Are microwave susceptors attenuated in viscoelastic mediums?

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Discussion Overview

The discussion revolves around the performance of microwave susceptors, specifically iron oxide and silicon carbide, when incorporated into viscoelastic mediums like silicone rubber compared to traditional ceramic formulations. Participants explore the factors affecting the heating efficiency of these materials in microwave applications, considering aspects such as dielectric constants, impedance matching, and the physical properties of the materials involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes a significant reduction in temperature rise of microwave susceptors in silicone rubber compared to ceramic formulations, suggesting that the viscoelastic nature of silicone may absorb or attenuate vibrational energy.
  • Another participant expresses doubt about the viscoelastic effect, proposing that differences in dielectric constants between silicone and ceramic could explain the observed heating performance.
  • Reflective losses and the need for impedance matching to air are mentioned as potential factors affecting energy absorption in the composite materials.
  • Concerns are raised about the thermal processing of the ceramic and the potential changes in the grains or presence of voids that could influence performance.
  • One participant highlights the interesting properties of silicon carbide, including its frequency-dependent dielectric constant, and questions whether dielectric constant information was collected for the materials used.
  • Another participant discusses the polarity of silicone rubber compared to other polymers and its potential impact on microwave performance.
  • The distinction between dielectric constant and impedance is clarified, with a suggestion that frequency-dependent dielectric permittivity may be a more accurate term.

Areas of Agreement / Disagreement

Participants express differing views on the primary reasons for the reduced heating in silicone rubber, with no consensus reached on whether viscoelastic effects, dielectric constants, or other factors are most significant.

Contextual Notes

Participants mention the lack of control tests and specific dielectric constant values for some materials, which may limit the conclusions that can be drawn from the observations made.

Who May Find This Useful

This discussion may be of interest to researchers and practitioners in materials science, engineering, and applied physics, particularly those focused on microwave applications and the properties of composite materials.

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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 ?
 
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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.
 
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.
 
Baluncore said:
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 . . .
 
Baluncore said:
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.
 
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?
 
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 . . . .
 
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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?
 
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