Thursday, May 15, 2014

Explore the Susceptor

This is a part of the first law of thermodynamics, aka the law of conservation of energy. It states that the total energy of an isolated system cannot be changed. Energy cannot be created nor destroyed only converted from one form to another.  A susceptor merely changes one form of energy into another. A susceptor is a material that can absorb electromagnetic energy (i.e. radio waves) and convert it to heat energy. The most recognizable example of the susceptor in your home is the one presently wrapped around a burrito in your microwave.

It's intuitive when you think about the physics. The heating element in the toast oven operates on a similar principle. When electricity is applied to the element, the electrical resistance of the metal converts some of the current into heat energy. But converting RF is another more nuanced animal. In that same context a susceptor is a lossy material with a resistivity of around 200 Ω/sq. The one which ensconces your Hot Pocket consists of a paper substrate coated in aluminum or nickel and coverd in a layer of polyethylene terephthalate or PET, a polyester film. Similar warmers units might use molybdenum, niobium, or even stainless steel.

Believe it or not, but susceptors do have a use beyond crisping the underside of cheap pizza at your local 7-11. They can be used to apply heat through induction to non-conductive materials. There are situations in manufacturing where a metal surface would interact with the material being produced and a non-conductive container, crucible etc. is preferable. For example, in metalorganic vapour phase epitaxy (MOCVD) a chemical vapour deposition method is used to deposit a polycrystalline film in the manufacture of semi-conductors. In this case a graphite susceptor is often used but carborundum is sometimes also used.

Sometimes radio waves are good for unexpected things...