The molybdenum crucible must be strictly controlled during the cooling process to avoid thermal stress damage, oxidation and structural deformation. The following are specific treatment methods and precautions:
- Cooling rate control
Principle: Cool down slowly to avoid thermal cracks or deformation caused by rapid cooling.
Recommended rate: ≤ 50℃/min (can be achieved by program-controlled furnace temperature or natural cooling).
Example: Cooling from 1800°C to room temperature requires ≥ 3 hours.
Segmented cooling:
Stage 1: High temperature section (> 1000℃) cooling at 30-50℃/min;
Stage 2: Medium temperature section (500-1000℃) cooling at 10-20℃/min;
Stage 3: Low temperature stage (< 500°C) can be cooled naturally or by forced ventilation.
- Cooling environment management
Inert gas protection:
Cool in an inert atmosphere such as argon or nitrogen to prevent oxidation.
Flow control: ≥ 5 L/min, ensuring the oxygen content in the furnace is < 10 ppm.
Vacuum cooling:
If the equipment supports it, vacuum cooling (vacuum degree ≤ 1×10 ⁻ ³ Pa) can be used to completely isolate oxygen.
Avoid water cooling:
Direct water cooling is prohibited, as it may cause the crucible to crack due to sudden cooling.
- Operation Specifications
Preheating and cooling symmetry:
The rates and paths of heating and cooling must be kept consistent to reduce thermal stress concentration.
Crucible placement:
Keep the crucible upright during cooling to avoid placing it sideways which may cause excessive local stress.
Cooling post-processing:
Surface inspection: After cooling to room temperature, visually inspect the crucible surface for cracks or deformation.
Dimensional measurement: Use a vernier caliper to measure key dimensions (e.g. wall thickness, diameter) to ensure that no irreversible deformation has occurred.
- Abnormal situation handling
| Anomalies | Possible causes | Countermeasures | Anomalies |
| crack | Rapid cooling or thermal stress | Scrap and re-evaluate cooling process | crack |
| Deformation | Uneven cooling rate | Adjust the cooling program and add segmented cooling steps | Deformation |
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