石墨包裹奈米鎳晶粒的緻密化之初步研究

Abstract

石墨包裹奈米晶粒(Graphite Encapsulated Metal Nanoparticles，簡稱GEM)是顆粒粒徑介於5-100 nm的球狀複合材料，其內核為金屬、外殼為石墨，因為特殊的結構與尺寸，使GEM成為科學界頗受矚目的材料之一。 在許多的研究中發現，由於奈米顆粒常常會有團聚的現象產生，讓顆粒與顆粒之間分散情況不佳，以至於在一般的情況之下無法形成緻密的堆積，為了能將奈米顆粒材料緻密化的堆積，根據材料性質的不同，需要經過各種高溫(攝氏一千度以上)與高壓(數個GPa)的製程下才可以形成。在本研究中偶然地發現經由磁性收集過程，可以讓石墨包裹奈米鎳顆粒自行形成一塊塊的緻密塊體，藉由阿基米得法的密度測量以及SEM照片上的初步觀察，判斷這些塊體的緻密程度可以達到80%以上。 本研究主要分為兩部份，第一部份是合成高緻密度石墨包裹奈米鎳晶粒塊體，並且從製造過程中找出可能會影響其形成的因素，以這些因素為變因進行實驗以研究這些因素與高緻密度石墨包裹奈米鎳晶粒塊體形成之關係。第二部份是由第一部份實驗後得到的結果推論出高緻密度石墨包裹奈米鎳晶粒塊體的形成機制。 實驗所得到的結果中，酸溶、磁力及以甲醇為分散劑與毛細作用是影響高緻密度石墨包裹奈米鎳晶粒塊體形成的重要因素。高緻密度石墨包裹奈米鎳晶粒塊體的形成過程及形成機制如下：酸溶及超音波震盪兩個步驟破壞了原本石墨包裹奈米鎳晶粒與其他產物(奈米金屬顆粒與碳雜質)間的部份團聚，並讓打散的顆粒的分散。接著石墨包裹奈米鎳晶粒浸泡在甲醇中時，甲醇分子會吸附在石墨包裹奈米鎳晶粒與碳雜質外圍，讓顆粒與顆粒間產生靜電斥力及增加顆粒間距，並降低了顆粒與顆粒間互相的吸引力(凡得瓦爾力及磁力)，同樣有助於分散。同時外加磁場的磁力會使部分的石墨包裹奈米鎳晶粒照磁場方向發生運動(轉動及滑動等) 讓顆粒的排列更接近磁場的方向。最後在甲醇蒸散的過程中，由於毛細作用力產生向內的拉力，讓顆粒與顆粒堆疊的更緊密，即形成了高緻密度石墨包裹奈米鎳金屬晶粒塊體。

Graphite Encapsulated Metal (GEM) nanoparticles 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. Because they easily aggregate together, nanoparticles can’t compact very well in normal situation. Depends on various materials, it may need more than one thousand centigrade or several GPa to make a high density bulk. In this research, we accidentally find a simple way to make high density GEM bulk via magnetic processing. By measuring the density of GEM bulks and observing the pictures taken by SEM, we estimate the porosity of the bulks is lower than 20%. The study can be divided two parts: First is to synthesize the high density graphite encapsulated nickel nanoparticles and to find out the possible affecting factors. Then we design a number of experiments based on the possible affecting factors as variable parameters. Second is to derive the possible mechanisms of the formation of the high density graphite encapsulated nickel nanoparticles. Though it is impossible to directly observe the rearranging and densification processes of GEM particles in a magnetic field, the following four factors may all play an important role: First is acid bath and ultrasonic mixing, which break up the aggregation of GEM, nickel nanoparticles and amorphous carbon, and disperse the particles. Second is when the GEM immersed in methanol, the methanol molecules adsorb on the surface of nanoparticles and decrease the van der Waals forces between them. Third is when the GEM put into an external magnetic filed, the ferromagnetic nickel nanoparticles could slide, rotate, and move themselves into a more compact position. Finally is when the methanol evaporates, and GEM becomes even denser through capillarity forces.