Hermetically Sealed Magnetic Assemblies for Vacuum and Hydrogen Applications

Hermetic Magnetic Solutions for Vacuum and Hydrogen Environments

Hermetic sealing is essential in applications where preventing gas, liquid, and particulate ingress or egress is critical. This article highlights the importance of hermetically sealed magnetic assemblies, particularly in vacuum environments and those requiring hydrogen protection. Hydrogen can aggressively attack neodymium-iron-boron (NdFeB) magnets, and hermetic sealing offers effective solutions.

Hermetic Seals: An Overview

Hermetic seals create an airtight barrier to prevent substance passage into or out of a protective vessel. Widely used in industries such as food packaging, medical devices, electronics, MEMS, sensors, and aerospace, hermetic seals protect sensitive components from environmental factors and safeguard the environment from potentially harmful contents.

Rare earth magnets, such as neodymium (NdFeB) and samarium cobalt (SmCo) magnets, can generally be used in vacuum environments, but their compatibility depends on several factors.

Magnets in Vacuum Applications:

Magnets can be used in vacuum applications; however, some special considerations are required in the following areas:

Selection of the Magnet Material:

Select a permanent magnet that is not from the bonded group. Bonded magnet materials are made by using resins to bind the powder. The resins are polymeric materials, such as thermoplastics or thermosetting resins, rubber, or elastomers, such as nylon. These materials have high TML (Total Mass Loss) and CVCM (Collected Volatile Condensable Material) values. Rare-earth magnets (Samarium Cobalt or Neodymium Iron Boron), Alnicos and Ceramics are suitable.

Selection of a Surface Treatment:

Magnet materials may be utilized without coating in low vacuum conditions, but under high vacuum conditions, pressures lower than 10mTorr, surface treatment should be considered. Metallic surface treatments are the most desirable, since polymer coatings will outgas, however, our PR1010 has been tested and found to meet standards considered very good for space applications (see outgassing characteristics on the technical data sheet for PR1010). We offer the following types of surface treatments that are vacuum compatible:

1. Nickel Plating
2. Aluminum Chromate IVD
3. PR1010 

Selecting the Pole Piece Material:

The pole piece or return path should be selected carefully. Consider using stainless steel CRES410, or if the material is prone to oxidation, consider Nickel plating. If you are using CRS1010, CRS1018 or other low carbon steel, or specialized steels such as Vanadium Permendur, Hiperco 50, Carpenter 50 or equivalent, Nickel plating is recommended.

 Securing the Magnet:

Magnets can be secured in one of two ways:

1. Mechanically
2. By use of bonding materials

If the magnet will be secured mechanically, outgassing is less of an issue. Consider the use of vented fasteners to prevent trapped gases in threaded holes.

If the magnet is secured using bonding materials, use advanced epoxies and adhesives with low TML and CVCM values. We offer several epoxies. Contact us before deciding, and we will work with you to select the correct adhesive or epoxy.

Putting the parts together:

Putting the parts together is very important. The parts MUST be carefully cleaned. Handling should be done by use of clean room compatible gloves. In very low pressures, even a fingerprint is a problem. We do not suggest doing this yourself. We have the experience necessary to put together an assembly that will be vacuum compatible.

1. Outgassing: In vacuum applications, outgassing is a significant concern. Outgassing refers to the release of trapped gases from materials when they are subjected to low-pressure environments. This can lead to contamination of the vacuum and affect the performance of sensitive equipment. As previously mentioned, rare earth magnets can outgas if not properly coated or treated, contaminating sensitive equipment in high-vacuum environments. To mitigate this, rare earth magnets are often coated with materials like nickel, gold, or epoxy, which reduce outgassing and protect the magnet from the vacuum’s effects.
2. Surface Oxidation: Neodymium magnets are particularly prone to oxidation, which can be exacerbated in a vacuum due to the absence of air and moisture. Coatings are also essential in preventing oxidation, especially when these magnets are used in ultra-high vacuum (UHV) conditions.
3. Mechanical Stability: In vacuum conditions, the absence of air pressure can impact the mechanical integrity of the magnet, particularly if the magnet has been bonded or if it relies on adhesives. Selecting the right adhesives that are vacuum-compatible and ensuring proper hermetic sealing are crucial for maintaining the magnet’s structural integrity.
4. Temperature Considerations: Vacuum environments often involve extreme temperature conditions. Rare earth magnets can lose their magnetism at high temperatures, so it’s important to consider the thermal properties of the magnet material and the specific application environment.

Materials and Methods for Hermetic Sealing

Hermetic seals can be made from metals, ceramics, glasses, and polymers. Metals and ceramics are preferred for their impermeability to gases like water vapor, hydrogen, and helium. Common hermetic sealing methods include:

1. Welding: Techniques like laser welding and electron beam welding join metals for high hermetic performance.
2. Thermocompression Bonding: Uses heat and pressure to join metals, often in semiconductor packaging.
3. Compression Seals: Formed by pressing two parts together, ideal for high-pressure and high-temperature applications.
4. Adhesion: Bonds formed by surface attraction between substrates and adhesives, such as epoxy.

Hermetically Sealed Magnetic Assemblies for Vacuum Applications