Microstructural and Thermal Properties of Nanocrystalline Silica Xerogel Powders Converted from Sago Waste Ash Material

Abstract

In the present investigation, nanocrystalline silica xerogel (NSX) powders were produced from an amorphous silica xerogel (ASX) extracted from sago waste ash. The NSX powders have been calcined at 1200oC, milled and then annealed at temperatures ranging from a room temperature to 1200oC. Their properties (and most notably the size of the particles) have been characterized on the basis of the experimental data obtained using thermal analysis (DSC/TGA), X-ray diffraction (XRD), Infrared and Raman spectroscopy. For the crystalline silica xerogel powders the results show a narrow distribution of the particle sizes centered around an average value of 636  67 nm. The DSC analysis of NSX indicates that in the temperature range from a room temperature to 300oC five distinct stages of the crystallization process take place, which are delimited by the transition temperature of 38oC, 92oC, 129oC, 168oC, and 246oC, respectively. Above 300oC, the crystalline phase is similar to an amorphous silica xerogel (ASX), i.e. cristoballite-like and tridymite-like crystalline silica phases confirmed by the XRD analysis. It has been observed that the characteristic band of cristoballite is strongly dependent on the thermal history and the NSX transforms into a stable form at a temperature of 1200oC. Both the Raman and the FTIR spectra elucidate the bonding system of the constituent atoms and groups (such as Si, O and OH) and throw light on their underlying structure. The obtained results are important for optimization of the parameters of the technological processes for production of nanocrystalline silica glass ceramics used as a host matrix for luminescence materials, each of which requires a specific porosity and structure.