Preparation of Acicular Ferrite Fine Particles Using an Emulsion Liquid Membrane System

Abstract

Listen

Translate article

Acicular ferrite fine particles (CoFe2O4 and NiFe2O4) were prepared by using an emulsion liquid membrane (ELM, water-in-oil-in-water (W/O/W) emulsion) system, consisting of Span 83 (sorbitan sesquioleate) as the surfactant and D2EHPA (bis(2-ethylhexyl)phosphoric acid) as the extractant (cation carrier). Co2+, Ni2+, and Fe2+ were extracted from the external water phase and stripped into the internal water phase to make precursor composite oxalate particles. The particles obtained in the ELM system were acicular, less than 1.5 μm in length, and were much smaller than those prepared in homogeneous aqueous solution. The molar composition of the particles, (M/Fe)p, (M = Co or Ni), prepared in the ELM system, was almost proportional to that of the feed external solution, (M/Fe)f. XRD, SIMS, and thermal analyses indicated that the particles obtained in the ELM system have a structure in which Co or Ni atoms are more likely to be located near the surface of the particles than in the core. Whereas in a homogeneous aqueous solution, for the Co−Fe system, more Fe atoms are located near the surface of the particles and, for the Ni−Fe system, more Fe atoms are located inside the particles. The relative transport rate into the internal water droplets (reaction area) and the precipitation rate for each metal ion are considered to be the principal factors causing the variation in composite structure. Precursor composite oxalate particles were calcined to give acicular ferrite fine particles. Using the ELM system, the transformation of the cubic solid solutions into a mixture of cobalt ferrite and hematite in the particles was retarded, as compared to that using the homogeneous system. The coercivity and saturation magnetization of resultant particles, calcined at various temperatures, were also investigated.