Effect of nucleating agent additions on gahnite based glass-ceramic glazes

Abstract

The effect of ZrO2 and TiO2 addition in gahnite-based glass-ceramic glazes was investigated by equimolar nucleating agents substituting 5 mol% SiO2. The effect of this substitution on thermal behavior was studied by means of differential thermal analysis and hot stage microscopy. The sintering behavior of samples changed with the type of nucleating agent added. Compacted samples sintered between 850 and 1200 °C for 2 min at 40°Cmin−1 heating and cooling rate for determining the crystallization sequence with using X-ray diffraction. Prepared glaze samples fired at 1180 °C for 6 min in the industrial roller kiln. Total firing time is 58 min from cold to cold. The effect of nucleating agent additions of gahnite-based glass-ceramic glazes was investigated by X-ray diffraction, scanning electron microscopy, spectrophotometer, and gloss meter. Glaze without nucleating agents shows heterogeneous surface crystallization from the frit surface to the inside with dendritic gahnite growth. With the nucleating agent, volume crystallization was seen with phase separation or seeding effect, which depends on the type of nucleating agent. Glaze with TiO2 has two different morphology; one region has fine gahnite crystals below 100 nm, and another region has clusters of nano gahnite crystals in flake-like morphology. ZrO2 provides fine gahnite crystallization, which is smaller than 80 nm. While ZrO2 added glaze is translucent, the other glazes are opaque due to different crystal sizes. It is observed that nano crystallization has a positive effect on pore swelling behavior due to its high crystal surface area. With the addition of the ZrO2 and TiO2, fine-sized nanocrystalline gahnite based glass-ceramic glazes can be obtained for the first time in industrial fast-firing conditions.