Mullite transformation

Differential Thermal Analysis. Details revealed in the differential thermograms of the 4 MA-Y zeolites (Figure 2) are consistent with the TGA results. They all exhibited an endotherm (122°-190°C) caused by loss of adsorbed water. Over the region where slow decomposition occurred, there were weak endotherms or inflections in the curve, and the rapid decomposition was confirmed by a sharp endotherm (516°-577°C). Dehydroxylation was clearly revealed by the endotherm at 785°C in MMA-Y, but was not as well resolved in the others. The exotherm (977°-1026°C) was owing to mullite transformation (I). However, all the samples were found to be amorphous (by x-ray) after 1 hour at 900°C. 

W. H. Flank (Houdry Laboratories, Marcus Hook, Pa. 19061) All of your samples, after a 1-hour treatment at 900°C, were found by x-ray examination to be amorphous. The Na20 content reported for your samples ranged from about 3-13% by weight. The mullite transformation... 

Much of the quartz in the fly ash originates from the coal as silt- and sand-sized particles, and it remains in the ash because it survives thermal transformation during the combustion process (Helmuth 1987). Small amounts of volatilized Si may also oxidize to form very fine crystals of quartz within the fly ash glass (Diamond 1984 Hubbard et al. 1984). Although bituminous coal ash may contain more than 50 wt% analytical Si02, only 5-10 wt% of it is present in the form of quartz (McCarthy et al. 1990). Some Si is present in the mineral mullite, but the majority of it is in the amorphous glass phase. 

Note that reaction of Al(OH)3 with triethanolamine produces a tetrameric alumatrane159. Despite the fact that this material is likely to be only a homogeneous mixture, it behaves as if it is atomically mixed. The following sections provide an example of how this mixture pyrolytically transforms to phase pure mullite. The purpose is to provide an example of... 

FIGURE 13. Diffuse reflectance infrared Fourier transform spectra of OOPS mullite precursor heated to selected temperatures (air/10°C/min/l h dwell)... 

As mentioned previously, the main body of research on whisker-reinforced composites was concerned with alumina, mullite, and silicon nitride matrix materials. None the less, selected work examined zirconia, cordierite, and spinel as matrix materials.16-18 The high temperature strength behavior reported for these composites is summarized in Table 2.5. As shown, the zirconia matrix composites exhibited decreases in room temperature strength with the addition of SiC whiskers. However, the retained strength at 1000°C, was significantly improved for the whisker composites over the monolithic. Claussen and co-workers attributed this behavior to loss of transformation toughening at elevated temperatures for the zirconia monolith, whereas the whisker-reinforcement contribution did not decrease at the higher temperature.17,18... 

Kaolinite is transformed into X-ray amorphous state when activated in air. According to authors [14,15], amorphization involves the destruction of bonds between tetrahedral and octahedral layers inside the package, till the decomposition into amorphous aluminium and silicon oxides. Other researchers [ 16,17] consider that amorphized kaolinite conserves the initial ordering of the positions of silicon atoms while disordering of the structure is due to the rupture of A1 - OH, Si - O - A1 bonds and the formation of molecular water. Endothermic effect of the dehydration of activated kaolinite is shifted to lower temperatures while intensive exo-effect with a maximum at 980°C still conserves. When mechanically activated kaolinite annealed at 1(X)0°C, only mullite (3Al20j-2Si0j) and X-ray amorphous SiOj are observed. In this case, the phase with spinel structure which is formed under thermal treatment of non-activated kaolinite is not observed thus, mechanical activation leads to the formation of other phases. 

The effect of mechanical activation on mullite synthesis from the mixtures of aluminium hydroxide and silica gel was studied in [32,39]. They demonstrated that under heating of nonactivated mixtures, the components undergo phase transformations similar to those observed for individual compounds. Mullite is synthesized at 1400°C. 

The heating of the mentioned mixture leads to the formation of the phase with spinel structure. This phase is transformed into mullite at 1100°C instead of 1400°C in conventional thermal method. Since synthesis temperature is much lower, thus formed mullite is in finely dispersed state... 

At the same time, very fine mullite will begin to form in the clay component in the 1000 —J 200 C range. Its formation and crystal growth then proceeds by reaction between the partially transformed clay component and the feldspar melt. The resulting acicular mullite crystals are several pm in length. Extensive sintering due to the melt proceeds simultaneously. 

M. Sales and J. Alarcon, Synthesis and phase transformations of mullites obtained from Si02-Al203 Gels J. Eur. Ceram. Soc. 16, 781-789 (1996). 

H. Schneider and A. Majdic, Kinetics and mechanism of the solid state high temperature transformation of andalusite (Al2SiC>5) into 3/2 mullite, 3Al203-2Si02, and silica, Si02, Ceramurgia Int. 5, 31-36 (1979). 

When heated, kaolinite undergoes a complex series of chemical and physical changes that transform the layered mineral to a combination of crystalline mullite and an amorphous siliceous phase. Though simple conceptually, the study of this reaction sequence... 

I.W.M. Brown, K.J.D. MacKenzie, M.E. Bowden, and R.H. Meinhold, Outstanding problems in the kaolinite-mullite reaction sequence investigated by 29Si and 27 AI solid-state nuclear magnetic resonance II, high temperature transformations of meta-kaolin, J. Am Ceram. Soc. 68(6), 298-301 (1985). 

Metallic silver is clearly evidenced by XRD beside mullite and corundum phases. The crystallite sizes are reported in Table 2 as well as the specific surface area. After the evaluation tests, we can observe a strong decrease of the intensity of the silver diffraction peaks disclosing a loss of the active phase during the hydrogen peroxide flow. In the case of sample Bl, an important weight loss is observed (- 8.1 %, Table 1), whereas for sample C, the weight loss is much lower (- 0.9 %) and a part of metallic silver transforms into silver oxide. 

Synthetic silica-alumina catalysts containing 25% alumina are converted to gamma-alumina and mullite when thermally treated at 700-1260°. The phase transformation is accompanied by loss of catalytic activity and by collapse of the porous structure of the catalyst. The gamma-alumina phase is formed apparently by crystallization of the amorphous alumina in the catalyst, while the mullite formation apparently results from a combined silica-alumina amorphous phase. At sufficiently high temperature all alumina is converted to mullite. Silica-alumina catalysts made from more stable silicas have a greater tendency to form gamma-alumina. Such catalysts have lower initial catalytic activity and maintain relatively high catalytic activity after steam deactivation. 

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