In demanding operational conditions requiring temperatures from room temperature up to 250°C, the presence of a magnetic environment, and an ultra-high vacuum (typically defined as pressures below 10⁻⁷ Pa), the selection of appropriate sealing rings is paramount. Such conditions are commonly found in advanced scientific research installations (e.g., particle accelerators, fusion experimental devices), semiconductor manufacturing equipment (e.g., etching machines, ion implanters), and aerospace propulsion systems.
Core Challenges and Sealing Requirements
Achieving effective sealing necessitates simultaneously meeting the following critical requirements:
- High-Temperature Resistance: The material must withstand long-term operation at 250°C, maintaining elasticity and sealing performance without decomposition or softening.
- Low Outgassing Rate: In ultra-high vacuum environments, the material’s total outgassing rate must be extremely low (typically <1×10⁻⁸ Pa・m³/s) to avoid releasing volatile substances that could contaminate the vacuum.
- Magnetic Interference Resistance/Compatibility: In magnetic environments, the sealing ring material itself should be non-magnetic or not interfere with the magnetic field, usually requiring the use of non-ferromagnetic materials.
- Radiation Resistance (if applicable): If ionizing radiation is present (e.g., in some experimental setups), the material must resist radiation damage.
- Mechanical Properties: Sufficient elastic recovery rate (typically requiring above 80%) and resistance to compression set are essential to cope with system pressure fluctuations and thermal cycling.
Suitable Sealing Ring Types and Materials
Based on the search results, the following sealing ring types and materials are preferred solutions for these苛刻 conditions:
1. Metal Seals
Metal seals are considered the gold standard for ultra-high vacuum environments, perfectly meeting the requirements of low outgassing, high-temperature resistance, and magnetic compatibility.
- Material Selection:
- Oxygen-Free Copper: This is the most common choice. It exhibits excellent plastic deformation capability, achieving sealing by plastically flowing under compression to fill minor imperfections on flange surfaces. It is non-magnetic, offers superior high-temperature resistance, and can endure high-temperature baking (often well above 250°C) to accelerate outgassing for achieving higher vacuum levels, making it the primary choice for widespread application.
- Pure Aluminum: Also non-magnetic and relatively inexpensive. It is softer and easier to form and seal, but its mechanical strength at higher temperatures may be inferior to oxygen-free copper.
- Silver / Gold: These metals offer exceptional performance and extremely low outgassing rates. However, their very high cost typically restricts their use to special or extreme research applications.
- Common Configurations:
- Conflat Flange (CF) Seal: Utilizes an oxygen-free copper gasket paired with a stainless steel knife-edge flange. Under bolt preload, the copper gasket plastically deforms and bites into the knife-edge, forming a static seal with extremely high integrity. This is a standard configuration in ultra-high vacuum systems.
- Spring-Energized Seals (e.g., Helicoflex): Consist of a metal jacket (e.g., oxygen-free copper, silver, stainless steel) and an internal spring. The spring provides continuous compensating force, enabling adaptation to thermal expansion/contraction and minor deformations within the system, resulting in very high sealing reliability. They are particularly suitable for applications with temperature cycling or vibration.
2. Perfluoroelastomer (FFKM)
If the system design is more suited for elastomeric seals or requires greater installation convenience, Perfluoroelastomer (FFKM) represents the top-tier choice among polymer materials, albeit at a very high cost.
- Characteristics: It can be regarded as the ultimate version of fluorocarbon rubber. Since almost all hydrogen atoms in its molecule are replaced by fluorine atoms, FFKM possesses excellent high-temperature resistance (can withstand over 300°C) and astonishing chemical resistance, capable of withstanding most harsh chemical media and plasma.
- Vacuum Performance: FFKM sealing rings manufactured through special formulation and clean processes exhibit extremely low outgassing rates and extractable content, meeting the stringent requirements of semiconductor and ultra-high vacuum equipment.
- Magnetic Properties: Elastomeric materials are generally non-magnetic and will not interfere with magnetic fields.
- Applications: Commonly used in vacuum chambers and corrosive gas delivery systems of semiconductor lithography and etching machines, as well as for oxidizer sealing in aerospace engines.
3. Fluorocarbon Rubber (FKM/Viton)
Fluorocarbon rubber is a commonly used elastomeric sealing material for high-temperature vacuum environments, representing a balance between performance and cost.
- Characteristics: It offers good high-temperature resistance (typically -20~250°C), oil resistance, and resistance to most chemicals.
- Vacuum Performance: The outgassing rate of standard FKM is higher than that of metals and FFKM. It is generally suitable for high vacuum (10⁻⁴ ~ 10⁻⁷ Pa) environments. For ultra-high vacuum applications, products with a low-outgassing-rate formulation must be selected, and high-temperature baking for degassing might be necessary (attention must be paid to its maximum baking temperature limit).
- Magnetic Properties: Non-magnetic.
- Note: It is not resistant to strong alkalis, ketones, and some ester solvents.
Comparison of Key Properties: The primary sealing options discussed—Oxygen-Free Copper metal seals, Perfluoroelastomer (FFKM), and Fluorocarbon Rubber (FKM)—differ significantly in their key characteristics. Oxygen-Free Copper seals withstand temperatures exceeding 400°C and exhibit extremely low outgassing, making them ideal for ultra-high vacuum (<10⁻⁷ Pa) applications. They are non-magnetic and offer good radiation resistance, but their elasticity and compensation rely on plastic deformation or internal springs. Their relative cost is high. Perfluoroelastomer (FFKM) seals can operate up to approximately 320°C. With extremely low outgassing (requiring clean-grade versions), they are also suitable for ultra-high vacuum (<10⁻⁷ Pa), are non-magnetic, offer good radiation resistance, and possess excellent inherent elasticity and compensation ability. However, their relative cost is very high, potentially exceeding ten times that of FKM. Fluorocarbon Rubber (FKM) seals have a lower maximum operating temperature of around 250°C. They exhibit a medium outgassing rate (requiring low-outgassing formulations) and are suitable for high vacuum (~10⁻⁴ – 10⁻⁷ Pa). While also non-magnetic and offering fairly good radiation resistance, their elasticity is good, and they represent a medium-cost option.
Selection and Usage Recommendations
- Priority Selection:
- For pure, utmost demanding ultra-high vacuum systems (e.g., particle accelerators, space environment simulation chambers), metal seals (oxygen-free copper) are the preferred and most reliable solution.
- For ultra-high vacuum environments that also involve corrosive media (e.g., semiconductor etching gases) or require better elasticity and easier installation, Perfluoroelastomer (FFKM) is the high-performance elastomeric choice, but it must be confirmed as an ultra-high vacuum clean-grade product.
- If the vacuum requirement is slightly lower (e.g., high vacuum) and the temperature range is within 250°C, Fluorocarbon Rubber (FKM) is an economical and practical choice.
- Design and Installation Key Points:
- Surface Quality: The surface roughness (Ra) of the sealing surface is crucial. For metal seals, Ra ≤ 0.8 μm or even lower is typically required. For elastomeric seals, a higher finish (Ra ≤ 0.4 μm) helps reduce wear and potential leakage points.
- Compression Ratio Control: The compression ratio of the sealing ring must be strictly controlled during installation. Over-compression can cause permanent deformation or damage, while insufficient compression leads to leakage.
- Uniform Tightening: Employ a symmetrical, multi-bolt tightening sequence to ensure even force distribution on the flange, preventing warping or distortion of the sealing surface.
- Baking: Ultra-high vacuum systems often require baking. Always confirm that the selected sealing ring material can withstand the system’s baking temperature.
Summary
Under the conditions of room temperature to 250°C, presence of a magnetic field, and requirement for ultra-high vacuum, oxygen-free copper metal seals (particularly in Conflat flange or spring-energized configurations) are typically regarded as the most reliable and primary technical solution due to their extremely low outgassing rate, excellent high-temperature resistance, and non-magnetic properties. If elastomers are necessary due to system design or the need to handle corrosive media, then Perfluoroelastomer (FFKM) is the only elastomeric material that can simultaneously meet these extreme demands, but one must be prepared for its high cost.
The final choice should be based on a comprehensive trade-off considering the specific vacuum level indicators, budget, system structure, and requirements for maintenance and reliability. In all cases, priority should be given to technical advice and support from professional sealing component suppliers.
Post time: Sep-05-2025