結論 - 奈米尺度下Bi-Sn共晶合金之凝固機制探討

本研究使用硫酸、草酸、磷酸製備20nm、45nm、170nm 之陽極氧化鋁為基板進行真空壓鑄法，鑄入液相之Bi-43Sn 共晶合金，使之冷卻，

得到有節狀結構之奈米線；並藉由改變鑄入溫度與冷卻方式、以對節狀奈米線之凝固機制有進一步了解。

1. 以壓鑄溫度 533K、水冷所製備之 20nm、45nm、170nm Bi-43Sn 奈米線進行 XRD 分析發現，隨不同直徑的奈米線，Bi 與 Sn 的結晶會有不同的優選方向。

2. 以壓鑄溫度 533K、水冷所製備之 20nm、45nm、170nm Bi-43Sn 共晶奈米線進行 DSC 熱分析發現，共晶溫度隨奈米線直徑變小而降低。

3. 由於高 G/V 值定方向凝固，因此經由 TEM 分析，各組不同直徑、

壓鑄溫度與冷卻方式所製備之 Bi-43Sn 奈米線，均為 Bi-rich 相與 Sn-rich 相的單晶交錯所組成的節狀結構。

4. 經由 TEM 之 EDS 成分分析，以不同壓鑄溫度、以水冷所製備之 170nm Bi-43Sn 奈米線，Bi-rich 相中的 Bi 比例與 Sn-rich 相的 Sn 比例隨壓鑄溫度增加提高。這是由於冷卻速度因為壓鑄溫度增加而降低。顯示出奈米尺度下，Bi-43Sn 奈米線的凝固行為受到基板壓鑄溫度影響。

5. 根據目前研究的結果與凝固理論得知，由於在奈米尺度下，高深寬比的基板限制住溶質的對流，以及極高 G/V 值的定方向凝固，導致Bi-43Sn 共晶合金液在凝固時，大量的 Bi 或 Sn 先成核晶出、另一個Sn 或 Bi 成分被排入液相中，但又因無法對流，而隨後晶出；

因此最後形成Bi-rich 相與 Sn-rich 相的單晶交錯所組成的節狀結構奈米線。

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