A study on mechanical properties, electrical conductivity and EMI shielding performance of syntactic foams.
Date of Issue2013
School of Materials Science and Engineering
Syntactic foam is a special class of light weight composite materials. It has been found useful in many areas, such as aerospace and submarine. In order to further widen its application spectrum, the enhancement in the mechanical properties of syntactic foams is essential. Besides mechanical properties, their electromagnetic interference (EMI) shielding has not been explored because of the non-conductive nature of the traditional fillers and matrices of syntactic foams. However, due to their light weight advantage, syntactic foams become an attractive candidate for EMI shielding applications, for electronic devices and electrical equipments. Therefore, developing syntactic foams with good mechanical properties and/or EMI shielding performance would expand their applications for future composite materials. In this work, hollow carbon microspheres (HCMs), instead of the traditional non-conductive microspheres, were employed to fabricate syntactic foams with phenolic resin as matrix. In the attempts to improve mechanical properties and/or EMI shielding performance of the resultant foams, three different approaches, namely coupling agent, carbonization and carbon nanofiber (CNF) reinforcement, were applied. In the first approach, the effect of coupling agent on mechanical properties and EMI shielding performance of syntactic foams was studied. Results showed that better interfacial adhesion could be induced from the coupling agent treated HCMs, which led to the enhancement in compressive strength, flexural strength and fracture toughness of the syntactic foams. Toughness mechanisms, including crack deflection, crack bowing and debonding, were proposed. However, EMI testing results showed that the introduction of coupling agent had no effect on the EMI shielding performance, because a three-dimensional electrically conductive network was not formed. In the second approach, the effect of carbonization on mechanical properties and EMI shielding performance of the syntactic foams was studied. The electrical conductivity was increased by approximately seven orders of magnitude, which resulted in a significant enhancement in shielding effectiveness (SE) by a factor of 16. The SE of 30 dB meant a shielding of over 99.9% of incident electromagnetic (EM) radiation. The shielding mechanisms were discussed in detail. However, it was also found that compressive and flexural strengths of the foams decreased due to the formation of glassy carbon and oversized internal voids after fully carbonization. The third approach encompassed the inclusion CNFs. Results showed that no enhancement in compressive strength with the addition of CNFs was observed. Flexural strength and fracture toughness were increased with increasing CNFs content and decreased beyond 1.5 vol% of CNFs. The decreasing trend was due to agglomeration and clustering of the CNFs. Toughening mechanisms, such as crack deflection, step structure and debonding of the CNFs, were proposed. It was also found SE of the CNF reinforcement syntactic foams (CNFRSFs) was increased with increasing CNFs content and was superior to those of the composites having either CNFs or HCMs only. SE of 25 dB was achieved in the syntactic foam having 2.0 vol% CNFs, which is good enough for most practical applications. The shielding mechanisms were discussed in detail.