Review.5 – Thermal Properties of the Hybrid Graphene-Metal NanoMicro-Composites: Applications in Thermal Interface Materials

Appl. Phys. Lett. 100 (2012) 073113 

Authors: Vivek Goyal and Alexander A. Balandin 

As electronic devices become smaller, power densities increase and so do hotspot temperatures, leading to a need for materials that can exchange heat better. The materials that facilitate this heat exchange are called thermal interface materials (TIMs). These types of materials are used to eliminate air gaps between semiconductors and heat sinks and thus improve heat transfer between these electronic components. In this journal article a TIM is prepared through the use of graphene, silver particles and an epoxy matrix. 

Article Summary: A novel thermal interface material (TIM) was prepared by the addition of few-layer graphene (FLG) to an epoxy matrix containing silver particles. The FLG was prepared by the density gradient ultra-centrifugation (DGU) process, using sodium cholate (SC) as a surfactant. Through the DGU process, the thickness of FLG can be controlled to produce SC-encapsulated FLG sheets of varying concentrations. The FLG was integrated into the silver epoxy by high shear mixing and ultrasonication. By using this method, composites were prepared  from 0.5 to 3 wt% of FLG. As a comparative study, carbon black was also used as a filler to produce carbon black/silver epoxy composites in the same wt% as the FLG composites. Scanning electron microscopy (SEM) was used to elucidate the nano and micro-scale physical characteristics of the composites. Using the transient plane source (TPS) technique, the thermal properties of the prepared composites were investigated. It was determined that with 5 vol. % FLG filler, a thermal conductivity of 9.9 W/mK, which is more than a 500% increase over the commercial silver epoxy (1.67 W/mK) without any filler. In addition to room temperature measurements, the thermal conductivity of the TIMs were studied up to 400 K, where they showed improved thermal conductivity values as compared to room temperature. The carbon black filler at its highest concentration tested did not show a noticeable increase in thermal conductivity.  

Some interesting points to be taken from this journal article include: 

• The use of FLG resulted in a much larger increase in thermal conductivity of the composite when compared to the same amount of carbon black; although carbon black is less expensive to use as a filler. 
• The thermal conductivity of the FLG-silver particle-epoxy composite is positively correlated with temperature. As a semiconductor produces more heat, the thermal conductivity of the composite will increase to help dissipate that heat. 
• The thickness of the FLG sheets can be controlled by the DGU process. Testing different thickness sheets might result in the discovery that sheets of certain thickness give better thermal conductivity values once integrated into the epoxy matrix.

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