To avoid the reaction between the molybdenum crucible and the melt, comprehensive measures should be taken from four dimensions: material selection, process control, environmental management, and surface treatment. The following are specific measures:
- Material selection
Matching melt material
do not react or react very little with molybdenum are preferred, such as gold, silver, copper, nickel and other low-melting-point metals.
Avoid highly active metals: Materials such as aluminum, titanium, and zirconium that easily form intermetallic compounds with molybdenum (such as MoAl ₂ and MoTi ₂) are prohibited.
Alloy selection: When melting alloys, it is necessary to ensure that there are no highly active elements in the alloy composition, or to inhibit the reaction by adding stabilizers (such as chromium).
Crucible purity control
Use high-purity molybdenum crucibles (purity ≥ 99.95%) to reduce the risk of impurities (such as iron, carbon) reacting with the melt.
- Process Control
Temperature Management
Upper temperature limit: The heating temperature should be strictly controlled below the temperature limit of the molybdenum crucible (usually ≤ 1800°C) to avoid the reaction rate increasing at high temperature.
Temperature gradient: Use slow heating/cooling process to reduce structural damage caused by thermal stress.
Time Control
Shorten the melting time: reduce the contact time between the melt and the crucible and reduce the probability of reaction.
Intermittent use: Avoid long-term continuous heating and cool the crucible periodically to restore surface stability.
Melt volume control
Filling rate limit: The melt volume shall not exceed 70% of the crucible volume to prevent the melt from expanding and overflowing or excessively contacting the crucible wall.
- Environmental Management
Atmosphere protection
Inert gas: Heating in an inert atmosphere such as argon or nitrogen to isolate oxygen and prevent oxidation reactions.
Vacuum environment: Use vacuum furnace for heating to completely eliminate the possibility of gas participating in the reaction.
Humidity Control
Dry environment: Ensure that the humidity in the furnace is ≤ 10% RH to prevent water vapor from reacting with the melt or crucible.
- Surface treatment
Coating protection
Zirconia coating: Spray zirconium oxide (ZrO ₂) coating on the inner wall of the crucible to form a physical barrier to isolate the melt from the molybdenum matrix.
Silicon carbide coating: suitable for high temperature environments, with a temperature resistance of up to 2000°C, but it is necessary to ensure that the coating is firmly bonded to the molybdenum substrate.
Pre-oxidation treatment
Surface passivation: Before the crucible is used, a dense MoO₂ layer is generated by low -temperature oxidation to slow down the oxidation rate at high temperatures.
- Operation Specifications
Preheating
Slow heating: It takes ≥ 2 hours from room temperature to working temperature to avoid thermal shock causing coating peeling or crucible cracking.
Cleaning and maintenance
Cleaning of melt residue: After each use, thoroughly clean the melt residue on the inner wall of the crucible to prevent reaction during the next heating.
Regular inspection: After every 50 uses, check the oxidation degree and coating integrity of the crucible inner wall.
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