Chemistry and structure of mixed magnesium ferric oxide aerogels
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S.A. Oliver Center for Electromagnetics Research, Northeastern University, Boston, Massachusetts 02115
G. Oliveri and G. Busca Istituto di Chimica, Facolta di Ingegneria, Universitd di Genova, P. le Kennedy, 16129 Genova, Italy (Received 20 August 1992; accepted 8 February 1993)
Fine particles of mixed oxides having the composition MgxFe2-2*O3_2x (x between 0 and 1.0) were prepared via the sol-gel supercritical drying route (aerogels). Samples were then postprocessed by heating in air at 773 K and 1073 K. The structure and chemistry of the samples were deduced through x-ray diffraction, thermal gravimetric and differential thermal analysis, infrared, and specular UV-visible measurements. Surface areas were measured by the BET method. The volume saturation magnetization and particle volume distribution of MgFe 2 O 4 samples were deduced from vibrating sample magnetometer measurements. The samples with x = 0 and x = 0.5 were well-crystallized a-¥e2O5 (hematite) and MgFe 2 O 4 (magnesioferrite), respectively. Samples with 0 < x < 0.5 consisted of disordered lacunar spinels containing an excess of Fe 3+ with respect to the spinel stoichiometry and a small amount of Fe 2+ . Samples heated above 773 K showed progressive segregation of a-Fe 2 O3. The sample with x = 0.66 was a mixture of a spinel phase and a hydrotalcite-like mixed Fe-Mg hydroxy compound containing methoxy and acetate ions. By heating in air, a monophasic Mg-excess spinel-type structure was obtained. The samples with 0.66 < x < 0.86 were mixtures of a spinel-type phase and another mixed Mg-Fe compound. When heated in air, the samples gave a mixture of MgO (probably containing Fe ions) and MgFe2O4. The saturation magnetization of MgFe2O4 samples is comparable to those previously reported, with the as-produced sample having a mean particle radius of 5.5 nm.
I. INTRODUCTION Fine powders are being used for a variety of applications, including catalysis, composite materials, and as starting materials for the production of high-quality bulk ceramics. Such applications require powders having good material properties, which may vary with the processing conditions used to produce the powder. Aerogels, powders obtained by the removal of the liquid phase of colloidal gels under supercritical conditions,1 may be useful for these applications. This report presents the first characterization of the material properties of magnesia iron-oxide aerogels. The purpose of this research is twofold. First, to demonstrate that fine powders of mixed oxides having good material properties can be produced through the solgel supercritical drying route; and second, to examine the solid-state characteristics of these aerogels over a broad range of compositions. The material system chosen for this initial study consists of samples having the formula MgxFe2_2^O3_2x, where x ranges between 0 and 1. Included in the system are the well-known oxide hematite ( a - F e 2 O 3 ) and the prototypic ferrimagnetic ferrite MgFe 2 O 4 . Fine powders of these materials 1418 http://journals.cambrid
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