Single-crystal Bi-Sb-Te based thermoelectric material with a nearly zT value of 1 is a promising candidate for either cooling application or electricity generation at room temperature region. Nevertheless, the brittle nature of single crystal Bi-Sb-Te is not suitable for device assembly. Mechanically improved polycrystalline Bi-Sb-Te compounds suffer a slight deterioration in thermoelectric properties. Recent researches make possible a higher zT value of 1 with nanocrystalline Bi-Sb-Te powders compacted by spark plasma sintering (SPS) process. Therefore, the importance of applied electric current cannot be neglected.
In this study, Bi-Sb-Te compounds were prepared by a powder metallurgical method with electrical sintering (ES) process. An electric current of density ~100 A/cm2 was introduced through a cold-pressed Bi-Sb-Te bulk during sintering. The applied electric current was found to be effective in elimination of crystal defects, leading to a high Hall mobility but moderately low carrier concentration in the sintered specimen. Theoretical calculation and experimental results indicate the carrier concentration of electrically sintered cold-pressed Bi0.4Sb1.6Te3 sample is optimized and gives rise to a zT value of 0.78. The cold-pressed specimen deteriorates the zT value due to its low relative density (~90%), while the electrically sintered hot-pressed sample can reach a zT value of 0.91 by increasing relative density (~99%) of the compacted bulk.

製備成單晶結構的Bi-Sb-Te 熱電材料具有在室溫下接近1的熱電優值(zT值)，此種材料可以作為冷卻或者是發電的應用。然而此種單晶結構因為機械性質不佳，作為元件使用時，切削成特定尺寸容易沿著特定結晶方向破裂。利用粉末冶金製備多晶結構的Bi-Sb-Te熱電材料雖然有較差的zT值，卻可以提高其機械強度。最近的研究指出，利用放電等離子燒結技術(Spark Plasma Sintering, SPS)製備Bi-Sb-Te奈米粉末成塊材具有超越1的zT300K值。所以在SPS製程中，額外加入的電流必定在燒結過程中扮演某種程度的效應。
本研究中利用粉末冷壓成的Bi-Sb-Te熱電塊材，再搭配電流燒結的方法。實驗過程中在冷壓試片中通入密度為100(A/cm2)的電流，實驗結果發現額外的電流有助於消除晶體內部的缺陷，使得燒結後Bi-Sb-Te試片中載子遷移率大幅度的上升而載子濃度小幅度的下降。經由理論以及實驗證實Bi0.4Sb1.6Te3冷壓試片經過電流燒結後有最佳的載子濃度，得到的zT300K值為0.78。而此zT值相較於文獻較低，是由於冷壓試片的相對密度較小(~90%)的關係，之後利用熱壓通電流處理的Bi-Sb-Te試片中，在相對密度接近理論密度(~99%)的試片得到的zT300K值為0.91。

[1] Http://old.npf.org.tw/PUBLICATION/SD/091/SD-R-091-029.htm
[2] D. M. Rowe, CRC handbook of thermoelectrics (1995) p19-25
[3] G. S. Nolas, J. Sharp and H. J. Goldsmid, Thermoelectrics: Basic Principles and New Materials Developments (2001) Ch1-Ch3
[4] G. J. Snyder and E. S. Toberer, Complex thermoelectric materials, Nature Materials 7 (2008), no. 2, 105-114.
[5] H. J. Goldsmid and R. W. Douglas, The use of semiconductors in thermoelectric refrigeration, British Journal of Applied Physics 5 (1954), 386-390.
[6] B. Poudel, Q. Hao, Y. Ma, Y. Lan, A. Minnich, B. Yu, X. Yan, D. Wang, A. Muto, D. Vashaee, X. Chen, J. Liu, M. S. Dresselhaus, G. Chen and Z. Ren, High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys, Science 320 (2008), 634-638.
[7] X. Tang, W. Xie, H. Li, W. Zhao, Q. Zhang and M. Niino, Preparation and thermoelectric transport properties of high-performance p-type Bi2Te3 with layered nanostructure, Applied Physics Letters 90 (2007), no. 1, 012102.
[8] Y. Q. Cao, X. B. Zhao, T. J. Zhu, X. B. Zhang and J. P. Tu, Syntheses and thermoelectric properties of Bi2Te3/Sb2Te3 bulk nanocomposites with laminated nanostructure, Applied Physics Letters 92 (2008), no. 14, 143106.
[9] W. M. Yim and F. D. Rosi, Compound tellurides and their alloys for Peltier cooling-A Review, Solid-State Electronics 15 (1972), 1121-1140.
[10] B. R. Nag, Electron Transport in Compound Semiconductors (1980) p7
[11] J. O. Sofo and G. D. Mahan, Optimum band gap of a thermoelectric material, Physical Review B 49 (1994), no. 7, 4565-4570.
[12] M. J. Smith, R. J. Knight and C. W. Spencer, Properties of Bi2Te3-Sb2Te3 Alloys, Journal of Applied Physics 33 (1962), 2186-2190.
[13] H. W. Jeon, H. P. Ha, D. B. Hyun and J. D. Shim, Electrical and thermoelectrical properties of undoped Bi2Te3-Sb2Te3 and Bi2Te3-Sb2Te3-Sb2Se3 single crystals, Journal of Physics and Chemistry of Solids 52 (1991), no. 4, 579-585.
[14] F. D. Rosi, B. Abeles and R. V. Jensen, Materials for thermoelectric refrigeration, Journal of Physics and Chemistry of Solids 10 (1959), 191-200.
[15] D. M. Rowe, CRC Handbook of Thermoelectrics (1995) p212
[16] G. F. Wang and T. Cagin, Electronic structure of the thermoelectric materials Bi2Te3 and Sb2Te3 from first-principles calculations, Physical Review B 76 (2007), no. 7, 075201.
[17] G. R. Miller and C.-Y. Li, Evidence for the existence of antistructure defects in bismuth telluride by density measurements, Journal of Physics and Chemistry of Solids 26 (1964), 173-177.
[18] L. R. Testardi and J. R. Wiese, Density anomalies in the Bi2Te3-Sb2Te3 system, Transactions of the American Institute of Mining, Metallurgical, and Petroleum Engineers, Incorporated 221 (1961), 647.
[19] J. Horak, K. Cermak and L. Koudelka, Energy formation of antisite defects in doped Sb2Te3 and Bi2Te3 crystals, Journal of Physics and Chemistry of Solids 47 (1986), no. 8, 805-809.
[20] T. Caillat, M. Carle, P. Pierrat, H. Scherrer and S. Scherrer, Thermoelectric properties of (BixSb1-x)2Te3 single crystal solid solutions grown by the T.H.M. method, Journal of Physics and Chemistry of Solids 53 (1992), no. 8, 1121-1129.
[21] J. Jiang, L. D. Chen, S. Q. Bai, Q. Yao and Q. Wang, Thermoelectric properties of p-type (Bi2Te3)(x)(Sb2Te3)(1-x) crystals prepared via zone melting, Journal of Crystal Growth 277 (2005), no. 1-4, 258-263.
[22] L. D. Ivanova and Y. V. Granatkina, Properties of single crystals in the Sb2Te3-Bi2Te3 solid-solution system, Inorganic Materials 31 (1995), no. 6, 678-681.
[23] O. Yamashita, S. Tomiyoshi and K. Makita, Bismuth telluride compounds with high thermoelectric figures of merit, Journal of Applied Physics 93 (2003), 368-374.
[24] X. A. Fan, J. Y. Yang, R. G. Chen, H. S. Yun, W. Zhu, S. Q. Bao and X. K. Duan, Characterization and thermoelectric properties of p-type 25%Bi2Te3-75% Sb2Te3 prepared via mechanical alloying and plasma activated sintering, Journal of Physics D-Applied Physics 39 (2006), no. 4, 740-745.
[25] W. Xie, X. Tang, Y. Yan, Q. Zhang and T. M. Tritt, Unique nanostructures and enhanced thermoelectric performance of melt-spun BiSbTe alloys, Applied Physics Letters 94 (2009), no. 10, 102111.
[26] Y. Ma, Q. Hao, B. Poudel, Y. C. Lan, B. Yu, D. Z. Wang, G. Chen and Z. F. Ren, Enhanced thermoelectric figure-of-merit in p-type nanostructured bismuth antimony tellurium alloys made from elemental chunks, Nano Letters 8 (2008), no. 8, 2580-2584.
[27] K. Kishimoto and T. Koyanagi, Preparation of sintered degenerate n-type PbTe with a small grain size and its thermoelectric properties, Journal of Applied Physics 92 (2002), no. 5, 2544-2549.
[28] C.-N. Liao, K.-M. Liou and H.-S. Chu, Enhancement of thermoelectric properties of sputtered Bi-Sb-Te thin films by electric current stressing, Applied Physics Letters 93 (2008), 042103.
[29] N. Keawprak, Z. M. Sun, H. Hashimoto and M. W. Barsoum, Effect of sintering temperature on the thermoelectric properties of pulse discharge sintered (Bi0.24Sb0.76)2Te3 alloy, Journal of Alloys and Compounds 397 (2005), 236-244.
[30] L. R. Testardi and McConnel.Gk, Measurement of the Seebeck Coefficient with Small Temperature Differences, Review of Scientific Instruments 32 (1961), no. 9, 1067-1068.
[31] A. A. Joraide, Thermoelectric properties of fine-grained sintered (Bi2Te3)25-(Sb2Te3)75 p-type solid solution, Journal of Materials Science 30 (1995), no. 3, 744-748.
[32] R. Ionescu, J. Jaklovszky and N. Nistor, Grain size effects on thermoelectrical properties of sintered solid solutions based on Bi2Te3, Physica Status Solidi A-Applications and Materials Science 27 (1975), 27-34.
[33] Z. Stary, J. Horak, M. Stordeur and M. Stolzer, Antisite defects in Sb2-xBixTe3 mixed crystals, Journal of Physics and Chemistry of Solids 49 (1988), no. 1, 29-34.
[34] H. Noro, K. Sato and H. Kagechika, The thermoelectric properties and crystallography of Bi-Sb-Te-Se thin films grown by ion beam sputtering, Journal of Applied Physics 73 (1993), no. 3, 1252-1260.
[35] M. Stordeur, M. Stölzer, H. Sobotta and V. Riede, Investigation of the Valence Band Structure of Thermoelectric (Bi1-xSbx)2Te3 Single Crystals, Physica Status Solidi B-Basic Research 150 (1988), 165-176.
[36] J. P. Fleurial, L. Gailliard, R. Triboulet, H. Scherrer and S. Scherrer, Thermal-properties of high-quality single crystals of bismuth telluride - part 2 mixed-scattering model, Journal of Physics and Chemistry of Solids 49 (1988), no. 10, 1249-1257.
[37] B. R. Nag, Electron Transport in Compound Semiconductors (1980) p221
[38] X. A. Fan, J. Y. Yang, W. Zhu, S. Q. Bao, X. K. Duan and Q. Q. Zhang, Thermoelectric properties of p-type Te-doped (Bi,Sb)2Te3 alloys by mechanical alloying and plasma activated sintering, Journal of Alloys and Compounds 448 (2008), no. 1-2, 308.
[39] G. S. Nolas, J. Sharp and H. J. Goldsmid, Thermoelectrics: Basic Principles and New Materials Developments (2001) p61
[40] J. Yang, T. Aizawa, A. Yamamoto and T. Ohta, Thermoelectric properties of p-type (Bi2Te3)(x)(Sb2Te3)(1-x) prepared via bulk mechanical alloying and hot pressing, Journal of Alloys and Compounds 309 (2000), no. 1-2, 225-228.
[41] K. Kishimoto, K. Yamamoto and T. Koyanagi, Influences of potential barrier scattering on the thermoelectric, properties of sintered n-type PbTe with a small grain size, Japanese Journal of Applied Physics 42 (2003), no. 2A, 501-508.
[42] D. B. Hyun, T. S. Oh, J. S. Hwang and J. D. Shim, Effect of excess Te addition on the thermoelectric properties of the 20% Bi2Te3-80% Sb2Te3 single crystal and hot-pressed alloy, Scripta Materialia 44 (2001), no. 3, 455-460.