Preparation of monolithic transparent mullite-based glass-ceramics by the sol–gel method

Monolithic transparent mullite-based glass ceramics were prepared in the system SiO 2 –Al 2 O 3 –B 2 O 3 –ZnO–K 2 O by the sol–gel method. Heat treatment of the resulted monolithic xerogels at 950 °C for 6 h, in air atmosphere resulted dense, homogeneous and transparent glass-ceramics. The activation energy for crystallization, the Avrami constants and the mechanism of crystallization were determined by the Marotta, Augis-Bennet and the modified Kisinger methods, using the differential thermal analysis. X-ray diffraction and transmission electron microscope were also used to determine the crystallinity of the specimens, and the morphology of the precipitated crystalline phase. Also, transparency of the prepared sol and the synthesized glass and glass ceramics were evaluated by UV–vis spectrophotometer. In addition, the volume percent of the precipitated mullite, which was calculated by the internal standard method, was 34%. The transmission of the samples in the range of visible wavelengths was about 79%. According to the results, the Avrami constant for crystallization was 1.24 that means mullite was precipitated in the glassy phase through the surface crystallization mechanism.