現今中溫型固態氧化物燃料電池(IT-SOFC)於工作環境下(600℃~800℃)，常使用含Cr之不鏽鋼作為金屬連接器，然而高Cr含量之金屬連接器易於表面形成Cr2O3等高電阻之氧化物，不僅增加連接器之電阻率，隨著使用時間其厚度不斷增加，並可能由於熱膨脹行為的差異而產生氧化層剝落等現象；另外此氧化物將與氧氣及水氣反應形成含Cr6+離子的氣體，並進一步於陰極及電解質表面還原而形成Cr2O3氧化物，此產物亦會增加陰極與電解質間之接觸電阻，進而降低IT-SOFC之使用效率及壽命。現今常見的改善方式為在連接器基材表面塗覆高導電率的氧化物，以期能減緩基材的氧化現象並維持連接器之高導電性。
本實驗的研究目的為改善金屬連接器於高溫環境之氧化現象，並降低其長時間使用下之電阻，首先選擇尖晶石型氧化物Mn1.5Co1.5O4(MCO)作為保護層材料，並以鎳基合金Inconel 625與鐵基合金Crofer 22 H作為連接器基材，利用電漿熔射噴塗將MCO披覆於基材表面，接著進行高溫長時間熱處理，發現MCO與Crofer 22 H之熱膨脹係數較相近，分別為12.10及12.25(10-6/℃)，因此熱處理後MCO保護層仍與Crofer 22 H緊密鍵結；而Inconel 625之熱膨脹係數為14.40(10-6/℃)，與MCO及SOFC零組件之差異皆較大，造成MCO保護層剝落的現象，因此後續以Crofer 22 H連接器作為研究的對象。
本研究亦選用鈣鈦礦氧化物La0.8Sr0.2MnO3(LSM)作為保護層材料，並以單一LSM與單一MCO保護層及LSM與MCO複合組成(分別以7:3、1:1以及3:7三種莫耳比例混合)披覆於Crofer 22 H基材表面，減緩基材氧化現象，並提升基材之導電性質。
Crofer 22 H為含有Cr含量22 wt.%及Mn含量0.8 wt.%與少許添加物之鐵基不鏽鋼。在800℃熱處理時氧化過程分為兩個階段，首先由於高Cr含量，Crofer 22 H表面形成corundum結構之Cr2O3，接著於Cr2O3的上層形成MnCr2O4尖晶石氧化物，其原因為Crofer 22 H的Mn元素由基材擴散至Cr2O3，且由於在Cr2O3的晶格中Mn的擴散係數(~10-13(cm2s-1))大於Cr元素(~10-15(cm2s-1))，Mn將穿越Cr2O3並於Cr2O3之上層與Cr2O3及外界的氧氣產生反應並形成MnCr2O4，此兩種氧化物的產生順序是因為MnCr2O4的自由能(−1818027 (Jmol-1K-1))遠低於Cr2O3(−841320 (Jmol-1K-1))。而本研究觀察Crofer 22 H經過空氣氣氛下800℃熱處理576小時後，其表面所形成之氧化層厚度約為4m。
為了改善連接器Crofer 22 H之氧化現象，本研究選用LSM與MCO作為保護層材料，而實驗結果顯示此兩者對於Crofer 22 H基材之氧化改善皆有明顯的效果，於保護層與基材間介面所形成氧化物之厚度分別約為2m及1m，且以元素分布分析發現保護層端無Cr元素的訊號，顯示此兩種保護層皆能有效減緩基材氧化現象，尤以MCO之保護效果更顯著。而LSM噴塗Crofer 22 H之試片經熱處理後其氧化層厚度較厚的原因在於保護層披覆階段產生的微裂縫在長時間熱處理後無法完全消除，此微裂縫的存在將導致氧離子較容易由外部空氣擴散至介面處並與基材反應。本研究亦觀察到MCO保護層於剛噴塗完的階段為MnO/CoO岩鹽結構，此現象是由於電漿熔射噴塗為以2000~3000℃之電子弧將粉末吹送至基板並以約106 Ks-1之速度急速冷卻，此溫度變化將使MCO維持高溫熔融態，而經800℃熱處理後轉變回尖晶石結構，此間之結構轉變伴隨著約25%的體積膨脹，且導電率方面有103~104倍數的提升，此導電率的提升是因為在岩鹽結構中，Mn/Co離子之間距過大，且Mn/Co離子之價數皆為二價，此兩者現象不利於價電子轉移；而Mn1.5Co1.5O4尖晶石結構中，八面體間隙存在相異價數之陽離子，其電子轉移所需克服之活化能遠小於在岩鹽結構中，因此導電率將大幅提升；另外MCO保護層之結構轉變與體積膨脹現象將消除保護層披覆階段所產生的微裂縫，因此能有效減緩介面處氧化層的生長速度。而在長時間電阻量測方面，於400小時後，Crofer 22 H裸材之電阻ASR為11.61 (mΩcm2)，具MCO保護層試片之ASR為3.75 (mΩcm2)，而具LSM保護層試片之ASR為9.83 (mΩcm2)，其電性表現可由氧化層的形成厚度來加以說明。
為了結合LSM材料本身的高導電率(於800℃為180 S/cm)及MCO作為保護層時優異的抗氧化性，將LSM與MCO混合後形成複合保護層並噴塗於Crofer 22 H表面，經過800℃長時間電阻量測並與先前試片相較起來，可發現其電阻顯著下降，在LSM與MCO以7:3、1:1及3:7的混合比例下其電阻ASR分別為1.1 (mΩcm2)、1.9 (mΩcm2)及6.3 (mΩcm2)。此現象顯示出此複合保護層不但具有MCO保護層之優良抗氧化性質，亦能顯現出LSM保護層之良好導電性，其中LSM與MCO以7:3之混合比例具有最低之電阻，因此對於連接器之抗氧化改善而言，LSM與MCO以7:3之比例作為保護層為較可行之組成。

Since oxide layer composed of Cr2O3 and MnCr2O4 with high resistance formed on surface of interconnect Crofer 22 H at SOFC operating temperature, it is necessary to deposit highly conductive oxide layer to reduce the oxidation rate of substrate. In this study, dense and well adherent coating were applied through air plasma-spraying on Crofer 22 H The result shows that the oxidation of Crofer 22 H at 800℃ was significantly suppressed. The coating materials used were La0.8Sr0.2MnO3(LSM) and Mn1.5Co1.5O4(MCO) due to their high conductivity and good oxidation resistance respectively. Mixtures of these two oxides with molar ratio 7:3, 1:1 and 3:7 were deposited on substrate as well in order to obtain desired the oxidation resistance property of MCO and high conductivity of LSM.
The oxidation rate of Crofer 22 H were effectively suppressed by LSM and MCO coating that the thickness of oxide were reduced from 4m of unprotected Crofer 22 H to 2m and 1m of protected sample, respectively. The crystal structure of MCO coating layer transformed from rock-salt to spinel that was followed by conductivity increment of 104 S/cm and volume expansion during annealing. The resistance ASR(Area-Specific-Resistance) for unprotected Crofer 22 H, LSM-coated, MCO-coated, composite coating MCO:LSM=7:3, 1:1 and 3:7 was 11.8, 9.8, 4.2, 6.4, 1.6 and 1.1(mΩcm2), respectively. The results indicated that the composite coating MCO:LSM=3:7 showed the lowest resistance from the attribution of high conductivity of LSM and good oxidation resistance of MCO.

[1] M. Smith and A. J. McEvoy “Sulfur-Tolerant cermet anode” Solid Oxide Fuel cell IX, 2005 p1437
[2] 陳楷中，“LSM 塗覆於固態氧化物燃料電池連接板之高溫氧化研究”，國立中央大學光機電工程研究所碩士論文，中華民國九十七年。
[3] Antoni, Laurent. "Materials for solid oxide fuel cells: the challenge of their stability." Materials Science Forum. Vol. 461. 2004.
[4] Jasinski, Piotr. "Micro solid oxide fuel cells and their fabrication methods." Microelectronics international 25.2 (2008): 42-48.
[5] McIntosh, Steven, and Raymond J. Gorte. "Direct hydrocarbon solid oxide fuel cells." Chemical Reviews 104.10 (2004): 4845-4866.
[6] N.Q. Minh, T. Takahashi, Science and technology of ceramic fuel cells (1995).
[7] 敖青、李德輝、孫良成、李勝利、劉偉明，“固態氧化物燃料電池鉻酸鑭連接材料研究現狀”，金屬熱處理，Vol.27 No.11，p.8-10 (2002).
[8] 孫良成、李德輝、李勝利、付貴福、敖青，周天亮，”鉻酸鑭材料熱膨脹機埋研究現狀”，工業加熱，Vol.33, No.3，pp.27-31.
[9] 韓敏芳、彭蘇萍，固體氧化物燃料電池材料與製備，科學出版社，2004年
[10] F. Tietz, H.-P. Buchkremer, and D. Stover, ”Components manufacturing for solid oxide fuel cells”Solid State Ionic, 152/153, 373 (2002).
[11] D.B. Meadowcroft, in: T. Gray, (Ed.), International Conference on Strontium Containing Compounds, Atlantic Research Institute, Halifax, Canada, 119 (1973).
[12] I. Yasuda and T. Hikita, J. Electrochem. Soc., 140, 1699 (1993).
[14] Sakai, N., Horita, T., Yamaji, K., Xiong, Y. P., Kishimoto, H., Brito, M. E., & Yokokawa, H. (2006). Material transport and degradation behavior of SOFC interconnects. Solid State Ionics, 177(19), 1933-1939.
[15] Hilpert, K., Das, D., Miller, M., Peck, D. H., & Weiss, R. (1996). Chromium vapor species over solid oxide fuel cell interconnect materials and their potential for degradation processes. Journal of the Electrochemical Society, 143(11), 3642-3647.
[16] Huang, K., Hou, P. Y., & Goodenough, J. B. (2000). Characterization of iron-based alloy interconnects for reduced temperature solid oxide fuel cells. Solid State Ionics, 129(1), 237-250.
[17] Zhu, W. Z., & Deevi, S. C. (2003). Opportunity of metallic interconnects for solid oxide fuel cells: a status on contact resistance. Materials Research Bulletin, 38(6), 957-972.
[18] Yang, Z., Hardy, J. S., Walker, M. S., Xia, G., Simner, S. P., & Stevenson, J. W. (2004). Structure and conductivity of thermally grown scales on ferritic Fe-Cr-Mn steel for SOFC interconnect applications. Journal of the Electrochemical Society,151(11), A1825-A1831.
[19] Wagner, C. (1952). Theoretical analysis of the diffusion processes determining the oxidation rate of alloys. Journal of the Electrochemical Society,99(10), 369-380.
[20] Geng, S., & Zhu, J. (2006). Promising alloys for intermediate-temperature solid oxide fuel cell interconnect application. Journal of Power Sources, 160(2), 1009-1016.
[21] Qiao, J., & Yang, C. Y. (1995). High-< i> T</i>< sub> c</sub> superconductors on buffered silicon: materials properties and device applications.Materials Science and Engineering: R: Reports,14(4), 157-201.
[22] Kolomiets BT, Sheftel J, Kurlina E. Electrical properties of some compound oxide semiconductors. Sov Phys Tech Phys. (1957);2:40–58.
[23] Jabry EH, Rousset A, Lagrange A. Preparation and characterization of manganese and cobalt based semiconducting ceramics. Phase Transit. (1988);13:63–71.
[24] Rios E, Gautier JL, Poillerat G, Chartier P. Mixed valency spinel oxides of transition metals and electrocatalysis : case of the MnxCo3-xO4 system. Electrochim Acta. (1998);44:1491–7.
[25] Restovic A, Rios E, Barbato S, Ortiz J, Gautier JL. Oxygen reduction in alkaline medium at thin MnxCo3-xO4 (0≤x ≤ 1) spinel films prepared by spray pyrolysis. Effect of oxide cation composition on the reaction kinetics. J Electroanal Chem. (2002);522:141–51.
[26] Yang Z, Xia GG, Li XH, Stevenson JW. (Mn, Co)3O4 spinel coatings on ferritic stainless steels for SOFC interconnect applications. Int J Hydrogen Energy. (2006);32:3648–54.
[27] Hagen A, Mikkelsen L. Xanes study of the oxidation state and coordination environment of manganese, chromium and cobalt in spinel type materials. Proceedings of the 26th Risø international symposium on material science: Solid State Electrochem (2005); p. 197–202.
[28] Philip J, Kutty TRN. Colossal magnetoresistance of oxide spinels CoxMn3-xO4. Mater Lett. (1999);39:311–7.
[29] Buhl, R. J. Phys. Chem. Solids (1969), 30, 805.
[30] Boucher, B.; Buhl, R.; di Bella, R.; Perrin, M. J. Phys. (1970), 31, 113.
[31] Naka, S.; Inagaki, M.; Tanaka, T. J. Mater. Sci. (1972), 7, 441.
[32] Yamamoto, N.; Kawano, S.; Achiwa, N.; Higashi, S. J. Jpn. Soc. Powder Metall. (1983), 30, 48.
[33] Yamamoto, N.; Higashi, S.; Kawano, S.; Achiwa, N. J. Mat. Sci. Lett. (1983), 2, 525.
[34] Wright, P. A.; Natarajan, S.; Thomas, J. M.; Gai-Boyes, P. L.Chem. Mater. (1992), 4, 1053.
[35] Yang, B. L.; Chan, S. F.; Chang, W. S.; Chen, Y. Z. J. Catal. (1991), 130, 52.
[36] D. Jarosch, Miner. Petrol. 37 (1987) 15.
[37] H.A. Jahn, E. Teller, P. Roy, Soc. A-Math. Phys. 161 (1937) 220.
[38] Yang, Z., Xia, G. G., Li, X. H., & Stevenson, J. W. (2007). (Mn, Co)< sub> 3</sub> O< sub> 4</sub> spinel coatings on ferritic stainless steels for SOFC interconnect applications. International Journal of Hydrogen Energy, 32(16), 3648-3654.
[39] Yang, Z., Xia, G., Simner, S. P., & Stevenson, J. W. (2005). Thermal growth and performance of manganese cobaltite spinel protection layers on ferritic stainless steel SOFC interconnects. Journal of the Electrochemical Society,152(9), A1896-A1901.
[40] Petric, A., Huang, P., & Tietz, F. (2000). Evaluation of La–Sr–Co–Fe–O perovskites for solid oxide fuel cells and gas separation membranes. Solid State Ionics, 135(1), 719-725.
[41] Li, J., Huang, Q., Li, Z. W., You, L. P., Xu, S. Y., & Ong, C. K. (2001). Enhanced magnetoresistance in Ag-doped granular La 2/3 Sr 1/3 MnO 3 thin films prepared by dual-beam pulsed-laser deposition. Journal of Applied Physics, 89(11), 7428-7430.
[42] Gu, J. Y., Kwon, C., Robson, M. C., Trajanovic, Z., Ghosh, K., Sharma, R. P., ... & Noh, T. W. (1997). Growth and properties of c-axis textured La 0.7 Sr 0.3 MnO 3-δ films on SiO 2/Si substrates with a Bi 4 Ti 3 O 12 template layer.Applied physics letters, 70(13), 1763-1765.
[43] Broussard, P. R., Browning, V. M., & Cestone, V. C. (1999). X-ray photoemission characterization of La 0.67 (Ca x Sr 1-x) 0.33 MnO 3 films.Journal of applied physics, 85(8), 5414-5416.
[44] Gu, J. Y., Ogale, S. B., Rajeswari, M., Venkatesan, T., Ramesh, R., Radmilovic, V., ... & Noh, T. W. (1998). In-plane grain boundary effects on the magnetotransport properties of La 0.7 Sr 0.3 MnO 3-δ. Applied physics letters,72(9), 1113-1115.
[45] Tiwari, A., Chug, A., Jin, C., Kumar, D., & Narayan, J. (2002). Integration of single crystal La< sub> 0.7</sub> Sr< sub> 0.3</sub> MnO< sub> 3</sub> films with Si (001). Solid state communications, 121(12), 679-682.
[46] Chakraborty, A., Devi, P. S., & Maiti, H. S. (1994). Preparation of La< sub> 1− x</sub> Sr< sub> x</sub> MnO< sub> 3</sub>(0⩽< i> x</i>⩽ 0.6) powder by autoignition of carboxylate-nitrate gels. Materials Letters, 20(1), 63-69.

[47] Zhang, N., Ding, W. P., Guo, Z. B., Zhong, W., Xing, D. Y., Du, Y. W., ... & Zheng, Y. (1997). Large lattice compression coefficient and uniaxial piezoresistance in CMR perovskite La1− XSrXMaO3 (0.15≤ x≤ 0.25). Zeitschrift für Physik B Condensed Matter, 102(4), 461-465.
[48] Villafuerte, M. J., Duhalde, S., Terzzoli, M. C., Polla, G., Leyva, G., & Correra, L. (1999). Structural and transport properties of La0.67Sr0.33Mn1-xSnxO3 thin films. Applied Physics A, 69(1), S565-S567.
[49] Yang, Z., Xia, G., Singh, P., & Stevenson, J. W. (2006). Electrical contacts between cathodes and metallic interconnects in solid oxide fuel cells. Journal of Power Sources, 155(2), 246-252.
[50] Cao, X. Q., Vassen, R., Schwartz, S., Jungen, W., Tietz, F., & Stöever, D. (2000). Spray-drying of ceramics for plasma-spray coating. Journal of the European Ceramic Society, 20(14), 2433-2439.
[51]Quadakkers, W. J., Piron-Abellan, J., Shemet, V., & Singheiser, L. (2003). Metallic interconnectors for solid oxide fuel cells–a review. Materials at high temperatures, 20(2), 115-127.
[52] G.V. Samsonov (Ed.), The Oxide Handbook, Plenum Data Corporation, New York, 1973, p. 524.
[53] Qu, W., Jian, L., Hill, J. M., & Ivey, D. G. (2006). Electrical and microstructural characterization of spinel phases as potential coatings for SOFC metallic interconnects. Journal of Power Sources, 153(1), 114-124.
[54] Jian, L., Jian, P., Bing, H., & Xie, G. (2006). Oxidation kinetics of Haynes 230 alloy in air at temperatures between 650 and 850 C. Journal of Power Sources, 159(1), 641-645.
[55]J.-Y. Kim, N.L. Canfield, L.A. Chick, K.D. Meinhardt, and V.L. Sprenkle, Proceedings of 29th Inter.
Conf. on Adv. Ceramics and Composites, ACeRS, 2005 (in press).
[56] Han, S. J., Chen, Y., & Sampath, S. (2014). Role of process conditions on the microstructure, stoichiometry and functional performance of atmospheric plasma sprayed La (Sr) MnO< sub> 3</sub> coatings. Journal of Power Sources, 259, 245-254.
[57] R. Hui, Z.W. Wang, O. Kesler, L. Rose, Jankovic, S. Yick et al. Thermal plasma spraying for SOFCs: applications, potential advantages, and challenges J Power Sources, 170 (2007), pp. 308–323
[58] Chen, L., Magdefrau, N., Sun, E., Yamanis, J., Frame, D., & Burila, C. (2011). Strontium transport and conductivity of Mn1. 5Co1. 5O4 coated Haynes 230 and Crofer 22 APU under simulated solid oxide fuel cell condition. Solid State Ionics,204(Complete), 111-119.