Optimizing the thermal stress distribution of molybdenum crucibles is a key measure to extend their service life, improve thermal stability and prevent cracking failure. The following are system optimization strategies from multiple perspectives of design, materials, processes and usage conditions:
1. Structural design optimization
1. Uniform wall thickness design
Uneven wall thickness can lead to uneven temperature gradients, which can create stress concentrations;
Optimize wall thickness to within **±5%** to avoid hot or cold spots.
2. Rounded corner transition structure
A large rounded corner (≥ R5 mm) is used at the junction of the bottom and the side wall to avoid thermal stress concentration caused by sharp corners;
Effectively reduce the probability of crack source formation.
3. Conical or spherical bottom design
For situations that require high heat flux density (such as crystal growth), using a slightly tapered or spherical bottom can reduce heat flux concentration and improve structural stability.
2. Material and Organization Control
1. Fine grain structure
Fine-grained structures (especially submicron or nanometer grains) can disperse thermal stress more evenly;
materials prepared by hot isostatic pressing (HIP) sintering or high-energy ball milling.
2. High purity materials
Impurities such as Fe, Si, and O can cause grain boundary embrittlement and reduce thermal stability;
TZM alloy or high-purity molybdenum material with a purity of ≥99.95% is preferred.
3. Manufacturing and heat treatment process
1. Annealing
**Vacuum annealing (1800~2000℃)** is used to eliminate cold working stress;
After annealing, the grains are stable, which is conducive to the release of thermal stress.
2. Multiple intermediate annealing + equal diameter deformation
During the spinning or forging process, adding multiple intermediate annealing can improve stress uniformity;
Control the balance of overall stress field distribution to avoid local hardening and crack initiation.
4. Temperature control and working condition adaptation
1. Slowly increase the temperature
It is recommended to control the heating rate to ≤5~10 ℃/min, especially in the temperature range of 200℃~600℃;
Avoid stress cracks caused by uneven thermal expansion.
2. Set the warm-up phase
Add a slow-rise interval before high-temperature operation (e.g., maintain the temperature at 300-400°C for 30-60 min);
Allow the entire crucible to be heated evenly to reduce thermal stress concentration.
3. Symmetry of working conditions
Ensure that the heating method (such as resistance heating, induction heating) is arranged symmetrically;
If a graphite heating element is used, it can be supplemented with a ceramic insulation tube or a thermally conductive filler to improve the uniformity of the temperature field.
5. Advanced technical means
1. Finite element simulation (FEA) thermal stress simulation
to predict temperature and stress fields during the design phase;
Helps to identify areas of stress concentration and optimize the structure.
2. Surface strengthening treatment
laser cladding, PVD coating (such as ZrO₂, Y₂O₃) and other processes can slow down the thermal conduction gradient;
Improve surface heat resistance and heat flow distribution.
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