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A new approach for production of anode microtubes as solid oxide fuel cell support
Author’s Accepted Manuscript A new approach for production of anode microtubes as solid oxide fuel cell support Ali Murat Soydan, Osman Yağız Akduman, Recep Akdeniz, Ömer Yıldız www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(18)32564-1 https://doi.org/10.1016/j.ceramint.2018.09.100 CERI19499
To appear in: Ceramics International Received date: 27 July 2018 Revised date: 4 September 2018 Accepted date: 10 September 2018 Cite this article as: Ali Murat Soydan, Osman Yağız Akduman, Recep Akdeniz and Ömer Yıldız, A new approach for production of anode microtubes as solid oxide fuel cell support, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.09.100 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new approach for production of anode microtubes as solid oxide fuel cell support Ali Murat Soydana, Osman Yağız Akdumanb, Recep Akdenizb, Ömer Yıldızc* a
b
Gebze Technical University, Institute of Energy Technologies, Gebze-Kocaeli, Turkey
Gebze Technical University, Department of Materials Science and Engineering, Gebze-Kocaeli, Turkey
c
Kocaeli University, Engineering Faculty, Metallurgical and Materials Engineering, Kocaeli, Turkey
Abstract In this study, a new inexpensive and simple technique has been developed to produce the anode microtubes used as a cell-support in the tubular SOFC system. NiO-GDC (nickel oxide–gadolinium doped ceria) rods with different binder contents were fabricated via thermoplastic extrusion. The rods were transformed into the tubular shape by wicking out the binder during the controlled debinding processing. This was achieved via processing at 600°C and sintering at 1250°C or 1400°C respectively. The wall thickness of microtubes was determined by the content of the binder and the ceramic powders. The mean wall thickness values of fabricated microtubes were 219 µm, 272 µm, and 314 µm. The three-point bending test results show that the best mechanical strength with a bending stress of 164.52 MPa was achieved with the microtubes prepared from the batch 1 containing 20 wt.% binder+surfactant and sintered at 1400°C for 5 h.
Keywords: Rod extrusion, Wicking process, Capillary effect, Microtube, Anode support
1. Introduction The CO2 emission rate continues to increase globally since the energy demand is mostly supplied by the fossil fuels in power generation [1]. The environmental concerns are driving the energy choices towards clean sources and renewable energy systems. Therefore, it is very important to develop cost______________________________
*Corresponding Author: Ömer Yıldız E-mail: [email protected]
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competitive green technologies to replace oil-based energy technologies. The solid oxide fuel cell (SOFC) is one of the promising types of the renewable energy systems to use the power generation. Their main drawbacks are initial higher manufacturing and investment costs involved [2-7]. SOFCs are electrochemical energy conversion devices that can operate with various fuel types at high temperatures [2-7]. SOFC is one of the best candidates for clean, reliable power production due to its high efficiency, and low emission rate [2-7]. There are different kinds of design configurations in terms of cell geometry, but the planar design and especially the tubular designs are mostly preferred [2-25]. The tubular or planar cells can be mostly fabricated as an electrode, electrolyte or dual layers, and as metal current collector support systems [2-26]. If a tubular configuration is compared to a planar system, it can be seen that the tubular system has better thermal stability, higher mechanical strength, and much more good sealing properties because of its structure and optimal system design [2,5,7,8,20,23,24]. On the other hand, the tubular type cells have higher manufacturing costs while the planar cells have much more operating costs [2,3,7,24]. Tubular cells can be fabricated by using one of the following methods: pressing, slip casting, injection molding and extrusion [5-25]. Extrusion is the most prevalent method for the production of the SOFC tubes because this method is very suitable for high volume manufacturing [6,7,19,23-25]. Different kinds of ceramic extrusion methods such as ram extrusion, screw extrusion, single layer extrusion, coextrusion, and hollow fiber extrusion were continuously investigated [5-25]. Injection molding is another widely used method to produce tubular SOFC. Injection molding is similar to extrusion; a paste is forced to flow into a die by an applied pressure to shape green ceramic bodies, only difference is the form of metal dies. Injection molding and extrusion dies are expensive and the friction between the die and the paste is a major problem. The maintenance costs are highly related to the abrasion of the barrel and the die. In the case of ceramic tube extrusion, the abrasion in the extrusion die becomes more significant as the extrudate wall gets thinner due to the need for the higher compression rate of the paste. The higher pressure results in a higher friction rate between the paste and the walls of the die and the barrel. In other words, the selection of simple extrudate
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geometries and thicker wall dimensions lower the applied pressure and reduce the wear of the machine parts. Extruding simple solid rod form is more cost effective than extruding hollow form ceramics because the energy consumption and maintenance costs are lower depending on the lower operating pressures and lower abrasion rates. This is also valid for injection molding. Pressure and viscosity of the paste have great influence on the green body and final product quality, but no matter how good the quality of the green body, the sintering of the extrudate must be carried on very carefully. Binder removal is a very critical and important stage for the sintering of the extruded ceramic green body to produce the ceramic microtubes without any defects because the deformation can occur during the debinding and the sintering processes according to the conditions [27-37]. Different processes for the removal of the binders can be used before or during the sintering process: thermal heat treatment, the wicking, the solvent extraction, the supercritical extraction, and the catalytic process [27-37]. The decomposition of polymers occurs in the cycle of melting, oxidation or combustion by the thermal treatment during the debinding process in the oxygen-containing atmosphere [30,32,34,35,37]. During the debinding by heat treatment, a very high gas pressure may build-up inside the shaped body due to uncontrolled reactions with very high rates. Therefore, this may lead to often cracked or fractured compact bodies [30,31,33,34,36-38]. The structural optimization of the binders and the modification of the decomposition conditions are the important parameters for the polymers and the compacts to form without any defects [30,37]. The decomposition can start at a defined temperature depending on the binder properties and continue as a chain reaction to form the gas phase in a high pressure and volume [30,37]. We have used the “wicking model” with a capillary flow of the binder from the green body which has been previously explained by German [27]. The main reason behind this choice is that the other debinding models such as the solvent extraction, the supercritical extraction, and the catalytic process, have other major phenomena serves as accelerating effect on debinding of the polymers only but the viscous flow of the polymer content is not present. Debinding process is performed by wicking out the
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binder in the extrudate rod body using capillary flow effect [27,28,30]. Capillary forces are generated by the surface tension in presence of solid and liquid together during the heat treatment of the green body above the softening point of the binder [28,30,33]. As a result of this, two different attractive forces occur between particles which can lead rearrangement of the particles and densification in the case of the contact angle < 90° [22,28,30]. The first force is on account of growing negative pressure in the melted polymer, the second force is caused by liquid/vapor interface energy which is perpendicular to both surfaces [38]. The melted binder spreads on the particles and pulls them closer by the aid of these two attractive forces as shown in Fig. 1. The permeability of the green body for the melted liquid can be different according to the capillary extraction forces that are affected by the absorption and adsorption potency of the particles [33,37]. The interactions of the ceramic particles, molten polymer fluid and gas phase are previously described in detail in the literature [27,37,38].
Fig. 1: Illustration of the attractive forces on the binder for wetting of particles; θ<90°. If the green rod body is embedded in the free powder mass in a heat treatment furnace at a certain temperature, the melted binder is drawn into the center of the surrounding powder in rod [29,31]. In this way, the densification of the anode powders occurs as a tube in a certain diameter with a certain wall thickness. But, the non-rigid rod body could be deformed during the suction of the binder into porous powder as explained by Gorjan et al. [31]. In our application, the ceramic particles within the green rod body can be rearranged by the capillary effect while the deformation of the extrudate is prevented by surrounding of the green rod body with the supported powders. During the transfer of the binder into the surrounding powders, ceramic particles are dragged through the surface of the extrudate by the first force that is mentioned above for binder removal [27-37], due to the negative pressure which is related to the negative curvature in the meniscus. The particles in the rod can be transformed into a tube by rearrangement of the capillary force as shown in Fig. 2.
Fig. 2: Illustration of the wicking and the debinding process steps (I-IV) for the transformation of the rod into the tube: lateral and cross-sectional view of the body (above and below respectively).
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In this study, a new inexpensive and simple production process has been developed to produce the anode microtubes used as the main support structure in microtubular SOFC systems. The microtubes were firstly produced as a green body in a rod shape with a diameter of 2.27 mm by extrusion. The rods were transformed into microtubes after the debinding of green body by controlling of the wicking process and by sintering at a certain heat treatment temperature. The mechanical properties and the microstructure of the tubes were investigated as a function of the wall thickness, the sintering temperature and also to the content of the binder and ceramic powders.
2. Experimental procedures 2.1 Materials Commercial nickel oxide with gadolinium doped ceria (NiO-GDC) powders with 60 wt.% of NiO, cyclohexanone, polyethylene (PE) and stearic acid were used as the main phase, solvent, binder, and surfactant with a purity of +99% and 98% from NexTech and Alfa Aesar respectively.
2.2 Production of the anode microtubes The microtubes were initially produced as a green body in a rod shape with a diameter of 2.27 mm by extrusion of the homogeneous mixtures of the anode powders and polymer binders. Three different batches, batch 1, 2 and 3 containing 20 wt.%, 16.7 wt.% and 13.4 wt.% binder+surfactant mixtures respectively, given in Table 1, were used to produce the green rods by an extruder as starting materials to fabricate the anode microtubes. PE was dissolved in cyclohexanone during the mixing for 30 min. at 70°C. NiO-GDC powders and stearic acid were added to the PE solution and stirred for a short time. The prepared batch mixtures were transferred into suitable containers where the homogenization took place by ball milling. Retsch PM 200 planetary mill was used to generate the homogeneous mixture for 20 minutes. After the mixing process, the solvent in suspension mixture was evaporated during the stirring at 90°C in air atmosphere. The dried powders coated by polymer mixtures were placed in the feeder for continuous extrusion process with the screw system and extruded in a rod shape with a diameter of 2.27 mm by a Mini-Extruder from Rondol Technology Ltd. Five different temperature zones were used during the extrusion of the rods as shown in Table 2. 5
Table 1: Compositions of the prepared batches. Table 2: Temperature zones of extrusion process steps. The rods were transformed into microtubes after the debinding of the green bodies by controlling of the wicking process with a capillary flow effect as shown in Fig. 2 and by sintering at a certain heat treatment temperature given in Fig. 3. The produced green rod bodies were placed in the NiO-GDC loose powders on an alumina plate and embedded in NiO-GDC loose powder mass to prevent the distortion of the rod bodies during the debinding by heat treatment at 600°C in an oven. The embedded rod bodies were sintered at two different temperatures of 1250°C or 1400°C for 5 hours with firing regimes as shown in Fig. 3 to obtain the anode microtube structure with a certain porosity between 3040%.
Fig. 3: Debinding and sintering regimes of the samples.
2.3 Characterization The morphology and microstructure on the outer surface and cross-section surface of the sintered and broken tubes was studied using Philips XL 30 SFEG Scanning Electron Microscopy (SEM). Threepoint bending method was used to investigate the effect of binder content and the sintering temperature on the mechanical properties of sintered tubular anode supports. The mechanical strength tests were performed using an Instron 5569 tensile tester on the five different samples with a length of 25 mm. The bending strength (
) was calculated using the following equation: [25,39]
where F is fracture force; L, D1, and D2 are the lengths, the outer and inner diameter, respectively. The specific surface areas (SSA) of the sintered samples were determined via the BET method using a Quantachrome Autosorb 1 instrument. The porosity of the sintered anode microtubes was determined by using Archimedes’ principle. The properties of the sintered microtubular anode support were studied with respect to the content of the binder and ceramic powders, the sintering temperature, the SSA, porosity, also the wall thickness, and mechanical strength. 6
3. Results and discussion 3.1 Microstructure of the anode microtubes The microtubes with tubular structure were produced successfully for the first time in this study by the debinding process with a wicking force at 600°C and then by sintering process performed at 1250°C or 1400°C. The shape, structure and surface morphology of the final product are highly dependent on the constituents of the starting materials and also the preparation conditions. Although Ismael et al. [31] produced the extruded lead zirconate titanate fibers in dense rod body by debinding at 600°C and by sintering at 1100°C previously, we produced the microtubes with desired interior hollow structure without any cracks by debinding and then sintering processes successively as described above. The thermal behavior of the polymer was also the determining factor in this case [27,30,33]. During the heat treatment, the binder within the embedded rod generated the main flow channels due to the ongoing thermal expansion and the acting capillary force. The binder in the fluid form flowed out of the compact via a capillary force with the wicking effect which is dependent on the binder form and the viscosity that is decreasing with the rise of the heat treatment temperature [27,30,33]. As mentioned above, those ceramic particles in the extrudate were expected to be dragged through the surface by capillary effect as shown in Fig. 2. Summers et al. [28] depicted that “all of the debinding processes described previously leave pendular bonds, giving the component sufficient strength to be handled prior to sintering”. Therefore, “the pendular bonds must be removed by evaporation prior to sintering and cannot be removed by wicking” [28]. In our study, the debinding and sintering processes were successively performed and there was no noticeable deformation problem. The transformation of the rod body to the tubular structure took place as anticipated and the tubular anode supports with closed in both ends were obtained without any deformation as shown in Figs. 4-6. When both sides of the closed tube were cut-off, the microtube with both open ends was produced as shown in Figs. 5 and 6. But, a small number of large open porosities were formed on the surface of the tubes marked with red circles on the image as shown in Fig. 6a, because it was believed that the main flow of the liquid binder and/or gas phase occurred through the big channels on the surface during the heat treatment at the debinding temperature. Furthermore, the residues of the surrounding powders on the surface of the
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tubes were observed as defects as shown in Fig. 5 and 6 b-d. The debinding process can be optimized to obtain the microtubes with desired surface properties, the microstructure, the porosity size, and distribution. In addition, if the surfaces of the sintered anode tubes are sanded with 320 mesh or more suitable sandpaper, the porosity structure and distribution in the anode surface can be homogenized which will enhance a smoother surface. As a result of this, the surface of the anode is ready to be coated with electrolyte. Coating of the electrolyte and the cathode layer on the anode can be done either by dip coating or slurry coating techniques. After the coating of each layer (electrolyte and cathode), the final microtube is produced by sintering all layers. Determining the sanding procedures and conditions for the examination of anode and electrolyte interface properties are a separate research topic. Fig. 4: Photo images of the sintered and untreated anode microtubes produced from the extruded green rod body. Fig. 5: SEM image of the anode microtube produced from the extruded green rod body and sintered at 1400°C. Fig. 6: SEM images of the anode microtubes sintered at 1400°C: outer surface (a), and cross sections of the samples batch 1 (b), 2 (c) and 3 (d).
From a geometrical perspective, the geometries and especially the wall thickness of the tubes were not perfectly regular as expected, although all green rod bodies had a diameter of 2.27 mm. After sintering, the tubes have different diameters in inverse proportion to the content of the binder as shown in Fig. 5b-d. The mean wall thicknesses of the samples from the batch 1, 2 and 3 were 219 µm, 272 µm, and 314 µm respectively. However, these results obtained in this study are quite successful as an initial process trial outcome. The wall thickness could be adjusted and the desired uniform wall thickness could be achieved in further studies, by optimization of the debinding process as explained in the literature [27,28,30,33] and also by changing of the amount of the binder mixtures and by optimization of the binder and ceramic powder properties. On the other hand, the execution time of the debinding process must be optimized.
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Carrying out a rapid debinding process (in a very short period of time), as the particles forming the anode structure will be dragged out of the center all the way outwardly (Fig. 2) all together rapidly and depending on the conditions, the tube produced will have a larger internal diameter, a thinner wall thickness and lower porosity ratio will result in the production of a high bulk density tube. Rapid debinding can result in a complete deformation. If the debinding process is carried out slowly, the NiO-GDC particles will not be dragged at a lower speed because of the controlled movement of the organic components depending on the binder properties. As a consequence, the tube produced will have both a smaller inner diameter and a larger wall thickness. Due to the slow movement of the particles, an anode structure with higher pores will be obtained due to the gas spaces between the particles. One of the most important results obtained in this study is that; when the binder ratio in the batch is too high, an excessive amount of gas generated during debinding causes NiO-GDC particles to move more strongly and cause particles to be pressed. As a result of the pressure, both a dense and less porous anode microstructure (high bulk density) can be obtained. Therefore, the mechanical strength of the produced anode microtubes will be higher. However, the only critical criterion is not the mechanical strength; the electrochemical performance of the cells obtained should be taken into consideration while optimizing the pore size and quantity of the produced tubes. On the other hand, the design of the heat treatment furnace, the heat transfer in the furnace, the placement of the green rod bodies in the powder, the type, quantity and properties of the binder, the grain size and size distribution of the NiO-GDC powders also affect the wall thickness of the produced tubes and the homogeneity of the structure. Effects of all given parameters on the wall thickness and the homogeneity of the microstructure will be investigated in a separate study.
3.2 Mechanical strength of the anode microtubes The lifetime of a fuel cell is highly dependent on the mechanical strength of the cell tubes [25]. The three-point bending strength tests of the anode microtubes as a support of the SOFC system were performed on the samples which were prepared with different binder content and sintered at 1250°C and 1400°C as shown in Table 1 and Fig. 3 respectively. The mechanical bending strength results display a proportional relationship with the sintering temperature, thickness, and density of the anode 9
support tubes as shown in Fig. 7 and they are comparable to values reported in the literature [25]. The thicknesses of the anode microtubes provided in Figs. 5 and 6 are inversely proportional to the binder content of the green rod bodies given in Table 1. The sample batch 1 with 20 wt.% binder content has shown best performance at both temperatures. The mechanical strength of the samples has been improved dramatically with the increasing sintering temperature. The samples sintered at 1400°C have demonstrated the best mechanical performance with an average value of 164.52 MPa (min. 164.22, max. 165.34 MPa) as shown in Fig. 7. Yang et al. [14] explained that the samarium doped ceria (SDC) microtubes have a bending strength from 20 MPa to 208 MPa increased as a function of the sintering temperature between 1300°C and 1500°C and the microtubes were found to be very fragile when the sintering temperature was lower than 1300°C. As the heat treatment temperature gets higher, the sintering effect becomes clear and thus the strength of the tubes is increased. The sintering effect of the heat treatment by sintering at 1250°C and 1400°C is clearly evident from the grain growth of the particles (crystallite size) according to the peak intensity determined by XRD as shown in Fig. 8-a and -b. The sintering temperature has an important influence on the mechanical bending strength and on the densification of the anode microstructure which was obtained by decreasing of the SSA and porosity determined by BET and Archimedes methods as shown in Fig. 9. However, it should not be forgotten that the only critic criteria for microtubular SOFC cells is not the mechanical strength and the electrochemical cell performance is one of the most important criteria. After the reduction of the anode microtubes in a hydrogen atmosphere, reduction of NiO to metallic Ni was expected [5,10,18,24]. However, as Maher et al. [40] explained, it was seen that GDC was reduced also to metallic Ce as shown in Figure 8c after reduction by heat treatment at 700°C in a dry hydrogen atmosphere at a pressure of 2 bar. Fig. 7: Mechanical bending strength of the samples prepared with different binder+surfactant contents (batch 1: 20 wt.%, batch 2: 16.7 wt.% and batch 3: 13.4 wt.%), and sintered at different temperatures.
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Fig. 8: XRD analysis of the NiO-GDC samples sintered at 1400°C (a), 1250°C (b) and then reduced at 700°C in a hydrogen atmosphere (c).
3.3 Active surface area of the anode microtubes SSA of the anode microtubes determined by BET method is not directly proportional with the binder content. The prepared prescriptions have met the expectations depending on the sintering temperature and binder contents. The expected declines for SSA were observed for all samples that sintered at 1400°C as shown in Fig. 9. The decreasing of the SSA means that the sintering effect occurs between the particles in the microstructure, the particles are strongly bonded together and the anode tube has the desired high mechanical strength. Considering the rise in the mechanical strength of the sample from batch 3, this was over threefold while SSA decreased only by 11% at 1400°C. Fig. 9: SSA and porosity (POR) results of the samples prepared with different binder+surfactant contents (batch 1: 20wt.%, batch 2: 16.7 wt.% and batch 3: 13.4 wt.%) and sintered at different temperatures. 4. Conclusions Tubular anode supports have been manufactured via a new approach in which the debinding and sintering processes were successively performed with a remarkably shortened batch preparation time. The self-transformation of green rod body to the anode tube has been accomplished, by the debinding of the binder using the capillary force of the melted polymer. Reorientation of ceramic powder within the melted polymer has allowed the body to change its geometry. It is found that the wall thickness of the tubes can be tailored by changing the binder to powder ratio, the wall thickness changed inversely proportional to the binder content. The obtained mechanical strength values of the sintered anode microtube supports were comparable to the results in the literature. The SSA tends to decrease as the sintering temperature increases, but the flexural strength of tubes increased. While dramatic change observed in flexural strength, attenuation of SSA was not significant. The process developed in this investigation can be optimized in further studies to produce a complete cell with a perfect geometrical tube structure with good dimensional tolerances and characterize its electrochemical properties. 11
Literatures [1] BP Statistical Review of World Energy, 2014. p. 1-2. [2] Singhal S.C., K. Kendal. High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications. 2003; ISBN 1-85617-387-9, Elsevier Ltd., Oxford, UK. [3] Yamamoto O., Solid oxide fuel cells: fundamental aspects and prospects. Electrochimica Acta. 2000;45(15–16):2423-2435. [4] Badwal S.P.S., Foger K. Solid Oxide Electrolyte Fuel Cell Review. Ceramics International. 1996;22:257-265. [5] Kim J.-H., Song R.-H., Song K.-S., et al. Fabrication and characteristics of anode-supported flattube solid oxide fuel cell. Journal of Power Sources.2003;122(2):138-143. [6] Chen Z., Takeda T., Kikuchi K., et al. Preparation of Anode/Electrolyte Ceramic Composites by Coextrusion of Pastes. Journal of the American Ceramic Society. 2004;87:983-990. [7] Sammes N.M. and Du Y., Fabrication and Characterization of Tubular Solid Oxide Fuel Cells. International Journal of Applied Ceramic Technology. 2007;4(2):89-102. [8] Sin Y.-W., Galloway K., Roy B., et al. The properties and performance of micro-tubular (less than 2.0 mm O.D.) anode supported solid oxide fuel cell (SOFC). International Journal of Hydrogen Energy. 2011;36(2):1882-1889. [9] Funahashi Y., Shimamori T., Suzuki T., et al. Fabrication and characterization of components for cube shaped micro tubular SOFC bundle. Journal of Power Sources. 2007;163(2):731-736. [10] Lee B.-T., Rahman A.H.M.E., Kim J.-H. Novel Design of Microchanneled Tubular Solid Oxide Fuel Cells and Synthesis Using a Multipass Extrusion Process. Journal of the American Ceramic Society. 2007;90:1921-1925. [11] Du Y., Sammes N.M., Tompsett G.A., Optimisation parameters for the extrusion of thin YSZ tubes for SOFC electrolytes. Journal of the European Ceramic Society. 2000;20:959-965. [12] Suzuki T., Yamaguchi T., Fujishiro Y., et al. Fabrication and characterization of micro tubular SOFCs for operation in the intermediate temperature. Journal of Power Sources. 2006;160(1):7377. [13] Yamaguchi, T., Suzuki T., Shimizu S., et al. Examination of wet coating and co-sintering technologies for micro-SOFCs fabrication. Journal of Membrane Science. 2007;300:45-50.
12
[14] Yang N., Tan X., Ma Z., et al. Fabrication and Characterization of Ce0.8Sm0.2O1.9 Microtubular Dual-Structured Electrolyte Membranes for Application in Solid Oxide Fuel Cell Technology. Journal of the American Ceramic Society. 2009;92:2544-2550. [15] Yamaguchi T., Shimizu S., Suzuki T., et al. Design and Fabrication of a Novel ElectrodeSupported Honeycomb SOFC. Journal of the American Ceramic Society. 2009;92:S107–S111. [16] Roy B.R., Sammes N.M., Suzuki T., et al. Mechanical properties of micro-tubular solid oxide fuel cell anodes. Journal of Power Sources. 2009;188:220-224. [17] Powell J., Blackburn S. The unification of paste rheologies for the co-extrusion of solid oxide fuel cells. Journal of the European Ceramic Society. 2009;29(5):893-897. [18] Othman M.H.D., Wu Z., Droushiotis N., et al. Single-step fabrication and characterisations of electrolyte/anode dual-layer hollow fibres for micro-tubular solid oxide fuel cells. Journal of Membrane Science. 2010;351:196-204. [19] Othman M.H.D., Droushiotis N., Wu Z., et al. Electrolyte thickness control and its effect on electrolyte/anode dual-layer hollow fibres for micro-tubular solid oxide fuel cells. Journal of Membrane Science. 2010;365:382-388. [20] Yang C., Li W., Zhang S., et al. Fabrication and characterization of an anode-supported hollow fiber SOFC. Journal of Power Sources. 2009;187(1):90-92. [21] Gil V., Gurauskis J., Campana R., et al. Anode-supported microtubular cells fabricated with gadolinia-doped ceria nanopowders. Journal of Power Sources. 2011;196:1184-1190. [22] Hsieh W.-S., Lin P., Wang S.-F., Fabrication of electrolyte supported micro-tubular SOFCs using extrusion and dip-coating. International Journal of Hydrogen Energy. 2013;38:2859-2867. [23] Li T., Wu Z., Li K., A dual-structured anode/Ni-mesh current collector hollow fibre for microtubular solid oxide fuel cells (SOFCs). Journal of Power Sources. 2014;251:145-151. [24] Jamil S.M., Othman M.H.D., Rahman M.A., et al. Recent fabrication techniques for microtubular solid oxide fuel cell support: A review. Journal of the European Ceramic Society. 2015. 35(1): p. 1-22. [25] Li T., Wu Z., Li K., Co-extrusion of electrolyte/anode functional layer/anode triple-layer ceramic hollow fibres for micro-tubular solid oxide fuel cells-electrochemical performance study. Journal of Power Sources. 2015;273:999-1005. [26] Fernández-González R., Hernández E., Savvin S., et al. A novel microstructured metal-supported solid oxide fuel cell. Journal of Power Sources. 2014;272:233-238. [27] German R.M., Theory of Thermal Debinding. International Journal of Powder Metallurgy. 1987;23(4):237-245. 13
[28] Summers E.M., Akinc M. Wick Debinding of Molybdenum-Silicon-Boron Extrudates. Journal of the American Ceramic Society. 2000;83:1670-74. [29] Somasundram I.M., Cendrowicz A., Wilson D.I., et al. Phenomenological study and modelling of wick debinding. Chemical Engineering Science. 2008;63(14):3802-3809. [30] Clemens F. Thermoplastic Extrusion for Ceramic Bodies, Chapter 17 in "Extrusion in Ceramics". Edit. by F. Händle, 2009, Springer-Verlag, Berlin Heidelberg. [31] Gorjan, L., Dakskobler A., T. Kosmač. Partial wick-debinding of low-pressure powder injectionmoulded ceramic parts. Journal of the European Ceramic Society. 2010;30(15):3013-3021. [32] Ismael M.R., Clemens F., Wyss P., et al. Processing and Properties of Co-Extruded Lead Zirconate Titanate Fibers. Journal of the American Ceramic Society. 2012;95:108-116. [33] Gorjan L. Wick Debinding – An Effective Way of Solving Problems in the Debinding Process of Powder Injection Molding, in “Some Critical Issues for Injection Molding”, Edit. Jian Wang, 2012, ISBN 978-953-51-0297-7, Publisher: InTech. [34] Ani S.Md., Muchtar A., Muhamad N., et al. Binder removal via a two-stage debinding process for ceramic injection molding parts. Ceramics International. 2014;40:2819-2824. [35] Gorjan L., Kosmač T., Dakskobler A. Single-step wick-debinding and sintering for powder injection molding. Ceramics International. 2014;40:887-891. [36] Sharmin K., Schoegl I. Optimization of binder removal for ceramic micro fabrication via polymer co-extrusion. Ceramics International. 2014;40:3939-3946. [37] Becker F.H. Debinding processes - physical and chemical conclusions and their practical realisations.
http://www.riedhammer.de/System/00/01/42/14224/633776345920000000_1.pdf,
(visited 22.11.2017). [38] Barsoum M.W. Fundamentals of Ceramics. Series in Material Science and Engineering. 2002; CRC Press. 624. [39] Cernica J.N., Strength of materials. Published by Holt Rinehart & Winston, 1977, New York, ISBN-13: 978-0030531958. [40] Maher R.C., Shearing P.R., Brightman E., et al. Reduction Dynamics of Doped Ceria, Nickel Oxide, and Cermet Composites Probed Using In Situ Raman Spectroscopy. Advanced Science. 2016;3:1500146 (1-8).
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Tables Caption Table 1: Compositions of the prepared batches. Table 2: Temperature zones of extrusion process steps.
Figures Caption Fig. 1: Illustration of the attractive forces on binder for wetting of particles; θ<90°. Fig. 2: Illustration of the wicking and debinding process steps (I-IV) for the transformation of the rod into the tube: lateral and cross-sectional view of the body (above and below respectively). Fig. 3: Debinding and sintering regimes of the samples. Fig. 4: Photo images of the sintered and untreated anode microtubes produced from the extruded green rod body. Fig. 5: SEM image of the anode microtube produced from the extruded green rod body and sintered at 1400°C. Fig. 6: SEM images of the anode microtubes sintered at 1400°C: outer surface (a), and cross sections of the samples batch 1 (b), 2 (c) and 3 (d). Fig. 7: Mechanical bending strength of the samples prepared with different binder+surfactant contents (batch 1: 20 wt.%, batch 2: 16.7 wt.% and batch 3: 13.4 wt.%), and sintered at different temperatures. Fig. 8: XRD analysis of the NiO-GDC samples sintered at 1400°C (a), 1250°C (b) and then reduced at 700°C in a hydrogen atmosphere (c). Fig. 9: SSA and porosity (POR) results of the samples prepared with different binder+surfactant contents (batch 1: 20 wt.%, batch 2: 16.7 wt.% and batch 3: 13.4 wt.%) and sintered at different temperatures.
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TABLES
Table 1: Compositions of the prepared batches.
Components
Batch 1 (wt.%)
Batch 2 (wt.%)
Batch 3 (wt.%)
NiO-GDC powder (A)
46.6
50
53.3
Binder + Surfactant (B)
20
16.7
13.4
Solvent
33.3
33.3
33.3
A : B ratio
70 : 30
75 : 25
80 : 20
Table 2: Temperature zones of extrusion process steps.
Zones Temperature (°C)
Feeding
1
2
3
Die
89
177
170
170
165
16
FIGURES
Fig. 1: Illustration of the attractive forces on binder for wetting of particles; θ<90°.
Fig. 2: Illustration of the wicking and debinding process steps (I-IV) for the transformation of the rod into the tube: lateral and cross-sectional view of the body (above and below respectively).
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Temperature ( °C )
1400 1200 1000 800 600 400
Regime 2
200
Regime 1
0 0
5
10
15
20
25
30
Duration ( h )
Fig. 3: Debinding and sintering regimes of the samples.
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Fig. 4:
Photo images of the sintered and untreated anode microtubes produced from the extruded green rod body.
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Fig. 5: SEM image of the anode microtube produced from the extruded green rod body and sintered at 1400°C.
Fig. 6: SEM images of the anode microtubes sintered at 1400°C: outer surface (a), and cross sections of the samples batch 1 (b), 2 (c) and 3 (d). 20
Fig. 7: Mechanical bending strength of the samples prepared with different binder+surfactant contents (batch 1: 20 wt.%, batch 2: 16.7 wt.% and batch 3: 13.4 wt.%), and sintered at different temperatures.
Fig. 8: XRD analysis of the NiO-GDC samples sintered at 1400°C (a), 1250°C (b) and then reduced at 700°C in a hydrogen atmosphere (c).
21
Fig. 9: SSA and porosity (POR) results of the samples prepared with different binder+surfactant contents (batch 1: 20 wt.%, batch 2: 16.7 wt.% and batch 3: 13.4 wt.%) and sintered at different temperatures..
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PII: DOI: Reference:
S0272-8842(18)32564-1 https://doi.org/10.1016/j.ceramint.2018.09.100 CERI19499
To appear in: Ceramics International Received date: 27 July 2018 Revised date: 4 September 2018 Accepted date: 10 September 2018 Cite this article as: Ali Murat Soydan, Osman Yağız Akduman, Recep Akdeniz and Ömer Yıldız, A new approach for production of anode microtubes as solid oxide fuel cell support, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.09.100 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new approach for production of anode microtubes as solid oxide fuel cell support Ali Murat Soydana, Osman Yağız Akdumanb, Recep Akdenizb, Ömer Yıldızc* a
b
Gebze Technical University, Institute of Energy Technologies, Gebze-Kocaeli, Turkey
Gebze Technical University, Department of Materials Science and Engineering, Gebze-Kocaeli, Turkey
c
Kocaeli University, Engineering Faculty, Metallurgical and Materials Engineering, Kocaeli, Turkey
Abstract In this study, a new inexpensive and simple technique has been developed to produce the anode microtubes used as a cell-support in the tubular SOFC system. NiO-GDC (nickel oxide–gadolinium doped ceria) rods with different binder contents were fabricated via thermoplastic extrusion. The rods were transformed into the tubular shape by wicking out the binder during the controlled debinding processing. This was achieved via processing at 600°C and sintering at 1250°C or 1400°C respectively. The wall thickness of microtubes was determined by the content of the binder and the ceramic powders. The mean wall thickness values of fabricated microtubes were 219 µm, 272 µm, and 314 µm. The three-point bending test results show that the best mechanical strength with a bending stress of 164.52 MPa was achieved with the microtubes prepared from the batch 1 containing 20 wt.% binder+surfactant and sintered at 1400°C for 5 h.
Keywords: Rod extrusion, Wicking process, Capillary effect, Microtube, Anode support
1. Introduction The CO2 emission rate continues to increase globally since the energy demand is mostly supplied by the fossil fuels in power generation [1]. The environmental concerns are driving the energy choices towards clean sources and renewable energy systems. Therefore, it is very important to develop cost______________________________
*Corresponding Author: Ömer Yıldız E-mail: [email protected]
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competitive green technologies to replace oil-based energy technologies. The solid oxide fuel cell (SOFC) is one of the promising types of the renewable energy systems to use the power generation. Their main drawbacks are initial higher manufacturing and investment costs involved [2-7]. SOFCs are electrochemical energy conversion devices that can operate with various fuel types at high temperatures [2-7]. SOFC is one of the best candidates for clean, reliable power production due to its high efficiency, and low emission rate [2-7]. There are different kinds of design configurations in terms of cell geometry, but the planar design and especially the tubular designs are mostly preferred [2-25]. The tubular or planar cells can be mostly fabricated as an electrode, electrolyte or dual layers, and as metal current collector support systems [2-26]. If a tubular configuration is compared to a planar system, it can be seen that the tubular system has better thermal stability, higher mechanical strength, and much more good sealing properties because of its structure and optimal system design [2,5,7,8,20,23,24]. On the other hand, the tubular type cells have higher manufacturing costs while the planar cells have much more operating costs [2,3,7,24]. Tubular cells can be fabricated by using one of the following methods: pressing, slip casting, injection molding and extrusion [5-25]. Extrusion is the most prevalent method for the production of the SOFC tubes because this method is very suitable for high volume manufacturing [6,7,19,23-25]. Different kinds of ceramic extrusion methods such as ram extrusion, screw extrusion, single layer extrusion, coextrusion, and hollow fiber extrusion were continuously investigated [5-25]. Injection molding is another widely used method to produce tubular SOFC. Injection molding is similar to extrusion; a paste is forced to flow into a die by an applied pressure to shape green ceramic bodies, only difference is the form of metal dies. Injection molding and extrusion dies are expensive and the friction between the die and the paste is a major problem. The maintenance costs are highly related to the abrasion of the barrel and the die. In the case of ceramic tube extrusion, the abrasion in the extrusion die becomes more significant as the extrudate wall gets thinner due to the need for the higher compression rate of the paste. The higher pressure results in a higher friction rate between the paste and the walls of the die and the barrel. In other words, the selection of simple extrudate
2
geometries and thicker wall dimensions lower the applied pressure and reduce the wear of the machine parts. Extruding simple solid rod form is more cost effective than extruding hollow form ceramics because the energy consumption and maintenance costs are lower depending on the lower operating pressures and lower abrasion rates. This is also valid for injection molding. Pressure and viscosity of the paste have great influence on the green body and final product quality, but no matter how good the quality of the green body, the sintering of the extrudate must be carried on very carefully. Binder removal is a very critical and important stage for the sintering of the extruded ceramic green body to produce the ceramic microtubes without any defects because the deformation can occur during the debinding and the sintering processes according to the conditions [27-37]. Different processes for the removal of the binders can be used before or during the sintering process: thermal heat treatment, the wicking, the solvent extraction, the supercritical extraction, and the catalytic process [27-37]. The decomposition of polymers occurs in the cycle of melting, oxidation or combustion by the thermal treatment during the debinding process in the oxygen-containing atmosphere [30,32,34,35,37]. During the debinding by heat treatment, a very high gas pressure may build-up inside the shaped body due to uncontrolled reactions with very high rates. Therefore, this may lead to often cracked or fractured compact bodies [30,31,33,34,36-38]. The structural optimization of the binders and the modification of the decomposition conditions are the important parameters for the polymers and the compacts to form without any defects [30,37]. The decomposition can start at a defined temperature depending on the binder properties and continue as a chain reaction to form the gas phase in a high pressure and volume [30,37]. We have used the “wicking model” with a capillary flow of the binder from the green body which has been previously explained by German [27]. The main reason behind this choice is that the other debinding models such as the solvent extraction, the supercritical extraction, and the catalytic process, have other major phenomena serves as accelerating effect on debinding of the polymers only but the viscous flow of the polymer content is not present. Debinding process is performed by wicking out the
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binder in the extrudate rod body using capillary flow effect [27,28,30]. Capillary forces are generated by the surface tension in presence of solid and liquid together during the heat treatment of the green body above the softening point of the binder [28,30,33]. As a result of this, two different attractive forces occur between particles which can lead rearrangement of the particles and densification in the case of the contact angle < 90° [22,28,30]. The first force is on account of growing negative pressure in the melted polymer, the second force is caused by liquid/vapor interface energy which is perpendicular to both surfaces [38]. The melted binder spreads on the particles and pulls them closer by the aid of these two attractive forces as shown in Fig. 1. The permeability of the green body for the melted liquid can be different according to the capillary extraction forces that are affected by the absorption and adsorption potency of the particles [33,37]. The interactions of the ceramic particles, molten polymer fluid and gas phase are previously described in detail in the literature [27,37,38].
Fig. 1: Illustration of the attractive forces on the binder for wetting of particles; θ<90°. If the green rod body is embedded in the free powder mass in a heat treatment furnace at a certain temperature, the melted binder is drawn into the center of the surrounding powder in rod [29,31]. In this way, the densification of the anode powders occurs as a tube in a certain diameter with a certain wall thickness. But, the non-rigid rod body could be deformed during the suction of the binder into porous powder as explained by Gorjan et al. [31]. In our application, the ceramic particles within the green rod body can be rearranged by the capillary effect while the deformation of the extrudate is prevented by surrounding of the green rod body with the supported powders. During the transfer of the binder into the surrounding powders, ceramic particles are dragged through the surface of the extrudate by the first force that is mentioned above for binder removal [27-37], due to the negative pressure which is related to the negative curvature in the meniscus. The particles in the rod can be transformed into a tube by rearrangement of the capillary force as shown in Fig. 2.
Fig. 2: Illustration of the wicking and the debinding process steps (I-IV) for the transformation of the rod into the tube: lateral and cross-sectional view of the body (above and below respectively).
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In this study, a new inexpensive and simple production process has been developed to produce the anode microtubes used as the main support structure in microtubular SOFC systems. The microtubes were firstly produced as a green body in a rod shape with a diameter of 2.27 mm by extrusion. The rods were transformed into microtubes after the debinding of green body by controlling of the wicking process and by sintering at a certain heat treatment temperature. The mechanical properties and the microstructure of the tubes were investigated as a function of the wall thickness, the sintering temperature and also to the content of the binder and ceramic powders.
2. Experimental procedures 2.1 Materials Commercial nickel oxide with gadolinium doped ceria (NiO-GDC) powders with 60 wt.% of NiO, cyclohexanone, polyethylene (PE) and stearic acid were used as the main phase, solvent, binder, and surfactant with a purity of +99% and 98% from NexTech and Alfa Aesar respectively.
2.2 Production of the anode microtubes The microtubes were initially produced as a green body in a rod shape with a diameter of 2.27 mm by extrusion of the homogeneous mixtures of the anode powders and polymer binders. Three different batches, batch 1, 2 and 3 containing 20 wt.%, 16.7 wt.% and 13.4 wt.% binder+surfactant mixtures respectively, given in Table 1, were used to produce the green rods by an extruder as starting materials to fabricate the anode microtubes. PE was dissolved in cyclohexanone during the mixing for 30 min. at 70°C. NiO-GDC powders and stearic acid were added to the PE solution and stirred for a short time. The prepared batch mixtures were transferred into suitable containers where the homogenization took place by ball milling. Retsch PM 200 planetary mill was used to generate the homogeneous mixture for 20 minutes. After the mixing process, the solvent in suspension mixture was evaporated during the stirring at 90°C in air atmosphere. The dried powders coated by polymer mixtures were placed in the feeder for continuous extrusion process with the screw system and extruded in a rod shape with a diameter of 2.27 mm by a Mini-Extruder from Rondol Technology Ltd. Five different temperature zones were used during the extrusion of the rods as shown in Table 2. 5
Table 1: Compositions of the prepared batches. Table 2: Temperature zones of extrusion process steps. The rods were transformed into microtubes after the debinding of the green bodies by controlling of the wicking process with a capillary flow effect as shown in Fig. 2 and by sintering at a certain heat treatment temperature given in Fig. 3. The produced green rod bodies were placed in the NiO-GDC loose powders on an alumina plate and embedded in NiO-GDC loose powder mass to prevent the distortion of the rod bodies during the debinding by heat treatment at 600°C in an oven. The embedded rod bodies were sintered at two different temperatures of 1250°C or 1400°C for 5 hours with firing regimes as shown in Fig. 3 to obtain the anode microtube structure with a certain porosity between 3040%.
Fig. 3: Debinding and sintering regimes of the samples.
2.3 Characterization The morphology and microstructure on the outer surface and cross-section surface of the sintered and broken tubes was studied using Philips XL 30 SFEG Scanning Electron Microscopy (SEM). Threepoint bending method was used to investigate the effect of binder content and the sintering temperature on the mechanical properties of sintered tubular anode supports. The mechanical strength tests were performed using an Instron 5569 tensile tester on the five different samples with a length of 25 mm. The bending strength (
) was calculated using the following equation: [25,39]
where F is fracture force; L, D1, and D2 are the lengths, the outer and inner diameter, respectively. The specific surface areas (SSA) of the sintered samples were determined via the BET method using a Quantachrome Autosorb 1 instrument. The porosity of the sintered anode microtubes was determined by using Archimedes’ principle. The properties of the sintered microtubular anode support were studied with respect to the content of the binder and ceramic powders, the sintering temperature, the SSA, porosity, also the wall thickness, and mechanical strength. 6
3. Results and discussion 3.1 Microstructure of the anode microtubes The microtubes with tubular structure were produced successfully for the first time in this study by the debinding process with a wicking force at 600°C and then by sintering process performed at 1250°C or 1400°C. The shape, structure and surface morphology of the final product are highly dependent on the constituents of the starting materials and also the preparation conditions. Although Ismael et al. [31] produced the extruded lead zirconate titanate fibers in dense rod body by debinding at 600°C and by sintering at 1100°C previously, we produced the microtubes with desired interior hollow structure without any cracks by debinding and then sintering processes successively as described above. The thermal behavior of the polymer was also the determining factor in this case [27,30,33]. During the heat treatment, the binder within the embedded rod generated the main flow channels due to the ongoing thermal expansion and the acting capillary force. The binder in the fluid form flowed out of the compact via a capillary force with the wicking effect which is dependent on the binder form and the viscosity that is decreasing with the rise of the heat treatment temperature [27,30,33]. As mentioned above, those ceramic particles in the extrudate were expected to be dragged through the surface by capillary effect as shown in Fig. 2. Summers et al. [28] depicted that “all of the debinding processes described previously leave pendular bonds, giving the component sufficient strength to be handled prior to sintering”. Therefore, “the pendular bonds must be removed by evaporation prior to sintering and cannot be removed by wicking” [28]. In our study, the debinding and sintering processes were successively performed and there was no noticeable deformation problem. The transformation of the rod body to the tubular structure took place as anticipated and the tubular anode supports with closed in both ends were obtained without any deformation as shown in Figs. 4-6. When both sides of the closed tube were cut-off, the microtube with both open ends was produced as shown in Figs. 5 and 6. But, a small number of large open porosities were formed on the surface of the tubes marked with red circles on the image as shown in Fig. 6a, because it was believed that the main flow of the liquid binder and/or gas phase occurred through the big channels on the surface during the heat treatment at the debinding temperature. Furthermore, the residues of the surrounding powders on the surface of the
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tubes were observed as defects as shown in Fig. 5 and 6 b-d. The debinding process can be optimized to obtain the microtubes with desired surface properties, the microstructure, the porosity size, and distribution. In addition, if the surfaces of the sintered anode tubes are sanded with 320 mesh or more suitable sandpaper, the porosity structure and distribution in the anode surface can be homogenized which will enhance a smoother surface. As a result of this, the surface of the anode is ready to be coated with electrolyte. Coating of the electrolyte and the cathode layer on the anode can be done either by dip coating or slurry coating techniques. After the coating of each layer (electrolyte and cathode), the final microtube is produced by sintering all layers. Determining the sanding procedures and conditions for the examination of anode and electrolyte interface properties are a separate research topic. Fig. 4: Photo images of the sintered and untreated anode microtubes produced from the extruded green rod body. Fig. 5: SEM image of the anode microtube produced from the extruded green rod body and sintered at 1400°C. Fig. 6: SEM images of the anode microtubes sintered at 1400°C: outer surface (a), and cross sections of the samples batch 1 (b), 2 (c) and 3 (d).
From a geometrical perspective, the geometries and especially the wall thickness of the tubes were not perfectly regular as expected, although all green rod bodies had a diameter of 2.27 mm. After sintering, the tubes have different diameters in inverse proportion to the content of the binder as shown in Fig. 5b-d. The mean wall thicknesses of the samples from the batch 1, 2 and 3 were 219 µm, 272 µm, and 314 µm respectively. However, these results obtained in this study are quite successful as an initial process trial outcome. The wall thickness could be adjusted and the desired uniform wall thickness could be achieved in further studies, by optimization of the debinding process as explained in the literature [27,28,30,33] and also by changing of the amount of the binder mixtures and by optimization of the binder and ceramic powder properties. On the other hand, the execution time of the debinding process must be optimized.
8
Carrying out a rapid debinding process (in a very short period of time), as the particles forming the anode structure will be dragged out of the center all the way outwardly (Fig. 2) all together rapidly and depending on the conditions, the tube produced will have a larger internal diameter, a thinner wall thickness and lower porosity ratio will result in the production of a high bulk density tube. Rapid debinding can result in a complete deformation. If the debinding process is carried out slowly, the NiO-GDC particles will not be dragged at a lower speed because of the controlled movement of the organic components depending on the binder properties. As a consequence, the tube produced will have both a smaller inner diameter and a larger wall thickness. Due to the slow movement of the particles, an anode structure with higher pores will be obtained due to the gas spaces between the particles. One of the most important results obtained in this study is that; when the binder ratio in the batch is too high, an excessive amount of gas generated during debinding causes NiO-GDC particles to move more strongly and cause particles to be pressed. As a result of the pressure, both a dense and less porous anode microstructure (high bulk density) can be obtained. Therefore, the mechanical strength of the produced anode microtubes will be higher. However, the only critical criterion is not the mechanical strength; the electrochemical performance of the cells obtained should be taken into consideration while optimizing the pore size and quantity of the produced tubes. On the other hand, the design of the heat treatment furnace, the heat transfer in the furnace, the placement of the green rod bodies in the powder, the type, quantity and properties of the binder, the grain size and size distribution of the NiO-GDC powders also affect the wall thickness of the produced tubes and the homogeneity of the structure. Effects of all given parameters on the wall thickness and the homogeneity of the microstructure will be investigated in a separate study.
3.2 Mechanical strength of the anode microtubes The lifetime of a fuel cell is highly dependent on the mechanical strength of the cell tubes [25]. The three-point bending strength tests of the anode microtubes as a support of the SOFC system were performed on the samples which were prepared with different binder content and sintered at 1250°C and 1400°C as shown in Table 1 and Fig. 3 respectively. The mechanical bending strength results display a proportional relationship with the sintering temperature, thickness, and density of the anode 9
support tubes as shown in Fig. 7 and they are comparable to values reported in the literature [25]. The thicknesses of the anode microtubes provided in Figs. 5 and 6 are inversely proportional to the binder content of the green rod bodies given in Table 1. The sample batch 1 with 20 wt.% binder content has shown best performance at both temperatures. The mechanical strength of the samples has been improved dramatically with the increasing sintering temperature. The samples sintered at 1400°C have demonstrated the best mechanical performance with an average value of 164.52 MPa (min. 164.22, max. 165.34 MPa) as shown in Fig. 7. Yang et al. [14] explained that the samarium doped ceria (SDC) microtubes have a bending strength from 20 MPa to 208 MPa increased as a function of the sintering temperature between 1300°C and 1500°C and the microtubes were found to be very fragile when the sintering temperature was lower than 1300°C. As the heat treatment temperature gets higher, the sintering effect becomes clear and thus the strength of the tubes is increased. The sintering effect of the heat treatment by sintering at 1250°C and 1400°C is clearly evident from the grain growth of the particles (crystallite size) according to the peak intensity determined by XRD as shown in Fig. 8-a and -b. The sintering temperature has an important influence on the mechanical bending strength and on the densification of the anode microstructure which was obtained by decreasing of the SSA and porosity determined by BET and Archimedes methods as shown in Fig. 9. However, it should not be forgotten that the only critic criteria for microtubular SOFC cells is not the mechanical strength and the electrochemical cell performance is one of the most important criteria. After the reduction of the anode microtubes in a hydrogen atmosphere, reduction of NiO to metallic Ni was expected [5,10,18,24]. However, as Maher et al. [40] explained, it was seen that GDC was reduced also to metallic Ce as shown in Figure 8c after reduction by heat treatment at 700°C in a dry hydrogen atmosphere at a pressure of 2 bar. Fig. 7: Mechanical bending strength of the samples prepared with different binder+surfactant contents (batch 1: 20 wt.%, batch 2: 16.7 wt.% and batch 3: 13.4 wt.%), and sintered at different temperatures.
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Fig. 8: XRD analysis of the NiO-GDC samples sintered at 1400°C (a), 1250°C (b) and then reduced at 700°C in a hydrogen atmosphere (c).
3.3 Active surface area of the anode microtubes SSA of the anode microtubes determined by BET method is not directly proportional with the binder content. The prepared prescriptions have met the expectations depending on the sintering temperature and binder contents. The expected declines for SSA were observed for all samples that sintered at 1400°C as shown in Fig. 9. The decreasing of the SSA means that the sintering effect occurs between the particles in the microstructure, the particles are strongly bonded together and the anode tube has the desired high mechanical strength. Considering the rise in the mechanical strength of the sample from batch 3, this was over threefold while SSA decreased only by 11% at 1400°C. Fig. 9: SSA and porosity (POR) results of the samples prepared with different binder+surfactant contents (batch 1: 20wt.%, batch 2: 16.7 wt.% and batch 3: 13.4 wt.%) and sintered at different temperatures. 4. Conclusions Tubular anode supports have been manufactured via a new approach in which the debinding and sintering processes were successively performed with a remarkably shortened batch preparation time. The self-transformation of green rod body to the anode tube has been accomplished, by the debinding of the binder using the capillary force of the melted polymer. Reorientation of ceramic powder within the melted polymer has allowed the body to change its geometry. It is found that the wall thickness of the tubes can be tailored by changing the binder to powder ratio, the wall thickness changed inversely proportional to the binder content. The obtained mechanical strength values of the sintered anode microtube supports were comparable to the results in the literature. The SSA tends to decrease as the sintering temperature increases, but the flexural strength of tubes increased. While dramatic change observed in flexural strength, attenuation of SSA was not significant. The process developed in this investigation can be optimized in further studies to produce a complete cell with a perfect geometrical tube structure with good dimensional tolerances and characterize its electrochemical properties. 11
Literatures [1] BP Statistical Review of World Energy, 2014. p. 1-2. [2] Singhal S.C., K. Kendal. High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications. 2003; ISBN 1-85617-387-9, Elsevier Ltd., Oxford, UK. [3] Yamamoto O., Solid oxide fuel cells: fundamental aspects and prospects. Electrochimica Acta. 2000;45(15–16):2423-2435. [4] Badwal S.P.S., Foger K. Solid Oxide Electrolyte Fuel Cell Review. Ceramics International. 1996;22:257-265. [5] Kim J.-H., Song R.-H., Song K.-S., et al. Fabrication and characteristics of anode-supported flattube solid oxide fuel cell. Journal of Power Sources.2003;122(2):138-143. [6] Chen Z., Takeda T., Kikuchi K., et al. Preparation of Anode/Electrolyte Ceramic Composites by Coextrusion of Pastes. Journal of the American Ceramic Society. 2004;87:983-990. [7] Sammes N.M. and Du Y., Fabrication and Characterization of Tubular Solid Oxide Fuel Cells. International Journal of Applied Ceramic Technology. 2007;4(2):89-102. [8] Sin Y.-W., Galloway K., Roy B., et al. The properties and performance of micro-tubular (less than 2.0 mm O.D.) anode supported solid oxide fuel cell (SOFC). International Journal of Hydrogen Energy. 2011;36(2):1882-1889. [9] Funahashi Y., Shimamori T., Suzuki T., et al. Fabrication and characterization of components for cube shaped micro tubular SOFC bundle. Journal of Power Sources. 2007;163(2):731-736. [10] Lee B.-T., Rahman A.H.M.E., Kim J.-H. Novel Design of Microchanneled Tubular Solid Oxide Fuel Cells and Synthesis Using a Multipass Extrusion Process. Journal of the American Ceramic Society. 2007;90:1921-1925. [11] Du Y., Sammes N.M., Tompsett G.A., Optimisation parameters for the extrusion of thin YSZ tubes for SOFC electrolytes. Journal of the European Ceramic Society. 2000;20:959-965. [12] Suzuki T., Yamaguchi T., Fujishiro Y., et al. Fabrication and characterization of micro tubular SOFCs for operation in the intermediate temperature. Journal of Power Sources. 2006;160(1):7377. [13] Yamaguchi, T., Suzuki T., Shimizu S., et al. Examination of wet coating and co-sintering technologies for micro-SOFCs fabrication. Journal of Membrane Science. 2007;300:45-50.
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[14] Yang N., Tan X., Ma Z., et al. Fabrication and Characterization of Ce0.8Sm0.2O1.9 Microtubular Dual-Structured Electrolyte Membranes for Application in Solid Oxide Fuel Cell Technology. Journal of the American Ceramic Society. 2009;92:2544-2550. [15] Yamaguchi T., Shimizu S., Suzuki T., et al. Design and Fabrication of a Novel ElectrodeSupported Honeycomb SOFC. Journal of the American Ceramic Society. 2009;92:S107–S111. [16] Roy B.R., Sammes N.M., Suzuki T., et al. Mechanical properties of micro-tubular solid oxide fuel cell anodes. Journal of Power Sources. 2009;188:220-224. [17] Powell J., Blackburn S. The unification of paste rheologies for the co-extrusion of solid oxide fuel cells. Journal of the European Ceramic Society. 2009;29(5):893-897. [18] Othman M.H.D., Wu Z., Droushiotis N., et al. Single-step fabrication and characterisations of electrolyte/anode dual-layer hollow fibres for micro-tubular solid oxide fuel cells. Journal of Membrane Science. 2010;351:196-204. [19] Othman M.H.D., Droushiotis N., Wu Z., et al. Electrolyte thickness control and its effect on electrolyte/anode dual-layer hollow fibres for micro-tubular solid oxide fuel cells. Journal of Membrane Science. 2010;365:382-388. [20] Yang C., Li W., Zhang S., et al. Fabrication and characterization of an anode-supported hollow fiber SOFC. Journal of Power Sources. 2009;187(1):90-92. [21] Gil V., Gurauskis J., Campana R., et al. Anode-supported microtubular cells fabricated with gadolinia-doped ceria nanopowders. Journal of Power Sources. 2011;196:1184-1190. [22] Hsieh W.-S., Lin P., Wang S.-F., Fabrication of electrolyte supported micro-tubular SOFCs using extrusion and dip-coating. International Journal of Hydrogen Energy. 2013;38:2859-2867. [23] Li T., Wu Z., Li K., A dual-structured anode/Ni-mesh current collector hollow fibre for microtubular solid oxide fuel cells (SOFCs). Journal of Power Sources. 2014;251:145-151. [24] Jamil S.M., Othman M.H.D., Rahman M.A., et al. Recent fabrication techniques for microtubular solid oxide fuel cell support: A review. Journal of the European Ceramic Society. 2015. 35(1): p. 1-22. [25] Li T., Wu Z., Li K., Co-extrusion of electrolyte/anode functional layer/anode triple-layer ceramic hollow fibres for micro-tubular solid oxide fuel cells-electrochemical performance study. Journal of Power Sources. 2015;273:999-1005. [26] Fernández-González R., Hernández E., Savvin S., et al. A novel microstructured metal-supported solid oxide fuel cell. Journal of Power Sources. 2014;272:233-238. [27] German R.M., Theory of Thermal Debinding. International Journal of Powder Metallurgy. 1987;23(4):237-245. 13
[28] Summers E.M., Akinc M. Wick Debinding of Molybdenum-Silicon-Boron Extrudates. Journal of the American Ceramic Society. 2000;83:1670-74. [29] Somasundram I.M., Cendrowicz A., Wilson D.I., et al. Phenomenological study and modelling of wick debinding. Chemical Engineering Science. 2008;63(14):3802-3809. [30] Clemens F. Thermoplastic Extrusion for Ceramic Bodies, Chapter 17 in "Extrusion in Ceramics". Edit. by F. Händle, 2009, Springer-Verlag, Berlin Heidelberg. [31] Gorjan, L., Dakskobler A., T. Kosmač. Partial wick-debinding of low-pressure powder injectionmoulded ceramic parts. Journal of the European Ceramic Society. 2010;30(15):3013-3021. [32] Ismael M.R., Clemens F., Wyss P., et al. Processing and Properties of Co-Extruded Lead Zirconate Titanate Fibers. Journal of the American Ceramic Society. 2012;95:108-116. [33] Gorjan L. Wick Debinding – An Effective Way of Solving Problems in the Debinding Process of Powder Injection Molding, in “Some Critical Issues for Injection Molding”, Edit. Jian Wang, 2012, ISBN 978-953-51-0297-7, Publisher: InTech. [34] Ani S.Md., Muchtar A., Muhamad N., et al. Binder removal via a two-stage debinding process for ceramic injection molding parts. Ceramics International. 2014;40:2819-2824. [35] Gorjan L., Kosmač T., Dakskobler A. Single-step wick-debinding and sintering for powder injection molding. Ceramics International. 2014;40:887-891. [36] Sharmin K., Schoegl I. Optimization of binder removal for ceramic micro fabrication via polymer co-extrusion. Ceramics International. 2014;40:3939-3946. [37] Becker F.H. Debinding processes - physical and chemical conclusions and their practical realisations.
http://www.riedhammer.de/System/00/01/42/14224/633776345920000000_1.pdf,
(visited 22.11.2017). [38] Barsoum M.W. Fundamentals of Ceramics. Series in Material Science and Engineering. 2002; CRC Press. 624. [39] Cernica J.N., Strength of materials. Published by Holt Rinehart & Winston, 1977, New York, ISBN-13: 978-0030531958. [40] Maher R.C., Shearing P.R., Brightman E., et al. Reduction Dynamics of Doped Ceria, Nickel Oxide, and Cermet Composites Probed Using In Situ Raman Spectroscopy. Advanced Science. 2016;3:1500146 (1-8).
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Tables Caption Table 1: Compositions of the prepared batches. Table 2: Temperature zones of extrusion process steps.
Figures Caption Fig. 1: Illustration of the attractive forces on binder for wetting of particles; θ<90°. Fig. 2: Illustration of the wicking and debinding process steps (I-IV) for the transformation of the rod into the tube: lateral and cross-sectional view of the body (above and below respectively). Fig. 3: Debinding and sintering regimes of the samples. Fig. 4: Photo images of the sintered and untreated anode microtubes produced from the extruded green rod body. Fig. 5: SEM image of the anode microtube produced from the extruded green rod body and sintered at 1400°C. Fig. 6: SEM images of the anode microtubes sintered at 1400°C: outer surface (a), and cross sections of the samples batch 1 (b), 2 (c) and 3 (d). Fig. 7: Mechanical bending strength of the samples prepared with different binder+surfactant contents (batch 1: 20 wt.%, batch 2: 16.7 wt.% and batch 3: 13.4 wt.%), and sintered at different temperatures. Fig. 8: XRD analysis of the NiO-GDC samples sintered at 1400°C (a), 1250°C (b) and then reduced at 700°C in a hydrogen atmosphere (c). Fig. 9: SSA and porosity (POR) results of the samples prepared with different binder+surfactant contents (batch 1: 20 wt.%, batch 2: 16.7 wt.% and batch 3: 13.4 wt.%) and sintered at different temperatures.
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TABLES
Table 1: Compositions of the prepared batches.
Components
Batch 1 (wt.%)
Batch 2 (wt.%)
Batch 3 (wt.%)
NiO-GDC powder (A)
46.6
50
53.3
Binder + Surfactant (B)
20
16.7
13.4
Solvent
33.3
33.3
33.3
A : B ratio
70 : 30
75 : 25
80 : 20
Table 2: Temperature zones of extrusion process steps.
Zones Temperature (°C)
Feeding
1
2
3
Die
89
177
170
170
165
16
FIGURES
Fig. 1: Illustration of the attractive forces on binder for wetting of particles; θ<90°.
Fig. 2: Illustration of the wicking and debinding process steps (I-IV) for the transformation of the rod into the tube: lateral and cross-sectional view of the body (above and below respectively).
17
Temperature ( °C )
1400 1200 1000 800 600 400
Regime 2
200
Regime 1
0 0
5
10
15
20
25
30
Duration ( h )
Fig. 3: Debinding and sintering regimes of the samples.
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Fig. 4:
Photo images of the sintered and untreated anode microtubes produced from the extruded green rod body.
19
Fig. 5: SEM image of the anode microtube produced from the extruded green rod body and sintered at 1400°C.
Fig. 6: SEM images of the anode microtubes sintered at 1400°C: outer surface (a), and cross sections of the samples batch 1 (b), 2 (c) and 3 (d). 20
Fig. 7: Mechanical bending strength of the samples prepared with different binder+surfactant contents (batch 1: 20 wt.%, batch 2: 16.7 wt.% and batch 3: 13.4 wt.%), and sintered at different temperatures.
Fig. 8: XRD analysis of the NiO-GDC samples sintered at 1400°C (a), 1250°C (b) and then reduced at 700°C in a hydrogen atmosphere (c).
21
Fig. 9: SSA and porosity (POR) results of the samples prepared with different binder+surfactant contents (batch 1: 20 wt.%, batch 2: 16.7 wt.% and batch 3: 13.4 wt.%) and sintered at different temperatures..
22