Study of the Effect of Methanol Evaporation Rate on Packing of Graphite Encapsulated Nickel Nanoparticles

石墨包裹奈米鎳晶粒(Graphite Encapsulated Nickel Nanoparticles，簡稱Ni-GEM)是顆粒粒徑介於5-100 nm 的球狀複合材料，其內核為鎳金屬、外殼為石墨，也是目前本團隊所研究鐵、鈷、鎳三種磁性金屬中能以磁力方式自行排列呈高緻密塊體的GEM 材料，其緻密程度高達80%以上。奈米粉末的團聚問題嚴重，所以相當難使之緻密，但本研究利用熟知的毛細現象就使Ni-GEM 形成高緻密塊體，其主要驅動力就是來自粒子間彎曲液面的毛細作用力(capillary force)，由毛細作用力驅動微粒聚集排列，將以往難以緻密的奈米顆粒形成高緻密度的塊體。 本研究包括兩部份，第一部份有別於以往利用磁力方式製備Ni-GEM塊體，改以離心機、石膏模型、壓坯成型法、自然沉澱法等方式製備Ni-GEM之緻密塊體，接著比較這些方法所製造出塊體的緻密度差異，並嘗試控制Ni-GEM 緻密塊體之外型，做出不同外型之Ni-GEM 塊體。第二部份為本研究之重點，針對在自然沉澱法中，甲醇不同的蒸散速率可得到不同緻密度之Ni-GEM 塊體，在這過程中對其Ni-GEM 顆粒間所受到的毛細力作討論，找出塊體緻密與毛細力之間的趨勢與關係。 由5 組不同溫度條件的實驗結果中發現，Ni-GEM 塊體的緻密度並非受到溫度直接的影響。從Lian 等人研究中所提出之液橋作用力模型發現，顆粒間之甲醇揮發過程中產生液橋力(liquid bridge force)。甲醇溫度越高，顆粒間產生之液橋力越小，但在高溫下液橋力較小所產生的Ni-GEM塊體相對密度卻都較低溫液橋力較大高，因此溫度顯然並非Ni-GEM 緻密之主要因素而應為蒸散速率。由多組實驗結果後證實利用自然沉澱法，單改變甲醇蒸散速率即可以製備出相對密度高達88-90%之塊體。此外由理論的推算可以發現顆粒大小58 nm 間的吸引力與磁鐵對於顆粒的吸引力大約分別是10-14 與10-12(N)，比液橋作用力10-9-10-10(N)還小。因此本研究推論在高揮發速率的環境下由於顆粒間的液橋快速的減少，顆粒與顆粒靠近的相對速度也會提升，但由於液橋消失的快，所以所形成之塊體也趨於破碎，這個現象在溫度越接近甲醇沸點越是明顯。在低溫(0℃)的環境下由於甲醇揮發速率慢，顆粒間受到液橋力的時間也相對增長且低溫下液橋力較大，因此這兩種溫度條件下所形成之Ni-GEM 塊體密度都較室溫(20℃)條件下來的高，因此不管是利用改變溫度的方式或者是改變環境氣壓的方式，只要透過改變甲醇蒸散速率就可以使石墨包裹奈米鎳金屬達到高緻密。Graphite Encapsulated Metal (GEM) nanoparticles is a spherical composite material with a core(metal)/shell(carbon) structure and its diameter is in a range of 5 to 100 nm. Because the outer shell protects the inner metal nanoparticles, the materials remain stable in strong acid as well as at high temperature in an oxygen-free environment. It is widely used in environmental implement, scientific research, biomedical applications, and military. GEM can be packed to form high density bulk by the magnet force and the problem of aggregation can be solved in our previous lab studies. This study not only utilizes other forces or set up several parameters to form high density bulk, but also investigates the mechanism of forming high density bulk of GEM. These experiments are designed as following: (1) centrifugal force, (2) gypsum casting, and (3) deposition method to form high density bulk. The results of centrifugal force show that high density, large volume bulk of GEM can be formed but the results of gypsum casting cannot. The results of deposition method obviously show that the density of GEM is increasing with increasing temperature, whether the magnet force was conducted or not. In fact, the liquid bridge force can explain the different evaporation rates will affect Ni-GEM bulk density when changing temperatures. Besides, this study also provides the theoretical calculation of the effects among Van der Waals force, the magnetic force, and the liquid bridge force. The results suggest that the liquid bridge force predominates of dense GEM bulk forming in the process. The liquid bridge force is a hundred times larger than the magnetic force and 10 thousand times larger than Van der Waals force. In summary, this study not only provides experimental results and theoretical calculations to prove that the dominated force is the liquid bridge force, but also provide another method to form the high density bulk of Ni-GEM