Microplasma Devices Research Articles

Decomposition of Volatile Organic Compounds Using Surface-Discharge Microplasma Devices

Five volatile organic compounds (VOCs), n-octane, ethyl acetate, toluene, p-xylene and ethyl benzene, were decomposed with a newly developed surface-discharged microplasma device (SMD). The SMD consisted of a micropatterned electrode on one side of a mica substrate and an inductive electrode printed on the other side. A piezoelectric, transformed high-voltage (66.7 kHz, 3.5 kV) was applied to four SMDs placed in a batch reactor containing VOC in a gas mixture; the aim was to generate a surface-discharged microplasma through a localized dielectric barrier discharge for the decomposition of the VOCs. The decay in VOC concentration (CVOC) during the discharge was evaluated by gas chromatography–mass spectrometry. For all the five VOCs, the decomposition rates can be treated as first-order reactions against CVOC after a discharge time of 30 min. Decomposition rate was dependent on the type of compound; the reaction rates of aromatic compounds were approximately twice as large as those of aliphatic compounds. Ion concentration measurements during the microplasma operation revealed that reaction rate showed linear relationship with the VOC ion concentration, which suggests that the ionization of the VOCs closely correlates with the rate-determining steps of decomposition reactions.

18.3: Microcavity Plasma Devices for Display Applications: Independently Addressable Arrays with Improved Luminous Efficacy

Independently addressable arrays of microcavity plasma devices have been fabricated and tested. This presentation reports data demonstrating that these devices provide a basis for alternative PDP cell structures of improved luminosity. The radiant output of Si arrays yields values for the luminous efficacy exceeding 7 lumens/W at 800 Torr of a Ne/50% Xe mixture. Measurements of the unoptimized radiant output of 500 × 500 (250,000) arrays of Si microplasma devices, operating in Ne/(5–50)% Xe mixtures and photoexciting (in transmission mode) a 20 μm thick film of green phosphor, yield values of the luminous efficacy up to 7.2 ± 0.6 lumens/W for a Ne/50% Xe mixture (total pressure of 800 Torr) excited by a 20 kHz sinusoidal voltage waveform. Sustaining voltages ranging from ∼250 to 340 V (RMS) yield luminance values up to ∼2000 cd-m−2 for Ne/50% Xe mixtures without incorporation of MgO. Recent laser spectroscopic measurements of Xe2 (a3∑u+ absorption in the visible and near-infrared suggest steps to be taken in PDP cell design, particularly as the Xe content in Ne/Xe mixtures is increased.

Large Arrays of Microcavity Plasma Devices for Active Displays and Backlighting

Developments of the past several years in the technology of microcavity plasma devices having characteristic dimensions of 10-100 /spl mu/m suggests their applicability to the next generation of active and passive displays. Two examples of device structures that are well suited for economically manufactured arrays of large active area are presented. Arrays as large as 500/spl times/500 (2.5/spl middot/10/sup 5/) pixels of Si inverted pyramid microplasma devices, with emitting apertures of 50/spl times/50 /spl mu/m/sup 2/ and designed for AC or bipolar excitation, have been designed and operated successfully in the rare gases at pressures up to and beyond one atmosphere. Multilayer Al/nanostructured Al/sub 2/O/sub 3/ microplasma devices having 100-300 /spl mu/m diam. cylindrical microcavities are robust and operate in the abnormal glow mode for rare gas or Ar/2-5% N/sub 2/ mixture pressures of 500-700 torr. Grown by a wet chemical process, the nanoporous Al/sub 2/O/sub 3/ dielectric yields a lightweight, flexible structure that produces intense visible or ultraviolet emission when driven by sinusoidal AC or bipolar voltage waveforms.

Arrays of microplasma devices fabricated in photodefinable glass and excited AC or DC by interdigitated electrodes

Arrays of microplasma devices, fabricated in a photodefinable glass and driven ac or dc by interdigitated electrodes outside the microcavities, have been fabricated and characterized. Operating in the abnormal glow mode for Ne pressures between 200 and 700 Torr and having characteristic cross-sectional dimensions of 75-100 /spl mu/m, the devices are well behaved and arrays as large as 25/spl times/25 pixels have been tested to date. Focused on ease of assembly and minimizing cost, the design of these arrays makes them attractive for structures having emitting areas of hundreds of square centimeters.

Decomposition of Toluene with Surface-Discharge Microplasma Device

A surface-discharge microplasma device (SMD) was developed for the decomposition of volatile organic compounds (VOCs) in the gas phase. The device is composed of a microscale-patterned electrode, a dielectric substrate, and a ground electrode. As a result of localized dielectric barrier discharge (DBD), surface-discharge microplasma was generated when a piezoelectric-transformed high voltage (66.7 kHz, 3.5 kV) was applied to the microscale-patterned electrode. The discharge current and voltage characteristics of the DBD were analyzed under atmospheric conditions. Toluene decomposition rate was evaluated by gas chromatography–mass spectrometry and nondispersive IR-absorption CO2 analysis. A decomposition efficiency of more than 99% was achieved in batch experiments. When the SMD was operated in a flow reactor, 30–80% of toluene was reduced with the percentage depending on residence time. The carbon balance between the toluene starting material and the CO2 product indicates that toluene was almost completely decomposed into CO2 by atomic oxygen in the microplasma.

Nanoporous alumina as a dielectric for microcavity plasma devices: Multilayer Al/Al2O3 structures

Nanostructured Al2O3 films with mean pore diameters of 20 nm, 5–30μm in thickness, and grown onto Al foil by a multiple step electrochemical process, provide a dielectric having superior properties for microplasma devices and arrays. Multilayer Al/nanoporous Al2O3 structures with a 100–300μm diameter (dia.) cylindrical microcavity and an overall thickness of ∼200μm are robust and operate in the abnormal glow mode for Ne or Ar/2–5% N2 mixture gas pressures (300 K) of 500–700 Torr. When driven with a sinusoidal ac waveform at frequencies of 5–20 kHz, small arrays (3×3→10×10) of 100μm dia. devices operate in Ne at rms voltages and currents of ∼160–270V and 0.4–4.5 mA, respectively. Arrays as large as 10×10 have been fabricated to date and generate azimuthally uniform discharges in each pixel without the need for external ballast. For an rms voltage of ∼275V, 5×5 arrays of 100μm dia. devices produce a luminance of 2700cdm−2 in 600 Torr of Ne for a sinusoidal ac excitation frequency of 20 kHz.

Recent advances in microcavity plasma devices and arrays: a versatile photonic platform

Selected highlights in the recent development of microplasma devices are reviewed with emphasis on large arrays of Si-based hybrid plasma/semiconductor pixels. Arrays of 40 000 (200 × 200) pixels, excited by sinusoidal ac waveforms at frequencies of 5–20 kHz, have now been realized. The fabrication of these arrays and their electrical and optical performance with rare gases and Ar/N2 mixtures are briefly described. Metal/dielectric/metal devices having a piezoelectric dielectric (BaTiO3), a cylindrical microcavity 50 µm in diameter, and a total thickness of ∼ 110 µm are also discussed. Finally, the introduction of multiwall carbon nanotubes into microdischarge devices as an auxiliary source of current is presented as being exemplary of the opportunities afforded by the integration of nanotechnology into microcavity plasma structures.

Optical and RF electrical characteristics of atmospheric pressure open-air hollow slot microplasmas and application to bacterial inactivation

We report electrical properties of radio frequency (RF)-driven hollow slot microplasmas operating in open air but with uniform luminous discharges at RF current densities of the order of A cm−2. We employ interelectrode separations of 100–600 µm to achieve this open-air operation but because the linear slot dimension of our electrode designs are of extended length, we can achieve, for example, open-air slot shaped plasmas 30 cm in length. This creates a linear plasma source for wide area plasma driven surface treatment applications. RF voltages at frequencies of 4–60 MHz are applied to an interior electrode to both ignite and sustain the plasma between electrodes. The outer slotted electrode is grounded. Illustrative absolute emission of optical spectra from this source is presented in the region from 100 to 400 nm as well as total oxygen radical fluxes from the source. We present both RF breakdown and sustaining voltage measurements as well as impedance values measured for the microplasmas, which use flowing rare gas in the interelectrode region exiting into open air. The requirement for rare gas flow is necessary to get uniform plasmas of dimensions over 30 cm, but is a practical disadvantage. In one mode of operation we create an out-flowing afterglow plasma plume, which extends 1–3 mm from the grounded open slot allowing for treatment of work pieces placed millimetres away from the grounded electrode. This afterglow configuration also allows for lower gas temperatures impinging on substrates, than the use of active plasmas. Work pieces are not required to be part of any electrical circuit, bringing additional practical advantages. We present a crude lumped parameter equivalent circuit model to analyse the effects of changing RF sheaths with frequency of excitation and applied RF current to better understand the relative roles of sheath and bulk plasma behaviour observed in electrical characteristics. Estimates of the bulk plasma densities are also provided. Finally, we present results of afterglow plasma based bacteria inactivation studies (Escherichia coli, Bacillus atrophaeus and B. atrophaeus spores) in which we employ the flowing afterglow plume from a hollow slot microplasma device rather than the active plasma itself, which is fully contained between electrodes.

40 000 pixel arrays of ac-excited silicon microcavity plasma devices

Arrays of (50μm)2 Si microcavity plasma devices, as large as 40000pixels (200×200) with a packing density of 104pixelscm−2, have been operated continuously in 500–900Torr of Ne with ac excitation (5–20kHz). More than two orders of magnitude larger than previously reported microplasma device arrays, 200×200pixel structures produce uniform glow discharges in each pixel, have a power consumption of ∼0.85Wcm−2 for pNe=700Torr and an excitation frequency of 5kHz, and exhibit maximum radiative efficiency for a Ne pressure of pNe≅700Torr and an excitation frequency of 10–15kHz. All arrays examined (10×10→200×200) are characterized by pixel-to-pixel emission intensities constant over the entire array to within ±10%, and no barriers to the successful operation of much larger arrays are apparent.

Implications of microcavity plasma devices for new plasma-display-panel cell structures with improved luminosity

— The unique properties of microcavity plasma devices, and their potential to provide the basis for alternative PDP cell structures of improved luminosity, are described. Arrays as large as 500 × 500 (250,000) inverted pyramid microcavity devices, each with an emitting aperture of 50 × 50 μm2 and designed for AC or bipolar excitation, have been fabricated in Si and operated in the rare gases and Ar/N2 mixtures at pressures up to and beyond 1 atm. For a device pitch of 100 μm, the array filling factor is 25% and the device packing density is 104 cm−2. Measurements of the unoptimized radiant output of 500 × 500 arrays of Si microplasma devices, operating in Ne/(5–50)% Xe mixtures and photoexciting (in transmission mode) a 20-μm-thick film of green phosphor, yield values of the luminous efficacy up to 7.2 ± 0.6 lm/W for a Ne/50% Xe mixture (total pressure of 800 Torr) excited by a 20-kHz sinusoidal voltage waveform. Sustaining voltages ranging from ∼250 to 340 V (RMS) yield luminance values up to ∼2000 cd/m2 for Ne/50% Xe mixtures but the incorporation of field emitters or MgO into the microcavity is expected to significantly reduce the required operating voltage. Also, the fabrication of microplasma devices in ceramic multilayer structures or glass for scaling the display area is discussed briefly. Recent laser spectroscopic measurements of Xe(a 3Σu+) absorption in the visible and near-infrared suggest steps to be taken in PDP cell design, particularly as the Xe content in Ne/Xe mixtures is increased.

20.1: Micro-discharge Arrays for High Resolution Plasma Display Panel

Arrays of microplasma devices, fabricated in a photodefinable glass and driven ac or dc by interdigitated electrodes outside the microcavities, have been fabricated and characterized. Operating in the abnormal glow mode for Ne pressures between 200 and 900 Torr and having cross-sectional dimensions of 75–100 μm, the devices are well behaved and arrays as large as 25×25 pixels have been tested to date. Focused on ease of assembly and minimizing cost, the design of these arrays makes then attractive for structures having emitting areas of hundreds of cm2.

Carbon nanotube-enhanced performance of microplasma devices

Incorporating multiwall carbon nanotubes (CNTs) directly into the cylindrical cathode of Ni screen/BN/Ni microplasma devices significantly improves all device performance parameters—operating and ignition voltages, as well as radiative efficiency. Having a cathode diameter of 200 μm, these devices exhibit operating voltages as much as 30 V (∼22%) lower than those required for an identical structure without CNTs. For Ne pressures of 100–300 Torr, ignition voltages are reduced by 14%–18% with the introduction of CNTs. In contrast, radiative efficiencies in the 300–800 nm spectral region are increased with CNTs by 6%–9% over the entire pressure range studied (200–600 Torr Ne). Voltage–current characteristics for two device configurations suggest that electrons generated within the cathode microcavity by CNT field emission are most effective in impacting device performance.

Integration of carbon nanotubes with microplasma device cathodes: reduction in operating and ignition voltages

Situating carbon nanotubes (CNTs) near the cathode of a microplasma device, or integrating CNTs directly into the device cathode, reduces significantly the operating and ignition voltages for the device. Radiant output is also improved and the greatest benefit to device operation is realised when the CNTs are deposited onto the interior wall of the cylindrical cathode microcavity.

Experimental study of breakdown voltage and effective secondary electron emission coefficient for a micro-plasma device

A micro-plasma device is used as a gas sensor. The micro-plasma device produces dc discharges with a parallel-plane electrode configuration. In this paper the breakdown voltage is measured for argon with tin oxide electrodes, for three different electrode distances (0.01 cm, 0.025 cm and 0.05 cm) and for pressure between 1.3 kPa and 13.3 kPa. The effective secondary electron emission coefficient is then determined from breakdown voltage results; its behaviour is analysed and compared with other experimental results. Differences between experimental breakdown voltage and a widely accepted analytical form of the breakdown law are also shown and the influence of the secondary electron emission coefficient is underlined.

Efficiency improvement of high-pressure microplasma by an electron beam

Efficiency has been a paramount issue for the past few years in high pressure microplasma devices such as Plasma Display Panels (PDP). In order to achieve a better efficiency, a novel method based upon electron beam emission has been investigated by a two-dimensional (2-D) fluid simulation. The injection of electron beam inside the PDP cell gives rise to substantially large density of excited Xe species (Xe*), while reducing ionized xenon (Xe/sup +/) and electron densities. This, in turn, enhances the efficiency, luminance, and reduces power consumption. The primary reason for generating more Xe* species could be attributed to the formation of low electric field region inside the PDP cell, which consequently improves its operational efficiency. The validity of 2-D fluid simulation results is ensured through a qualitative comparison between the one-dimensional fluid and the kinetic results.