MadSci Network: Engineering

Re: Material of microwave oven walls?

Date: Sat May 15 09:57:49 1999
Posted By: Edward Peterson, Staff, Chemical Engineering, S&B Engineers and Constructors
Area of science: Engineering
ID: 926534442.Eg

There are lots of questions to answer, so I will discuss them in order.

I would like to know the material out of which the walls of microwave ovens are made after looking at all the different problems posted about the behaviour of metals in microwave ovens.

Microwave ovens are made with an aluminum interior. Aluminum has a very low microwave absorptivity. Metals that have lower absorptivity to microwave energy include copper, silver and gold. Aluminum is used because it is cheap. The aluminum walls do absorb some of the heat, about 2 to 6%, but that is generally not considered too severe. The main issues with metals in a microwave oven involves:

  1. Metals that are good absorbers of microwave energy (i.e. iron, nickel, tungsten, zinc, etc.). These will heat up very quickly and can even melt!
  2. Metal with a miniscule surface thickness or powdered metal. Metals, even aluminum, have a penetration depth on the order of a few microns. When the metal is the same thickness or diametr as the depth of microwave penetration, a resonance effect can cause very rapid heating.
  3. Sharp edged metal in close proximity. The electromagnetic energy in a microwave oven is continuous. At a metal surface, the electrical potential goes to zero. If two metal points come in close to one another in a microwave oven, the intervening space can have a very large field gradient, which leads to sparking.
The walls of most microwave ovens are aluminum which are also coated with a plastic that does not absorb microwave energy. Examples of polymers that do not absorb microwave energy are: polyethylene, polypropylene and polystyrene. The plastic coated aluminum is easier to keep clean that the aluminum wall would be by itself. Otherwise, stainless steel can be used without a surface covering. More expensive microwave ovens have stainless steel interiors.

Also, how is the oven made leak-proof?

First, the welds are continuous on the metal surfaces so there is no leak. Second, microwaves are relatively big, so they can't get through holes too easily. The screen that you look through in the door does not let microwaves through because the holes are about 1/20 to 1/100 of the microwave wavelength, which is about 5.3 cm. Microwaves can get through an infinitesimally thin crack that is greater than a quarter of a wavelength in length. Therefore, the door should leak very badly. To solve this problem, microwave oven builders use a quarter wave choke method. Although quite complex mathematically, it is a simple technique not unlike designing an organ pipe (music) to give no sound at all by cutting it off at a point where the sound wave has zero amplitude. Most manufacturers add some microwave absorbing polymer around the door to make sure no microwave energy escapes.

Doesn't the wall material heat up? I have investigated the glass door (does not heat up) and some of the front rods inside (does heat up), but how about the walls (my hand never reached that far).

Not very much when the oven is used normally. If food is placed in the oven that strongly absorbs microwave oven, then the energy is dissipated in the food preferentially. If material is put in the oven that poorly absorbs microwave energy, then more energy will be available to heat the oven walls. One object that you do not see that can be heated by microwave energy more easily than the walls is the magnetron itself. The magnetron is the source of microwave energy. It gets very hot when operating. If the energy that it sends out comes back to it, it will get even hotter. The hotter it gets, the quicker the magnetron will lose life and fail. Newer ovens have positive air cooling devices (fans) to keep the magnetron from getting too hot too quickly.

And how would you think different materials would act, like metals, plastcs, ceramics, composites...

Bulk metals heat with respect to their magnetic susceptibility. If powdered, the metals will heat according their penetration depth and magnetic susceptibility.

Plastics heat up according to the polarity of the polymer molecule. If there is no dipole in the polymer, it will not absorb microwave energy. Polyvinyl chloride, polyacrylic acid and polyethers heat up well. Polyethylene and polystyrene have no dipole, so they do not get hot. Other polymers do not absorb microwave energy if their temperature is below the glass transition temperature, even if they have strong dipoles. For example, polycarbonate and some nylons.

Ceramics are a more complex matter, but in general, pure oxides do not heat well. Materials like carbides heat enormously well and other ceramics like nitrides heat almost as well. Mixtures of oxides can be heated. If you make a molecular mixture of silicon and aluminum oxide, it will absorb microwave energy whereas the oxides of the pure elements will not absorb microwave energy.

I hope I have answered your questions. If not, please let me know. Recently, marvelous developments have taken place in applying microwave energy to chemistry and welding of ceramics. There is still a lot about microwave interactions of materials that is not understood well, so there is a lot to learn and explore.

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