合成石墨包裹奈米金屬晶粒製程中熔融金屬內碳原料變化之初步研究

Preliminary study on the variation of carbon source during synthesizing graphite encapsulated metal nanoparticles (GEM)

蕭崇毅(Chung-I Hsiao)

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(2006年)

https://doi.org/10.6342/NTU.2006.00246

摘要

石墨包裹奈米晶粒(Graphite Encapsulated Metal Nanoparticles, 簡稱GEM)為粒徑介於5-100 nm的球狀複合材料，其內核為金屬，外殼為石墨。由於其特殊的結構與尺寸，使得這項新材料成為學術界積極研究的對象。
歸納本實驗室數年來用鎢電極電弧法合成GEM的研究成果，雖然在產量與良率方面已大幅提升，但對電弧實驗當中坩堝內原料的變化過程了解不足，這也造成對未來改進電弧製程與發展理論模型產生重大障礙。本研究因此針對坩堝內原料的變化等問題，設計並進行一系列的實驗與分析。
本研究主要分為兩部分，第一部分用鎢電極電弧法合成GEM；電弧的高溫會將坩堝中的金屬塊與石墨碎片熔融，並使金屬與石墨蒸發為混合蒸氣，在離開電弧區後冷凝成GEM。第二部分則是檢視分析在電弧實驗後仍留在坩堝內的金屬塊。首先將金屬塊切割並從縱切面觀察其內的石墨產狀與分布狀況，然後再用酸將金屬塊溶解，分析殘餘石墨粉的粒徑大小與型態，以得到金屬塊的成分與內部產狀等微細結構資訊。
實驗結果發現，金屬塊的上表面形態與電弧作用密切相關。中心亮面處是電弧電漿直接接觸的區域，而周圍霧面處則是電弧外圍蒸氣冷凝成長的區域。兩者的成分都以碳為主，但霧面處則有冷凝後因表面張力而形成向中心呈輻射狀排列之金屬顆粒。金屬塊的內部形態則與製程中的溫度梯度變化相關，中央部份的溫度始終高於周圍與底部，造成(一)高熔點的石墨由邊緣較冷區域成核並向中央生長，不僅石墨的數量較多粒徑也較大。(二)中央的金屬與碳被蒸發後，四周補充過來的金屬較多，碳(石墨)較少，因此最後凝固的中央部分，不僅石墨的數量少，粒徑也較小。另外由金屬塊中石墨片的分布情形，並未發現有明顯對流作用的證據，但由其粒徑呈現對數常態(log-normal)分布，顯示在金屬塊內部的石墨片是從溶在金屬中的碳析出與結晶生長而成。此外本研究所採用的人造鑽石與石墨兩種碳原料在熔融金屬中的作用並不相同。鑽石粉末在鐵、鈷、鎳等催化金屬中會相轉變成石墨，形成從鑽石表面一片片剝離的小片石墨。此過程增加了石墨與金屬接觸的面積，也增加了碳的溶解量並降低了碳的濃度梯度。此結果會使蒸發出來的蒸氣中，金屬與碳的比例在空間分布上更均勻，因而提高被石墨完整包裹GEM的良率。石墨粉末則在金屬中很安定，僅會因高溫而溶解少量的碳，而且只會在石墨粉末周圍達到飽和，因此會造成較高的碳濃度梯度。此結果會造成碳在蒸氣中局部集中，不僅GEM的良率降低，而且會生成許多碳屑雜質。

關鍵字

石墨包裹奈米金屬晶粒 ； 電弧 ； 鑽石 ； 石墨化 ； 碳

並列摘要

Graphite Encapsulated Metal (GEM) nanoparticle is a new spherical composite material with a diameter ranging between 5 and 100 nm. It has a core/shell structure, where the core is metal and the shell is graphite. Having the special structure and nanosizes, GEM has become an interesting research subject for the academic community.
For the last several years, we have been using a tungsten arc-discharge method to synthesize GEM, and have successfully increased the yield and the recovery ratio of the GEM. However, we still know very little about the changes of the raw materials inside the crucible during the runs. The lack of knowledge could be a barrier keeping us from both to improve the synthesis process and to develop a mathematic model for the system. To fill up the gap, this study specifically designed to investigate what has been happening to the raw materials in the graphite crucible.
The work can be divided into two parts: First is to design and do a number of synthesis experiments that will produce suitable samples for the later analysis. Second is to analyze the metal blocks left in the crucible after the synthesis experiments. The metal blocks were first cut, polished and observed under the microscope; finally, the blocks were put into an acid-bath and fully dissolved. Only the insoluble graphite flakes stayed intact in the solution for the size analysis.
The morphology on the upper surface of the metal block is closely related to the arc-discharge during the runs. The center smooth and shining area directly contacts with the arc plasma, while the surrounding rough and foggy area, on which the carbon and metal vapor condensed and deposited, stays outside of the arc. Chemical analysis shows both areas are covered with carbon (i.e., graphite.) In addition, many radial oriented small metal blobs can be found at the rough area. Many graphite flakes existed inside the metal blocks, and their distribution and shapes are related to the temperature gradient caused by the arc-discharge. The temperature at the center portion of the block is the highest, therefore only smaller pieces and less amount of graphite can be found. The temperature at the bottom and wall portions is relatively lower, where many larger graphite flakes can be found. The log-normal size distribution of the graphite flakes in the metal blocks indicates the graphite formed by a nucleation and growth process.
Using diamond carbon source has greatly improved the synthesis efficiency. Evidence shows that small graphite flakes were formed and come off from the surface of the diamond powder. The existence of these small graphite flakes has significantly increased the contact area between graphite and metal, thus increased the amount of carbon dissolved into the metal. When evaporated from the melting metal pool, the carbon and metal vapor mixed more uniformly, and therefore synthesize more well-encapsulated GEM.

並列關鍵字

Graphite encapsulated metal nanoparticles (GEM) ； diamond ； graphite ； carbon ； graphitization ； arc discharge system

參考文獻

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Host, J. J., Dravid, V. P., Teng, M. H., Elliott, B. R., Hwang, J. H., Mason. T. O., Johnson, D. L. (1997) Graphite encapsulated nanocrystals produced using a low carbon: metal ration. J. Mater. Res., 12, 1268-1273.