Neutron-diffraction study up to 1600 °C of 3:2 mullite

Abstract.

Neutron-diffraction studies were carried out on pure and Cr-doped 3:2 (‘sinter’) mullite. To learn about structure–property relationships of this refractory material, thermal expansion coefficients and structural parameters were determined by in situ high-temperature investigations. Neutron experiments were performed at the powder diffractometers D2B/ILL, E9/HMI, and HRPT/PSI. All data gave a rather uniform picture of the thermal expansion coefficients: a linear behaviour below 1000 °C and significantly increased (mean) expansion between 1000 and 1600 °C. There is a marked anisotropy of the highest values along the b-axis of the orthorhombic mullite structure. Concerning the structural parameters and the derived bond lengths, there is an expansion of the Al1O₆ octahedra, whereas the cross-linking Al2SiO₄ tetrahedra stay more or less rigid, but may slightly rotate. Thus a major expansion along the b direction may be qualitatively understood. A structural discussion, however, is affected by the presence of disorder in mullite, in particular in Cr-doped mullite, as it is also reflected by the presence of a modulated diffuse background in the powder patterns.