石墨包裹鎳奈米晶粒在高溫高壓下合成鑽石的初步探討

Preliminary study on the synthesis of diamond by using graphite-encapsulated nickel nanoparticles at high pressure and high temperature

蕭敦仁(Tun-Jen, Hsiao)

指導教授 ： 鄧茂華

共同指導教授 ： 宋健民

國立臺灣大學/理學院/地質科學研究所/碩士(2005年)

https://doi.org/10.6342/NTU.2005.02809

摘要

自Tracy Hall在1955年成功地以環狀高壓機將石墨與硫化鐵（FeS）觸媒合成出鑽石以來，科學家即不斷地研究鑽石合成原理，但對於其生長機制尚未全然了解。至目前為止的高溫高壓下合成鑽石技術，所用之原料通常為微米級的石墨以及金屬催化劑，其中的催化劑通常為熔融的過渡元素，包括ⅧB族的Fe、Co、Ni、Ru、Th、Pd、Os、Ir、Pt和另三個過渡元素Mn、Cr、Ta。為了對鑽石形成機制有更深入的了解，本研究採用石墨包裹金屬奈米晶粒（Graphite Encapsulated Metal Nanoparticles, 簡稱GEM）作為全新的碳來源及催化劑，以高溫高壓法來合成鑽石晶粒。GEM為粒徑10-100 nm的球狀複合材料，其內核為金屬，外殼為石墨層。根據傳統鑽石合成機制理論的預測，以GEM合成鑽石的優點主要有以下三點:一、具有極大的比表面積，二、內核金屬即為合成鑽石所需要的催化劑，不需要另外添加，三、外層石墨與內核金屬緊密接觸，應可加速反應速率。
本實驗主要分為兩階段，第一階段為製造足夠數量的鎳GEM奈米晶粒作為合成鑽石的原料，第二階段則是將GEM粉末進行兩組高溫高壓實驗。A組實驗為將初產物粉末直接進行高溫高壓實驗，B組之修正程序實驗則先將初產物進行酸溶、真空熱處理，使GEM外層的石墨殼層增厚且增加結晶度，再進行高溫高壓實驗。合成鑽石所使用的高溫高壓設備為六面頂高壓機，其產生的溫度、壓力可達到鑽石穩定相區（1300℃與5.2 GPa）。
實驗結果發現，雖然以傳統石墨與催化金屬粉末配置的對照組成功的合成出微米級的鑽石，但是實驗組以X光繞射與拉曼光譜分析均很意外的未觀察到明顯的鑽石訊號，必須利用穿透式電子顯微鏡（TEM）之電子繞射分析才發現產物中含有少量的奈米鑽石。推測其生長機制為在高溫高壓實驗時，GEM的鎳核心會熔化析出，使外層的石墨殼層崩解，形成許多結晶度差、粒徑為奈米級的石墨微片。而只有部分結晶度較好之奈米級的石墨微片受到催化劑作用才會形成奈米鑽石，其他結晶度差的石墨必須要先經過再結晶，才有機會被催化成鑽石，因此樣本中不會析出並大量的鑽石微片，也因此導致先生成的奈米鑽石晶粒，故無法繼續長成微米級的鑽石晶粒。另外在實驗區中也意外發現結晶度良好具六邊形外形的石墨晶體，本研究推測（1）因為奈米粉末的孔隙度高，難以完全緻密化，在高溫高壓實驗時，容易造成壓力分佈不平均，而且（2）用來分隔實驗區的石墨隔片，因電阻較高也會導致溫度高於對照組。因此使得實驗區中的溫壓條件落入石墨的生長區內，加上受到鎳之觸媒催化作用，隨著實驗時間增加與石墨再結晶作用，漸漸形成微米級又具良好外形的石墨晶體。綜合上述，本研究以GEM當作合成鑽石的原料，但實驗結果未發現微米級的鑽石，推究其原因，包括溫度、壓力及石墨層結晶度不夠等，都可以歸因於原料的奈米結構所導致。

關鍵字

石墨包裹鎳奈米晶粒 ； 合成鑽石

並列摘要

Since 1955 when Tracy Hall had first successfully synthesized diamond by using a belt high-pressure apparatus from a mixture of troilite (FeS) and graphite raw materials, many scientists have been studying on the formation mechanism of diamond and have gained some significant progress. The raw materials used in modern diamond synthesis technology are usually micrometer sized graphite powder and metal catalyst. This research, however, uses a new raw material, the graphite-encapsulated metal nanoparticle (GEM), as the carbon source and metal catalyst in the synthesis of diamond. GEM is a composite material, whose grain size is 10-100 nm and with a core/shell structure, i.e., the core is metal, and the shell is graphite. The three possible advantages of using GEM to synthesize diamond are: its large specific area, the already existed catalytic core metal, and the close contact of core/shell structure.
Though the comparing section of the sample that was composed of industrial graphite and metal catalyst had successfully produced diamonds, in the experiment-section of the sample it was a surprise that we could not find any diamond with either XRD or Raman spectrum analysis, only by TEM that we found some nanodiamonds. The possible formation mechanisms of nanodiamond are: first, the nickel core melts and the nickel precipitates outside of the graphitic shells. Second, graphite shells become disintegrated and forming many crystallized graphite flakes. Third, the graphite flakes are catalyzed by the nickel catalyst and transformed into nanodiamond. In addition, we found a well-crystallized graphite single crystal by TEM. This may indicate that first, the pressure during the experiments was actually lower than the required high pressure. Second, because the graphite pieces that used in the experiments to prevent any contamination have a higher resistance, they would probably produce more heat and create a higher temperature. Thus, the relatively lower pressure and higher temperature experimental conditions would lead to the recrystallization of graphite, and produce the hexagon graphite crystal observed.
In summary, the nanostructure of the GEM nanopaticles may probably have deviated the required experimental temperature and pressure. As a result, we cannot find any larger sized diamond other than nanodiamond.

並列關鍵字

graphite-encapsulated nickel nanoparticles ； the synthesis of diamond

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