碲化鍺材料為現今被廣泛使用的熱電半導體材料，具有優異的熱電傳輸性質，但偏高的熱傳導係數仍有待進一歩改善。在眾多改善方法中，多元素的摻雜(multi-element alloying/doping)是一個能夠有效降低晶格熱傳導係數的方法。除此之外，具類磷烯結構的四族硫族化物由於其二維的階梯狀晶體結構，因此具有顯著的低熱傳導係數，但同時也因其半導體能隙較寬，使其載子濃度較低，因此具有高的Seebeck係數，但是也連帶造成了較低的導電性。本研究根據此兩項材料的特性，研發出一(Ge/Sb)(S/Se/Te)多元合金系統，利用本實驗室自製的電流輔助燒結熱壓系統，製備出熵值為1.32R的高熵熱電合金。在實驗第一部分中，我們針對不同的熔煉與熱壓製程參數進行嘗試，最後選擇性質最佳的元素配比，在700 ℃水淬下得到鑄錠，以及採450 ℃熱壓60分鐘的參數製備出標準樣品。在第二部分則是探討此標準樣品的高溫熱電性質，並以熱退火方式對樣品進行優化。最終研究結果發現，經退火處理過後，樣品內部會有兩種現象發生，其一是較大尺寸偏析相的回溶，其二是內部原子的再結晶現象。而經350 ℃退火樣品具有最好的熱電性質，室溫下之熱傳導係數為0.79 W/mK，在687 K下更可以低至0.57 W/mK，比現今常見的熱電材料之熱傳導係數低上許多；而在電阻率部分，退火樣品只有16.9 mΩcm，與其它同樣具低熱導率之硫族化物材料相比是十分低的數值；Seebeck係數方面，室溫下為111 µW/K，在687 K下則會上升至210 µW/K；因此最終無因次熱電優值在室溫下從標準樣品的0.022，經退火後上升到0.032，在687 K下也從標準樣品的0.287，變為退火樣品的0.339，數值有顯著的提升。

GeTe-based compounds exhibit promising thermoelectric properties in middle-high temperature regime. A further reduction of lattice thermal conductivity is required to enhance the thermoelectric figure-of-merit of the GeTe-based compounds. Among various approaches, multi-element alloying or doping is effective in decreasing lattice thermal conductivity of thermoelectric materials due to severe distortion in crystal lattice. Additionally, group-IV monochalcogenides with phosphorene-like structure exhibit significant low thermal conductivity owing to their two-dimensional zig-zag structure, and high Seebeck coefficient due to their low carrier concentration. In this study, a multi-element alloy (MEA) system, (Ge/Sb)(S/Se/Te), was prepared through the combination of current-assisted sintering and post thermal treatment. The MEA has a configurational entropy of 1.32R as compared to the GeTe or GeS compound of 0.69R. The MEA exhibits uniformly dispersed nanoscale composites of GeTe-rich phase and GeS-rich phase. We have optimized the conditions of ice quenching, hot pressing and thermal annealing processes to achieve the best thermoelectric properties of the MEA. Next, the thermoelectric properties of the MEA were measured at elevated temperatures. It was found that dissolution of large-scale precipitates and recrystallization occurred during the thermal annealing. Finally, we found the MEA annealed at 350 ℃ has the best thermoelectric properties. The thermal conductivity was found to decrease from 0.91 W/m·K for the as-prepared MEA to 0.79 W/m·K for the annealed MEA at room temperature. Additionally, its electrical resistivity decreased from 24.8 to 16.9 mΩcm. Hence, the dimensionless figure-of-merit of the MEA raised from 0.022 to 0.032 at room temperature, and from 0.287 to 0.339 at 687K after thermal annealing.

[1] D. M. Rowe, Thermoelectrics handbook: macro to nano, (CRC press), 121-125 (2018).
[2] G. J. Snyder, E. S. Toberer, Complex thermoelectric materials, (World Scientific : Nature Publishing Group), 101-110 (2011).
[3] G. S. Nolas, J. Sharp, J. Goldsmid, Thermoelectrics: basic principles and new materials developments, (Springer Science & Business Media), 145-149 (2013).
[4] Y. Pei, X. Shi, A. LaLonde, H. Wang, L. Chen, G. J. Snyder, Convergence of electronic bands for high performance bulk thermoelectrics. Nature 473, 66-69 (2011).
[5] M. Samanta, K. Biswas, Low thermal conductivity and high thermoelectric performance in (GeTe)1–2x(GeSe)x(GeS)x: competition between solid solution and phase separation. Journal of the American Chemical Society 139, 9382-9391 (2017).
[6] D. Tan, H. E. Lim, F. Wang, N. B. Mohamed, S. Mouri, W. Zhang, Y. Miyauchi, M. Ohfuchi, K. Matsuda, Anisotropic optical and electronic properties of two-dimensional layered germanium sulfide. Nano Research 10, 546-555 (2017).
[7] Q. Tan, L.-D. Zhao, J.-F. Li, C.-F. Wu, T.-R. Wei, Z.-B. Xing, M. G. Kanatzidis, Thermoelectrics with earth abundant elements: low thermal conductivity and high thermopower in doped SnS. Journal of Materials Chemistry A 2, 17302-17306 (2014).
[8] Y. Xiao, C. Chang, Y. Pei, D. Wu, K. Peng, X. Zhou, S. Gong, J. He, Y. Zhang, Z. Zeng, Origin of low thermal conductivity in SnSe. Physical Review B 94, 125203 (2016).
[9] M. Sist, C. Gatti, P. Nørby, S. Cenedese, H. Kasai, K. Kato, B. B. Iversen, High‐temperature crystal structure and chemical bonding in thermoelectric germanium selenide (GeSe). Chemistry–A European Journal 23, 6888-6895 (2017).
[10] Q. Tan, J.-F. Li, Thermoelectric properties of Sn-S bulk materials prepared by mechanical alloying and spark plasma sintering. Journal of Electronic Materials 43, 2435-2439 (2014).
[11] B. Lipin, G. McKay, Geochemistry and mineralogy of rare earth elements. Reviews in Mineralogy, Mineralogical Society of America, Chantilly, Virginia 21, 348 (1989).
[12] S. LeBlanc, S. K. Yee, M. L. Scullin, C. Dames, K. E. Goodson, Material and manufacturing cost considerations for thermoelectrics. Renewable and Sustainable Energy Reviews 32, 313-327 (2014).
[13] Y. Jien-Wei, Recent progress in high entropy alloys. Ann. Chim. Sci. Mat 31, 633-648 (2006).
[14] Y. Pei, A. D. LaLonde, H. Wang, G. J. Snyder, Low effective mass leading to high thermoelectric performance. Energy & Environmental Science 5, 7963-7969 (2012).
[15] J. Li, Z. Chen, X. Zhang, Y. Sun, J. Yang, Y. Pei, Electronic origin of the high thermoelectric performance of GeTe among the p-type group IV monotellurides. NPG Asia Materials 9, 353-353 (2017).
[16] Z. Chen, X. Zhang, Y. Pei, Manipulation of phonon transport in thermoelectrics. Advanced Materials 30, 1705617 (2018).
[17] K. Biswas, J. He, I. D. Blum, C.-I. Wu, T. P. Hogan, D. N. Seidman, V. P. Dravid, M. G. Kanatzidis, High-performance bulk thermoelectrics with all-scale hierarchical architectures. Nature 489, 414-418 (2012).
[18] J. He, M. G. Kanatzidis, V. P. Dravid, High performance bulk thermoelectrics via a panoscopic approach. Materials Today 16, 166-176 (2013).
[19] Z. Zheng, X. Su, R. Deng, C. Stoumpos, H. Xie, W. Liu, Y. Yan, S. Hao, C. Uher, C. Wolverton, Rhombohedral to cubic conversion of GeTe via MnTe alloying leads to ultralow thermal conductivity, electronic band convergence, and high thermoelectric performance. Journal of the American Chemical Society 140, 2673-2686 (2018).
[20] H. Okamoto, Ge-Te (germanium-tellurium). Journal of Phase Equilibria 21, 5 (2000).
[21] S. Perumal, S. Roychowdhury, K. Biswas, High performance thermoelectric materials and devices based on GeTe. Journal of Materials Chemistry C 4, 7520-7536 (2016).
[22] J. Li, X. Zhang, X. Wang, Z. Bu, L. Zheng, B. Zhou, F. Xiong, Y. Chen, Y. Pei, High-performance GeTe thermoelectrics in both rhombohedral and cubic phases. Journal of the American Chemical Society 140, 16190-16197 (2018).
[23] Y. Tung, M. L. Cohen, Relativistic band structure and electronic properties of SnTe, GeTe, and PbTe. Physical Review 180, 823 (1969).
[24] J. Li, X. Zhang, Z. Chen, S. Lin, W. Li, J. Shen, I. T. Witting, A. Faghaninia, Y. Chen, A. Jain, Low-symmetry rhombohedral GeTe thermoelectrics. Joule 2, 976-987 (2018).
[25] J. Li, Z. Chen, X. Zhang, H. Yu, Z. Wu, H. Xie, Y. Chen, Y. Pei, Simultaneous optimization of carrier concentration and alloy scattering for ultrahigh performance GeTe thermoelectrics. Advanced Science 4, 1700341 (2017).
[26] A. H. Edwards, A. C. Pineda, P. A. Schultz, M. G. Martin, A. P. Thompson, H. P. Hjalmarson, C. J. Umrigar, Electronic structure of intrinsic defects in crystalline germanium telluride. Physical Review B 73, 045210 (2006).
[27] J. Lewis, Band structure and nature of lattice defects in GeTe from analysis of electrical properties. Physica Status Solidi (B) 35, 737-745 (1969).
[28] R. Tsu, W. Howard, L. Esaki, Optical and electrical properties and band structure of GeTe and SnTe. Physical Review 172, 779 (1968).
[29] Y. Pei, A. LaLonde, S. Iwanaga, G. J. Snyder, High thermoelectric figure of merit in heavy hole dominated PbTe. Energy & Environmental Science 4, 2085-2089 (2011).
[30] X. Zhang, L.-D. Zhao, Thermoelectric materials: Energy conversion between heat and electricity. Journal of Materiomics 1, 92-105 (2015).
[31] L. C. Gomes, A. Carvalho, Phosphorene analogues: Isoelectronic two-dimensional group-IV monochalcogenides with orthorhombic structure. Physical Review B 92, 085406 (2015).
[32] T.-R. Wei, G. Tan, X. Zhang, C.-F. Wu, J.-F. Li, V. P. Dravid, G. J. Snyder, M. G. Kanatzidis, Distinct impact of alkali-ion doping on electrical transport properties of thermoelectric p-type polycrystalline SnSe. Journal of the American Chemical Society 138, 8875-8882 (2016).
[33] J. W. Yeh, S. K. Chen, S. J. Lin, J. Y. Gan, T. S. Chin, T. T. Shun, C. H. Tsau, S. Y. Chang, Nanostructured high‐entropy alloys with multiple principal elements: novel alloy design concepts and outcomes. Advanced Engineering Materials 6, 299-303 (2004).
[34] M.-H. Tsai, J.-W. Yeh, High-entropy alloys: a critical review. Materials Research Letters 2, 107-123 (2014).
[35] R. Liu, H. Chen, K. Zhao, Y. Qin, B. Jiang, T. Zhang, G. Sha, X. Shi, C. Uher, W. Zhang, Entropy as a gene‐like performance indicator promoting thermoelectric materials. Advanced Materials 29, 1702712 (2017).
[36] S. Shafeie, S. Guo, Q. Hu, H. Fahlquist, P. Erhart, A. Palmqvist, High-entropy alloys as high-temperature thermoelectric materials. Journal of Applied Physics 118, 184905 (2015).
[37] J. Vetter, J. Frühauf, H. G. Schneider, On the morphology of< 100>‐textured germanium crystallites in Zn—Ge eutectic alloy. Crystal Research and Technology 19, 499-505 (1984).
[38] L. Hu, Y. Zhang, H. Wu, J. Li, Y. Li, M. Mckenna, J. He, F. Liu, S. J. Pennycook, X. Zeng, Entropy engineering of SnTe: Multi‐principal‐element alloying leading to ultralow lattice thermal conductivity and state‐of‐the‐art thermoelectric performance. Advanced Energy Materials 8, 1802116 (2018).
[39] S. Perumal, S. Roychowdhury, K. Biswas, Reduction of thermal conductivity through nanostructuring enhances the thermoelectric figure of merit in Ge1−xBixTe. Inorganic Chemistry Frontiers 3, 125-132 (2016).
[40] Y. Zhang, Y. J. Zhou, J. P. Lin, G. L. Chen, P. K. Liaw, Solid‐solution phase formation rules for multi‐component alloys. Advanced Engineering Materials 10, 534-538 (2008).
[41] H.-S. Kim, Z. M. Gibbs, Y. Tang, H. Wang, G. J. Snyder, Characterization of Lorenz number with Seebeck coefficient measurement. APL materials 3, 041506 (2015).
[42] M. Seki, K. Hachiya, K. Yoshida, Photoluminescence and states in the bandgap of germanium sulfide glasses. Journal of Non-crystalline Solids 315, 107-113 (2003).
[43] N. Bauer, K. Fajans, The molar dispersion and refraction of free and bonded ions. Journal of the American Chemical Society 64, 3023-3034 (1942).
[44] H. Wiedemeier, P. Siemers, The thermal expansion of GeS and GeTe. Zeitschrift Für Anorganische und Allgemeine Chemie 431, 299-304 (1977).
[45] S. Mahadevan, R. Manojkumar, T. Jayakumar, C. Das, B. Rao, Precipitation-induced changes in microstrain and its relation with hardness and tempering parameter in 17-4 PH stainless steel. Metallurgical and Materials Transactions A 47, 3109-3118 (2016).
[46] L. Yang, J. Li, R. Chen, Y. Li, F. Liu, W. Ao, Influence of Se substitution in GeTe on phase and thermoelectric properties. Journal of Electronic Materials 45, 5533-5539 (2016).