二氧化矽氣凝膠與複合氣凝膠之製備與物理性質探討

Preparation and physical properties of silica-based aerogels and composite aerogels

魏得育(Te-Yu Wei)

指導教授 ： 呂世源

國立清華大學/工學院/化學工程學系/碩士(2006年)

摘要

本論文主要研究的對象分為兩大部分，第一為二氧化矽氣凝膠的製備，第二為金屬氧化物-二氧化矽複合氣凝膠的製備與物理性質的探討。在二氧化矽氣凝膠的製備上，我們採用表面改質的方式，再配合常壓乾燥進行，如此可以大大的降低傳統使用超臨界乾燥所需要的極高成本。以過去常壓乾燥的文獻做一個基礎，開發出多次改質的製程程序，製備出具有低密度、高孔隙率和低熱傳導係數的二氧化矽氣凝膠，其中孔隙率高達97%，熱傳導係數低達0.036W/m-K，物理性質遠比過去文獻中，同樣利用常壓乾燥製程所製備之二氧化矽氣凝膠來的好。另外也針對其它物理化學性質做分析與比較，如比表面積、疏水性和表面官能基檢測，由檢測分析的結果得知，經過多次改質的二氧化矽氣凝膠，因為表面官能基充份的反應形成疏水的鍵結，所以在常壓乾燥的過程中具有較低的體積收縮率，獲得的二氧化矽氣凝膠孔洞體積較大，孔隙率因此大大的提升，進而降低了熱傳導係數，經過接觸角的量測，發現疏水角度達143o。另外也針對製備參數進行探討，以EtOH/TEOS為8的製備參數下具有較佳的物理性質，改變溶膠-凝膠反應的pH值探討中，發現以高pH值下的製程製備之二氧化矽氣凝膠具有較大的孔洞體積，也因此其孔隙率更大，熱傳導係數更是低達0.03W/m-K。
在金屬氧化物-二氧化矽的複合氣凝膠研究上，我們製備出兩種不同種類的複合氣凝膠，其中包含了二氧化鈦-二氧化矽複合氣凝膠、二氧化錫-二氧化矽複合氣凝膠。二氧化鈦-二氧化矽複合氣凝膠的製備上採用粉體直接添加進行溶膠-凝膠反應，配合開發之多次改質常壓製程技術製備而得，並針對添加後的複合氣凝膠進行物理性質分析，在本研究上，我們成功的製備出常溫熱傳導係數0.04W/m-K的複合氣凝膠塊材，改善了過去文獻上，以粉體為主的製程，並具有些微的疏水性，經過化學的分析，二氧化鈦粉體是以物理鑲嵌的方式存在於氣凝膠中，與二氧化矽無化學上的鍵結。
二氧化錫-二氧化矽複合氣凝膠研究上，主要是針對複合氣凝膠在PL發光的特性上做探討，進而計算量子產率與檢測光觸媒效應做比較。採用兩種不同的製備方式，在溶膠-凝膠法的製備方式下，在多步驟的改質程序中會損耗大量的前驅物，以添加最多莫耳數(0.1mole)的二氧化錫前驅物所製備的複合氣凝膠，其量子產率最佳。熱處理溫度上升，會因為晶體成長較佳，結晶性較好，而獲得較強的PL發光與較大的量子產率，其中以高溫700oC下所製備的複合氣凝膠，其中二氧化錫所貢獻的量子產率為0.0086為最佳，且在光觸媒的檢測上比純二氧化矽氣凝膠具有12%的轉化率提升。利用液相沉積法所製備之二氧化錫-二氧化矽複合氣凝膠在PL發光上的表現比以溶膠-凝膠法製備的複合氣凝膠強許多，因為其塞填的量遠大於溶膠-凝膠法。在熱處理溫度的影響上，因為結晶性較佳與晶粒較小而接觸面積大，我們發現熱處理溫度500oC下具有較強的PL發光強度，二氧化錫貢獻之量子產率也較大，約0.0157，對比純二氧化矽氣凝膠具有77%的光觸媒轉化率的提升。

關鍵字

氣凝膠 ； 二氧化矽 ； 二氧化鈦 ； 二氧化錫

並列摘要

Monolithic silica aerogels with thermal conductivities as low as 0.036 W/m-K and porosities as high as 97% were successfully prepared with ambient pressure drying through a multiple surface modification approach. The tetraethoxysilane (TEOS)-derived wet gel was made hydrophobic with multiple surface treatments of trimethylchlorosilane (TMCS) and dried under ambient pressure. It was found that the contact angle could reach 143o after multiple surface modifications, which was higher than the products prepared with the single surface modification procedure. This multiple surface modification procedure led to lower volume shrinkages during the ambient pressure drying and the silica aerogels thus obtained possessed higher porosities. As to the effect of the solution pH, it was found that higher pH resulted in higher pore volumes and lower thermal conductivities. When the pH was controlled at 10~11, silica aerogels with thermal conductivities as low as 0.03W/m-K were obtained.
In this research, metal oxide-SiO2 composite aerogels were also prepared and studied. Monolithic TiO2-SiO2 composite aerogels with thermal conductivities as low as 0.04W/m-K together with better mechanical strengths were successfully prepared with the multiple surface modification procedure at ambient pressure drying.
SnO2-SiO2 composite aerogels were also prepared and studied for their photoluminescence and photocatalysis performance. In the sol-gel reaction procedure, it was found that higher incorporation concentrations of SnO2 precursor produced SnO2-SiO2 composite aerogels with higher quantum yields of PL because of the more well crystallized SnO2 in the aerogels. The SnO2-SiO2 composite aerogels heat-treated at higher temperatures exhibited higher quantum yields of PL and higher photocatalytic conversions, because the crystal growth achieved in heat treatment of higher temperatures was more complete than in lower temperatures. For SnO2-SiO2 composite aerogels produced from the liquid deposition procedure, products of heat treatment at 500oC gave higher quantum yields of PL and photocatalytic efficiency than produced of 400oC, due again to the better crystallinity. For products obtained at heat treatment temperature of 700oC, the quantum yields of PL was however lower than those prepared at 500oC, because the grain size of tin oxide became larger and the specific surface area thus was lower.

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