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We have developed a method for production of high-density MgAlO ceramics characterized by low dielectric losses. The method is based on sintering of ultrafine, high-purity powder materials in the electromagnetic field of intense microwave radiation. The initial nanodimensional magnesium-aluminate powder was synthesized by the sol-gel method and sintered in a gyrotron complex operated at a frequency of 24 GHz. The results of studying the conditions and regimes of microwave sintering of MgAlO ceramics are presented, along with its phase features, microstructural peculiarities, and mechanical and dielectric characteristics. The possibility of controlling the microstructure of the sintered material by varying the regime of RF heating is demonstrated. [ABSTRACT FROM AUTHOR]

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 AlO-YO 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 AlO-5 vol.% ZrO layers, and yttria layers, such as sintered coarse-grained samples or fine YO powder layers on the AlO 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 AlO-YO 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 YO oxide because of the unipolar diffusion of Al ions to YO. The validity of the proposed mechanism is confirmed by numerical evaluations. [ABSTRACT FROM AUTHOR]

The interdiffusion and microstructural evolution of the CrO-AlO (5 vol.% ZrO) diffusion couple are studied in the temperature range 1600-1800°C under microwave heating (24 Hz) and, for comparison, under traditional heating using electron microprobe analysis and microscopic analysis. It is found that the concentration of chromium is distributed differently in AlO in diffusion zones under microwave and traditional heating. This is due to greater contribution of grain-boundary diffusion to the effective diffusion flux under microwave heating. Bulk diffusion and average grain-boundary diffusion coefficients are calculated. The grain size in the diffusion zone toward AlO is smaller after microwave heating. Traditional heating induces grain growth by recrystallization, whereas two processes, recrystallization and polygonization, are superimposed during microwave heating. The polygonization is due to the generation of dislocations under thermal stresses originating from nonuniform temperature distribution in the diffusion zone with variable concentrations of the components. The calculated bulk and grain-boundary diffusion coefficients can be used to predict the kinetics of various diffusion mass-transfer processes in AlO and CrO oxides and their mixtures. [ABSTRACT FROM AUTHOR]