Influence Of Preparation Process On Formation Of Zircon Grain And Glaze Properties

Zirconium glaze is the most widely used opaque glaze due to its advantages of easypreparation, low cost and good decorative effect. Zircon crystal with high refractive index andMohs hardness can not only produce a high opacity, but also impart excellent mechanicalproperties to the glaze. Therefore, understanding the crystallization mechanism and factorsaffecting the crystallization of zircon is important to opitimize the appearance and propertiesof zirconium glaze. In this research, the author studied the crystallization mechanism ofzircon, and the influence of compostion and process parameter on zircon crystallization andglaze properties, as well as the corrosion resistance and cleanability of different glazes. Thisresearch can provide a reference for the study of crystal nucleation and grwoth in glaze andthe development of related products.First, a based raw glaze was obtained by the orthogonal method. However, SEM imageshowed that zircon crystals distibuted unevenly on the surface of raw glaze. It was found thatthe uneven distribution of zircon occurred during the densification of the raw glaze and wascaused by the releasing of gas. The distribution of zircon has a significant effect on glaze’soptical property and surface roughness. Compared with the frit glaze that containedapproximative amount of zircon, the raw glaze had lower opactiy and higher roughnessthough it had smaller, spherical crystals. Based on these facts, a zirconium-based frit glaze ismore suitable for a sanitary ware.The crystallization mechanism of zircon from a frit glaze was studied. Zircon not onlycan form from the conversion of t-ZrO2and Ca2ZrSi4O12, but also can directly crystallizefrom the melt. The transformation of t-ZrO2to zircon mainly occurred at low temperature,and the growth of zircon had two mechanisms: the inward growth and the outward growth.The former was the migration of Si4+from the melt to the zircon-ZrO2interface, and thelatter was the result of nucleation-growth process. Ca2ZrSi4O12may directly break down intozircon, and the excess CaO and SiO2dissolved into the melt. SEM images showed thatzircon were rod-shaped (or fibrous) crystals in the final glazes. The calculated activationenergy for zircon was311.2kJ/mol. The Avrami index (n) was0.37, which meant thecrystallization of zircon was one-dimensional diffusion-controlled growth. This can explainwhy zircon eventually grow into a rod.The promotion effect of ZnO on zircon crystallization was studied. XRD analysisrevealed that the amount of zircon was proportional to the addition amount of ZnO and aappropriate amount of ZnO can lead to a complete conversion of ZrO2to zircon.The three roles of ZnO in zircon formation was discussed. Moreover, the effect of ZnO/CaO andAl2O3/R2O on zircon crystallization under a slow firing schedule was studied. Since all ZrO2was transformed to zircon due to the long firing time and high firing temperature, thecomposition had a negligible influence on zircon content. The glaze containing more ZnO andAl2O3also crystallized ZnAl2O4except zircon. However, the presence of two kinds of crystalsincreased the surface roughness but decreased the wettability.The addition of TiO2, CaF2and P2O5can significantly change the type and crystallizationtemperature of the crystals. P2O5can clearly low the crystallization temperature of zircon andfavor the formation of small, spherical zircon. The glaze containing P2O5had the highestopacity. The result also showed that additives had no clear impact on surface roughness andcontact angle of the studied glazes.The effect of particle size, heating rate, sintering temperature and holding time on thecrystallization and glazed performance was explored. The results showed that particle sizehardly affected the high-temperature softening behavior of glaze, but had a large impact onthe appearance of the final surface. The crystallization was maily related to the firingtemperature and holding time. Zircon always appeared to be rod-like crystals, but sphericalcrystals can be obtained by increasing firing temperature, which leaded to the partialdissolution of zircon. The L*a*b*test showed that a good opacity can be got whether the mainphase was zircon or t-ZrO2.Finally, the author compared the corrosion resistance and cleanability ofzircon-containing raw glaze, zircon-containing frit glaze, transparant glaze, and TiO2film.The chemical resistance of glazes closely depended on the existence of crystals and theirdistribution. The raw glaze had a good chemical resistance in strong acid solution but wereattacked in strong alkali solution. The frit glaze showed a good chemical resistance in all testsolution. Both of strong acid and alkali solution can attack the transparant glaze due to itsabsence of crystal. The TiO2film had similar performance to the raw glaze, but can beseverely attacked by the alkali solution. As the difference in surface roughness and contactangle of the tested samples was small, there is no significant difference in terms ofcleanability. Considered all aspects of performances, the zircon-containing frit glaze is a goodcandidate for sanitary ware.