Page 15 - Science Focus (issue20)

有穩妥的結構之餘亦非常輕巧。理所當然地，世界上已知最 膠被用於收集星際塵埃，因為高速移動的塵埃分子能在氣
輕的固體亦是一種氣凝膠，名為石墨烯氣凝膠（graphene​ 凝膠像海棉般的多孔結構中逐漸減速並嵌入其中。此外，在
aerogel）或空氣石墨（aerographene），它在真空下比空 火星探索任務中，氣凝膠被用作保護火星探測器中的電子
氣輕 7.5 倍​[3]。另一個有趣的特性是其絕佳的隔熱能力。 電路，確保其能在火星極大的日夜溫差下正常運作。 The Serendipitous
空氣本身是一個差勁的熱導體，但它仍然可以透過對流傳 TM
熱。然而，氣凝膠中狹窄的奈米結構能阻礙空氣分子自由和 從氣凝膠的故事，我們可以了解到一個突破性的發明最 Invention of Teflon :
有效地流動，令裡面的空氣不能透過對流傳熱 [4]，使氣凝 初可能只來自一個簡單而有趣的想法。誰知道你的想法有
膠成為甚至比空氣更佳的熱絕緣體。 一天可能亦會帶來科技上的突破呢？ An Appreciation of Unexpected Results

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氣凝膠其中一個最受大眾歡迎的應用，大概是美國太空 欣賞不一樣的實驗結果：鐵氟龍 的意外發明
總署在多次太空任務上的應用了 [5]。在星塵任務中，氣凝 References 參考資料：
[1] Gurav JL, Jung IK, Park HH, Kang ES, Nadargi DY. Silica By Henry Lau 劉以軒
Aerogel: Synthesis and Applications. J Nanomater.
2010;(2010):1-11.
[2] Kistler SS. Coherent Expanded Aerogels and Life rarely goes according to plan. The same can novel compounds. He produced the gas and stored
甚麼是超臨界流體？ Jellies. Nature. 1931;127(3211):741. doi:10.1038/127741a0 be said for science. As scientists have painfully learned them in small containers. One day, while retrieving the
[3] Sun H, Xu Z, Gao C. Multifunctional, Ultra-Flyweight, (and will continue to learn), more often than not, it gas, one container released less gas than it previously
你應該對（在某一壓力下的）沸點和熔點，以及物質三 Synergistically Assembled Carbon Aerogels. Adv. Mater.
態的概念非常熟識。可是，當物質處於其「臨界點」以上的 2013;25(18):2554-2560. takes a few tries to get your expected experimental contained while the same container weighed as much
高溫和高壓下，它也可以以「超臨界流體」的狀態存在，當中 [4] Veritasium. World's Lightest Solid! YouTube. results. But sometimes, unexpected results turn out to as it did before. Puzzled by this mystery, Plunkett sawed
Updated June 1, 2019. https://www.youtube.com/
的分子完全氣化，但被壓縮至像液體分子一樣高的密度。因 watch?v=AeJ9q45PfD0. be the best results. Such is the case of one Dr. Roy J. open the container and found a white powdery
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此，超臨界流體能同時展示以上兩個物態的特性，液相和氣 [5] NASA. Aerogel – Catching Comet Dust. STARDUST – Plunkett, the inventor of Teflon which you may know substance, which explained the missing gas. After
NASA’s Comet Sample Return Mission. Updated March to be a key component of non-stick frying pans. This is conducting some chemical tests, Plunkett confirmed
相之間的區別變得模糊，包括視覺上的分界。酒精甲醇的臨 31, 2005. https://stardust.jpl.nasa.gov/tech/aerogel.html.
界壓力和溫度分別為8.1035 x 10 Pa（大約80 atm，即是 [6] Vesovic V. METHANOL. Thermopedia. Updated February the story of how an experimental blunder turned into that the substance is in fact polytetrafluoroethylene
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大氣壓力的80倍）和512.6 K（239.45 C）[6]。 14, 2011. http://www.thermopedia.com/content/952/. a scientific wonder. Stick around to find out more! created from the polymerization of the “missing gas”.
o
doi:10.1615/atoz.m.methanol
You may have hea rd of Teflon TM befo re. Upon further examination, Plunkett discovered the
Scientifically called polytetrafluoroethylene (PTFE), it is material to be highly resistant to heat and corrosive
a polymer made up of identical repeating “monomer” chemicals. The most unique property was that
units joined together. The monomer in question is polytetrafluoroethylene was very slippery. Marketed as
called tetrafluoroethylene. While that sounds like a Teflon since 1945, a French engineer, Marc Grégoire,
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mouthful, it’s actually a simple structure: “-ethylene” attempted to coat his aluminum pan with the material
(IUPAC name: “-ethene”) implies that the monomer at his wife’s suggestion and created the first non-
has a backbone of two carbons, joined together by stick pan in 1954 [2]. To this day, non-stick pans are
a carbon-carbon double bond. The breaking of one still made by coating the metal cooking surfaces with
such bond allows monomeric tetrafluoroethylene to Teflon .
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join with one another, forming polymers. “Tetrafluoro-”
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refers to the four fluorine atoms attached to the two The slipperiness of Teflon is made possible
carbon atoms, substituting the hydrogen atoms in thanks to the high electronegativity of fluorine along
regular ethylene. the polymer chain [3]. For a
substance to attach to
a n oth e r s u bs ta n ce,
s o m e k i n d o f
molecular interaction

Now that we’ve covered the chemical nature of
Figure 1. Phase diagram of methanol and the phase change in supercritical drying (see the red arrow). Teflon , we can delve into the accidental invention
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圖一 甲醇的相圖和超臨界乾燥時的物態改變（見紅色箭頭）
of Teflon [1]. In 1938, Roy J. Plunkett, a recent PhD
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* The graph is not necessary drawn to scale.
* 圖未必依比例畫。 graduate, was working on creating new forms of non-
toxic refrigerants. In a study, a considerable amount
of tetrafluoroethylene gas was needed to synthesize
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