Microstructure and topological analysis of Co : Al2O3 nanocermets in new FCC and BCC metastable Co-structures

Co : Al₂O₃nanocermets are synthesized by co-reducing Co²⁺-cations dispersed in a mesoporous AlO(OH) · H₂O matrix (amorphous) in a pure H₂atmosphere at 850–1150 K. The dispersed Co²⁺-cations in pores co-reduce to separated Co-particles of controlled size, as small as 50 nm, encapsulated in thin Al₂O₃films, which are formed in-situ by molecular decomposition of the matrix, 2AlO(OH) · H₂O Al₂O₃+ (2 + 1)H₂O. The Al₂O₃film which coats Co-particles has an amorphous structure. This is possible only if it is extremely thin limited to a thickness of t< 2r_c, with r_c 1.9 nm the critical size of its nucleation and growth into a stable crystallite. The thin Al₂O₃surface film supports the formation and existence of Co-particles in a modified FCC or BCC crystal structure. As a result, unusually, large crystallites of an average 100 nm diameter could be observed. Normally, such large particles of pure Co-metal exist in an HCP structure which undergoes a reversible martensitic transformation to FCC structure at 695 K. The results are analyzed and discussed in terms of microstructure, x-ray diffraction and XPS studies of nanocermets prepared under selected conditions.