The influence of solvents on sol–gel derived calcium aluminate

High purity calcium aluminate (CA) samples were derived by a sol–gel method using aluminium-sec-butoxide (ASB) and calcium nitrate tetrahydrate (CN). In order to facilitate the controllable synthesis process an ethyl acetoacetate (EAA) was used as a chelating agent for ASB (EAA:ASB=3:2). Synthesis was performed using different solvents: ethanol (EtOH), 2-propanol (PrOH), and 2-butanol (BuOH) in order to influence the early stage of CA sol–gel synthesis. The gels were examined using Fourier transform infrared (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopy. Al was found to be highly prone to change of coordination states as a consequence of different synthesis parameters and thus various processes occurring; oligomerisation, hydrolysis, condensation, and chelation.The derived gels were dried and thermally treated to form CA. The thermal evolution, crystallisation behaviour and final microstructure of dried gels were monitored by means of Differential Thermal and Thermogravimetric Analysis (DTA/TG), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) with Energy dispersive X-ray spectroscopy (EDS), particle size distribution and N2 adsorption/desorption isotherms. It was determined that solvents influence gel composition, structure and particularly the morphology of CA precursor powders. The influence of solvents on the properties of sols and gels also reflects in the properties of calcined CA powders like phase composition, crystallisation parameters and the morphology.EtOH based sample is characterised with emphasised condensation stage on behalf of which gel thermal stability is enhanced. Thereby final CA microstructure may differ upon calcination. EtOH based sample yields Scherrer crystallite size of 150 Å and bimodal particle size distribution with size between 5 and 20 µm while specific surface area was 5 m2 g−1, differing to samples where other solvents were used.It was shown that better utilisation of the CA sol–gel synthesis process can be achieved by simple modification of synthesis parameters.