Enhancing graphene reinforcing potential in composites by hydrogen passivation induced dispersion

Yingchao Yang, William Rigdon, Xinyu Huang & Xiaodong Li★

Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, South Carolina 29208, USA.

★E-mail:

Section 1. TEM images of the graphene sheets dispersed in absolute ethanol with different concentrations.

Figure S1│TEM images of graphene sheets dispersed in absolute ethanol with different concentrations using the combined hydrogen passivationand ultrasonication. (a) 1 mg/ml. (b) 2 mg/ml. (c) 3 mg/ml. (d) 5 mg/ml.

Graphene sheets with different concentrations weredispersed into absolute ethanol using the combined HP and ultrasonication. Details of the dispersion can be found in the TEM images (Figure S1). The thickness of the dispersed graphene sheets could bedown to a few layers when the concentration wasless than 2 mg/ml (Figs. S1a and S1b). We often found graphene flakes containing tens of layers or even more when the concentration wasincreased up to 3 mg/ml or 5 mg/ml (Figs. S1c and S1d). The stable solution wasachieved when the graphene concentration was~2 mg/ml.

Section 2. TEM images ofthe graphene sheets dispersed in epoxy.

Figure S2│TEM images of the graphene sheets dispersed in epoxy. (a) Dispersed by the coupled hydrogen passivationand ultrasonication for 2h. (b) Dispersed by ultrasonication only for 2h.

The composite block wastrimmed under light microscope. The trimmed block was then installed into a microtome (Sorvall MT-2B Ultramicrotome) to obtain thin sections. The thickness wascontrolled to be ~100 nm. Several pieces of thin sections weretransferred to a copper grid (300 mesh). The TEM images of the composite reinforced by dispersedgraphene sheets are shown in Fig. S2. No gap was found along graphene edges and on the sides, which prove that the graphene and epoxy are in intimate contact. The numbers of graphene layers can be examined by the edges of the sheets. For example, Fig. S2a shows that graphene sheets in epoxy dispersed by the coupled hydrogen passivationand ultrasonication contain ~5graphene layers. Whereas the graphene sheets in epoxy dispersed by ultrasonication only contain a bunch of graphene layers, as shown in Fig. S2b and majority of thegraphene sheets tend to agglomerate and form flakes in the epoxy.

Section 3. TEM image of bridging graphene sheets in epoxy.

Figure S3│TEM image of bridging graphene sheets in epoxy.

Figure S3 shows that the graphene sheets bridge the hole, which is thought to be one kind of reinforcing mechanismsin composites. The graphene bridging wasoften seen on the fracture surface,pointing toward strong epoxy-graphene interaction and enhanced mechanical properties.

Section 4.Calculation of fracture energy.

Figure S4│Representative load-displacement curve for calculating fracture energy.

The fracture energy can be estimated by integrating the shaded area under the loading curve1. Note that the fracture energy measured here consists of the energy used for both crack initiation and propagation. The fracture energy can be calculated by the equation (1) as follows:

(1)

where E, f(x), and d are the fracture energy, function of load and displacement curve, and the maximum of displacement, respectively.

Section 5.Microtribometer.

Tribometers are used to measure tribological quantities of materials between two surfaces in contact, such as coefficient of friction, friction force, and wear volume2-6.Testing can be carried out in various environments to simulate real life scenarios. In addition testing rigs can take numerous geometries but generally take the form of a loaded probe that impinges the sample. Friction can be calculated by measuring the forces acting between the two materials and wear by measuring the volume of material lost during the test. According to the maximum load, the load sensor includes0.05kg, 0.5kg, 2 kg, 100 kg, or even larger. After the loading force is loaded onto the sample, the stage holding the sample begins to run in cycle. The lateral force can be recorded, which is friction force or wear force. Then, the coefficient of friction (COF) can be calculated. If the sample iswith coating, which can be cut into a small island, the lateral force can be loaded onto the side ofthe island. The lateral force will increase until the island breaks. The sensor can record the lateral force, which can be used to calculate the shear strength of the coating7.

In this study,three-point bending testswere carried out to measure the mechanical properties of the composites, such as elastic modulus, fracture strength, and fracture energy using the Universal Macro-Tribometer (Model UMT-2). During a typical test, a beam sample with the dimension of 20 × 2.5 × 2.0 mm (length × width × height) was mounted onto a stainless steel test cell with two adjustable support fulcrums on the base. Both ends of the beam were left free. Another stainless steel pinwas held stationary in a holder which was mounted to a sensor that measured both vertical force (load) and vertical displacement. The Model DFH-100 force/load sensor was selected for such three-point bending tests with a load range from 1 to 100 kg and the loadresolution of 0.1 kg. The Basic Encoder was used for displacement sensing, with a displacement range of 100 mm and the displacement resolution of 0.5 µm. The sensor was attached to a motorized carriage which loaded the pin against the beam specimenat the center. The vertical displacementwas increased up with the incremental rate of 0.1 mm/s until the beam fractured. The elastic modulus of the composites was calculated from the initial linear portion of the three-point bending loading-displacementcurve. The force at failure in bending test was used to calculate the fracture strength of the beam.The integration of the load-displacement curve up to the fracture failure was used to determine the fracture energy.

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