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

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 (Y₃Fe₅O₁₂), Barium Ferrite (BaFe₁₂O₁₉), and Nickel Aluminate (NiAl₂O₄) 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 Y₂O₃/Fe₃O₄ diffusion couple showed a unidirectional diffusion of Fe species into Y₂O₃, forming a different sequence of intermediate phases.