Cobalt incorporation in mullite

Abstract

Mullite samples doped with cobalt were derived from diphasic gels with constant atomic ratio (Al + Co)/Si = 3:1, where 0, 1, 2, and 3 at% of aluminum was replaced by cobalt. X-ray powder diffraction showed that the samples contained mullite phase and some amount of α-Al₂O₃ (for pure and doped samples) and CoAl₂O₄ (for doped samples). Cobalt doping caused an increase in unit-cell parameters of the mullite phase. Transmission electron microscopy and energy dispersive X-ray spectroscopy were used for sample microanalysis and determination of the chemical composition of the Co-doped mullite phase. The Rietveld method was performed for quantitative phase analysis of the samples and for structure refinement of the mullite phase in the samples. It was found that a small amount of Co²⁺, 0.36 wt%, substituted for Al³⁺ in the AlO₆ octahedra of the mullite structure. Simultaneously, the same amount of tetrahedral Al³⁺ ions was likely substituted by Si⁴⁺ in the (Al,Si)O₄ tetrahedra for the purpose of charge compensation. The remaining cobalt reacted with alumina forming CoAl₂O₄, and dissolved in the glassy phase. The proposed formula for Co-doped mullite is Co_yAl_4+2x-2ySi_2-2x+yO_10-x