Preparation and characterization of graphene/metal composites

Licentiate thesis, 2021

Since the isolation of graphene in 2004, much research has been conducted to understand this novel material and how its properties can be utilized in different applications. One type of venture involves graphene as a reinforcing filler in metal matrix composites (MMC) which is becoming increasingly prevalent in the automotive and aerospace industries. Such composites combine the machinability and processing flexibility of metals with the unique properties of graphene. In fact, copper-graphene composites have demonstrated ameliorated mechanical strength with thermal conductivities elevated beyond pristine copper. However, the challenges that remain to commercialize copper-graphene composites are numerous. The most challengeable one is that graphene must be uniformly dispersed in the matrix and adhere to copper through an industrially scalable and affordable process. Moreover, the volume fraction of graphene must be efficiently controlled, lest superfluous amounts lead to structural detriment.

In this regard, the emphasis of this study was to investigate a scalable and simple method to obtain such MMC via powder metallurgy. Specifically, gas atomized copper powder was functionalized with 3-aminopropyl-triethoxysilane (APTES) in toluene (APTES-Cu), resulting in a positively charged surface; then aqueously dispersed and negatively charged graphene oxide (GO) could then be self-assembled on the surface APTES@Cu via electrostatic interaction (Cu@APTES-Cu). The thickness of GO layers and morphology on the powder was controlled by modulating APTES grafting duration and APTES concentration in toluene. Cu@APTES-Cu powders were thermally annealed before compaction and sintering in inert atmosphere.

The results show that surface modification of metal powders serves as a scalable and versatile approach to coat graphene on metal particles for the preparation of graphene/metal composites. Surface modification of copper with 0.2 vol% APTES in toluene for 30 minutes was sufficient to obtain composite powders with incomplete GO coating, which nonetheless demonstrated improved hardness. However, cold working of sintered composites was essential to densify the porous structure created by reduced GO during sintering. On the other hand, sintered composite samples that exhibited higher thermal conductivity than copper was obtained with higher APTES and GO loading. After thermal annealing, these thicker GO coatings were found to improve thermal conductivity in sintered composites by acting as thermal bridges between individual composite particles. Despite incomplete sintering of these composites, a 20% increase in thermal conductivity was attainable. Finally, both polarization scans and etching measurements in concentrated HCl and ammonium persulfate (APS) indicate that the GO coating decomposes on the outer surface during sintering. However, the reduced GO coating can retard corrosion of the internal composite structure by diffusion inhibition.