Electronic structure and thermoelectric properties of Ti and Si doped Fe2VGa

熱電材料被視為其中一種可以解決能源問題的材料，其中具有高功率因子(power factor)的Heusler系統近年來被廣泛的研究。這篇論文中，我們利用取代效應探討鐵釩鎵(Fe2VGa) Heusler系統的熱電性質以及磁性性質，其中包括鈦原子(Ti)取代釩原子(V)跟矽原子(Si)取代鎵原子(Ga)。我們使用電弧熔煉法合成所有樣品，包括Fe2V1-xTixGa (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25) 和 Fe2VGa1-xSix (x = 0, 0.05, 0.1, 0.15, 0.2)。在X光繞射的分析中，我們展示了所有樣品都是L21的晶體結構還有每個樣品的晶格常數；同時，我們利用能量分散式光譜儀揭露了樣品的化學計量式。當取代濃度大於0.1時，兩個不同取代系統的功率因子(power factor=S2/ρ)皆會大幅度的提升，這現象可以歸功於能態密度中費米能階的移動。由能帶計算中我們得知鐵釩鎵系統的費米能階坐落在pseudo gap中，然而取代效應使費米能階移出pseudo gap，進而跟能態密度有交錯，導致Seebeck常數上升，而功率因子又與Seebeck常數成平方正比的關係，所以兩個不同取代系統的功率因子皆大幅度的提升。因為合金效應的關係，使所有有取代樣品的傳熱性都大幅度被壓抑，其中Fe2VGa0.8Si0.2的熱傳導性被抑制了兩倍。因為傳熱性的抑制以及同時功率因子的提升，使得Fe2V0.8Ti0.2Ga的熱電優值在420 K時較未被Ti取代之母材Fe2VGa提高了10倍。另外，我們也介由觀察樣品的磁化率以及磁化量探討了這些樣品的磁性性質。Thermoelectric application has been considered as a possible solution for electric crises, and, recently, Heusler alloys have been studied for its large power factor near room temperature. In this thesis, we investigate the thermoelectric and magnetic properties of Ti-substituted (p-type) and Si-substituted (n-type) Heusler alloy Fe2VGa. All samples including Fe2V1-xTixGa (with x = 0, 0.05, 0.1, 0.15, 0.2, 0.25) and Fe2VGa1-xSix (with x = 0, 0.05, 0.1, 0.15, 0.2) are prepared through arc-melting method. The X-ray refinement shows their L21 crystal structure and corresponding lattice parameters, while the Energy-dispersive X-ray spectroscopy (EDX) reveals the stoichiometry. With proper substitution, with x > 0.1, the power factor of both systems is dramatically enhanced, which can be attributed to the Fermi level shifting. According to the theoretical calculation for the density of state, the Fermi level of the un-doped sample is located within the pseudogap, while the Fermi level starts to move out the pseudogap since the substituting effect applying, and it will consequently intercept with the conduction or valence band. Due to the alloying effect, the thermal conductivity of Fe2VGa0.8Si0.2 sample is significantly suppressed by a factor of 2. Therefore, we observed that the figure of merit (zT) in Fe2V0.8Ti0.2Ga sample is enhanced by 10 times at 420 K as compared with the parent compound Fe2VGa. Their magnetic properties are also investigated by means of susceptibility and magnetization measurements