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Page 1: Surface cleaning of metal wire by atmospheric pressure plasma

Applied Surface Science 256 (2009) 1096–1100

Surface cleaning of metal wire by atmospheric pressure plasma

T. Nakamura a,d,*, C. Buttapeng b, S. Furuya c, N. Harada d

a Electronic-Mechanical Engineering Department, Oshima National College of Maritime Technology, 1091-1 Komatsu, Suo-Oshima, Yamaguchi, Japanb School of Electrical and Energy Engineering, University of the Thai Chamber of Commerce, 126/1, Vibhavadee-Rungsit, Dindaeng, Bangkok 10400, Thailandc Faculty of Education, Gunma University, 4-2 Aramaki, Maebashi, Japand Department of Electrical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Japan

A R T I C L E I N F O

Article history:

Available online 6 June 2009

Keywords:

Atmospheric pressure discharge plasma

Plasma application

Surface cleaning

Annealing

A B S T R A C T

In this study, the possible application of atmospheric pressure dielectric barrier discharge plasma for the

annealing of metallic wire is examined and presented. The main purpose of the current study is to

examine the surface cleaning effect for a cylindrical object by atmospheric pressure plasma. The

experimental setup consists of a gas tank, plasma reactor, and power supply with control panel. The gas

assists in the generation of plasma. Copper wire was used as an experimental cylindrical object. This

copper wire was irradiated with the plasma, and the cleaning effect was confirmed. The result showed

that it is possible to remove the tarnish which exists on the copper wire surface. The experiment reveals

that atmospheric pressure plasma is usable for the surface cleaning of metal wire. However, it is

necessary to examine the method for preventing oxidization of the copper wire.

� 2009 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Applied Surface Science

journal homepage: www.e lsev ier .com/ locate /apsusc

1. Introduction

The atmospheric pressure plasma used in this study is a non-equilibrium plasma, and differs significantly from others in termsof particle and electron temperature. Although atmosphericpressure plasma shows electron temperature of about 10,000 K,the temperature of ion and neutral particles is as low as severaltens of 100 K. This result in atmospheric pressure non-equilibriumplasma being considered most obviously suited for environmentalapplications and energy [1].

Additional inherent advantages of using this atmosphericpressure plasma are as follows: (1) relatively high density tominimize the treatment time, (2) lower temperature of non-equilibrium plasma suitable for treating plastic films, (3) does notrequire a vacuum vessel and evacuation facilities, and (4) ability totreat a relatively wide area.

The manufacturing process of fine wire mainly consists of thethree following processes: (1) drawing the material through dices,(2) cleaning off the lubricant used in the drawing process, and (3)annealing to give malleability and ductility in order to improvedurability. In the manufacturing process of present thin metallicwire, steam and ultrasonic cleaning, are carried out in order toremove the lubricant which is used in the drawing process. Also, in

* Corresponding author at: Electronic-Mechanical Engineering Department,

Oshima National College of Maritime Technology, 1091-1 Komatsu, Suo-Oshima,

Yamaguchi, Japan.

E-mail address: [email protected] (T. Nakamura).

0169-4332/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2009.05.121

the annealing, it energizes the metallic wire, and the annealing isdone by Joule heating.

In the previous study, it was possible to carry out rapid surfacecleaning by irradiating the processing object with the plasma [2–4]. Therefore, the main purpose of the current study is toinvestigate the surface cleaning effect for a cylindrical object. Inthis study, the experiment was carried out from the viewpoint ofthe cleaning effect.

2. Cleaning principle

The aqua pure cleaning and the organic solvent cleaning arecalled ‘‘general wet wash.’’ A lot of equipment and a large supply ofenergy are needed for processing the wastewater fluid. However,in this study, plasma is used for the cleaning, so it is called ‘‘drycleaning.’’

In general, plasma energy is extremely high so that a radical,which easily reacts with other molecules, is produced. Based onthis property, it is possible to carry out the cleaning at a lowtemperature by removing the organic pollutants on the substratesurface. The principle is shown in Fig. 1.

The DP method (direct plasma) is the chemical reactionbetween the oxygen radical and organic contamination. However,such a chemical reaction may not be able to counter the inorganicfouling. Therefore, an ion revitalization is made to collide with thepollution at high speed. Here, the RIE method (reactive ion etching)of peeling is used to physically remove the dirt.

The fouling which exists on the substrate used in this study isregarded as very tiny ‘‘particles,’’ ‘‘metal pollution,’’ ‘‘organic

Page 2: Surface cleaning of metal wire by atmospheric pressure plasma

Fig. 1. Image of the cleaning principle. (a) Direct plasma and (b) reactive ion etching.Fig. 3. Experimental system.

Table 1Experimental parameter.

Applied voltage 10 kV

Frequency 40 kHz

Dielectric and the thickness Alumina, 4 mm

Gap length 5 mm

Inert gas He

Gas flow rate 5 l/min

Carrier speed of copper wire 0 m/min

Plasma exposure time 5 s

T. Nakamura et al. / Applied Surface Science 256 (2009) 1096–1100 1097

contamination,’’ ‘‘a natural oxidation film’’ and ‘‘dust,’’ which arevisible.

3. Experimental outline and setup

3.1. Atmospheric discharge plasma

There are various surface treatment methods using plasmas. Inthe present study, the direct irradiation of the surface of materialswith the atmospheric dielectric barrier discharge plasma isproposed for the treatment. The conceptual configuration isshown in Fig. 2.

The active species (radical ion, electron, etc.) generated in theplasmas under the atmospheric pressure are used. The effects areas follows [2]:

� Cleaning effects such as contamination removal.� Roughened surface effect which forms an uneven effect on the

surface.� Active effects such as the surface treatment.

This device consists of electrode and dielectric. By using thedielectric, transition to the arc is prevented. The characteristics ofthe atmospheric dielectric barrier discharge plasma are as follows:

� This system is composed of simple devices.� No requirement for a vacuum vessel and evacuation facilities.

3.2. Experimental setup

For the experimental setup, the present system was composedof a gas tank, plasma reactor, and power supply with control panel,as shown in Fig. 3. The power supply is controlled through thecontrol panel. The high frequency and high voltage is applied to the

Fig. 2. Conceptual layout of atmospheric dielectric barrier discharge plasma.

reactor, and the plasma is generated in the gap between electrodes.An inert gas such as helium or argon is supplied to the electrodegap from the gas tank to assistant in plasma generation.

The experiment parameters are listed in Table 1. The conduct-ing wire used in this study is copper with a diameter of 0.2 mm. Forthe lubricant, the following were used: copper and copper alloywater solubility drawing oil (Roux bright #1700 for the fine line;phi 0.9–0.1 actually used in the drawing process). The surfacecleaning effect by exposure to the plasma was evaluated, after oilhad adhered to the copper wire. In addition, the plasma wasirradiated without carrying copper wire in this study.

In the previous study, the report showed that there was asurface cleaning effect by irradiating the processing object withplasma [5]. On the other hand, in this study, as a method forreplacing the cleaning by steam, the effect of the cylindrical surfacecleaning by plasma is evaluated.

In general, to confirm the effect of the surface cleaning, thecontact angle method is introduced and measured [3]. As thecontact angle method is best suited for the measuring of a flatsurface, it is not appropriate for this study, since the object is a finewire.

Therefore, the surface of the copper wire is examined using thesecondary electron image (SEI) and reflection electron image bythe SEM function added to electron probe microanalysis (EPMA;Shimadzu EPMA-1600).

In addition to the above observation method, a compositionanalysis of the copper wire surface was also performed. This

Fig. 4. Reactor form. (a) Direct type and (b) indirect type.

Page 3: Surface cleaning of metal wire by atmospheric pressure plasma

Fig. 5. Schematic view of the reactor adopted for this study.

Fig. 6. Applied voltage and current waveforms.

T. Nakamura et al. / Applied Surface Science 256 (2009) 1096–11001098

analysis can obtain the density of an unknown sample bycomparison with same characteristic X-rays strength of the kindobtained from a standard value and an unknown. It is thought, byadopting this method, the surface cleaning effect can be furtherclarified.

3.3. Reactor form

The surface treatment of the object is performed by usingatmospheric discharge plasma. In the cleaning experiments both a,direct and indirect reactor are used. The direct reactor is used toinsert the substrate into the plasma, while the indirect reactor isused to extrude the plasma that arises between electrodes and theworking gas. The schematics of both the direct-and-indirectreactor are shown in Fig. 4.

In the direct reactor (Fig. 4(a)), substrate is directly irradiatedwith the plasma. As a result, the cleaning surface is larger.However, the damage to the substrate seems to be a significantproblem. This is because there is a property which concentrates inthe salient part of the plasma and undergoes transition to the arcdischarge resulting in larger damage.

In contrast, in the indirect reactor (Fig. 4(b)); substrate damageis reduced. Plasma and radical (material that is revitalized in achemical reaction) have a longer life due to the cleaning effect.Therefore, it is probable that the damage decreases because there istoo great an interval between the electrode part and the substrate.

The reactor configuration adopted in this study is shown inFig. 5. The reason for adopting this reactor is that the previousannealing experiment could not be performed using the reactorshown in Fig. 4(a) and (b). In this reactor, the energy of thegenerated plasma is intensively irradiated to the wire.

The waveforms of the applied voltage and the electrode currentare shown in Fig. 6. As we can see from the figure, the high

Fig. 7. Evaluation by the secondary electron image. (a) After t

frequency voltage with the pulse component is applied betweenthe electrodes. It is known that the plasma is generally generated inthe initial-rise part of the voltage. Therefore, it seems to beadvantageous that the power supply contains the pulse componentand high frequency for plasma generation [6].

4. Experimental results and discussions

When the surface of the copper wire is prepared by the additionof a lubricant, the copper wire is irradiated with plasma. Thesecondary electronic image obtained by the scanning electronmicroscope (SEM) function appended in electron probe micro-analysis (EPMA) shown in Fig. 7. Fig. 7(a) shows the surface ofcopper wire after the lubricant adhesion, while Fig. 7(b) shows thecopper wire after the plasma treatment.

In Fig. 7(a), an incrustation is observed on the wire surface. Thisis known as a dried lubricant. The incrustation can be removed byplasma irradiating (Ref. Fig. 7(b)).

The reflection electron image by EPMA is shown in Fig. 8.Fig. 8(a) shows the surface of copper wire after the lubricantadhesion, while Fig. 8(b) shows the copper wire after the plasmatreatment.

In Fig. 8(a), the incrustation (marked with the white circle) isobserved on the copper wire surface. However, after the surface isirradiated with the plasma, it is found that the incrustation hasbeen removed (Fig. 8(b)).

The reasons why the incrustation is removed from the wiresurface is speculated as follows: (1) the electron and the ion that

he lubricant adhesion and (b) after the plasma exposure.

Page 4: Surface cleaning of metal wire by atmospheric pressure plasma

Fig. 8. Evaluation by the reflection electron image. (a) After the lubricant adhesion and (b) after the plasma exposure.

T. Nakamura et al. / Applied Surface Science 256 (2009) 1096–1100 1099

exists in plasma collide with the surface of the copper wire at highspeed. As a result, the incrustation is physically exfoliated; (2) thematerial which was on the surface melts because the copper wire isheated to annealing temperature. The radical generated in theplasma and the melted material reacted and the attachment isremoved. However, which is the most likely or major effect is noteasily judged from this study.

In this study, the results show that material that adheres to thesurface of the copper wire is removed by using the plasmatreatment. As a result, the surface looks smooth and clean.

The results of the surface composition analysis are presented inFig. 9. Fig. 9(a) shows the surface of copper wire after the lubricant

Fig. 9. Chemical composition analysis of the copper wire surface. (a) After the

lubricant adhesion and (b) after the plasma exposure.

adhesion, while Fig. 9(b) shows the copper wire after the plasmatreatment.

In Fig. 9(a), the reasons why the lubricant spectrum could not bedetected are as follows. The X-ray transmitted was absorbed by thelubricant and only analyzed the copper wire.

The spectrum of oxygen emerges by irradiating the surface withthe plasma. On the ratio of copper and oxygen in Fig. 9(b), thecopper was 60.272 wt% in analysis results, and the oxygen was39.728 wt%. By irradiating the plasma, the copper wire tempera-ture rises until the annealing point. Afterwards, the surface isthought to oxidize by rapidly cooling in the atmosphere. When thesurface is oxidized, it is not possible to market the copper wire.Devising a countermeasure is, therefore, a problem for futureresearch.

5. Conclusions

This paper has presented the basic experiment of surfacecleaning of copper wire observed by using the EPMA.

The results showed that it was possible to remove theattachment which exists on the copper wire surface by plasmairradiation. As a reason for removing this attachment, the cleaningeffect of the plasma like the RIE method or DP method seems to beoriginal. Which method is predominant is cannot easily beevaluated from this study. However, from achieved result it canbe said that the plasma cleaning is possible for cylindrical materiallike a copper wire.

The formation of copper oxide resulting from irradiating thecopper wire with atmospheric pressure plasma was a majorproblem of the current study. Research is ongoing to find a solutionto this predicament.

Acknowledgments

I wish to express my gratitude to all of my co-authors for theircontributions to this paper. Moreover, my sincere appreciation andacknowledgment goes to Dr. Yazawa, Dr. Sakamoto, and studentsat Harada Lab in Nagaoka University of Technology, who greatlyassisted in the experiments.

References

[1] T. Nozaki, K. Okazaki, Creation of atmospheric pressure non-equilibrium plasmasfor efficient material processing, The High Temperature Society Explanation 28 (3)(2002) 1–10.

[2] T. Uchikoshi, The Illustration Machine Material (In Japanese), Tokyo Denki Uni-versity Press, 1996, pp. 26, 106.

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[3] K. Fukuda, Y. Kondo, K. Oishi, Y. Toda, The development of adhesion technologyusing atmospheric pressure glow plasma processing, Konica Minolta TechnologyReport, vol. 1, 2004, pp. 35–38.

[4] T. Nakamura, T. Suto, S. Furuya, N. Harada, Annealing of metal wireby atmospheric pressure plasma, 2007 IEEE Shin-etsu Session, 2007,219 pp.

[5] N. Harada, M. Yoshida, T. Nakamura, Improvement of tear strength using dielectricbarrier discharge plasma, in: Proceedings of the 3rd Japan-US Symposium onPulsed Power and Plasma Applications, 2006, pp. 41–46.

[6] T. Nakamura, C. Buttapeng, S. Furuya, N. Harada, Application of atmosphericpressure discharge plasma in environmental protection, in: Proceedings of theInternational Symposium on Environmental Management, 2008, pp. 34–35.