(Invited) Expectations for High-Power Semiconductors in Microwave-Assisted Chemistry
Authors: Mitani Tomohiko , Shinohara Naoki 
Published: February 2020

Microwave-assisted chemical reactions have attracted significant attention over many years, primarily because the microwave heating mechanism, so-called dielectric heating, is quite different from conventional conduction heating. Microwave heating has two interesting features: internal heating and selective heating. The internal heating takes place because microwaves propagate deeply into materials and simultaneously generate heat on both their internal and external regions. Selective heating is caused by dielectric property of materials. Microwaves can heat the material more preferably with a larger dielectric loss. These features will lead to shorter chemical reaction time and more efficient reaction.
Since a microwave oven was invented nearly 70 years ago, a vacuum tube device i.e. magnetron has accounted for an overwhelming share of microwave heating applications. High-power semiconductors, on the other hand, are rapidly developing throughout the 21st century. Semiconductor-type microwave ovens with LDMOS have been developed these days. Although they are still inferior to the magnetron in cost and output power, the high-power semiconductors are expected to enhance microwave heating applications, especially microwave-assisted chemistry, thanks to their high spectrum purity, high frequency stability, wide frequency variability, and functionality.
Semiconductor amplifiers will work more effectively in a resonant-type (so-called single mode) applicator than magnetrons because of their aforementioned features. In a single mode applicator with a high-quality resonator, microwaves can heat materials the most efficiently at the resonant frequency. Then frequency stability and adjusting capability are quite important because the resonator has narrow passband and the resonant frequency is easily shifted due to physical properties and volume of the materials. Hence semiconductor amplifiers are more suitable than magnetrons as a microwave power source of the single mode applicator.
With respect to frequency allocation, the ISM bands of 2.45 GHz or 915 MHz (only used in several countries) are generally used for microwave heating. However these frequency bands are not always optimal for chemical processing because permittivity and permeability of materials are dependent on frequency and temperature as well as their physical and chemical properties. For this reason, we have developed batch-type microwave heating applicators which one can irradiate liquid samples with microwaves over a wide frequency range from 800 MHz to 2.7 GHz. Then high-power semiconductor amplifiers play an essential role in examining chemical reactions by using our developed applicators, which the magnetron can never assume.
A challenging problem on microwave-assisted chemistry is scale-up of the microwave applicators for high-volume production. High-power, highly-efficient and compact semiconductor amplifiers, especially with AlGaN/GaN power devices, will contribute greatly to commercialization of microwave-assisted chemistry.
In the present paper, we introduce availability of the high-power semiconductors in the microwave-assisted chemistry field.