12 July 1996 Dynamic redox reactions and the formation of fine-grained polycrystalline oxide/silicate ceramics from inviscid melts
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Abstract
The dynamics of oxidation or reduction reactions in multicomponent, transition-metal-cation-bearing oxides involves the coupled diffusion of electron holes (polarons) and component cations. As a consequence, internal reactions, resulting in the nucleation/crystallization at a reaction front of higher-order oxides or of the more-noble metal component, respectively, dominate dynamic behavior. In the amorphous state, such reactions result in homogeneous nucleation of this product phase, suggesting an approach to the preparation of fine, uniform-grained ceramics directly from inviscid melts: the redox-reaction-front oxide or metal phase can act as a dispersed heterogeneity for nucleation of other, 'majority' phases. This approach, for which microgravity containerless processing is ultimately required, is illustrated with results from ferrous iron- bearing aluminosilicate melts and glasses. For example, oxidation of the melt at 1400 degrees C results in 'isothermal undercooling': the liquidus temperature and primary phase are a function of oxygen activity and thus sub-micrometer magnetite forms at the internal reaction front. The dynamics are rapid; chemical diffusion during oxidation is dominated/rate-limited by motion of the divalent, network-modifying cations. Dynamic reduction, i.e., formation of metal at an internal reduction front via rapid cation diffusion, occurs as well, a kinetical 'mirror image' of the oxidation process.
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Reid F. Cooper, Reid F. Cooper, } "Dynamic redox reactions and the formation of fine-grained polycrystalline oxide/silicate ceramics from inviscid melts", Proc. SPIE 2809, Space Processing of Materials, (12 July 1996); doi: 10.1117/12.244359; https://doi.org/10.1117/12.244359
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