Magnesium oxide coating: scientific explanation of magnesium hydroxide improving refractory aging
——In-depth analysis from molecular barriers to engineering protection
In the field of high-temperature industry, the aging stability of refractory materials directly determines the life of equipment and production safety. As the core protective barrier of the refractory system, the key to improving the performance of magnesium oxide (MgO) coating lies in the thermal conversion mechanism and microstructure regulation of magnesium hydroxide (Mg(OH)₂).
1. Thermal decomposition kinetics: synergistic effect of triple protection mechanism
Endothermic-cooling chain reaction
Magnesium hydroxide undergoes step-by-step decomposition at 340–490℃:
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Mg(OH)₂ → MgO + H₂O↑ ΔH = -44.8 kJ/mol
This process absorbs 1.3 kJ/g of heat, causing the surface temperature of the material to drop by more than 150℃. Taking fire-resistant gypsum board as an example, when 35% Mg(OH)₂ is added, its peak heat release rate (PHRR) is reduced by 62%, and the time for the material to reach the critical decomposition temperature is extended by 40%, which provides a critical window for personnel to escape.
Gas phase dilution and combustion inhibition
Decomposition releases 18.6% mass fraction of water vapor, achieving triple effects:
Dilute oxygen concentration to <15% (below the critical value of combustion);
Block free radical chain reaction (•OH + H₂O → H₂O₂);
Catalyze CO→CO₂ conversion to reduce toxic smoke.
In-situ construction of ceramic barrier
The generated MgO (specific surface area > 20 m²/g) forms a multi-level microporous ceramic layer on the surface of the material:
Physical barrier: thermal conductivity <0.5 W/m·K, reducing heat conduction efficiency;
Melt thickening: Increase polymer melt viscosity and suppress the risk of droplet ignition (UL94 drop count is reduced to 0).
2. Structural advantages of magnesium oxide coating: from atomic bonding to macroscopic performance
The foundation of stability of crystal structure
High melting point barrier: MgO has a melting point of 2852℃. In its ionic crystal, Mg²⁺ and O²⁻ are bonded by strong ionic bonds. The lattice vibration energy at high temperature needs to be >1200 kJ/mol to be destroyed, which is much higher than the steel smelting temperature (1600℃).
Dense lattice arrangement: The hexagonal close-packed structure (HCP) makes the atomic spacing only 0.21 nm, which effectively resists high-temperature thermal expansion stress.
Microscopic design of thermal shock resistance
Low expansion coefficient: α=13.5×10⁻⁶/K (20–1000℃), which is only 60% of alumina. During the start-up and shutdown of the glass melting furnace, when the temperature suddenly changes by 500℃, the thermal stress of the MgO coating is <3 MPa, avoiding peeling and cracking.
Defect engineering control: 2% yttrium oxide (Y₂O₃) doping introduces lattice distortion and improves fracture toughness to 4.5 MPa·m¹/² (↑40%).
3. Engineering efficiency enhancement path: precise control of the performance of the covering layer
1. Golden parameters of material design
Parameters Optimization target Time-to-effect improvement effect
Particle size D₅₀=0.8–1.5μm Nano-scale particles fill micro-cracks, and the flexural strength of fire-resistant gypsum board ↑8%
Morphology Hexagonal flakes dominate Layered stacking blocks the oxygen diffusion path
Purity ＞99.5% Reduce Fe₂O₃ and other impurities catalyze oxidation reactions
2. Composite synergistic system
Zinc borate catalyzes carbonization: Adding 2% Zn₃B₆O₁₂, the density of the carbonized layer is increased to 0.92 g/cm³, and the fire resistance limit is increased from 90 minutes to 120 minutes;
Nano clay loaded with magnesium: Montmorillonite loaded with nano Mg(OH)₂, the addition amount is reduced to 18%, and the peak heat release rate is reduced by 62%.
3. Interface strengthening technology
Silane coupling grafting: KH-570 modification reduces the surface energy of Mg(OH)₂ by 40% and increases the bonding strength with the silicate matrix by 300%;
Bionic mineralization coating: Zinc stearate coating simulates the shell structure and inhibits the agglomeration of high-temperature particles.
IV. Industrial evidence: from laboratory to high-temperature kiln
Steel industry
Converter magnesia carbon brick: High-purity MgO covering layer (content> 95%) makes the lining life exceed 3000 times, and the slag penetration depth is <1 mm (traditional materials>3 mm).
Glass manufacturing
Pool wall fused magnesia brick: The dense MgO layer resists the erosion of 1700℃ glass liquid, the melting furnace life is 8 years, and the annual maintenance cost is reduced by 40%.
Building fire protection
Tunnel fireproof board: 35% flower ball Mg(OH)₂+5% glass fiber, passed the 120-minute fire resistance test of EN 1364-1 standard, and the smoke density peak is <375.
5. Future breakthroughs: intelligent covering layer and circular design
Self-healing microcapsule technology
The polyurethane shell wraps the phase change material (PCM), which releases the repair agent when the temperature is greater than 300°C and automatically fills the microcracks.
Bio-based hybrid coating
Genetically engineered strains synthesize lipid-MgO composite layers, reducing VOC emissions by 67% and carbon footprint by 50%.
Magnesium hydroxide converts fire energy into harmless phase change energy and chemical energy through the third-order reaction of endothermic energy consumption → gas phase flame retardant → ceramic barrier. The magnesium oxide covering layer solidifies this protection into a long-lasting barrier with its strong bonded lattice, low expansion structure and intelligent interface. In the future, with the development of bionic mineralization and self-healing technology, fire resistance will break through physical limits and achieve precise editing of "molecular-level fireproof genes".