Are microwave susceptors attenuated in viscoelastic mediums?

[RMN]

Are microwave susceptors attenuated in viscoelastic mediums ?

I am experimenting with some widely known microwave susceptors like iron oxide and silicon carbide (materials that normally convert RF to heat quite effectively). In ceramic formulations they perform great, as expected. When formulated in silicone rubber, they do not perform like same weight % as ceramic formulations when subjected to identical watts/time in a microwave oven.

I am seeing less than 1/2 the temperature rise per identical runtime in the silicone matrix. Curious if the good performance of these materials in dry ceramic form could be due to the resonance or vibration of these materials in and against the dry ceramic matrix, and against each other (very frictional in dry ceramic ?) - - and in silicone rubber the medium is rather viscoelastic, so perhaps more of the vibrational energy is absorbed or attenuated by the more flexible nature of the medium ?

 

[Baluncore]

Sorry about the long delay.
I very much doubt it is a viscoelastic effect. Any vibration energy must become heat at some point.

I expect the difference observed is due to the different dielectric constant between the silicone and the ceramic. It may also depend on how the ceramic containing the grains was thermally processed. The grains may have changed in some way or the ceramic may contain voids.

 

[chemisttree]

Probably reflective losses are significant. See here for example. I believe you need to impedance match the composite (to air) to get full absorption otherwise the RF reflects.

 

[RMN]

Sorry about the long delay.
I very much doubt it is a viscoelastic effect. Any vibration energy must become heat at some point.

I expect the difference observed is due to the different dielectric constant between the silicone and the ceramic. It may also depend on how the ceramic containing the grains was thermally processed. The grains may have changed in some way or the ceramic may contain voids.

Not sure. The SC as a granule reacts strongly in microwave, but once mixed into silicone rubber no thermal phenomena at all occurs . . .

 

[RMN]

Sorry about the long delay.
I very much doubt it is a viscoelastic effect. Any vibration energy must become heat at some point.

I expect the difference observed is due to the different dielectric constant between the silicone and the ceramic. It may also depend on how the ceramic containing the grains was thermally processed. The grains may have changed in some way or the ceramic may contain voids.

Thanks for that link. Pure granules of silicone carbide placed as a heap in a microwave heat up rapidly, but when mixed into silicon rubber and microwaved, no thermal phenomena occurs.

 

[Baluncore]

Good to hear back.
As chemisttree pointed out, if the outer surface of the silicone reflects more energy than the surface of the ceramic then it would explain the reduced heating. That will come down to the dielectric constant of the silicone versus the ceramic.

My next question would be to ask if the silicone and ceramic were tested as controls without any extra material added.

Silicon carbide, SiC, has interesting properties. If I remember correctly, it can be a semiconductor and also have a strongly frequency dependent dielectric constant. So matching free space to SiC through a silicone rubber or ceramic could be quite interesting.

Did you collect dielectric constant information on your SiC, silicone rubber and ceramic?

 

[RMN]

Baluncore:

2.9 SI is dielectric constant for silicone rubber. I did not use controls.

I assume most of this is related to the polarity of the material, silicone rubber is very low polarity, styrene and PVC are examples of high polarity polymers.

Many chemists simply use the amount of oxygen present in the chemistry to get a rough idea of polarity.

I cannot find a dc for polycaprolactone, a nylon series aliphatic linear polyester, another polymer I am also interested in working with.

The ceramic is calcium aluminate refractory cement with perlite and sand, no dc published for it either, but with 10% silicon carbide added it heats up like a champ in a microwave . . . I can get >300F to 360F in 60 seconds exposure fairly easily (laser thermometer).

Heading off to work . . . .

 

[chemisttree]

Impedance is more than just the dielectric constant as I understand it. Impedance is frequency dependent, for example. Perhaps frequency dependent dielectric permittivity is the term you intend?