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I'm looking to demonstrate to class of students how microwave absorption works, but I need something relatively common which I can easily make into a board. Does anyone have any suggestions for substances which I could use (currently my best idea is sandwiching a plastic bag full of gel or cheese - something viscous enough not to leak easily, but with a high water content - in between two boards, effectively making an absorbing plane to contain the microwaves) for this?

Oh, and for more information I'm using a microwave transmitter, receiver and oscilloscope for the rest of the setup.

Thank you for your time!

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  • $\begingroup$ The most common microwave absorbing substance is plain ole' water. Anything that contains significant amounts of it will heat up in a microwave. Like cheese, for instance. Or highly hydrated salts like alum or aluminium sulphate hydrate... $\endgroup$ – Gert Dec 1 '18 at 21:22
  • $\begingroup$ What are you using as a source of microwaves? A microwave oven? What do you mean by "that can be easily made into a board"? $\endgroup$ – JackI Dec 1 '18 at 21:23
  • $\begingroup$ Why not use ice? $\endgroup$ – Farcher Dec 1 '18 at 21:29
  • $\begingroup$ This needs to last at least a fortnight for multiple classes to use it. So far my best idea has been to seal dough in clingfilm and sandwich it between two boards. $\endgroup$ – DoublyNegative Dec 1 '18 at 22:08
  • $\begingroup$ See Fig. 2 here: academia.edu/27070081/… $\endgroup$ – safesphere Dec 2 '18 at 0:58
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When I do demonstrations of this type, I use a hand and also a wet sponge. I demonstrate that a dry sponge doesn't absorb microwaves, then wet it and show that the wet sponge does. If you want to be able to do the prep and assembly in advance, I would just put a wet sponge inside a ziplock bag. If you need a larger size, I bet wet paper towels inside a freezer-bag ziplock would also be a very strong absorber. You also might be able to find rectangular sealable bottles that you could just fill with water.

In comments, people have suggested ice. Actually, I believe ice is a poor absorber. This is basically work = force x distance. The water molecules are locked in place and can't rotate.

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  • $\begingroup$ Thanks, wet paper towels sound like a really good idea (I was otherwise left mixing flour and water to make dough to bag as an absorbent!) $\endgroup$ – DoublyNegative Dec 2 '18 at 19:53
  • $\begingroup$ It's rather interesting to note that when water is placed on a thin wet paper towel, it actually seems to act more like a mirror than an absorbent, so would you know of any method/material (just for future reference), which is thin and still works to absorb microwaves (I know this wasn't the original question, but just wondered). $\endgroup$ – DoublyNegative Dec 10 '18 at 21:40
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Maybe I misunderstand what you need, but why not use specialized microwave absorber products (carbon/foam/magnetic iron based), such as that at http://stores.cumingmicrowave-online-store.com/c-ram-lf-75/ - flat panel 61x61cm, 20dB absorption (with metal backing).

If $95 is too expensive, you can look for something less efficient or less broad-band. Please read carefully manufacturer's notes.

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Plasmas absorb microwaves like crazy https://www.youtube.com/watch?v=G7lfzA7WzVI

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  • $\begingroup$ The OP wants to make a board out of it. $\endgroup$ – Ben Crowell Dec 2 '18 at 1:22

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