錫鉍共晶奈米線材之製備、生成機制與應用

Abstract

本研究利用真空液壓壓鑄製程，結合20 nm、70 nm與220 nm管胞孔徑的陽極氧化鋁模板(Anodic Aluminum Oxide, AAO)技術，成功地製造出錫鉍共晶 (Bi-43 wt % Sn)奈米線。有別於錫鉍塊材的層狀共晶結構，錫鉍共晶奈米線則是沿著線軸，呈現錫與鉍元素交替分佈的段狀共晶結構；在各個組成區段中，各元素均為單晶結構。比較不同管徑之錫鉍共晶奈米線之後，較細之奈米線傾向有較長的析出區段。而且，在單一線材中，錫與鉍區段的長度比例會接近3.3 :1。本製程的特色，不同於一般需要將材料離子或分子化的沉積製程；將合成好之材料加熱熔解，直接注入氧化鋁奈米孔洞，凝固後即成為一維奈米材料，可精準的控制並維持塊材所配置之材料成分配比。 參考固化理論，提出段狀結構之形成機制，並結合電子顯微鏡臨場觀察奈米線材之退火過程，詳細說明奈米線材微結構與凝固冷卻型態之關係。沿著線軸的方向性冷卻行為，促使共晶奈米線也沿著線軸的方向交替析出。在重新熔解凝固的退火實驗中，比較70 nm 與200 nm的線材，較大的管徑有利於流體的對流行為；因此，在相同退火條件下，尺寸效應造成凝固行為的差異，細管徑將限制融熔金屬的流動行為，使得固化之後的微結構產生變化。真空壓鑄冷卻過程是有方向性的，將會有利於段狀的共晶結構線材形成；相對地，退火過程則是等向性冷卻，有利於整體均勻分布的多晶結構。 最後，利用本方法所獲得的錫鉍共晶奈米線，直接於空氣爐中高溫退火，便可進一步地獲得氧化鉍-氧化錫(BiOx-SnOx)之複合奈米線材。將奈米線材由氧化鋁模板中溶出時，蝕刻液會在合金線表面造成氧化，此自然生成的氧化層即可與內部合金形成同軸管(core-shell)結構，並可防止內部合金在高溫製程中的流失。當此同軸管奈米線在空氣爐中，以每分鐘50℃的升溫速率加熱至700 ℃；經歷持溫一個小時的退火處理後，便可直接獲得所稱之複合氧化物線材。進一步分析氧化物之光學性質，陰極發光(Cathodoluminescence, CL)光譜與光導效應(Photoresponse)光譜也同時用以佐證氧化鉍與氧化錫的存在。

關鍵字：錫鉍共晶奈米線、固化過程、尺寸效應、氧化鉍-氧化錫之複合奈米線材

Eutectic Bi-43Sn (in weight percent) nanowires with diameters of 20 nm, 70 nm and 220 nm respectively, were fabricated by a vacuum hydraulic pressure injection process using anodic aluminum oxide (AAO) as templates. Novel eutectic microstructure was found within the fabricated nanowires, which composed of alternating Bi and Sn segments along their wire axes. Within the segments, the electron diffraction analysis revealed single crystalline structures of Bi and Sn elements respectively. Parameters that control the nanowire fabrication process were discussed. It was found that the as the wire diameter reduced, longer Bi and Sn segments formed. In addition, the alternative segmental compositions showed that the ratio of tin to bismuth is approximately 3.3:1. For this process, nanowires were formed from non-ion deposition with liquid alloy; in this way, the composition of nanowires can be controlled precisely in stoichiometry. The size effects on solidification and the formation mechanism of the segmented eutectic Bi-43Sn nanowires have also been investigated during the in situ reheating processes. A directional solidification along the wire axis limits the segmented eutectic nanowire to arrange axially due to directional solidification and the sequential enrichment of two elements. In 70 nm nanowires, the small size confines the convection in liquid, which results in differences in the microstructure and composition profiles between 70 and 200 nm nanowires during the reheating process. In the vacuum hydraulic pressure injection process, the directional cooling helps the formation of single crystal, and the isotropic solidification leads to polycrystalline microstructure. Finally, bismuth oxide-tin oxide (BiOx-SnOx) heterostructure nanowires with a diameter of 70 nm were fabricated by directly annealing Bi-Sn eutectic nanowires. After removal of AAO template with etching solution, a spontaneous oxide was formed on the nanowires, enclosing the Bi-Sn eutectic alloys. While these nanowires went through the annealing process at a heating rate of 50 ℃/ min, the well-annealed oxide nanowires remained solid, straight and segmental. The results of cathodoluminescence (CL) spectrum and photoresponse proved that the products comprised bismuth oxide and tin oxide. This fabrication methodology is a simple way to fabricate one-dimensional oxide nanomaterials.

Keywords: Bi-Sn eutectic nanowires, Solidification, Size Effect, BiOx-SnOx nanowires