奈米二氧化矽微粒用途廣泛,若能精確控制其生成粒徑的大小,那實際應用價值將更為提高。本論文主要是利用田口實驗法進行沈澱法製備二氧化矽奈米微粒之探討,以期得到較佳粒徑控制反應之參數組合及瞭解反應機制。 由特性要因圖的解析,選擇出8個實驗因子,分別為分散劑之種類、濃度、反應時加入之時間、攪拌速度、反應的起始溫度、pH值、矽酸鹽加入量(固含量)及反應時間;除了分散劑種類選擇2個水準外,其餘各因子皆選擇3個水準,並以L18直交表作為實驗配置。經由S/N比及靈敏度之分析結果顯示,以沈澱法製造二氧化矽奈米顆粒中,在不變更攪拌葉和矽酸鹽的濃度情況下,分散液的濃度、加入時間及矽酸鹽的加入量為控制顆粒成長所必須優先考慮的因子項目。從確認實驗中,在望大特性裡得到粒徑約60~80 nm,望目特性中則粒徑約50~55 nm,望小特性則得到粒徑約25 nm之二氧化矽微粒。 藉由FE-SEM和TEM圖形分析可知,二氧化矽的粉體聚集的情況相當的嚴重,這是由於二氧化矽分子間具有極強的化學鍵能存在,導致於二氧化矽一次顆粒僅在20~60 nm之間,而無法成為奈米材料的因素。由XPRD圖譜分析,以沈澱法合成的二氧化矽,因無明顯的特性峰存在,因此可以證實本實驗所合成的二氧化矽為非結晶相。 本實驗亦藉由黏度的變化來解釋二氧化矽反應形成的機制。實驗中在反應第38 min時,發現二氧化矽反應之黏度急遽升高,達到5800 cp後開始下降,至43 min後回覆至初始的狀態,可能是因為無法立即去除反應所生成的鹽類,造成反應粒子會相互鍵結形成所謂之三度網狀結構體。
The understanding of nanoparticle size distribution, porosity, reactivity of silica prepared by precipitation route may play a critical or important role in the applications of silica chemistry. Therefore, the main objectives of the present study were to investigate the synthesis of silica nanoparticles by precipitation method and to study the optimal controlling conditions of the nanoparticle size distribution by Taguchi method. Experimentally, the controlled key factors of the silica nanoparticle size distribution were the kind of dispersers, concentration of reactants, dosing times of dispersing solution, mixing rates, initial reaction temperatures, pH values, weights of silicate adding, and reaction times. The experimental results indicated that the significant key parameters of the silica naoparticle synthesis were the concentration of reactants, dosing time of dispersing solution, and quantity of added silicate for the nanoparticles size distribution of silica according to the signal to noise ratio. Based on the results of orthogonal array, the well-controlled experiments of the optimal key parameters were found. The nanoparticle size distributions of the silica synthesis by using Taguchi method were shown as following respectively: 25 nm of smaller-the-better characteristics, 50~55 nm of nominal-the-better characteristics, and 60~80 nm of larger-the-better characteristics. In addition the FE-SEM and TEM microphotos indicated that the size distribution of SiO2 primary nanoparticles could be controlled in the range of 20 to 60 nm. However, the nanophase SiO2 power was agglomerated because the strong chemical bonding was existed in the SiO2 molecules surface and therefore the nanostructures of SiO2 power were not formed. The SiO2 nanoparticles obtained from the precipitation method were amorphous confirmed by the XRPD patterns. Furthermore, 3-D network nanostructures were formed in the intermolecular SiO2 nanoparticles because the sodium sulfate by-products were not removed immediately in the second stage of precipitation. The values of viscosity increased and decreased notably at the reaction times of 38 and 43 minutes, respectively and finally went down to the initial state. It was suggested that the reaction mechanisms of silica formation might be affected by the changes of viscosity in the precipitation process.