Surface Modification of Polymethyl-methacrylate Using Atmospheric ...

IEEE Transactions on Dielectrics and Electrical Insulation

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Surface Modification of Polymethyl-methacrylate Using Atmospheric Pressure Argon Plasma Jets to Improve Surface Flashover Performance in Vacuum Tao Shao, Yixiao Zhou, Cheng Zhang, Wenjin Yang, Zheng Niu and Chengyan Ren Institute of Electrical Engineering, Chinese Academy of Sciences Key Laboratory of Power Electronics and Electric Drives, Chinese Academy of Sciences Beijing, 100190, China

ABSTRACT Hydrophilic modification of polymethyl methacrylate (PMMA) surface is performed by atmospheric pressure plasma jet (APPJ) in Ar gas for improving the PMMA surface flashover performance in vacuum. In the experiments, APPJ is driven by a microsecond-duration pulsed generator, which has voltages of 0-30 kV, a rise time of 300 ns and a full width at half maximum of 2ȝs. Characteristics of the APPJ are analyzed according to its voltage and current waveform, discharge image and optical emission spectrum. Furthermore, surface properties of the PMMA surface before and after the treatment are characterized by water contact angle measurements and morphology observations. Results show that the main species of the plasma jet are composed of N2, Ar, OH, and O, among which such polar groups as OH and O enhance the hydrophilic property of the PMMA surface. The water contact angle decreases from 68° to a minimum value (16°) after the treatment. In addition, all the surface flashover voltages in vacuum for the PMMA samples treated by APPJ are higher than those for the untreated PMMA samples. Index Terms - Pulsed power, microsecond pulse, gas discharge, pulsed discharge, non-thermal plasma, atmospheric pressure plasma jet, surface treatment, hydrophilic modification, surface flashover, flashover voltage.

1 INTRODUCTION VACUUM is widely used in high-voltage electrical equipments and high-power devices due to its excellent insulating properties. However, surface flashover always appears along vacuum insulators of electrical equipments, which may damage both insulators and equipments. Therefore, in order to guarantee high insulating performance of the vacuum insulators, it is necessary to enhance surface flashover performance of the insulators in vacuum. There are many factors influencing surface flashover performance in vacuum, such as dielectric constant of insulating material, shapes of both solid insulator and electrode, surface charge density, surface morphology of insulator, applied voltage, and so on. All these factors have been widely investigated in many studies [1-11]. Especially, the surface morphology of the insulator is closely related to its vacuum insulating performance [12-15]. Generally, most studies focus on the effect of surface charge, creepage distance, and surface roughness of the material on flashover Manuscript received on 10 March 2014, in final form 30 September 2014, accepted 5 December 2014.

voltage. For example, Sundara et al presented the relationship between the creepage distance and the surface flashover voltage in air [13]. Victoria et al studied the effect of roughness on the surface flashover voltages with different materials under different ambient conditions, and the date indicated a relationship between the roughness and the surface flashover voltage of the insulator [14]. There are several methods allowing one to modify the surface property of polymers: chemical surface treatment, arc spraying, laser remelting, and non-thermal plasma treatment. Among these methods, non-thermal plasma treatment is considered as an effective approach to modify the surface of polymer materials because of its simplicity, easiness of operation, environmental protection, and low energy cost [1617], especially, the plasma treatment can be carried out at atmospheric-pressure. Generally, researchers use dielectric barrier discharge (DBD) with a plate-in-parallel configuration to generate non-thermal plasmas for surface modification [20-22]. It has been confirmed that some modifications (nano-coating or fluorination) on the polymer material surfaces can improve the

DOI 10.1109/TDEI.2014.004637

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T. Shao et al.: Surface Modification of Polymethyl-methacrylate Using Atmospheric Pressure Argon Plasma Jets

surface insulation performance of insulators [23-25]. Fang et al treated a glass surface using DBD in the atmospherepressure air to improve the surface hydrophobicity and showed that the flashover voltage of the wet glass surface increased after the treatment [26]. However, such plasma source can only be applied to flat and thin materials, and sometimes discharge filaments in the DBD may damage the surface of the material. In recent years, atmospheric pressure plasma jet (APPJ) has attracted attention as a new plasma source. The APPJ has uniform spatial distributions of high energy ions and electrons. In addition, a flexible form of the APPJ can been used to treat complex surfaces. Kong et al used a ten-channel APPJ array to treat the complex three-dimensional substrates, and the experiment showed that the APPJ array with effective self-adjustment had excellent temporal and spatial uniformity [27]. Fang et al used the APPJ in argon to enhance surface hydrophilicity of polyethylene terephthalate (PET) and showed that the water contact angle decreased with the increase of surface energy of the treated PET [28]. Shao et al used a DBD-like plasma jet to treat a polytetrafluoro ethylene (PTFE) surface. Their observations using scanning electron microscopy and x-ray photoelectron spectroscopy indicated that some protuberances and hydrophilic groups, such as carbonyl, carboxyl, and hydroxyl, were introduced on the PTFE surface [29]. Currently, while the surface flashover behavior in vacuum has been extensively studied, the mechanism of surface flashover is still far from being very clear. Different methods on increasing the surface flashover voltage in vacuum have been proposed extensively. However, little attention has been paid to the relationship between the surface plasma treatment of the insulator and the surface flashover voltage in vacuum. In our work, the APPJ plasma in Ar is used to treat the polymethyl methacrylate (PMMA) surface. The electrical characteristics, the emission spectrum, and the surface flashover performance in vacuum before and after treatment are described in this paper.

2 EXPERIMENTAL SETUP OF SURFACE MODIFICATION Figure 1 shows the pin-ring electrode structure of the plasma jet devices, as well as the measuring systems. The Tshape plasma jet is with a quartz glass tube and gas inlet. The inner and the outer diameters of the quartz tube are 3 mm and 6 mm, respectively. A tungsten needle with 1 mm diameter and 150 mm length is inserted into the center of the quartz tube as a high-voltage electrode. A 10-mm wide copper tape wrapped on the outer surface of the quartz tube is used as a grounded electrode. The high voltage electrode is fixed at a distance 20 mm away from the open end of the tube. High purity argon (99.99 %) is fed into the reactor chamber from the bottom side of the quartz tube. This process is controlled by a Sevenstar mass flow meter within a range of 0-15 standard liter per minute (SLM) with a resolution of 0.01SLM. The plasma jet is powered by a microsecond-duration pulsed generator, which provides 0-30 kV voltage with rise time 300

Figure 1. Schematic diagram of the experimental setup.

ns and a full width at half maximum 2 ȝs. The pulse repetitive frequency (PRF) of the generator ranges from 1 Hz to 5 kHz, which is controlled by a pulse trigger. The trigger signals are generated by a programmable pulse generator (PG-F1) [30]. The voltage applied on the electrode is measured through a Tektronix P6051A voltage probe. The discharge current is measured through the current coil (Pearson 6595, 0.5 V/A). The discharge voltage and the current waveforms are recorded by a digital oscilloscope Lecroy 204Xi. Discharge images are taken by Canon EOS500D and TAMRON lens (70-200 mm f/2.8) with an exposure time of 1/2 s. Optical emission spectrum of the plasma jet is measured by an AvaSpec-36486ˈwhich has six channels with individual integral time. The measured wavelengths are in the 200-900 nm region with resolutions of 0.06 nm-0.09 nm. PMMA is commonly known as an organic glass being one of the best transparent polymer materials with excellent optical properties, electrical insulating properties, and mechanical strength. It is used extensively as an insulator. The material for the treatment is commercial PMMA. Before the experiment, a 2 mm-thick PMMA bulk is cut into small samples of 40 cm×40 cm, and then successively washed with acetone, alcohol, and deionized water. An ultrasonic washer is used to clean the PMMA surface. Finally, PMMA samples are placed into a vacuum drying oven for 3 hours. The temperature is fixed at 85 °C and the pressure is 5 Pa.

Figure 2. Discharge images at 10kV, 3 kHz with different gas flow rates.


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Figure 3. Voltage and current waveforms at 10 kV, 3 kHz with different gas flow rates of (a) 0.01 SLM, (b) 0.1 SLM, and (c) 2 SLM.

In our experiments, the generally used conditions are as follows if there is no special statement: the applied voltage is 10 kV, the PRF is 3 kHz and the flow rate is 2 SLM. In order to evaluate the plasma treatment, the surface energy of PMMA is an important parameter that determines most of surface properties, and can be characterized by contact angle measurements (JGW-360a, China). Generally, water contact angle measurement is the most common method for determining the hydrophilic property of the materials via the famous Young equation. The contact angle is the angle, conventionally measured by liquid, where a liquid/vapor interface meets a solid surface. It should be pointed out that the plasma jet and the PMMA sample are fixed, thus, an effective treatment region using APPJ is not sufficient to cover the whole sample. In this work, contact angles are measured from the profile of about 2 ȝL liquid drops of distilled water and polyethyleneglycol placed on the effective region of the PMMA surface immediately after treatment. This effective region has an estimated diameter of 1 cm around the centre of the PMMA samples [31]. The values of static contact angle are obtained by means of Laplace-Young curve fitting based on the imaged water drop profile and are the average of eight measured data on each sample. The water is left on the PMMA samples for about 15 s in all cases.

becomes to be bipolar with a positive pulse at the rising edge and a negative pulse at the falling edge of the applied voltage. The amplitudes of these two pulses are 1.5 A and 0.3 A, respectively, as shown in Figure 3b. When the flow rate is 2 SLM, there is only a positive pulse with an amplitude 2.4 A at the rising edge of the applied voltage, as shown in Figure 3c. Figure 4 shows the emission spectra of the APPJ. The experimental conditions are as follows: the applied voltage is 10 kV, the PRF is 3 kHz and the flow rate is 2 SLM. The optical fiber is placed 10 mm away from the nozzle in parallel. It can be observed that the main species in the APPJ are: the nitrogen second positive system N2 (C3Ȇu-B3Ȇg) [21], OH o

( A 2 ¦ X 2 3 , 306-318 nm), the atomic oxygen O ( 3 p 5 P o 3s 5 S , 777.4 nm; 3 p 3 P o 3s 3 S , 844.6 nm) and the excited states of Ar atoms emitting in the region between 680850 nm.

3 ELECTRICAL CHARACTERISITICS OF ARGON PLASMA JETS MODIFICATION Figure 2 presents typical images of plasma jets in argon under different flow rates, which range from 0 SLM to 2.0 SLM. It can be seen that the discharge occurs only in the tube when the flow rate is smaller than 0.1 SLM, and the plasma jet appears when the flow rate exceeds 0.1 SLM. Furthermore, the discharge behaves as corona discharge, DBD-like discharge, and plasma jet when the flow rates are 0.01 SLM, 0.1 SLM, and 2 SLM, respectively. The corresponding voltage-current waveforms are shown in Figure 3 when the flow rates are 0.01, 0.1 and 2 SLM, respectively. When the flow rate is 0.01 SLM, the displacement current is seen from 0 to 0.4 ȝs and the real discharge current is about 30 mA at 0.5 ȝs, as shown in Figure 3a. As the flow rate increases to 0.1 SLM, the current

Figure 4. Optical emission spectroscopy of APPJ in argon. Table 1. Data of selected spectral lines [25].

Ȝ (nm) 738.39 750.38 763.51 772.37 811.53 826.45

g 1.9 0.34 0.29 0.063 25 15.3

6 -1

A (10 S

) 14.7569 14.8092 14.9574 14.7804 13.1531 13.3278

E (eV) 9 5 5 7 3 3

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4 EXPERIMENTAL RESULTS OF PMMA SURFACE MODIFICATION Figure 6a shows the photograph of the APPJ treatment experiment. The intensity of the APPJ decreases with spreading it out from the center. The treatment area on the PMMA can be up to 500 mm2. The PMMA is treated for 90 s under a distance of 5 mm. Under this condition, the images of the water contact angles before and after the treatment are shown in Figures 6b and 6c. Figure 7 shows the contact angles after the treatment under different distances away from the nozzle (1-20 mm). It can be seen that in all cases, the contact angle of the PMMA surface is smaller than its original value before the treatment. The minimum value is 16° when the distance is 5 mm. The variation of the static water contact angles with treatment time is shown in Figure 8. The treatment distance was fixed at 5 mm. It is observed that the water contact angle rapidly decreases with the increase of the treatment time. When the treatment time exceeds 90 s, the value of the water contact angle saturates. This is because the surface reactions between plasma and the PMMA surface gradually reaches a steady state with the increase of the treatment time.

Figure9.5.The Fitting result of electron Figure water contact angle atemission differenttemperature storage times.

Figure 6. The image of treatment by APPJ (a) and the surface water contac angles of before (b) and after 90s (c) treatment.

Electron excitation temperature (Texc) is one of the important parameters in the APPJ. It is just slightly lower than the electron temperature, so it can be used to estimate the electron temperature [32]. Texc is calculated from some characteristic Ar optical emission spectral lines by using the Boltzmann slope method according to the following equation [33]: § O I ln ¨ i i ¨ A g © i i

· ¸ ¸ ¹

1 c kT exc

Figure 7. The water contact angle at different treatment distances.

(1)

where, Ii and Ȝi are the fluorescence intensity and the wavelength of recorded spectrum line. g and A are the statistical weight and the transition probability at an energy level E, respectively. k is the Boltzmann’s constant, and C is a constant between 0.92 and 1.6. By using the self-absorption characteristics between two wavelengths, signal-to-noise ratio, and mutual interference based on the spectral lines in Fig.4, seven characteristic wavelengths are chosen to calculate Texc. The calculation parameters are shown in Tab. 1. The fitting results are shown in Figure 5. It can be calculated that the Texc is about 0.62 eV.

Figure 8. The water contact angle at different treatment times.


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Figure 9. The water contact angle at different storage times.

In order to investigate the aging characteristics of the treatment, the PMMAs treated by the APPJ at different distances are placed in open air for some days. In our cases, the conditions of the room environment are as follows: the room temperature is about 20 °C, the pressure is about 100 kPa, and the humidity is about 20%. Figure 9 shows the water contact angle dependence on the storage time in open air. It is observed that the water contact angle of the treated PMMA increases with the storage time. However, as compared with the untreated samples, the value of the water contact angle of the treated PMMAs placed in open air for 10 days is still smaller. The aging effect is due to the contact of the treated PMMA with the air. During the storage treatment, the dust and the hydrophobic organic contaminants in air attach the surface of the treated material, and the gas molecules react with the hydrophilic group. Therefore, the water contact angle of the treated PMMA recovers with the storage time increase.

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Figure 10. The scheme of the experimental setup of surface flashover in vacuum.

Figure 11. The waveforms of voltages before and after the flashover.

5 EXPERIMENTS OF SURFACE FLASHOVER IN VACUUM In order to study the surface flashover over the PMMA surface in vacuum before and after the treatment, an experimental system is built as shown in Figure 10. In the experiments, the water on the PMMA is absorbed by a absorbent paper immediately after the contact angle measurement and then laid in a vacuum dryer for 1 h to dry the PMMA sample completely (5 Pa, 85 ć). After that, the PMMA sample is fixed in the experiment between two fingershaped electrodes to proceed to the flashover experiments. It should be pointed out that all the samples have not been stored in open air before surface flashover in vacuum. The gap between two electrodes varies from 0 to 15 mm. The vacuum system consists of a RVP-8 mechanical pump and a FF 160/500F molecular pump. The vacuum degree of this system is maintained at the level of 5×10-4 Pa. Figure 11 shows the surface flashover voltage over the PMMA surface before and after the treatment. For each range of water contact angle, 5 samples are used. The applied

Figure 12. The image of flashover with a gap of 1.5 mm and the applied voltage of 12 kV.

voltage is 12 kV and the gap width is 1.5 mm. As compared to the applied voltage before the flashover, the flashover voltage decreases from 12 kV to 10.5 kV, and the flashover occurs at the rising edge of the applied voltage. The corresponding discharge image is shown in Figure 12. It is observed that there are bright spark channels appearing between the electrodes. In our case, if surface flashover occurs five times during 10 experiments, the voltage is defined as the initial flashover voltage. If surface flashover occurs 10 times in 10 experiments, the voltage is defined as the stable flashover voltage.

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T. Shao et al.: Surface Modification of Polymethyl-methacrylate Using Atmospheric Pressure Argon Plasma Jets Table 2. Ra, Rrms, and Rmax obtained by AFM for the untreated and the treated (at 15, 60, 90 s by APPJ) PMMA surfaces. Sample untreated 15 s treatment 60 s treatment 90 s treatment

Ra 3.43 3.35 3.81 6.96

Surface roughness (nm) Rrms 4.63 4.87 4.96 9.34

Rmax 46.4 80.8 85.6 121

6 DISCUSSIONS

Figure 13. The change of vacuum surface flashover with different water contact angle at the distance of 1.5 mm, 3 mm, 5 mm, respectively.

Figure 14. The dependence of water contact angle on the electric field at different gaps.

Figure 13 shows the surface flashover voltage over the PMMA surface with different water contact angles. The gap width varies from 1.5 mm to 5 mm. It can be seen that the surface flashover voltage increases with the decrease of the water contact angle. When the water contact angle is 15-20°, the increase of the surface flashover voltages at 1.5 mm, 3 mm, 5 mm gaps are 70.7, 41.7, and 20.7%, respectively. Figure 14 shows the dependence of the water contact angle on the flashover electric field at different gaps. It is seen that the increase of the surface flashover electric field at the gaps of 3 and 5 mm is small. When the gap width is 1.5 mm, the surface flashover voltage increases by 62.5% from the initial value (8 kV/mm) up to 13 kV/mm. This can be attributed to the decrease of the effective treatment area of the small APPJ. Because the inner diameter of the tube is 3 mm, the main treatment area should be below 9 mm2 (3 mm × 3 mm), though it is found that the effective treatment area is about 500 mm2. The treatment should be not uniform when the gap distance is relatively long (above 3 mm), which may affect the surface flashover voltage. Our further work will aim at the development of an APPJ array capable to treat bigger samples.

According to our measurement during the surface modification using the APPJ, the polymer molecules are activated and a large number of free radicals are generated in the plasmas. These free radicals make the PMMA surface etched, cross-linked, and the surface chemical composition changed. The intensity of the free radicals action is enhanced with the increase of treatment time and discharge power. When the discharge power reaches a certain value, the free radicals reach a maximum intensity, and the APPJ has a sufficient reaction with the polymer surface. Furthermore, the roughness of the PMMA surface increases after the treatment. In order to study the surface properties of the PMMA before and after the treatment, some samples are investigated by using Atomic Force Microscope (AFM). Figure 15 shows some typical AFM images before and after the treatment. The scanning area is 5 ȝm×5 ȝm. The untreated PMMA surface in Figure 15a presents a very smooth surface, and the surface roughness is lower than that of the treated surface. Figures 15b, 15c and 15d show the surface of the treated PMMAs using APPJs during 15, 60, and 90 s, respectively. The values of surface roughness are about 2-3 times rougher than that of the untreated PMMA. It can be attributed to that plasmas induced by the APPJ impact with the PMMA surface. During this interaction, the particles restructure the chemical bonds of the surface and etch the PMMA surface, making the surface roughness increase. Furthermore, the change of the surface roughness can be calculated by the Nanoscope software. The values of the mean surface roughness (Ra), root mean square surface roughness (Rrms), and maximum surface roughness (Rmax) obtained from the AFM scans are shown in Table 2. It is observed that the surface roughness increases after the APPJ treatment. In addition, the surface roughness increases with the treatment time. In our experiments, results of the flashover test indicate that the APPJ surface modification method can effectively improve the surface insulating performance of the PMMA. In our opinion, this is due to the characteristic change of the PMMA surface. The change of surface roughness is on an order of nanometer. The roughness of the PMMA surface increases due to the etching. The movement distance of the electrons driven by the electric field is extended, which increase the creepage distance along the surface. Also, the collision frequency between electrons and PMMA surface is increased due to the increase of the surface roughness. Therefore, more electrons are absorbed during the collisions, and the accumulation of surface charge is inhibited according to Yamamoto’s result [7]. Thus, the surface flashover voltage increases with the plasma treatment.


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7 CONCLUSION In this paper, some experimental results on the PMMA surface modification using the APPJ are presented. Results show that the APPJs in Ar can efficiently improve the surface hydrophilicity of the PMMA. During the treatment, the surface water contact angle decreases with the increase of treatment time, and reaches saturation at a certain time. It is found that a distance of 5 mm between the nozzle and the material can lead to a decrease of the water contact angle of the PMMA from 68° to 16°. The increase of hydrophilicity on the PMMA surface is mainly due to the introduction of oxygen-containing hydrophilic polar groups on the surface and the surface roughness. Furthermore, the treatment changes the surface characteristics of the PMMA, and the surface roughness increases after the treatment. The PMMA surface after the treatment extends the creepage distance along the surface. It also affects the secondary electron emission yield, thus the surface flashover voltage may increase. In further work, a large treatment area will be used and some explanation about the increase of surface flashover voltage after using the APPJ array will be reported.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China under contracts 51222701, 51207154, 11076026, the National Basic Research Program of China under contract 2014CB239505-3, and the State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources under contract LAPS14009.

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Figure 15. AFM images of untreated and treated PMMA materials (5 ȝm × 5 ȝm). (a) Untreated, (b) after 15s of APPJ treatment, (c) after 60s of APPJ treatment, and (d) after 90s of APPJ treatment.

In addition, it is known that the secondary electron avalanche plays a crucial role in the development of flashover. The surface treatment also may result in the change of the surface composition and the surface charge traps, which can affect the secondary electron emission yield. The change of the secondary electron emission yield induced by the treatment will be further investigated by measuring variation of the secondary electron emission coefficient.

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Tao Shao (M’10–SM’12) was born in Hubei, China, in 1977. He received the B.Sc. degree from the Wuhan University of Hydraulic and Electrical Engineering, Wuhan, China, in 2000, the M.Sc. degree in electrical engineering from Wuhan University, Wuhan, in 2003, and the Ph.D. degree in electrical engineering from the Graduate University, Chinese Academy of Sciences (CAS), Beijing, China, in 2006. He was a Visiting Scholar with the Department of Electrical and Computer Engineering, University of New Mexico, Albuquerque, NM, USA, from 2011 to 2012. He is currently with the Institute of Electrical Engineering, CAS. His current research interests include high-voltage insulation, gas discharge, plasma application, and measurement. Dr. Shao is a Member of the Dielectrics and Electrical Insulation Society of the IEEE and the Chinese Society of Electrical Engineering. He is an Editorial Board Member of the Laser and Particle Beams, the Transaction of China Electrotechnical Technology, the High Voltage Engineering, and the Insulating Materials. He was a recipient of the 2012 Lu Jiaxi Young Talent Award at CAS K. C. Wong Education Foundation. He is the corresponding author and may be reached at [email protected]. Yixiao Zhou was born in Nantong, Jiangsu, China, in 1987. He received the B.S. degree in automation from Nanjing University of Technology, Nanjing, China, in 2010, where he is currently pursuing the M.S. degree. His current` research interests focus on the atmospheric pressure cold plasma jets and their application in polymer surface modification.

Cheng Zhang (M’13) was born in Wuxi, Jiangsu, China, in 1982. He received the Ph.D. degree in electrical engineering from the Graduate University, Chinese Academy of Sciences, Beijing, China, in 2011. He is currently with the Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing China. His current research interests focus on gas discharge and application. Dr. Zhang is a member of the Dielectrics and Electrical Insulation Society of the IEEE. Wenjin Yang was born in Xinxiang, Henan, China, in 1987. He received the B.Sc. degree in electronic and information engineering from the Xi’an Jiaotong University, Xi’an, China, in 2010, and the M.Sc. degree in electronic science and technology from the Key Laboratory for Physical Electronics and Devices of the Ministry of Education at Xi’an Jiao tong University, Xi’an, China in 2013. He is currently with, the Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, China. His current research interests focus on Plasma technology, gas discharge and application. Zheng Niu was born in Zhengzhou, Henan, China, in 1987. He received the B.Sc. degree in electrical engineering at Huazhong University of Science and Technology, Wuhan, China in 2009. He is currently pursuing the Ph.D. degree in the Univeristy of Chinese Academy of Sciences, Beijing, China. His current research interests focus on gas discharge and application.

Chengyan Ren was born in Queshan, Henan, China, in 1979. She received the B.Sc. degree from Yanshan University, Hebei, China, in 2002 and the M.Sc. degree in electrical engineering from Xi’an Jiaotong University, Xi’an, China, in 2005. She is currently with the Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, China. Her current research interests focus on high voltage insulation and pulsed power technology.