Ind. Eng. Chem. Res. 2004, 43, 5483-5488
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Surface Modification of Fine Powders by Atmospheric Pressure Plasma in a Circulating Fluidized Bed Reactor Soon Hwa Jung,† Sang Min Park, Soung Hee Park,‡ and Sang Done Kim* Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea
Surface modification of fine alumina powders (60 µm) by plasma-enhanced chemical vapor deposition (PECVD) at atmospheric pressure was carried out in a circulating fluidized bed reactor (18 mm i.d. by 1 m height). A stable glow discharge under atmospheric pressure can be attained successfully at a source frequency of 13.56 MHz with ring-shaped electrodes. The deposited thin film on alumina powder from TEOS/O2 plasma was characterized by FTIR-ATR, XPS, and SEMEDS. The organic silicon films, SiOxCyHz, are deposited by TEOS plasmas at atmospheric pressure. The FTIR spectra of deposited films exhibit the three characteristic bands of SiO2 near 1072, 800, and 450 cm-1. These CH and Si-C bonds disappear with increasing input power. Input power and oxygen content in the reaction gas control carbon and oxygen contents in the deposited film. The film deposition on the particle surface is evenly distributed, and the layer thickness is approximately 2-4 µm. Introduction The macroscopic properties of powders are strongly influenced by their surface properties. Therefore, surface modification is an important issue in powder technology and also becomes an interesting research field for materials that are used in powder form. However, research works on fine powder treatment are comparatively scarce since powder has a tendency to aggregate and a large surface area/unit mass that makes it difficult to handle. For the plasma treatment of powders, the combination of reactor geometry and plasma generation must be considered carefully. Remote plasma systems where plasma jets are fed into the powder beds are found in the literature.1,2 A spout bed with remote plasma jet was also adopted.3,4 Another approach used rotary drums with low-energy plasmas.5,6 However, dispersion is not satisfactory in both cases. In the third concept, plasma is generated at the upper part of a stationary bubbling fluidized bed.7-10 Recently, a circulating fluidized bed, which belongs to a fast fluidization regime, is introduced as a plasma reactor.11-13 Fast fluidized beds provide lowtemperature and -pressure gradients at high gas velocity where fine powders can be dispersed successfully.14 The plasma is generated directly in the riser part of the reactor by coupling rf13 or microwaves.11-15 Low-pressure plasma is a well- and long-established process for material treatment. However, the major drawbacks are the requirements of expensive and complicated vacuum systems with limitation of the vacuum-compatible materials. The atmospheric pressure glow (APG) discharge generates stable plasma at * To whom correspondence should be addressed. Tel.: 8242-869-3913. Fax: 82-42-869-3910. E-mail:
[email protected]. † Present address: Chemical Process & Catalysis Research Institute, LG Chemical Ltd., Research Park, 104-1 Moonjidong, Yuseong-gu, Daejeon 305-380, Korea. ‡ Present address: Department of Chemical Engineering, Woosuk University, 490 Hujung-ri, Samrye-up, Wanju-gun, Chonbuk 565-701, Korea.
Figure 1. Typical configurations of the volume discharge (a, b) and the schematic diagram of plasma electrode for powder treatment (c) by the Kogoma group20,21 and (d) in this study.
atmospheric pressure that reduces the cost of the equipment and extends the application range of plasma technology. Okazaki and co-workers16-18 developed APG discharge by using helium as a dilution carrier gas with the insertion of a dielectric barrier. It is known that the glow discharge with a dielectric occurs between two electrodes with ac, at least one of which should be covered by a dielectric.19 The volumetric glow discharge can be achieved by two typical configurations of the electrodes as shown in Figure 1: (a) two parallel dielectric plates with outside-fitted electrodes; (b) the two electrodes on the same side of the dielectric.19 Recently, the Kogoma group20,21 developed an rf electrode for powder treatment as shown in Figure 1c. The coaxial electrode is centered, and the ground electrode is wrapped outside of the tubular reactor. The discharge gap between the centered electrode and dielectric is narrower than 5 mm. The insertion of an electrode in the reactor has inherent disadvantages such as hindrance of solids motion by the electrode and unintended deposition on the electrodes. Therefore, a different electrode configuration for powder treatment is needed to avoid the hindrance of solid motion and be free from electrode contamination. Okazaki and Kogoma group used a pair of metallic tapes wrapped around the reactor tube.22 In the present study,
10.1021/ie034216w CCC: $27.50 © 2004 American Chemical Society Published on Web 03/25/2004
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Figure 2. Schematic diagram of a circulating fluidized bed reactor and plasma discharge facilities.
a stable glow discharge under atmospheric pressure was successfully attained with ring-shaped electrodes, as shown in Figure 1d, at a source frequency of 13.56 MHz. By insertion of the riser of the CFB reactor into the inner part of the electrodes, the quartz wall functions as a dielectric. The plasma glow volume is related to the reaction conditions such as applied rf power, reaction gas species, gas volume, etc. Basically, the plasma glow filled from the wall boundary to the center of the reactor. The objectives of this study are to generate plasma at atmospheric pressure in a circulating fluidized bed (18 mm i.d. by 1 m height) and to make a thin film deposition on alumina powders by plasma-enhanced chemical vapor deposition (PECVD). PECVD and fluidization are the dominant techniques in film deposition and the particle treatment process, respectively. The combination of both techniques is expected to lead to flexible and cost-effective particle coating processes.23 Experimental Section Materials. Alumina (Al2O3, dp ) 60 µm, Fs ) 4950 kg/m3) powder was used as a specimen to be modified since alumina is a general support of catalysts with a broad size selection. Helium gas (99.999%) was used as a carrier gas for the generation of plasma glow. Argon gas (99.999%) was also mixed up to 20% of the reaction gas to extend the glow volume. TEOS (Si(OC2H5)4, Aldrich) was used without further purification as a Si precursor. Oxygen (99.999%) was added to increase decomposition of TEOS. Reactors. A circulating fluidized bed reactor (CFB) and the plasma discharge facilities are shown schematically in Figure 2. It consists of a CFB reactor, reaction gas-supply section, precursor supply, plasma electrode, and rf plasma matching sections. The CFB reactor was composed of a riser (18 mm i.d. by 1 m height, quartz
glass), cyclone, downcomer (18 mm i.d., Pyrex glass), and loop- seal as a solids feeding system. The riser was placed on the inner part of the ring-shaped electrode and served as a dielectric barrier. The loop-seal can control solid flow rate by adjusting aeration rate at the bottom of the loop-seal. During the operation, solid particles were entrained by upflowing gas from the bottom of the riser and the entrained particles were separated in a cyclone and flow back into the riser through a downcomer and the loop-seal. The role of precursor supply system is to generate a vapor of precursors and then deliver it to the reactor. To introduce the precursor, TEOS solution was placed into a glass bubbler and then helium gas was passed through the bottle to vaporize and carry away the precursor. The bubbler and gas inlet lines were kept at 75 °C. The fluidizing gas was regulated and measured with mass flow controllers (MKS). Plasma System. Plasma was used to enhance the deposition rate and to decrease the mean operating temperature compared to that of the conventional thermal CVD system. The plasma was generated with an automatching network (New Power Plasma, Seoul, Korea) and a power supplier (ENI power systems, OEM, 1.2 kW) of a radio frequency (rf) of 13.56 MHz. The discharge voltage between the rf powered and ground electrodes was measured by using a high-voltage probe (×1000, Tektronix), and the input power was varied from 150 to 350 W. The high-voltage electrodes were made of stainless steel ring (20 mm i.d., 24 mm o.d., 2 mm thick), which was insulated by coating to prevent arcing at the ambient air. Five of the electrode rings were connected to the rf-powered axis, and four of them were connected to ground axis. These rf-powered and ground electrode rings were placed alternately. Characterization. The properties of deposited film on alumina powders were determined by XPS, IR, and sorption properties, respectively. The XPS spectra were calibrated against the C1s line from adventitious carbon (284.5 eV) that was determined by a VG Scientific ESCALAB 200R spectrometer using a Mg KR photon source at 12.5 kV and 20 mA. The quantitative atomic surface composition was deduced from the numerical fits of the different experimental peaks in XPS spectra. IR spectra of the plasma-deposited film were recorded on an FTIR spectrometer (Bio-Rad FTS 3000) with golden gate single reflection ATR. A total of 100 successive scans were recorded with a 4 cm-1 step and then averaged. The morphology of the deposited film was measured with a field emission scanning electron microscope (FESEM, Hitachi S-4800). The deposition thickness was measured by the line scanning profile of Si KR1 with energy-dispersive X-ray spectrometry (SEMEDS). Experimental Procedure. Initially, alumina powders (52 g) were loaded into the reactor. The mixture of He, Ar, O2, and TEOS vapor was injected into the bottom of the riser, and a part of the helium gas was injected into the bottom of the loop-seal for powder circulation. The total flow rate of the reaction gas was kept at 1200 cm3/min. When the system reached steady state, the rf power was adjusted to ignite glow discharge. The plasma-enhanced chemical vapor deposition (PECVD) was performed at the ambient temperature. The treatment time, input power, and gas velocity were varied. At the completion of a run, the plasma was cut
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Figure 3. Effect of input power on discharge voltage across the electrodes.
off, the reactor was flushed with He gas, and then the treated powders were withdrawn from the reactor. Results and Discussion Plasma Generation. Once electrical breakdown occurs, the discharge makes a transition to the glow discharge regime, in which the amount of excitation of the neutral background gas is great enough such that the plasma is visible to the eyes.24 The characteristics of the plasmas were determined as a function of input power and carrier gas composition by using a highvoltage probe. Figure 3 shows the effects of input power on the discharge voltage across the electrodes where voltage increases with the input power. As soon as the electrical breakdown occurs, the discharge voltage sharply drops and then the voltage increases again with the input power. The breakdown of pure He was easily achieved with small input power, and the breakdown voltage of He is 1.03 kV at 5 W. The breakdown of pure Ar occurs about 2.5 kV and is followed by the arc like filamentary discharge. These values of breakdown voltage are similar to that of other atmospheric sources that are operated at ac or dc. Plasma discharges at nearatmospheric and higher pressure have a tendency to create plasma filaments which are followed by a rapid formation of arc or spark. The kind of discharge gas determines the stability of the glow discharge.25 Helium gives a stable homogeneous glow discharge, whereas nitrogen, oxygen, and argon easily cause the transition into a filamentary glow discharge. In the preliminary experiment, the effect of the contents (0-40%) of Ar in the He carrier gas on the plasma glow discharge was determined. With increasing content of Ar in the carrier gas, the breakdown voltage, the plasma glow volume, and the light intensity increase. In case of a 20% Ar mixture, the breakdown voltage is 1.6 kV at 75 W. For argon contents above 40%, the breakdown occurs at the input power higher than 200 W and has a tendency to create plasma filaments perpendicular to the electrode with increasing input power. Thus, we selected a 20% Ar mixture for the stable glow discharge with economic considerations. FTIR Analysis of the Film Deposition with APG. To characterize the chemical bonding nature of the deposited films, infrared adsorption was measured by
Figure 4. FTIR spectra for (a) the TEOS precursor, film deposited by PECVD of TEOS (O2/TEOS, 30) on the reactor wall without powder for 3 h at (b) 150 W, (c) 250 W, and (d) 350 W.
using a FTIR spectrometer. Figure 4 shows the FTIR spectra for (a) the TEOS precursor, deposited film by plasma with TEOS (O2/TEOS of 30) on the reactor wall without powder for 3 h at (b) 150 W, (c) 250 W, and (d) 350 W, respectively. The other conditions were the same as those for typical powder treatment. The spectra of deposited films (Figure 4b-d) exhibit three characteristic bands of Si-O-Si near 1072, 800, and 450 cm-1.26-29 The broad band is centered approximately at 3400 cm-1, which associated with the O-H stretching vibration mode of SiO-H.26 These O-H bonds indicate the presence of bonded water in all of the films.26 Also, a weak bending vibration of H2O near 1650 cm-1 is detected in all of the deposited films. Water incorporation into the film has been suggested as coming from water that accumulated on the layer as a reaction product and then was incorporated into the film and/or water that was taken up from air after treatment.30 With a low input power at 150 W, as shown in Figure 4b, the band at 3650 cm-1 is additionally observed, which is assigned to O-H stretches of isolated SiOH groups,31 and the weak CH band is detected. Due to the similarities between the FTIR spectra of the films derived from TEOS plasmas and TEOS precursor, it is likely that carbon atoms are mainly incorporated in ethoxy groups (O(C2H5), 2870, 2930, 2950 cm-1).28 The weak absorption band at 800-820 cm-1 is due to the stretching of Si-C.26,29 The intensity of the SiOH bonds decreases with increasing discharge power via the decomposition of ethyl groups of TEOS as can be seen in Figure 4c,d. The gas-phase reaction of the precursor decompositions is enhanced by corresponding increases in plasma energy. It leads to the increase in density of the cross-link of the films,28 and then the intensity of the OH bands decreases. There is no detection of C-H bands in alkyl groups (1350, 1420 cm-1) or ethoxy groups (2870, 2930, 2950 cm-1) and also no evidence for a Si-H absorption band at 2260 cm-1 in Figure 4c,d. Figure 5 shows (a) FTIR spectra with ATR for the raw alumina powders, (b) FTIR spectra with KBr for deposited film on the reactor wall without powder by the plasma with TEOS, O2/TEOS of 30 at 350 W for 3 h, and (c) FTIR spectra with ATR for surface-modified powder by PECVD from TEOS with solid holdup of
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Figure 5. (a) FTIR spectrum with ATR for the raw alumina powders, (b) FTIR spectra with KBr for deposited film by PECVD of TEOS on the reactor wall without powder at 350 W for 3 h (O2/ TEOS, 30), and (c) FTIR spectra with ATR for surface-modified powder by PECVD of TEOS at 350 W for 3 h (solid holdup, 0.002; O2/TEOS, 30).
Figure 7. (a) SEM image of the surface-modified powder by PECVD of TEOS for 3 h at 350 W (magnification: ×300) and (b) the corresponding EDS map of Si KR1 (solid holdup, 0.002; O2/ TEOS, 30). Table 1. Atomic Ratios of O/Si and C/Si for the Surface-Modified Powders by PECVD from TEOS with a Solid Holdup of 0.002 as a Function of the O2/TEOS Ratio applied rf power (W) (O2/TEOS flow rate) Figure 6. Low-resolution XPS spectra of the surface-modified powder by PECVD of TEOS for 3 h at (a) 250 W and (b) 350 W (solid holdup, 0.002; O2/TEOS, 30).
0.002 and O2/TEOS of 30 at 350 W for 3 h, respectively. The difference of the spectra in range of 400-900 cm-1 is due to the different window of the instrument. ATR covers a shorter range of wavenumber between 650 and 4000 cm-1. The spectra of Figure 5b,c exhibit the characteristic bands of SiO2 near 1072 cm-1.26-29 A broad absorption peak in the wavenumber between 3200 and 3700 cm-1, which is assigned to associated O-H, is found to be low, and it below the detection limit in the plasma-treated powders. The broad absorption peak indicates the presence of bonded water in films. The water accumulated on top of the powder surface as a reaction product that may not exceed a critical value for surface coverage which can incorporate into the film. The water incorporated into the film may not be detected since the circulating fluidized bed enhances the mass transfer between solid and gases and the reaction extent on the powder is lower than the deposited film on the reactor wall due to the short contact time. XPS Analysis for the Surface-Modified Powders. Figure 6 shows the low-resolution XPS spectra for the
at. ratio
250 (30/1)
350 (10/1)
350 (20/1)
350 (30/1)
O/Si C/Si
2.83 0.61
3.02 0.18
3.15 0.11
2.81 0.10
surface-modified powder by PECVD from TEOS for 3 h with solid holdup of 0.002 and O2/TEOS of 30 at (a) 250 W and (b) 350 W, respectively. After the TEOS plasma deposition, the observed Al2p peak in the alumina powder nearly disappears and Si2p and Si2s peaks appear. The small C1s spectra possibly assigned to the main peak (285 eV), and the O1s is positioned at about 533 eV. The Si2p peak is symmetrical and positioned at 103 eV,31 which means the film has the Si(-O)4 environment.28 The atomic ratios of O/Si and C/Si obtained from the composition ratios of O1s, C1s, and Si2p peaks for plasmatreated powders as a function of O2/TEOS ratio at 250 and 350 W are shown in Table 1, respectively. As can be seen in Figure 6a, the XPS spectrum has a sharp C1s peak, which may be originated from the ethoxy group in the precursor. With increasing discharge power, the C1s peak decreases via the decomposition of ethyl groups of TEOS. Also, as the oxygen fraction increases in the reaction gas, carbon content in the films
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fied as an organic SiOxCyHz film, follows the results from the low-pressure PECVD.26-29,33 SEM Analysis for TEOS Plasma-Treated Powders. Figure 7 shows (a) the cross-sectional SEM image of the surface-modified alumina powders by PECVD from TEOS with solid holdup of 0.002 and O2/TEOS of 30 at 350 W (magnification: ×300) and (b) the corresponding EDS map of Si KR1, respectively. Comparison between the particle surface shape in Figure 7a,b indicates that a thin film is evenly deposited despite low sphericity of alumina due to high solid-gas contact in the circulating fluidized bed. Figure 8 shows (a) the SEM image of surface modified powders (magnification: ×2000) and (b) the corresponding EDS line scan profile of Si KR1. The layer thickness of the deposited film on the alumina powder can be determined from Figure 8b and found to be approximately 2-4 µm. Conclusions
Figure 8. (a) A SEM image of the surface-modified powder by PECVD of TEOS at 350 W (magnification: ×2000) and (b) the corresponding EDS line scan profile of Si KR1 (solid holdup, 0.002; O2/TEOS, 30).
decreases, whereas the oxygen and silicon contents increase. Generally, oxygen is assumed to be involved in TEOS dissociation and responsible for elimination of C and H atoms from the growing film.33 Oxygen becomes excited and enhances the gas-phase reaction of the precursor decomposition to form intermediate species,33 which are believed to be partially oxidized organosilicon fragments of starting precursors.29 On the other hand, the elimination ability of oxygen may imply a competitive effect between the deposition and etching process.33 Thus, the O/Si ratio could decrease with a further increase in oxygen content. The O/Si atomic ratio of 2 indicates surface of the deposited film consists of a chemical structure similar to SiO2.27,28 However, as can be seen in FTIR data, part of the oxygen-silicon bonds (-O-Si-) can be replaced by oxygen-hydrogen (-O-H) and oxygen-carbon (-OC-) bonds. The substitution of bonds accounts the fact that the O/Si ratio is greater than 2. As noticed, the film deposited from the TEOS plasma at atmospheric pressure has the Si(-O)4 environment and a part of the oxygen-silicon bonds (-O-Si-) are replaced by organic species. The characterized qualities of the deposited films by FTIR and XPS are very similar to those obtained in the low-pressure PECVD,26-29,33 although, in the APG discharge, excitation of these molecules is based on the Penning reaction between these molecules and metastable helium atoms.34 Thus, it can be classi-
The surface modification of fine alumina powder was performed in a circulating fluidized bed with atmospheric pressure plasma. A stable glow discharge under atmospheric pressure can be successfully attained at a source frequency of 13.56 MHz with the insertion of a dielectric quartz tube into the inner part of the ringshaped electrodes. The FTIR spectra of deposited films with TEOS atmospheric pressure plasma exhibit the three characteristic bands of SiO2 near 1072, 800, and 450 cm-1 and a broad absorption peak of the OH bond. As the input power increases, these CH bond and Si-C bonds disappear. By increase of the oxygen fraction, the carbon content in the films decreases, whereas the oxygen and silicon contents increase. The organic film, SiOxCyHz, is deposited on the fine alumina powder from the atmospheric pressure TEOS plasmas in the circulating fluidized bed. The film deposition on the powder is evenly distributed, and the layer thickness is approximately 2-4 µm. Acknowledgment We acknowledge a Grant-in-Aid for Research to S.D.K. through the Green Production Project by the Ministry of Commerce, Industry, and Energy of Korea. Literature Cited (1) Matsukata, M.; Oh-hashi, H.; Kojima, T.; Mitsuyoshi, Y.; Ueyama, K. Vertical Progress of Methane conversion in a D. C. Plasma Fluidized Bed Reactor. Chem. Eng. Sci. 1992, 47, 29632968. (2) Vivien, C.; Wartelle, C.; Mutel, B.; Grimblot, J. Surface Property Modification of Polyethylene Powder by Coupling Fluidized Bed and Far Cold Remote Nitrogen Plasma Technologies. Surface Interface Anal. 2002, 34, 575-579. (3) Hanabusa, T.; Uemiya, S.; Kojima, T. Surface Modification of Particles in a Plasma Jet Fluidized Bed Reactor. Surf. Coat. Technol. 1996, 88, 226-231. (4) Flamant, G. Hydrodynamics and Heat Transfer in a Plasma Spouted Bed Reactor. Plasma Chem. Plasma Process. 1990, 10, 71-85. (5) Iriyama, Y.; Ikeda, S. Plasma-induced Graft Polymerization onto Powders. Polym. J. 1994, 26, 109-111. (6) Reuter. VDI-Fortschrittsberichte 5; VDI-Verlag: Dusseldorf, Germany, 1997; No. 464. (7) Shin, H. S.; Goodwin, D. G. Deposition of Diamond Coatings on Particles in a Microwave Plasma-Enhanced Fluidized Bed Reactor. Mater. Lett. 1994, 19, 119-122.
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of Nitrides on Fluidized Particles. Powder Technol. 2001, 120, 134-140. (24) Roth, J. R. Industrial Plasma Engineering; IOP Publishing: Philadelphia, PA, 1995. (25) Massines, F.; Rabehi, A.; Decomps, P.; Gadri, R. B.; Se´gur, P.; Mayoux, C. Experimental and Theoretical Study of a Glow Discharge at Atmospheric Pressure Controlled by Dielectric Barrier. J. Appl. Phys. 1998, 83, 2950-2957. (26) Song, Y.; Sakurai, T.; Kishimoto, K.; Maruta, K.; Matsumoto, S.; Kikuchi, K. Syntheses and Optical Properties of LowTemperature SiOx and TiOx Thin Films Prepared by Plasma Enhanced CVD. Vacuum 1998, 51, 525-530. (27) Aumaille, K.; Valle´e, C.; Granier, A.; Goullet, A.; Gaboriau, F.; Turban, G. A. Comparative Study of Oxygen/Organosilicon Plasmas and Thin SiOxCyHz Films Deposited in a Helicon Reactor. Thin Solid Films 2000, 359, 188-196. (28) Valle´e, C.; Goullet, A.; Nicolazo, F.; Granier, A.; Turban, G. In situ Ellipsometry and Infrared Analysis of PECVD SiO2 Films Deposited in an O2/TEOS Helicon Reactor. J. Non-Cryst. Solids 1997, 216, 48-54. (29) Pai, C. S.; Chang, C. P. Downstream Microwave PlasmaEnhanced Chemical Vapor Deposition of Oxide Using Tetraethoxysilane. J. Appl. Phys. 1990, 68, 793-801. (30) Gruska, B.; Wandel, K. The Influence of Temperature and Pressure on the Structure of Remote Plasma Enhanced Chemically Vapor Deposited SiO2 Investigated by Spectroscopic Ellipsometry. Thin Solid Films 1993, 233, 240-243. (31) Theil, J. A.; Tsu, D. V.; Watkins, M. W.; Kim, S. S.; Lucovsky, G. Local Bonding Environments of Si-OH Groups in SiO2 Deposited by Remote Plasma-Enhanced Chemical Vapor Deposition and Incorporated by Post Deposition Exposure to Water Vapor. J. Vac. Sci. Technol. 1990, A8, 1374-1381. (32) Goullet, A.; Charles, C.; Garcia, P.; Turban, G. Quantitative Infrared Analysis of The Stretching Peak of SiO2 Films Deposited from Tetraethoxysilane. J. Appl. Phys. 1993, 74, 68766882. (33) Sawada, Y.; Ogawa, S.; Kogoma, M. Synthesis of PlasmaPolymerized Tetraethoxylane and Hexamethyldisiloxane Films Prepared by Atmospheric Pressure Glow Discharge. J. Phys. Appl. Phys. 1995, 28, 1661-1669. (34) Yokoyama, T.; Kogoma, M.; Okazaki, S.; Moriwaki, T. The Mechanism of the Stabilization of Glow Plasma at Atmospheric Pressure. J. Phys. Appl. Phys. 1990, 23, 1125-1127.
Received for review October 28, 2003 Revised manuscript received February 6, 2004 Accepted February 13, 2004 IE034216W
