20Khz Ultrasonic Dispersion of Graphene Nanoplatelets

20Khz Ultrasonic Dispersion of Graphene Nanoplatelets

Graphene has outstanding mechanical properties due to its unique structure, and is regarded as an ideal reinforcement of metal matrix composites. However, it is always in an agglomerate form due to its large specific surface area, and thus, it must be first dispersed prior to combining with a matrix, and ultrasonic treatment is considered to be the most effective way. In this work, the effects of parameters of tip ultrasonic treatment, such as ultrasonic time, ultrasonic power, solvent kind, and its temperature, on dispersion and structure of graphene nanoplatelets (GNPs) were studied. The results show that increasing ultrasonic time or ultrasonic power can enhance the dispersion and exfoliation effects of GNPs, but also increase fragmentation degree and disorder degree of C-atom distribution simultaneously. Solvents with low temperature, low viscosity, or high surface tension have similar effects to those of increasing ultrasonic time or power. However, for tap water, a high-surface-tension solvent, it has relatively low fragmentation degree, and good dispersion and exfoliation effects due to the hydrophilicity of GNPs. However, ethyl alcohol is a more suitable solvent because it has excellent volatility and inert reaction characteristics with GNPs and matrix alloys besides a good dispersion effect. The GNPs can achieve the expected status when they are ultrasonically treated for 4 h under a power of 960 W in EA solvent at 35 °C.

Principle of Ultrasonic Graphene Dispersion
There are two types of ultrasonic equipment, tip and bath sonicator. The power of the tip sonicator is always higher than that of the bath one, and thus, the tip sonicator is much more efficient for dispersion than the bath sonicator under the same conditions . However, most investigations emphasize the microstructure and mechanical properties of the achieved graphene reinforced composites . As for the fabrication of the composites, especially for the dispersion of graphene, only a set of parameters was provided, and the detailed effects of the parameters such as the ultrasonic powder and time, the viscosity, surface tension and temperature of the solvents on the graphene dispersion, are still unclear. Therefore, the employed parameters in their investigation might not be the optimum, and the mechanical properties of the composites are unsatisfactory due to the resulting inhomogeneous distribution of graphene. Previous investigations indicated that ultrasonic treatment could disperse GNP agglomerates, but simultaneously lead them to fragmenting . The fragmentation not only reduces the aspect ratio of graphene and decreases its load transfer efficiency, and thus, impairs its strengthening role , but also increases C-atoms with dangling bonds at the edge of GNPs; such C-atoms always have high chemical activity and can easily react with matrix-alloying elements to form brittle carbides at the graphene/matrix interface , which also impairs the strengthening role of GNPs . In addition, some investigations have suggested that vacancies might form during ultrasonic treatment and the structure integrity of graphene was then destroyed, and thus the strengthening role was also decreased . Cheng et al. found that the ultrasonic dispersion of carbon nanotubes was dependent on the solvent physical properties such as vapor pressure, viscosity, and surface tension . Furthermore, solvent temperature rise is a common phenomenon during ultrasonic treatment, and the vapor pressure of a solvent has a close relationship with its temperature, i.e., the solvent temperature also can affect the dispersion of graphene. However, unfortunately, there are no investigations on these aspects.

Graphene dispersion purpose
There are a lot of graphite materials in nature, and graphite with a thickness of 1 mm contains about 3 million layers of graphene. Single-layer graphite is called graphene, which does not exist in the free state, and it exists in the form of graphite sheets laminated with multiple layers of graphene. Since the interlayer force of the graphite sheet is weak, it can be exfoliated layer by layer by external force, thereby obtaining a single-layer graphene with a thickness of only one carbon atom.

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