用於日間輻射冷卻應用的軟質材料工程

Engineering of Soft materials for Daytime Radiative Cooling Applications

施薇雅(Selva Brindha Sermaraj)

指導教授 ： 陳學禮

共同指導教授 ： 萬德輝(Derhui Wan)

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202103013

摘要

在許多工程領域的應用中，過熱通常是重要問題，包括建築、工業應用、電子設備。輻射熱調節技術可以呈現以特定波長的熱量形式放射熱輻射來冷卻系統的一種能力。本論文提出了一種軟、薄、高透明的聚丙烯醯胺水凝膠，用於結合蒸發和輻射冷卻的應用。理想的輻射冷卻材料主要是在8微米至13微米的大氣窗口區域具有高吸收和高放射率。本研究的聚丙烯醯胺水凝膠可以在8微米至13微米的波長區域放射中紅外輻射，並產生冷卻效果。我們證明了通過滴落塗佈法實現膨脹厚度控制在100微米以下的水凝膠。其展現了93%的吸收在中紅外光區域，證明了的水凝膠塗料具有很強的冷卻效果。與其他輻射冷卻材料相比，它結合輻射冷卻與蒸發降溫應用，以提高散熱效率和低成本。
在第二個主題中，我們研究了聚丙烯醯胺水凝膠的水收集技術，通過在水凝膠中加入潮解鹽類；鹽類沉澱在水凝膠的多孔基質中，有助於在環境溫度下收集水分。眾所周知，水具有好的輻射冷卻特性，此外，在散熱過程中，水凝膠開始蒸發，這可能會導致水凝膠變乾，但其富含吸濕性鹽的水凝膠可以長期保持水分，並且可以在正常室溫下重新收集水分。這種行為將有助於水凝膠進行水合，並在8微米至13微米的中紅外光區域放出熱輻射。
在第三個主題中，討論了通過應變來拉長和排列聚合物鏈的異向性行為來改善聚合物薄膜的導熱性。尼龍66聚合物和聚四氟乙烯（PTFE）膠帶被用於這個實驗展示。結果顯示，尼龍66薄膜在伸長率為90％下的導熱性增加。這表現出每公分的加熱速率為0.0352 ˚C/s。伸長率為700%的PTFE聚合物薄膜，其導熱性表現為每公分的加熱速率增為0.18688 ˚C/s。本研究也展現了具有不同伸長率的PTFE和尼龍66薄膜樣品的紅外光譜。從結果來看，我們提出了通過增加伸長率來提高聚合物薄膜的導熱性。聚合物薄膜沿鏈方向的伸長也將有助於將輻射冷卻材料內部侷限的熱量散發出去，產生連續有效的冷卻效果

關鍵字

水凝膠 ； 輻射冷卻 ； 大氣光窗 ； 潮解鹽 ； 熱導率 ； 尼龍66 ； 聚四氟乙烯

並列摘要

In many engineering fields overheating is the major problem in different applications that include commercial buildings, industrial applications, electrical and electronic devices. There are even different varieties of radiative cooling materials owing to the process in nanophotonic and metamaterials for radiative cooling applications. Inventive thermal regulation technologies can present a terrific capacity for emitting thermal radiation in the form of heat at a specific wavelength to cool down the system. Radiative cooling is an innovative thermal regulation technique for cooling the system by emitting thermal radiation. Here we propose a soft thin highly transparent polyacrylamide hydrogel for combined evaporative and radiative cooling applications. It is commonly believed that the ideal radiative cooling materials should possess extremely high reflectivity in near-infrared (NIR) wavelengths mainly in 0.3 μm -2.5 μm, high infrared absorption, and high emissivity in the atmospheric window region of 8 μm -13 μm. Our polyacrylamide hydrogel can emit the mid-infrared radiation (MIR) in the wavelengths region of 8 μm –13 µm and will produce a strong cooling effect. We demonstrated that the hydrogel with controlled swelling thickness below 100 μm was achieved by the drop-casting method. Here we showed 93% of absorption in the mid-infrared region results demonstrating that the fabricated hydrogel paint has a strong cooling effect and helps to improve energy efficiency. It assists in many radiative cooling applications to improve the system efficiency and low cost as compared with other radiative cooling materials.
In the second topic, we studied the moisture harvesting technique of polyacrylamide hydrogel by enriching the hydrogel with hygroscopic salts; the salt precipitated inside the hydrogel porous matrix and it helped to collect the moisture at the ambient temperature. As it is known, the water has better radiative cooling characteristics, moreover, during the heating process the hydrogel starts to evaporate and it may cause the hydrogel to get dried, but our hygroscopic salt enriched hydrogel can hold the water for a long time, and it can recollect the water from the normal ambient temperature. This optimistic behaviour will help the hydrogel to get hydrated and emit the thermal radiation in the mid-infrared region of 8 μm -13 μm and it will deliver a strong cooling effect during the whole process.
In our third topic, we discussed the thermal conductivity improvement in the polymer thin film by applying the strain to elongate and align the polymer chain in an anisotropic behaviour. The nylon 66 polymer and the polytetrafluoroethylene (PTFE) tape were employed for this experiment demonstration. The result showed the increase in thermal conductivity of nylon 66 thin film with an elongation percentage of 90 %. This exhibits the heating rate per centimetre value of 0.0352 ˚C/sec. The thermal conductivity of PTFE polymer thin film with an elongation percentage of 700% exhibits the heating rate per centimetre value of 0.18688 ˚C/sec. We also characterized the infrared spectrum of the PTFE and nylon 66 thin film samples with different elongation percentages. From the results, we presented the increase in the thermal conductivity of polymer thin film by increasing the elongation percentage. Elongation of the polymer thin film along the chain direction will also aid in emitting the heat suppressed inside the radiative cooling materials to produce a continuous cooling effect.

並列關鍵字

hydrogel ； radiative cooling ； atmospheric window ； hygroscopic salts ； thermal conductivity ； nylon 66, ； polytetrafluoroethylene (PTFE)

參考文獻

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