Oxide-metal eutectic composite synthesis

Synthesis of oxide-metal eutectic composites has been investigated with respect to the most basic melt chemistry principles and directional solidification phenomenon. The internal zone growth (IZG) technique has been extensively utilized in this study and in turn significantly deve-loped and better understood. The growth of composite samples 5-cm in diameter has been demonstrated.

The growth of oxide-metal eutectics of both the M_xO_y-M and M_xO_y-M’ (where M’ ≠ M) types have been considered. The Cr₂0₃-Cr and Cr₂0₃-Mo eutectic systems were used as the experimental model systems to test the influence of partial oxygen pressure and solidification parameters on composite composition control and composite microstructure. It was found that partial oxygen pressure of the composite growth environment influences the composition of the melt in a way that can be predicted by regular solution thermodynamics. Also basic thermodynamic principles were utilized to predict the growth of more complex eutectic systems: (M,M’)_x0_y- M, e.g. (Cr,Al)₂O₃-Cr, and M’_xMO_z-M", e.g. LaCrO₃-Mo.

The results of controlled directional solidification experiments indicate that oxide-metal eutectics obey the same coupled growth theories that have been well established for metallic eutectic systems. In the coupled growth studies it was observed that eutectic composite structure could be grown from an extremely metal rich melt at unusually fast growth rates which is indicative of a large thermal gradient at the growth interface. Since solute redistribution can occur in a thermal gradient called the Soret Effect, the eutectic constitutional supercooling criterion was rederived to include the Soret diffusion term. The analysis reveals that the constitutional supercooling parameter "[G/R] minimum" is independent of the magnitude of the thermal gradient through the Soret Effect.