化學還原法合成奈米金屬（鎳、鐵、銀）材料之研究與應用

The Chemical Synthesis and Application of Nanosized Metallic Materials (Nickel, Iron and Silver)

黃國政(Kuo-Cheng Huang)

指導教授 ： 周更生

國立清華大學/工學院/化學工程學系/博士(2007年)

https://doi.org/10.6843/NTHU.2007.00051

摘要

本研究利用化學還原法，以金屬鹽為前驅物，選擇適當的還原劑與分散劑，在水溶液中合成奈米鐵微粒與鎳絲等材料，並開發其相關應用。
鎳絲是以聯胺為還原劑，在磁場中還原鎳離子而得。觀察其結構，可以發現鎳絲是由鎳微粒在磁場中受磁場作用，而排列連接成絲。研究顯示，鎳絲的型態受到兩個重要因素影響，分別為成核速率與成長速率。在快速成核的條件下，所獲得的鎳絲直徑較細，而在較慢速成核的條件下，鎳絲直徑會較粗。在成長速率較快時，鎳絲的結構會較為鬆散脆弱，相反地，在成長速率較慢時，鎳絲的結構會較為緊密，強度也會較高。而所製得之鎳絲，可應用在電磁波遮蔽材料上的應用，在相同的混煉程序下，鎳絲由於有較佳的形狀係數，因此較同粒徑之鎳粉，展現出較佳之電磁波遮蔽效果。
奈米鐵微粒是以硼氫化鈉為還原劑，在水溶液中還原鐵離子而得。研究發現，水溶性高分子PAA（polyacrylic acid）有助於鐵微粒的分散，能夠確實阻止鐵微粒的聚集。此外，藉由成核促進劑鈀離子的添加，提供大量的成核點，能夠有效減小鐵微粒的粒徑。同時也發現，溶液的pH值對鐵微粒的大小也有很大的影響，研究顯示這是因為分散劑的分散能力受到pH值的影響所致。在較高的pH值時，PAA的分散能力較佳，因此能夠維持成核點的穩定，溶液中的成核點數量較多，最後，所成長的鐵微粒粒徑較小，相反的，若pH較低，以致於成核點分散不佳，成核點數量會減少，最後成長的鐵微粒則粒徑較大。奈米鐵微粒做為鎳鐵電池中鐵電極的活性材料時，其首次電容量最高達510 mAh/g-Fe，而放電電流也可達200 mA/g-Fe以上。顯示微小粒徑的確有助於提高鐵的利用率，而巨大的比表面積，則可有效提供大電流的輸出。
另外，我們利用旋轉塗佈的方式，將奈米銀微粒成功平均分佈在基板上，藉由控制銀膠體濃度，可控制基板上銀微粒濃度從單獨分開到單層緊密堆積，甚至多層緊密堆積結構。在不同溫度下熱處理，發現多層之奈米銀堆積結構在100℃的熱處理下，即已導電，隨著溫度提高電阻先降低到一最小值，然後增加，最後又變為不導電。觀察其結構變化，可以發現奈米銀粒子隨著溫度提高而燒結，因此展現出導電特性。隨著溫度繼續增加，燒結程度繼續提高，但最後由於收縮成大顆粒，才又變得不導電。

關鍵字

奈米 ； 鎳 ； 鐵 ； 銀 ； 電極

並列摘要

In this study, chemical reduction method was adopted to produce ultrafine metallic materials including iron nanoparticles and nickel fibers in aqueous solution from their respective metal salt precursors. Related applications of these metallic materials were also investigated.
Nickel fibers were reduced from nickel chloride by hydrazine in the presence of magnetic field. It was found that nickel fibers were composed of nickel particles connected linearly under the effect of the magnetic field. This work showed the morphology of nickel fibers was mainly affected by two factors: nucleation rate and growth rate. In the fast nucleation case, thinner nickel fibers would be obtained, and on the contrary, thicker fibers would be obtained in the slow nucleation case. When the growth rate was fast, the nickel fibers would be loose and weak, and when the growth rate was slow, the nickel fibers would be tight and strong. In the application as electromagnetic interference (EMI) shielding materials, nickel fibers showed a better effectiveness than nickel particles with the same diameter, this is benefited from the high aspect ratio of nickel fibers in the composite materials.
Iron nanoparticles can be synthesized by using sodium borohydride as the reducing agent and iron chloride as the precursor in the aqueous solution. In was found that the water-soluble polymer PAA (polyacrylic acid) is beneficial to the dispersion of iron nanoparticles, hence stopping the agglomeration of iron nanoparticles. In order to decrease the diameter of iron nanoparticles, palladium iron considered as the nucleation promoter can be added to increase the nucleation sites. In addition, it was also found that the pH of the solution affected the iron nanoparticle size noticeably, which directly caused from the dispersing capability of dispersing agent under different pH values. When the pH was high, the dispersing capability of PAA increased and the nucleation sites were well-dispersed. The numerous nucleation sites therefore subsequently led to the small iron nanoparticles. On the contrary, when the pH was low, the dispersing capability of PAA decreased and the nucleation sites were agglomerated. Consequently, the loss of nucleation sites led to the large iron nanoparticles. The iron nanoparticle electrode showed the capacity as high as 510 mAh/g-Fe with the discharge current of over 200 mAh/g-Fe. This result indicated the utilization ratio of iron was indeed improved by the decrease of iron nanoparticle size as well as the discharge current was enhanced by the large surface area of iron nanoparticles.
The silver nanoparticles could be dispersed on a substrate by the spin-coating method. By controlling the silver colloid concentration, the distribution of silver nanoparticles on the substrate could be varied from dispersed separation to mono-layered close packing, eventually to multi-layered close packing. After heat treatment at different temperatures, the multi-layered silver nanoparticles structure started to exhibit conductivity after 100℃ treatment. The resistivity initially decreased with temperature and then increased to become finally insulated again. On the SEM observation, the silver nanoparticle structure exhibited sintering effect at the increase of temperature and became conductive. But when the temperature continuously increased, the silver nanoparticles structure shrank into large particles and became insulated again.

並列關鍵字

nanoparticle ； nickel ； iron ； silver ； electrode

參考文獻

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