The Role of Industrial-Grade Magnesium Oxide in Ceramics
In ceramic production, industrial-grade magnesium oxide (MgO) serves as a versatile auxiliary raw material. Although its addition to ceramic bodies and glazes is typically limited to 1% to 5%, it helps optimize various aspects of production, ranging from the sintering process and yield rate to the final performance of the product. Its applications span architectural, daily-use, and specialized industrial ceramics, making it a functional additive worthy of consideration as the ceramic industry transitions toward low-temperature fast-firing, energy conservation, and high efficiency.
1. Fluxing Action: Reducing Sintering Energy Consumption and Widening the Process Window
The primary components of ceramic raw materials are silicates, which generally require high melting temperatures. Magnesium oxide acts as a mild flux. During the heating stage, it reacts with components like silica, alumina, and calcium oxide in the body or glaze to form low-melting-point eutectic mixtures. This disrupts the lattice barriers on the surface of raw material particles and reduces inter-particle bonding resistance, thereby lowering the sintering temperature of the ceramics. Experimental data indicates that the sintering temperature of ordinary ceramics, which is typically between 1250°C and 1350°C, can be reduced to 1150°C–1280°C with the addition of magnesium oxide—a reduction of 50°C to 100°C.
At the same time, the sintering temperature range for conventional ceramics is often narrow, typically only 30°C to 50°C, meaning slight fluctuations in kiln temperature can cause under-firing or over-firing. Magnesium oxide can widen the sintering temperature range to 80°C–100°C, extending the effective holding time during the soaking stage. This allows for more complete and uniform mineral reactions within the body and glaze, reducing the impact of temperature fluctuations on sintering quality and improving the yield rate in mass production.
2. Crystal Phase Regulation: Inhibiting Excessive Grain Growth and Introducing Reinforcing Phases
During the ceramic sintering process, high temperatures can easily cause grains such as quartz and feldspar to coarsen, leading to structural defects. By adsorbing onto grain surfaces, magnesium oxide impedes abnormal grain growth, resulting in a more uniform microstructure and preventing localized stress concentration that can cause brittle fracture.
In alumina-containing ceramic formulations, magnesium oxide reacts with alumina to form a magnesium aluminate spinel phase. Magnesium aluminate spinel has a high melting point of 2135°C, high hardness, and good fracture toughness, while matching well with the thermal expansion coefficient of the matrix. Through the “pinning effect,” it enhances the bonding strength between crystal phases and reduces the risk of interfacial cracking. It also refines the glass phase network so that the glass phase is uniformly distributed within the grain boundaries during cooling, forming a continuous and thin glass phase film. This helps prevent the rapid propagation of cracks typically caused by thick glass phase layers.
3. Improving Mechanical Properties and Thermal Stability
Through the combined effects of fluxing and crystal phase regulation, industrial-grade magnesium oxide can noticeably improve the mechanical and thermal properties of ceramics.
- Daily-use ceramics: Adding 3% to 5% magnesium oxide can increase the bending strength of porcelain from 80–100 MPa to 120–150 MPa, and improve its thermal shock resistance from 10–15 cycles to 20–30 cycles.
- Architectural ceramics: Introducing 1% to 3% industrial-grade magnesium oxide into tile bodies or glazes can raise the Mohs hardness of the tiles from 5–6 to over 7, while also enhancing frost resistance and lowering the risk of cracking caused by freeze-thaw cycles in winter.
- Structural ceramics: Introducing 10% to 15% magnesium oxide into alumina ceramics can increase fracture toughness from
3 MPa⋅m1/23 MPa⋅m1/2to4–5 MPa⋅m1/24–5 MPa⋅m1/2, making them suitable for manufacturing wear-resistant components.
Regarding thermal stability, magnesium oxide has a melting point of approximately 2800°C, and its thermal expansion coefficient matches many ceramic matrices. This helps reduce internal stress during high-temperature sintering, preventing cracking and deformation. Studies show that incorporating 5% to 15% magnesium oxide into ceramic bodies can improve thermal shock resistance by more than 30%.
4. Whitening Effect and Improvement of Glaze Quality
Magnesium oxide itself is a white powder of high purity and minimal impurities. When added to ceramic bodies or glazes, it can enhance the whiteness and glossiness of ceramic products, which is particularly beneficial for high-whiteness requirements in white porcelain and celadon. In enamel glazes, magnesium oxide helps lower the melting temperature and promotes the precipitation of anatase crystals, improving the whiteness and color tone of titanium glazes, giving the glaze surface a clean, bluish-white hue. Regarding glaze optimization, magnesium oxide also reduces the viscosity of the glass phase, ensuring that the glaze slurry spreads evenly over the body surface, thereby preventing defects such as glaze crawling and pinholes.
5. Key Application Points in Different Ceramic Categories
Different ceramic categories require varying addition levels and types of magnesium oxide:
- Daily-use ceramics: Typically, 0.5% to 2% of light-burned magnesium oxide (caustic calcined magnesia) is added to the base formulation to improve bending strength and thermal shock resistance.
- Industrial refractory ceramics: These utilize dead-burned magnesium oxide (with a purity of no less than 95%) as the primary raw material, combined with a small amount of alumina to produce magnesium aluminate spinel ceramics capable of withstanding temperatures above 1800°C.
- Electronic ceramics: These incorporate 0.1% to 1% high-purity magnesium oxide (purity of no less than 99%) into raw materials like barium titanate to refine grains, stabilize the dielectric constant, and enhance insulation performance.
- Architectural ceramics: These involve adding 1% to 3% industrial-grade magnesium oxide to tile bodies or glazes to improve wear resistance, stain resistance, and frost resistance.
6. Conclusion
Although the addition of industrial-grade magnesium oxide in ceramic production is relatively small, it plays an important role in reducing sintering temperatures, enhancing mechanical strength and thermal stability, and improving appearance quality through the combined effects of fluxing, crystal phase regulation, and whitening. As the ceramic industry continues to prioritize energy conservation, emission reduction, and high-performance products, the application value of industrial-grade magnesium oxide is expected to expand further.