前瞻性材料的機械性質—以熱電材料與金屬玻璃為例

Abstract

金屬玻璃的獨特性質，使其可成為熱電材料模組的擴散阻障層，因此瞭解這兩種不同材料的機械性質，是往後將此結合在一起的關鍵。本論文分成兩部分，第一部分研究熱電材料的常溫機械性質，包括N型的ZrNiSn與P型的Tetrahedrite。第二部分以獨特的形變方法研究帶狀金屬玻璃的剪切帶形成機制。 在探討N型ZrNiSn系統之機械性質的研究中，不同的粉末製備方式以及不同的燒結溫度被用來製造不同的微結構與孔隙率，此兩種因素對機械性質的影響皆在此論文被探討。實驗結果發現材料的彈性模數隨著孔隙率呈指數遞減。另外多餘的鎳含量也在本實驗中被鑑定出，使ZrNiSn的理論密度上升。在高能球磨之粉末製成的試片擁有較高的楊氏模數，可歸因於antisite defect的生成。P型Tetrahedrite熱電材料的常溫機械性質亦被研究。天然礦物的tetrahedrite以及50%天然礦物50%合成元素的化合物都被納入探討。Slow crack growth的現象也在本篇論文中被探討，在放置於高濕度環境長達兩個禮拜的過程中，並沒有顯著的裂縫成長被發現。另外運用SiC與graphene nanoplatelet提升破壞韌性的方法也在此論文中被嘗試使用。 金屬玻璃因為其非晶特性帶來的許多優異性質，被學界與業界所重視。塊狀的金屬玻璃擁有良好的彈性形變範圍，接近理論值的強度，其抗腐蝕性，抗磨耗性更優於一般金屬。他擁有玻璃轉換溫度，可以在過冷液相區對其輕易的加工，開展了金屬二次加工的另一種可能。然而，縱使具有種種的優越性能，金屬玻璃受限於他微乎其微的塑性變形能力，使得它在破壞時，幾乎都是粉碎式的破壞。金屬玻璃由於其非晶的特性，無法用傳統結晶金屬的差排理論來解釋其塑性變形機制，他的塑性變形能力是由剪切帶的成核與成長所控制。本篇論文第二部分將運用特殊的剪刀形變方式，在帶狀鐵基金屬玻璃上製造出規則的剪切帶，並配合有限元素法模擬材料的受力狀況，分析驅動剪切帶成核與成長的機制。期望能從本質上提升金屬玻璃的塑性變形能力。

Metallic glasses have unique properties that can be served as a diffusion barrier and can be put in thermoelectric devices. The investigation of the basic mechanical properties of both materials is implemented in this thesis for the promising combination of these two materials in the future. In the first part of this thesis, the room temperature mechanical properties of N-type ZrNiSn and P-type tetrahedrite systems were investigated. To study the mechanical properties of ZrNiSn, different powder processing methods and sintering temperatures were applied to introduce different porosities and microstructures. The elastic moduli were measured by analyzing the resonant ultrasonic spectra, and they were found to decrease exponentially as the porosity increased. Excess Ni has been characterized to cause the elevation of theoretical density of the material. The high energy ball milled specimens had higher Young’s modulus with the antisite defect introduced by this processing method. The room temperature mechanical properties of tetrahedrite system were also investigated, including natural mineral and 50-50 natural synthetic tetrahedrite. Slow crack growth behavior was monitored in this study. There was no obvious crack growth even in the high humidity environment. Experiments of adding SiC nanoparticles and graphene nanoplatelet into the tetrahedrite as promising strategies to enhance the fracture toughness were performed. However, no distinct improvement was found in this study. In the second part of this thesis, the unique deformation method, scissors cutting was applied to Fe-based metallic glass ribbon, and the corresponding shear band behavior was characterized with the aid of finite element simulations. Metallic glass alloys exhibit impressive mechanical properties including large elastic strain to failure and high tensile strength resulting from their amorphous nature that have attracted people’s attention to this new generation of material. However, their plasticity is dominated by the shear band behavior and always appears to be nearly brittle. A regular shear band patter was observed after the scissors cutting reached a steady-state. The result indicates that the shear band shows its stability under specific stress state. Furthermore, the result provides a simple and attainable way to control shear banding. The finite element analysis was performed to simulate the stress field resulting from cutting the metallic glass ribbon. The result verified that the shear band formation is controlled by the shear stress.