Masateru Nishioka

We demonstrated soldering of an electrical component (the metal-oxide semiconductor field effect transistor (MOSFET)) on a printed circuit bord (PCB) via solder paste heated by microwave irradiation. The behavior of the object soldered with solder heated in a microwave magnetic field and electric field were evaluated by using various microwave resonators. In the magnetic field, the solder paste was selectively heated by microwave irradiation. We confirmed that the MOSFET was connected onto the PCB without any damage and that the MOSFET operated normally. However, in an electric field, the solder paste cannot be heated by microwave irradiation because the edges of the PCB were selectively heated. Consequently, the MOSFET could not be connected onto the PCB. The strengths of the electromagnetic fields produced by different microwave resonators were numerically simulated to determine the optimal positions for microwave soldering. Based on the simulation results, microwave heating with separate magnetic and electric fields was carried out and we investigated the heating characteristics of the samples. The appearance and cross sections of the samples were observed by using scanning electron microscopy and energy dispersive X-ray spectroscopy. To estimate any damage by microwave irradiation of the MOSFET, we evaluated the output and transfer characteristics of the MOSFET. We conclude that microwave heating in a magnetic field was superior to heating with an electric field.

A semiflow microwave (MW) heating reactor similar to a flow reactor system was developed. Slurry raw materials in the reaction tube were heated continuously and cooled rapidly by moving a thin MW resonator instead of flowing slurry raw materials. From highly viscous mother slurries, Linde-type A (LTA) and faujasite (FAU)-type zeolite nanoparticles of small crystal grains were synthesized quickly. Results show that this heating system can synthesize hydroxy-sodalite (SOD)-type zeolite from coal fly ash particles including those larger than 50 μm. Numerical calculations using the COMSOL Multiphysics program revealed the thermal distribution of liquids of various viscosities using the semiflow MW heating reactor.

We developed a method for in situ fabrication of copper nanoparticles inside cotton fibers. Copper nanoparticles can be fabricated mainly in the central part of the fiber by absorbing a raw material solution and by applying microwave heating in a state where the raw material solution is pressed with immiscible liquids. Surface SEM images and cross-sectional EDS mapping for the fabricated fibers clarified that copper nanoparticles fabricated on the cotton surface were suppressed considerably more by the hydrophobic raw material solution than by the hydrophilic raw material solution. These cotton fibers containing copper nanoparticles were found to have antiviral properties against the influenza A virus.

Microwaves (MWs) are often used to enhance various heterogeneous catalytic reactions. Here, we demonstrate real-time monitoring of a catalyst’s oxidation state in a microwave catalytic reaction using a resonance frequency. The changes in the catalyst’s oxidation state during the reaction induced changes in the resonance frequency in the cavity resonator. The resonance frequency was not affected by 2-propanol adsorption, while the frequency decreased with the reduction of WO₃ → WO_3−x. That is, the redox state of the WO₃ catalyst could be detected using the resonance frequency. The oxidation state of the WO₃ catalyst was then directly observed by the resonance frequency during the dehydration reaction of 2-propanol by microwaves as a model reaction. Resonance frequency monitoring revealed that the enhanced dehydration of 2-propanol by microwaves was attributable to the reduction of the WO₃ catalyst. Moreover, the temporal changes in the oxidation state of the WO₃ catalyst detected by the resonance frequency coincided with that observed by operando Raman spectroscopy. Therefore, real-time resonance frequency monitoring allowed facile detection of the bulk catalyst oxidation state under microwaves without using any spectroscopic apparatus.

We developed a simple fabrication method, which can emplace functional materials inside porous fibers. In contrast to conventional impregnation methods or surface coating, various functions can be included inside of natural or synthetic fibers. This fabrication method has three steps. First, the raw material solution is absorbed to the fiber. Then the fiber is immersed and pressurized in immiscible liquids with the raw material solution. Finally, the functional particles are fabricated in‐situ inside of the fiber by a chemical reaction such as microwave selective heating. As a model reaction, we fabricated silver‐nanoparticle‐containing cotton fiber. The elemental silver was distributed inside of the fiber in a cross‐sectional distribution as confirmed by SEM‐EDS. Furthermore, we fabricated fibers with zeolite particles inside of porous PTFE fibers.

We have developed a novel microwave (MW) soldering system using a cylindrical single-mode TM₁₁₀ MW cavity that spatially separates the electric fields at the top and bottom of the cavity and the magnetic field at the center of the cavity. This MW reactor system automatically detects the suitable resonance frequency and provides the optimum MW irradiation conditions in the cylindrical cavity via a power feedback loop. Furthermore, we investigated the temperature properties of electrodes by MW heating with the simulation of a magnetic field in the TM₁₁₀ cavity toward the mounting of electronic components by MW heating. We also developed a short-time melting technology for solder paste on polyimide substrate using MW heating and succeeded in mounting a temperature sensor using the novel MW heating system without damaging the electronic components, electronic circuits, and the substrate.