Molybdenum foil is used to produce cover plates in sapphire crystal growth furnaces, requiring high-temperature sealing and chemical stability. The process includes: 1) Material Selection: High-purity (≥99.95%) cold-rolled molybdenum foil (0.5-2 mm thick) with tensile strength of 800-1000 MPa and hardness of 160-200 HV is chosen. 2) Cutting: Laser cutting shapes the foil into circular or polygonal cover plates per furnace design, with ±0.02 mm precision. 3) Forming: Stamping or machining forms flat or slightly curved structures, followed by annealing (800-1200°C, vacuum) to relieve stress and enhance ductility (10-20%). 4) Surface Treatment: Acid washing removes oxides, and polishing (roughness Ra<0.1 μm) ensures sealing performance. 5) Assembly: The cover plate is connected to the furnace via molybdenum bolts or welding, withstanding 2000°C. Molybdenum’s high melting point (2623°C) and creep resistance ensure the cover plate remains undeformed, isolating atmospheres and protecting crystals. Its low thermal expansion coefficient (4.8 × 10⁻⁶/K) matches furnace materials, minimizing thermal stress. Thermal conductivity (138 W/m·K) maintains heat uniformity. Inert gas protection is needed above 600°C to prevent MoO₃ formation. Cover plates ensure a pure furnace atmosphere, preventing contamination. Advantages include high-temperature stability and sealing; disadvantages are oxidation sensitivity and high cost. Quality control involves dimensional inspection, surface analysis, and high-temperature testing.
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