奈米碳管複合材:熱電與電磁波屏蔽之應用

Carbon nanotubes based composites: Applications of thermoelectric power and electromagnetic shielding

賴耀成(Lai, Yao Cheng)

指導教授 ： 徐文光

國立清華大學/工學院/材料科學工程學系/博士(2015年)

https://doi.org/10.6843/NTHU.2015.00594

摘要

奈米碳管具有高穩定性、高比表面積與高導電性等特性，相當適合作為導電複合材之填充物。本文旨在奈米碳管複合材應用之研究，其內容分為多壁奈米碳管/二氧化鈦之熱電性質與多壁奈米碳管/乙基纖維素多孔複合材之電磁波屏蔽。二氧化鈦之導電率藉由多壁奈米碳管提升電子自由路徑而提升，並且碳摻雜之二氧化鈦的電子有效質量上升導致Seebeck係數提升，最後位於晶界之奈米碳管作為聲子散射中心，降低熱導率。因此，0.1wt%多壁奈米碳管ZT於室溫下提升了7380%。冷凍乾燥法製備之高重量百分比(50wt%)多壁奈米碳管/乙基纖維素多孔複合材有別於一般高導電率塊材對於電磁波呈現低反射率與高吸收率之特性。孔洞結構對此現象有三個貢獻。第一，電磁波可進入材料內部，降低反射率。第二，增加材料內部的散射，增加行進路徑。最後，提升比表面積，增加界面極化。其中，最後兩者提升吸收率。

關鍵字

奈米碳管 ； 熱電 ； 電磁波屏蔽 ； 多孔複合材 ； 二氧化鈦

並列摘要

Carbon nanotubes (CNTs) which have high stability, specific surface area, and conductivity are suitable for conductive composite. This research discuss the application of carbon nanotubes composite as thermoelectric (multi-walled CNTs (MWCNTs)/TiO2) and electromagnetic shielding (porous composite of MWCNTs/ ethylcellulose) materials. Seebeck coefficient, according to Ioffe’s approximation, is inversely proportional to carrier density and decreases with doping. Here we find that incorporation of multi-walled carbon nanotubes into rutile TiO2 improves electrical conductivity and Seebeck coefficient at low filling fraction of tubes; the former is owing to lengthening of mean free path and doping modified effective mass for the latter. Tube-oxide mixing also causes significant phonon scattering at interfaces and reduced thermal conductivity is verified by promoted figure of merit. In contrast to conductive bulk materials, ethylcellulose porous composite with high MWCNTs weight percentage, which is synthesized by freeze-drying, exhibits low reflectance and high absorbance. The contribution of porous structure can be sorted into three mechanisms. First, electromagnetic wave is able to penetrate into porous structure. It decreases the reflectance. Second, the increase of diffuse reflection expands the transport path. Third, the increment of specific surface area enlarges the interfacial polarizations. The latter two are contributed to the enhancement of absorbance.

並列關鍵字

Carbon nanotubes ； Thermoelectricity ； Electromagnetic shielding ； Porous composite ； Titanium dioxide

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