| dc.description.abstract | 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.
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