditions was an intermediate but stable product. Furthermore, the nonstoichiometric and nonequilibrium (dynamic) conditions are important for producing single‐phase BaO6Fe 2 O 3 particles. The single‐phase particles are highly stable and can be produced continuously in a reaction time of <1 min.