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Abstract
A semiconductor microwave device that generates a series of burst microwaves at a sub-microsecond duration has been successfully used in a breakdown plasma spectrometer in atmospheric conditions. Microwave delivery has been changed to couple the microwave with laser sparks and electric sparks which are typical plasma ignition sources in laser-induced breakdown spectroscopy (LIBS) and spark-induced breakdown spectroscopy (SIBS). A helical antenna was used for the laser spark, while a coaxial antenna was considered more appropriate for the electric spark. The weak and transient sparks in LIBS and SIBS are enlarged by the microwaves which are stably sustained in the air. The microwave's output power and pulse duration are easily controllable, resulting in tunable plasma intensity and sustained production of hydroxyl radicals (OH radicals). Even in continuous-wave operation by microwave, the low-energy system prevented the formation of high-temperature thermal plasma (>10,000 K) without any mechanical cooling system. The microwave-enhanced LIBS (MW-LIBS) and microwave-enhanced SIBS (MW-SIBS) could be applied to optical emission spectroscopy analyses. In analytical applications, MW-SIBS produces no shockwave in contrast with MW-LIBS which is a great advantage in powdered samples. The MW-SIBS successfully analyzed the direct introduction of copper metal powders.
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1. Introduction
Plasma is used in a wide range of applications in combustion, food, material processing, semiconductor production, biomedicine, and optical spectroscopy [1–14]. The plasma characteristics greatly depend on pressure due to the reduction of the mean free path at increased pressures [15]. At high pressures, the plasma is applied to air/fuel ignitions and nanoparticle synthesis benefiting from the decreased chemical reactions [1–3]. At atmospheric pressure, the chemical species in nonthermal plasmas efficiently generate the ions and radicals for sterilization, skin and cancer therapy, and optical spectroscopy [8,10,11]. In optical spectroscopy, the plasma breakdown is commonly induced by electrical spark discharge or laser-induced ablation and breakdown [16–21].
The laser-induced plasma temporarily shows continuum emission and ionic and atomic spectra with time (100 nsec to 100 µsec), followed by molecular spectra during its short lifetime (more than 100 µsec) [22]. The radical source ion is only observed at a few microseconds due to its highly energetic state and transient nature. Extending the lifetime of radicals, especially hydroxyl (OH)radicals, in plasmas is highly essential in the combustion, surface modification, and decomposition of toxic materials such as volatile organic compounds. Oxidation is a representative reaction in chemical plasma application, in which hydroxyl (OH) radicals and O3 could play an essential role as oxidants [23–26]. Notably, the OH radical has a higher oxidative potential than O3 (OH: 2.80 V, O3: 2.07 V) [26].
There are several methods to prolong the emission of OH radicals in laser-induced breakdown spectroscopy (LIBS), such as increasing the density through the decrease of driving frequency [25]. A potential method to increase OH production is by introducing microwaves into the transient plasma seed typically generated by spark ignition and laser ignition. The addition of microwave hypothetically increases the OH radical density and prolongs its lifetime. Figure 1 shows the increase of intensity of the LIBS emission and extension of the plasma lifetime. Higher energy consumption by the laser or electric spark intensifies the transient laser ablation or spark (blue line). However, increasing the power and pulse frequency could not significantly change the lifetime (blue arrow). A millisecond microwave pulse input after the spark or laser ignition produces a long-lived stable plasma, as indicated by the red line. The lifetime of breakdown plasmas is stably extended and sustained in the air. As indicated by the saturated emission profile, the resulting plasmas may be thermally and chemically stable in time.
Fig. 1. Schematic illustrating the concept of stably sustainable plasma. Blueline: Conventional breakdown plasma such as laser-induced or electric-discharge-induced plasma. Redline: Stable plasma sustained by microwaves.
We developed a hybrid plasma system that potentially controls the breakdown of plasma sources and extends the lifetime of laser-induced plasma and electric spark discharge [27–32]. The microwave system supplied the solution to the transient plasma seed formation generated by spark ignition and laser ignition [22,30,31]. The bursts of microwave pulsed enhance and sustain the plasma emission in time and space, but thermal problems limit the plasma lifetime to several milliseconds. The same hybrid plasma was also employed by using magnetrons which deliver high-power microwaves (<2000 W) and led to the generation of thermal plasmas (>10,000 K). The thermal plasma causes thermal problems such as system overheating and electrode damage such as erosion which is not suitable for the continuous operation of hydroxyl radical productions. The frequency instability of the MWs in magnetrons is usually 2.45 GHz ±20 MHz, and shockwaves in plasma generation are also not preferred for high-level stability of plasma emission.
For spectroscopic applications, two types of systems were introduced, microwave-enhanced laser-induced breakdown spectroscopy (MW-LIBS) and microwave-enhanced spark-induced breakdown spectroscopy (MW-SIBS) [28,33]. In these systems, a more compact semiconductor microwave was integrated, resulting in the generation of continuous nonthermal atmospheric plasmas with long lifetimes and high emission intensities. Figure 2 shows the high controllability of the semiconductor device output power and pulse characteristics compared with the conventional magnetron. In contrast with the fixed pulses in the magnetron, the semiconductor microwave forms square-wave pulses with adjustable pulse width and burst settings. The microwave output power can also be continuously controlled at 0–150 W. The generally low energy transmission of semiconductor microwaves has made integrating the microwave into the ignition systems difficult. Optimization of the microwave applicator by electromagnetic field simulations enabled the application of semiconductor MWs to a plasma generation system under atmospheric pressure in the air [34]. In MW-LIBS, the nonthermal plasma generation is realized by a single pulse msec duration of the microwave and a burst of pulsed 100 ns microwaves using the optimized designs of a helical antenna and capacitor-like antenna.
Fig. 2. Comparison of the controllability of magnetron and semiconductor MW generators. Magnetron: power: 2.4 kW (peak), 700 W (average), pulse on/off = 4 us/16 µs; semiconductor: power: 0–150 W, pulse 100 ns ∼ continuous wave.
We have applied the MW-LIBS system to enhance emissions for the applications in Fukushima decommissioning of nuclear debris [28,34–39]. The generation and sustainment of non-equilibrium plasma were reported by injecting microwaves into the plasma after the breakdown. Using a semiconductor microwave device, a method for maintaining the plasma after a breakdown in space in time generates the enlarged plasma without thermal problems, which potentially enables prolongs the lifetime of the electrode/antenna. We also reported on the optimum transmission of the antenna by changing the design of the antenna and microwave oscillation conditions [34]. The MW-LIBS has limited the type of materials analyzed, such as powders. This drawback is solved using MW-SIBS, which has successfully measured the detection limit of three mixtures of powders.
This study compares the OH radical production of these hybrid devices, MW-SIBS, and MW-LIBS. The MW-SIBS has been named the plasma-ball spectroscopy from the previous report, where the limits of detection of Pb, Cu, and Fe were measured [28]. We also compare the sustainment effects of the plasma in spatial and temporal dimensions using high-speed visualization. We further report on the measurement results of the metal elemental analysis using low energy breakdown.
2. Measurement methods
Two systems were used, MW-LIBS and a hybrid MW-SIBS/LIBS The experimental setup of the hybrid MW-SIBS/LIBS system is shown in Fig. 3. A commercially available automobile spark plug (LFR5A-11; NGK) was attached to the top of the cavity for the spark ignition for SIBS. The energy required for spark discharge was 20–30 mJ, and the voltage was 1–3 kV. At the opposite end of the spark plug, a locally intensified EM field was formed by a rod-shaped antenna that generates a 2.45 GHz microwave power. The system is enclosed by a mesh chamber, which functions as a microwave resonator and MW leakage shield. Two viewing windows in the mesh chamber serve as the aperture for the introduction of incident laser and receiving of emissions light into the fiber, which is coupled to the spectrometer.
To form the microwave enhanced LIBS in this hybrid system, the second-order harmonic of a Q-switched Nd: YAG laser (wavelength: 532 nm, pulse width: 8.0 ns, beam diameter 8.0 mm, Quanta-Ray ND1-004; Spectra-Physics) was focused by a plano-convex lens to produce the air plasma between the spark plug and antenna as shown in the figure. The maximum energy of the laser was 200 mJ at 10 Hz. Plasma emission spectra were measured using a CCD spectrometer (Oriel MS-257: Andor) coupled to an optical fiber with a grating of 300 lines/mm or 2400 lines/mm. The spectrometer was calibrated with a Hg lamp and had a spectral resolution of 0.9 nm (300 lines/mm) or 0.1 nm (2400 lines/mm). A photomultiplier tube (PMT) unit for detecting filtered light (SPB-1000, Imagineering) was used for the temporal measurements of the OH radical.
A delay pulse generator (DG535; Stanford Research Systems) controlled the timing of laser injection or spark discharge for synchronization with the MW irradiation. The MW impedance was experimentally adjusted for plasma generation by a three-stub tuner, including fine adjusting the gap between the MW antenna and the spark electrode. Therefore, MW energy was effectively applied to the laser-induced breakdown plasma or spark discharge, and a stable generation of MW plasma was achieved. The MW antenna can also be used as the ground electrode for spark discharge.
A high-speed microwave camera (Shimadzu's HPV-1) was used for visualization of the plasma. In the application of powder elemental analysis, all-metal powders were obtained commercially as reagent-grade chemicals from Kishida Chemical Co. Ltd.
The MW- LIBS experimental schematics are shown in Fig. 4. The pulse generator (M577, Berkeley Nucleonics Corporation, San Rafael, CA) synchronously triggers the four systems, laser source, microwave generator, spectrometer, and high-speed camera. A semiconductor laser source (L11038-11, Hamamatsu, Hamamatsu, Japan) generates the plasma ablation and breakdown of the Al2O3 sample by condensing the photons into the 50 mm lens. A helical coil propagates the high localized EM field supplied by the 2.45 GHz semiconductor microwave source, which enlarges the plasma ablation and simultaneously produces the air plasma. The enlarged plasma emission is collected by the receiving optics and was analyzed by the Echelle spectrometer (ME 5000, Oxford Instruments, Andor, Belfast, UK). The visualization of the plasma was captured by the high-speed camera (Fastcam SA-Z, Photron, West Wycombe, UK) using 100,000 frames per second. Detailed measurement systems were reported in the previous papers [38,39].
3. Results and discussion
3.1. Visualization of atmospheric air plasma
The breakdown of the air plasma at atmospheric pressure was achieved using 180 mJ and was visualized with and without the microwave. Figure 5 shows the time series evolution of the atmospheric pressure air plasma for different microwave propagation, using the magnetron and using the microwave semiconductor source. In the top row images, the short-lived air plasma with 1 mm in length is extinguished after 250 µs. In the middle row images, the plasma is irradiated with a 1000 W microwave using the magnetron source, which enlarges the plasma to 30 mm maximum length and prolongs the lifetime to more than 1000 µs. The lower row shows the effects of using the 150 W microwave supplied by the semiconductor source, which generates a more stable glow discharge plasma ball. This is in contrast with the unstable corona discharge generated by the high-power magnetron.
The three plasma images are also compared and shown. Since no optical filter is used, the spectrum of the plasma cannot be specified. Therefore, a comparison with spectroscopic data is performed in Fig. 8.
In the microwave enhancement of LIBS using alumina (Al2O3), we also observed the formation of air plasma at atmospheric pressure. Figure 6 shows the time evolution of the plasma formation using 2 mJ laser energy, 1.0 kW MW power, and 1.0 ms microwave pulse width. The plasma formation includes two time-dependent physical characteristics, breakdown, and enhancement. After irradiation of the laser, the initial breakdown plasma remains small, which is at the same volume without the microwave. The enhancements occurred in the succeeding timeline of plasma evolution after the initial shockwave of breakdown plasma, and it slowly expanded. The microwave injection time is 1000 µsec, during which the plasma with and length increased. Plasma grows with microwave injection linearly with an increase in microwave injection. From 10 to 1000 µsec, the same plasma shape and volume increased. After irradiation with microwaves, the plasma separates from the sample and disappears at about 1500 µsec. The plasma formation is discussed further in future reports [38,39].
The shape of the plasma was measured and shown in Fig. 7. With the increase in the microwave injection period, the height and width of the plasma grow linearly. The length is 4mm, and the width is 6.0 mm, while the height is changed significantly.
The actual images of the LIBS and SIBS are shown in Fig. 8. Laser-induced and spark-induced breakdowns were used as plasma seeds; then, microwaves were injected into the seed to generate the microwave atmospheric pressure air plasma. Because the output frequency of the semiconductor microwave device is highly stable, the resulting plasma shape was spherical; thus, we call it a plasma ball [33].
Fig. 8. Comparison of image and air plasma emission spectra initiated by laser and spark in air, with and without MWs. Light blue line: laser-induced breakdown plasma, light red line: spark-induced breakdown plasma, dark blue line: MW plasma initiated by laser-induced plasma, dark red line: MW plasma initiated by spark-induced plasma.
The OH and N2 emissions are compared for the MW-LIBS and MW-SIBS in Fig. 8. The SIBS and LIBS plasma results in the nonproduction of OH radicals without the addition of microwaves. High emissions of OH and N2 are both observed for MW-LIBS and MW-SIBS which may be due to the dissociation of the water vapors surrounding the open ignition system. In the future, it may be interesting to study the humidity limits in the production of OH.
Similar levels of intensity emissions were observed for both MW-LIBS and MW-SIBS. This spectral similarity means that the plasma did not depend on the initial plasma sources (lasers or sparks); hence, the bulk of succeeding chemical reactions are caused by microwaves. The enhanced form of the plasma, which is the plasma ball, is an unbounded microwave atmospheric pressure air plasma. In conventional microwave air plasma sources, the enlarged unbounded air plasma is difficult to produce without the aid of noble gases. Also, it is quite challenging to generate microwave atmospheric pressure air plasmas in the air without a well-adjusted waveguide, even with a high-power magnetron. Moreover, a commercially available microwave-induced plasma that is similar to the intended purpose of MW-SIBS uses large microwave power at 1 kW [40]. A constant gas supply is also required to sustain the plasma torch. This has been eliminated in the MW-SIBS.
In MW-SIBS, both lasers and sparks could therefore be used as the plasma seed since they yield similar spectral shapes and intensities. The spectra were enhanced compared to those without microwaves.
Figures 9 and 10 compare the emission spectra of using the magnetron versus the semiconductor microwaves in MW- SIBS. Qualitatively from the images, the magnetron produced denser air plasma than using the semiconductor microwave. Higher intensity emissions are therefore observed for the wavelength range of 250–500 nm—however, the emission of the metal atoms, Cu I and Ni I, are observed. The Cu and Ni are elemental compositions of the automobile spark plugs indicating erosions of the spark plug due to the thermal heating by the microwave. The MW plasma obtained by semiconductor MWs (100 W) exhibited enhanced emission of air molecules such as OH and N2 compared to the spark-discharge plasma. The amount of metallic emission is negligible, where the Cu peaks (325 nm, 327 nm) were less observed. These results indicated that the semiconductor MW plasma is better for analyzing gas molecular samples because of its negligible background effect. Focusing on the wavelength range of molecular gas emission (300–320 nm), as shown in the inset image, the plasma emission of OH emission is enhanced by the microwave. The OH spectrum is also well defined in contrast with the magnetron plasma spectrum, which overlaps the spectral peak of Ni I. The atomic emission of N2 did not appear in the spectrum. Therefore, the MW plasma produced by a semiconductor device yields higher accuracy in spectral analysis. From here onwards, the semiconductor microwave was used in the discussions.
Fig. 9. Comparison of magnetron and semiconductor MW plasma using the emission spectra of air from plasmas obtained by spark discharge (SP), SP + magnetron MWs (Mg), and SP + semiconductor MWs (SS).
Fig. 10. Comparison of magnetron and semiconductor MW source using OH emission spectrum at 300–320 nm.
The MW power dependence of the plasma dimensions and emission spectra of OH are shown in Fig. 11. Our previous magnetron systems consumed at least 600–1000 W of energy on average to generate plasma; however, a minimum of 15 W produced and sustained the atmospheric air plasma in the semiconductor system, where stable plasma is observed using 30 W. The plasma volume and emission intensity was increased with higher input energy, but the spectral measurements of OH are maintained. Since we focused on the controllability of OH generation in the plasma, the temperature characteristics were investigated using the OH emission peak (A2Σ+ - X2Π, 309 nm) as estimated by LIFBASE [41] and ROTEM [27] simulations. The simulation of plasma produced by semiconductor MWs of 100 W and 30 W indicated a temperature of around 3600 K, which is a rotational temperature. The rotational temperature is considered a higher estimates of gas temperature while the vibrational temperature is a lower estimate of the electron temperature [42]. Without the additional microwave energy, the vibrational temperature of the spark ignition is lower than with the microwave input. Increased vibrational temperatures may signify higher molecular dissociation, which is the case here, thus also results in higher plasma density.
Fig. 11. Power dependence of plasma appearance and spectra of air with gas temperature simulations. These spectra were measured under air, 1 atm. CCD exposure time was 100 ms with 20× accumulation.
In typical atmospheric air plasmas, the rotational temperature quickly become thermal equilibrium plasmas (>10,000 K) without cooling by airflow or short pulse control. During the lifetime of the air plasma ball, OH emission was steadily observed. The OH radical species typically have a short existence time, but the continuous cycle of excitation and emission of OH occurs, prolonging its lifetime. Lower energy consumption facilitates the recombination of atoms which results in high spectral emissions. Interestingly, the plasma temperature was not as cool as low-temperature non-equilibrium air plasma, around room temperature or lower. We considered that the low energy consumption of the semiconductor device could produce temperature saturation, and this plasma could be the so-called meso-plasma proposed by Kambara et al. [43]. According to their reports, meso-plasmas are neither thermal plasmas nor low-temperature air plasmas. In these medium-temperature atmospheric air plasmas, the formation of radical and ionic species such as OH can be facilitated.
The rotation temperature and vibration temperature were calculated from detailed spectroscopic measurements. The result shows that it is a non-equilibrium plasma that contributes to the generation of OH radicals in meso-plasma. A plasma system as an OH emission source is established for a wide variety of applications such as surface treatment of materials, sterilization of viruses, sterilization, semiconductor manufacturing equipment, decomposition, and synthesis of gas composition. It has become possible to maintain the OH emission source by non-equilibrium plasma and control its intensity with a microwave energy input of 30-100W, which is relatively small.
To detect the temporal change in the OH emission in time, the PMT with a bandpass filter measures the emissions (λc = 307.5 nm, λΔ = 8 nm, Optical Coatings, Japan). The delay pulse generator controlled the microwaves and sparks, and the electrical discharge started at 3 ms in Fig. 12(a). Then, the microwave input with a 1.3-ms delay and 10-ms duration after the discharge was applied. The emission characteristics are shown as an average of 20 pulses where the initial sharp signal (3–4.3 ms) in the plasma emission profile indicates spark emission, and the second increase in emission and the plateau (4.3–14.3 ms) indicate the formation of an MW plasma. After the spark discharge, the MW plasma rose sharply with the MW input, and the higher input energy of the MWs accelerated the formation of an MW plasma. The emission continued to increase slightly with time; however, the increase ratio depended on the MW power. Finally, it disappeared rapidly when the MWs were turned off (10 ms). The OH emission is plotted versus the MW power in Fig. 12(b). The OH intensity was nearly proportional to the MW power but exhibited a log approximation. The intensity limit might correspond to the limit of the plasma size, which is about 10 mm in diameter (Fig. 11).
Fig. 12. (a) Time profile of the emission of OH (A-X, 309 nm) The spark discharge started at 3 ms; then, MWs were injected with a 1.3-ms delay after the spark. MW input duration was 10 ms. (b) Power dependence of OH emission intensity. (c) Time dependence of 60-W MW plasma emission.
The OH plasma lifetime can be easily controlled by varying the MW duration, as shown in Fig. 12(c). The duration range of the MWs is 0.5–100 ms in the future, but it was experimentally more than a few microseconds with no upper limit. The increase in emission intensity in Fig. 12(a) became saturated 10 ms after the MW plasma generation, and the intensity remained constant if the MW power was on. These results indicated that the MW plasma produced by a semiconductor device could maintain steady emissions over long periods. The lifetime and intensity can be easily controlled by varying the MW input signal. Although the laser pulse duration is commonly within nanoseconds, and even previously reported MW-assisted plasmas had lifetimes of a few milliseconds, the semiconductor MW plasma lifetime appears boundless. We could obtain accurate spectra by stable accumulation and taking advantage of the unlimited plasma lifetime. Although spectral inaccuracy has been the biggest problem with LIBS owing to the transience of plasma generation, the plasma ball should be able to contribute to the development of a new light source for LIBS and SIBS. For instance, with 100× accumulation in conventional LIBS, each plasma cannot show a constant emission profile because of the stepwise emission measurements (emission of continuum → ions → atoms → molecules) [22].
In contrast, we could accumulate multiple emission spectra from a stable single plasma. Indeed, the effect of accumulation of the spectrum of air is shown in Fig. 13. The spectral intensity and S/N ratio were dramatically improved by 10× accumulation from a single shot with only 30 W of MW input power [Fig. 13(a)]. Because of the long stable plasma lifetime, the intensity increased linearly with the accumulation number [Fig. 13(b)]. Controlling the CCD exposure time and accumulation sample number also improved the accuracy of the spectrum). The total exposure time was 0.1 s, divided into 10, 100, and 1000 CCD shutter releases. A short exposure time per shot and a large sample number effectively produced a clear spectrum. The large sample number decreased the background noise.
Fig. 13. Accumulation effect in optical emission spectroscopy. (a) Comparison of single-shot and 10× accumulation emission spectra. (b) Accumulation number dependency of OH emission intensity.
To demonstrate a radical emission profile inside the microwave-enhanced atmospheric air plasma, OH and N2 spectra were measured by Cassegrain optics [22] with high spatial resolution, 50 um. The measurement was performed from the plasma centerline, as shown in Fig. 14. It was found that OH and N2 spectra can decrease with the radius from the centerline. The decreasing trend of the two radicals was almost the same. Within 1.5 mm from the centerline of the plasma, this radical distribution shows smoothness and is well confined to the plasma region.
Figure 15 demonstrates microwave-enhanced plasma formation and the sustainment of OH in the air by utilizing the spark produced by the ablation of a nearby sample. The conventional LIBS results in abrupt low OH emission intensity, which instantaneously drops. The addition of microwaves enhances the emissions and sustains the high emission in time.
Fig. 15. Utilizing the ablation in the production of microwave-enhanced air plasma which sustains the OH radicals in time. Without the microwave, the OH is produced momentarily at a very weak emission.
3.2. Powder metal detection: elemental analysis of metal powder was attempted using MW-LIBS
The standard LIBS has been applied to all types of materials except in their powder form, which is usually pressed into pellet forms before measurements. When plasma is generated by laser ablation, a shockwave blows off the sample, scattering the samples and resulting in an inefficient breakdown. The MW- SIBS offered a solution to this drawback of using the LIBS. The microwave-enhanced plasma is generated and sustained with low microwave energy of 30-100 W, where powdered samples can be inserted into the plasma volume. The spectroscopic measurement was therefore attempted using this method.
Because a continuous-wave (CW) plasma does not generate shockwaves, powder analysis without pretreatment should also be possible by using MW- SIBS. Figure 16 shows a schematic representation of a prototype system for powder analysis. A ceramic rod was dipped into a powder sample to attach the powder electrostatically. The rod was inserted into the plasma cavity to expose the powder to the plasma ball. A copper spectrum was successfully obtained by using a 30-W CW plasma ball, and sample scattering was not observed. Exemplary spectra were acquired during a 1ms exposure; however, the minimum exposure time to obtain the Cu spectrum was 10 µs, which is the minimum control time of the spectrometer (MS257, Oriel).
Fig. 16. Schematic of powder MW- SIBS and Cu powder spectrum. Spark-induced MW plasma ball (30 W, CW) in the air (1 atm) was used for the light source.
A highly sensitive digital weighing scale measures the weight of the copper powder before and after MW- SIBS to quantify the amount of sample lost during measurement. Surprisingly, after 20 s of plasma treatment, the weight increased slightly from 10 mg to 13 mg. The metallic luster of the copper changed to a dark color. The increase in weight was thought to correspond to oxidization; when copper (Cu) oxidizes to copper oxide (CuO), the formula weight increases 1.3-fold from 63.5 to 79.5 g/mol. The weight did not continue to increase at longer exposure times (60 s and 180 s); thus, MW-SIBS requires a small sample, which cannot be detected on this time scale. Plasma emission from metal powder may be caused by energy exchange between the plasma and partially vaporized metal. No need sample pellet is needed in this technique.
The characteristics of the emission of metal species in plasma were studied using the Cu spectra in Fig. 17. The emission intensity of the two peaks of Cu I (324.7 nm, 327.4 nm) increased with increasing MW power. The value of the peak top intensity is plotted at MW powers of 30–50 W in Fig. 17(b), and normalized spectra are shown in Fig. 17(c). These figures show that the spectral intensity was increased by increasing the MW power, but the ratio of the two Cu peaks was not changed. Therefore, the plasma temperature did not change in the metal atomic spectra, at least 30–50 W.
Fig. 17. Comparison of MW output power in Cu spectrum. (a) Emission spectra of Cu in 30- to 50-W MWs. (b) Cu's peak intensity (324.7 nm and 327.4 nm) and MW power. (c) Normalized spectra for comparison of spectral shapes.
4. Conclusions
We have developed a continuous nonthermal atmospheric air plasma formation system sustained by semiconductor microwaves. The spherical microwave atmospheric pressure air plasma, or plasma ball, could be generated by microwave enhancements of the plasma source(seed), either laser- or spark-induced breakdown. The OH emission intensity and lifetime were easily controlled by varying the microwave intensity and duration. The plasma sustained in the air potentially be applied to broad areas of plasma science as an oxidant owing to the low energy consumption of semiconductor microwaves, where a stable continuous plasma at 3600 K was successfully generated without any mechanical cooling system. A spectroscopic measurement using MW- LIBS, and MW-SIBS yielded a larger volume and longer lifetime than conventional LIBS or SIBS systems. The plasma results in molecular spectra measurements of gases and stable accumulation at a longer exposure time. This system can be applied to molecular analysis of gases and atomic spectroscopy of powder metals without any pellet compression. The continuous emission of a plasma ball did not produce a shockwave, thus, the plasma ball could be a powerful light source for powder element analysis. As an initial study, the copper powder was observed by MW-SIBS without pretreatment or special equipment. The MW-SIBS is beneficial in a wide range of industrial scanning and monitoring applications, such as steel, foods, soil, minerals, drugs, and radioisotopes in energy plants. Further studies are currently underway to quantitatively analyze various mixed powders.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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33. J. A. Ofosu and Y. Ikeda, “Elemental analysis and mixture ratio determination in fine powder metals using microwave-sustained plasma ball spectroscopy,” Spectrochim. Acta, Part A 160, 105693 (2019). [CrossRef]
34. Y. Ikeda, Y. Hirata, K. Soriano, and I. Wakaida, “Antenna Characteristics of Helical Coil with 2.45 GHz Semiconductor Microwave for Microwave-Enhanced Laser-Induced Breakdown Spectroscopy (MW-LIBS),” Materials 15(8), 2851 (2022). [CrossRef]
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37. Y. Ikeda, J. Soriano, and I. Wakaida, “Improvements of emission peak intensity and SNR in microwave-enhanced laser-induced breakdown spectroscopy (MW-LIBS),” Spectrochimica Acta - Part B Atomic Spectroscopy (Unpublished results).
38. Y. Ikeda, J. Soriano, and I. Wakaida, “Spatially and temporally resolved Al plasma formation in microwave-enhanced laser-induced breakdown spectroscopy,” Spectrochimica Acta - Part B Atomic Spectroscopy (Unpublished results).
39. Y. Ikeda, J. Soriano, and I. Wakaida, “The influence of microwave injection in alumina crater formation using microwave-enhanced laser-induced breakdown spectroscopy,” Appl. Surface Sci. (Unpublished results).
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