Anisothermal reaction synthesis of garnets, ferrites, and spinels in microwave field

Abstract

The experimental achievement of reacting two phases that are held at two different temperatures—the anisothermal reaction condition—is a radical innovation in materials chemistry. Details on the synthesis of Yttrium Iron Garnet (Y 3Fe 5O 12), Barium Ferrite (BaFe 12O 19), and Nickel Aluminate (NiAl 2O 4) in 1–10 min in a microwave field are provided. The starting precursor oxides were chosen such that they include a low and a high microwave absorbing phases. When these mixtures are exposed to a 2.45-GHz multimode microwave field the highly absorbing powder particles act as (micro)heat sources and the low absorbing powder particle act as (micro)heat sinks, and create the “anisothermal heat distribution,” a hitherto never attained phenomenon in materials science. A comparison study carried out comparing conventional reactions of the same phases suggested that the “anisothermal heating phenomenon” is also responsible for the very rapid reactions and product formations. Model experiments performed with an Y 2O 3/Fe 3O 4 diffusion couple showed a unidirectional diffusion of Fe species into Y 2O 3, forming a different sequence of intermediate phases.