碲化鍺屬於中高溫型熱電材料，近年，Ge0.87Pb0.13Te已被證明在GeTe為基底的熱電系統中具有優異的熱電性質，這歸因於碲化鍺本身良好的電性，以及鉛摻雜後產生的多尺度聲子散射中心，包括點缺陷、奈米級析出物和晶界等。現今Ge0.87Pb0.13Te研究的製備手法大多以水淬獲得合金後，再以高溫500~550℃熱壓或火花電漿燒結的方式將合金的粉末燒結成錠，其中火花電漿燒結具有較優異性質。然而部分研究火花電漿燒結的樣品性質卻難以穩定重現，另外電流效應對Ge0.87Pb0.13Te內部的晶體缺陷和微結構的影響也尚未完全了解。本研究據此目的分為兩部分探討：其一，為達到樣品製備的穩定性，探討燒結前熱處理對後續燒結的可能影響；其二，以低於常見燒結溫度的400℃進行電流輔助燒結，並探討電流在燒結中扮演的角色。實驗首先以「水淬」與「退火」製程製得之合金差異對後續熱壓燒結之影響進行探討，並分析討論「燒結前熱處理的差異」以及「燒結前熱處理差異對後續熱壓燒結的影響」。研究結果指出相較於水淬製程，退火製程製得之合金成分較為單一，使得後續熱壓燒結過程中較缺乏擴散驅動力，其母相組成仍留有較多Pb原子而有較低的載子濃度。試片室溫的載子濃度相較水淬熱壓樣品從5.54  1020 cm-3下降到4.28  1020 cm-3；同時因粉體間擴散較慢而使燒結密度較水淬樣品低，導致其載子遷移率由42 cm2/Vs 微幅下降到36 cm2/Vs。整體而言，儘管退火熱壓電阻率的上升，退火熱壓樣品載子濃度的下降，但較高的Seebeck係數可使Ge0.87Pb0.13Te材料在室溫的熱電功率因子由6.35 W/cmK2提升到6.75 μW/cmK2。另外，本實驗基於上述退火熱壓燒結之結果，進一步藉由電流輔助燒結，以相同於熱壓燒結的溫度製備Ge0.87Pb0.13Te樣品，嘗試探討電流對內部原子的影響及熱電傳輸性質的差異。研究結果指出電流輔助燒結過程中不僅引入了大量的焦耳熱，還促進了燒結過程中Ge0.87Pb0.13Te化合物的原子擴散，使系統整體更接近完全熱平衡狀態。與退火熱壓樣品相比，電流輔助燒結樣品室溫的載子濃度從4.28  1020 cm-3下降到3.85  1020 cm-3，Seebeck係數因此微幅提升；載子遷移率則因為燒結密度提升(由94.4%大幅提升至98.0% (理論密度))，導致其值由36 cm2/Vs 大幅上升到65 cm2/Vs；晶格熱導率則因大量〖Pb〗_Ge錯位缺陷的出現而由1.29 W/m·K大幅下降至0.46 W/m·K。最後，對於Ge0.87Pb0.13Te之高溫傳輸行為，本研究亦藉由變溫XRD，分析熱壓燒結與電流輔助燒結樣品的相變化溫度區間差異，並由相圖估算材料的相變化溫度區間隨成份均質度變化而改變之情形。整體而言，電流輔助燒結之Ge0.87Pb0.13Te因Seebeck係數的些微提升和電阻率進一步的降低，熱電功率因子可在773K達到34.6 W/cmK2、熱電優值達到1.5。

Germanium telluride (GeTe) is a class of thermoelectric materials suitable for operation at middle-high temperatures. Recently, it has been demonstrated that Ge0.87Pb0.13Te exhibits superior thermoelectric properties among various Ge-Te based compounds, which is attributed to superior electronic performance of GeTe matrix and the presence of multi-scale phonon scattering centers including point defects, nano-precipitation and grain boundaries in the material. Nowadays, most of Ge-Te based compounds prepared through consolidation of compound powders by hot-pressing technique or spark plasma sintering (SPS) method at the temperature around 500 ~ 550°C. The samples prepared by SPS usually had better properties. However, the properties of the SPS’ed samples are somewhat difficult to reproduce. In addition, how electric current affects lattice defects and microstructure of Ge0.87Pb0.13Te compound is still not fully understood. This study is divided into two parts: First, we intend to develop a pre-sintering heat treatment to achieve the sample stability during subsequent sintering and thermal operation. Second, we investigate the current effect on thermoelectric transport properties of Ge0.87Pb0.13Te compound prepared by current-assisted sintering method. In first part, we explored the effect of pre-sintering "water quenching" and "annealing" heat treatment on the properties of hot-pressed samples. The results indicated that the composition of the alloy prepared with the pre-sintering annealing process is relatively uniform, which makes more Pb atoms remain in crystal lattice during hot-pressing. Hence, the room-temperature carrier concentration of pre-sintering annealed sample is 4.28×1020 cm-3compared to the water-quenched sample with a value of 5.54×1020 cm-3. The carrier mobility decreases slightly from 42 cm2/V·s. to 36 cm2/V·s due to lower mass density. Although the resistivity slightly increased, the thermoelectric power factor of pre-sintering annealed sample still increased from 6.35 μW/cm·K2 to 6.75 μW/cm·K2 at room temperature owing to increased Seebeck coefficient. In the second part, we investigate the current effect on the thermoelectric properties of the samples prepared by current-assisted sintering. The results indicate that the electric current not only introduces extensive joule heating but also motivates atomic diffusion in the Ge-Te compounds during the sintering process. The carrier concentration of the electrically sintered sample decreases from 4.28×1020 cm-3 to 3.85×1020 cm-3, and hence the Seebeck coefficient is slightly increased. The carrier mobility is sharply increased from 36 cm2/V·s to 65 cm2/V·s due to the enhanced mass density after sintering, increasing from 94.4% to 98.0% of the theoretical density. The lattice thermal conductivity dropped sharply from 1.29 W/m·K to 0.46 W/m·K due to a large amount of 〖Pb〗_Ge anti-site defects. In addition, we have analyzed different temperature dependent thermoelectric properties of hot pressed and current-assisted sample. The phase change phenomena of the sintered Ge0.87Pb0.13Te samples are examined by the x-ray diffraction analysis. To sum up, the thermoelectric power factor of electrically sintered Ge0.87Pb0.13Te sample can reach 34.6 μW/cm·K2 and a zT value of 1.5 at the temperature of 773K. The improvement is mainly attributed to the increased Seebeck coefficient and the decreased resistivity of the electrically sintered samples.

摘要 I
ABSTRACT III
目錄 VII
圖目錄 IX
表目錄 XII
壹、緒論 1
1.1 研究背景 1
1.2 研究動機 5
貳、文獻回顧 6
2.1 碲化鍺系化合物 7
2.1.1 碲化鍺的晶體結構 7
2.1.2 碲化鍺的電子結構 8
2.1.3 碲化鍺之晶格缺陷 10
2.2 鉛摻雜之碲化鍺化合物 12
2.2.1 鉛摻雜碲化鍺之晶格缺陷 13
2.2.2 GeTe-PbTe之相圖 14
2.2.3 多尺度微結構效應與Ge0.87Pb0.13Te之微結構 16
2.3 電流輔助粉末燒結 19
2.4 鉛原子電致遷移現象 22
參、實驗流程及分析方法 23
3.1 試片製備 25
3.2 量測方法 27
3.2.1 Seebeck係數與四點式電阻量測 27
3.2.2 霍爾效應量測 28
3.2.3 熱傳導係數量測 30
3.2.4 表面形貌、析出物、成份分析 32
3.2.5 X光繞射分析 32
肆、結果與討論 34
4.1 GETE-PBTE系統粉末燒結前熱處理製程效應探討 34
4.1.1 水淬與退火製程對GeTe-PbTe熔煉合金成份及微結構之影響 34
4.1.2 水淬與退火製程對GeTe-PbTe熔煉合金後續熱壓燒結成份之影響 39
4.1.3 水淬與退火製程對GeTe-PbTe熔煉合金經熱壓燒結後之熱電性質影響 43
4.2 GETE-PBTE系統經熱壓與電流輔助燒結後傳輸性質比較 47
4.2.1 電流對Ge0.87Pb0.13Te載子濃度之影響 49
4.2.2 電流對Ge0.87Pb0.13Te載子遷移率的影響 54
4.2.3 電流對Ge0.87Pb0.13Te變溫電性之影響 56
4.2.4 電流對Ge0.87Pb0.13Te熱傳導率的影響 62
伍、結論 68
陸、參考文獻 70

[1] D. M. Rowe, Thermoelectrics handbook: macro to nano. (CRC press, 2018).
[2] G. S. Nolas, J. Sharp, J. Goldsmid, Thermoelectrics: basic principles and new materials developments. (Springer Science & Business Media, 2013), vol. 45.
[3] C.-N. Liao, K.-M. Liou, H.-S. Chu, Enhancement of thermoelectric properties of sputtered Bi–Sb–Te thin films by electric current stressing. Applied Physics Letters 93, 042103 (2008).
[4] C.-N. Liao, L.-C. Wu, J.-S. Lee, Thermoelectric properties of Bi–Sb–Te materials prepared by electric current stressing. Journal of Alloys and Compounds 490, 468-471 (2010).
[5] C.-N. Liao, L.-C. Wu, Enhancement of carrier transport properties of BixSb2− xTe3 compounds by electrical sintering process. Applied Physics Letters 95, 052112 (2009).
[6] Y.-H. Chen, C.-N. Liao, Transport properties of electrically sintered bismuth antimony telluride with antimony nanoprecipitation. Applied Physics Letters 111, 143901 (2017).
[7] S. Perumal, S. Roychowdhury, K. Biswas, High performance thermoelectric materials and devices based on GeTe. Journal of Materials Chemistry C 4, 7520-7536 (2016).
[8] 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 (2011).
[9] 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).
[10] 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, e353 (2017).
[11] 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 (2012).
[12] J. Li, X. Zhang, S. Lin, Z. Chen, Y. Pei, Realizing the high thermoelectric performance of GeTe by Sb-doping and Se-alloying. Chemistry of Materials 29, 605-611 (2016).
[13] J. He, M. G. Kanatzidis, V. P. Dravid, High performance bulk thermoelectrics via a panoscopic approach. Materials Today 16, 166-176 (2013).
[14] H. Okamoto, Ge-Te (germanium-tellurium). Journal of phase equilibria 21, 496-496 (2000).
[15] S. Perumal, S. Roychowdhury, D. S. Negi, R. Datta, K. Biswas, High thermoelectric performance and enhanced mechanical stability of p-type Ge1–xSbxTe. Chemistry of Materials 27, 7171-7178 (2015).
[16] 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).
[17] Y. Tung, M. L. Cohen, Relativistic band structure and electronic properties of SnTe, GeTe, and PbTe. Physical Review 180, 823 (1969).
[18] 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).
[19] 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).
[20] 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).
[21] J. Lewis, Band structure and nature of lattice defects in GeTe from analysis of electrical properties. physica status solidi (b) 35, 737-745 (1969).
[22] R. Tsu, W. Howard, L. Esaki, Optical and electrical properties and band structure of GeTe and SnTe. Physical Review 172, 779 (1968).
[23] 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).
[24] Y. Gelbstein, J. Davidow, E. Leshem, O. Pinshow, S. Moisa, Significant lattice thermal conductivity reduction following phase separation of the highly efficient GexPb1–xTe thermoelectric alloys. physica status solidi (b) 251, 1431-1437 (2014).
[25] D. Hohnke, H. Holloway, S. Kaiser, Phase relations and transformations in the system PbTe-GeTe. Journal of Physics and Chemistry of Solids 33, 2053-2062 (1972).
[26] P. Ramachandrarao, S. Misra, T. Anantharaman, Thermodynamic and constitutional studies of the PbTe-GeTe system. Journal of Materials Science 10, 1849-1855 (1975).
[27] S. G. Parker, J. E. Pinnell, L. N. Swink, Determination of the liquidus-solidus curves for the system PbTe-GeTe. Journal of Materials Science 9, 1829-1832 (1974).
[28] Y. Gelbstein, J. Davidow, S. N. Girard, D. Y. Chung, M. Kanatzidis, Controlling metallurgical phase separation reactions of the Ge0. 87Pb0. 13Te alloy for high thermoelectric performance. Advanced Energy Materials 3, 815-820 (2013).
[29] B. Qiu, H. Bao, G. Zhang, Y. Wu, X. Ruan, Molecular dynamics simulations of lattice thermal conductivity and spectral phonon mean free path of PbTe: Bulk and nanostructures. Computational Materials Science 53, 278-285 (2012).
[30] D. Wu, L.-D. Zhao, S. Hao, Q. Jiang, F. Zheng, J. W. Doak, H. Wu, H. Chi, Y. Gelbstein, C. Uher, Origin of the high performance in GeTe-based thermoelectric materials upon Bi2Te3 doping. Journal of the American Chemical Society 136, 11412-11419 (2014).
[31] M. J. Polking, H. Zheng, R. Ramesh, A. P. Alivisatos, Controlled synthesis and size-dependent polarization domain structure of colloidal germanium telluride nanocrystals. Journal of the American Chemical Society 133, 2044-2047 (2011).
[32] W. Pompe, X. Gong, Z. Suo, J. Speck, Elastic energy release due to domain formation in the strained epitaxy of ferroelectric and ferroelastic films. Journal of Applied Physics 74, 6012-6019 (1993).
[33] D. M. Hulbert, A. Anders, D. V. Dudina, J. Andersson, D. Jiang, C. Unuvar, U. Anselmi-Tamburini, E. J. Lavernia, A. K. Mukherjee, The absence of plasma in “spark plasma sintering”. Journal of Applied Physics 104, 033305 (2008).
[34] U. Anselmi-Tamburini, S. Gennari, J. Garay, Z. A. Munir, Fundamental investigations on the spark plasma sintering/synthesis process: II. Modeling of current and temperature distributions. Materials Science and Engineering: A 394, 139-148 (2005).
[35] P. S. Ho, T. Kwok, Electromigration in metals. Reports on Progress in Physics 52, 301 (1989).
[36] X. Wang, J. Guo, Y. Wang, X. Wu, B. Wang, Segregation of lead in Cu–Zn alloy under electric current pulses. Applied physics letters 89, 061910 (2006).
[37] Y. Liu, M. Pritzker, Effect of pulse plating on composition of Sn–Pb coatings deposited in fluoroborate solutions. Journal of applied electrochemistry 33, 1143-1153 (2003).
[38] L. Weintraub, J. Davidow, J. Tunbridge, R. Dixon, M. J. Reece, H. Ning, I. Agote, Y. Gelbstein, Investigation of the microstructural and thermoelectric properties of the (GeTe)0.95(Bi2Te3)0.05 composition for thermoelectric power generation applications. Journal of Nanomaterials 2014, 2 (2014).
[39] Y. Gelbstein, O. Ben-Yehuda, Z. Dashevsky, M. P. Dariel, Phase transitions of p-type (Pb, Sn, Ge) Te-based alloys for thermoelectric applications. Journal of Crystal Growth 311, 4289-4292 (2009).
[40] E. Levin, M. Besser, R. Hanus, Electronic and thermal transport in GeTe: A versatile base for thermoelectric materials. Journal of Applied Physics 114, 083713 (2013).
[41] J. Li, Y. Xie, C. Zhang, K. Ma, F. Liu, W. Ao, Y. Li, C. Zhang, Stacking Fault Induced Minimized Lattice Thermal Conductivity in the High-Performance GeTe-Based Thermoelectric Materials upon Bi2Te3 Alloying. ACS applied materials & interfaces, (2019).
[42] 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).