• Roy, Shibayan
• Ram, Shanker
• Ghosh, Dipankar

Abstract

Co : Al2O3nanocermets are synthesized by co-reducing Co2+-cations dispersed in a mesoporous AlO(OH) · αH2O 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) · αH2O → Al2O3+ (2α + 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< 2rc, with rc∼ 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.

Topics

• impedance spectroscopy
• microstructure
• pore
• amorphous
• x-ray diffraction
• x-ray photoelectron spectroscopy
• decomposition