Study on tape casting process for fabrication of large-area solid oxide fuel cells = 대면적 고체산화물연료전지의 제작을 위한 테이프 캐스팅 공정에 관한 연구

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Solid oxide fuel cells (SOFCs) are the devices which convert chemical energy of hydrogen and oxygen to electrical energy. This has advantages of high energy conversion efficiency, fuel flexibility, and no precious metal catalysts. Therefore, SOFCs are getting more and more attentions as a next-generation power generat-ing technology. The most popular manufacturing technology is tape casting for fabricating SOFC unit cells. Tape casting is the process that fabricates green tapes by casting the mixture of ceramic powder, binder, plas-ticizer, dispersant, and solvent. The fabricated green tapes are heat-treated to forms final unit cells. The tape casting process is composed of several sub-steps, and process conditions of each step influence the green tapes considerably. In addition, although tape casting is developed for fabricating small lab-scale unit cells, it is not easy to apply the technology to large-area unit cell fabrication directly because additional issues emerge. Therefore, detailed study on each step of tape casting is necessary. The main goal of this study is the fabrication and evaluation of large-area SOFC unit cells. To achieve the goal, minimum amount of organic additives, including binder and plasticizer, was determined to 20 wt%. Afterward, the required time of vacuum pumping for the removal of bubbles in tape cast slurry was deter-mined, and slurry and green tape characteristics were controlled by solvent ratio changes during the vacuum pumping. As a result, solvent ratio of 33 wt% was confirmed to have the most adequate slurry and tape characteristics. Therefore, it could be concluded that changing solvent ratio can be one of the effective way of controlling tape casting characteristics. In addition, it was confirmed that slurry characteristics are major factors which determines adequate solvent ratio other than tape characteristics. The thermal decomposition and shrinkage characteristics of the fabricated green tapes were analyzed to obtain minimized crack formation and best flatness of sintered tapes. The thermal decomposition of bind-er, plasticizer, and pore former was confirmed at $250^\circ C$, $350^\circ C$, and $500^\circ C$. Therefore, heat-treatment steps at these temperatures were added in the temperature profile to minimize crack formation. From the shrinkage behavior results, co-sintering temperature of $1250^\circ C$ was determined to minimize shrinkage rate mismatch between anode and electrolyte layers. The fabricated unit cells showed the maximum power densi-ty of $1.2 W/cm^{2}$ at $800^\circ C$. To fabricate large-area unit cells, additional emerging issues were confirmed. When binder and plasti-cizer were decomposed, serious crack occurred. This was believed that the reason of the crack is the tapes could not endure the internal stress caused by shrinkage mismatch between anode and electrolyte during the thermal decomposition. To solve the issue, this study proposed minimizing shrinkage mismatch and improv-ing mechanical strength of unit cells by increasing the strength of anode support layer, which composes most of the unit cell volume, by changing powder size and compositions. The shrinkage mismatch was minimized by substituting NiO powder with particle size of $0.86 \mu m$, and graphite flake was adopted to form horizontally oriented pore structure, leading to higher mechanical strength after thermal decomposition. Based on the re-sults, a large-area unit cell with a diameter of 5 cm was fabricated. This cell showed 1.05 V of open circuit voltage (OCV) which means a dense electrolyte layer. The maximum power was 4.3 W at $800^\circ C$. In summary, de-tailed studies on sub-steps of tape casting were conducted to fabricate SOFC unit cells. In this study, tape cast slurry and tapes were controlled by solvent ratio in slurry, and adequate process conditions were proposed. Finally, unit cells were successfully fabricated and tested based on the results of this study. In addition, to scale up tape casting to fabricate large-area unit cells, emerging issues were con-firmed, and solution to resolve the issues were suggested. A large-area unit cell was also successfully fabricat-ed and tested. From this study, it is expected that decrease in manufacturing cost and increase in yield rate can be achieved, which can help commercialization of SOFC. In addition, the result of this study can help new technologies developed in research organization to be applied in industry in a large scale. It is also be-lieved that this tape casting technology can be utilized in the field of ceramics, such as Multi-layer capacitor (MLC), Multi-layer ceramic package (MLCP), Solar cell, etc. to fabricate thin and flat substrates.
Bae, Joongmyeonresearcher배중면researcher
한국과학기술원 :기계공학과,
Issue Date

학위논문(석사) - 한국과학기술원 : 기계공학과, 2016.2 ,[vi, 59 p. :]


Solid oxide fuel cell; Tape casting; Large-area unit cell; Manufacturing process; Process conditions; 고체산화물 연료전지; 테이프 캐스팅; 대면적 단전지; 생산기술; 공정조건

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