Q: What are the best materials for electromagnetic applications?
A: This depends on the application, and on factors such as the performance, cost and specific geometry. Electromagnetic materials are commonly compared using B-H curves (B stands for induction and H for magnetizing force), which are basically a plot of how much magnetic flux a material will carry versus the intensity of the magnetic field. This can be experimentally seen by using an electromagnet and observing how strongly a material is attracted to it. Those that are attracted strongly are said to have high permeability to flux; e.g. plastic has an extremely low permeability while low carbon steels have very high permeability.
Among various steels and certain other alloys, there are differences in how much flux a given material will transmit (how strongly it is attracted to an electromagnet). Some steels are almost like plastic in this respect, while other steels hardly take any magnetic intensity at all to be attracted strongly. At the same time, some materials will be attracted strongly at low intensities, but as the intensity builds, the attraction (or flux flow) does not correspondingly increase. This is where a good B-H curve can be indispensable for easy comparison of materials.
Here is a list of some common applications and good materials for each:
Motors: Silicon steels have superb permeability in one plane but poor permeability in other directions. However, for motor laminations this is just fine.
Solenoids: Low carbon steels are great for many applications, especially due to their low cost; 430 F works very well when stainless steel is required; Hiperco 50 is one of the best materials available, but not many applications exist that warrant the increased performance versus the high cost.
Shielding: Supermalloy and Mumetal both have very high permeability at relatively low intensities. They become easily saturated with flux at higher magnetic intensities, but most shielding applications do not require high intensities.
For materials that do not respond when subjected to magnetic fields, 300 series stainless steel is great when high strength is required; brass works well when self-lubricating properties are needed, and plastic is a great option if high material stiffness or strength is not a concern.
Q: I am trying to seal a mixture of medical gases (helium and oxygen) under high pressure, but am encountering issues with slow leaks. Can you suggest some ideas for good sealing techniques?
A: The gas that is leaking is probably helium. Helium is such a small molecule that it is very loose and will fit through tiny spaces that other molecules cannot enter. Press fits can be problematic because the molecules can escape via the surface roughness, especially under high pressure. Welds are the best sealing option.
For movable seals, I am sure the seal vendors would be overly happy to inform you all about their latest products, although a properly-designed double o-ring seal is usually a decent solution.