Hydrothermal synthesis of ultrafine barium hexaferrite nanoparticles and the preparation of their stable suspensions

Abstract

The hydrothermal treatment of an appropriate suspension of Ba and Fe hydroxides in the presence of a large excess of OH(-) results in the formation of Ba hexaferrite at temperatures as low as 150 degrees C. This low formation temperature enables the synthesis of uniform, ultrafine Ba hexaferrite nanoparticles. These nanoparticles have a disc-like shape, approximately 10 nm wide, but only approximately 3 nm thick. When the temperature of the hydrothermal treatment is increased, large platelet Ba hexaferrite crystals appear as a consequence of secondary re-crystallization (Ostwald ripening). In this work, this undesired process of secondary re-crystallization has been evaluated. We show that the secondary re-crystallization can be totally suppressed with the use of an oleic acid surfactant. The addition of oleic acid enabled the synthesis of uniform, ultrafine nanoparticles at temperatures up to 240 degrees C. The nanoparticles were hydrophobic and could be suspended in nonpolar liquids to form relatively concentrated ferrofluids. Such stable suspensions of hexaferrite nanoparticles will be technologically important, especially as precursors for the preparation of new nanostructured materials, for example nanocomposites or nanostructured ceramic films.

One-step synthesis of barium hexaferrite nano-powders via microwave-assisted sol–gel auto-combustion

“…Probably, this isotropic characteristic in the shape of the product particles is due to the rapid simultaneous homogenous nucleation in the uniform temperature field and the limited secondary crystal growth with respect to the very short-lived auto-combustion process. 16 The magnetization measurements were carried out at room temperature with the obtained powders pre-dispersed in alcohol. As shown in Fig.…”

“…4, the as-synthesized BaM nano-powders show a magnetization loop characterized by rapidly reaching saturation at an external magnetic field of as low as 1000 Oe and a saturation magnetization of about 64.1 emu/g, which is still much lower than the theoretical value of BaM single crystal about 72 emu/g, but a little higher than or similar to the reported results in literatures. [6][7][8][9]16 Several theories, including surface effects, spin-canting phenomenon, purity and particle size effects, have been proposed to account for the relatively low magnetization of nano-particles. 17 Since barium hexaferrite demonstrates a high uniaxial anisotropic field of 3.3 × 10 5 J/m 3 , the critical volume of the nano-particles could be as small as approximately 300 nm 3 , much smaller than for the soft magnetic spinel magnetite (K = 1.4 × 10 4 J/m 3 ), 18 which is approximately 7200 nm 3 .…”

Surface analyses of barium hexaferrite particles for magnetic suspensions

“…The hydrothermal powders were prepared with the method described in Ref. [11]. For the hydrothermal synthesis, the barium and iron nitrates were dissolved in water and the hydroxides were precipitated with a large amount of sodium hydroxide.…”

Preparation of barium hexaferrite powders using oxidized steel scales waste

“…Commercial ceramic making or solid state reaction is a conventional way to synthesis barium hexaferrite powders using stoichiometric mixture of barium carbonate and iron oxide and firing at high temperature [4]. Other methods to prepare barium hexaferrite are through co-precipitation method [3,5], sol-gel method [6], and hydrothermal method [7]. Solid state reaction is chosen because of its low cost, no complex chemical involved and relatively has simple processes.…”