Structural Disorder and Thermal Dilatation Behavior in Cr-Doped Mullite
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ABSTRACT Lattice parameters of some different chromium-doped mullites obtained by a sol-gel route and sintered at 16500 C have been determined from X-Ray Diffraction data as a function of both temperature and Cr contents. Profile refinements on the X-ray diffraction diagrams have been carried out, showing that, at temperatures between 8000 and 15350 C, an anisotropic thermal expansion takes place for the a and b lattice parameters of the Cr-doped mullite. We show that this anisotropic expansion is dependent on the temperature increase and the chromium content for the different compositions of the mullite crystals. INTRODUCTION Mullite ceramic materials exhibit excellent mechanical and thermal properties at elevated temperatures. Therefore, the thermal behavior of polycrystalline mullite aggregates, obtained from alumina-silica powders under different sintering conditions and chemical compositions, has been the subject of several investigations [1], [2] in order to reach a better understanding on these materials. These properties depend, among other factors, on the degree of mullitization and the presence of residual phases at grain boundaries. In this work, the interactive behavior between several mullites with different degrees of chromium contents, and ruby (solid solution A120 3 -Cr 2O 3) when the temperature is increased, is analyzed by means of X-ray powder diffraction (XRD). The samples had been previously synthesized by the sol-gel route. The quantitative X-ray study has been carried out using the whole profile pattern refinement method with a two-phases model. The advantage that this procedure offers is the possibility of measuring the cell parameter dilatation in these Cr- doped mullites more accurately than with a single-phase model. Moreover, it has been possible to extend the thermal expansion analysis towards a high temperature region (around 15000 C), where the thermal decomposition of Crmullites takes place. However, an additional complication can arise when these analyses are performed namely, the existence of overlapped peaks belonging both to the precursor phases (particularly ax-A120 3) and the mullite. EXPERIMENTAL SET-UP AND DATA ANALYSIS. The materials used in this paper are different stoichiometric 3/2 type mullites synthesized by a sol-gel method. The complete formula for mullite is A14 +2xSi 2- 2xO l 0-x, where x ranges between 0.17 and 0.59. The mullites concerning this study were prepared with different chemical compositions, that is, containing Cr 20 3 amounts ranging from a non-doped material to a 9 wt.% doped one. Therefore, the total Cr-contents in the samples are 0, 1, 3, 6, and 9 wt.% Cr 2 0 3 . All 757 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
of them were sintered at 16500 C. The experimental X-ray powder diffraction profiles were recorded on a Philips diffractometer (PW-1830, Cu Ka, fixed slits, graphite monochromator). It was equipped with a hot-stage (Anton-Peer). All of the data were collected according to a stepped temperature schedule (17
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