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標題: | 合成石墨包裹矽奈米顆粒之初步研究 Preliminary Study on the Synthesis of Graphite Encapsulated Silicon Nanoparticles |
作者: | Jhih-Ying Chen 陳志穎 |
指導教授: | 鄧茂華(Mao-Hua Teng) |
關鍵字: | 石墨,矽,碳化矽,核-殼結構,電弧法,奈米顆粒, graphite,silicon,silicon carbide,core-shell structure,arc discharge method,nanoparticles, |
出版年 : | 2020 |
學位: | 碩士 |
摘要: | 石墨包裹矽奈米顆粒(graphite encapsulated silicon nanoparticles, GES)是一種大小約20-100 nm、具有核-殼結構的球狀奈米複合材料,其核心由奈米級矽或碳化矽多晶所組成,外部則由層狀石墨片相互堆疊而成。由於石墨的物化性質穩定,因此可保護奈米級的矽質核心不受環境氧化,而保留其獨特的奈米特性。近來發現具有核-殼結構的GES相當適合應用於鋰離子電池(lithium-ion batteries)的陽極材料中,為電池產業上具有應用潛力的新興材料。 自1994年開始,許多研究團隊利用電弧裝置進行石墨包裹矽奈米顆粒之合成研究,然而矽在高溫電弧作用下極易與碳化合成碳化矽(silicon carbide, SiC)晶粒,因此矽被認定為無法以電弧法合成包裹顆粒的元素。而本研究利用1999年美國西北大學所研發出的改良式鎢電弧法(modified tungsten arc discharge method)進行實驗,終於成功合成出GES。然而因缺乏GES合成之相關文獻,即使根據合成石墨包裹奈米顆粒理論中,最廣為人接受的二步驟機制模型(two-step mechanism model)也無法完整解釋其合成機制。因此本研究藉由設計不同實驗參數,並從初步分析結果中推導GES的組成與電弧下的反應等,提出可能的GES合成機制。 本研究之實驗設計參照本研究團隊對於石墨包裹金屬奈米顆粒(graphite encapsulated metal nanoparticles, GEM)的研究歷程,並利用TEM、XRD與TGA等儀器分析產物。由於矽的碳化傾向非常高,因此GES的核心主要以矽或碳化矽晶粒所組成。本研究為了了解GES內矽的比例及分布,先將XRD分析結果以矽-碳化矽檢量線對比,發現不論使用哪種碳源及碳源添加的方式,GES內矽的重量百分比皆不超過20 wt.%;接著利用TGA與退火方式進行熱處理,並從反應曲線推測GES內部分布,目前以碳化矽晶粒包覆矽晶粒的外-內核結構較為合理,因外核與碳層接觸,較易形成碳化矽晶粒。另外根據熱處理結果,發現GES的石墨化程度在退火後提升約10%,且碳化矽佔比下降,說明碳化矽在約550oC時分解成石墨與矽,因此本研究加以猜測:1. GES顆粒有機會受空氣對流滑過電弧表面,在碳層最內處形成石墨包裹;2. GES可藉由退火方式進行純化。 為了進一步了解電弧下碳與矽的合成反應,利用液滴法及蒸氣法兩種不同的碳源添加方式進行研究。從XRD結果可以發現,使用蒸氣法的GES之矽晶粒較液滴法多,分別約19 wt.%及13 wt.%。這是由於液滴法的碳源可直接進入電弧內與矽反應,電弧受干擾發生變形或中斷,矽蒸氣在電弧內碰撞時間與範圍不足,又因碳化傾向大而形成較多的碳化矽;反之蒸氣法的碳因在電弧外揮發,受向上對流及電弧的高溫影響而無法進入電弧內,使矽蒸氣可在電弧內合併成較大的碳、矽混合顆粒,並根據二步驟機制模型中的相分離步驟與石墨分離,最後核心表面的液態矽在電弧邊界與石墨形成碳化矽,保留內部矽晶粒不被碳化。 本研究綜合以上GES初步結果分析其基礎物化特性,期望能提供未來建立相關研究之基礎,提升GES在不同材料領域上的應用。 Graphite encapsulated silicon nanoparticles (GES) is a spherical nanocomposite with a core-shell structure of about 20-100 nm in size, the core of which is composed of nano-scale silicon or silicon carbide polycrystallines. The outside is made of layered graphite flakes stacked on top of each other. Due to the stable physical and chemical properties of graphite, it can protect the nano-scale silicon core from environmental oxidation reaction, while retaining its unique nano-size characteristics. Recently, it has been found that GES with a core-shell structure is quite suitable for use in anode materials of lithium-ion batteries, and is a promising material with application potential in the battery industry. Since 1994, many research teams have used arc devices to conduct the synthesis study of graphite-coated silicon nanoparticles. However, silicon is very easy to be carbonized into silicon carbide crystals under the high temperature of arc, so silicon has been deemed as an impossible element to synthesize this kind of encapsulation particles by arc method. In this study, the modified tungsten arc discharge method, developed by Northwestern University in 1999, was used to conduct experiments, and finally the GES was successfully synthesized. However, due to the lack of related literatures on GES synthesis, even the two-step mechanism model cannot fully explain the GES synthesis mechanism. Therefore, in this study, by designing different experimental parameters and deducing the composition of GES and the reaction under the arc from the preliminary analysis results, a possible synthesis mechanism of GES is proposed. The experimental design of this study is based on graphite encapsulated metal nanoparticles (GEM), and the products are analyzed using instruments such as TEM, XRD and TGA. Because the high tendency of silicon to be carbonized, the core of GES is mainly composed of silicon or silicon carbide polycrystallines. In this study, in order to understand the proportion and distribution of silicon in GES, the XRD analysis results were compared with the Si-SiC calibration line. And it was found that the weight percentage of silicon in GES does not exceed 20 wt.% no matter which carbon sources were used or the way of carbon sources addition method is used. Followed by heat treatment using TGA and annealing, and inferring the internal distribution of GES from the reaction curve. So far, the outer-core structure of the silicon carbide polycrystallines coated with silicon carbide polycrystallines is more reasonable. Because the outer core is in contact with the carbon layer, it is easier to form silicon carbide. In addition, according to the heat treatment results, it was found that the degree of graphitization of GES increased by about 10% after annealing, and the proportion of silicon carbide decreased, indicating that silicon carbide decomposed into graphite and silicon at about 550oC. Therefore, there are two guesses: 1. GES particles have the opportunity to slide across the arc surface by air convection, forming a graphite flakes at the innermost part of the carbon layer. 2. GES can be purified by annealing. In order to further understand the synthesis reaction of carbon and silicon under the arc, two different methods of carbon sources addition, the droplet method and the vapor method, were used for research. From the XRD results, the GES by using the steam method has more silicon crystals than the droplet method, which are about 19 wt.% and 13 wt.%, respectively. This is because the carbon sources of the droplet method can directly enter the arc and react with silicon, the arc is deformed or interrupted by interference, the collision time and range of silicon vapor in the arc are insufficient, and more silicon carbide is formed due to the large carbonization tendency. On the contrary, the carbon in the vapor method cannot enter the arc due to the upward convection and high temperature of the arc, so that the silicon vapor can coalesce into larger carbon and silicon mixed particles in the arc. According to the two-step mechanism model, graphite is separated by the phase segregation step. Finally, the liquid silicon on the core surface forms silicon carbide with graphite at the arc boundary, leaving the internal silicon grains not to be carbonized. This study analyzes the basic physical and chemical characteristics of the above preliminary GES results, and hopes to provide a basis for establishing relevant research in the future and enhance the application of GES in different material fields. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/48854 |
DOI: | 10.6342/NTU202003202 |
全文授權: | 有償授權 |
顯示於系所單位: | 地質科學系 |
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