(77a) Fluidization of Graphene Nanoplatelets: From Microstructure to Hydrodynamics

Graphene is a material consisting of a single two-dimensional layer of sp2 hybridized carbon atoms arranged in honeycomb lattice. Since its discovery in 2004, graphene has been attracting considerable attention due to its exceptional physical, mechanical, thermal, electronic and optical properties. In fact, it has an extremely high surface area (theoretically 2630 m²/g) and it is the lightest (with a mass of 0.77 mg for 1 m²), but at the same time the strongest material known (with an ultimate tensile strength of 130 GPa and a tensile stiffness of ∼1 TPa) [1]. Moreover, it is the best conductor of heat at ambient conditions (∼5,000 W m^-1 K^-1) and electricity (up to 200,000 cm² V^-1 s^-1), and it is optically transparent (with a light absorption of 2.3% of white light) [2-4]. One of the most common methods to synthesize graphene is liquid-phase exfoliation, where graphite is milled into a powder whose particles are separated into tiny flakes (also called platelets) by mechanical forces in liquid. These flakes which consist of a few layers of graphene are then sorted based on their size and thickness, i.e., number of layers. Yet, controlling the number of layers in graphene manufacturing remains challenging and often the final product consists of a powder containing multilayer (i.e., ≤ 10 layers) flakes. Still, the properties of multilayer graphene powders are relevant to practical applications such as electric batteries and field emitters.

The number of layers of graphene determines its microstructure which affects the powder bulk behaviour (e.g., flowability, dispersibility, fluidizability and agglomerability). One of the most interesting applications of graphene is its use in composite materials and, for example, graphene powders are emerging in the field of catalysis as excellent support materials. An attractive route for the functionalization of graphene with metal and metal-oxide nanoparticle catalysts is atomic layer deposition carried out in a fluidized bed reactor. In order to achieve an uniform deposition of nanoparticles for high-performance catalysts, ensuring a good fluidization behaviour of graphene powders is crucial. Therefore, understanding how graphene powders fluidize depending on their microstructure is vital.

In this work, we study for the first time the fluidization behavior of three kinds of graphene powders: graphene nanoplatelets of 150 m²/g (GNP150), graphene nanoplatelet aggregates of 300 m²/g (GNPA300) and graphene nanoplatelet aggregates of 750 m²/g (GNPA750). Graphene powders fall under group C of Geldart’s classification, i.e., cohesive powders. Therefore, their fluidization is expected to be prevented by the strong interparticle forces, which exceed the drag force induced by the gas flow. However, the graphene platelets do not fluidize individually, instead they form into agglomerates, due to the aforementioned cohesive forces, as earlier observed for semi-spherical nanoparticles [5, 6]. As a result, the fluidization behavior depends on the properties of the agglomerates. We correlate the morphology of the graphene nanoplatelets at nano-, micro- and macro-scales to their hydrodynamic behaviour.

GNP150 at the nanoscale consist of agglomerates of mostly 2D flakes (6-8 nm thick and 15 μm wide). The latter, in virtue of being relatively thick, tend to be rigid, or in other words they are not prone to fold and wrinkle. As such, they truly enjoy a 2D geometry. Rigid 2D-like objects pack poorly compared to 3D-like flexible objects. As a result of that, agglomerates consisting of 2D thick nanoplatelets are loosely packed, hence GNP150 have a low bulk density. At the macroscale, GNP150 form mm-sized spherical agglomerates, whose size distribution depends on the history of the powder. Upon vibration-assisted fluidization, the bed of particles segregates into mm-sized agglomerates at the bottom, increasingly smaller agglomerates along the bed height, and fines above the splash zone. No fluidization is observed without the use of vibration at any of the tested superficial velocities (i.e., up to 16.85 cm/s). The use of vibration results into bubbling fluidization at relatively low superficial velocities (1.35 cm/s).

GNPA300 and GNPA750 at the nanoscale are not found in 2D form. In fact, higher-surface-area graphene powder translates into thinner nanoplatelets that are more prone to fold and wrinkle. As a result of that, the nanoplatelets are closely packed, and thus have an higher bulk density. At the macroscale, the powders look very fine and the size of the agglomerates is hardly discernable by naked eye. For GNPA300, fluidization occurs only when assisted via vibration. Fluidization occurs at higher gas velocities than in the previous case, i.e., 2.7 cm/s. Once fluidization occurs, the bed height remains approximately constant with increasing gas velocity. High superficial velocities result in slugging behaviour and significant elutriation. Instead, for GNPA750, fluidization does occur even without the use of assisting methods upon breaking of channels. In fact, increasingly thinner nanoplatelets translate into more cohesive agglomerates and higher agglomerate density. Even though cohesiveness deteriorates fluidization quality, higher densities promote fluidization. In light of this, the fact that GNPA750 fluidizes even without vibration is no surprise, as we expect the increased cohesiveness between nanoplatelets to result into highly packed and thus dense agglomerates that act as micron-sized Geldart-B-like powders.

This work demonstrates the large influence of the structure at the nm- to mm-scale on the large-scale fluidization behaviour. Such insight is essential when scaling up processes for gas-phase production of nanostructured materials [7].

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