熱電元件能將將廢熱回收轉換成電能，以提升能源使用效率。熱電元件通常是由陣列P-N型熱電材料組成，因此在元件中有許多接點，接點的性質與熱電元件的性能息息相關。GeTe為重要的中溫型熱電材料，Ni與Co常作為熱電元件的擴散阻障層。本研究探討Ni與Co擴散阻障層對GeTe的界面反應，以了解Ni與Co做為阻障層的合宜性。本研究也測定了Ni-Ge-Te與Co-Ge-Te三元系統相圖，提供基礎的材料系統知識，也有助於了解界面反應。
使用純元素製備GeTe基材、Ni-Ge-Te三元合金與Co-Ge-Te三元合金。將合金試樣封入於抽真空的石英管中，放置於500℃與400℃的高溫爐中平衡。經過熱處理1至4個月後，淬冷於水中。以金相、組成、與粉末X-光繞射分析其平衡生成相。界面反應是以反應偶的實驗進行探討。利用電鍍方法於GeTe基材上鍍上Ni層與Co層。將Ni/GeTe與Co/GeTe反應偶，放置於高溫爐中。以金相分析與組成分析，分析生成相的種類、反應型態與厚度，並佐以相圖分析其反應機制。
本研究發現Ni-Ge-Te系統在500℃與400℃，有二個三元化合物Ni3GeTe2與Ni5.45GeTe2。依據本研究的相平衡實驗、文獻中的三元與組成二元系統的相圖資料，推測出Ni-Ge-Te的在500℃與400℃的等溫橫截面相圖。在500℃，Ni-Ge-Te含有14個單相區與14個三相區。單相區分別為Ni、Ge、Liquid、Ni3Ge、Ni2Ge、Ni5Ge3、NiGe、NiTe2、NiTe0.775、Ni3Te2、α-GeTe、β-GeTe、Ni3GeTe2與Ni5.45GeTe2。在400℃，含有15個單相區與15個三相區。與500℃相比較，單相區少了β-GeTe，多了Te與γ-GeTe。
Co-Ge-Te的三元系統在500℃與400℃，也有二個三元化合物Co2Ge3Te3與CoGeTe。在500℃，Co-Ge-Te含有15個單相區與15個三相區。單相區分別為α-Co、ε-Co、Ge、Liquid、Co2Ge、Co5Ge3、CoGe、Co5Ge7、CoGe2、CoTe2、Co2Te3、α-GeTe、β-GeTe、CoGeTe與Co2Ge3Te3。在400℃，包含了15個單相區與15個三相區。與500℃相比較，單相區少了β-GeTe與α-Co，多了Te與γ-GeTe。
本研究在製備GeTe基材的過程中，發現GeTe在高溫仍為菱面體結構之α-GeTe而非立方體結構之β-GeTe，此點與文獻的結果並不一致。在Ge-Te於500℃的相平衡實驗中，隨著合金的Te含量增加，GeTe由α-GeTe轉變為β-GeTe。
反應偶的實驗結果發現，界面反應生成多個且複雜的反應相，且反應層厚度隨著時間增加而增厚。Ni/GeTe在500℃下反應1天後，生成Ni3Te2、Ni3Ge、Ni5Ge3、NiGe、Ni5.45GeTe2與Ni3GeTe2，反應層厚度約為71 μm。在500℃下的擴散路徑為Ni/(Ni3Te2+Ni)/ Ni3Ge/(Ni3Ge+Ni5.45GeTe2)/(Ni5.45GeTe2+Ni5Ge3)/(Ni5Ge3+Ni3GeTe2)/(NiGe+Ni3GeTe2)/GeTe，Ni為主要擴散元素，其次為Te。在400℃下反應28天後，生成Ni5Ge3、NiGe、Ni5.45GeTe2與Ni3GeTe2，反應層厚度約為72 μm。在400℃下的擴散路徑為Ni/(Ni5Ge3+Ni5.45GeTe2)/ (Ni5Ge3+Ni3GeTe2)/ (NiGe+Ni3GeTe2)/ GeTe，Ni為主要擴散元素。
Co/GeTe在500℃下反應14天後，生成Co2Te3、Co5Ge3、CoGe與CoGeTe，反應層厚度約為80 μm。界面處的擴散路徑為Co/Co2Te3/Co5Ge3/(Co5Ge3+Co2Te3)/(CoGe+Co2Te3)/CoGeTe/GeTe，Co為主要擴散元素，其次為Te。在400℃下反應50天後，生成CoGe2、Co5Ge3與Co2Te3，反應層厚度約為12 μm。在400℃下的擴散路徑為Co/CoGe2/Co5Ge3/ (Co5Ge3+Co2Te3)/ GeTe，Co為主要擴散元素，其次為Ge。
在探討Co/GeTe的界面反應過程中，觀察到一個非常特殊的現象，在 Co層外圍亦觀察到反應層的生成。此生成相由內至外分別為(Co5Ge3+Co2Te3)、(CoGe+Co2Te3)、CoGeTe，本研究認為是由外圍Co層與氣化GeTe反應生成。

Thermoelectric materials can directly convert waste heat energy into electrical energy to improve energy efficiency. Thermoelectric modules are usually composed of an array of P-N type thermoelectric materials. There are many contacts in the module, and the properties of the contacts are closely related to the performance of the thermoelectric module. A barrier layer is usually introduced between the thermelectric material and the joining material to prevent damage to the thermoelectric materials caused by intensive interfacial reactions. GeTe is an important medium-temperature thermoelectric material, and Ni and Co are often used as diffusion barriers for thermoelectric modules. This study investigated the interfacial reactions between Ni and Co and GeTe to understand the suitability of Ni and Co used as barrier layers. This study also determined the phase diagrams of the Ni-Ge-Te and Co-Ge-Te ternary systems, which provide basic material information and assistance to understand interfacial reactions.
GeTe substrate, Ni-Ge-Te ternary alloy and Co-Ge-Te ternary alloy were prepared with pure Ni, pure Co, pure Ge and pure Te elements. Ternary alloys with various compositions were sealed in evacuated quartz tubes and equilibrated in furnaces at 500°C and 400°C. After heat treatment for 1 to 4 months, they were removed and quenched in water. The equilibrium phasee were analyzed by metallographic analysis, composition analysis and X-ray powder analysis. The Ni/GeTe and Co/GeTe couples were prepared by electroplating Ni and Co layers on the GeTe substrate obtained from quenching. The Ni/GeTe and Co/GeTe reaction couples were placed in a high temperature furnace. After various reaction times, the couples were removed from furnace and quenched. The morphologies, phases, and thickness of the reaction layers were examined by using metallographic analysis and composition analysis. The reaction mechanism was analyzed with the aid of phase diagrams.
Two ternary compounds, Ni3GeTe2 and Ni5.42GeTe2, were found at 500℃ and 400℃. Based on the phase equilibrium results of this study and the phase equilibria results of the ternary and binary systems in the literature, the isothermal sections of Ni-Ge-Te at 500°C and 400°C are determined. At 500°C, Ni-Ge-Te contains 14 single-phase regions, which are Ni, Ge, Liquid, Ni3Ge, Ni2Ge, Ni5Ge3, NiGe, NiTe2, NiTe0.775, Ni3Te2, α-GeTe, β-GeTe, Ni3GeTe2 and Ni5.45GeTe2, and 14 three-phase regions, At 400°C, Ni-Ge-Te contains 15 single-phase regions and 15 three-phase regions. Compared with the results of 500℃, β-GeTe is not in single-phase regions, but Te and γ-GeTe are included. The ternary system of Co-Ge-Te also has two ternary compounds, Co2Ge3Te3 and CoGeTe, at 500℃ and 400℃. At 500°C, Co-Ge-Te contains 15 single-phase regions, which are α-Co, ε-Co, Ge, Liquid, Co2Ge, Co5Ge3, CoGe, Co5Ge7, CoGe2, CoTe2, Co2Te3, α-GeTe, β-GeTe, CoGeTe, Co2Ge3Te3, and 15 three-phase regions.At 400°C, Co-Ge-Te contains 15 single-phase regions and 15 three-phase regions. Compared with the results of 500℃, β-GeTe and α-Co are not in single-phase regions, but Te and γ-GeTe are included.
It was found that the structure of the GeTe prepared in this study was rhombohedral, which was inconsistent with the results in the literature. In the phase equilibrium experiment of Ge-Te at 500℃, with the increase of Te content of the alloy, α-GeTe transformed into β-GeTe.
The reaction layers in the couples had complicated morphologies and various phases, and the thickness of the reaction layer increased with time. Ni3Te2, Ni3Ge, Ni5Ge3, NiGe, Ni5.45GeTe2, and Ni3GeTe2 were formed after Ni/GeTe reacted at 500 °C for 1 day, and the thickness of the reaction layer was about 71 μm. The diffusion path of Ni/GeTe at 500℃ is Ni/(Ni3Te2+Ni)/Ni3Ge/(Ni3Ge+Ni5.45GeTe2)/(Ni5Ge3+Ni5.45GeTe2)/(Ni5Ge3+Ni3GeTe2)/(NiGe+Ni3GeTe2)/GeTe, and Ni is the main diffusion element, followed by Te. Ni5Ge3, NiGe, Ni5.45GeTe2, and Ni3GeTe2 were formed after Ni/GeTe reacted at 400 °C for 28 day, and the thickness of the reaction layer was about 72 μm. The diffusion path of Ni/GeTe at 400℃ is Ni/(Ni5Ge3+Ni5.45GeTe2)/(Ni5Ge3+Ni3GeTe2)/(NiGe+Ni3GeTe2)/GeTe, Ni is the main diffusion element.
Co2Te3, Co5Ge3, CoGe, and CoGeTe were formed after Co/GeTe reacted at 500°C for 14 days, and the thickness of the reaction layer was about 80 μm. At 500℃, the diffusion path of Co/GeTe at the interface is Co/Co2Te3/Co5Ge3/(Co5Ge3+Co2Te3)/(CoGe+Co2Te3)/CoGeTe/GeTe, where Co is the main diffusion element, followed by Te. CoGe2, Co5Ge3 and Co2Te3 were formed after Co/GeTe reacted at 400°C for 50 days, and the thickness of the reaction layer was about 12 μm. The diffusion path of Co/GeTe at 400℃ is Co/CoGe2/Co5Ge3/(Co5Ge3+Co2Te3)/GeTe, where Co is the main diffusion element, followed by Ge.
In addition, the formation of reaction layers was also observed at the outer Co layer. From the inside to the outside, the reaction phases were (Co5Ge3+Co2Te3), (CoGe+Co2Te3), and CoGeTe. It is believed that the formation was due to the reaction between the outer Co layer and the vaporized GeTe.

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