探討奈米至微米尺度之鑽石氧化反應動力學機制

Abstract

珍貴且生成環境特殊的鑽石在基礎礦物學、岩石學與反應動力學領域中是相當受重視的研究課題。本研究藉熱重分析實驗數據觀察不同粒徑之鑽石氧化反應變化，以及利用動力學模型-主導曲線模型(Master Curve Model, MCM)與Avrami Equation分析，嘗試擬合出各反應之主導曲線以描述其反應過程之溫度、時間與反應歷程。更以主導曲線數據建立不同粒徑的鑽石氧化反應之反應量與溫度時間的定量關係。 實驗設計上可分為三部分，首先針對各不同粒徑之鑽石粉末以X光粉末繞射儀（XRD）確認其成分，使用掃描式電子顯微鏡（SEM）觀察鑽石表面並檢視鑽石顆粒尺寸的粒徑分佈。並利用BET法量測各不同鑽石粉末隨氧化程度而改變之比表面積；第二部分以熱重分析儀(TGA)進行等溫的鑽石氧化反應實驗。第三部分利用MCM與Avrami方程式擬合熱重分析所得到的反應數據。進一步以MCM所分析的主導曲線建立不同粒徑之鑽石氧化反應量、溫度與時間關係。並將TGA實驗結果與TGA模擬結果作比較，探討控制氧化反應之因素與其反應機制。 從研究結果得知，未分析出歷經加熱過程後按鑽石氧化反應機制假說中可能出現的石墨；由SEM觀察結果，觀察到鑽石氧化初步的表面變化因氧化晶面偏好性、表面缺陷與弱鍵結而有不同形貌之表面。但隨著氧化程度越來越高，其顆粒邊界鈍化，鑽石隨著氧化反應不斷縮小粒徑。藉由探討奈米鑽石團聚的情形與針對氧化反應放出的反應熱進行實驗校正，即可以MCM擬合不同粒徑之鑽石氧化反應結果，並且得到良好之主導曲線。但因整體反應放熱致使溫度不穩定，而無法以Avrami方程式得到良好的結果。另一方面，從MCM之推導原理看來它無法辨別反應機制，但依MCM所得之反應視活化能176.3-272.0 kJ/mol與其他相關研究之反應活化能相近，則很可能指向鑽石氧化反應為單一機制。再加上，TGA顯示重量隨時間和溫度變化的單一路徑與XRD未發現石墨的出現，初步排除鑽石氧化反應機制是由鑽石相變為石墨而後轉為二氧化碳的機制。綜合各定量分析與MCM的良好的擬合結果，進而推論鑽石氧化反應機制應由單一機制所主導，鑽石氧化是由氧氣直接與鑽石之表面碳原子結合形成二氧化碳。

Many studies have discussed the kinetic processes and the mechanism of diamond oxidation; however, there is no convincible theory concerning the reaction mechanism. Moreover, kinetic studies on the oxidation of nano-sized diamond are still lacking. The novelty of this work lies in the fact that we confirmed the mechanism of diamond oxidation controlled by one single mechanism. The research consists of three parts: First is to verify diamond powders crystal phases and purities by X-ray diffractometer (XRD). Observation of the morphology and size distribution of diamond powders is confirmed by scanning electron microscope (SEM). We also use mercury intrusion porosimetry (MIP) equipment to analyze the porosity of the diamond powder aggregates. Moreover, we obtained the specific surface area of diamond powder by BET method. Second is to oxidize a series of diamond powders by thermal gravitometry analyser (TGA) at isothermal conditions. Third is to analyze the weight loss data by kinetic models: Avrami equation and Master Curve Model (MCM). In conclusion, the composition of graphite was not detected by XRD. The fact that diamond oxidation starts from the edge, defect and dislocation was observed by SEM. By the MCM results, we can obtain good fitting curves presenting the relation between reaction percentage, time and temperature. The apparent activation energy of different sized diamond oxidation is around 176.3-272.0 kJ/mol. By XRD and MCM results, diamond oxidation is only controlled by one single mechanism that diamond oxidized by the oxygen directly and without the transformation of graphite. The study of diamond oxidation behaviors may have significant impact on all of the nanotechnology industries, as well as on basic science.