Crystal structure and microstructure of synthetic hexagonal magnesium–cobalt cordierite solid solutions (Mg_2−2xCo_2xAl₄Si₅O₁₈)

Abstract

Co 2+ -containing cordierite glasses, of nominal compositions (Mg 1Àx Co x ) 2 Al 4 Si 5 O 18 (with x = 0, 0.2, 0.4, 0.6, 0.8 and 1), were prepared by melting colloidal gel precursors. After isothermal heating at 1273 K for around 28 h, a single-phase -cordierite (high-temperature hexagonal polymorph) was synthesized. All materials were investigated using X-ray powder diffraction and field-emission scanning electron microscopy. The crystal structure and microstructure were determined from X-ray diffraction patterns. Rietveld refinement confirmed the formation of magnesium-cobalt cordierite solid solutions. The unit-cell volume increased with the increase of cobalt content in the starting glass. The crystalline microstructure of the cordierites was interpreted using a mathematical model of a polycrystalline material and characterized by prevalent crystallite shape, volume-weighted crystallite size distribution and second-order crystalline lattice-strain distribution. Hexagonal prismatic was the prevalent shape of -cordierite crystallites. Bimodality in the size distribution was observed and interpreted as a consequence of two paths of the crystallization: the nucleation from glass of -cordierite, which transformed into -cordierite with annealing, or the nucleation of -cordierite directly from glass at high temperatures. Scanning electron microscopy images agreed well with crystalline microstructure characteristics determined from the X-ray diffraction line-profile analysis.

Sintering and final properties of kaolinite‐magnesite tapes for the manufacture of cordierite‐mullite ceramics

“…Consequently, the sole occurrence of mullite was not enough to balance the lack of cohesion within the studied samples since the temperature was not appropriate to enhance the contribution of magnesium to lower the viscosity of amorphous phases (promotion of viscous flow sintering) and/ or to react with silica and alumina to form cordierite. The ternary phase diagram MgO‐Al 2 O 3 ‐SiO 2 (MAS), β‐cordierite could be expected in the temperature range 3,5,9‐11 1000°C to 1100°C, but we could not afford to reach ideal homogeneity within our mixture (powder with liquid route mixing) and thermodynamic equilibrium during our firing conditions. Therefore, this could justify the delay observed for the crystallization of cordierite (1200°C) in our samples, and thus related to the diffusion of magnesium.…”

“…With regard to the various industrial fields, synthetic and natural materials based on magnesium, aluminum, and silicon oxides were employed to produce cordierite ceramics 8 . Based on the stacking order of [SiO 4 ] and [AlO 4 ] tetrahedrons in [(Si 4 Al 2 )O 18 ] hexagonal rings, the cordierite is characterized by a complex polymorphism 9‐11 : the α‐cordierite, also known as indialite, has a hexagonal symmetry (space group P6/mcc ) and crystallizes at a high temperature (stable between 1447°C and 1457, at atmospheric pressure); the β‐cordierite has orthorhombic structure (space group Cccm ) and crystallizes at low temperature 950°C; the μ‐cordierite, metastable rhombohedra phase is obtained by the crystallization of cordierite glass below 925°C. …”