In a vacuum environment, the performance of molybdenum crucible is significantly better than that in an oxygen environment, mainly due to avoiding the problem of high temperature oxidation. The following are its performance characteristics in a vacuum environment:
High temperature resistance:
The melting point of molybdenum is 2623°C. In a vacuum environment, the molybdenum crucible can stably work at 1700-2000°C, or even higher (close to its melting point), depending on the purity of the crucible, the manufacturing process and the conditions of use.
The vacuum environment eliminates oxidation reactions and extends the life of the crucible.
Chemical stability:
Under vacuum, molybdenum crucibles show good chemical inertness to most molten metals and compounds, and are suitable for smelting rare metals (such as tungsten, tantalum), precious metals or semiconductor materials (such as silicon, sapphire).
react slightly with molybdenum at high temperatures, and compatibility needs to be evaluated.
Mechanical properties:
Molybdenum maintains high strength and toughness at high temperatures and is suitable for bearing the weight and thermal stress of materials in the crucible.
The vacuum environment reduces thermal shock and oxidation fatigue, and reduces the risk of crucible cracking or deformation.
Thermal conductivity and uniformity:
Molybdenum has good thermal conductivity (about 138 W/m·K) and can achieve uniform temperature distribution in a vacuum environment, making it suitable for high-precision heat treatment or crystal growth processes.
The vacuum environment avoids gas convection interference and further improves temperature control accuracy.
Service life:
The vacuum environment significantly prolongs the life of the molybdenum crucible because there is no oxidation loss, the crucible surface remains smooth, and material volatilization or structural degradation is reduced.
However, during long-term use, attention should be paid to the recrystallization of molybdenum (above about 1200°C), which may cause embrittlement and affect the reusability of the crucible.
Note:
Vacuum requirements: High vacuum (usually less than 10 ⁻ ³ Pa) must be ensured to completely avoid oxidation caused by trace amounts of oxygen.
Impurity control: Volatile impurities in the material in the crucible may be deposited on the inner wall of the crucible and need to be cleaned regularly.
Temperature gradient: Heat radiation is dominant in a vacuum environment, and the heating system needs to be optimized to avoid local overheating.
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