| http://www.w3.org/ns/prov#value | - grains including the surface inside the pores of the porous structure are covered with a thin and continuous film of an oxide selected from the group consisting of ZrO.sub.2, MgO, Al.sub.2 O.sub.3, SiO.sub.2, TiO.sub.2, Cr.sub.2 O.sub.3, MnO, Fe.sub.2
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