Scanning Microscopy


The review evaluates evidence of aqueous surface attack on glasses, titanate minerals and synthetic rock material (Synroc C) available from:- leach rates; surface analysis (XPS, SAM, SIMS); IR spectroscopy; and electron microscopy. Direct observations are described showing the formation of:- ion-exchanged, cation deficient layers; altered glass network and crystal line lattice layers and recrystallised, reprecipitated and surface-segregated layers. The titanate minerals react in the order perovskites > hollandite > fluorite structures (zirconolite, zirkelite, pyrochlore, polymigllyte) > rutile. The formation of amorphous Ti-O films, recrystallising to TiO2 (anatase and brookite), is observed on perovskite and hollandite surfaces. The surface reactivity of the titanate minerals is essentially the same in the ceramic assemblage Synroc C but additional microstructure (intergranular films, pores, triple points, minor phases) complicates the interpretation of first-day leach rates and depth profiles of leached discs. Reinterpretation of apparently congruent and incongruent dissolution data, using this evidence, is given for the glasses and titanate minerals. It is shown that solution analyses do not adequately describe the processes occurring in leaching and dissolution. The three major mechanisms of surface attack - ion-exchange, base catalysed hydrolysis of the network or lattice, and recrystallisation - are compared. Cation-exchange is very fast but limited to a layer no more than 20nm ahead of the reacted layer. Reaction of the titanate lattice is relatively slow and, in ceramics, appears to be limited by the grain size of the perovskite phase. In situ recrystallisation of amorphous TiOx films to TiO2 is fast above 100°C giving a polycrystalline discontinuous layer of varying depth (up to the grain size).

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