Scholarly article on topic 'Laser-sintered Porous Structures for Samarium-based Solid Oxide Fuel Cells'

Laser-sintered Porous Structures for Samarium-based Solid Oxide Fuel Cells Academic research paper on "Materials engineering"

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{"Green tape laser sintering" / "Nd:YAG laser" / "Solid oxide fuel cell" / "Samarium strontium cobaltite" / "Samarium-doped ceria" / "Sacrifice material."}

Abstract of research paper on Materials engineering, author of scientific article — Kazuhiko Yamasaki, Masashi Koizumi, Katsuhiro Maekawa

Abstract An attempt has been made to fabricate both fuel and air electrodes for solid oxide fuel cells (SOFCs) by utilizing the green-tape laser sintering (GTLS) method. We prepared two types of green pastes with a mixture of samarium strontium cobaltite (SSC) powders for the cathode, NiO/SDC (sub-micrometer-sized samarium-doped ceria) cermet powders for the anode, polystyrene particles as sacrifice material, polymer binder and solvent. The mask printing method was formed an electrode film on the SDC substrate. The printed paste was dried at 433K for 60min in an electric furnace, before a pulsed Nd:YAG laser was irradiated in a row on the pattered sample. The GTLS parameters were set at 1.06μm in wavelength, 80Hz in pulse frequency, 0.6-3.0ms in pulse width, 0.1-0.5mJ in pulse energy, about 0.4mm in diameter of beam spot, and 3.3-16.7mm/s in scan speed. A porosity of 22% in the SSC film has been attained although porous microstructures are influenced by pulse energy, pulse width, the defocus amount of laser beam, and substrate temperature during irradiation. The samarium-based electrode structure is less porous than conventional YSZ-based one. Adhesion between the electrolyte and the electrodes was insufficient.

Academic research paper on topic "Laser-sintered Porous Structures for Samarium-based Solid Oxide Fuel Cells"

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Materials Science

Procedia Materials Science 4 (2014) 98 - 103 ^^^^^^^^^^^^^^

www.elsevier.com/locate/procedia

8th International Conference on Porous Metals and Metallic Foams, Metfoam 2013

Laser-sintered porous structures for samarium-based solid oxide

fuel cells

Kazuhiko Yamasaki*, Masashi Koizumi, Katsuhiro Maekawa

Department of Mechanical Engineering, College of Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Ibaraki 316-8511, Japan

Abstract

An attempt has been made to fabricate both fuel and air electrodes for solid oxide fuel cells (SOFCs) by utilizing the green-tape laser sintering (GTLS) method. We prepared two types of green pastes with a mixture of samarium strontium cobaltite (SSC) powders for the cathode, NiO/SDC (sub-micrometer-sized samarium-doped ceria) cermet powders for the anode, polystyrene particles as sacrifice material, polymer binder and solvent. The mask printing method was formed an electrode film on the SDC substrate. The printed paste was dried at 433 K for 60 min in an electric furnace, before a pulsed Nd:YAG laser was irradiated in a row on the pattered sample. The GTLS parameters were set at 1.06 pm in wavelength, 80 Hz in pulse frequency, 0.6-3.0 ms in pulse width, 0.1-0.5 mJ in pulse energy, about 0.4 mm in diameter of beam spot, and 3.3-16.7 mm/s in scan speed. A porosity of 22% in the SSC film has been attained although porous microstructures are influenced by pulse energy, pulse width, the defocus amount of laser beam, and substrate temperature during irradiation. The samarium-based electrode structure is less porous than conventional YSZ-based one. Adhesion between the electrolyte and the electrodes was insufficient.

© 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of Scientific Committee of North Carolina State University

Keywords: Green tape laser sintering; Nd:YAG laser; Solid oxide fuel cell; Samarium strontium cobaltite; Samarium-doped ceria; Sacrifice material.

CrossMar]

* Corresponding author. Tel.: +81-294-38-5278; fax: +81-294-38-5278. E-mail address: kyama@mx.ibaraki.ac.jp

2211-8128 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of Scientific Committee of North Carolina State University doi: 10.1016/j.mspro.2014.07.607

1. Introduction

Metal or ceramics porous structures are utilized in various industrial products, such as shock absorbers, filters, lightweight vehicles, and fuel cells. In particular, solid oxide fuel cell (SOFC) necessitates metal/ceramics porous structures in anode/cathode electrode films.

Porous structure

Anode (Ni etc)

Electrolyte (YSZ Ceramics,

Cathode

((La1.x,Ax-)Mn03 (A=Ca,Sr;0x0.5),

Fig. 1. Schematic of solid oxide fuel cell (SOFC).

Fig. 1 schematically shows the principle of power generation by an SOFC. The cell basically has three layers: anode electrode, electrolyte, and cathode electrode, being composed of ceramics, and metal/ceramic or cermet (Singhal (2000), Tietz et al. (2002)). These three layers produce three-phase boundaries (TPBs) where anode/cathode electrode, electrolyte, and fuel/air meet. Chemical reactions take place at TPBs, which generates electricity. The oxygen (1/2O2) contacted with the TPBs on the air electrode side is separated into O2- to give two electrons. The O2-passes through the electrolyte layer, and binds with hydrogen (H2) at the TPBs on the fuel electrode side. These reactions lead to the formation of water (H2O) and two electrons. Consequently, power generation efficiency at the SOFCs depends on the length of TPBs and ion or electrical conductivity of electrolyte layer (Singhal (2000)). Understandably, porous structure with many TPBs is preferable to achieve high generating efficiency.

Yttrium-stabilized zirconia (YSZ) is widely used as an electrolyte, and NiO/YSZ cermet and lanthanum strontium manganite (LSM) are well adapted to the anode electrode (fuel electrode) and the cathode electrode (air electrode), respectively (Singhal (2000), Tietz et al. (2002), Joo and Choi (2006)). Traditional SOFCs with these materials operate at a high temperature of around 1,273 K. For middle- or low-temperature operation below 1,073 K, a samarium-doped or lanthanum-gallate-based electrolyte has been developed (Han Chen et al. (2011), Hong et al. (2013)). These electrolyte and electrode films for SOFCs have been manufactured by various methods, such as tape casting, slurry coating, electrochemical vapor deposition, and laser deposition (Singhal (2000), Tietz et al. (2002), Joo and Choi (2006), Han Chen et al. (2011), Hong et al. (2013)).

Recently, the green-tape laser sintering (GTLS) method has been proposed, enabling us to fabricate various structures with controlled porosity and pore size (Maekawa et al. (2007), Yamasaki and Maekawa (2010)). This laser processing can be utilized to make porous electrode films for SOFCs on an electrolyte substrate, where the size and location of the electrode can be adjusted as well. In the present study, the application of the GTLS method to the fabrication of porous structures for samarium-based SOFCs is investigated.

2. Experimental

YSZ has structural stability and a high ion-conductivity (electrical conductivity) at high temperatures: e.g., about 200 mS/cm at 1,273 K, so that high total generation efficiency together with cogeneration systems is obtained at high temperature operations around 1,073-1,273 K. High-temperature operation, however, give rise to some shortcomings, such as long start/finish time, the generation of thermal stress and crack during operation. Recently, many researches have developed materials for middle and low temperature operation of SOFCs (Han Chen et al. (2011), Hong et al. (2013)) : e.g., samarium-doped ceria (SDC), which has a high ion conductivity (electrical

conductivity) of higher than 1 mS/cm at 1,073 K. NiO/SDC cermet and samarium strontium cobaltite (SSC) are selected as the anode electrode and the cathode electrode, respectively, because coefficients of thermal expansions (CTEs) of these electrode materials are close to CTE of the SDC-type electrolyte.

In the present study, two kinds of green tapes were prepared: one made from NiO/SDC cermet powder (NiO/Sm0.2Ce0.8Ox, NiO/SDC, AGC Seimi Chemical Co., Ltd.) with 0.99 ¡¡m in diameter for the fuel electrode, and another from SSC powder (Sm0.5Ce0.5CoOx, SSC, AGC Seimi Chemical Co., Ltd.) with 1.0 ¡m in diameter for the air electrode. The electrolyte is a sheet of SDC (Sm0.2Ce0.8Ox, SDC-20, Japan Fine Ceramics Co., Ltd.) with 1 mm in thickness.

The green pastes were prepared by mixing these metal/cermet or ceramic powder with ethyl-cellulose as a polymer binder, ethanol, and oil. The polymer binder helps to keep the shape of the coated tape during the drying process. For the purpose of adjusting porous structures, we put additional particles into the paste, such as monodisperse polystyrene (PS, SX-350H, Soken Chemical & Engineering Co., Ltd.), with 3.5 ¡m in diameter, and graphite (GP, SGO-1 grade, SEC Carbon Ltd.) with 1-10 ¡m in diameter. Around 30vol% of each sacrifice material was mixed in each green paste. Fig. 2 shows SEM images of the sacrifice powders. It can be seen that PS is sphere-shaped, and GP is leaf-shaped.

These pastes were directly coated on the SDC electrolyte sheet by the mask printing method (Maekawa et al. (2007), Yamasaki and Maekawa (2010)). The thickness of a mask was set at about 22 ¡m. After being dried at 433 K for 60 min in an electrical furnace, the thicknesses of these green tapes were reduced to around 18 ¡m.

Laser sintering was carried out by irradiating a high-energy pulsed Nd:YAG laser to these green tapes coated on the SDC substrate. Fig. 3 shows a schematic of the GTLS method. In this experiment, a 300W-class laser system (LU300, Hitachi Construction Machinery Co., Ltd.) was employed with a chamber located on a CNC X-Y table. In order to sinter a wide area of the tape, a multimode beam with a top-hat shaped in intensity distribution was used.

The tape coated on the SDC-20 substrate was put on a hotplate to control temperature of the substrate during irradiation. The GTLS was carried out by scanning a pulsed Nd:YAG laser in a row under the conditions of 1.06 ¡m wavelength, 80 Hz pulse frequency, 0.6-3.0 ms pulse width, and about 0.4 mm beam spot diameter. The temperature range of the substrate surface was around 293-467 K. Sintering phenomena are accelerated as the substrate temperature is raised.

The pulse energy range was around 0.1-0.5 mJ, and the range of scan speed was around 3.3-16.7 mm/s (2001,000 mm/min). The overlapping rate of each pulse was estimated around 0.8, depending on the scan speed of the X-Y table, pulse frequency, and the spot size of the beam. Besides, to control the beam fluence, the focal point of the beam, Df, was set at a deeper position up to 16 mm. In the case of no substrate heating, a moderate laser irradiation was carried out twice at the same position, not only to obtain a firm adhesion between the porous films and the substrate, but also prevent cracking within the substrate. Such damage is induced by a rapid temperature change with a giant-pulse irradiation.

(a) v * < C * / 'Irrt-y rrA\ //#>1 v \ k

1 3 nm 3 um 1 ¿d * «^l

Fig. 2. SEM images of sacrifice powders: (a) polystyrene and (b) graphite powders.

Nd:YAG laser Wavelength: 1.06 Pulse widths: 0.6-3.0 ms Pulse frequency: 80 Hz

Green tape Substrate

Fig. 3. Schematic of green tape laser sintering (GTLS) method

Macro structure of the laser-irradiated film surface was examined by a laser-scanning microscope (an optical microscope; VK-8700, Keyence Corporation), whereas microstructure by a scanning electron microscope (SEM; VE-9800, Keyence Corporation). Porosity on the film surface was measured by image analysis software, in which a binary image of the surface is generated so as to fit modified (black) area to the one of pore at the surface in the original SEM image. A margin of error in the calculated porosity was within a few % because that undercut level was decided by a human operator in the modification process.

Surfaces of the laser-sintered film were observed by a laser-scanning microscope (LSM), and analyzed with an energy dispersive X-ray analysis (EDX; EMAX ENERGY EX-350, Horiba Ltd.).

3. Results and discussion

First, feasibility of the GTLS for cathode films was examined without using the sacrifice materials. The laser processing parameters were set as follows: 80 Hz pulse frequency, 1.8 ms pulse width, and 3.3 mm/s (200 mm/min) scan speed. The irradiated mean pulse energy, E, was around 0.14 mJ, and substrate temperature was 467 K.

Fig. 4 shows SEM images of laser-sintered SSC film on the SDC-20 substrate. In this case, Df was set at a deeper position to control the beam fluence: 8 mm (Fig. 4 (a, b)), 10 mm (Fig. 4 (c)), and 16 mm (Fig. 4 (d)). At Df= 8 mm, the pulse energy that is necessary for the evaporation of the polymer binder and the agglomeration of the powders in the green-tape was about 0.136 mJ. This value is similar to that in the case of YSZ-based materials (Maekawa et al. (2007)).

(a) E= 0.136 mJ, Df= 8 mm (b) E= 0.15 mJ, Df= 8 mm

(c) E= 0.154 mJ. Df= 10 mm (d) E= 0.154 mJ. D£= 16111111

Fig. 4: SEM images of laser-sintered SSC films on SDC-20 substrate at substrate temperature of 467 K.

As can be seen in the figure, the SSC ceramic materials are mostly melted because of a high-energy input. The structure becomes more porous with increasing Df, and beam fluence or Df influences porosity. As a result, the SSC film has a porosity of up to 11%, whereas the LSM cathode film that was laser-sintered on the conventional YSZ electrolyte attained up to 34% (Maekawa et al. (2007)). The thickness of the SSC films are estimated a few micrometers.

A firm adhesion between the electrode and the substrate is required for SOFC applications. It was found that adhesivity tended to increase with increasing substrate temperature. However, not only poor adhesion was recognized due to incomplete sintering of the ceramic material in the vicinity of the substrate, but also cracking within the electrolyte often took place at the substrate temperature of 467 K. These issues may be resolved by the use of thinner green-tapes below 10 p,m and a rapid cooling to room temperature.

As for the NiO/SDC anode electrode, it was not successful in obtaining porous structures by the GTLS process; porosity of the film was about 2-3% only. It seemed that nickel in the NiO/SDC paste was first decomposed, and then aggregated or flowed within the film before the SDC particles were fully sintered. Further investigation is needed from the viewpoint of temperature management, including adjusting laser parameters; beam defocusing, and substrate temperature.

(a) PS, Df= 16 mm, 298 K (b) PS, Df= 16 mm, 467 K

Fig. 5: SEM images of laser-sintered SSC films on SDC-20 substrate when using PS (a, b), and GP (c).

(a) SEM image, E= 0.15 mJ, Df= 2 mm

Fig. 6: EDX analysis of a laser-sintered NiO/SDC film on SUS316 substrate.

Aiming at increasing porosity in the electrode, we used the sacrifice materials in the process of GTLS. Fig. 5 shows SEM images of laser-sintered SSC films electrode on the SDC substrate when using PS (a, b) and GP (c). In the case of no substrate heating, the porosity of the SSC film was 20% although PS particles remain here and there inside the film. On the other hand, in the case of substrate temperature of 467 K, porosity of the laser-sintered SSC film was about 22%. It is not clear that there appears the effect of PS during the GTLS process because the substrate heating may have evaporated the particles.

As for graphite particles, the porosity of the film was about 13 %. The effect of GP addition is very limited although the oxidation of carbon takes place over 973 K.

EDX analysis of the laser-sintered NiO/SDC film was carried out. Instead of using the SDC substrate, the anode film was fabricated on the SUS316 substrate (0.2 mm in thickness) under the following conditions: 80 Hz in pulse frequency, 2.0 ms in pulse width, about 0.158 mJ in pulse energy, and 3.3 mm/s (200 mm/min) in scan speed.

In Fig. 6, the laser-sintered NiO/SDC film is located on the left-hand side. Laser sintering appears to be successful in the SEM image (Fig. 6(a)). Some elements, such as Sm(see in Fig. 6(b)), Ce (see in Fig. 6(c)), Ni (see in Fig. 6(d)), and Fe (see in Fig. 6(e)) are detected on the surface. The aggregated nickel can bee seen in Fig. 6(d), which means that temperature may instantaneously exceed the melting point of Ni.

4. Conclusions

Manufacturing of electrode films for SOFCs by the GTLS method using samarium-doped metal/ceramic powder have been developed. Controlling Nd:YAG laser conditions and using sacrifice materials have achieved the porous SSC films with about 22% in porosity. This porosity on the SDC substrate is lower than that on the YSZ substrate. On the other hand, the laser-sintered NiO/SDC film has a poor porosity of a few percent. Adhesion between the electrolyte and the electrodes is insufficient at the present stage.

References

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Films Prepared by Screen Printing Method, Journal Electrochem Society, 160, F375-F380. Joo, J. H., Choi, G. M., Electrical conductivity of YSZ film grown by pulsed laser deposition, Solid State Ionics 177, 1053-1057 Maekawa, K., Hayashi, T., Hanyu, K., Umeda, K., Murakami, T., 2007. The Spark Plasma Sintering Method using Laminated Titanium Powder

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Tietz, F., Buchkremer, H. P., Stover, D., 2002. Components manufacturing for solid oxide fuel cells. Solid State Ionics 152-153, 373-381. Yamasaki, K. Maekawa, K., 2010. Fabrication of electrode films for Solid Oxide Fuel Cells by the Laser Sintering Method. Proceedings of 2010 ISFA 2010 International Symposium on Flexible Automation, Tokyo, Japan, JPS-2555.