Correlation of the precursor type with densification behavior and microstructure of sintered mullite ceramics
Introduction
Although extensive work has been done related to the processing and characterization of sol-gel derived materials having mullite compositions, (AlVI[AlIV2+2xSi2−2xO10−x] where 0.22<x<0.58),1 a complete understanding in terms of the relationship between the initial process parameters and the sintering behavior and microstructure of mullite ceramics has not yet been fully achieved.
The general agreement is that the mixing scale in mullite sol-gel precursors actually controls the phase transformation sequence and the temperature of mullite formation as well as the properties of sintered bodies.2
Different types of mullite precursors were described depending on the degree of homogeneity in as-prepared powders.2, 3, 4, 5, 6, 7, 8 Single-phase mullite precursors, or type I precursors (Schneider's definition2) possess atomic or near atomic level of homogeneity and transform into tetragonal/or pseudotetragonal mullite at ∼980 °C. In gels with nanometer scale of homogeneity (diphasic gels), however, two different pathways of phase development are observed. Diphasic gels designated by Schneider as type II precursors consist of pseudo-boehmite and amorphous silica at room temperature. It is generally accepted that the transformation of pseudo-boehmite follows the same phase transformation sequence as that in boehmite forming γ-Al2O3, which transforms in δ-Al2O3. The latter phase reacts with amorphous silica forming mullite above 1250 °C. Diphasic gels designated as types III precursors are non-crystalline up to about 980 °C and mullite formation is preceded by the formation of a weak crystalline transient alumina such as cubic Al–Si spinel or γ-Al2O3 at 980 °C, which later reacts with amorphous silica forming mullite at temperatures lower than 1250 °C. However, most mullite gels consist of a combination of different types of precursors, rather than that of the intended end members.7
The effect of varying the scale of mixing on the densification and microstructure of mullite ceramics has been investigated in several studies.8, 9, 10, 11, 12 Single-phase mullite precursors do not sinter effectively without the application of high temperatures and/or high pressures. In contrast, mullite prepared from colloidal precursors can be sintered at much lower temperatures. Another way to achieve dense mullite ceramics at lower temperatures is a technique involving transient viscous sintering of microcomposites consisting of alumina particles coated with an amorphous silica layer. Sacks et al.13, 14 achieved dense ceramics by sintering of α-Al2O3 particles coated with an amorphous silica at 1600 °C. Bartsch et al.15 reduced processing temperatures by ≈300 °C with amorphous SiO2-coated γ-Al2O3 instead of α-Al2O3 particles. They pointed out that improved densification at lower temperatures is due to transient viscous flow sintering of amorphous silica.
The aim of this work was to study the influence of Al2O3 component (its crystalline form and particle size) on the crystallization pathway of diphasic gels containing alkoxy derived silica, and on the sintering behavior of compacted precursors. As alumina components used were: Al(NO3)3·9H2O, γ-Al2O3 and boehmite (γ-AlOOH). Special attention was given to differentiate the crystallization path and the microstructure of sintered bodies when precursors containing boehmite or in situ formed pseudoboehmite were used. The structural evolution with temperature has been studied by differential thermal analysis (DTA) and X-ray diffraction (XRD). The microstructure and morphology of sintered ceramics has been investigated by scanning and transmission electron microscopy (SEM, TEM) and energy dispersive X-ray spectrometry (EDX).
Section snippets
Gel preparation
Four precursors with the stoichiometric mullite composition (3Al2O3·2SiO2) but with different level of mixing were prepared as follows: The gel A was prepared by dissolving Al(NO3)3·9H2O in water (nitrate/water molar ratio equals 1:32). The solution was stirred and refluxed at 60 °C overnight. Tetraethylsilane (TEOS, Fluka >98%) previously mixed with ethanol (with TEOS/ethanol molar ratio of 1/4) was added dropwise to the nitrate solution. The mixture was heated at 60 °C under reflux condition
Results
TEM micrographs of dried gels A, B and D are shown in Fig. 1 along with the micrograph of γ-Al2O3 suspension. It is interesting to note that γ-Al2O3 in the as-prepared precursor C was not visible in TEM micrograph, although the XRD pattern confirmed it at room temperature. Therefore, its particle size was measured in the suspension just before TEOS was added. The EDX and B.E.T analyses of the gels are given in Table 1. The compositions of gels were measured at 10 different positions and the
Powder characterization
Depending on the starting materials and the methods applied (Table 1), the synthesized precursors have different properties, which in turn affect the resulting properties of the ceramics. All four gels have the same 3:2 mullite composition (within the error span of EDX analysis) and similar specific surface area. However, the as-dried gel A is amorphous at room temperature, while in the other three gels alumina component is present in a crystalline form; as pseudoboehmite in the sample B, γ-Al2O
Conclusion
Clear differences were found in the microstructure of diphasic precursors A and B in comparison to those of precursors C and D. Type III gel, and the combination of type II and type III precursors exhibited elongated mullite grains embedded into the “equiaxial mullite matrix”. This morphology is due to the overlapping of mullite crystallization and viscous flow sintering temperatures. The inferior densification behavior of gel A with respect to the gel B is caused by the lower temperature of
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