Synergistic effect of microwave plasma and ultrasonic wave on decomposition of organic compounds in water

Abstract

To improve the decomposition efficiency of organic compounds dissolved in water, an ultrasonic wave was synchronously superposed to a microwave plasma produced under water. By changing the phase difference between the microwave plasma pulse and the ultrasonic wave, enhancement of OH emission in the plasma as well as enhancement of organic decomposition was observed when the pulsed plasma was ignited at a phase of instantaneous reduced pressure induced by the ultrasonic wave. This result suggests that the ultrasonic power can enhance OH radical formation by choosing the timing of the synchronously-injected pulsed microwave power. The decomposition rate from the viewpoint of energy efficiency was about four times larger with the ultrasonic wave superposition compared with the case of ordinary microwave plasma treatment without the ultrasonic superposition.

An Attempt to Produce Electrical Discharges in Acoustic Cavitation Bubbles

“…The formation of bubbles by Joule heating becomes less efficient at a higher water pressure because of a higher boiling temperature. Actually, it has been observed experimentally that the optical emission intensity from plasmas in bubbles induced by microwave heating becomes weaker at the higher ultrasonic pressure [14].…”

“…Recently, electrical discharges inside bubbles in liquids attract much attention of many researchers in conjunction with pollution control [1,2], sterilization [3,4], treatments of biological tissues or plasma medicine [5,6], and material processing [7]. In many cases, the power supply for the electrical discharges firstly produces bubbles via the Joule heating of the liquid, and it is followed by electrical discharges inside the bubbles [8][9][10][11][12][13][14]. Another way is the artificial introduction of bubbles from gas nozzles.…”

Temporal Variations of Optical Emission Spectra in Microwave-Excited Plasma in Saturated Water Vapor under Reduced Pressure during Photoresist Removal

“…A water-cooled dummy load was equipped between the microwave generator and the plasma to protect the magnetron from damage [25]. Non-modulated 2.45 GHz microwaves were injected into the water vapor through the slot antenna [12][13][14][15]. Forward and reflected microwave powers were measured using a mean-responding power detector that had been set for use in high-frequency receiver and transmitter signal chains up to 2.5 GHz (MMO-220HP; Ohta Electronics Co. Ltd.).…”

“…Using a slot antenna, we developed microwaveexcited plasma produced in water bubbles under a reduced saturated vapor pressure condition [12][13][14][15] and applied it to photoresist removal [16]. Using this method, ultrapure water is used as a source gas for the microwave-excited plasma.…”

Atmospheric pressure water-vapor plasma in an air-shielded environment by water flow

“…Furthermore, the underwater discharge inevitably requires the production of the gas phase (bubble) which is very unstable and degrades the process reproducibility. We have reported effective production of a microwave underwater plasma in the reduced pressure environment [26][27][28] or microwave plasma at reduced pressures utilizing the Venturi effect. 29) Although these plasmas are rather effective at producing the plasma utilizing the reduced pressures, they are still not suitable for the large-area process, because the plasma is enclosed by solid materials.…”