材料 Aerogel

http://en.wikipedia.org/wiki/Aerogel

逸事

" Samuel Stephens Kistler在1931年發明氣凝膠。而這一切是因為他與Charles Learned之間的賭注,競爭看誰有辦法將凝膠裡的液體成分用氣體取代卻不使發泡的間壁收縮崩塌。最後Kistler辦到了。"


製備

"Production of aerogels is done by the sol-gel process.

The removal of the liquid from a true aerogel involves special processing. Gels where the liquid is allowed to evaporate normally are known as xerogels. As the liquid evaporates, forces caused by surface tensions of the liquid-solid interfaces are enough to destroy the fragile gel network. As a result xerogels cannot achieve the high porosities and instead peak at lower porosities and exhibit large amounts of shrinkage after drying.

In 1931, to develop the first aerogels, Kistler used a process known as supercritical drying. By increasing the temperature and pressure he forced the liquid into a supercritical fluid state where by dropping the pressure he could instantly gasify and remove the liquid inside the aerogel, avoiding damage to the delicate three-dimensional network. While this can be done with ethanol, the high temperatures and pressures lead to dangerous processing conditions. A safer, lower temperature and pressure method involves a solvent exchange. This is typically done by exchanging the ethanol for liquid acetone, allowing a better miscibility gradient, and then onto liquid carbon dioxide and then bringing the carbon dioxide above its critical point. "

由水玻璃經常壓乾燥製造氣凝膠之製程
" 本製程係由製程由水玻璃經超臨界乾燥製造氣凝膠之製程演變而來,僅乾燥的部分由超臨界乾燥改為常壓乾燥,但要進行常壓乾燥之前,濕凝膠必須先經改質,亦即須將濕凝膠分子中親水性的氫氧基改質至疏水性的矽烷基,常用的改質劑為Trimethylchlorosi-lane (TMCS),其反應式如下:

TMCS + HO-Si≡ → (CH3)3Si-O-Si≡ + HCl 

經改質後的濕凝膠便可進行常壓乾燥 (Ko, 1998),簡化的常壓乾燥流程圖顯示於圖三,改質後濕凝膠在沸騰的水中將所含溶劑排出,然後浮於水面而得氣凝膠。"

性質

" 其密度大約為3mg/cm3僅僅為空氣的三倍重。(由Larry Hrubesh領導的LLNL實驗室首先製備出世界上密度最小的CO2氣凝膠)

儘管氣凝膠裡有個膠字,但它其實是堅硬而乾燥的物質,就其物理性質與膠體一點也不類似。被稱為膠是由於它的製造過程提取於凝膠。提起指尖輕輕在凝膠表面按壓一下並不會留下痕跡;如果以加重的力道按壓會造成永久的凹陷;而加上足夠的力量會讓它如玻璃般破碎散落成塊。這個便是我們所知道的易脆性。

雖然它具有破碎的傾向,但是以結構來說它仍是非常堅固的。它優秀的負載能力是源於其分子的組成:大約尺寸為2到5奈米的球形聚合粒子互相結合成一個個小單元,進而組合成樹突狀的微觀立體結構。這些小單元會形成碎形鏈狀的三維結構,其間充塞著大量的孔洞,每個孔洞尺寸不大於100奈米。而這些孔洞的聚集密度以及個別的平均大小能夠經由製造過程加以控制。"

" 因為吸濕的自然性質,凝膠摸起來乾乾的,並表現出強大的吸水能力。為此長時間拿著凝膠的人要穿著手套,以預防皮膚表面出現乾燥脆化的現象。

因為瑞立散射的影響,凝膠裡的微小樹突結構會過濾可見光光譜中波長較短小的部份。而這就產生了它輕柔的顏色:在較暗的環境散發柔柔的藍光,在明亮的環境散發微微的黃光。

氣凝膠本身的性質是具親水性的,但是藉由化學製程可將它改變為具疏水性。"

廠商

母廠 (都有建材應用產品!)
http://www.cabot-corp.com/
http://www.aerogel.com/

代理商
http://www.alltech1995.com.tw/index-Chinese.html

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