Cobalt incorporation in mullite

Abstract

Mullite (nominally 3Al_2O_3.2SiO_2) is technologically important material for advanced ceramics applications. Depending on the synthesis procedure, mullite is able to incorporate considerable amounts of transition metal cations [1]. While Cr^3+, Fe^3+ and V^3+ have the strongest tendency of incorporation in mulllite, only low or very low amounts of Fe^2+, Co^2+, Mn^2+ and Zr^4+ ions can enter the mullite structure. A major goal of this work is to determine upper limit of Co^2+ incorporation in mullite. The samples of pure mullite and of Co-doped mullite were derived from diphasic precursors and sintered at 1600 o ; C for two hours. Four samples were prepared containing 0, 1, 2 and 3 at% Co. They were examined by XRD at room temperature. Samples contained mullite phase and small amounts of alpha-Al_2O_3 and CoAl_2O_4. Unit-cell parameters of the mullite phase were refined by the whole-powder-pattern fitting method [2]. They increased just slightly with increase of cobalt content in the samples. Quantitative phase analysis showed that the samples with 1, 2, and 3 at% Co contained 0.8, 2.5 and 5.1 wt% CoAl_2O_4, respectively. These means that ~0.6 at% Co was incorporated in mullite. Same value of upper limit of Co^2+ incorporation in mullite was obtained on the basis of intensity ratio I_(311, CoAl_2O_4) / I_(111, mullite phase), which was linearly dependent on the Co content in the examined samples.