Graphene has been considered one of promising alternatives to EMI shielding material. The graphene, which features its outstanding electrical conductivity and large surface area, has advantages of density and corrosion resistance compared to conventional shielding materials. In order to accommodate graphene to practical use, as EMI shielding material, the agglomeration of graphene nanoplatelets must be overcome and the impedance of graphene has to be matched with that of free space. By using the molecular-level mixing process, graphene nanoplatelets decorated by magnetic nanoparticles are fabricated.
In this research, RGO/Ni and RGO/Co nanocomposite powders are fabricated by a molecular-level mixing process by growing Ni nanoparticles on graphene surface for EMI shielding applications. Nanoparticles of uniform size are well distributed by interfacial bonding with functional groups on GO. Also, Graphene nanoplatelets (GNP) / Metal were fabricated with the same process to fabricate RGO/metal. GNP/Ni/Wax nanocomposites, fabricated by the molecular-level mixing process, showed SE value of 40dB in 8-12GHz. This is attributed to good electrical properties of GNP and functional groups that played an important role in the improvement of homogeneity of equal-sized nanoparticles and formation of efficient network improving the conductivity of composites.
GNP/Ni/PMMA nanocomposites are fabricated by the solution blending process. Also, multi-layered functionally graded composition of GNP/Ni/PMMA nanocomposites are fabricated by the process in which mixed and degassed each layers of GNP/Ni/PMMA are packed layer by layer in a mold and pressed isostatically under a pressure.
Analysis of EMI shielding mechanisms for multi-layered GNP/PMMA is carried out confirming calculation and experiment results of SER, SEA and SE have an aligned tendency with each other. Based on this analysis, multi-layered structure of GNP/Ni/PMMA nanocomposites are fabricated to maximize EMI shielding properties of GNP/Ni composites in the polymer matrix. In case of the multi-layered system with an alternating stack of dielectric polymer and conductive GNP fillers, the major shielding mechanism results from uneven distribution which increases the number of reflections between internal interfaces in the material. Absorption includes this effect from the sum of all interfaces formed on the conducting and insulating layers. While, the major shielding mechanism of the system of the conductivity gradient with GNP concentration is the absorption by conductive dissipation, which shows enhanced EMI shielding effectiveness. The progressive increase in conductivity from the surface causes the absorption of most of the power by conductive dissipation. In addition, additional shielding capability results from increased absorption from additional reflection with concentration gradient in the material.
In particular, the multi-layered system ({15 wt% Ni/PMMA}/{20 wt% GNP/Ni/PMMA}/{30 wt% GNP/Ni/PMMA}/{40 wt% GNP/Ni/PMMA}) shows shielding effectiveness of 64 dB. These results suggest that GNP/Ni composites are highly promising EMI shielding material with the simple molecular-level mixing process at a low cost.
전자파 차폐란 표면에서 전자기파를 흡수 및 반사시켜 전자파가 내외부로 전이되는 것을 방지하는 것을 말하며 차폐재로는 전도성 물질이 이용이 된다. 최근 그래핀이 새로운 전자파 차폐 소재로서의 가능성을 보이고 있다. 분자수준혼합공정기술을 적용하여 그래핀에 나노자성입자를 형성시켜 복합분말을 제조하는 방법은 그래핀의 응집을 막아줄 수 있을 뿐만 아니라 자성특성으로 인해 전자파 차폐성능을 높일 수 있다. 그래핀의 표면에 기능기를 부여하여 금속 이온을 성장시켜 그래핀/금속 나노복합소재를 제조한 결과 40dB의 성능을 보였다.
또한 GNP/Ni 나노복합소재를 PMMA 고분자 수지에 복합하여 GNP/Ni/PMMA 나노복합재를 제조하였다. 다층구조 시스템을 GNP/Ni/PMMA 나노복합재에 적용하여, GNP/Ni 분율이 다른 각 층을 각각 제조하여 한 층으로 쌓아 hot pressing 공정을 거쳤다. 이렇게 제조된 GNP/Ni/PMMA 다층 구조 시스템은 절연층과 전도층이 교차되는 형태와 전자파 전파 방향에 따라 GNP/Ni 분율이 점차 증가되는 형태가 있다. 첫 번째 형태의 경우 층간 임피던스 차이에 의한 내부 전자파 반사가 주요 차폐 메커니즘이 된다. 두 번째 형태의 경우 전자파 입사 방향에 따라 분율이 증가되는 전도성 필러에 의해 전자파가 열로 소실된다. 또한 층간의 분율 차이에 따른 재료 내부 층간 반사가 더욱 향상된 전자파 차폐성능을 보이게 하였다. 또한 GNP/PMMA 다층구조의 이론적 분석과 실험 결과가 일치함을 보였다.
GNP/Ni 나노복합소재를 PMMA에 섞어 함량이 증가하는 다층구조로 제작한 결과 64 dB의 차폐 성능을 보였다. 이는 입사층의 임피던스를 높게 할 뿐만 아니라 전기 전도에 의한 전자파의 열 소실을 주 메커니즘으로 하여 함량 변화에 따른 층 사이의 계면 반사를 추가로 발생시킴에 의한 것이다.
