There are several microwave absorber depending on the frequency range of the microwave, M-type barium hexagonal ferrite-based wide band microwave absorber are used most commonly.

Check these publications

1) http://www.sciencedirect.com/science/article/pii/S0304885303007959

2) http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=801112&tag=1

3) http://www.sciencedirect.com/science/article/pii/S0304885399007556

You can use of kind of ceramics in the microwave. If your microwave is not like the laboratory devise i.e kitchen microwave . So you should use of brick ceramic (Al2O3) inside of your system. anyway, please let me know, what kind of materials you want synthesis in the microwave

Thanks for your reply

I think that I should explain more. I want to put graphene oxide in microwave(kitchen type) and it needs a solution or some materials to absorb microwave. for example Ethylene glycol but I am not sure about safety.

In view of my point, usage of solution e.g Ethylene can be so dangerous due to explosion. So you should another materials which SiC can be more suitable.

Please feel free if you have any question.

I need microwave absorbent to convert microwave irradiation to heat. ceramics like Al2O3 can not convert microwave to heat. irradiation will be damped by ceramics. it is not applicable for this test.

I am not line with you. Since, I synthesized kind of materials with microwave which coated with brick Al2O3. The temperature can be reached up 1200C after 1h. Please pay attention, you should use bulk SiC as absorption of irradiation, i.e. you put your samples on SiC bulk.

1 hour is too long. how did you keep a hot brick in microwave for 1 hour?! reduction should be done in less than 3 min. I tested refractory brick before, nothing happened. is there any other alternative?

We synthesized ceramic by microwave. Holding time is depend on your materials. As mentioned, SiC can absorbed irradiation.