The crystalline microstructure of titanium dioxide doped mullites prepared from monophasic gels thermally treated at temperatures 900^oC, 1100^oC, 1200^oC, 1400^oC, 1500^oC and 1600^oC was analysed. Two series of gels with nominal compositions 3(Al_2-xTi_xO₃)•2(SiO₂) and 2(Al_2-xTi_xO₃)•(SiO₂) for x=0.00, 0.02, 0.05, 0.07, 0.10 and 0.15 were prepared [1,2].
X-ray diffraction analysis was performed by using an X-ray diffractometer in the Bragg-Brentano geometry, equipped with Cu K? X-ray tube and a graphite monochromator, operating at 30kV and 40mA. X-ray diffraction patterns were recorded in the range between 15^o and 90^o (2θ), with a step size of 0.02^o (2θ) and a counting time of 10s per step. Lattice parameters were determined by using ZnO as standard and least square fit. The instrumental peaks were recorded from lanthanum hexaboride (NIST 660a SRM) and from them the standard line profiles corresponding to Bragg angles for mullite were calculated. Pure X-ray diffraction line profiles were computed by using a stable deconvolution algorithm. Several pure line profiles, corresponding to the strongest reflections, were simultaneously analysed for each sample [3] to estimate prevalent crystallite shape and to determine a volume-weighted crystallite size distribution and a second-order crystalline lattice strain distribution. The crystallites were modelled in four different shapes, as tetragonal or orthorhombic prisms, cylinders and spheres; the shape was modelled as the ratio of characteristic dimensions (such as prism edge lengths). The shape anisotropy of the crystallites was found. The dependence of crystalline microstructure of the solid solutions on the titanium contents was observed. A characteristic feature of all volume-weighted crystallite size distributions was their bimodality; each distribution could be well approximated as the sum of two logarithmic-normal distributions. This bimodality is explained as a result of nucleation and initial growth of crystallites in two stechiometric forms of mullite that happened during thermal treatment. Mean crystallite sizes, calculated from the size distributions, were in agreement with results of scanning electron microscopy observations. In general, crystallite were larger with increasing either the content of dissolved titanium oxide or the final treatment temperature. A mechanism of formation of titania-doped mullite solid solutions is suggested and the limit of solubility of titania in mullite is estimated on the basis of the interpretation of the structural and microstructural evolution from gels to doped mullites.

[1] E. Ruiz de Sola, F. Estevan, J. Alarcón: Low-temperature Ti-containing 3:2 and 2:1 mullite nanocrystals from single-phase gels; J. Eur. Ceram. Soc. 27 (2007), 2655-2654.
[2] E. Ruiz de Sola, F. J. Serrano, E. Delgado-Pinar, M. M. Reventós, A. I. Pardo, M. A. Kojdecki, J. M. Amigó, J. Alarcón: Solubility and microstructural development of TiO₂-containig 3Al₂O₃.2SiO₂ and 2Al₂O₃.SiO₂ mullites obtained from single-phase gels; J. Eur. Ceram. Soc. 27 (2007), 2647-2654.
[3] M. A. Kojdecki, E. Ruiz de Sola, F. J. Serrano, E. Delgado-Pinar, M. M. Reventós, V. J. Esteve, J. M. Amigó, J. Alarcón: Microstructural evolution of mullites produced from single-phase gels; J. Appl. Cryst. 40 (2007), 260-276.

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/15462 must be provided.

1. Comparison of two standards for powder X-ray diffraction
2. Evolution of size and shape of mullite crystallites in triaxial porcelains