This chapter presents a study in which an amorphous-to crystalline transition of anodic niobia is examined by using magnetron-sputtered niobium and its alloy substrates, with particular attentions paid to the preexisting surface oxide layer as a nucleation site of crystalline oxide and incorporation of foreign species hindering its nucleation. Niobium and Nb-N films were prepared by magnetron sputtering on to glass, silicon and aluminum substrates. Aluminum substrates were electropolished and subsequently anodized in 0.1 mol dm-3 ammonium pentaborate electrolyte to 200 V to provide flat surfaces. The niobium and niobium alloy films prepared were anodized at a constant current density of generally 50 A m-2 to selected voltages with and without current decay in stirred 0.1 mol dm-3 ammonium pentaborate and 0.1 mol dm-3 phosphoric acid electrolytes at 60. o C. Platinum sheet was used as a counter electrode. Amorphous-to-crystalline transition of anodic niobia formed on the sputter-deposited niobium films, which are free from preferable sites, such as inclusions and surface roughness, of the transition, initiated at approximately outer 30% of the film thickness where air-formed oxide is located. The growth of the crystalline oxide to the metal/film interface is rapid and the phosphorus species incorporated from the phosphoric acid electrolyte suppress effectively the amorphous-to-crystalline transition. Similarly, the foreign species incorporated from the substrate, such as nitrogen, silicon, and tungsten species suppress the crystalline oxide formation.
ASJC Scopus subject areas
- Chemical Engineering(all)