Background

1.2.1 Introduction to DBD

Dielectric barrier discharges, or silent discharge, have been known for more than a century. First experimental investigations were reported by Siemens in 1857, see figure1.1. They concentrated on the generation of ozone. This was achieved by

subjecting a flow of oxygen or air to the influence of a dielectric barrier discharge (DBD) maintained in a narrow annular gap between two coaxial glass tubes by an alternating electric field of sufficient amplitude. The novel feature of this discharge apparatus was, that the electrodes were positioned outside the discharge chamber and were not in contact with the plasma. In his later years Werner von Siemens considered his discharge configuration for the generation of ozone as one of his most important inventions.

Dielectric barrier discharge (DBD) or silent discharge is a typical non-equilibrium high pressure ac gas discharge, see figure1.2. It is known that the DBD can be occurred between two electrodes, at least one of which should be covered with dielectric, when an ac high voltage is applied on the electrodes as figure1.3. Dielectric barrier discharge (DBD) are coupled high pressure discharges operated from an a.c. high voltage supply at frequencies from 60Hz up to about 1MHz. With increasing voltage, the reaction gas of the system will breakdown, and product many streamers between the electrodes. We can observe luminous phenomenon at the same time. Due to the high pressure of 0.1 to beyond 1 bar the discharge consists of narrow transient plasma microstreamers, which are often distributed statistically both in time and within the space between the electrodes. The microstreamers have diameters of the order of 100μm and usually very short-lived, on the order of 100 ns or loss. Both the exact plasma wire diameter and

precise time duration depend upon the gas used and the reactor pressure. In DBD, the dielectric surfaces serve the role of a capacitor in series with the plasma. The plasma in DBD consists of a lot of microstreamers, see figure1.4. When the microstreamers cross the gap and impinge on the dielectric, the dielectric charges up. Since the transverse mobility of charge on the dielectric is extremely low, the charging of the dielectric is restricted to the local vicinity of the streamer. When the local dielectric charges and reduces the voltage across the gap, the streamer is quenched, thereby preventing formation of an arc. The discharge gap itself has a typical width ranging from less than 0.1mm to several centimeters, depending on the application. To initiate a discharge in such a discharge gap filled with a gas at about atmospheric pressure, voltages in the range of a few hundred V to several kV are required. The gas can either flow through the DBD (ozone generation, surface treatment, pollution control) or it can be recycled (CO2 lasers) or fully encapsulated (excimer lamps, excimer based fluorescent lamps and light panels, plasma display panels).

1.2.2 Introduction to rare gas excimer lamp

The recent use of silent discharges to generate narrow band UV radiation based on excimer formation in the plasma provides a new optical source or lamp for investigating photo-physical and photochemical processes involving the interaction of

UV radiation with matter. Many UV systems based on this technology are used for germicide treatment of surfaces, surface modification, UV curing, and material deposition processes, as well as for UV-induced chemical synthesis and decomposition.

For industrial applications excimer UV sources driven by silent discharges may have definite advantages, of high reliability, scalability to large area and very high UV powers, and reduced costs per UV photon. Table 1.1 lists the molecular species which emit VUV and various UV wavelengths.

Excited dimers or "excimers" of argon, krypton and xenon emit narrow-band VUV radiation between 100 and 200 nm. These pure rare gas excimers are efficient fluorescers, converting the electron energy into VUV radiation. Molecular xenon has a slightly bound excimer state at about 8 eV, as shown in figure 1.5. Into this bound state,

a large population of states converges via bi-atomic collisions:

*

*

Xe + Xe → Xe

2 ﹙1.1 ﹚

The spontaneous emission of photons resulting from a transition from this level to the ground state peaks provides photons around 7.2 eV or 172 nm. In the microdischarges the electron energy has to be optimized for efficient excitation of the

atomic

Xe

^*level which can react with neutral Xe atoms to form the

Xe

^*₂ excimer.

e + Xe → Xe

*

+ e

﹙1.2 ﹚

e + Xe → Xe

**

+ e

﹙1.3 ﹚

2

Since excimer formation is usually a three body reaction, higher pressures favor

excimer formation with corresponding decrease in

Xe

^*densities. For this reason non-equilibrium discharges at pressures above 50 torr are required. In addition to the

direct path of reactions that form upper states (1.2) and (1.5). We also have state formation by all reactions starting from higher lying excited and ionic atomic and molecular states. Extended kinetic models treating the interaction of high-energy electron beams with xenon are based on the assumption that the various excited states of the xenon atom and molecule can be represented by two (fictitious) excited atomic

and molecular states:

*

Xe

2

Xe

₂^*

Xe

*, , , . Silent discharges in xenon can be sources of VUV radiation peaking at 172 nm, having a half-width of 12–14 nm and emitting practically no other radiation in the wavelength region between 180 and 800 nm. Thus, for the purposes of photochemistry, this is a VUV source of high spectral purity. Silent discharge configurations are quite flexible with respect to the geometrical shape and the electrode configuration. See figure 1.6. Figure 1.6(a) is a planar lamp geometry that could be meters in extent. Figure 1.6(b) is an open co-axial geometry for irradiating rods or cylinders placed within the open cylinder with UV light emitted by

Xe

**

Xe

^*₂

Xe

^**₂

the outer annulus. Figure 1.6(c) could be used to irradiate the inner surfaces of cylinders meters long, provided we insert the smaller diameter lamp into the larger diameter cylinder. Figure 1.6 (b) shows this geometry, which has a great potential for irradiating fiber surfaces or gas and liquid flows. Hence, a major advantage of the silent discharge is its flexibility to achieve lamps radiating in unique geometric directions with respect to the geometrical shape of surface to be irradiated. As Figure 1.6 shows we can adapt the form of the UV source to the intended surface. In addition to forming plane panels for the irradiation of large surfaces and cylindrical sources radiating outwards or even radiating inwards can be built.

在文檔中 以實驗的方法研究真空紫外光準分子燈管之紫外光放射 (頁 12-17)