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Silicon Carbide (SiC)
Silicon Carbide (SiC) is a ceramics material which strongly absorbs microwave irradiation. It can therefore be used to enhance microwave heating of microwave transparent reaction mixtures.
Microwave chemistry generally relies on the ability of a reaction mixture to efficiently absorb microwave energy, taking advantage of microwave dielectric heating phenomena, like dipolar polarization or ionic conduction mechanism. Consequently, the reaction mixture must be microwave-absorbing in order to be efficiently heated in the microwave field.
In fact many commonly used organic solvents are almost microwave-transparent. If the dissolved compounds (starting materials, reagents, catalysts, etc.) are not microwave–absorbing either, the reaction mixture cannot be heated in the microwave field.
While Teflon vessels are completely microwave-transparent, the usual borosilicate glass vials are microwave-absorbing to some extent and can therefore help to passively heat a reaction mixture in the microwave field. Nevertheless, in some cases sufficient heating will still not be possible. Therefore, several strategies have been employed to overcome this problem, like switching to more polar solvents or the addition of strongly absorbing materials (e.g. salts or ionic liquids). However, all of these invasive methods have a clear disadvantage in that the polarity of the original solvent system is modified, which may lead to different reaction pathways or significantly lower product yields. Furthermore, if nevertheless used, polar additives can significantly complicate the work-up.
Properties and advantages of Silicon Carbide in microwave synthesis
As a perfect non-invasive alternative, SiC has turned out to be the ideal material for enhancing heating rates of non-polar reaction mixtures. It strongly absorbs microwave energy and subsequently and rapidly transfers the generated thermal energy via conduction phenomena to the reaction mixture. This allows microwave-transparent or poorly absorbing reaction mixtures to be effectively heated to extremely high temperatures and pressures. Furthermore, due to its high melting point (approx. 2700 °C) and very low thermal expansion coefficient it can be employed up to extremely high temperatures. Additionally, due to the fast heat distribution within the material, hot spots can be excluded and homogeneous temperature distribution and uniform heating within the whole material are guaranteed.
Practically, SiC is a virtually indestructible and fully recyclable solid material, and different shapes of microwave reactor accessories are provided (see Figure 1), which can be employed in both single-mode and multimode microwave reactors.
Conventional heating vs. microwave heating
An interesting aspect associated with the use of SiC has to be mentioned. Since SiC strongly absorbs microwave irradiation, the vessel is heated rather than its content. Therefore, the mixture itself is purely heated by conventional conduction and convection principles and not by direct in-core heating. Nevertheless, it has been shown that the major advantages of microwave chemistry are still present due to the rapid superheating to high temperatures in closed vessels.
These findings are the basis of conventionally heated reactors for high-speed synthetic chemistry.