Browsing by Subject "Molecular beam mass spectrometry"

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    Molecular beam mass spectrometric investigation of plasma-material interactions motivated by plasma-catalysis
    (2022-01) Jiang, Jingkai
    Low-temperature atmospheric pressure plasmas are a source of many different highly reactive species that enable a variety of plasma-material processing applications. Nonetheless, the non-equilibrium nature and limited indirect tuning possibilities make it difficult to control reactive species production, particularly for atmospheric pressure plasmas. The underlying mechanisms of plasma-material interactions are currently not fully understood and more in-depth insights of the plasma-material interaction mechanisms would be beneficial for the advancement of several applications enabled by atmospheric pressure plasmas. For a better control and optimization of the plasma sources, quantitative measurements of these plasma-produced gas-phase reactive species are needed as a starting point. Molecular beam mass spectrometry (MBMS) was used in this study as a powerful diagnostic technique to perform in-situ measurements of both long-lived and short-lived (radicals, ions) species fluxes at the plasma-material interface. An adjustable MBMS system was designed, built and implemented to fulfill optimal conditions of both neutral and ionic species measurements. Firstly, considering that the MBMS sampling is intrusive, the influence of the flow fields in free and substrate-impinging atmospheric pressure plasma jets (APPJs) on the distribution of reactive species density was investigated and models to account for reactions in the near surface boundary layer were developed. In addition, the capability of MBMS was further extended by developing detection and calibration approaches for the absolute measurement of singlet delta oxygen, O2(a1Δg) the first electronically excited state of O2, vibrationally-excited nitrogen N2(v), as well as the absolute density of ions. These advances enabled the following key findings reported in the first part of the thesis. ● The MBMS measurements of O2(a1Δg) showed that O2(a1Δg) is the dominant reactive species in the effluent of an RF-driven atmospheric pressure plasma jet (APPJ). The ability to measure axial and radial profiles of O2(a1Δg) impinging on a substrate in the effluent of the APPJ is a key advantage of the MBMS diagnostic method over well-established optical diagnostics. ● The spatially resolved measurements of N2(v) in the effluent of an APPJ were enabled by fitting the mass spectrometry signals with the electron-impact ionization cross sections of N2(v) as a function of electron energy, assuming a Treanor-like vibrational distribution function. The approach provides a complementary diagnostic technique for detecting N2(v) near substrates with excellent spatial resolution and detection limits, and also shows that RF-driven plasmas can produce large fluxes of vibrationally-excited nitrogen that is reported to be important in plasma catalysis. ● Absolute densities of positive ions in the effluents of an APPJ were obtained through calibration with a dc corona discharge with a well-known current density profile [3]. Positive ion densities in the effluent of the APPJ were found to be more than 4 orders of magnitudes lower than the densities of the dominant reactive neutral species (O, O2(a1Δg), O3) in the afterglow region suggesting that plasma-surface interactions in this case are dominated by neutral radical interactions. These results are examples of extended diagnostic capability in atmospheric pressure plasma that have a large potential to enable a better understanding of plasma-surface interactions. With the established MBMS, the control of plasma produced species fluxes by tuning plasma operating parameters as well as the ion dynamics in the effluents of APPJs were also investigated in the second part of the thesis. An extensive parameter study shows that for a fixed feed gas composition and plasma dissipated power, the ratio of short-lived (O, ions) and long-lived (O3 and singlet oxygen) species fluxes can be changed by orders of magnitude by changing the treatment distance and the plasma modulation frequency. Furthermore, changing the gas flow rate is found to be potentially an effective approach to tune the ratio of the O and ion fluxes. The results presented in this contribution can be a valuable tool to control the reactive species fluxes to substrates by tuning plasma operating parameters for plasma-surface interactions. A characterization of ion fluxes impinging on substrates as produced by a modulated RF-driven APPJ operating in a homogenous gas environment (Ar+1% O2) was performed using MBMS. The influence of the RF modulation frequency (100 Hz-20 kHz) upon the ion fluxes was investigated by time-resolved measurements, and lifetimes of the dominant ions were obtained. Significant differences in the dynamics of the positive and negative ions were found and explained by large electron densities in the afterglow produced by electron detachment reactions from negative ions due to the large concentrations of atomic oxygen and singlet delta oxygen. Quantitative measurements of the ion densities causing these memory effects were reported. The results highlight the tremendous impact of memory effects on plasma propagation and their corresponding pre-ionization densities which were measured for the first time. The third part of this dissertation focuses on the interaction of plasma and catalysts in the context of partial oxidation of methane. Promising plasma catalysis synergy effects are observed for the conversion of chemicals, showing enhanced conversion compared with the plasma-only or catalysis-only conditions. The underlying mechanisms responsible for these highly beneficial synergistic effects are to date not understood but are typically suggested to be due to the production of reactive species. The established MBMS enabled the quantitative in situ measurements and a better control of reactive species (reactive neutral species and ions) in the gas phase which are needed to understand the fundamental mechanism of plasma-catalyst interaction. The MBMS results were correlated with the in situ measurements of changes in surface properties by collaborators at the University of Maryland. The key finding of this collaborative work is that a strong correlation between atomic O flux and CH4 conversion was identified suggesting its importance for the oxidation of CH4 to CO and CO2, and that the formation of surface CHn might be the rate-determining step of the production of CO and CO2 at 500 oC in plasma catalysis. In addition, we showed for oxygenates that the heating effects initiated by the plasma significantly impacted the desorption rates of methanol from particle surfaces but the contribution from thermal catalysis was excluded. MBMS measurements and estimates of species lifetimes suggested that the synergistic effect in methanol production was caused by radical species most likely CH3O2, indicating surface reactions induced by secondary more long-lived radicals such as alkylperoxy radicals might be less impacted by transport limitations.