Page 12 - Science Focus (issue20)

Overview network collapses due to capillary action caused by 簡介
the attractive forces between the liquid molecules. What is a supercritical fluid?
In 1931, an American chemical engineer, Dr. Imagine when solvent molecules constantly vaporize 在 1931 年， 美 國 化 學 工 程 師 Samuel​Stephens​
You are probably familiar with the concepts of boiling
Samuel Stephens Kistler bet his colleague, Dr. Charles from the gel, the intermolecular forces between point and melting point (at a given pressure), and the three Kistler 博士與同事 Charles​Learned 博士打賭，說他們可
Learned, that they could replace the liquid inside the remaining liquid molecules keep pulling the states of matter. However, a substance can also exist as 以把啫喱內的液體換成氣體而不會使它塌陷。他們把這個
a jelly with gas without shrinking it. Dubbing this molecules together to fill the vacancy produced “supercritical fluid” when subjected to high temperature 概念稱為「氣凝膠」，並最終使用二氧化矽造出史上第一塊
concept “aerogel”, they managed to make the first in order to maintain the density of the gel. This also and pressure above its “critical point”, beyond which the
aerogel with silica [1]. Afterwards, other materials, induces an inward stress on the delicate network of molecules are completely vaporized but compressed to the 氣凝膠​[1]。此後，其他材料，譬如氧化鋁、纖維素、卵白蛋
extent that is as dense as those in the liquid. A supercritical
such as alumina, cellulose, egg albumin, rubber and the gel, causing the network to collapse and shrink. fluid therefore shows the properties of both states, and the 白、橡膠和瓊脂都被用於製作氣凝膠​[1,​2]。你知道氣凝膠
agar, were also used to produce aerogels [1, 2]. Do clear distinction, including the visual boundary, between 是怎樣製造的嗎？讓我們探索一下氣凝膠的科學和它一系
you know how aerogel is made? Let’s discover more In Dr. Kistler’s paper, he proposed a method the liquid and gas phases disappears. For methanol (an 列的應用，這全都要歸功於它的密度和隔熱特性。
about the science of aerogel and its wide range called “supercritical drying”, which consists of two alcohol), the critical pressure and temperature are 8.1035 x
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of applications thanks to its density and thermal steps [2]. First, a water-based hydrogel can be 10 Pa (around 80 atm, i.e. 80 times atmospheric pressure)
o
and 512.6 K (239.45 C) respectively [6].
insulating properties. 如何製造氣凝膠
submerged in alcohol to substitute the water in the
jelly with alcohol via diffusion. Then, the hydrogel 要明白如何製造氣凝膠，我們首先要了解果凍的結構​
How to Make Aerogel is subjected to a high temperature and pressure The Exciting Properties and Applications —​對，是香甜有彈性，大家都喜愛的果凍啊！想像你正在廚
beyond its critical point in a machine called 房準備果凍，你需要用上三種材料：果凍粉、水和糖。我們可
In order to understand how aerogel is produced, autoclave. The alcohol inside the now alcohol- Hooray – we have discovered how to produce 以暫且忽略糖，因為它只為果凍提供甜味。製作出來的果凍
we first need to understand the structure of jelly based hydrogel is induced to reach its “supercritical aerogel, now what? Let’s look at some of the most 含有 95%​水份和少量的多孔固體結構，這樣的結構被定義
– yes, the chewy, sweet jelly that everyone loves. fluid” phase, in which the adhesive forces between interesting properties, namely the density and
Imagine cooking jelly in your kitchen, there are three the fast moving molecules become insignificant thermal insulation. In fact, aerogel can even be 為水凝膠（hydrogel）。要製作氣凝膠，我們會希望把水凝
main ingredients you need: jelly powder, water and due to the overwhelming kinetic energy gained. 膠當中的水轉換成氣體。
sugar. In this case, sugar can be taken out of the If we hold the temperature above the critical made with up to around 99.9% air [3], which makes
equation because it provides only the sweet taste. temperature but depressurize the autoclave at this the aerogel very light, while allowing it to maintain 要把水凝膠中的水換走這個步驟聽起來很簡單，但其實
The resulting jelly, which is 95% water with a small stage, all the fluid inside the hydrogel becomes gas a sturdy structure. Naturally, the lightest known 非常複雜。如果你僅是把凝膠加熱令當中的液體汽化，液體
amount of porous solid support, is defined as one of solid is also an aerogel, called graphene aerogel, 分子之間的吸引力會令凝膠的固體網狀結構因毛細管作用
the hydrogel structures. To make aerogel, you want (Figure 1). By converting the liquid alcohol into gas or aerographene, which is 7.5 times lighter than 塌陷，令整塊凝膠收縮。試想像溶劑分子不斷從凝膠汽化，
to replace the water in hydrogel with air. via this intermediate supercritical fluid phase, the air in vacuum [3]. Another interesting property is 剩下的液體分子藉著分子間的吸引力不停把餘下的分子拉
solid jelly structure is preserved due to the absence its excellent thermal insulation ability. Air itself is a
Swapping water for air in hydrogel sounds simple, of the capillary action brought by intermolecular poor thermal conductor, but it can still transfer heat 在一起，以彌補分子流失所產生的空位以維持凝膠的密度。
but it’s actually very complex. If you simply vaporize attractions. Voilà – you have produced aerogel from by convection. However, in aerogel, the narrow 這會在纖巧的凝膠骨架產生一股向內收縮的壓力，令網狀
the liquid by heating the jelly, it shrinks as its solid hydrogel. space in the nanostructure hinders air molecules 結構塌陷並收縮。
from moving freely and effectively, and hence
prevents convection of the air trapped inside [4], 在 Kistler 博士的論文中，他提出了一種名為「超臨界
which enables aerogel to be an even better thermal 乾燥（supercritical​drying）」的方法，過程涉及兩個步
驟​[2]。首先，把水凝膠浸進酒精裡，凝膠中的水可以通過
氣凝膠 — is the use by NASA in some of their space missions [5]. 擴散作用被換成酒精。然後，把水凝膠放進一部名為高壓釜
insulator than air.
One of the most popular applications of aerogel
（autoclave）的機器裡，並把溫度和壓力提高至其臨界點

體（supercritical​fluid）」的狀態，當中獲得極大動能的分
由空氣構成的物質及其特性 In the Stardust mission, aerogel was used to collect （critical​point）以上，令水凝膠中的酒精處於「超臨界流
samples of interstellar dust because the rapidly
子會高速移動，分子間的吸引力因而變得微不足道。如果我
moving dust particles can decelerate gradually 們在這時把溫度維持在臨界溫度以上，但把高壓釜減壓，水
and embed itself in the sponge-like porous structure 凝膠內的所有流體都會變成氣體（圖一）。透過超臨界流體
Aerogel – of aerogel. In addition, in the Mars Rover mission, 這個相把液體變成氣體，固體的凝膠結構能得以保存，因為
aerogel is used as a layer of insulation to protect
An Air-Based Material and electronic circuits inside the Mars rover against the 我們避免了分子間吸引力所帶來的毛細管作用。瞧，你已經
成功從水凝膠製造出氣凝膠了。
huge diurnal temperature variation on Mars.
Its Exciting Properties From the story of aerogel, we can see that a 有趣的特性和應用

groundbreaking invention may just stem from a
simple and fun idea. Who knows – perhaps your 好耶！我們知道怎樣製造氣凝膠了，然後呢？讓我們認
idea can make a breakthrough in science and 識一下其有趣的特性，分別是密度和隔熱能力。事實上，氣
By Randy Stefan Tanuwijaya
technology one day! 凝膠最多可以含有高達 99.9%​的空氣​[3]，這使氣凝膠擁

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