Nicholas Heeder

Sensitivity and dynamic electrical response of CNT-reinforced nanocomposites

A series of dynamic compressive experiments were performed to experimentally investigate the electrical response of multi-wall carbon nanotube (CNT)-reinforced epoxy nanocomposites subjected to split Hopkinson pressure bar (SHPB) loading. Low-resistance CNT/epoxy specimens were fabricated using a combination of shear mixing and ultrasonication. Utilizing the CNT network within, the electrical resistance of the nanocomposite was monitored using a high-resolution four-point probe method during each compressive loading event. In addition, real-time deformation images were captured using high-speed photography. The percent change in resistance was correlated to both strain and real-time damage. The results were then compared to previous work conducted by the authors (quasi-static and drop weight impact) in order to elucidate the strain rate sensitivity on the electrical behavior of the material. Furthermore, the percent change in conductivity was determined using a Taylor expansion model to investigate the electrical response based on both dimensional change as well as resistivity change during mechanical loading within the elastic regime. Experimental findings indicate that the electrical resistance is a function of both the strain and deformation mechanisms induced by the loading. The bulk electrical resistance of the nanocomposites exhibited an overall decrease of 40–65% and 115–120% during quasi-static/drop weight and SHPB experiments, respectively.

Massive electrical conductivity enhancement of multilayer graphene/polystyrene composites using a nonconductive filler.

We report a massive increase in the electrical conductivity of a multilayer graphene (MLG)/polystyrene composite following the addition of nonconducting silica nanoparticles. The nonconducting filler acts as a highly effective dispersion aid, preventing the sheetlike MLG from restacking or agglomerating during the solvent casting process used to fabricate the composite. The enhanced dispersion of the MLG leads to orders of magnitude enhancement in electrical conductivity compared to samples without this filler.

Electro-mechanical Behavior of Graphene–Polystyrene Composites Under Dynamic Loading

An experimental investigation was conducted to understand the electro-mechanical response of graphene reinforced polystyrene (PS) composites under static and dynamic loading. Graphene/PS composites were fabricated using a solution mixing approach followed by hot-pressing. Absolute resistance values were measured with a high-resolution four-point probe method for both quasi-static and dynamic loading. A modified split Hopkinson (Kolsky) pressure bar apparatus, capable of simultaneous mechanical and electrical characterization, was developed and implemented to investigate the dynamic electro-mechanical response of the composites. In addition to measuring the change in electrical resistance as well as the dynamic constitutive behavior, real-time surface damage and global deformation was captured using high-speed photography. The real-time damage was correlated to both stress–strain and percent change in resistance profiles. The experimental findings indicate that the bulk resistance of the composite increased significantly due to the brittle nature of the PS matrix and the presence of relative agglomerations of graphene platelets which resulted in micro-crack formations. Scanning electron microscopy imaging gives further insight into the various damage mechanisms that occur within the composites subjected to a static or dynamic load. The results show that the change in transport properties can provide further insight into the micro-structural evolution of composite materials during loading.

Particle Templated Graphene-Based Composites with Tailored Electro-mechanical Properties

A capillary-driven particle level templating technique was utilized to disperse graphite nanoplatelets (GNPs) within a polystyrene matrix to form composites that possess tailored electro-mechanical properties. Utilizing capillary interactions, highly segregated composites were formed via a melt processing procedure. Since the graphene particles only resided at the boundary between the polymer matrix particles, the composites possess tremendous electrical conductivity but poor mechanical strength. To improve the mechanical properties of the composite, the graphene networks in the specimen were deformed by shear. An experimental investigation was conducted to understand the effect of graphene content as well as shearing on the mechanical strength and electrical conductivity of the composites. The experimental results show that both the mechanical and electrical properties of the composites can be altered using this very simple technique and therefore easily be tailored for desired applications.