Paper
Development of Corrugated Ceramic Sheet for SOFC Electrolyte by Micro Imprint Process
2016 Volume 63 Issue 7 Pages 519-523
Details
2016 Volume 63 Issue 7 Pages 519-523
Yttria-stabilized zirconia (YSZ) has been used for an electrolyte of solid oxide fuel cells (SOFC). To enhance the efficiency of SOFC, we developed a corrugated, or wavy-shaped, YSZ sheet for the electrolyte. As the corrugated sheet has larger surface area than a flat-type sheet, higher energy density can be obtained. We have proposed micro powder imprint (μPI) with multi-layer imprint process to fabricate micro scale pattern on the both surfaces of a thin YSZ sheet. The μPI is a combined process of nano imprint lithography and powder metallurgy; the resolution is high, and the process is mass-productive. In this work, we selected a compound material containing YSZ powder and a binder consisting of thermoplastic resin as a starting material. The compound sheet was prepared by tape casting from slurry and was imprinted by a fine-patterned mold with stacked on a silicone rubber sheet. The silicone rubber was so flexible that micro patterns on the both sides of the compound sheet was obtained after imprint. In the present work, the process condition of μPI and the heat program of debinding and sintering were also considered. As a result, a wave-type sintered YSZ sheet without significant defects was successfully obtained.
SOFC (Solid Oxide Fuel Cells) is a kind of fuel cells. SOFC has highest efficiency in fuel cells and mainly used for power plants. Fig. 1 shows schematic of SOFC, which is composed of anode, cathode and electrolyte made of ceramics. Some reports about the interface structure of electrodes and electrolyte have been reported by H. Iwai1–3). Today, yttria-stabilized zirconia (YSZ) sheet with thickness of under 100 μm is widely used for the electrolyte. In Iwai’s reports, a wavy-electrolyte layer was developed to improve the reaction efficiency by expansion of the surface area of the sheet and to maximize current density by decreasing movement resistance of oxygen ion. As the YSZ is fragile and difficult to be machined, we propose a new method to form a wavy electrolyte layer. The aim of our study is to develop a processing method of the thin and wavy YSZ sheet for SOFC electrolyte. We will introduce advanced process based on micro powder imprint (μPI)4–8) in this paper.
Image of SOFC.
μPI is a combined process of nano imprint lithography (NIL)9) and powder metallurgy. NIL is a process to transcribe a nano-pattern on a resin film using a mold, and has been proposed by S. Y. Chou. The process can produce a fine pattern on the surface in high resolution, and is mass-productive. NIL is a process for polymer as a work material, while μPI is a process to process inorganic materials. In μPI, starting material is a compound sheet containing powder of inorganic materials and resin as a binder. Fine pattern can be transcribed on a compound sheet by pressing with a mold as similarly with NIL. After imprinting, the resin in the compound is decomposed by heating, and inorganic powder is bonded together during the following sintering step.
Authors already have reported fabrication of electrolyte sheet which has micropattern on one side of the sheet by μPI. In this study, manufacturing process of the wavy electrolyte is proposed. The process is composed of 3 processes. The first process is sheet casting for production of compound sheets to be worked. The second process is imprinting. In this pressing process, compound sheet was put on a soft rubber sheet to shape a micropattern not only on one side of the sheet but also the other surface. Finally, debinding and sintering process are performed as the third process. By controlling a heating program, a wave-type sintered YSZ sheet without defects could be obtained.
YSZ compound sheet was fabricated by sheet casting10). At first, slurry of YSZ powder and binder was prepared with water solvent, and the slurry was applied onto substrate. After drying, a compound sheet was formed. In this paper, YSZ powder (TZ-8Y, Tosoh Corp.), poly-vinyl alcohol (PVA, degree of polymerization 500) and glycerin were prepared. PVA is main component of binder, and glycerin acts as plasticizer to control formability of the compound.
The YSZ powder was supplied as granulated powder. To break up the granules and agglomerates, the powder was milled with a beads mill apparatus after adding dispersant (A-6270, Toagosei Co., LTD.) to avoid reagglomeration. Fig. 2 shows particle size distributions before and after milling. The particle size of the premixed powder, or granule, was about 300 nm. After 15 min milling, the peak shifted to about 100 nm, and particle size distribution became narrow. This result shows that agglomeration of the original powder was well broken.
Particle size distribution of YSZ.
YSZ slurry after milling was mixed with PVA, glycerin and water using a stirring deaerator (SK-350T, SHASHIN KAGAKU Co., LTD.). The ratio of YSZ, PVA and glycerin in the final compound sheet was controlled to be 50:35:15 in volume. Water was added so that the ratio of water and other materials was 7:2 in mass. After stirring, the slurry was formed into a sheet by a table coater. The gap between a blade and a substrate polyethylene terephthalate (PET) sheet was set to 400 μm, and the forwarding speed of the blade was set to 33 mm/s. The applied slurry was heated for drying out water by an oven at 80 °C for 300 s. Thickness of the obtained compound sheet was 50 μm.
In imprinting, the compound sheet was put on top of a silicone rubber sheet and imprinted by fine patterned mold as shown in Fig. 3. The silicon sheet under the compound sheet plays a role as a “deformable mold”, and wavy patterns are obtained on the both sides of the YSZ compound sheet.
Imprint process.
Fig. 4 shows an image of the mold used for imprinting. Polyimide sheet was formed by laser machining, and was used as a mold. As polyimide is heat-resistant material which is available at a temperature over 200 °C, so that it can be used as a material for the mold of imprinting. The pattern of the prepared mold was roundish triangular line and space. The depth of valley was 60 μm and the pitch of line was 135 μm.
Image of the mold.
A silicone sheet of 100 μm thickness was prepared for the lower layer. Rubber hardness degree of this sheet was 47. Silicone is suitable for the present use since it is superior in heat resistance and chemical stability, and can deform largely. Also silicone has low surface energy that means easy to be removed from the sheet sample after imprinting.
Imprinting was performed by servo press machine (DT-J330, Micro Fabrication Laboratory, LLC.) with heater attached on the press stage. Mold, YSZ compound sheet and silicone sheet were stacked and were put in metallic container to prevent from flowing out of the compound material. The imprinting temperature was set to 120 °C which is higher than the softening point of compound sheet. The press keeping time was 20 s.
Imprinting depth was precisely controlled by the servo press machine. Fig. 5 shows cross sections after imprinting with changing imprinting depth. Fig. 5 (a) shows imprinted sample with 60 μm imprinting depth which was same as the depth of valleys of the mold. In this case, depth of the pattern on the upper surface was 30 μm, and that on the lower surface was only 5 μm because of contraction of the silicone sheet. Fig. 5 (b), (c), (d) and (e) correspond to imprinting at 70, 80, 90 and 100 μm, respectively. Depth of patterns was deeper as imprinting depth was increased. Fig. 6 shows relationship between imprinting depth and depth of patterns. Depth of patterns reached the depth of the mold when the imprinting depth was 100 μm. Fig. 7 shows a cross-sectional image of imprinted YSZ sheet. Wavy shape was formed without any defect. The depth of the pattern of the upper side, or mold side, was 60 μm; i.e. the shape of the mold was transcribed exactly onto the YSZ compound. It is noted that there was no damage in the mold and the lower silicone sheet after imprinting, and they were reusable.
Images of patterns by changing imprinting depth; (a) Imprinted sample with 60 μm imprinting depth, (b) 70 μm, (c) 80 μm, (d) 90 μm, and (e) 100 μm.
Result of imprinting tests.
Result of imprinting depth with 100 μm.
After sheet samples were formed, binder was removed and samples were sintered using an electric furnace in air atmosphere. While debinding and sintering, YSZ compound sheet bends under the influence of heat. To prevent a warpage, the compound sheet was put between yttria plates.
During debinding process, organic materials are decomposed into gas materials, which sometimes cause high inner pressure in the compact to break up the sample. To prevent the rapid gas formation, temperature program was considered using thermogravimetric analysis (TGA). Glycerin and PVA begins to decompose at 200 °C and 300 °C, respectively, and both of them are completely removed at 600 °C. From this result, we set keeping temperatures at 200 °C, 350 °C, and 600 °C each for two hours in debinding process. The rate of temperature increase in debinding was set to 50 °C/h.
YSZ powder was sintered after debinding. YSZ is sintered at over 1300 °C in general11,12). We set the sintering temperature at over 1400 °C and the rate of temperature increase in sintering to 100 °C/h. Fig. 8 are SEM images of a surface of sintered YSZ at 1400 °C for 4 h (a) and at 1450 °C for 20 h (b). There are many pores on the surface of sintered at 1400 °C for 4 h (a). The size of pores are about 400 nm and this pores may lead to a leak of hydrogen which is inadmissible for SOFC. The number of pores on the surface of YSZ sintered at 1450 °C for 20 h (b) decreased. This is because sintering was promoted by heating at higher temperature for longer time13,14). This result means the sample sintered at 1450 °C for 20 h was fully dense.
The surface of sintered YSZ.
Fig. 9 shows SEM image of a cross section of the sintered YSZ sheet. The sample was sintered without distortion or bending. The thickness of sintered YSZ sheet was 30 μm. From result of image analysis, the surface area of wavy YSZ sheet increased to 133 % from flat sheet and improvement of efficiency is expected.
Cross section of wavy YSZ sheet.
We fabricated samples for performance test of SOFC. Fig. 10 shows a flow of sample preparation process. Wavy YSZ sheet was extremely thin and fragile. To prevent breakage, we attached another YSZ compound sheet as supporting structure to wavy YSZ compound sheet before sintering. After sintering, pastes of anode and cathode were applied, and the piece was heated again.
Flow of making process of test sample.
Fig. 11 shows a fabricated test piece after applying a NiO-YSZ anode layer. It is noted that we could handle the sample by hand since the sample had enough strength by the supporting structure.
Image of trial peace.
A thin and wavy YSZ sheet was fabricated by improved μPI process with a soft rubber sheet. A shape of wave patterns can be formed exactly by changing imprinting depth. The YSZ sheet does not have crack and may be available as an electrolyte of SOFC. A surface area of wavy YSZ sheet was increased to 133 % from flat sheet. This means a reaction surface area can increase and a rise of efficiency is expected by the present process. Performance test of SOFC is our next work.
The authors would like to thank the following financial supports: JSPS KAKENHI Grant Number 15H04161, and JST A-STEP program Grant Number AS262Z01235L.
