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Microstructural evolution of mullites produced from single‐phase gels
Author(s) -
Kojdecki Marek Andrzej,
Ruiz de Sola Esther,
Serrano Francisco Javier,
DelgadoPinar Estefanía,
Reventós María Mercedes,
Esteve Vicente Javier,
Amigó José María,
Alarcón Javier
Publication year - 2007
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s0021889807000295
Subject(s) - crystallite , microstructure , mullite , materials science , nucleation , transmission electron microscopy , scanning electron microscope , crystallography , diffraction , analytical chemistry (journal) , phase (matter) , mineralogy , composite material , chemistry , nanotechnology , chromatography , optics , ceramic , metallurgy , organic chemistry , physics
The crystalline microstructure of mullites obtained by heating monophasic gels has been investigated. Gels with alumina to silica molar ratio of 3:2 (as in secondary mullite) and 2:1 (as in primary mullite) were prepared by gelling mixtures of aluminium nitrate and tetraethylorthosilicate. Phase transformations were induced by heating the gel precursors, with different final treatment temperatures between 1173 and 1873 K. The mullites formed as a result of thermal treatment were studied by means of X‐ray diffraction, scanning electron microscopy and transmission electron microscopy. The crystalline structure (unit‐cell parameters) and microstructure were determined from X‐ray diffraction patterns. The formation of mullites of homogeneous chemical composition and with unit‐cell parameters depending almost linearly on the treatment temperature was found. Their compositions, expressed as alumina to silica molar ratio, were determined from the unit‐cell parameters and were in the range of those characterizing primary and secondary mullites. Mullites processed at lower temperatures were accompanied by small amounts of vitreous phase. The crystalline microstructure of the obtained mullites was interpreted by means of a mathematical model of polycrystalline material, involving prevalent crystallite shape, volume‐weighted crystallite size distribution and second‐order crystalline lattice strain distribution as model parameters. The model parameters were determined for each sample by modelling its X‐ray diffraction pattern and fitting it to a measured pattern. Bimodality of the size distribution was observed and explained as a consequence of two crystallite nucleation and growth processes, which started from small alumina‐rich and alumina‐poor domains, spontaneously formed in a precursor gel at early stages of heating. Images produced by scanning and transmission electron microscopy agreed well with the characteristics obtained from the analysis of the X‐ray diffraction patterns.