Brunauer, G.; Frey, Friedrich; Boysen, Hans; Schneider, H.; Fischer, P.; Hansen, Th.; Többens, D. M.; Ehrenberg, H.
Neutron-diffraction study up to 1600°C of 3:2 mullite.
In: Applied Physics A: Materials Science and Processing, Vol. 74, Nr. SUPPL.II:
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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.