Background and Motivation

A frame work of the background and motivation is revealed in Figure 1. 1. Three categories of basic concept for plasma are introduced: characteristics of the plasma, characteristics of the atmospheric pressure glow discharge and features of the dielectric-barrier discharges.

1.1.1 Characteristics of the Plasma

The term "plasma" was coined by Irving Langmuir in 1928, who was the pioneers in gas discharges and defined plasma to be a region containing balanced charges of ions and electrons not influenced by its boundaries.

1.1.1.1 Definition of Plasmas

The plasma state is often referred to as the fourth state of matter. It is distinct from other lower-energy states of matter; most commonly solid, liquid, and gas. Much of the visible matter in the universe is in the plasma state.

A plasma is a collection of free charged particles moving in random directions that is, on the average, electrically neutral. Plasmas are ionized gases. Hence, they consist of positive (and negative) ions and electrons, as well as neutral species. The ionization degree can vary from 100% (fully ionized gases) to very low values (e.g.

1e-4~1e-6; partially ionized gases).

1.1.1.2 Type of Plasmas

We can distinguish two main groups of plasma in Table 1, i.e. the

high-temperature plasmas and the so-called low-temperature plasmas. Further, based on the relative temperatures of the electrons, ions and neutrals, low-temperature plasmas are classified as "thermal" or "non-thermal". Thermal plasmas have electrons and the heavy particles at the same temperature, i.e., they are in thermal equilibrium with each other. Non-thermal plasmas on the other hand have the ions and neutrals at a much lower temperature. Because of the large difference in mass, the electrons come to thermodynamic equilibrium amongst themselves much faster than they come into equilibrium with the ions or neutral atoms. For this reason, the "ion temperature"

may be very different from (usually lower than) the "electron temperature". This is especially common in weakly ionized technological plasmas, where the ions are often near the ambient temperature.

1.1.1.3 Applications of Plasmas

Plasmas have been utilized in many well-established industrial applications (e.g.

for surface modification, lasers, lighting, among others). In the following, we will describe some of the most widespread applications:

First, surface treatment and modification with the possibility to treat (and to coat) a surface at low temperature and at pressure close to atmospheric is an important advantage for industrial applications. The operating gases included air, He, N₂, N₂ + O2, Ar, CF4, NH3, Cl2, etc. Second, ozone is a potent germicide and one of the strongest known oxidants. The main applications of ozone generation are in water treatment (drinking water plants using ozone for disinfection) and in pulp bleaching.

The operating gases include dried clean air or dry oxygen. Third, plasma is used to provide reactive species such as N2*, O2*, O(¹D), O(³P), N(⁴S) in pollution control applications. These species initially formed by electron collisions in the microdischarge filaments subsequently provide a number of reaction paths to generate

additional O and OH. These radicals can subsequently react with hazardous compounds to form non-hazardous or less hazardous substances such as O2, O3, CO, CO₂, H₂O₂. Fourth, gas discharges are also used for laser applications, more specifically as gas lasers (e.g. high speed welding and cutting of metal plates and other materials is the main application of silent discharge CO2 laser). Fifth, The lamps especially used for high-speed printing on heat sensitive substrates. Large numbers of xenon excimer lamps are now routinely used for ‘‘UV cleaning’’ of substrates in display and semiconductor manufacturing. Finally, plasma display panels (PDPs) displays utilizing Xe VUV radiation to excite phosphors are the most recent addition to dielectric-barrier discharge applications.

1.1.2 Characteristics of the Atmospheric Pressure Glow Discharge

When the applied voltage exceeds the breakdown strength of the ambient gas, an avalanche is formed. During the short breakdown period, the non-conducting gas becomes conductive and, as a result, generates different kinds of plasmas. The understanding of kinetic processes in plasmas of atmospheric gases is of great interest in various branches of modern physics and chemistry, such as discharge physics, plasma chemistry, chemistry and optics of the atmosphere. In order to get the appropriate plasma parameters, the experimental and theoretical investigations are quite important tools for understanding the plasma properties.

The measurements in a discharge at atmospheric pressure are related to light intensity (spectroscopic measurements and short exposure time photo) and electrical characteristics (measurements of the discharge current and construction of the Lissajous figures). But spectroscopic or electric diagnostics have the limitation in the investigations of discharge behavior such as distribution of electric field and plasma particle density over discharge gap. Recently, simulation has become an important

method in understanding the plasma physics and chemistry of gas discharges since the direct quantitative measurements inside the discharge volume are either very difficult or very costly. Not only can an efficient and accurate modeling provide detailed plasma physics and chemistry within complex gas discharges, but also may it be used as an optimization tool for designing a new plasma source.

1.1.3 Features of the Dielectric-Barrier Discharges (DBD) 1.1.3.1 Structures

A sketch of dielectric barrier discharge is showed Figure 1. 2. The DBD usually is generated in the space between two electrodes covered with the insulating dielectric material. The most frequently used dielectric materials being Pyrex, quartz, polymers and ceramics, in some applications additional protective or functional coatings are applied. When the applied voltage exceeds the breakdown strength of the ambient gas, an avalanche is formed. Sources that operate under vacuum are at a disadvantage with respect to those that operate at 1 atm because of the increased capital costs and the requirement for batch processing of workpieces associated with vacuum systems.

1.1.3.2 Advantages

Types of non-equilibrium atmospheric-pressure plasma (APP) are generally classified based on the power sources, which may include radio frequency (RF) capacitively coupled discharge, AC dielectric barrier discharge (DBD) and microwave discharge. Among these, the parallel-plate DBD driven by AC power supply (10-100 kHz) may represent one of the most attractive discharges because of: 1) its easier implementation as compared with low-pressure plasmas, 2) low operational cost 3) possibility of the production of homogeneous plasma, 4) lower working temperature.

1.1.3.3 Applications

Dielectric-barrier discharges, or simply barrier discharges, have been known for more than a century. First experimental investigations were reported in 1857. The DBD at atmospheric pressure driven by a alternating current power source has been widely used in various industrial fields, such as surface treatment, thin film deposition, pollution control, plasma display cell production. The DBD is generated in the space between two electrodes, which were covered with insulating dielectric layers.