輸入微量液態碳源對合成石墨包裹奈米鎳晶粒及電弧型態轉變之初步研究

Effect of Injecting Minor Liquid Carbon Sources on Graphite Encapsulated Nickel Nanoparticles Synthesis and Arc-shape Conversion

許舜婷(Shun-Ting Hsu)

指導教授 ： 鄧茂華

國立臺灣大學/理學院/地質科學研究所/碩士(2016年)

https://doi.org/10.6342/NTU201603026

摘要

石墨包裹奈米晶粒(Graphite Encapsulated Metal Nanoparticles, GEM)為粒徑介於5-100 nm的球狀複合材料，其内核一般為鐵磁性金屬、外殼為石墨層，內核受到外層石墨保護，可抵抗氧化、酸鹼等極端環境的侵蝕，保持核心金屬原有的特性。隨著核心金屬種類的不同，GEM可展現相當多元的特性，例如磁性、儲氫能力等，近年來已被廣泛地應用於磁性紀錄、醫學上的人體試蹤劑以及藥物載體、能源產業上的儲氫材料等，顯示GEM在各個領域方面的潛力。
　　本研究室目前使用電弧法來合成石墨包裹奈米晶粒，其中又以1995年由Teng et al.以及Dravid et al.發展的改良式鎢-碳電弧法為目前最具有工業化量產潛力的方法，此方法改善了以往碳-碳電弧法中，初產物碳雜質過多的問題，然而依然無法有效提升初產物的包裹良率與粒徑控制。直到2012年，本研究室根據探討GEM形成機制的二步驟模型機制，以液態碳源取代原本鎢-碳電弧法中的固態碳源，於電弧放電時將液態碳源對艙內電弧中心注入，利用電弧的高熱將碳源分解，大幅提升GEM合併區內的碳與金屬比例，成功使GEM的包裹良率由以往的20-30%提升至85%，並且藉由調配液態碳源的種類，提出初步的粒徑控制方式，使GEM的合成開發效率躍升至全新的視野。
　　然而，改用液態碳源後卻面臨了兩個棘手的問題：首先，於電弧放電時加入液態碳源雖然能讓GEM的包裹良率提升，卻會造成電弧放電中斷，導致實驗停擺而無法持續進樣；再者，改用液態碳源後會使鎢電極熔耗速率由使用固態碳源時的1 mm/hr大幅增加至420 mm/hr，使得單次合成GEM的時間下降，無法一次大量地製造GEM，對於GEM的基礎科學研究與相關應用造成極大阻礙。除此之外，對於加入液態碳源後，電弧組成的改變以及電弧運作的模型尚不清楚，無法將其與使用固態碳源時的情況以一貫之。
　　本研究的目的即為瞭解加入液態碳源後艙內電弧的變化，並解決使用液態碳源後造成電弧中斷與鎢棒熔耗的問題。首先以加裝微量進樣幫浦的方式，改善每次液態碳源進樣量與進樣方向，使碳源可定向噴灑至主要合成GEM的區域，此法可以避免液態碳源沿鎢棒滴落時造成的瞬間電阻，讓電弧放電能穩定而持續地進行。除此之外，本研究以穿透式電子顯微鏡(Transmission Electron Microscope, TEM)證實使用微量進樣幫浦進樣所合成的GEM保有完整的核殼結構，並用熱重分析儀(Thermogravimetric analysis, TGA)的重量變化計算出碳源使用效率，計算結果顯示使用微量進樣幫浦可將碳源使用率由原本的20%大幅提升至64%。
　　其次，本研究中列出加入液態碳源後造成鎢棒熔耗量增加的各種因素，並逐一以實驗及理論計算的方式驗證。藉由發射光譜分析(Optical Emission Spectrosopy)可推測液態碳源加入後會造成電弧中心的氣體由氦氣轉變成以氫氣為主，而電弧的溫度也呈現較高溫的藍白色，代表電弧溫度升高是造成鎢棒熔耗速率增加420倍的主因。在經由熱傳導計算與實驗後，本研究證實了增加鎢棒直徑可有效減少鎢棒熔耗的問題。
　　由於微量進樣幫浦可控制碳源進樣速率，本研究藉由調控碳源進樣速率與其合成的GEM包裹良率變化，提出合成GEM時包裹良率隨時間的變化分布。在相同的實驗參數下，將100μL的液態碳源分別以10μL/min進樣10分鐘，以及100μL/min進樣1分鐘的方式實驗比較，發現前者的GEM包裹良率較後者高5倍。根據探討GEM形成機制的二步驟機制理論，本研究推測在實驗過程中加入液態碳源會快速提高合併區內的碳與金屬比例，但是碳蒸氣會快速隨著艙內熱對流離開合併區，因此在相同碳源進樣量下，多次進樣可大幅提升碳源使用效率與包裹良率。
　　最後，本研究根據目前所得的電弧實驗分析數據，進一步對於加入液態碳源後，電弧形狀由鐘狀轉變成柱狀的原因提出一個假設模型，藉以提升GEM在基礎科學與材料應用領域的潛力。

關鍵字

石墨包裹 ； 奈米晶粒 ； 正丙醇 ； 微量進樣

並列摘要

Graphite Encapsulated Metal (GEM) nanoparticles are spherical core-shell structured composite material with a diameter ranging from 5–100 nm. The core of GEM is metal, and its outer shell is composed of several layers of graphite/graphene which can preserve the inner core in a severe environment, such as from acid erosion and oxidation. It is well known that many different functional groups, including carboxyl and hydroxyl, can be easily attached to the surface of carbon materials. Recently, several studies have revealed that GEM has a great potential to become a novel material including, for example, in hydrogen storage and biomedical materials due to its unique properties. For instance, Wu et al.(2007) used polyethylene glycol and folic acid grafted on Fe-GEM for the heat treatment of cancer, and Chung et al. (2009) used Co-GEM as an electrochemical hydrogenation material.
The modified tungsten arc-discharge method was developed by Teng et al. and Dravid et al. in 1995. This is the most practical method for producing a large quantity of GEM because it reduces the amount of carbon debris origin compared to the Krätschmer–Huffman method. However, the encapsulation efficiency of GEM remains low. Until 2012, with the help of the two-step mechanism model, we used n-propanol as the liquid carbon source to synthesize GEM, significantly increasing its encapsulation efficiency from 20–30 wt% to around 80 wt%, and presenting a preliminary method for controlling the particle size of GEM through different liquid carbon sources.
However, we faced two difficult issues after switching the carbon source from solid to liquid. First, this method could disturb the arc discharge which causes the discontinuity of the experiment, leading to the lockout unsustainable injection. Second, the consumption rate of the tungsten rod rose from 1 mm/h to 420 mm/h, making it difficult to synthesize large quantities of well-encapsulated GEM. In addition, the detailed mechanism, after entering the liquid carbon source, still remains unclear.
The purposes of this study are to realize the changes of arc in the cabin and to resolve problems after using the liquid carbon source. In order to solve the problems, this research has installed a liquid metering pump to regulate the amount and direction of each injection, so that the carbon source can be directed to mainly spray the synthetic region of GEM, which is called the coalescence region. This method avoids the resistance caused by dripping liquid along the tungsten rod, and successfully sustained the experiment. The TEM images show that the synthesized GEMs, using a liquid metering pump, retain a complete core-shell structure, and the utilization of carbon source calculated by TGA data shows significant improvement, from 20% to 64%.
Furthermore, we listed the possible reasons causing the high consumption rate of tungsten rod, and verified them by theoretical calculation and manner of experiments, one by one. It can be speculated by OES data that the dominant gas in the center of the arc changed from helium to hydrogen. In the meantime, the arc temperature rose show by the color changing into blue and white, representing the higher arc temperature is the main reason causing the tungsten melting rate to increase 420-fold. After calculating the heat conduction, we confirmed that increasing the diameter of the tungsten rod can immediately solve this problem.
Since it is feasible to control the injection rate through the use of a liquid metering pump, we tried to figure out the encapsulation efficiency of GEM over time when synthesizing GEM. Under the same experimental parameters and total liquid injection volume, we compared the results of two different injection rates, 10μL/min and 100μL/min, and found that using the former injection rate can result in 5-fold higher encapsulation efficiency. According to the two-step mechanism of GEM, we speculated that adding liquid carbon source during arc discharge would rapidly increase the carbon proportionate of the coalescence region; however, the carbon vapor will quickly leave the coalescence region via convection. Thus, for the same total liquid injection volume, taking a small amount and injecting it a few times is the best way to inject the liquid carbon source; it can significantly improve the encapsulation efficiency and the utilization rate of the carbon source.

　　Lastly, based on the experiment results, we proposed a model that can explain the transformation of the arc body from bell-shaped to columnar, after injecting the liquid carbon sources. Furthermore, our model raises the potential of employing GEM to fundamental science and applied material fields.

並列關鍵字

Graphite encapsulated ； nanoparticle ； n-propanol ； liquid metering pump

參考文獻

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