| 摘要: | 2008年吳茂昆院士的研究團隊發現了新的「鐵基超導體」(FeSe)，其超導溫度TC約為8 K，當溫度低於90 K時會伴隨著結構相變從Tetragonal變為Orthorhombic結構，且隨著外加壓力至7 GPa超導溫度TC從8 K提升至37 K。二維FeSe之薄膜鍍於不同溫度的基板其超導溫度TC與膜厚的關係也有顯著的差異。鍍於320℃基板，薄膜容易沿著C軸成長，隨著膜厚減小超導溫度Tc也有降低的趨勢；於500℃基板上成長FeSe薄膜，其容易沿著(101)方向成長，超導溫度Tc不易隨膜厚而改變。本文研究一維FeSex奈米微粒其磁性以及熱力學等物理性質與塊材、薄膜樣品比較。藉由脈衝雷射將塊材FeSex製作成奈米微粒。並將X光繞射數據利用GSAS軟體擬合，擬合結果指出奈米微粒中包含了FeSe之tetragonal、 hexagonal結構，純鐵、純硒以及氧化鐵。從高解析度穿透式電子顯微鏡以及選區電子繞射之結果可得知FeSex –tetragonal結構奈米微粒粒徑分部約為5 nm。從300 至10 K變溫X光繞射實驗中FeSex –tetragonal結構之奈米微粒並未發生結構相轉變。 溫室氣體排放量持續增高以及能源逐漸短缺，新興能源不斷被開發，也因此熱電材料逐漸地被廣泛研究。在室溫下常見的熱電材料以Bi2Te3¬最為廣泛研究，故本文研究Bi2Te3¬單晶奈米線製備方式以及 量測單根奈米線之熱電性質(電阻率、Seebeck係數、熱傳導率)。本研究之Bi2Te3奈米線為Rhombohedral結構並沿(110)方向成長，於直徑150 nm之奈米線其電阻率(ρ=5.16 μΩ-m)小於塊材一倍。然而奈米線Seebeck係數之絕對值(S= 43.44μV/K)低於塊材(S= 213.84μV/K)，其主因為Seebeck係數容易受到Bi與Te原子百分比知些微差異而有劇烈變化。本文研究結果發現奈米線之熱電優質係數ZT座落於0.12~0.34之間。 In 2008, a novel Iron-based superconductor-FeSe was discovered by M.K. Wu. The superconductor has a transition temperature, Tc, of around 8 K. It is accompanied by a structural phase change, from Tetragonal to Orthorhombic, as the temperature lowers below 90 K. Moreover, with applied pressures up to 7 Gpa, an increase in Tc, to 37 K, was observed. In two dimensional FeSex thin films, the superconducting properties change with deposition temperature. Films grown at 320°C prefer to orient along the c-axis, and its Tc decreased with film thickness. Contrasting this, films grown at 500°C orient aptly along the (101) direction without obvious thickness dependence. In this investigation the magnetic and thermal properties of one dimensional FeSex nanoparticles were studied; the results were compared to bulk. The nanoparticles were fabricated using pulse laser deposition (PLD) using bulk FeSex targets. Refinements of X-ray diffraction (XRD) data were performed using GSAS software. The results revealed the nanoparticles composition: phases of tetragonal and hexagonal FeSe, pure iron, pure selenium, and FeO. The sizes of nanoparticles are about 5 nm in tetragonal FeSe, as confirmed by transmission electron microscopy (TEM) and Selected Area Electron Diffraction Pattern (SAED). Structural phase transition was not observed in nanoparticles. The particles maintained its tetragonal phase from temperatures around 300 K down to 10 K. With the perpetuating increase in green house gases and the gradual shortage of conventional energy sources, new and more efficient energy substitute is unceasingly being developed and sought after. Below room temperature, the thermoelectric material Bi2Te3 is one such candidate widely studied. In this investigation, the fabrication of single crystal Bi2Te3 nanowires is studied along with the TE properties of a single nanowire (Seebeck coefficient, resistance, and thermal conductivity). In this study, Rhombohedral structured Bi2Te3 nanowires grown along (110) direction were prepared. The typical resistivity of the as prepared nanowires (150 nm in diameter) was around 5.16 μΩ-m, half that of its bulk counterpart. However, the absolute value of Seebeck coefficient in the nanowires (S= 43.44μV/K) is lower than the bulk value (S= 213.84μV/K).This is due to the sensitivity of Seebeck coefficient to the atomic composition of Bi and Te. From our results, ZT values in the range of 0.12~0.34 were obtained. |
