Why can calcined zinc oxide effectively reduce the thermal expansion coefficient of glazes and improve the thermal shock resistance of ceramic products?
Publish Time: 2025-10-15
During the ceramic firing process, the matching of glaze and body is a key factor in determining the quality of the finished product. When ceramic products cool after high-temperature firing, or when subjected to alternating hot and cold temperatures during daily use, the glaze and body generate stress due to the different degrees of thermal expansion and contraction. If this stress exceeds the material's tolerance limit, it can cause cracking, flaking, and even fracture of the body. This phenomenon, known in the professional field as "thermal shock damage," is a long-standing challenge in ceramic production. The introduction of calcined zinc oxide is one of the keys to solving this problem. It is not only a flux but also a functional material that can adjust the physical properties of glazes. It plays an irreplaceable role in reducing the thermal expansion coefficient of glazes and improving the thermal shock resistance of ceramics.
This ability of calcined zinc oxide stems from its unique chemical structure and high-temperature behavior. During the glaze melting process, zinc oxide doesn't simply flow as a low-melting-point component. Instead, it integrates into the silica-oxygen network as a stable cation, forming a denser and stronger glassy phase structure with key components like silica and alumina. This structure provides greater bond strength and lower free volume, resulting in smaller molecular vibrations and less overall expansion when heated. In other words, the addition of calcined zinc oxide "tightens" the glaze's microstructure, making its volume changes with temperature more moderate, thereby effectively reducing its coefficient of thermal expansion.
When the glaze's coefficient of expansion is close to or slightly lower than that of the base, a slight compressive stress forms during cooling. This condition, known as "glaze prestressing," significantly enhances the glaze's mechanical strength. Even when subjected to extreme external temperature fluctuations, the glaze is less likely to crack due to excessive tensile stress. This matching relationship is particularly important in architectural ceramics such as wall and floor tiles, as these products are often used in environments with large temperature fluctuations, such as kitchens and bathrooms, and must exhibit excellent thermal shock resistance. Calcined zinc oxide stabilizes the expansion behavior of glazes, ensuring that products remain intact after repeated hot and cold cycles, preventing fine cracks or "glaze shock."
In addition, the physical form of calcined zinc oxide enhances its effectiveness in glazes. After high-temperature calcination, zinc oxide particles become denser, with increased crystallinity, higher specific gravity, and better fluidity. During glaze slurry preparation, it disperses more evenly within the suspension, reducing agglomeration and precipitation, and ensuring consistent composition across all areas after glazing. This uniformity is particularly important during firing, as it avoids stress concentrations caused by local compositional variations, further improving overall thermal stability and yield.
In practical applications, this property of calcined zinc oxide not only enhances product durability but also opens up new possibilities for optimizing ceramic processing. For example, during rapid firing, the accelerated heating and cooling rates facilitate the accumulation of thermal stress between the body and glaze. Glazes using calcined zinc oxide are better able to withstand these intense thermal cycles, reducing defects and supporting the development of a more efficient and energy-efficient modern ceramics industry. It also works synergistically with other oxides to enhance the glaze's gloss, whiteness, and density, making the ceramic not only stronger but also more beautiful.
Ultimately, the value of calcined zinc oxide lies not only in its chemical composition but also in its profound control over the material's macroscopic properties. Like a "structural mediator," it reshapes the glaze's network structure at the microscopic level, making it more resilient to temperature fluctuations. This inherent stability is the foundation of ceramic products' longevity. Behind every flawless tile, in every piece of pottery that has withstood the test of heat and cold and remains intact, calcined zinc oxide silently fulfills its mission: to safeguard the beauty of ceramics through the power of science.
