Aqueous tape casting of 10 mol%-gd 2 O 3 -doped CeO 2 nano-particles

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1 Materials Science and Engineering A 429 (2006) Aqueous tape casting of 10 mol%-gd 2 O 3 -doped CeO 2 nano-particles L.H. Luo, A.I.Y. Tok, F.Y.C. Boey School of Materials Science & Engineering, Nanyang Technological University, Nanyang Avenue, Singapore, Singapore Received 26 July 2005; received in revised form 4 May 2006; accepted 11 May 2006 Abstract The aqueous tape casting process was systematically developed and investigated for the tape casting of 10 mol%-gd 2 O 3 -doped CeO 2 nanoparticles. This study focused on obtaining the optimum slurry formulations and on the effects of the processing parameters such as the stability, the rheology and the wetting properties on the tape characteristics. High solid loading (55 vol.%) and stable slurries of 10GDC nano-particles were achieved using poly(acrylic acid) (PAA) as a dispersant; poly(vinyl alcohol) (PVA) as a binder; poly(ethylene glycol) (PEG) as a plasticizer; octanol as a defoamer and 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate as a surfactant. The ph values of the slurries were maintained in the range of Homogeneous, defect-free green tapes (thickness m) with smooth surface and high relative green density (51.5%) were thus successfully obtained Elsevier B.V. All rights reserved. Keywords: Aqueous tape casting; Nano-particles; 10GDC; Fuel cells 1. Introduction Tape casting produces a thin layer (or coating) on a carrier surface via the doctor blade technique [1 3]. The applied coating dries to form a flexible film that consists of a particle-filled, polymeric matrix with appreciable porosity, which can be cut, punched or stacked to produce multilayered structures. Some potential applications include solid-oxide fuel cells [4,5], multilayered ceramic packages (MLCP) [6 8], multilayered ceramic capacitors (MLCC) [9 11], sensors/actuators and transducers. Traditionally, tape casting was done using organic solvents. However, due to environmental, health, safety and economic reasons, aqueous (water-based) tape casting is rapidly replacing the non-aqueous form of tape casting. Despite the abovementioned advantages, several problems are often encountered. These include slow drying of the tape, higher crack sensitivity, flocculation and poor wetting of the slips due to the high surface tension of water [12,13]. Therefore, the development of such formulations has been a subject of recent research [14 16]. However, the majority of research has still been focused on the development of aqueous tape casting systems using micrometer or sub-micrometer ceramic particles. Research on the tape casting of nanosized ceramic powders have not been reported Corresponding author. Tel.: ; fax: address: miytok@ntu.edu.sg (A.I.Y. Tok). much in journal papers. The stability and rheological properties of nanosized ceramic powders are more complex due to particle interactions as a result of the small inter-particle distances [16,17]. A moderate increase in the ionic strength of these slurries will lead to a decrease in viscosity [18]. Dietrich and Neubrand [18] have observed that rheological properties of SiO 2 nano-particles depend strongly on the particulate size and the molecular weight of dispersant, which indicate strong steric effects. Dispersed slurries containing SiO 2 nano-particles can be prepared only within a very limited range of dispersant concentration. However, nanoscale ceramic powders offer the possibility of manufacturing dense ceramic materials with few microstructural defects at low sintering temperatures. Thus, the research and development of aqueous tape casting of nanoparticles have great significance in the field of high-performance ceramics. Gadolinium oxide-doped cerium oxide (fluorite structured), 10 mol% Gd 2 O 3 CeO 2 (hereafter referred to as 10GDC for convenience), is a solid solution formed by replacing the Ce 4+ sites of the CeO 2 lattice by Gd 3+ cations. A 10GDC has been recognized as a low temperature ( C operating temperature) electrolyte material for applications in solid-oxide fuel cells (SOFC) as 10GDC has a higher ionic conductivity [19]. In order to increase the ionic conductivity of fuel cells, electrolytes of 10GDC are required to be multilayered. A 10GDC nano-particles can be sintered at a low temperature, therefore, they can be co-fired with the anode/cathode at low tempera /$ see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.msea

2 L.H. Luo et al. / Materials Science and Engineering A 429 (2006) ture ( C) and in addition the nanostructure greatly increases its mechanical properties [20]. Thus, the research and development of tape casting of 10GDC nano-particles are significant in the SOFC field. In this paper, an aqueous solution of PAA with molecular weight of 5000 g/mol was used as an electrostatic dispersant. This PAA can also aid ceramic dispersion because of the presence of non-adsorbed PAA species in solutions; the surface tension was measured to be 58 dyn/cm by the Wilhelmy method [21]. The binder chosen was partially hydrolyzed (87 89%) PVA with molecular weight of 80, ,000 g/mol. Commercial PVA is usually made from polyvinyl acetate and characterized by its hydrolysis level and molecular weight. The percentage of acetate groups converted to alcohol groups determines the hydrolysis level of PVA, which affects the degree of polymer crystallinity [22]. Only partially hydrolyzed PVA can be dissolved in water completely. PEG with molecular weight of 400 g/mol was used as a plasticizer due to its good compatibility with PVA. The degree of plasticization also depends on the molecular size of the plasticizer. Rambaldini et al. [23] measured the bending strength of green bodies containing wt.% of PVA and PEG. It was found that PEG 300 has an improved plasticizing effect as compared to PEG 1000 and PEG 10,000. Several reports [24,25] have addressed the effects of binder quantity and binder/plasticizer ratio on the mechanical strength and density of ceramic green tapes as well as on the rheological properties of the slurries. 2. Experimental procedure 2.1. Starting materials A 10GDC [Ce 0.1 Gd 0.9 O 1.95 ] ceramic nano-particles, supplied by Advanced Material Resources Ltd., Canada (AMR), were synthesized by the carbonate co-precipitation. Details of synthesis can be found in a previous work [20]. Poly(acrylic acid) (PAA) (Sigma Aldrich chemie GmbH) with molecular weight of 5000 g/mol was used as an electrostatic dispersant. Binder chosen was partially hydrolyzed (87 89%) poly(vinyl alcohol) (PVA) (Sigma Aldrich chemie GmbH) with molecular weight of 80, ,000 g/mol and poly(ethylene glycol) (PEG) (Sigma Aldrich chemie GmbH, Germany) with molecular weight of 400 g/mol was used as a plasticizer. The ph value of the suspensions was adjusted by analytical grade NH 3 H 2 O and HNO 3 (J.T. Baker). KNO 3 (J.T. Baker) was used to adjust the ionic strength of the suspension for zeta potential measurement. Surfactant used was 2,4,7,9-tetramethyl-5- decyne-4,7-diol ethoxylate (Sigma Aldrich chemie GmbH) and defoamer selected was octanol (Sigma Aldrich chemie GmbH) As-received nano-particle characterization X-ray diffraction (Rigaku Dmax 2200) analysis of the 10GDC nano-particles showed that the Gd 3+ replaced the Ce 4+ lattices in the fluorite structure (FCC) of CeO 2, resulting in a Ce 0.1 Gd 0.9 O 1.95 solid solution [20]. Multipoint Brunauer Emmett Teller (BET) measurements with nitrogen as Fig. 1. TEM image of the 10GDC nano-particles calcined at 800 C for 2 h. the adsorption molecule (Micromeritics ASAP 2000, Norcross, GA) yielded a nano-particle specific surface area of m 2 /g, which corresponded to an equivalent spherical particle diameter of 33.6 nm. The morphology of the calcined nano-particles was observed using Transmission Electron Microscopy (TEM JEOL 2010, Tokyo Japan), and is shown in Fig Zeta potential analysis Zeta potential of the suspensions was determined by Zetaplus (Brookhaven Instruments Corp., USA). Suspensions were prepared by dispersing 10GDC nano-particles (0.01 vol.%) in deionized water both in the absence and presence of the dispersant at various ph levels. The ionic strength of each suspension was adjusted using M KCl solution. The ph levels were then adjusted with analytical grade NH 3 H 2 O and HNO 3. Subsequently, the suspension was ultrasonically agitated and stirred for 30 min to allow for deflocculation, and to achieve equilibrium between the powder surface and the dispersant Settling experiments Sedimentation measurements were conducted by adjusting the ph level. After adjusting the ph level, the suspensions of 0.01 vol.% 10GDC with 1.5 vol.% PAA were ultrasonically agitated and stirred for 30 min. The suspensions were then poured into 10 ml graded measuring cylinders. The readings of the sedimentation volume were recorded over a period of 4 days Rheological measurements The rheological behavior of the slurries with various solid loading of 10GDC nano-particles and optimum dispersant (1.5 vol.% PAA), and the slurry produced from the optimum formulation as shown in Table 1, were characterized using a controlled stress rheometer (HAAKE VT550) with sensor

3 268 L.H. Luo et al. / Materials Science and Engineering A 429 (2006) system MV 2 : radii ration = 1.14, gap width = 0.96 mm, sample volume > 46 cm 3 at 22 C with a double concentric cylinder measurement. In order to avoid undesirable influence from different mechanical histories, the samples were pre-sheared at an identical shear rate of 300 s 1 for 2 min and left standing for an additional 2 min before measurement. The apparent viscosities were measured as a function of shear rate in ascending order form 0 to 300 s 1 during a period of 3 min Slurry preparation and tape casting The tape casting slurries were prepared in two stages. First, 10GDC nano-particles were dispersed using the dispersant in deionized water with the ph adjusted within the range of Subsequently, the slurry was subjected to wet ball milling for 24 h. Ball-to-weight ratio used was 20:1 and milling speed was rpm. Next, the binder and plasticizer solution, surfactant and defoamer were added, and the slurries were ball milled for another 24 h to achieve good homogeneity. Before the addition of PVA, the PVA was pre-dissolved in a sufficient amount of deionized water by heating the PVA solution to 75 C for at least 10 h and stirred to ensure complete dissolution. Surfactant (2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate) was added to decrease the surface tension of water. According to the supplier s data sheet, N/m of water surface tension can be decreased to N/m by adding 0.01 wt.% of this surfactant. This is useful as it enhances the deposition onto a silicone-coated mylar carrier film. The surface energy of the mylar carrier film was N/m [26]. The surfactant and defoamer are necessary for water-based systems due to the high surface tension of water and its foaming tendency [9]. Plasticizers are additives with low molecular weight and are used to reduce the T g of binders to room temperature or less. Poly(ethylene glycol) (PEG) is an effective plasticizer for PVA. The plasticizer to binder ratio must be adjusted so that the tape is flexible, resistant and easily released from the carrier film. In present study, the binder: plasticizer ratio is held at 1:1 by volume. The optimum composition of the slip used for tape casting is shown in Table 1. The slurry was maintained at a ph value above 9.0 to ensure that the slurry was well-dispersed, and was de-gassed in vacuum to remove entrapped air bubbles. A customized laboratory-scale tape caster [27] was used and the tape casting process utilized was adapted and modified from the ERC-105 process devel- Table 1 Slurry formulation of 10GDC aqueous tape casting Additive Function vol.% 10GDC Powder 55 PAA Dispersant 1.5 PVA Binder 4.5 PEG Plasticizer 4.5 Water Solvent ,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate Surfactant 0.1 Octanol Defoamer 0.1 Fig. 2. Flowchart of tape casting process. oped by Mistler et al. [28]. A casting rate of 300 mm/min and gap height of m were chosen. The suspensions were cast onto a silicone-coated mylar carrier film instead of glass. Investigations carried out show that mylar was a better substrate than glass because the dried tapes can be released from the mylar film easily. This was due to the repulsion force between the nonpolar bond (mylar) and polar bond (ceramic tape). After tape casting, the tapes were left to dry at room temperature for 24 h. To determine the bulk density of the dried tape, at least 10 samples (2.5 cm 2.5 cm) were cut from different locations of the tape. The volume of the tape was determined by a geometrical method, where the thickness was measured using a micrometer accurate to mm; the weight of the samples was measured using a laboratory scale accurate to 0.01 mg. Fig. 2 shows the flowchart of the tape casting procedure. 3. Results and discussions 3.1. Effects of dispersant addition Zeta potential measurement Zeta potential studies were performed to identify the optimum ph value and dispersant content for stable slurries with and without the addition of PAA. As shown in Fig. 3, 10GDC has an isoelectric point (IEP) at ph 4.4. However, for this ph range, the maximum zeta potential (absolute value) does not exceed 30 mv, which means that the electrostatic repulsion between the 10GDC nano-particles is insufficient for stabilizing suspensions if no dispersant is added. A dramatic shift of IEP occurs when the PAA dispersant was added to the suspension. The zeta potential became more negative and the IEP was displaced towards acidic values, reaching a ph of around 2.4 (the ph value changes by 2.0 units). For the suspensions containing dispersant, the maximum zeta potential (absolute value) was 60 mv at ph 10. Obviously, the PAA dispersant greatly influenced the interaction between colloidal particles in the aqueous suspensions. PAA is

4 L.H. Luo et al. / Materials Science and Engineering A 429 (2006) Fig. 3. Effect of dispersant as a function of ph value. an electrosteric type dispersant with electrostatic and steric stabilization mechanisms. The steric stabilization mechanism of the dispersant results in an increase in the distance and repulsive interaction between particles because organic molecules were adsorbed onto the surface of the particles. Distinct changes in surface charge of PAA-dispersed-10GDC suspensions were due to the adsorption of dispersant onto the surface of the particle, inducing a more negative charge to the particle double layer, since this kind of dispersant can dissociate in aqueous solution with ph value higher than 3 according to the following reaction [29]: RCOOH H 2O RCOO + H + (1) With such an enhanced negative surface charge, the suspension can be stabilized more easily and sustained for a fairly long time. Thus, the optimum conditions for the stable dispersed slurries are a dispersant coverage of 1.5 vol.% at ph value of Stability of the suspension The stability of the suspension at various times was measured from the change of sediment volume as a function of ph and time. Sedimentation of well-dispersed slurries takes a long time, and is seen by an opaque medium on the graded tube. On the other hand, sedimentation will be observed rapidly with weakly dispersed slurry. Fig. 4 shows the effect of ph on the sedimen- Fig. 5. Viscosity of 10GDC suspension with 1.5 vol.% PAA at various solid loading as a function of shear rate. tation volume of slurries with dispersant. The results show that the slurries were not well-dispersed at a low ph. For 0.01 vol.% concentration 10GDC nano-particles in the ph range of 1 8, sedimentation was observed after 2 days and the sedimentation was complete after 4 days. After 4 days, the suspension at ph 9 10 was able to sustain at a loss of approximate 40% sedimentation volume with respect to the first day Rheological properties of the slurries About 1.5 vol.% PAA was employed for the stabilization of slurries with 65, 70, 75, 80 vol.% solid loading with an adjusted ph of As seen in Fig. 5, the curves for 65, 70, 75 vol.% loading exhibited characteristics close to Newtonian behavior, although a slight shear-thinning was observed. The characteristics are typical of a stable colloidal suspension. When the solid loading was increased to 80 vol.%, the suspension showed highshear-rate Newtonian behavior in the low-shear-rate region; the shear-thinning behavior was obvious. A low viscosity of 0.3 Pas at a low shear rate of 75 s 1 was observed, thus confirming the good dispersing ability of the PAA for 10GDC nano-particles aqueous suspension, even for high solid loading. Fig. 4. Sedimentation volume of 10GDC vs. ph value (with 1.5 vol.% PAA) (ph 1.7, 3.65, 8.7, from left to right). (a) After 2 days and (b) after 4 days.

5 270 L.H. Luo et al. / Materials Science and Engineering A 429 (2006) Fig. 8. The tape caster and 10GDC tapes after dried at 100 C for 4 h. Fig. 6. Influence of binder on the zeta potential of 10GDC suspension with 1.5 vol.% dispersant of PAA Effects of binder and plasticizer Fig. 6 shows the zeta potential measurements of the 10GDC nano-particles in the presence and absence of PVA. The ph IEP was nearly unaffected with the addition of PVA. PVA appeared to have very limited influence on the charge density of the 10GDC nano-particles in the presence of PAA. This could be due to the high-affinity adsorption type and the irreversible adsorption nature of PAA. These results also indicated that the adsorption of PVA onto the 10GDC nano-particles was driven by non-electrostatic mechanisms such as hydrogen bonding or hydrophobic interactions, similar to that reported by Paik et al. [30]. As Fig. 7 shows the rheological properties of 10GDC suspensions produced from the optimum formulation listed in Table 1 showed shear-thinning behavior, which was in good agreement with the requirement for the tape casting process. During the casting process, slurry viscosity decreased under the shear stress generated by the doctor blade. When the shear stress was released, the slurry viscosity immediately returned to its previous high level. This prevented the settling of the particles and preserved the homogeneous distribution of the ceramic particles by reducing the mobility of the constituents Tape casting and drying Various ratios of dispersant, binder and plasticizer were evaluated at ph range of 9 10 and the optimum formulation for 10GDC slurries was determined (see Table 1). Due to the addition of the surfactant, uniform thickness across the as-dried tape could be achieved. The as-dried tapes released from the mylar carrier film easily. The green density of as-dried tapes depended on the viscosity, the solid content of the suspension as well as the tape thickness. The average relative green density of a asdried tapes was 51.5% determined by a geometrical method. The as-dried tapes were flexible, defect-free and had smooth surface. In order to confirm that there was no reaction between the 10GDC nano-particles and the deionzed water in this slurry, XRD measurements of the tape after de-binding (heat treatment at 550 C for 1 h) was carried out and the results showed that all the peaks belonged to cubic CeO 2 phase confirming no phase change. Fig. 8 shows the customized tape caster and 10GDC green tapes after being dried at 100 C for 4 h. 4. Conclusions Fig. 7. Rheological properties of 10GDC suspensions produced from the formulation listed in Table 1. Flexible, defect-free and smooth tapes of 10 mol%-gd 2 O 3 - doped CeO 2 nano-particles were successfully produced via aqueous tape casting process. Optimum formulation for the tape was investigated and it was shown that high solid loading (55 vol.%) would lead to high relative green density (51.5%). In the presence of PAA dispersant, the IEP of 10GDC nanopowders shifted from ph 4.4 to 2.4 and the maximum zeta potential absolute value increased from 30 to 60 mv, this was attributed to charge reversal as PAA was adsorbed on the 10GDC surface, thus achieving a high zeta potential value of 60 mv. It was noted that PVA binder did not affect the adsorption of PAA. For aqueous 10GDC slurries, the ph value would greatly affect the rheology of the slurry. The optimum ph value of the slurry was found to be in the range of 9 10, with optimum dispersant (PAA) content at 1.5 vol.%. Shear-thinning behavior of the slurry was observed, which is desirable in the tape casting process.

6 L.H. Luo et al. / Materials Science and Engineering A 429 (2006) Acknowledgement We are thankful to Advanced Materials Resource, Ltd. (Canada) for supplying the 10GDC nano-particles and their continued support in this project. References [1] E.P. Hyatt, Am. Ceram. Soc. Bull. 65 (4) (1986) [2] A.I.Y. Tok, F.Y.C. Boey, Y.C. Lam, Mater. Sci. Eng. A 280 (2000) [3] A.I.Y. Tok, F.Y.C. Boey, K.A. Khor, J. Mater. Process. Technol. 89 (1999) [4] S. Ramanathan, K. Krishnakumar, P.K. De, S. Banerjee, J. Mater. Sci. 39 (2004) [5] S. Majumdar, T. Claar, B. Flaudermeyer, J. Am. Ceram. Soc. 69 (8) (1986) [6] A.I.Y. Tok, F.Y.C. Boey, K.A. Khor, J. Mater. Eng. Per. 8 (4) (1999) [7] Y.P. Zeng, D.L. Jiang, G. Peter, J. Eur. Ceram. Soc. 20 (2000) [8] M. Sen, Y. Juan, M.F.F. Jose, J. Eur. Ceram. Soc. 24 (2004) [9] D.-H. Yoon, I. Lee Burtrand, J. Eur. Ceram. Soc. 24 (2004) [10] D.-H. Yoon, I. Lee Burtrand, J. Eur. Ceram. Soc. 24 (2004) [11] D.P. Greil, Mater. Sci. Eng. A 202 (1995) ; N. Ushifusa, M.J. Cima, J. Am. Ceram. Soc. 74 (10) (1991) [12] T. Chartier, A. Bruneau, J. Eur. Ceram. Soc. 12 (1993) [13] X.Zh. Jiang, D.L. Jiang, Sh.H. Tan, L.H. Gui, M.L. Ruan, J. Am. Ceram. Soc. 84 (11) (2001) [14] A. Navarro, J.R. Alcock, R.W. Whatmore, J. Eur. Ceram. Soc. 24 (2004) [15] Y.P. Zeng, A. Zimmermann, L. Zhou, F. Aldinger, J. Eur. Ceram. Soc. 24 (2004) [16] Y. Liu, L. Gao, J. Am. Ceram. Soc. 86 (7) (2003) [17] J.A. Lewis, J. Am. Ceram. Soc. 83 (10) (2000) [18] A. Dietrich, A. Neubrand, J. Am. Ceram. Soc. 84 (4) (2001) [19] T.S. Zhang, J. Ma, L.B. Kong, S.H. Chan, X.J.A. Chan, Solid State Ionics 170 (2004) [20] A.I.Y. Tok, L.H. Luo, F.Y.C. Boey, Mater. Sci. Eng. A 383 (2004) [21] N. Pallas, B. Pethica, Colloids Surf. 6 (3) (1983) [22] J.H. Feng, F. Dogan, Mater. Sci. Eng. A 283 (2000) [23] P. Rambaldini, A. Morell, O. Sagnes, 1990, p [24] C.J. Martinez, J.A. Lewis, J Am. Ceram. Soc. 85 (10) (2002) [25] J.X. Zhang, D.L. Jiang, S.H. Tan, L.H. Gui, M.L. Ruan, J. Mater. Res. 17 (8) (2002) [26] J.E. Smay, J.A. Lewis, J. Am. Ceram. Soc. 84 (11) (2001) [27] A.I.Y. Tok, PhD Thesis, 2000, School of Mechanical and Production Engineering, Nanyang Technology University, Singapore. [28] R.E. Mistler, R.B. Runk, D.J. Shanefield, in: G.Y. Onada, L.L. Hench (Eds.), John Wiley & Sons, New York, 1978, pp [29] Q. Tan, Z. Tang, Zh. Zhang, W. Yao, K. Fang, Mater. Sci. Eng. B 99 (2003) [30] U. Paik, V.A. Hacley, H.-W. Lee, J. Am. Ceram. Soc. 82 (4) (1999)