Plasma catalysis is an improved conversion of the input gas flow by employing plasma and catalytic process. The applied electric field turns the gas into a conductor indicating the formation of plasma state with free electrons, excited molecules, and ions. The resulting low temperature, non-equilibrium plasma with energetic electrons interacts with the catalytic surface. Plasma-catalyst combination has a surplus effect, called synergy. Due to the synergistic effects of plasma catalysis, it has many applications including the destruction of volatile organic compounds, the production of fertilizers, the synthesis of value-added chemicals and the conversion of greenhouse gases. Many studies in the literature have presented an outstanding enhancement in the process of conversion. However, due to the multifaceted interaction between plasma and catalyst, the understanding of the fundamental mechanism is missing. In this study, we will focus on basic molecular interactions (e.g., adsorption, desorption) at the plasma-catalyst interface. The key outcome of this research will be the development of a novel reaction chamber in FTIR to investigate the interactions at the molecular level. The in situ FTIR studies will show how plasma species cooperate with catalyst and how those mechanisms are implemented to produce the desired products.
Category: Paper Reviews
Review: UNIQUE SOLUTIONS TO BOUNDARY VALUE PROBLEMS IN THE COLD PLASMA MODEL
In this article, Otway provides a solution to the closed Dirichlet problem which is a mixed eliptic-hyperbolic equation. This type of equations are encountered in electromagnetic wave propagation in cold plasmas.
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Summary: INTRODUCTION TO GAS DISCHARGES
This article describes plasma sources, the possible movements of particles created inside the plasma, the origins of these particles, the definitions of thermal/non-thermal plasmas, surface and volume interactions, the processes of electrical breakdown, plasma discharge with boundary relations, 0-D model for density, temperature and electric field, and 1-D model for different pressure levels.
Summary: PARTIALLY IONIZED GASES
The article presents introductory materials for the processes inside atoms and between atoms based on energy levels. Firstly, the particles which have a probability to interact with the other species are described. Among the interacting species, photon and electron are elementary particles. The energy of a photon is calculated depending on its frequency. Electron has only translational kinetic energy calculated by translational speed. For atoms and molecules, their total energy is a summation of translational, vibrational and rotational energies. There are many energy levels representing excited levels. If we consider an atom, the minimum energy above the ground level causes excitation and constitutes a free electron. When energy transmitted to atom exceed the ionization energy (series limit), a free ion is created. After ionization, particles can attain any energy level so they create a continuum. All energy levels correspond to a configuration of possible energy states, which is called as degeneracy. It can be defined as the number of different quantum states with the same energy. For electron, there are only two possible states resulting from electron spin. Atoms can have larger degeneracy values depending on their quantum states.
Summary: ATOMIC COLLISIONS
In this book chapter, particle collisions are examined. Electrons and ions can experience elastic collisions by preserving total momentum and energy. Otherwise, they lose their energy in the form of ionization or excitation and this type is named inelastic collision. Electrons and fully stripped ions have only kinetic energy but excited and ionized atoms possess internal energy, analogous to potential energy. While their internal energy is constant, kinetic energy is redistributed between the colliding particles in elastic collisions. For the super elastic collisions, an excited atom can be de-excited and the total kinetic energy becomes larger.
Summary: EEDF FOR MODELLING THE PLASMA KINETICS IN DIELECTRIC BARRIER DISCHARGES
In this article, different electron energy distribution functions (EEDF) for plasma conditions in xenon dielectric barrier discharge (DBD) are explored in plasma modelling. At the beginning, ionization and excitation rates resulting from electron-neutral collisions are discussed. Generally, local-field approximation (LFA) is used for those collisions by assuming electrons gain energy in nanoseconds and reaches equilibrium. For the LFA models, electron energy distribution function is governed by the Boltzmann equation for the primary elastic and inelastic collisions. Although LFA is useful to calculate the primary ionization and excitation rates, diffusion and mobility coefficients, the secondary processes are omitted. These processes can be superelastic collisions, stepwise ionization, electron-ion recombination etc. The models using LFA neglect the secondary processes to reduce the computational cost.
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Summary: MEASUREMENT OF TOWNSEND COEFFICIENT
Sanders investigated the Townsend coefficient (α) in the regions with different pressure and distance levels in his two papers. For the former one, the author criticized the early measurements of Townsend, Bradbury and Paavola to give an explanation to the current values for higher X/p (the ratio of electric field strength [V/cm] to the pressure [mm]). Townsend explained the energetic ions cause more ions and introduced value which represents new pairs of ions per centimeter. However, his experiments were conducted in a low pressure, small distance, high electric field environment and the results failed to explain lower X/p values in high pressure conditions. The researchers proposed that at higher pressure level, the number of the created ions increases due to electron impact ionization in the dark current just before the breakdown. The distorted fields with space charge satisfy the Townsend equation with new values.
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Summary: PHYSICS OF CATHODE PROCESS
In this chapter, the types of electron emissions from the surface of a metal, which are thermionic emission, field-enhanced thermionic emission, field emission and thermo-field emission, are introduced. While cathode is emitting electrons, anode collects electrons passively and this process simply determines the roles of anode and cathode in gas discharges. The electric current is mostly carried by electrons because of their lower mass and higher energy compared to ions. Also, current density is critical to evaluate power density distribution and energy balance which are responsible for all the processes such as electron emission, phase transition and plasma production.
Summary: SURFACE PROCESSES
In this part, surface adsorption-desorption reactions and secondary electron emission process are explained in detail. Surface processes have a key role on surface etching, for example. The gas phase on top of the surface interacts with the surface layer. Adsorption-desorption reactions affect gas-phase species concentration. Also, positive ion neutralization and secondary electron emission have effects on gas discharge. Therefore, gas and surface reactions are coupled.
Summary: THE EFFECTS OF THE FERMI LEVEL ON ION INDUCED ELECTRON EMISSION FROM CHEMICALLY AND SPUTTER CLEANED SEMICONDUCTERS
In the article, low energy ion induced electron emission (IIEE) is investigated on Si and Ge semiconductors. Plasma interactions on semiconductors are recently used for etching and deposition processes. According to the previous studies, the IIEE strongly depends on the surface process but this research focuses on the sub-surfaces processes such as doping type, Fermi level of the material and cleanliness level. The studies from the literature show that the IIEE measurements are affected by the electron density in the conduction band of the semiconductor. They claim that more electrons near the vacuum potential result in larger IIEE. However, the IIEE theory predicts less dependence on electron density by assuming all the emitted electrons from the valance band. Thus, there should be no direct relation between the IIEE and doping density and type for semiconductors, on the contrary to metals.
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