Gases content

Fig. 4-4 shows the firing voltage measurement as a function of operating pressure and gas content on various direct current bipolar pulsed excitation waveforms frequencies. As shown in Fig. 4-4, the firing voltage varies substantially with the various gas contents. The voltage difference between neon and argon are 40 to 140 V at gas pressures from 300 Torr to 800 Torr. With the addition of argon in pure neon, the firing voltage is reduced about 10 V at whole gas pressure range. At Ne+Ar(2%) gas pressure of 300 Torr, the firing voltage decreases from 261 V in pure neon to 250 V with a direct current bipolar excitation frequency of 20 kHz. A Penning effect which provides a lower igniting voltage plays the role in this phenomenon.

A Penning gas mixture consists of an inert gas containing another gas which has lower ionization potential than or equal to the metastable potential of the origin inert gas.

300 400 500 600 700 800 280

300 400 500 600 700 800

300 400 500 600 700 800 240

Fig. 4-4. The relationships between the firing voltage and operating pressure of the devices on different gas content at gas pressures from 300 to 800 Torr on various

excitation frequencies of (a) 4 kHz, (b) 10 kHz, and (c) 20 kHz.

4.4 Memory coefficient

Fig. 4-5 illustrates the dependence of memory coefficient (α_m) and firing voltage (Vf) on the operating pressure from 300 Torr to 800 Torr with different pulsed excitation frequencies at 2, 4, 10, and 20 kHz. Memory coefficient is widely used to estimate the margin performance of plasma display panel (PDP). The definition of memory coefficient (α_m) derived from the quality of voltage margin is shown by the following equation [25-27]:

The memory coefficient (α_m) is related to the stability that is to sustain a discharge at a voltage lower than that required for initiating it. When the memory coefficient (α_m) value is large, the higher probability of stable discharge phenomena is acquired. Regardless of gas content, the voltage required to ignite a discharge decreases significantly with the increase of excitation frequency. In addition, higher memory coefficient (α_m) is obtained with excitation frequency on 20 kHz, and the similar calculated results are acquired in neon, argon and Ne+Ar(2%). On the contrary, with excitation frequency on 2 kHz, lower memory coefficient (α_m) is obtained and the glow discharge phenomenon is too weak to sustain. Consequently, with a relatively high excitation frequency, a low firing voltage can be acquired and an intense glow discharge phenomenon can be found stably.

300 400 500 600 700 800

300 400 500 600 700 800

200

Fir ing V oltage ( V ) Memory coeffic ient (

)

Pressure (Torr)

300 400 500 600 700 800 200

250 300 350

Ne/Ar (2%)

Fir ing Voltage (V) Memor y c o ef fic ient (

)

Pressure (Torr)

2 kHz 4 kHz 10 kHz 20 kHz

0.0 0.5 1.0 1.5 2.0

(c)

Fig. 4-5. Dependence of the firing voltage and memory coefficient (αm) on working pressures of the devices with different excitation frequencies of 2, 4, 10, and 20 kHz:

(a) neon, (b) argon, (c) Ne+Ar(2%).

4.5 Optical appearance

Fig. 4-6 exhibits the discharge charged-coupled device (CCD) images of the operated nano-tip enhanced microplasma devices at 400 Torr of neon gas with varying bipolar pulsed excitation and frequencies of 4, 10, and 20 kHz. The images are captured by an image observation system including a color charged-coupled device (CCD) camera (Watec, WAT-202D), a zoom lens set (NAVITAR, Zoom 6000), a digital camera, and a personal computer. The operating voltages of the discharge CCD images are 300 V and 600 V in Fig. 4-6. With higher excitation frequency and applied voltage, the optical appearance of the discharge changes from relatively dark to bright and the light emission also become more intensive.

In addition, the standing striation phenomena from the metal electrode edge are observed from the CCD images. The striation means the traveling waves or stationary perturbations in the electron number density which occurs in partially ionized gases.

Striation is known in positive column and ionospheric plasma system, and it is related to the wave mechanism. Striation in typical dielectric microdischarge device is basically governed by the ionization-dominated α-process.

Fig. 4-7 presents photographs of the entire operated nano-tip enhanced microplasma devices operating with neon, argon and Ne+Ar(2%). The gas pressure and excitation frequency were fixed at 400 Torr and 20 kHz. Because there is only a dielectric with ITO electrode and dielectric can provide wall charge effect, the discharge expands with the pattern of ITO transparent electrode. High intensity of glow discharge in proposed devices reveals its potential of extended development in future work.

(a) (b)

(c) (d)

(e) (f)

Fig. 4-6. CCD images of the glow discharge for the device operating in 400 Torr of neon gas at, (a) 4 kHz, 300 V, (b) 4 kHz, 600 V, (c) 10 kHz, 300 V, (d) 10 kHz, 600 V,

(e) 20 kHz, 300 V, (f) 20 kHz, 600 V.

(a)

(b)

(c)

Fig. 4-7. Photographs of the glow discharge for the device operating with 20 kHz pulsed waveform and 400 Torr of gas pressure: (a) neon, 450 V, (b) argon, 500 V,

(c) Ne+Ar(2%), 450 V.

Chapter 5

Summary and Future Work

5.1 Summary

High pressure glow discharges stabilized by operating with relatively small characteristic dimensions have been given the term “microplasma”. With the intersection of plasma science, photonics and materials science, microplasma device science and technology offers not only a new realm of plasma application but also device capability. The first and second conferences about microplasma are organized in 2003 and 2004 respectively and the third International Workshop on Microplasmas (IWM) is held in Germany, 2006. The growth of this series of microplasma conference shows the developing potential of microplasmas. Recent examples of microplasma devices developed with many configurations, such as inverted pyramid, planar electrode and three dimensional structures. Moreover, materials of microplasma device are also the critical research topics in the development of microplasma device. Research and development on microplasma devices reveal the characteristics which are well-behaved electrically and optically and appear to be valuable and feasible applications such as active display and backlighting [27].

This work was devoted to the characterization of nano-tip enhanced microplasma device in neon, argon and Ne+Ar(2%). Using nano-tip is to locally enhance the electric field and reduce the discharge voltage. This article presents the panel type microplasma devices with nano-tip enhanced electrodes and demonstrates that the

proposed devices have been successfully fabricated and operated with various direct current bipolar pulsed excitation frequencies from 2 to 20 kHz in neon, argon and Ne+Ar(2%) gas mixture. The nano-tip enhanced microplasma devices are characterized by abnormal glow discharge properties and low discharge voltages were acquired due to the fact that the local electric field distribution is heightened by the nano-tips. Operating at pressure up to 800 Torr at voltage as low as 250 V provides stable glow discharge phenomena. Large voltage margin of our proposed devices reveals the potential of wide operating range in future device. Simple fabrication of the devices is an obvious advantage to procure the larger area panel type nano-tip enhanced microplasma devices. The nano-tip enhanced microplasma device technology provides advantages which are discharge voltage, lifetime, brightness, and efficiency. The high resolution patterns with nano-tips, low discharges voltage obtained from the short distance between panel electrodes and the strong emission from the nano-tips can be provided from the proposed devices. Furthermore, with the advantages which are discharge voltage, lifetime, brightness, and efficiency, the nano-tip enhanced microplasma technology provides possible opportunity to integrate with broad applications.

5.2 Future work

Despite the encouraging results of this study as to the positive of using nano-tip enhanced electrodes in the microplasma devices, future research is required in a number of directions. The work we presented is an exciting first step and among the many topics to be explored in future research and some important ones can be discussed as following sections.

5.2.1 Phosphor

In this work, the proposed nano-tip enhanced microplasma device is only one line device. In the future, multi-line and full size panel microplasma device are desired. Because Red, Green, Blue and White (RGBW) colors are desired for display application, vacuum ultra-violet (VUV) excitable phosphors is capable of achieving this future work. To avoid destroying the nano-tip, a spraying method with spray nozzle is commented to grow phosphor layer in the proposed devices. According to the full size panel microplasma device with phosphors, it can provide potential of application on flat plasma light source

5.2.2 Dielectric layer

From the observation of long time operation, there is some erosion which comes from the ion bombardment phenomena on the nano-tip. In order to avoid the erosion, a dielectric layer can be deposited on rear metal electrode to protect the nano-tip enhanced electrode. Base on the plasma display panel (PDP) technology, Magnesium Oxide (MgO) is the best candidate in this future work. MgO can provide low sputtering rate and high secondary emission coefficient. With depositing the MgO thin film, the life time of proposed device is confidently longer.

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