In the field of materials science, crucibles play a pivotal role in high-temperature processes, especially in deposition techniques such as evaporation coating. Appropriate crucible materials should be selected according to the specific type of deposition material being used. 
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In  Physical Vapor Deposition (PVD) processes, crucibles are employed to hold solid metals or compounds, which are then heated to the point of evaporation or sublimation. This allows the vaporized material to condense as a thin film on the substrate. Many molten materials require a crucible liner, and the actual charge capacity of the crucible depends on the volume of liner.

Common Issues and Solutions of Crucibles in Material Deposition Processes

Chemical Compatibility Issues

Crucibles and their liners must be chosen carefully to avoid unwanted chemical reactions at high temperatures. Incompatible crucible materials can corrode or contaminate the melt, which may compromise product quality.

Recommended Solutions:

• Use of Crucible Liners:Lining the crucible with a suitable material can prevent chemical reactions between the crucible and molten material, preserving the purity of the evaporated material. But not all crucibles are equipped with liners. For the evaporation of molten metals such as gold (Au) and the deposition of boron (B), cobalt (Co), chromium (Cr), iron (Fe), germanium (Ge), palladium (Pd), and nickel (Ni), crucible liners are generally not required.
• Selecting Appropriate Liner Materials:Depending on the molten material, liners made fromTantalum (Ta),Nickel (Ni),Molybdenum (Mo),Pyrolytic Boron Nitride (PBN), orTungsten (W)are commonly used to mitigate reactions and protect the crucible from damage.
  
  

Thermal Expansion Mismatch

A critical issue in high-temperature processes is the thermal expansion mismatch between the crucible and the molten material. During heating, molten materials typically expand more rapidly than the crucible, exerting pressure on the walls and potentially leading to cracks or deformation. Similarly, during cooling, stress can develop as the material contracts, especially if there is an excessive amount of residual material in the crucible.

Solutions:

• Matching Thermal Expansion Coefficients:Ensure that the thermal expansion coefficients of the crucible and the material are compatible to minimize stress.
• Limit Material Volume:Keep molten material confined to a small pool at the bottom of the crucible during cooling to avoid contact with the sidewalls, or exhaust all material before cooling.
• Regular Inspections:After each process, inspect the crucible for cracks or structural damage.
  

Crucible Filling Recommendations

The degree to which a crucible is filled depends on the type of deposition material, and it can vary significantly between sublimating materials, molten materials, and chemical compounds.

Sublimating Materials:Fill the crucible to 90% capacity for optimal performance. In constant-power or temperature evaporation, vapor distribution and rate change as the material is consumed, but sublimating materials (e.g., salts) usually exhibit minimal issues.

Molten Materials/Non-Sublimating Materials:The recommended fill level for non-sublimating materials is 75% of the crucible's capacity. Overfilling can lead to cracks due to the difference in thermal expansion between the crucible and materials like aluminum oxide when reheated.

Chemical Compounds:For chemical compounds, the best results are typically achieved when the crucible is filled to about 50%, with the material reducing to around 17% during evaporation. If vapor quality (e.g., dissociation, vapor temperature, or rate) is critical, further adjustments may be required to optimize the evaporation process.

Crucible Cleaning Best Practices

To avoid cross-contamination, it is recommended that each crucible is dedicated to heating a single type of material. While some organic compounds can be thoroughly removed with proper cleaning, others may be more difficult to eliminate. The success of cleaning depends on the specific organic compound and the firing temperature.

Recommended Cleaning Procedure:

• Soak the crucible in a 0.1M potassium hydroxide solution for 15-20 minutes.
• Rinse with deionized water.
• Clean in an ultrasonic bath with a fine powder detergent (2 tablespoons dissolved in 200 mL of water) for 60 minutes.
• Rinse thoroughly with deionized water again.
• Submerge the crucible in acetone and ultrasonically clean for 20 minutes.
• Repeat the process with isopropanol for an additional 20 minutes.
• Dry the crucible at 70°C for at least 40 minutes, or longer if needed.

Alternatively, heating the crucible to temperatures well above the normal evaporation point of the organic materials can allow for self-cleaning by burning off residues.

Custom Crucible Materials and Solutions

For custom crucible shapes, pore sizes, materials, or other specialized requirements, please contact us for tailored solutions.