奈米碳/高分子複合材料熱性質與電磁波屏蔽效能之研究

INVESTIGATIONS OF POLYMER COMPOSITES REINFORCED WITH NANO-CARBON MATERIALS ON THERMAL PROPERTIES AND ELECTROMAGNETIC INTERFERENCE SHIELDING EFFICIENCY

陳盈儒(Yin-Ju Chen)

指導教授 ： 戴念華 ； HASH(0x933fee0) ； 葉銘泉

國立清華大學/工學院/材料科學工程學系/博士(2013年)

摘要

本研究探討奈米碳/高分子複合材料之熱性質與電磁波屏蔽效能，研究中成功製備數種奈米碳材料做為補強材料，如單壁奈米碳管、多壁奈米碳管、奈米石墨薄片、石墨烯與鍍金屬石墨烯等，並將其分別加入至白蠟與聚苯胺二種高分子基材中。
本論文一主題旨在探討添加奈米石墨薄片於白蠟中之熱傳導與熱性質分析，並比較奈米石墨薄片排列方向(有序排列及無序排列)之影響。研究結果顯示僅添加1.0 wt%的奈米石墨薄片即可在白蠟中形成一導通網絡，有助於此複合材料之熱傳導行為。本文中利用Maxwell-Euken與rule of mixtures 方程式預測無序與有序奈米石墨薄片排列之理論熱傳導值。實驗中證實此複合材料之熱傳導值與奈米石墨薄片之添加量呈正比關係，其有序與無序排列奈米石墨薄片添加量為5.0 wt%之熱傳導值分別為4.47±0.15 與1.68±0.07 W/m K，遠高於純白蠟之熱傳導值(0.33±0.04 W/mK)，兩組複合材料之熱傳導實驗值皆十分相近理論值。熱性質方面，複合材料之熔點與固、液相變化溫度分別近似於53與60 0C，此二溫度並不會因為添加不同含量的奈米石墨薄片而有明顯的變化。此複合材料之潛熱則與添加材料之含量呈反比關係，說明此複合材料在相變化時所需之能量較低，成為具相變化功能之高導熱複合材料。
本論文另一主題為探討聚苯胺複合材料於電磁波屏蔽效能之應用。本研究係分別添加不同比例(0.1、0.5、1.0 wt %)之單壁奈米碳管、奈米石墨薄片與奈米碳管/奈米石墨薄片三種補強材料於導電高分子聚苯胺製備複合材料，探討此類複合材料之微結構、表面形貌、導電值以及電磁波屏蔽效能。結果方面，添加1.0 wt %的奈米碳管/奈米石墨/聚苯胺複合材料之屏蔽效能可達27.0 dB，高於純聚苯胺的11.3 dB，導電值亦增加了五個數量值，是為本實驗之最高值。單比較奈米碳管與奈米石墨薄片之結果，由於兩種補強材料維度之差異，而後者具有較大的表面積更易於在基地材料中形成導電網絡，故添加奈米石墨薄片之導電值與屏蔽效能較佳。而本章中亦針對電磁波屏蔽之吸收與反射效果進行討論，証實吸收為此複合材料之主要機制。
為更進一步探討聚苯胺複合材料之電磁波屏蔽效能，本章分別以石墨烯與鍍金屬石墨烯(銀與鎳)做為補強材料，亦探討其微結構、表面形貌與導電性等性質，使用之補強材料添加量分別為0.5、1.0、3.0與5.0 wt%。實驗結果顯示，添加5.0 wt%的鍍銀石墨烯於聚苯胺時具有最佳之導電度與屏蔽效能，分別為20.32 S/cm與29.33 dB。經表面形貌分析後得知，所添加之補強材料能夠均勻地分散在聚苯胺基地中形成導電網絡，而覆鍍於石墨烯的銀與鎳顆粒亦擔任良好的導電媒介。分析結果顯示吸收仍是此聚苯胺複合材料之主要屏蔽機制，其主因為較高的介電常數所致。經比較後，認為鍍銀石墨烯/聚苯胺複合材料所具有之屏蔽效果幾乎符合業界標準，此類複合材料日後極具發展潛力。
為發展一低成本、製程簡要與使用方便之電磁波屏蔽材料，本章研究中使用一簡要之浸鍍方法製備具電磁波屏蔽效果之多孔複合材料。研究中使用市售海綿做為研究對象，藉由浸鍍製程將奈米碳材料(石墨烯、多壁奈米碳管、石墨烯/多壁奈米碳管)吸附於海綿纖維上形成導電網絡，並比較於吸附奈米碳材前有無鍍覆銀奈米顆粒對電磁波屏蔽效能之影響。研究結果顯示，未先鍍覆銀顆粒之電磁波屏蔽效能大約在14 dB左右，其值不因所吸附碳材料之不同而有明顯改變，但屏蔽值在先行鍍覆銀顆粒後卻有很明顯的提升，研究中之最高值為24.33 dB。經分析後，可知此海綿複合材料之電磁波屏蔽機制係以反射為主，推測係由海綿本身的多孔結構所致。

關鍵字

高分子複合材料 ； 奈米碳管 ； 奈米石墨薄片 ； 石墨烯 ； 熱傳導 ； 電磁波屏蔽效能

並列摘要

In this dissertation, the investigations on the thermal properties and electromagnetic interference shielding efficiency (EMI SE) of nanoscale carbon materials/polymer composites have been studied. The fillers such single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), graphite nanosheets (GNS), graphene (GP), and the graphene decorated with Ag (Ag@GP) and Ni (Ni@GP) were synthesized successfully, and incorporated with various polymers including paraffin and polyaniline (PAni), respectively.
Two paraffin composites filled separately with randomly distributed graphite nanosheets (R-GNS) and oriented graphite nanosheets (O-GNS) were fabricated, and their thermal properties and structural characteristics were investigated. The experimental results show that a conductive network at 1.0 wt% GNS loading was found. The thermal conductivities of the R-GNS/paraffin and the O-GNS/paraffin composites are 4.47±0.15 and 1.68±0.07 W/m K, respectively, when 5.0 wt% GNS were introduced. The Maxwell-Euken model and the modified rule of mixtures model were proposed to predict the thermal conductivities of the R-GNS/paraffin and the O-GNS/paraffin composites, respectively. The melting point and the solid-liquid phase transition temperature of the R-GNS/paraffin composites are approximately 53 and 60 0C, respectively, and neither of these values was significantly affected by the presence of GNS. Decreases in the latent heat of the R-GNs/paraffin composites with increased GNS loading were also found.
This work demonstrates the fabrications and characterizations of PAni composites containing SWCNTs, GNS, or hybrid fillers SWCNTs/GNS. The characterization of microstructure, examination of fracture surface morphologies, and measurement of electric conductivity and EMI SE were performed. It was found that both the electric conductivity and the EMI SE increase with filler loading, and the nanocomposites filled with 1.0 wt% SWCNTs/GNS possessed the highest electric conductivity of 16.2 S/cm and total EMI SE of 27.0 dB. The experimental results also show that absorption is the primary mechanism of EMI SE for all of the loadings and fillers.
To develop novel EMI shielding materials, PAni composites filled with GP, GP decorated with silver nanoparticles (Ag@GP), and GP decorated with nickel nanoparticles (Ni@GP) were prepared, and the microstructures, morphologies, electrical conductivities, and EMI SE of the composites with different filler loadings (0.5, 1.0, 3.0, and 5.0 wt%) were investigated. The PAni composite containing 5.0 wt% Ag@GP showed the best electrical conductivity of 20.32 S/cm and highest EMI SE of 29.33 dB. The uniform dispersion of fillers significantly enhanced the formation of conductive pathways in the PAni matrix, and the presence of metal nanoparticles on the GP surface and between the GP layers also increased the electrical conductivity. The results of this study show that absorption is the primary factor governing EMI shielding, which is attributed to the high permittivity of the composites. This study reveals that the Ag@GP/PAni composite is promising for applications as an EMI shielding material.
Porous composites fabricated through a simple dip-coating method demonstrated excellent performance in EMI shielding. A commercial sponge was coated with silver nanoparticles before being dip-coated with GP, MWCNTs, or hybrid GP/MWCNTs to form Ag/carbon nanomaterial hybrid composites. Herein, we found an insignificant difference in EMI SE among the porous composites without the Ag nanoparticle coating, with values of approximately 14.4 dB. Interestingly, the hybrid composites with the Ag nanoparticle coating exhibited excellent EMI shielding (24.33 dB). The EMI SE measurements showed that reflection dominates the EMI SE for all the sponge composites studied in this work due to their porous structure.

並列關鍵字

polymer composites ； carbon nanotubes (CNTs) ； graphite nanosheets (GNSs) ； graphene (GP) ； thermal conductivity ； electromagnetic interference shielding efficiency (EMI SE)

參考文獻

[75] 洪志緯，”抗EMI無電鍍銅聚丙烯腈高分子奈米複合材料之製備與研究”，大同大學材料工程研究所碩士論文，p. 36，2005。

[69] 凌國銓，”奈米碳管/環氧樹脂複合材料之電磁屏蔽與機電性質研究，國立清華大學碩動力機械工程學系碩士論文”， p. 56，2007。

[72] 邱國斌、蔡定平，”金屬表面電漿簡介”，物理雙月刊，28，pp. 472-485，2006。

[100] Kaye, George William Clarkson, “Mechanical properties of materials”, Kaye and laby tables of physical and chemical constants, National physical laboratory (http://www.kayelaby.npl.co.uk/).

[4] S. Iijima,“Helical microtubules of graphitic carbon”, Nature, 354, pp. 56-58, 1991.