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  • 學位論文

碳基複合材料應用於X頻段之電磁波屏蔽效能分析

Investigations on the electromagnetic interference shielding effectiveness of carbon-based composites in X-band

指導教授 : 戴念華

摘要


電磁污染是現今急需解決的重要議題之一,因為它不僅會干擾電子設備的運作,更會對人體健康造成影響,可能導致如白血病、腦腫瘤等嚴重的疾病。對此而言,碳及其複合材料在電磁波屏蔽領域引起各界廣大關注,其具有優良的電性、可撓性、低成本、環境友善、容易製造及化學惰性等優點。然而,高導電度容易造成屏蔽機制以反射為主,這可透過調整奈米材料的微結構、屏蔽物結構設計或添加外來物質來調整,如使用具有介電性質或磁偶極子之材料來增強電磁波吸收。本研究中,以碳奈米材料作為屏蔽X頻段電磁波的主要材料,並深入探討微結構、介電材料和磁性材料於電磁波屏蔽中扮演之角色與機制。 首先,本研究將討論介電材料和磁性材料於電磁波屏蔽中的作用。我們開發出一種輕量化的複合墊,其孔隙率約為40%,由非晶碳、氧化鋅奈米棒和鐵鋅鎳氧體(Zinc oxide nanorods-nickel zinc ferrite, ZNF)組成,其於X頻段的電磁波屏蔽主要由吸收所貢獻。由振動樣品磁強計的測量可證實,複合材料的飽和磁化量會隨著ZNF粉末的重量百分比增加,而使電磁波的磁損耗增加。當樣品厚度為1.0 mm時,非晶碳複合材料測得之電磁波屏蔽效能為25.70 dB,而當樣品摻入ZNF粉末後,電磁波屏蔽效能可進一步提高至53 dB,效能的提升主要來自複合材料較佳的磁性、界面極化和介電性質之貢獻。適當的材料搭配,可使其具有高電磁波吸收率,而本研究研發之複合材料可屏蔽99.999%的電磁波,其反射率為8.394%、吸收率為91.605%。此外,可透過控制複合材料中ZNF粉末的量,來調整複合材料的電磁性質及電磁波屏蔽性質。 為了探討孔洞與添加介電材料及磁性材料對電磁屏蔽所造成的影響,我們進一步製備具有三維連通孔洞的石墨烯氣凝膠,並複合鈷鐵氧體(Cobalt ferrite nanoparticles, CFO)奈米粒子和氧化鋅奈米棒,用於吸收機制主導的電磁波屏蔽材料。由於其低密度、高孔隙率以及高表面積的特性,石墨烯氣凝膠在5 mm的厚度下,總屏蔽效能可達25.07 dB,而在摻雜CFO奈米粒子後,可進一步提升至42.10 dB。複合磁性鐵鈷氧體不僅能夠提升氣凝膠的總屏蔽效能,且能夠將電磁波吸收率從37.738%提升至87.788%。藉由複合ZnO奈米棒以及鈷鐵氧體奈米粒子於氣凝膠,總屏蔽效能可被提高至48.56 dB,同時具有93.655%的電磁波吸收率。此外,氣凝膠的高表面積和高孔隙率有利於電磁波在材料內部進行多重反射,因此能夠達成電磁波的高吸收率。 除此之外,許多團隊也針對穿戴式電磁波屏蔽材料如導電織品進行開發,以保護人類和敏感的電子設備免於電磁污染。我們進一步在棉織品上複合氧化鋅和還原氧化石墨烯(Reduced graphene oxide, rGO),用於製備穿戴式電磁波屏蔽材料。透過簡單且低成本的溶膠-凝膠法(sol-gel method),可以均勻地將氧化鋅奈米顆粒塗覆於棉織品表面,藉由噴塗rGO溶液並進行熱還原可以簡單並均勻地塗覆rGO。由結果可以觀察到總屏蔽效能隨著rGO塗覆量增加,然而反射損耗會因為棉織品的導電度提高而降低。rGO/ZnO塗覆棉 (ZnO + 7 wt% rGO)的總屏蔽效能最高可達99.999%,其中反射率為17.783%,吸收率為82.216%。此材料之高吸收率可歸因於ZnO奈米顆粒的高介電性質、rGO片的高導電度和rGO/ZnO塗覆棉織品的核-殼結構。由實驗結果得知,rGO的過量添加會阻塞棉織品的所有孔隙,這對於電磁波的吸收是不利的。當ZnO塗覆棉的rGO塗覆量為3 wt%時,可達到90%的高吸收率。

並列摘要


Electromagnetic (EM) pollution is a serious concern to be addressed because it not only disturbs the functioning of EM devices, but also affects human health and may lead to serious disease such as leukemia and brain tumor. In this regard, carbon materials and their composites have gained much attention in the field of electromagnetic interference (EMI) shielding because of their advanced electrical properties, excellent flexibilities, low production cost, environmental friendliness, easy fabrication and chemical inertness. However, high electrical conductivity often results in reflection dominant EMI shielding. Thus, the microstructure of the nanomaterials, the structure of the shield and the inclusion of the foreign materials such as materials with dielectric or magnetic dipoles play an important role in the absorption of the EM waves. In this research work, carbon nanomaterials were used as a primary material for the investigation of EMI shielding effectiveness in X-band. The role of the microstruture of the shield, the effects of the inclusion of dielectric and magnetic materials in the shield, and the mechanism of shielding have been discussed in details. Initially, we focus on understanding the role of the inclusion of dielectric and magnetic materials in the shield. Therefore, we developed lightweight hybrid composite mats, having porosity around 40%, composed of amorphous carbon, zinc oxide nanorods and nickel zinc ferrite for excellent absorption dominant EMI shielding effectiveness in the X-band. The vibrating sample magnetometer measurement confirmed that the saturation magnetization value of the composite materials enhances with the weight percentage of zinc oxide nanorods-nickel zinc ferrite (ZNF) powder, which leads to enhanced magnetic loss of the EM waves. With the thickness of 1.0 mm, the total EMI shielding effectiveness of the amorphous carbon composite was measured to be 25.70 dB, which was further enhanced to 53 dB by the incorporation of the ZNF powder. Such high increment attributes to the enhanced magnetic properties, interfacial polarization and dielectric properties of the composite. The synergistic combination of the materials results in the high absorption loss of the EM waves. Thus, the composites can shield up to 99.999% power of the EM waves which is shared by the 8.394% reflection and 91.605% absorption. Moreover, the magnetic, electrical and EMI shielding properties of the composites can be tuned by controlling the amount of ZNF powder in composites. To better understand the role of porosity along with the inclusion of dielectric and magnetic materials in the shield, we further developed three dimensional interconnected graphene aerogels decorated with cobalt ferrite (CFO) nanoparticles and ZnO nanorods for the application of absorption dominant EMI shielding. Because of low density, high porosity and large surface area, the graphene aerogel showed total EMI shielding effectiveness of 25.07 dB at a thickness of 5 mm, which was further improved to 42.10 dB with the inclusion of CFO nanoparticles. The incorporation of magnetic CFO not only enhanced the total EMI shielding of the aerogels, but also improved the power absorption from ∼37.738% to ∼87.788%. By further incorporating ZnO nanorods along with CFO nanoparticles into the aerogels, the total EMI shielding effectiveness and power absorption enhanced to 48.56 dB and ∼93.655%, respectively. Moreover, the large surface area and porosity of the aerogels facilitated multiple reflections of the EM waves inside the material, thus provided high absorption of the EM waves. Other than that, the wearable EMI shielding materials such as conducting textiles have also emerged to protect human as well as sensitive electronics from EM pollution. We further employed zinc oxide and reduced graphene oxide (rGO) coatings on cotton fabrics to prepare wearable EMI shielding materials. The uniform coating of zinc oxide nanoparticles is achieved by the easy and cost effective in-situ sol-gel method, whereas the uniform coating of rGO is achieved by simple spraying of graphene oxide solution followed by thermal reduction. It is observed that the total EMI shielding effectiveness increases with the rGO loading, however, the reflection loss decreases owing to improved conductivity of the cotton fabric. The rGO/ZnO coated cotton (ZnO + 7 wt% rGO) achieves the highest total EMI shielding effectiveness of ~99.999%, which is shared by ~17.783% of reflection and ~82.216% of absorption. Such high absorption dominant EMI shielding is attributed to highly dielectric ZnO nanoparticles, highly conductive rGO sheets and the core-shell structure of the coated cotton fabric. It is also concluded that the excessive loading of rGO can block all the pores of the cotton fabric which is not beneficial for high absorption of the EM waves. The high absorption of ~90% is demonstrated when the ZnO coated cotton is loaded with 3 wt% rGO.

參考文獻


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