Uniform DC Glow DischargeUsing CNT- Nanostructured Cathodes for UV Photons Production in Ar-H2O Mixes

Leron Vandsburger, Nathan Hordy, Sylvain Coulombe, and Jean-Luc Meunier

Abstract : Carbon nanotube covered stainless steel meshes are being explored as potentialcathode materials for use in low-pressure DC glow discharge systems using Ar-H2O-N2 mixtures.CNT-nanostructured materials have been chosen based on CNTs stability in water vapor, which isconsidered as a replacement for mercury vapor in glow discharge-based light sources (i.e. fluorescentlamps). Mesh cathodes have been tested in a DC discharge setup in Ar-H2O and Ar-N2 gasmixtures. I-V characteristics, optical emission spectra, digital photographs, and SEM micrographshave been collected for CNT meshes, polar-functionalized CNT meshes, and bare stainless-steelmeshes. For all types of CNT samples, the introduction of N2 causes a significant increase in UVemission, as compared to water vapor alone. Results indicate that dense CNT coatings are notdegraded by interaction with H2O-containing discharges. However, ion bombardment, which isunavoidable in DC operation, causes significant degradation of CNT structures. CNTs have beencoated with a plasma polymer layer. This has been found to protect against chemical attack, butnot against ion bombardment. Samples prepared in this way show a reduced operating voltage inpure N2 as compared to functionalized bare stainless steel.

Keywords : carbon nanotubes, nanomaterials, mercury-free glow discharge, fluorescent lamp

I. INTRODUCTION

Fluorescent lighting has recently become a renewed focusof research efforts, due mainly to the legislative prohibitionof incandescent light bulbs in both Canada and the UnitedStates. Despite their energy efficiency and lifetime, fluores-cent lamps present an imminent problem, considering thescale of future use. The operating mechanism, and indeedthe efficiency, of fluorescent lamps depends on the emissionof UV photons at 254 nm from Hg vapor. The typicalfluorescent lamp contains approximately 3-5 mg of Hg;making clear the imminent environmental issues associatedwith disposal of spent lamps, which in 2004 numbered 60million in Canada alone [1]. Replacing Hg is not a simpletask, however, due to its high efficiency and absence ofsignificant chemical reactivity with lamp electrodes. Thus,the search for a gas fill replacement is intrinsically linkedto the search for new electrode materials and structures.

With the aim of finding a common solution to bothproblems, a new electrode design has been selected basedon work done with CNT-stainless steel materials. Hy-drophilicity is vital to this effort, for not only does watervapor emit photons in the near-ultraviolet, but it alsooxidizes bare metal. For that reason, considering the stablephysical-chemical interaction of water vapor with func-tionalized CNTs and the broad molecular emission fromOH radicals in the near UV range, an argon/water vapormix has been selected as a replacement for Hg vapor and aCNT-stainless steel nanostructured mesh material has beenchosen to replace coiled tungsten as the cathode material.

As a result of the geometry of the new electrodematerial, the negative glow can be used as the principalphoton source, rather than the positive column as in thecurrent design. This is a significant advantage, because thenegative glow is considerably brighter than the positivecolumn. In this way, a lamp designed with the newelectrode material will be more luminous, and can be usedas planar light-sources. The planar geometry of CNT-meshelectrodes makes them ideal for use in handheld devices

or flat televisions. Whereas current lamps emit light in alldirections uniformly and require diffuser plates, the newdesign will produce nearly unidirectional light, furtherincreasing efficiency.

II. BACKGROUND

Theoretical and experimental work on CNT-materialcathodes has focused on field emission devices. This isdue mostly to theoretical predictions of field-enhancementand reduced work function resulting from the aspect ratioof CNTs [2]. In a field emission lamp, electrons emittedfrom the surface of the cathode strike a phosphor screen toproduce visible light. A plasma is not generated betweenthe electrodes, but rather a high vacuum is necessary, aswell as high electric field strength. The following work hasbeen published on CNT-cathode lamp prototypes usingthe cold cathode direct scintillation approach [3-5]. Muchmore work has been published on field-emission fromCNTs, but will not be summarized here.

The other side of the current project is to replacemercury as the UV-photon source in fluorescent dischargelamps. Work has been published, within the last few years,investigating glow discharges in pure water vapor and inmixtures of water vapor and rare gases [6]. These studiesall used conventional tungsten electrodes or some othermetal electrode. No work has yet reported a combinedCNT electrode and water vapor glow discharge lamp.

Modification of the surface chemistry of CNTs wasfirst developed using wet chemical methods, wherebyCNT samples would be suspended in strong acids andchemically degraded [7]. Plasma treatment was explored asan alternative to wet chemical functionalization, in orderto reduce both waste and process complexity. Plasmafunctionalization has been used in this project, so it will beaddressed specifically. Glow discharge treatment in argon-ethane-oxygen mixes has been shown to hydrophilize CNTsby covalently bonding alkane and carbonyl groups into the

2

FIG. 1. TEM image of MWNT grown by CVD

outer shells of MWNTs [8]. Thus far this approach hasbeen used to render stable CNT suspensions in polar liquidslike water or ethanol, which would otherwise be impossible.

III. METHODS

Mesh cathode samples are prepared by a single-stepthermal chemical vapor deposition method that produces auniform layer of multi-walled carbon nanotubes (MWNTs).The diameter and length of the MWNTs thus produced canbe controlled by adjusting the parameters of the growthprocedure. An example of the type of CNT produced bythis method is shown below (Fig.1). Once grown, MWNTscan be hydrophilized by RF-plasma functionalization inthe interest of maximizing their interaction with watervapor according to a procedure developed previously [8].

After testing in Ar-H2O vapor mixes showed the hy-drophilized tubes to be more susceptable to chemical attackthan untreated CNT samples, a new approach was adoptedto chemically shield them by dual-functionalization. Theprocedure involved a primary non-polar treatment, coatingthe CNTs with a plasma polymer layer, and a secondarytreatment functionalizing the polymer rather than theCNTs themselves.

A direct-current low-pressure discharge chamber hasbeen assembled to test the CNT-covered mesh samples.Samples are prepared for use by attaching a contact that isconnected to an electrical feedthrough. An AMETEK XG600-1.4 model 850 W DC power supply drives the setup.The circuit is closed to ground, meaning that the mesh sam-ple acts as the cathode and the counter-electrode, actingas the anode, is in contact with the chamber housing andground. The pressure and gas composition in the chamberis controlled manually by adjusting the flow rates of H2Ovapor, N2 and Ar and by a manual control valve on the vac-uum line. Both a rotary vane pump and a molecular drag

pump are installed in the system, for which the pressureis measured by piezo-transducer and miniature ion gauges,respectively. A schematic of the setup is shown in Figure 2.

FIG. 2. Schematic of DC discharge setup *by Alex Emmott

Breakdown and sustaining voltage values are recordedfor a set current. Emission spectra are collected using aPrinceton Instruments Acton Spectrapro 2300i monochro-mator equipped with a PIXIS 256 CCD detector and areanalyzed by the WinSpec software package. Followingtesting, samples are examined using a Phenom tabletopSEM to asses the effects of plasma-surface interactions onCNT surface integrity. All experiments are carried out ata constant discharge current of 10 mA.

IV. RESULTS

The breakdown and discharge voltage for all types ofsamples are similar within 30 V. Average values for adischarge in 1:1 mixture of Ar and H2O are given in Table1. Surface durability is assessed by SEM micrographsfor CNT samples. A tableau of images representing thestate of the surfaces before and after treatment is shownin Figure 3. A second tableau, showing photographs ofthe discharges formed by the three types of samples inboth pure Ar and Ar-H2O, is given in Figure 4. Opticalemission spectra collected for each type of sample inAr-H2O mixes are given in one chart in Figure 5.

Cathodes used in Ar-N2 discharges were dual-functionalized, unlike in the Ar-H2O tests, but thesame information was collected. Spectra from both CNTsamples and functionalized stainless steel are given inFigure 6; and SEM micrographs of the dual-functionalizedsample in Figure 7. Sustaining and breakdown voltagesare reported in table 1 above.

Ar-H2O VBreakdown VSustaining

Bare Stainless Steel 373 303

CNTs 388 270

Hydrophilized CNTs 328 306

Ar-N2-H2O

Dual Func. Stainless Steel 503 429

Dual Func. CNTs 500 376

TABLE I. Voltage measurements for DC glow discharges

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FIG. 3. SEM images of a,b) CNTs and c,d) functional-ized CNTs before and after DC discharge

FIG. 4. Photoarray of Samples in DC discharge

V. DISCUSSION:

The limitation of using CNTs in DC discharges is clearfrom the SEM images in Fig. 2. For such samples, signifi-cant degradation of the CNT structure was observed after 5min of use as a cathode. This was a result of a combinationof plasma oxidation and ion bombardment. The absence ofan oxygen peak (777 nm) from emission spectra and thesmall presence of hydroxyl band emission (310- 340 nm)shows that, although dissociation of water did take place

FIG. 5. Spectra of samples in Ar-H2O DC discharge

FIG. 6. Spectra of samples in Ar-H2O-N2 DC discharge

FIG. 7. Dual functionalized samples in Ar-H2O-N2 DCdischarge a) before, b) after

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FIG. 8. Red surface glow in a CNT covered mesh sample

as seen from the strong presence of atomic hydrogen, oxy-gen groups were either not being exited in the plasma orwere absent entirely. The presence of a strong red glow, cor-responding to the Balmer Hα line (653.6 nm), near the sur-face of CNT samples indicates that oxidation of CNTs is apossible explanation for the absence of oxygen species fromthe emission spectra. An example of this red surface glowis shown in Figure 8. Furthermore, the presence of CNTs inthe crevices between steel threads indicates that ion bom-bardment plays a considerable role in the degradation pro-cess, since ion bombardment occurs by a line-of-sight pro-cess that follows the electric field lines between the anodeand cathode. Introduction of water vapor into the dis-charge chamber caused a rapid change in the emission spec-trum (i.e. color), as seen clearly in Fig. 3. The CNT sam-ples showed a stronger blue color than bare stainless steelfor the same compositions, while hydrophilized CNTs didnot show a noticeable difference from bare stainless sam-ples. The effect of introducing N2 into the chamber was animmediate emission in the near-UV, which is desirable forapplications in lighting. For this reason, N2 was added toall subsequent experiments. When compared to bare stain-less steel, theplasma polymer coated CNT samples showeda slightly reduced sustaining voltage of 363 V compared to408 V, indicating a possible effect of field enhancement.

The spectra produced are similar in that the emissionin the UV is comparable in intensity to the emission fromAr. The peaks in the near-UV are all produced by variousnitrogen species [9]. The presence of the plasma polymerlayer did protect the CNTs from chemical attack, asplanned. This can be seen from the SEM images of a dualfunctionalized sample that was used as a cathode. CNTswere not damaged as thoroughly as in previous samples,and the ion bombardment shadow was not visible. Thegreatest degradation occurred in areas where the dischargewas brightest, indicating that the plasma polymer layeris not impervious to either chemical oxidation or ionbombardment. In the interest of deciding the optimumthickness for the polymer layer, several deposition timeswere investigated. After a certain thickness the coated-CNT material could not function as an electrode, due to

the non-conductive nature of the coating. In this way itwas decided that there was an optimum thickness of thelayer, which has not yet been measured.

VI. CONCLUSION:

CNT-mesh covered samples provide an important oppor-tunity to study the interaction between energized species ina DC plasma. For use as cathodes in H2O containing sys-tems, however, the CNTS must be protected to shield themfrom chemical oxidation. Growth of bamboo-like CNTs andplasma polymer coating can reduce exposure of the innerand outer walls to energized species, which helps maintainthe lattice integrity of MWNTs. Ion bombardment,however, is a more difficult matter to avoid when used inDC applications. To that end, work will soon begin on RFoperation to limit the ion flux near the cathode surface.

VII. REFERENCES

1. Pollution Probe, Background Study on In-creasing Recycling of End-of-Life Mercury-Containing Lamps from Residential and Com-mercial Sources in Canada, October 31, 2005:http://www.pollutionprobe.org/Reports/merclamps-report.pdf!

2. Dionne, M., S. Coulombe, and J.-L. Meunier. Low-Pressure Gas Discharge UV Source Using a Thermo-Field Emission Carbon Nanotube Array Cathode. inLightsources 12. 2010. Eindhoven, Netherlands.

3. Chen, J., et al. Flat-panel luminescent lamp using car-bon nanotube cathodes. 2003. Lyon, France: AVS.

4. Huang, J.X., et al., Field-emission fluorescent lamp us-ing carbon nanotubes on a wire-type cold cathode anda reflecting anode. Journal of Vacuum Science & Tech-nology B: Microelectronics and Nanometer Structures,2008. 26(5): p. 1700-1704.

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6. Artamonova, E., et al., Low pressure water vapour dis-charge as a light source: I Spectroscopic characteristicsand efficiency. Journal of Physics D: Applied Physics,2008. 41(15): p. 5206.

7. Hummers, W.S. and R.E. Offeman, Preparation ofGraphitic Oxide. Journal of the American Chemical So-ciety, 1958. 80(6): p. 1339-1339.

8. Vandsburger, L., et al., Stabilized aqueous dispersionof multi-walled carbon nanotubes obtained by RF glow-discharge treatment. Journal of Nanoparticle Research,2009. 11(7): p. 1817-1822.

9. Boudam, M.K. and et al., Characterization of the flowingafterglows of an N2-O2 reduced-pressure discharge: set-ting the operating conditions to achieve a dominant lateafterglow and correlating the NOβ UV intensity varia-tion with the N and O atom densities. Journal of PhysicsD: Applied Physics, 2007. 40(6): p. 1694.