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A polymeric Al-modified bentonite (B-OHAl) obtained by intercalation of OH-Al species was used as precursor for mullite-containing coatings. Raw clay and B-OHAl were characterized by X-ray fluorescence (XRF), Infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG-DTA). Rheological and electrokinetic behavior of B-OHAl dispersions were optimized by the addition of sodium polyacrylate (NaPA) for its use in dip coating on ceramic substrates. Moderate densification and mullite development of B-OHAl by thermal treatment up to 1200 °C was observed by mercury intrusion porosimetry (MIP), XRD and scanning electron microscopy SEM. NaPA addition on dispersions and an increase in porosity values in substrate led to higher amounts of coating retained in dip coated composites sintered at 1200 °C. [ABSTRACT FROM AUTHOR]

Aluminum-pillared montmorillonites are useful materials for their application as catalysts, adsorbents and ceramic composites. The precursor is a pillared montmorillonite that is not thermally stabilized. The precursor preparation methods, textural properties and catalytic activity have been extensively investigated, but comparatively, studies concerning their thermal transformations at high temperature are limited. In this work, precursors were prepared using two types of montmorillonites, Cheto (Ch) and Wyoming (W), and using two different OH-Al polymer sources: hydrolyzed (H) and commercial (C) solutions. Structural and thermal transformations of the precursors with heating up to 1200 °C were determined by X-ray diffraction and thermogravimetric analysis. Thermal analysis of these precursors below 600 °C revealed the influence of OH-Al polymers from the two solutions. The major phases developed at 1200 °C from the original montmorillonites were mullite for W and cordierite for Ch. The content of these phases depended on the aluminum in the octahedral sheet of the pristine montmorillonites. Amorphous phase, cristobalite, spinel, sapphirine and others phases were also found. The intercalation of OH-Al polymers in montmorillonites caused an increase in amorphous content after treatment at 1030 °C; however, it favored mullite development above 1100 °C. Although total aluminum content of both W and Ch precursors was similar, the transformation to mullite was directly related to the octahedral aluminum/magnesium ratio. The phase composition of the products at 1200 °C was not dependent on the type of intercalated OH-Al polymers. The increase in mullite content of the thermally treated precursors contributes to its possible application as advanced ceramic products. [ABSTRACT FROM AUTHOR]

Abstract: Two montmorillonitic clays were put into contact with a solution containing inorganic OH-Al polymers and subsequently washed and dried. The obtained solid (i.e.the clays with the OH -Al species in the montmorillonite interlayer) which was called the precursor, was subjected to a thermal treatment up to 1000 ¡ C using DTA-TG for analysis at low temperature. Additionally, the structural changes of the precursor by heating at 1030, 1100 and 1200 ¡ C for 2hours and that of the natural clays were examined. The crystallinity and formation of new phases was followed by x-ray diffraction analysis. Mullite and cordierite were originated depending on the different contents of aluminium and magnesium in the structure of natural montmorillonite, respectively, accompanied with other phases. The intercalated OH-Al species in both clays favored the increase in the mullite content in the final products. However, the transformation to this phase was directly related to the octahedral aluminium of the natural clay and thus mullite was found to be the dominant phase from montmorillomite containing higher octahedral Al. [Copyright &y& Elsevier]