Effect of microwave heating on the mass transfer, phase formation, and microstructural transformations in the [Y.sub.2][O.sub.3]-[Al.sub.2][O.sub.3] diffusion couple

The phase composition, phase growth kinetics, and structures of diffusion zones formed under microwave heating (24 GHz) (MWH) and conventional heating (CH) in two-layer [Al.sub.2][O.sub.3]-[Y.sub.2][O.sub.3] samples are studied by optical and scanning electron microscopy and electron microprobe analysis. Diffusion annealing was conducted at 1700°C for 5 h in vacuum, the heating rate being 10°C/min in all experiments. The diffusion couples included alumina layers, such as coarse-grained polycore or sintered [Al.sub.2][O.sub.3]-5 vol. % Zr[O.sub.2] layers, and yttria layers, such as sintered coarse-grained samples or fine [Y.sub.2][O.sub.3] powder layers on the [Al.sub.2][O.sub.3] 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, particularly 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 [Al.sub.2][O.sub.3]-[Y.sub.2][O.sub.3] system, which involves both the change in the phase composition of the diffusion 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 permeability increases under MWH. The accelerated GB diffusion under MWH promotes the YAG phase growth in both oxides as a result of opposite diffusion flows of Al and Y ions along GBs. Under TH the YAG phase is formed only in [Y.sub.2][O.sub.3] oxide because of the unipolar diffusion of [Al.sup.3+] ions to [Y.sub.2][O.sub.3]. The validity of the proposed mechanism is confirmed by numerical evaluations. Keywords: microwave heating, reaction diffusion, microwave effect, alumina and yttria, diffusion couple, microstructure, yttrium aluminum garnet (YAG), perovskite (YAP), monoclinic phase (YAM).