不同粒徑大小的石墨包裹奈米鎳晶粒在NP-9膠體系統中之分散研究

Dispersion of the Graphite Encapsulated Nickel Nanocrystals with Different Particle Sizes in the NP-9 Colloidal System

李尚實(Shang-Shih Li)

指導教授 ： 鄧茂華

國立臺灣大學/理學院/地質科學研究所/碩士(2006年)

https://doi.org/10.6342/NTU.2006.02924

摘要

奈米顆粒的分散技術已經廣泛應用在現今許多科技產業上，包括鍍膜、化妝用品、食材藥物和染劑塗料上。良好的分散和粒徑分佈可以讓材料的物理性質均一，並使反應面積和速率增加，使材料和藥物達到有效的利用和吸收。石墨包裹奈米鎳晶粒(Ni-GEM)是一種粒徑為1~100奈米(nm)的球狀複合材料，其內核為Ni金屬而外層為石墨殼層。外層的石墨層可以幫助奈米級的Ni金屬顆粒不至於氧化或被酸侵蝕。但是由於受到GEM中心之鐵磁性金屬的磁力和奈米粒徑下的凡得瓦力影響，導致GEM產生團聚，形成微米(μm)級大小甚至更大的團聚顆粒，此現象對於原本在奈米尺度下所應具有的高比表面積、顆粒滲透反應和鍍膜的均勻分佈等優異特性都造成很大的阻礙。
為了解決團聚的問題，本實驗先對GEM顆粒間的作用力進行初步理論計算，針對磁力和凡得瓦力的相對大小做比較。當顆粒粒徑小於40 nm，兩作用力的比例大約接近1:1；隨著顆粒粒徑的下降，雖然磁力的強度降低較多，但兩作用力的強度都會明顯降低，因此較小粒徑的GEM應該較易分散。本研究設計了一套實驗的流程，第一步先降低合成GEM實驗環境中的壓力(從300 torr降為50 torr)，讓生產出的GEM的顆粒平均粒徑從約23.3 nm 縮小至14 nm，藉以降低顆粒間磁能的大小(根據大部分磁性材料特性可知磁化強度M(emu/cm3)和材料的體積成正相關)。其次將生產出的粉末利用超音波震盪(ultrasonic bath)配合NP-9非離子型界面活性劑(nonionic- surfactants)進行分散，並使用流變儀(rheometer)來檢測顆粒的分散程度。實驗中發現當NP9的重量百分濃度約在40%時可以達到微胞的飽和濃度(critical micelle concentration – CMC)。如果改變所添加GEM的顆粒粒徑，較小粒徑的GEM在相同濃度NP9的膠體系統中，會有較高的黏滯性，這是因為當顆粒具有相同的總體積時，平均顆粒粒徑較小的GEM比表面積較大，所形成的微胞也會較多，因此導致黏度的上升。藉由流變儀檢測黏度可得知，低艙壓下生產的小粒徑粉末，恰如理論計算的預測，確實降低磁力相吸的效應，而獲得較佳的分散效果。
本研究藉由GEM外層石墨親油特性，採用非離子型界面活性劑NP-9作為分散劑，確實可以將GEM顆粒長期懸浮於溶液中形成微胞而不沉澱。利用流變儀對分散的實驗進行檢測，是一套有效的分析流程與步驟，可以利用不同模式的檢測成功分析出奈米顆粒懸浮的情況，在應用上便可以清楚了解奈米材料的特性，針對問題進行解決，應用層面將可以開拓的更廣。

關鍵字

分散 ； 石墨包裹金屬晶粒 ； 膠體系統 ； 界面活性劑

並列摘要

Graphite encapsulated metal (GEM) nanoparticles is a relatively new material. With an inner ferromagnetic metal core and several layers of outer graphitic shells, GEM (1-100 nm in diameter) can survive in severe environments and still preserve its nanocrystalline properties. GEM has many potential applications, and some of which require dispersive particles, e.g. as a dispersive catalyst on a substrate when making carbon nanotubes; yet before this type of applications could ever become reality one major problem must be solved. The problem is the severe agglomeration of the GEM nanoparticles, which is due to both the van der Waal’s forces among the particles and the strong magnetic forces of their ferromagnetic cores. Because the sizes of agglomerated particles are much larger than nanometer scale, the characteristic properties of nanoparticles, such as high surface to volume ratio and better absorption, will be lost.
To effectively disperse the GEM nanoparticles, many organic solvents such as methanol, oleic acid and acetone had been used but with little success. The best results came when a non-ionic surfactant – nonylphenol ethoxylate (NP-9) – was used, combined with an improved synthesis technique to control the average particle sizes by adjusting total pressure, that GEM nanoparticles uniformly suspended in NP9 solution and formed a colloid. Colloids are distinguished from true solutions by the presence of particles that are too small to be observed under a microscope yet are much larger than common molecules.
The viscosity of GEM colloids of various weight percentage of NP-9, with fixed temperature and pH, were analyzed by a rheometer. The viscosity apparently decreases with the particle size of GEM. In addition, the solution becomes saturated jelly-like substance as the weight percentage of NP-9 is equal to 40%-50%. Preliminary results show that the key of success is to reduce the average particle sizes of GEM, e.g. from 25 nm to 14 nm such as in this work, thus minimize the magnetic interaction between GEM and increase the effective reaction surface area with surfactant NP-9. Other factors such as temperature and pH value will be included in future experiments.

並列關鍵字

dispersion, ； graphite encapsulated metal ； colloidal system ； surfactant

參考文獻

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