元素選擇性填佔對含第六族元素(硫、硒、碲)固態化合物熱電性質的影響

Abstract

本計畫的主要目的為合成具有高熱電優值(Figure of Merit, ZT = σS2/κ ; T = temperature; S = Seebeck coefficient; σ = electrical conductivity; κ = thermal conductivity) 的新穎固態化合 物，並對其結構, 物理與化學性質做詳細的解析。這些材料在未來的能源相關的元件，例如 微型發電機與無聲冷/熱循環器具有相當高的潛力。固態化合物具有高熱電優值需要具備低電 阻，低導熱度與高Seebeck 係數。最近的文獻指出半導體材料包含元素週期表上的重元素有 可能具有優異的熱電優值。由於新化合物的結構，組成與物性均無法正確掌握，因此本計畫 將使用理論計算與實驗相互運用，期望能瞭解固態材料的結構與物性的關係，進而提高材料 的熱電優值。初步將尋找具混合填佔的固態化合物的電子結構做基礎研究，使用電子組態的 理論計算(Extended Huckel type)探討其原子位置的相對密力根的總數(relative Mulliken population)與物理性質的關係，並進一步推測新的實驗方向去合成具有特殊的性質的新熱電 材料。主要著眼的目標將針對四與五元素系統的固態化合物。具體研究方向包含以下兩個主 要系統: 1)T-M1-M2-X 與2) M1-M2-M3-X。其中T 為過渡金屬元素；M1、M2 與M3 為第三 至第五族元素；X 為第六族元素(S、Se、Te)。在實驗的部份將從本實驗室先前所合成之化合 物為基礎，針對可能的3 元以及4 元的固態系統做有系統的研究。在實驗的過程將以元素與 二元化合物為起始反應物、利用 1) 真空高溫合成(Tmax = 1050 °C)；2) 在Ar 的環境下以高 溫電弧合成 (Tmax~2000 °C)；或以 3) 融熔化的鹽類做為溶液在真空、較低的溫度下反應 (Tmax~ 600 °C)，使假設的反應進行並產生預期的固態化合物。所合成之化合物將進行晶體結 構分析；熱分析(TG/DTA)；元素分析(SEM/TEM/EDS)；反射光譜(UVVIS，IR)，磁性、導 電、導熱與熱電性質(Seebeck coefficient)的測量以決定是否為好的熱電材料。

This project is dealing with the synthesis and characterization of new solid state compound exhibiting high figure-of-merit (Figure of Merit, ZT = σS2/κ ; T = temperature; S = Seebeck coefficient; σ = electrical conductivity; κ = thermal conductivity). Compound with high figure-of-merit (low resistivity, low thermo conductivity and high Seebeck coefficient) is applicable for energy related devices, such as electric power generator and refrigerator. It has been shown that semiconducting materials with heavy elements could exhibits excellent thermoelectric property. This project will combine electronic calculations and experiments on a know compound with some mixed occupied atomic sites to understand the relationship between the site preference and physical structure of thermoelectric materials. The extended Huckel type band structure calculation will be employed to calculate the relative Mulliken populations as a function of electron count on a know structure type, which will be used to derive the possible trend on electronic structure, bonding, site preference and physical properties of synthesized compound. The results may shed light on new directions to synthesize new compound with desire thermoelectric properties. Multinary metal chalcogenide systems are chosen for the synthesis of new compound. The target solid state systems include: 1) T-M1–M2-X and 2) M1-M2-M3-X (M1, M2, M3 = group III-V elements; T = transition metals; X = group VI elements(S、Se、Te)). The reactions will be carried out by mixing stoichiometric ratio of elements/binary phases in an argon filled glove box and heating under vacuum by sealed silica tube, Arc melt methods well as using metal halide fluxes. Products will be characterized by single/powder crystal x-ray diffraction, thermo analyses (TG/DTA), electron microscopy (SEM/TEM, EDS), electrical/thermal conductivity, UV-VIS-NIR diffuse reflectance, magnetic susceptibility, and Seebeck coefficient measurements to find the best thermoelectric material.