Effects Of Composition And Process Conditions On Crystallization Of Zirconium-based Frit Glaze

Recently, glass-ceramic glazes developed from glass-ceramics have become a hot topicin the field of ceramic preparation. The aim of these researches is to improve glaze surfaceproperties by forming specific microstructures in different glazes through controllingcrystallization. Zirconium-based frit glaze is among the most widely used glazes in traditionalceramics because of its high opacity, high whiteness, excellent wear resistance and goodadaptability with body. Therefore, it’s necessary to study the crystallization characteristics ofzirconium-based frit glaze to obtain better glaze surface properties. This paper investigatedthe effects of glaze composition, fluoride nucleating agent and process conditions on types ofcrystalline phases, the amount of zircon, crystal size and morphology in zirconium-based fritglaze, which can be used to guide the future researches in this field.We firstly discussed the effect of molar ratio of SiO₂to ZrO₂on crystallization behaviorof the zirconium-based frit glaze. The crystallization percentage of zircon gradually increasedwith the increasing of SiO₂/ZrO₂, and the morphology of zircon crystal turned from thickprismatic to small needles. When SiO₂/ZrO₂was8, there was no zircon formed in the glaze,while large amounts of spherical tetragonal zirconia and monoclinic zirconia were found.When SiO₂/ZrO₂was9, small amount of prismatic zircon was formed and its crystallizationpercentage was approximately16%. When SiO₂/ZrO₂was ranging from14to18, thecrystallization percentage of zircon reached to100%, and the morphology of zircon was smallneedles.On the basis of SZ18whose molar ratio of SiO₂to ZrO₂is18, we further discussed theeffects of molar ratio of Al₂O₃to R₂O, Li₂O and fluoride nucleating agent on crystallization ofzirconium-based frit glaze. The results showed that Al₂O₃/R₂O had a great effect on themorphology of zircon. When Al₂O₃/R₂O was0.86and1, the aspect ratio of needle-like zirconcrystal was increased. When Al₂O₃/R₂O was0.51, the morphology of zircon crystal becameflaky. When Al₂O₃/R₂O was1.59and2.01, zircon crystal was low aspect ratio, even becamespherical. The increase in Li₂O content leaded to the gradual decrease in zircon content, as well as the whiteness of the glaze. With the increasing of Na₂SiF₆content, the crystallizationcharacteristics of the glaze was similar to that with reducing Al₂O₃/R₂O, indicating fluoridecould not precipitate fine crystal size in the zirconium-based frit glaze.Additionally, the effect of bivalent metal oxides on crystallization of zirconium-based fritglaze was studied by their equimolar substitution. The results showed that the crystallizationof zircon from the zirconium-based frit glaze consisted of two steps: formation of tetragonalzirconia （t-ZrO₂） at low temperatures and the reaction between t-ZrO₂and SiO₂at highertemperatures. Since divalent metal ions could affect the crystallization process, there were alot of differences on crystalline phases and contents of zircon in glazes with different divalentmetal oxides composition. As Ca²⁺could make glaze melt precipitate other zirconium-basedcompound, Ca2Zr（Si4O12） crystal was formed in these glazes containing a higher content ofCaO. As Ba²⁺could suppress the crystallization of tetragonal zirconia in glaze melt, there wasno zircon in these glazes containing a higher content of BaO, which resulted in sometransparent glazes. As Sr²⁺could make glaze melt precipitate less t-ZrO₂before1050°C, thecrystallization percentage of zircon was lower in these glazes containing a higher content ofSrO, and the value was lower than25%. As Zn²⁺could not only make glaze melt precipitatemore t-ZrO₂before1050°C, but also facilitate the reaction between t-ZrO₂and SiO₂to formzircon after1150°C, the crystallization percentage of zircon was the highest in these glazescontaining a high content of ZnO, and the value was higher than90%. Although Mg²⁺couldmake glaze melt precipitate more t-ZrO₂before1050°C, the crystallization percentage ofzircon was not higher in these glazes containing a higher content of MgO because the reactionbetween t-ZrO₂and SiO₂was suppressed, and the crystallization percentage of zircon waslower than60%. The crystallization characteristics of glazes containing two divalent metaloxides had considerable changes compared with those of the glazes containing only onedivalent metal oxide, especially in microstructures of glaze surface, which resulted in a part ofmatt glazes.Finally, the effects of process conditions on crystallization of SZ18and MO wereinvestigated. The results showed that process conditions had little influence on crystallization of SZ18, but had a significant influence on crystallization of MO. When median diameter （D50）of the frit particle was reduced from8.75to2.20μm, the surface crystallization of t-ZrO₂waspromoted before1000°C in the glaze melt of MO, generating large amounts of finelycrystalline t-ZrO₂, which in turn inhibited the formation of zircon; When D50was increasedfrom8.75to15.80μm, the melting temperature of the MO shifted towards highertemperatures, which also reduced the formation of zircon. Since both of the decrease inheating rate and the increase in holding time favored the crystallizaion of zircon in MO. Therewas more zircon in MO prepared under the sanitary-ware firing condition and refiringcondition than that in MO prepared by fast firing. Moreover, the crystallization mechanism,crystallization percentage and morphology of zircon were different in zirconium glazes withdifferent adding modes of Zr sources. Based on the MO, the content of zircon was low whenwhen ZrO₂crystal material were added to the batch as mill addition, because zircon wasformed from the reaction between ZrO₂and SiO₂. When raw zircon material were added intothe batch as mill addition, the crystallization percentage of zircon were higher because allzircon derived from the undissolved zircon crystals, and the morphology of zircon wasspherical.