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Oxide thermoelectric materials have been considered to be potential candidates in high-temperature thermoelectric power generation, however, their high thermal conductivity renders them inferior to the conventional thermoelectric materials and limit their practical application. In this paper, we successfully reduce the thermal conductivity of CdO polycrystals through Ca2+ doping, and the improvement in ZT is also obtained due to the low thermal conductivity. Cd1-xCaxO (x=0, 0.01, 0.03, 0.08) polycrystals are synthesized by adding CaCO3 into CdO via conventional solid-state reaction method and their high-temperature thermoelectric properties are studied. XRD results reveal that all samples are composed of CdO polycrystals, and the lattice parameters increase with Ca2+ content due to the larger radius of Ca2+ as compared with that of Cd2+. Addition of CaCO3 can induce the formation of point defects as well as pores in the CdO polycrystals, thus inhibits the grain growth of CdO and induces the increase of grain boundaries. The main electron carriers in CdO are reported to be shallow level donor impurities formed by oxygen vacancies; as the Ca2+ concentration in Cd1-xCaxO increases, the conduction band minimum of the samples shifts upward and the level of donor impurity becomes deeper, finally resulting in the decrease of electron carrier concentration. Meanwhile, the reduced carrier concentration in the doped samples leads to the increase of both the electrical resistivity ρ and the absolute Seebeck coefficient |S|, while the electrical thermal conductivity κ e will decrease with increasing Ca content. Investigations on the thermal properties of the obtained samples demonstrate that the introduction of Ca2+ is effective to suppress the thermal conductivity. The increment of pores and grain boundaries in the doped samples will enhance the long-wavelength phonon scattering, resulting in the decrease of phonon thermal conductivity κ p. Furthermore, the point defects, which come from the mass and size differences between Ca and Cd atoms, also act as scattering centers and lead to a considerable decrease in phonon thermal conductivity. Due to the simultaneous reduction of both electrical and phonon thermal conductivity, the total thermal conductivity κ may substantially be suppressed, for example, the total thermal conductivity of Cd0.95Ca0.05O reaches 2.2 W·m-1·K-1 at 1000 K, a remarkable decrease as compared with pristine CdO, which is 3.6 W·m-1·K-1 measured at the same temperature. Benefiting from the drastically reduced thermal conductivity, Cd0.99Ca0.01O polycrystals can achieve a high ZT of 0.42 at 1000 K, 27% higher than the pure CdO, which is one of the best n-type oxide TE materials reported so far.
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Keywords:
- thermoelectric /
- CdO /
- thermal conductivity /
- ZT
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[16] Wang S F, L Q, Li L J, Fu G S, Liu F Q, Dai S Y, Yu W, Wang J L 2013 Scripta Mater. 69 533
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[30] He Q Y, Hu S J, Tang X G, Lan Y C, Yang J, Wang X W, Ren Z F, Hao Q, Che G 2008 Appl. Phys. Lett. 93 042108
[31] Wan C L, Pan W, Xu Q, Qin Y X, Wang J D, Qu Z X, Fang M H 2006 Phys. Rev. B 74 144109
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[1] Zhang X, Ma X Y, Zhang F P, Wu P X, Lu Q M, Liu Y Q, Zhang J X 2012 Acta Phys. Sin. 61 047201 (in Chinese) [张忻, 马旭颐, 张飞鹏, 武鹏旭, 路清梅, 刘燕琴, 张久兴 2012 物理学报 61 047201]
[2] Heremans P J, Jovovic V, Toberer S E, Saramat A, Kurosaki K, Charoenphakdee A, Yamanaka S, Snyder J G 2008 Science 321 554
[3] He Y, Day T, Zhang T S, Liu H L, Shi X, Chen L D, Snyder J G 2014 Adv. Mater. 26 3974
[4] Zhang H, Luo J, Zhu H T, Liu Q L, Liang J K, Rao G H 2012 Acta Phys. Sin. 61 086101 (in Chinese) [张贺, 骆军, 朱航天, 刘泉林, 梁敬魁, 饶光辉 2012 物理学报 61 086101]
[5] Ohta H, Kim S, Komune Y, Mizoguchi T, Nomura K, Ohta S, Momura T, Ikuhara Y, Hirano M, Hosono H, Koumoto K 2007 Nat. Mater. 6 129
[6] Lan J L, Liu Y C, Zhan B, Lin Y H, Zhang B P, Yuan X, Zhang W Q, Xu W, Nan C W 2013 Adv. Mater. 25 5086
[7] Zhu X B, Shi D Q, Dou S X, Sun Y P, Li Q, Wang L, Li W X, Yeoh W K, Zheng R K, Chen Z X, Kong C X 2010 Acta Mater. 58 4281
[8] Wang H C, Wang C L, Su W B, Liu J, Sun Y, Peng H, Zhang J L, Zhao M L, Li J C, Yin N, Mei L M 2011 Acta Phys. Sin. 60 087203 (in Chinese) [王洪超, 王春雷, 苏文斌, 刘剑, 孙毅, 彭华, 张家良, 赵明磊, 李吉超, 尹娜, 梅良模 2011 物理学报 60 087203]
[9] Wu Z H, Xie H Q, Zhai Y B, Gan L H, Liu J 2015 Chin. Phys. B 24 034402
[10] Ohtaki M, Araki K, Yamamoto K 2009 J. Electron. Mater. 38 1234
[11] B'erardan D, Guilmeau E, Maignan A, Raveau B 2008 Solid State Commun. 146 97
[12] Liu Y, Lin Y H, Lan J L, Xu W, Zhang B P, Nan C W, Zhu H M 2010 J. Am. Ceram. Soc. 93 2938
[13] Lubeck C R, Han T Y-J, Gash A E, Satcher J H, Jr, Doyle F M 2006 Adv. Mater. 18 781
[14] Yu H J, Jeong M, Lim Y S, Seo W-S, Kwon O-J, Park C-H, Hwang H-J 2014 RSC Adv. 4 43811
[15] Wang S F, Liu F Q, L Q, Dai S Y, Wang J L, Yu W, Fu G S 2013 J. Eur. Ceram. Soc. 33 1763
[16] Wang S F, L Q, Li L J, Fu G S, Liu F Q, Dai S Y, Yu W, Wang J L 2013 Scripta Mater. 69 533
[17] Li L J, Liang S, Li S M, Wang J L, Wang S F, Dong G Y, Fu G S 2014 Nanotechnology 25 425402
[18] Ohta S, Nomura T 2005 Appl. Phys. Lett. 87 092108
[19] Bocher L, Aguirre M H, Logvinovich D, Shkabko A, Robert R, Trottmann M, Weidenkaff A 2008 Inorg. Chem. 47 8077
[20] Lan J-L, Liu Y, Lin Y-H, Nan C-W, Cai Q, Yang X 2015 Sci. Rep. 5 7783
[21] Lin C-J, Wei W-C J 2008 Mater. Chem. Phys. 111 82
[22] Park K, Seong J K, Kim G H 2009 J. Alloys Compd. 473 423
[23] Liu H, Fang L, Wu F, Tian D X, Li W J, Lu Y, Kong C Y, Hang S F 2014 Surf. Rev. Lett. 21 1450033
[24] Burbano M, Scanlon D O, Watson G W 2011 J. Am. Chem. Soc. 133 15065
[25] Pelatt B D, Ravichandran R, Wager J F, Keszler D A 2011 J. Am. Chem. Soc. 133 16852
[26] Francis C A, Detert D M, Chen G, Dubon O D, Yu K M, Walukiewicz W 2015 Appl. Phys. Lett. 106 022110
[27] Guibin C, Yu K M, Reichertz L A, Walukiewicz W 2013 Appl. Phys. Lett. 103 041902
[28] Mun H, Choi S-M, Lee K H, Kim S W ChemSusChem. Published online: 17 MAR 2015, DOI: 10. 1002/cssc. 201403485
[29] Zhou X, Wang G, Zhang L, Chi H, Su X, Sakamotob J, Uher C 2012 J. Mater. Chem. 22 2958
[30] He Q Y, Hu S J, Tang X G, Lan Y C, Yang J, Wang X W, Ren Z F, Hao Q, Che G 2008 Appl. Phys. Lett. 93 042108
[31] Wan C L, Pan W, Xu Q, Qin Y X, Wang J D, Qu Z X, Fang M H 2006 Phys. Rev. B 74 144109
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