電化學剝離奈米石墨片性質研究

葛春明(Chuen-Ming Gee)

指導教授 ： 陳志臣

國立中央大學/工學院/機械工程學系/博士(2014年)

摘要

本研究提出以天然石墨與瀝青經熱壓成型製作石墨胚體，後續經不同溫度熱處理，以此製備的石墨塊材做為電極，並以不同石墨原料作比較，利用電化學剝離製程，將塊狀石墨材或不同石墨原料剝離出石墨薄片，探討熱處理溫度與不同石墨原料對剝離出石墨薄片性質影響，以拉曼光譜儀(Raman)鑑定材料的特性、光學顯微鏡(OM)與掃瞄式電子顯微鏡(SEM)觀察剝離石墨薄片型態、原子力顯微鏡(AFM)掃瞄厚度與表面形貌及化學分析電子能譜(XPS/ESCA)量測表面官能基團鍵結。
電極材料不選用價格昂貴之高純度石墨棒、高結晶度天然石墨片或高方向性熱裂解石墨，改用一般工業用天然石墨片與脈石墨及瀝青作為起始原料並製備為石墨電極，以電化學剝離製程，可獲得大量石墨薄片。
本研究中，自製石墨電極經不同熱處理溫度後，利用電化學剝離出之石墨薄片，尺寸差異性不大。若自製石墨電極以Z軸方向切割，增加石墨結構邊緣的暴露面積，以電化學剝離出之石墨薄片，其尺寸增加約1.5–2倍，並可得到層數較少、均勻的石墨薄片。
比較自製石墨電極與天然石墨以電化學剝離出之石墨薄片，其尺寸分佈約十微米左右，而脈石墨剝離出之石墨薄片尺寸較小，約數百奈米至數微米，這與脈石墨中石墨層排列方向較不一致有關，但厚度尺寸大致相同。
以噴塗方式將穩定分散之石墨烯墨水製作成透明導電膜，試樣之透光率約為70%，片電阻約為1.35 x 105 Ω/square。進一步將試樣退火處理，相同透光率之電阻值下降，退火處理後試樣之電阻約降低16-24%。另以抽濾方式將穩定分散之石墨烯墨水製備石墨烯紙，厚度約5μm之石墨烯紙，X-Y平面與Z垂直方向熱傳導係數各約為~600 W/m•K與5.5 W/m•K，X-Y平面之熱傳導係數較傳統金屬塊材高，且其價格便宜，局部高溫熱源可快速地於水平方向散開，另其厚度薄，未來可考慮取代熱界面材料應用，成為具有潛力之導熱材料。

關鍵字

電化學剝離 ； 雙層石墨烯片 ； 寡層石墨烯 ； 石墨烯墨水 ； 透明電極 ； 熱傳導

並列摘要

This study proposes an approach to synthesis graphite nano-plates by electrochemical exfoliation (EC) with graphite bulks as raw materials, where the graphite was manufactured artificially from thermoforming of natural graphite and pitch with different temperatures. Based on such electrochemical exfoliation method, the effects of thermal treatment temperatures and different graphite raw materials on the properties of exfoliated nano-plates were investigated. The quality of graphite nano-plates was identified by Raman spectroscopy. The morphologies of the samples were carried out by optical microscope (OM) and scanning electron microscope (SEM). Atomic force microscopy (AFM) was used to measure the thickness of graphite nano-plates. X-ray Photoelectron Spectroscopy (XPS) or Electron Spectroscopy for Chemical Analysis (ESCA) was used to identify the surface functional groups.
In this study, the expansive high-purity graphite rods, single-crystal natural graphite flake or highly oriented pyrolytic graphite (HOPG), were frequently used in industry, were replaced by our prepared artificial graphite. The as prepared graphite, as electrode in EC process, was made from natural graphite flakes, vein graphite and pitch as starting materials. The results indicate that graphite nano plates can be successfully obtained with high quality and large-scale production by using as-prepared artificial graphite. Moreover, it was found out that the lateral size of exfoliated nano-plates was independent to the heat treatment temperatures of as-prepared graphite. With specific cutting orientation in Z-axis direction, the graphite edge were observed to be highly exposed, leading to larger size (1.5-2 times), few-layered and excellent uniform of graphene nano-plates after subjected to EC process. Compare our prepared graphite with natural graphite, it was found out the lateral size distribution of exfoliated nano-plates was about ten microns, which was larger than that of vein graphite(about several hundred nanometers to several microns). This is possibly due to the lower ordering of graphitic orientation on vein graphite. However, it was worthy noting that the thickness of nano-plates was roughly the same.
For particle application, it was demonstrated that transparent conducting films could be made by the suspended graphite nano-plates, the so-called graphene-ink. The sample was coated on transparent substrate by air spraying method. The results show that the sheet resistance was 1.35 x 105 Ω/square with 70% light transmittance. The resistance of the films can be significantly decreased by post-treatment, indicating that the sheet resistance may decrease to 16–24% after thermal treatment. The graphene paper fabricated through filtration process provides the highly quality and massive graphene sheets. The paper thickness was about 5 μm. The in-plane and cross-plane thermal conductivity of the graphene paper can reach about 600 and 5.5 W/m•K measured by the laser flash method respectively. The in-plane thermal conductivity was higher than most metal and lower cost. The local heat spot was transferred much faster to be spread out in the horizontal direction. Moreover, the additional advantage of graphene paper was mainly on the ultra-thin thickness when compared to that of most commercial thermal interface materials (TIMs). The as-prepared graphene paper shows potential to be high performance thermal conductivity material in the future.

並列關鍵字

Electrochemical exfoliation ； Bilayer graphene sheets ； Few-layer graphene ； Graphene ink ； Transparent electrodes ； Heat conduction

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