Metal ion plasma generation under strong magnetic field in microwave resonator

Plasma is widely implemented for welding and other applications; it is generated and maintained from gases using various methods, such as DC discharge, capacitive discharge, inductive discharge, microwave discharge, and ECR.¹ Most of these methods employ gases or liquids as the plasma sources. Among these methods, plasma generation through microwave excitation has gained considerable interest since it enables the generation of radical atoms required for efficient chemical reactions; it also enables the use of the luminescence spectra of the elements. Consequently, it has been implemented in laser light sources, semiconductor manufacturing equipment using ECR, diamond deposition equipment, and Anntena Excited Microwave Discharge (AMED).^2–7 In this method, a strong electric field is used to generate a spark discharge based on Paschen’s law. In the microwave region, a microwave resonance structure is used to generate and maintain the plasma in a strong electric field.

In recent years, plasma has been applied as a reducing agent for oxides. For example, thermogravimetric measurements of the reduction reaction temperature of copper oxide with carbon demonstrated a lower reaction temperature, indicating the Ar plasma contributed to the reaction.⁸ In addition, the low-temperature reduction of scandium oxide at 660 °C and reduction of vanadium oxide using Mg as the reductant have been reported as reduction reactions by microwave irradiation.⁹ It was also reported that the reduction of V₂O₅ to vanadium was successfully carried out in a short span of 1 h at 1000 °C by means of a reaction with magnesium vapor using microwave irradiation.¹⁰ These reduction reactions produced a significant reduction in the reaction temperature, in contrast to conventional thermal reduction reactions. The optical spectroscopy measurements indicate that this can be attributed to the generation of Mg plasma in this microwave-assisted process with the Mg ions functioning as the reducing agent.

The microwave irradiation devices used for scandium oxide and vanadium oxide in the previous experiments were multimode applicators in which the Mg generation and reduction reactions were performed in a single microwave source. This increases the difficulty of controlling the plasma generation and reaction temperature. Therefore, experiments were conducted using the single-mode applicators of the TM010 and TM110 modes, and the results demonstrated that the TM110 mode, which generates a strong magnetic field, was more optimal than the TM010 mode applicator.¹¹ This is a new and unconventional method of utilizing plasma, which we consider to be crucial for energy reduction in future material processes. However, the plasma cannot be stabilized for more than 10 min to produce a reduction reaction. Therefore, we have redesigned the TM110 applicator and achieved plasma excitation directly from the metal, which improved the stabilization for a longer period of time.

The design of the plasma generator section comprises a double structure of quartz tubes because induction heating causes metals to evaporate and adhere to the quartz tubes, which insulates the heated area and produces large fluctuations in the impedance in the cavity. Furthermore, the magnetic field strength is sufficient to re-evaporate the metal through induction heating even if the metal is reattached. Subsequently, 1 gm of Ca metal powder or Mg-taped alumina was placed in the alumina boat located in the inner quartz tube, and the pressure was reduced to ∼1–2 Pa using a rotary pump. The microwave energy was supplied from the microwave oscillator to the TM110 through a λ/4 antenna probe, which was coupled to the bottom of the cylinder using a coaxial cable. The microwave generator comprised an oscillator (ISC-2425-25+, Mini-Circuits Laboratory, Inc.) and amplifier module (ZHL-2425-250X, Mini-Circuits Laboratory, Inc.). The temperature of Ca metal powder or taped Mg was measured through the quartz tube using a two-color infrared radiation thermometer (IGAR6, LumaSense Technologies Inc.). The wavelengths of the infrared ray were ∼1.55 and 2.25 µm to directly measure Ca metal powder or taped Mg, respectively. Plasma spectroscopy was performed using a spectrometer (EV-2.0-STD, Horiba Stec. Co., Ltd.). Figure 1 depicts the experimental apparatus used in this study. Figure 2 shows the magnetic and electric field distribution at an eigenfrequency of 2.435 GHz obtained by the eigenvalue calculation using COMSOL Multiphysics—a piece of commercial finite element software.

Figure 3 presents the power input and reflected waves vs time from the microwave power input, Fig. 4 depicts the sample temperature, and Fig. 5 shows the tuned frequencies of the resonator when the electric and magnetic field configurations are used in the plasma generator and reaction section. For Mg, the plasma emission was stabilized when the sample temperature exceeded 300 °C, and this stabilization continued for ∼85 min or more until Mg was exhausted. For Ca, the luminescence was observed at ∼600 °C, which required ∼10 min. When the plasma emitted light, the tuned frequencies improved by ∼40 MHz for Mg and 10 MHz for Ca, as shown in Fig. 5. The proportion of conductors in the cavity increases when plasma is generated since it is a conductor. Assuming a plasma conductivity of ∼1000 S/m, the electromagnetic field simulation demonstrates a mode conversion from the TM110 mode to the TM111 mode, which increases the frequency by ∼60 MHz, as shown in Fig. 6. However, since both the modes have a high magnetic field at their center, we believe that microwave energy can be supplied to the plasma in a stable manner despite this mode change.

Figure 7 shows the spectrometer results; the plasma-emitting species were identified using the National Institute of Standards and Technology (NIST) database. A significant fluctuation was observed in the plasma emission intensity due to the uncontrolled Mg temperature and the fogging of the quartz glass due to Mg deposition. Therefore, the plasma emission intensity was set to a logarithmic scale, and the spectrometer measurement was relatively stable since Ca has a low vapor pressure and the quartz glass is not fogged. Figure 8 shows a photograph of Mg and Ca plasma emission.

The mechanism of plasma generation is explained as follows: A magnetic field generates an induced current in a metal, which initiates the emission of thermal electrons and atoms from the metal surface when it exceeds a certain temperature. This temperature is determined by the work function and vapor pressure of Mg and Ca. Subsequently, the electrons begin to rotate, collide with the evaporating atoms, and split into ions and electrons due to the magnetic field; these ions and electrons are generated in a torrential manner to form the plasma. The stability of the plasma is ensured by maintaining the TM110 and/or TM111 mode and supplying energy to the plasma by increasing the frequency during the generation of the plasma.

The generation of stable Mg and Ca ions, which have sufficient Gibbs free energy, helps in realizing the reduction of scandium oxide and vanadium oxide at low temperatures, which has been a major challenge thus far due to the use of Eryngium. Furthermore, energy-saving processes can be realized for material synthesis and reduction reactions by implementing new material processes.

This work was supported in part by the Grant-in-Aid for Scientific Research (Grant Nos. 21K04781 and 22H03779) from MEXT, Japan. This article is also based on the results obtained from a project, Grant No. JPNP0622001, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).

The authors have no conflicts to disclose.

Satoshi Fujii: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (lead); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (lead); Writing – review & editing (lead). Jun Fukushima: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Validation (equal); Visualization (equal); Writing – original draft (equal).

The data that support the findings of this study are available within the article.