Applied Science and Convergence Technology 2019; 28(4): 107-112
Published online July 31, 2019
Copyright © The Korean Vacuum Society.
Se Youn Moon*
Department of Quantum System Engineering, Chonbuk National University, Jeollabuk-do 54896, Republic of Korea
Correspondence to:E-mail: email@example.com
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.
Gas temperature is one of the most important parameters for atmospheric pressure plasma applications. Based on the fact that this parameter is closely related to the rotational temperature of diatomic molecules at atmospheric pressure, a spectroscopic method of measuring the rotational temperature was used by analyzing the OH, O2, N2+, CH, and CN molecular spectra, which are frequently observed in various atmospheric pressure plasmas. In this work, a semi-automatic program for the determination of the rotational temperature was developed, allowing the use of modest resolution monochromators, generally used in industries and laboratories. Different atmospheric pressure plasma sources were used for application to the various gas temperatures in the range of 400–2500 K. Different diatomic molecular spectra emitted under the same plasma condition showed almost the same gas temperature. Through a sensitivity study of the method, it was found that the diagnostic conditions, such as molecular species and optical resolution, should be carefully selected for more accurate measurements.
Keywords: Atmospheric-pressure plasma, Rotational temperature, Gas temperature, Diatomic molecular spectrum, Optical emission spectroscopy
Atmospheric pressure plasma sources have recently received increased attention due to their many advantages. These advantages include the fact that they do not require expensive vacuum equipmentand are low-cost and simple systems that are easy to operate. Because of these advantages, many types of atmospheric pressure plasma sources have been developed using various frequency range powers from direct current to microwave, or with a short pulse. For example, the microwave and radio frequency (RF) plasma torch, dielectric barrier discharge, arc plasma torch, and the atmospheric-pressure capacitively coupled plasma (CCP) jet are well known examples of atmospheric pressure plasma sources [1–5]. As atmospheric pressure plasmas have many applications, particularly for surface modification of thermally weak materials, such as fabrics, polymers, or glass plates for display industries, remediation of hazardous gases utilizes the gas temperature effect of the plasma; thus, accurate measurement and control of the gas temperature are very important to optimize the performance of the plasma treatment [6,7]. The gas temperature, which is defined as the kinetic temperature of heavy particles, can be obtained thorough spectroscopic methods, such as the Doppler broadening method and the Boltzmann plot method, in which optical emission spectroscopy has been generally used because of its simplicity and nonintrusive nature. The gas temperature measurement by the Doppler broadening of the spectral lines emitted from neutral particles is the most straightforward method, given by the following equation.
where Δ1/2 is the FWHM of Doppler broadening,
The second method, the Boltzmann plot of diatomic molecular spectrum, is based on the fact that the gas temperature is closely related to the rotational temperature of an atmospheric pressure plasma [8–10]. The gas temperature can be obtained by measuring the rotational temperature of the diatomic molecular spectra in equilibrium plasmas, as well as in none-quilibrium plasmas at atmospheric pressure because the rotational-translational relaxation is sufficiently fast to equilibrate the rotational and the gas temperatures [11,12]. Figures 1(b) and 1(c) show a typical OH molecular spectra calculated with a given temperature and broadening value, and the Boltzmann plot diagram obtained by analyzing the intensity ratio according to each spectral line, respectively. Similarly, because several tens of molecular lines exist in the very short wavelength region, a high-resolution spectrometer is also required to obtain the well-resolved Boltzmann plot. In recent studies, therefore, an improved method using the synthetic spectrum has been used to overcome the limitation of resolution [11–14]. In this work, to achieve this, a semiautomatic program for the gas temperature measurement was developed using the synthetic spectrum method. In particular, three kinds of diatomic molecules (OH, N2, and O2), frequently observed in many atmospheric pressure plasmas due to their open-air operation, were selected. Furthermore, the CH and CN emission spectra, as sometimes observed by the addition of CH4 gas to deposit carbon-based materials, were investigated [15,16]. By comparing the measured values, the accuracy and usefulness of the different spectra were examined. In section 2, theories regarding the Boltzmann plot and synthetic spectrum method are described, and numerical code is introduced. In section 3, we present the experimental results and the validity of the numerical code; the conclusion follows in section 4.
If an atmospheric pressure plasma is produced in ambient air, the hydroxyl (OH) molecular band (A2S+ − X2P, 309 nm) and the nitrogen monopositive ion (N2+) first negative system (B2Su+ − X2Sg+, 391.4 nm) are observed in the emission spectrum due to water molecules in the air and nitrogen from the air composition, respectively. Meanwhile, O2 (b1Sg+ − X3Sg−, 759.4 nm), CH (A2Δ − X2P, 431.4 nm), and CN (B2S+ − X2S+, 388.3 nm) are emitted from the plasma by the addition of a small fraction of O2 and CH4 gas for specific applications. The theoretical spectrum intensity corresponding to any rotational temperature (
The powerful advantage of the synthetic method is the possibility of a low-resolved spectrometer for the
In this work, we used different kinds of atmospheric pressure plasma sources, such as RF atmospheric pressure CCP  and atmospheric pressure microwave-induced plasma . Detailed descriptions for the plasma source and configuration are in references [13,15]. The spectroscopic setup consisted of a Chromex 250 spectrometer with 1200 and 600 grooves/mm gratings and a convex lens of 6.3 cm focal length. The instrumental broadening of the detection setup was measured using a He-Ne laser and/or a mercury lamp.
Figure 4 shows a comparison of the
Until the above results were obtained, the
Another noticeable information from Fig. 6 is the temperature dependence of the spectra. The rotational spectra of the diatomic molecules are more suitable for low-gas temperature plasma. By increasing the temperature, the slope of
In this work, a rotational temperature measurement program was developed to measure the gas temperature of atmospheric pressure plasmas. The semiautomatic program analyzed OH, O2, N2+, CH, and CN molecular spectra, which are frequently emitted from atmospheric pressure plasmas. According to the results applied to different plasma sources, the method shows high accuracy of measurement in the range of 400–2500 K. From the simultaneous investigation of the different molecular spectra emitted from the same plasma source, almost the same value of the rotational temperature was obtained. Through the sensitivity study of the method at various rotational temperatures and instrumental broadening values, it was found that the molecular type, the rotational temperature region, and the optimum instrumental broadening value should be decided for high-accuracy measurements depending on the experimental environment.
This work was supported by the National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (No. NRF-2017M1A7A1A01015893).