Selecting the right ultra-high vacuum diffusion pump oil requires a comprehensive consideration of the vacuum system's operating conditions, performance requirements, and compatibility. This can be determined from the following key perspectives:
1. Vacuum Requirements
The key consideration is the oil's saturated vapor pressure, which must match the system's target vacuum range. For example, for ultra-high vacuum applications between 10⁻⁸ and 10⁻¹⁰Pa, choose an oil with a vapor pressure ≤10⁻¹¹Pa (at 20°C). If the vacuum requirement is slightly lower (e.g., 10⁻⁶Pa), choose an oil with a slightly higher vapor pressure, balancing cost and performance.
2. Operating Temperature Range
Diffusion pump oil typically operates at temperatures between 200°C and 300°C. Ensure that the oil exhibits excellent thermal stability within this range, preventing decomposition, carbonization, or increased volatility. For example, methylphenyl silicone oil with a higher phenyl content offers better high-temperature resistance than pure methyl silicone oil, making it more suitable for high-temperature applications. 3. System Compatibility
Avoid chemical reactions with system materials (such as metals, rubber seals, and other vacuum greases). For example, some fluorinated silicone oils may corrode certain metals, so material compatibility must be confirmed in advance.
If the system involves gases (such as oxygen or corrosive gases), choose a more chemically inert oil to prevent oxidation or degradation.
4. Service Life and Maintenance Cost
High-quality oils should have low volatility and resistance to cracking, reducing downtime costs associated with frequent oil changes. For example, high-molecular-weight silicone oils generally have a longer service life than low-molecular-weight types due to their stable molecular structure.
5. Industry Application Specifications
High-precision applications such as semiconductors and optical coatings may have strict requirements for oil purity (e.g., residual metal ions and volatiles). Therefore, select a specialized type that meets industry standards (such as SEMI specifications) to avoid contamination of the workpiece.
In summary, when selecting an oil, it is important to prioritize key system parameters such as vacuum level, temperature, and media. Then, consider compatibility and long-term cost considerations. If necessary, consult the pump manufacturer's recommended oil type.