Abstract
Calcium aluminate (CaAl2O4, CA) powders were prepared by sol–gel technique at low sintering temperatures. Aluminium-sec-butoxide (Al(OsBu)3, Asb) and calcium nitrate tetrahydrate (Ca(NO3)2 × 4H2O) were used as starting materials. Ethyl acetoacetate (C6H10O3, Eaa) was used as a chelating agent in order to control the rate of hydrolysis of Al(OsBu)3. Three gels with Eaa/Asb molar ratios of 1/1, 3/2 and 2/1 were prepared. The dried gels and thermally treated samples were characterized by means of Fourier Transform Infrared spectroscopy (FTIR), 1H, 13C Nuclear Magnetic Resonance (NMR) spectroscopy, solid-state 27Al Magic Angle Spinning (MAS) NMR, 3Q MAS NMR spectroscopy, and X-Ray Diffraction (XRD). From the results obtained, the effect of modification of the starting Asb on the hydrolysis process, hydrolyzed gel structure and crystallization behavior is discussed. It has been established that Eaa reacts completely with Asb forming chelate. Various chelate units are formed including trichelated, Al(Eaa)3 units. Spontaneous gellation has been observed in the sols slowly hydrolyzed by exposing to air moisture. The reactivity towards hydrolysis of chelated alkoxide depends on the number of chelating ligands bonded to aluminium. Sec-butoxy groups were primarily hydrolyzed; ethyl acetoacetate groups in less chelated units are much less susceptible to hydrolysis, while trichelated units were not hydrolyzed. Thus, in hydrolyzed gels a partially chelated oligomers and trichelated Al(Eaa)3 units exist. The crystal phase, not described previously, related to Al(Eaa)3 chelate crystallizes in the gels with higher Eaa/Asb ratio. Hydrolysis leads to formation of three kinds of Al coordination sites: six coordinated Al(Eaa)3, and five and six coordinated Al atoms in oligomers. The Eaa/Asb ratio strongly influences the relative ratios between the various coordination states of aluminium atom. After thermal treatment of the gels at 1,000 °C for 2 h, CA was obtained along with minor CaAl4O7, (CA2) and Ca12Al14O33(C12A7) compounds. Thermal treatment at higher temperature increases the amount of CA and decreases the amount of minor components.
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