Effect of Microwave Heating on the Mass Transfer, Phase Formation, and Microstructural Transformations in the Y₂O₃–Al₂O₃ Diffusion Couple

Abstract

The phase composition, phase growth kinetics, and structures of diffusive zones formed under microwave heating (24 GHz) (MWH) and conventional heating (CH) in two-layer Al₂O₃–Y₂O₃ samples are studied by optical and scanning electron microscopy and electron microprobe analysis. Diffusive annealing was conducted at 1700 °C for 5 h in vacuum, the heating rate was 10 °C/min in all experiments. The diffusion couples were alumina layers, such as coarse-grained polycore or sintered Al₂O₃ - 5 vol.% ZrO₂ layers, and yttrium oxide layers, such as sintered coarse-grained samples or fine Y₂O₃ powder layers on the Al₂O₂ surface. It is shown that the phases formed during reactive diffusion do not uniquely correspond to the phase diagram, but depend on the initial structure of contacting layers and the type of heating. This is attributed to the contribution of kinetic factors to the competitive phase growth, namely, to the structural sensitiveness of diffusion coefficients whereby the diffusive phases grow and the stresses appearing when new phases form. It is found that MWH influences the competitive phase growth in the Al₂O₃–Y₂O₃ system, which consists in both the change in the phase composition of the diffusive zone compared to that formed under CH and the acceleration of phase growth. The maximum effect of the phase growth acceleration under MWH is observed for the YAG phase, which is 30 times as fast as that under CH. It is suggested that the structure of grain boundaries changes and, accordingly, their diffusive permeability increases under MWH. The accelerated GB diffusion under MWH promotes ( or may provide) the YAG phase growth in both oxides as a result of opposite directed diffusion fluxes of Al and Y ions along GBsUnder TH the YAG phase is formed only in Y₂O₃ oxide because of the unipolar diffusion of Al³⁺ ions to Y₂O₃. The reasonableness of the offered mechanism is confirmed by numerical evaluations.