高導電性雙晶修飾Bi-(Sb/Se)-Te奈米結構薄膜及其於高效能熱電元件之應用

Abstract

熱電材料具有尺寸小、結構簡單、可靠度高之特點，且可以作為發電或致冷器之用，但由於傳統 塊材之熱電轉換效率過低，因而使其應用受到極大之限制。近年來由於奈米科技的發展，透過奈米結 構合成技術，使得熱電材料轉換效率得以大幅提升，部分熱電材料系統已達商業應用之要求。然而， 隨著奈米結構尺度的縮減，激增之晶界或介面嚴重影響奈米熱電材料之導電度，進而造成整體熱電效 能大幅下降。如果無法根本解決高介面電阻之問題，則奈米結構熱電材料之ZT 值將難以有所突破。 為此，本研究計畫從結構改良的觀點提出一可行之「雙晶改質奈米結構」概念，亦即利用具較低 電阻之雙晶結構介面，來取代一般高缺陷晶界或介面，以期在不顯著影響熱傳導的前提下，大幅提昇 雙晶奈米結構之導電性。本研究計畫是在既有之脈衝雷射沉積技術下，利用製程參數之設計與調控， 找出並強化具雙晶結構之各式奈米結構熱電薄膜，來達到提昇熱電材料ZT 值之目的。內容包括兩大 研究主題：利用斜向沉積技術成長具雙晶結構之高方向性Bi-(Sb/Se)-Te 基奈米結構熱電薄膜，以及高 效能薄膜型熱電轉換元件之開發。 本實驗室現階段已成功製備一系列皆可調控「尺度」、「維度」、「排列方向」及「結晶優選方位」 之Bi-(Sb/Se)-Te 基奈米結構熱電薄膜。為了更進一步控制Bi-(Sb/Se)-Te 基奈米熱電薄膜之結晶優選方 位，以期形成「類磊晶結構」，來達到有效形成雙晶結構與強化熱電性質之目的，本研究計畫擬於PLD 系統中搭配斜向沉積(glancing angle deposition; GLAD)技術。另外，針對具有雙晶結構之各式奈米結構 熱電薄膜進行一系列之製程最佳化，以期能夠調控雙晶結構之形態，並探討雙晶結構之模型及其對奈 米結構熱電薄膜導電率及熱傳導係數之影響。最後，更將合成所得具雙晶結構之N 型及P 型奈米結構 熱電薄膜，應用於高效能薄膜型熱電轉換元件之製作。

Thermoelectrics have very attractive features, such as small size, simplicity, and reliability, and have important applications for power generation or cooling. However, the low thermoelectric conversion efficiencies of conventional bulk materials limit their applications. Recently, due to the development of nanoscience and nanotechnology, the thermoelectric conversion efficiencies have been greatly improved by the nano-structured synthesis technique to meet the requirements for commercial use. However, as the sizes or dimensions of the nanoblocks are designedly reduced, the accompanied grain boundary or interface effects become more significant to seriously sacrifice the electrical conductivity as well as the thermoelectric performance. Without fundamentally resolving the high-resistance problem of nanoassemblies, remarkable advances in ZT will never be expected. Here, we propose a facile concept in the viewpoint of structural modification, i.e. twin-modified nanostructures. By replacing the defective grain boundaries or interfaces with perfect twins, the electrical conductivity of the twined nanostructures can be greatly enhanced without significantly affecting the thermal conductivity. In this research plan, by using the pulsed laser deposition (PLD) with precisely designed and controlled deposition parameters, a series of twined nanostructured films with distinct morphologies and dimensions will be synthesized and investigated for achieving the purpose of high ZT value. Two main subjects are included: the fabrication of highly oriented and twinned Bi-(Sb/Se)-Te nanostructured films and their applications for high-performance thin-film thermoelectric conversion devices. We have successfully synthesized a series of Bi-(Sb/Se)-Te nanostructured films with well controlled sizes, dimensions, alignments and preferential orientations. In order to further control the preferential orientation of the Bi-(Sb/Se)-Te nanostructured films to form an epitaxial like structures for effectively obtaining the twin structures and enhancing the thermoelectric properties, glancing angle deposition (GLAD) will be introduced to the present PLD system. Additionally, the deposition parameters for preparing the twined-nanostructures will be focused and optimized for controlling the growth style of the twins. The structural models of the twined structures as well as their effects on the electrical conductivity and thermal conductivity will be investigated. Finally, the prepared highly-ordered N-type and P-type Bi-(Sb/Se)-Te films will be applied for the fabrication of high-performance thin-film thermoelectric devices.