Formation mechanism of nanoporous structures through the crystallization of amorphous oxides

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A high density of nanovoids between 2 and 13 nm in size were produced during the crystallization process as amorphous Al_2O_3 and WO_3 20～30 % lower in density than their crystalline phases were annealed. By contrast, no formation or growth of nanovoids was observed during the crystallization of amorphous TiO_2 and Cr_2O_3, which have a density 2% lower than its crystalline phase. These results suggest that large difference in density between the amorphous and crystalline oxides can be utilized to produce nanoporous structures.