Application of electrophoretic deposition for inner

SCT-16038; No of Pages 7 Surface & Coatings Technology xxx (2010) xxx–xxx

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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t

Application of electrophoretic deposition for inner surface coating of porous ceramic tubes L. Kreethawate a,b, S. Larpkiattaworn c, S. Jiemsirilers a,b, L. Besra d, T. Uchikoshi e,⁎ a

Research Unit of Advanced Ceramics, Department of Materials Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand National Center of Excellence for Petroleum, Petrochemicals and Advanced Materials, Chulalongkorn University, Bangkok 10330, Thailand Thailand Institute of Scientific and Technological Research (TISTR), Technopolis, Khlong-Luang, Pathumthani 12120, Thailand d Institute of Minerals and Materials Technology, Bhubaneswar 751 013, Orissa, India e National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b c

a r t i c l e

i n f o

Article history: Received 31 March 2010 Accepted in revised form 17 August 2010 Available online xxxx Keywords: Electrophoretic deposition Porous ceramic Alumina Tube Polypyrrole

a b s t r a c t The direct coating of a nano-porous alumina layer on the inner surface of micro-porous alumina tubes was performed by electrophoretic deposition (EPD). A thin layer of polypyrrole (Ppy) was synthesized on the inside wall of the porous tubes by the chemical polymerization of pyrrole (Py) to give the wall electric conduction for the EPD electrode. The bimodal suspension of alumina powders with 0.6 μm and 30 nm average particle sizes was selected to control the nano-porous structure. The thickness of the coating layer was controlled by altering the applied voltage and deposition time. The interfacial connection of the coated layer and the substrate was observed by SEM before and after sintering. The pore size of the coated layer was characterized by its pore size distribution. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Porous ceramics are often used as gas/liquid separators, catalyst supports, and molecular sieves which can be used under severe environmental conditions throughout the industry, medicine and the home because of its superior properties such as high-temperature stability, strength, catalytic activity, erosion resistance and corrosion resistances when compared to the polymeric membrane. In spite of these excellent properties, the potential of porous ceramics has not been fully utilized because of the lack of established technologies to control the pore size and the surface functionality by modification with other functional materials. Therefore, many processes have been used to improve these qualities. Controlling the pore size of porous ceramics has been performed by slip casting techniques under N2 gas bubbling [1] or using the powers of different sizes [2,3]. Unidirectionally oriented pores have been introduced by adding nylon66 fibers, and mesoporous ceramic supports have been prepared by adding starch [4–6]. In addition, sol–gel methods have been used for forming a multilayer membrane on a porous alumina support [7,8].

Abbreviations: EPD, electrophoretic deposition; Ppy, polypyrrole; Py, pyrrole; APS, ammonium peroxodisulfate; NDA, 2-6-naphthalenedisulfonic acid disodium salt. ⁎ Corresponding author. Tel.: +81 29 859 2460; fax: +81 29 859 2401. E-mail address: [email protected] (T. Uchikoshi).

Electrophoretic deposition (EPD) is a type of colloidal processing in ceramic production wherein ceramic bodies are directly shaped from a stable colloidal suspension by a DC electric field. It has the advantages of a short formation time, needs simple equipment, and little restriction of the substrate shape. Compared to other advanced shaping techniques, the EPD process is very versatile since it can be easily modified for a specific application; i.e., deposition can be made on a flat, cylindrical or any shaped substrate with only minor changes in the electrode design and positioning. In particular, EPD offers easy control of the thickness and morphology of a deposited film through simple adjustment of the deposition time and applied potential [9]. Therefore, EPD can be used for the uniform surface coating of complex-shaped materials [10–14]. One of the prerequisites for the EPD process is that the substrates are electrically conductive; however, the deposition on nonconductive ceramic substrates has been tried by some researchers. Matsuda et al. and Besra et al. used a NiO-YSZ porous flat substrate with one side coated with a graphite or carbon sheet and obtained a YSZ deposit on the uncoated non-conductive side [15–17]. In the case of the deposition on non-conductive porous tubes, Hamagami et al. obtained an alumina deposit on the outer surface of a porous alumina tube by arranging a noncontact cathodic Pt wire at the center of the tube and a noncontact anodic Pt mesh as the counter electrode at the outside of the tube [12,18]. A similar technique was used by Negishi et al. to prepare a La0.8Sr0.2Co0.8Fe0.2O3 − δ (LSCF) thin layer on the outer surface of a porous alumina tube [19].

0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.08.069

Please cite this article as: L. Kreethawate, et al., Surf. Coat. Technol. (2010), doi:10.1016/j.surfcoat.2010.08.069

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L. Kreethawate et al. / Surface & Coatings Technology xxx (2010) xxx–xxx

APS

NDA

Py

substrate distilled water

ice water

magnetic stirrer water

overnight adhered Ppy powder

Ppy-coated substrate

Fig. 1. Flowchart of the Ppy coating process on the ceramic substrates.

Their methods are to capture electrophoretically migrating particles on the surface of the porous substrate by a filtration technique. This method is useful, but the density of the deposits is sometimes inferior to that of the directly consolidated deposits on an electrode. Recently, electrically conductive polymers, such as polyaniline, polypyrrole and polythiophene, have attracted much attention for use in electronic devices, electrochemical devices, functional electrodes, sensors, etc. [20]. Polypyrrole (Ppy) is one of the most extensively studied electric conductive polymers since the monomer, pyrrole (Py), is commercially available, water soluble, easily oxidized, and, moreover, Ppy can be synthesized and coated as a monolithic thin film on various metal and ceramic surfaces. Ppy is expected to be a good electrode material for fabricating shape-designed ceramics by the EPD process since it has a high electric conductivity and can be burnt out in air at b500 °C. In this study, the formation of a nanoporous alumina layer on the inner surface of micro-porous tubular alumina supports was performed by EPD to fabricate a ceramic membrane filter which is designed for use in water purification. We also demonstrate the effectiveness of the Ppy coating on nonconductive porous ceramic substrates as a suitable pretreatment for the subsequent electrophoretic deposition (EPD).

The Ppy coating on the substrates was conducted by the chemical polymerization of Py in an aqueous solution according to the following reaction [21]:

(1)

Ammonium peroxodisulfate (APS, (NH4)2S2O8), an oxidant, and the 2-6naphthalenedisulfonic acid disodium salt (NDA, C10H6(SO3Na)2), a doping agent, were added to distilled water and stirred until the

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+ DC power supply

Stainless steel electrode

2. Materials and methods The porous tubular alumina supports used in this study were made by extrusion of a paste prepared by mixing alumina powders (with average sizes of 0.7 μm and 58 μm) with carboxymethyl cellulose (CMC) as the binder. The extruded alumina tubes were subsequently dried in air at room temperature, then sintered at 1650 °C for 1 h. The porosity of the alumina tubes after sintering was about 46%, and the pore diameter of the tube was in the range of 0.1 and 10 μm with the mode at 1 μm. Flat glass slides were also used for the preliminary experiments to determine the appropriate EPD conditions of the bimodal suspension on Ppy as an electrode substrate that will be shown later.

Plastic holder

Porous Al2O3 tube

Fig. 2. Experimental setup for EPD on the inner wall of the porous alumina tube.



3

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1 µm

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Ppy-coated

non-coated

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Fig. 3. Images of the Ppy-coated substrates: (a) Ppy-coated glass slide, (b) fracture surface of the Ppy film on a glass slide, (c) appearance of the porous alumina tube, (d) Ppy-uncoated inner wall of the tube, and (e) Ppy-coated inner wall of the tube.

600 500

Thickness (µm)

formation of a homogeneous solution. Two types of substrate materials, namely, silane-coated glass slides (26 mm× 76 mm) and porous alumina tubes (O.D.=13 mmϕ, I.D.= 7 mmϕ, L =30 mm) were suspended and soaked in this solution, and then 0.01 M Py (C4H4NH) was added to it via a syringe. The outer surface of the alumina tubes was covered with a polytetrafluoroethylene (PTFE) thread seal tape to prevent the Ppy coating. The exposed surfaces of the materials were covered with Ppy films produced by the polymerization of Py at 0 °C for 15–18 h. The Ppycoated substrates were then removed and dried at room temperature in air. The lightly adhered Ppy powder on the material surfaces was brushed off. The Ppy coating process on the ceramic substrates is schematically shown in Fig. 1. The required property as a water purification filter is to remove bacteria (size 0.2–1 μm), fine particles, sediments, and other impurities. In this study, a bimodal alumina suspension was used to reduce the pore (void) size to smaller than 0.2 μm, minimize the shrinkage during sintering, and decrease the sintering temperature by improving the green density. The mixture of alumina powders with average

400 300 200 10 V

100

100 V 0

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10

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40

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Deposition time (min) Fig. 4. Thickness of the deposited layer versus deposition time at different applied voltages.


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(a)

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3 µm

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3 µm

(e)

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3 µm

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Fig. 5. Microstructure of the as-deposited layer on the Ppy-coated glass slide substrates prepared at different applied voltages for a fixed time: (a) 10 V, 1 min, (b) 10 V, 5 min, (c) 10 V, 10 min, (d) 10 V, 30 min, (e) 10 V, 1 h, (f) 100 V, 1 min, (g) 100 V, 5 min and (h) 100 V, 10 min.

particle sizes of 0.6 μm (AKP-15, Sumitomo Chemical Co., Ltd., Japan) and 30 nm (Nanotek Al2O3, C.I. Kasei. Co., Ltd, Japan) in the weight ratio of 9:1 was selected [21]. The suspensions of a 30 wt.% solid content were prepared in ethanol. To make the alumina surface positively charged, butoxyethyl acid phosphate (JP-506H, Johoku

Chemical Co., Ltd., Japan) [22] and polyethyleneimine with the average molecular weight of 10,000 (PEI, Wako Pure Chemical Industries, Ltd., Japan) were used as dispersants. Poly vinyl butyralco-vinyl alcohol-co-vinyl acetate (PVB, Aldrich Chemical Company, Inc., Germany) was also added as a binder. The solution was



(b)

(a)

1 mm

5

(c)

1 mm

(d)

1 mm

(e)

alumina substrate

alumina substrate deposited layer

deposited layer

100 µm

100 µm

Fig. 6. Cross-sectional image of the coated layer deposited at 100 V followed by sintering at 1250 °C: (a) and (d) thin deposit ~200 μm, (b) and (e) thick deposit ~500 μm, and (c) thickest deposit ~1 mm.

ultrasonicated (Modal US-1200, Nihonseiki Kaisha., Ltd., Japan) to improve the dispersion of the powders. The electrophoretic deposition characteristics of the bimodal alumina suspension were first investigated using the Ppy-coated glass slides as deposition electrodes (cathode). DC voltages of 10 V and 100 V were applied using a Source Meter (Model 2400, Keithley Instruments, Inc., USA). A stainless steel sheet (2 cm × 2 cm) was used as the counter electrode (anode). For the deposition on the Ppycoated inner surface of the alumina tubes, a stainless steel rod (ϕ = 2 mm) passing through the tube was used as the counter electrode. The experimental setup for EPD on the inner wall of the porous alumina tube is schematically shown in Fig. 2. The porous alumina tube was placed in the cylindrical plastic holder with several openings at the upper and lower parts for circulation of the suspension. After the deposition for a predetermined time, the deposits along with the tubular substrates were dried at 80 °C, followed by sintering at 1250 °C for 2 h in air. X-ray diffraction (XRD, XRD-6000, Shimadzu Corp., Japan) was used to characterize the phase transformation of the coated layer before and after the sintering. The thickness and microstructure of the sintered layer prepared on the inner wall of the porous tube were observed using an optical microscope and a scanning electron microscope (SEM, S-5000, Hitachi, Ltd., Japan). A pore size analyzer (Pore master, Quantachrome Instruments, USA) was used to characterize the pore size distribution of the tubular alumina support before and after the formation of the inner layer. 3. Results and discussion A typical Ppy film synthesized on a glass slide and the inner surface of a porous alumina tube is shown in Fig. 3. The Ppy films which were formed on the substrates appear black in color. The bumpy surface of the Ppy film covering the inner wall of the tube (Fig. 3e) reflects the original rugged surface. Ppy film has been estimated to have about a 0.5 μm thickness and 5.9 S/cm electrical conductivity under this synthesis condition [21]. Fig. 4 shows the average thickness of the alumina deposit versus time during the constant voltage deposition on the Ppy-coated glass slides. The thickness of the deposited layer increased as a function of the applied voltage and time. The control of its thickness was very

easy by changing the applied voltage and the deposition time. The adhesion of the deposited layer on the Ppy surface was fairly good. The surface microstructure of the as-deposited layers for various deposition times is shown in Fig. 5. During the initial stage of the deposition shown in Fig. 5(a), when the thickness of the layer was thinner than 5 μm, more large particles were deposited on the substrate than the smaller ones; however, the ratio of small particles increased with the deposition time and the microstructure of the deposits became very similar (Fig. 5(b)–(e)). This is probably because of the higher electrophoretic mobility of the large particles: when a DC voltage is applied to the suspension, the large particles are first assembled, but the small particles soon accumulate. After the transition time, the proportion becomes almost constant. The difference in the microstructure during the initial stage of the deposition was not observed when the applied voltage was 100 V (Fig. 5(f)–(h)). This result suggests that a nearly uniform component deposit in the depth direction can be prepared even from the bimodal suspension by EPD. The XRD analysis revealed that the small amount of the γalumina phase contained in the green deposits (~10%) transformed to the α-phase during the sintering at 1250 °C. Fig. 6. shows the cross-sectional images of the porous tubular alumina supports along with the inner coating layers of different thicknesses. The inner layers were deposited at 100 V followed by sintering at 1250 °C. The Ppy layer which is composed of C, H and N was completely burnt out during the heating and the deposited layers were grafted with the substrate. The interface between the deposited layer and the porous alumina matrix shows a good junction without peeling off for the sample with the thickness of ~200 μm (Fig. 6 (a),(d)). However, microscopic separation at the interface was revealed for the sample with the thickness of ~500 μm (Fig. 6(b),(e)). The thickness of the deposit became more non-uniform with cracks and peeling off appearing when the thickness of the deposit was about 1 mm (Fig. 6 (c)). These results suggest that the thick layer of deposits exhibited a higher shrinkage and caused cracking and de-lamination during the sintering. Fig. 7 shows the microstructure of the coated layer at different points in the depth direction. The distribution of small and large particles in many areas seemed to be similar (Fig. 7 (c)–(g)). The small particles were distributed throughout the pores between the large particles and seemed to connect the large particles


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(h)

(a)

deposited layer

alumina substrate

(e) (d) (c) (b)

(b)

(g) (f)

3 µm

100 µm

(c)

(d)

3 µm

3 µm

(e)

(f)

3 µm

(g)

3 µm

(h)

3 µm

3 µm

Fig. 7. The microstructure of the coated layer at different points in the depth direction: (a) cross-sectional image of the coated layer, (b) substrate, (c) microstructure of the layer near the interface, (d) 50 μm from the interface, (e) 100 μm from the interface, (f) 150 μm from the interface, (g) 200 μm from the interface, and (h) outermost surface.

together. The morphology of the outermost surface of the layer looked very different from the other cross-sectional points; i.e., many of the small particles disappeared (Fig. 7(h)). It is likely that the small particles are combined and form new large grains. The outermost surface of the coated layer (Fig. 7(h)) has a nano-porous

structure with a pore size smaller than the inner positions of the coated layer (Fig. 7(c)–(g)). The pore size distribution of the fabricated porous alumina filter characterized by a pore size analyzer is shown in Fig. 8. The pore size of the deposited alumina layer is smaller than 50 nm, which is small enough for removing bacteria



0.025

7

Acknowledgments

Mercury volume (cc/g)

uncoated substrate 0.02

The authors would like to thank Dr. T. S. Suzuki at NIMS for his SEM observations, Ms. A. Miki and Mr. H. Yamada at NIMS for their experimental support, and Prof. Y. Sakka at NIMS for his valuable comments. L.K. would like to thank the Research Unit of Advanced Ceramics, Chulalongkorn University and NIMS for their financial support. This work was partially supported by a Grant-in-Aid for Scientific Research (C) (No. 20560687) from the Japan Society for the Promotion of Science (JSPS).

inner surface-coated substrate

0.015 0.01 0.005

References

0 0.001

0.01

0.1

1

Pore size (µm) Fig. 8. Pore size distribution of the porous alumina tube before and after the inner surface coating.

[1] [2] [3] [4] [5] [6]

whose size is in the range of 0.2–10 μm [18,23]. This pore size is too large to remove virus in air; however, a high ratio (N4-log) removal of virus in water has been achieved by a coagulation–filtration process using a membrane with the pore size of 0.1 μm [24]. This indicates that the EPD is a useful technique to fabricate a nano-porous secondary layer, which can be used as filter for water purification. 4. Conclusions The direct deposition of alumina particles onto the inner wall of a porous alumina tube was successfully made from a nano-submicro bimodal suspension by EPD. Crack-free porous layers of good interfacial joining with the inner surface of the porous alumina tube were obtained when the thickness of the layer was less than ~200 μm. A higher thickness of deposits invariably exhibited cracks and delamination from the substrate surface. The pore size of the prepared porous layer was less than 50 nm.

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