新能源的開發及節能減碳，例如：燃料電池、生質酒精、太陽能及能源轉換等，已經成為現今人類發展的重要議題。熱電材料即為一受到期待的能源轉換方式的材料，熱電材料在溫差發電及熱電致冷方面都具有廣泛的應用價值，其中陶瓷熱電材料具有成本低廉、無毒、無污染等之優點，是深受期待的材料。陶瓷材料中之鈦酸鍶是功能性非常多元的材料，具有介電常數高、介電損耗低、熱穩定性好等之優點，是被期待的熱電材料之一。
稀土元素的摻雜為提升鈦酸鍶熱電性質的方式之一，其中以鑭摻雜之鈦酸鍶在前人的研究中具有最佳熱電性質，但現今的研究成果仍無法將其應用於實際熱電產品中。為提升摻鑭鈦酸鍶塊材之熱電性質，本研究選用奈米級YSZ添加物作為聲子散射源以降低塊材的熱傳導率。
摻鑭鈦酸鍶粉體是以有機前導物法合成，粉體經混合、成形後，於氬氫還原氣氛(Ar-5%H2)中以1400 ℃燒結4小時形成塊材。塊材之結晶相及微觀組織是分別以XRD及SEM分析，熱電性質之量測範圍為298 - 673 K。
研究結果顯示，YSZ添加量小於3 wt %有助於提升塊材之相對密度、導電率及power factor值，當YSZ添加量為≧3 wt %時，塊材之相對密度、導電率及power factor值皆開始降低。此外，YSZ添加對於塊材之Seebeck係數並無顯著影響，且於量測溫度區間塊材皆維持n型半導體之特性，所有添加YSZ之樣品中，以2 wt % YSZ添加量之試樣具有最佳之power factor值，在673 K時power factor值為0.71 mW/m-K2。
熱傳導率量測結果顯示，在各組添加YSZ的試樣中，隨YSZ添加量之增加，塊材之熱傳導率減少，顯示YSZ的添加足以對於聲子熱傳導造成影響。未添加YSZ之試樣因密度最低孔隙率最大而呈現最低之熱導率值。
綜合各項熱電性質後計算試樣之ZT值，結果顯示，2 wt % YSZ添加量之試樣在673 K時具有最高之ZT值，0.1。

To improve the thermoelectric properties of La-doped SrTiO3 bulk, YSZ (Yttria-stabilized zirconia) were chosen as the additives in this study. La-doped SrTiO3 powders prepared by polymerized complex process were mixed with nano YSZ powders by ball milling. The mixture powders were CIP-ed and then sintered at 1400 °C for 4 hours in a 5% H2/Ar atmosphere. The density of samples were evaluated by Archimedes’ method. The crystal structure and microstructure were confirmed by XRD and SEM, respectively. The electrical conductivity and Seebeck coefficient were measured from 300 K to 673 K. Both the relativity density and electrical conductivity of La-doped SrTiO3 bulk samples were obviously enhanced by < 3 wt % YSZ additives from 85 % to 95 % and 145 S/cm to 414 S/cm, respectively. These improvement results were from the Y3+ and Zr4+ ions diffusion into SrTiO3 lattices during sintering. When YSZ added up to 3 wt %, the relative density and electrical conductivity of bulk decrease because the excess YSZ particles restrict the movement of grain boundary during sintering. The Seebeck coefficient did not change a lot despite the addition of YSZ. The maximum value of power factor was 0.75 mW/m‧K2 at 673 K for the sample with 2 wt % YSZ. Thermal conductivity of bulk decreased with the increase of YSZ amount. 2 wt % YSZ addition bulks showed the highest ZT value of 0.097 at 673 K.

[1] D.M. Rowe, CRC handbook of thermoelectrics, CRC press (1995)
[2] T.C. Harman, P.J. Taylor, M.P. Walsh, B.E. LaForge, Quantum dot superlattice thermoelectric materials and devices, Science, 297, 2229-2232 (2002).
[3] H. Ohta, S. Kim, Y. Mune, T. Mizoguchi, K. Nomura, S. Ohta, T. Nomura, Y. Nakanishi, Y. Ikuhara, M. Hirano, H. Hosono, K. Koumoto, Giant thermoelectric Seebeck coefficient of a two-dimensional electron gas in SrTiO3, Nature Materials, 6, 129-134 (2007).
[4] C.H. Kuo, H.S. Chien, C.S. Hwang, Y.W. Chou, M.S. Jeng, M. Yoshimura, Thermoelectric properties of fine-grained PbTe bulk materials fabricated by cryomilling and spark plasma sintering, Materials Transactions, 52, 795-801 (2011).
[5] N. Okinaka, L. Zhang, T. Akiyama, Thermoelectric Properties of Rare Earth-doped SrTiO3 Using Combination of Combustion Synthesis (CS) and Spark Plasma Sintering (SPS), ISIJ International, 50, 1300-1304 (2010).
[6] K. Park, J.S. Son, S.I. Woo, K. Shin, M.W. Oh, S.D. Park, T. Hyeon, Colloidal synthesis and thermoelectric properties of La-doped SrTiO3 nanoparticles, Journal of Materials Chemistry A, 2, 4217-4224 (2014).
[7] 陳映璉, 鑭摻雜鈦酸鍶之粉末製備, 燒結行為及其塊材熱電性質之研究, 成功大學材料科學及工程學系碩士學位論文, (2015).
[8] H. Obara, A. Yamamoto, C.-H. Lee, K. Kobayashi, A. Matsumoto, R. Funahashi, Thermoelectric properties of Y-doped polycrystalline SrTiO3, Japanese Journal of Applied Physics, 43, 540 (2004).
[9] S. Hashimoto, F.W. Poulsen, M. Mogensen, Conductivity of SrTiO3 based oxides in the reducing atmosphere at high temperature, Journal of Alloys and Compounds, 439, 232-236 (2007).
[10] P.K.Gallagher, F. Schrey, F.V. Dimarcello, Preparation of semiconducting titanates by chemical methods, Journal of the American Ceramic Society, 46, 359-365 (1963).
[11] A. Kikuchi, N. Okinaka, T. Akiyama, A large thermoelectric figure of merit of La-doped SrTiO3 prepared by combustion synthesis with post-spark plasma sintering, Scripta Materialia, 63, 407-410 (2010).
[12] S.C. Zhang, J.X. Liu, Y.X. Han, B.C. Chen, X.G. Li, Formation mechanisms of SrTiO3 nanoparticles under hydrothermal conditions, Material Science and Engineering:B-Solid, 110, 11-17 (2004).
[13] Y. Mao, S. Banerjee, S.S. Wong, Hydrothermal synthesis of perovskite nanotubes, 3, Chemical Communications, 408-409 (2003).
[14] W.D. Yang, K.M. Hung, Optimization of the experimental conditions for the preparation of a thin strontium titanate film by hydrothermal process, Journal of Material Science, 37, 1337-1342 (2002).
[15] P. Durán, F. Capel, J. Tartaj, C. Moure, Low-temperature fully dense and electrical properties of doped-ZnO varistors by a polymerized complex method, Journal of the European Ceramic Society, 22, 67-77 (2002).
[16] M. Kakihana, M. Yoshimura, H. Mazaki, H. Yasuoka, L. Börjesson, Polymerized complex synthesis and intergranular coupling of Bi-Pb-Sr-Ca-Cu-O superconductors characterized by complex magnetic susceptibility, Journal of Applied Physics, 71, 3904 (1992).
[17] M. Ito, T. Nagira, S. Katsuyama, S. Hara, Synthesis of NaxCo2O4 thermoelctric oxide by the polymerized complex method and SPS methods, Scripta Materialia, 317-320 (2005).
[18] M. Ito, T. Matsuda, Thermoelectric properties of non-doped and Y-doped SrTiO3 polycrystals synthesized by polymerized complex process and hot pressing, Journal of Alloys and Compounds, 477, 473-477 (2009).
[19] R. Richman, J. Stringerb, Prospects for efficient thermoelectric materials in the near term, San Diego, CA, DARPA/DOE High Efficient Thermoelectric. (2002).
[20] TE Technology Inc. Frequently asked questions on thermoelectrics. [Online]. http://www.tetech.com/techinfo/#faqs. (2005).
[21] F. Stabler, Automotive applications of high efficiency thermoelectrics, Proceedings of DARPA/ONR/DOE High Efficiency Thermoelectric Workshop, 1-26 (2002).
[22] S.B. Riffat, X. Ma, Thermoelectrics: a review of present and potential applications, Applied Thermal Engineering, 23, 913-935 (2003).
[23] G. Nolas, H. Goldsmid, A comparison of projected thermoelectric and thermionic refrigerators, Journal of Applied Physics, 85, 4066-4070 (1999).
[24] S. Sõmiya, Hydrothermal Reactions for Materials Science and Engineering: An Overview of Research in Japan, Springer Science & Business Media (2012).
[25] J. Hannay, On the artificial formation of the diamond, Proceedings of the Royal Society of London, 30, 450-461 (1879).
[26] K. Byrappa, M. Yoshimura, Handbook of hydrothermal technology, William Andrew (2012).
[27] J.O.Eckert, C.C.Hung‐Houston, B.L. Gersten, M.M. Lencka, R.E. Riman, Kinetics and mechanisms of hydrothermal synthesis of barium titanate, Journal of the American Ceramic Society, 79, 2929-2939 (1996).
[28] W. Zhu, C. Wang, S. Akbar, R. Asiaie, Fast-sintering of hydrothermally synthesized BaTiO3 powders and their dielectric properties, Journal of Material Science, 32, 4303-4307 (1997).
[29] M.P. Pechini, Barium titanium citrate, barium titanate and processes for producing same, Google Patents, (1966).
[30] J. Liu, Z. Zeng, Q. Zheng, H. Lin, Effective transfer integrals for the Jahn-Teller distortion in LaMnO3, Physical Review B, 60, 12968 (1999).
[31] P.A. Lessing, Mixed-cation oxide powders via polymeric precursors, American Ceramic Society Bulletin, 68, 1002-1007 (1989).
[32] 高弘任, 檸檬酸法製備鋁酸鍶鈣螢光粉體及其光性質研究, 成功大學材料科學及工程學系碩士學位論文, (2008).
[33] D. Hennings, W. Mayr, Thermal decomposition of (BaTi) citrates into barium titanate, Journal of Solid State Chemistry, 26, 329-338 (1978).
[34] G.A. Hutchins, G.H. Maher, S.D. Ross, Control of the Ba: Ti ratio of BaTiO3 at a value of exactly 1 via conversion to BaO• TiO2• 3C6H8O7• 3H2O, American Ceramic Society Bulletin, 66, 681-684 (1987).
[35] M. Kakihana, M.M. Milanova, M. Arima, T. Okubo, M. Yashima, M. Yoshimura, Polymerized complex route to synthesis of pure Y2Ti2O7 at 750°C using Yttrium‐Titanium mixed‐metal citric acid complex, Journal of the American Ceramic Society, 79, 1673-1676 (1996).
[36] S. Cho, P. Johnson, R. Condrate Sr, Thermal decomposition of (Sr, Ti) organic precursors during the Pechini process, Journal of Material Science, 25, 4738-4744 (1990).
[37] J. Coutures, P. Odier, C. Proust, Barium titanate formation by organic resins formed with mixed citrate, Journal of Material Science, 27, 1849-1856 (1992).
[38] T. Okuda, K. Nakanishi, S. Miyasaka, Y. Tokura, Large thermoelectric response of metallic perovskites: Sr1-xLaxTiO3 (0≦x≦0.1), Physical Review B, 63, (2001).
[39] S.L.F. Syh-Yuh Chang, Chung-Chuang Wei, Sintering of SrTiO3 with Li2O3 addition, Ceramic International, 15, 231-236 (1989).
[40] K.S. Liu, I.N. Lin, Enhanced densification of SrTiO3 perovskite ceramics, Applications of Ferroelectrics, 261-264, (1994).
[41] S. Cho, P. Johnson, Evolution of the microstructure of undoped and Nb-doped SrTiO3, Journal of Material Science, 29, 4866-4874 (1994).
[42] Q. Fu, S. Mi, E. Wessel, F. Tietz, Influence of sintering conditions on microstructure and electrical conductivity of yttrium-substituted SrTiO3, Journal of the European Ceramic Society, 28, 811-820 (2008).
[43] F. Gao, H. Zhao, X. Li, Y. Cheng, X. Zhou, F. Cui, Preparation and electrical properties of yttrium-doped strontium titanate with B-site deficiency, Journal of Power Sources, 185, 26-31 (2008).
[44] L. Amaral, A.M. Senos, P.M. Vilarinho, Sintering kinetic studies in nonstoichiometric strontium titanate ceramics, Materials Research Bulletin, 44, 263-270 (2009).
[45] C.N. George, J. Thomas, R. Jose, H.P. Kumar, M. Suresh, V.R. Kumar, P.S. Wariar, J. Koshy, Synthesis and characterization of nanocrystalline strontium titanate through a modified combustion method and its sintering and dielectric properties, Journal of Alloys and Compounds, 486, 711-715 (2009).
[46] K. Maca, V. Pouchly, Z. Shen, Two-step sintering and spark plasma sintering of Al2O3, ZrO2 and SrTiO3 ceramics, Integrated Ferroelectrics, 99, 114-124 (2008).
[47] M.S. Dresselhaus, G. Chen, M.Y. Tang, R. Yang, H. Lee, D. Wang, Z. Ren, J.P. Fleurial, P. Gogna, New Directions for Low‐Dimensional Thermoelectric Materials, Advanced Materials, 19, 1043-1053 (2007).
[48] X. Tang, W. Xie, H. Li, W. Zhao, Q. Zhang, M. Niino, Preparation and thermoelectric transport properties of high-performance p-type Bi2Te3with layered nanostructure, Applied Physics Letters, 90, 012102 (2007).
[49] H. Li, X. Tang, X. Su, Q. Zhang, Preparation and thermoelectric properties of high-performance Sb additional Yb0.2Co4Sb12+y bulk materials with nanostructure, Applied Physics Letters, 92, 202114 (2008).
[50] H. Li, X. Tang, Q. Zhang, C. Uher, High performance InXCeyCo4Sb12 thermoelectric materials with in situ forming nanostructured InSb phase, Applied Physics Letters, 94, 102114 (2009).
[51] Y. Pei, N.A. Heinz, A. LaLonde, G.J. Snyder, Combination of large nanostructures and complex band structure for high performance thermoelectric lead telluride, Energy & Environmental Science, 4, 3640 (2011).
[52] N. Wang, H. Li, Y. Ba, Y. Wang, C. Wan, K. Fujinami, K. Koumoto, Effects of YSZ Additions on Thermoelectric Properties of Nb-Doped Strontium Titanate, Journal of Electronic Materials, 39, 1777-1781 (2010).
[53] N. Wang, H. Chen, H. He, W. Norimatsu, M. Kusunoki, K. Koumoto, Enhanced thermoelectric performance of Nb-doped SrTiO3 by nano-inclusion with low thermal conductivity, Scientific reports, 3, 3449 (2013).
[54] E. Li, N. Wang, H. He, H. Chen, Improved Thermoelectric Performances of SrTiO3 Ceramic Doped with Nb by Surface Modification of Nanosized Titania, Nanoscale Research Letters, 11, 188 (2016).
[55] Z. Shao, S. Saitzek, P. Roussel, A. Ferri, É. Bruyer, A. Sayede, M. Rguiti, O.Mentré, R.Desfeux, Microstructure and Nanoscale Piezoelectric/ Ferroelectric Properties in La2Ti2O7 Thin Films Grown on (110)-Oriented Doped Nb:SrTiO3 Substrates, Advanced Engineering Materials, 13, 961-969 (2011).
[56] C. Chen, T. Zhang, R. Donelson, T.T. Tan, S. Li, Effects of yttrium substitution and oxygen deficiency on the crystal phase, microstructure, and thermoelectric properties of Sr1−1.5xYxTiO3−δ (0⩽x⩽0.15), Journal of Alloys and Compounds, 629, 49-54 (2015).