以人造鑽石及噴氣式電弧法合成石墨包裹奈米鐵晶粒之初步結果

Abstract

石墨包裹奈米顆粒（GEM）因為具有極大的比表面積及穩定的物性， 所以是一種非常具有應用潛力的奈米材料。GEM之結構分為內核及外殼，內核是奈米級的金屬顆粒，外殼則是多層的石墨，顆粒大小介於10-100 nm之間。由於外層的石墨層可保護內部的金屬顆粒免於氧化或酸侵蝕，因此可以長期保持金屬顆粒在奈米尺度下的行為與特性。 本實驗室使用電弧法合成GEM多年，經過前人（Lin et al., 2000及 Cheng et al., 2001）改良製程，發現只要對產物做真空熱處理或在實驗進行中加入甲烷以增加額外碳源，都可以大幅提高GEM的產量。但是在穿透式電子顯微鏡的觀察下，產物的顆粒仍有分布不集中，大小差異過大（同一次實驗中顆粒大小從10 nm至200 nm都有）等現象，對於後續的基礎性質研究及應用是很大的問題。雖然有許多不同的方法可以控制GEM的粒徑大小，包括改變實驗中的環境氣體種類與壓力等，但是在理論上其效果都比不上直接對電弧噴氣。由於GEM的成長主要是發生在電弧區電漿外圍的一薄層空間，顆粒在此區間會彼此碰撞聚合成長，噴氣除了可以降低顆粒在此區間彼此碰撞聚合的機會，也會壓縮此區間的範圍，從而達到控制並降低平均粒徑大小的效果。Teng等人在1995年曾經使用過噴氣電弧製造奈米 鎳顆粒，雖然成功讓顆粒粒徑減小到10 nm以下，但是卻因為噴氣的冷卻作用，使得原料的蒸發量驟降而降低產量。 本研究工作分為兩大部分，第一是為了要能夠控制產物的粒徑，我們設計製造了全新的二號真空艙，除了改善原來一號艙的缺點，更重要的是我們率先採用了環型噴氣的設計來合成GEM。初步實驗結果發現，當使用環型噴氣合成石墨包裹奈米鐵晶粒，產物會自燃而形成氧化鐵；然而在一號艙未噴氣條件下並無此現象，顯示顆粒變小後反而促進了鐵的自燃反應，目前雖以水中收集法可解決初產物在一開艙收集時就立刻自燃氧化的問題，但在後續酸溶純化過程中仍會發生全部顆粒迅速氧化的現象，此問題目前仍無解，有待未來解決。在第二部份的工作則是為了要發展出有效的方法以避免因為噴氣而減低產量。我們根據合成GEM的機制，設計一序列的實驗，結果發現使用人造鑽石作為碳源時，初產物的產量提高了50%，而酸溶後純化之良率也提高為250%。由於人造鑽石在常壓下是亞穩定相，所以在實驗中比石墨（原始實驗所採用的碳源）更容易溶進金屬中，因而提高了石墨在金屬中的溶解度與均勻度，使得電弧所蒸發出的金屬與碳混合蒸氣也更均勻，有效減低了因為碳含量過少或過多所導致的包裹不完全或是碳質碎屑過多的問題。

Graphite Encapsulated Metal (GEM) nanoparticle is a new composite material first found in 1993. GEM has a shell (graphite layers) and core (metal nanocrystals) structure, and a 10~100 nm size range. Because the graphite layers can protect the inner metal cores form oxidation and acid erosion, the new material provides an excellent opportunity to study the property and behaviors of nanoparticles. We have been using the modified arc-discharge method to synthesize GEM for some years, and have come up with some effective processes to improve the efficiency of the method. For example, by adding methane into the arc-discharge or by annealing the synthesized raw powder, both processes can increase the yields of GEM. However, neither process can control the size distribution of the GEM, i.e., the properties of the powders are not uniform. To control the sizes of the powder, using a blown-arc may be the best method. Unfortunately, the blown-arc method will also decrease the production rate dramatically. To control the sizes of GEM, a new arc-discharge system with a circular blown jet was designed and built. The preliminary results show that the sizes of the Fe-GEM produced in this new system are so small that once exposed to the air the GEM spontaneously oxidize into iron oxides. Though the oxidation can be prevented by collecting the GEM into water, the powder will still oxidize during the acid-bath process. To maintain an adequate production rate while controlling the sizes of the GEM, a modified arc-discharge method using synthetic diamond powder as carbon source (in stead of graphite) was developed. Using diamond carbon 3 source proved to be an effective modification that the amount of the as-made powder has increased by 50%, and the after acid-bath powder has also increased by 250%. Because diamond is a metastable phase under ordinary temperature and pressure, it dissolved into the metal much easier than graphite. As a result that the carbon distributes more evenly in the melting metal, and thus the carbon vapor mixes more uniformly with the metal vapor during the evaporation.