Typical microwave ovens do a lousy job of defrosting because liquid water absorbs their radiation far better than ice. So once a spot melts, it will quickly rise to cooking temperature while the rest of the food remains frozen. Would it be possible to build an oven that uses microwaves absorbed preferentially by ice instead, so it would defrost well? Such an oven would presumably be inefficient for cooking, but still valuable.
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It would be very difficult to do so. Microwaves heat by adding energy at resonant frequencies of the molecules. Ice and water have very different ranges:
The ease of the movement depends on the viscosity and the mobility of the electron clouds. In water, these rely on the strength and extent of the hydrogen-bonded network. In free liquid water, this movement occurs at GHz frequencies (microwaves) whereas in more restricted 'bound' water it occurs at MHz frequencies (short radiowaves) and in ice at kHz frequencies (long radiowaves).
A radio wave in the GHz region is less than 1m long, which makes it easy to work with in the spaces a microwave has to work with, and easy to generate with reasonable sized antennas. As it turns out, microwaves must operate in the 2.450 GHz band because that's the band allocated to microwaves by the FCC. That's a wavelength of roughly 12cm, so the antennas are very reasonable indeed.
Closer to 1kHz, a region known as VLF, we find wavelengths of almost 3000km. This means our antennas have to be much shorter than their associated wavelength, which makes them much less efficient. Most of the energy of such an antenna doesn't actually get emitted from the antenna. It is typically sent to ground as "waste." I'd have to consult an antenna expert to get a real answer, but 10% efficiency is not unheard of for VLF antennas.
In order to “tune” a microwave oven to handle defrosting different from cooking, I think you would need more than one microwave frequency and I don't think that’s going to happen since the FCC sets the frequency range and the frequency of 2450 MHz has become the industry standard. Even if the permitted range by the FCC would allow another frequency more favorable to ice, it would probably drive up the cost more than it would be worth to consumers.
As far as I know the defrost cycle on all microwave ovens involve varying the on and off times. Shorter on and longer off times defrost more evenly. However, it would also take longer to defrost and users now days expect everything to happen quickly in a microwave oven. Winds up being a compromise.
Hope this helps.
There is a way to do this, as follows. A trap for the melted water can be built inside the microwave cavity using the same material (thin perforated metal sheet) used to block microwave transmission through the window in the door of the oven. Inside that trap you position a glass beaker to catch the melt, which drips into the trap through a small hole in its top.
Once inside the trap, the water cannot be heated anymore by the microwaves inside the cavity and those microwaves can instead be used to melt the ice.