Ultrasonic spraying allows you to get high-quality graphene coating without post-processing

    The unique properties of graphene - its electrical and thermal conductivity, as well as its mechanical strength - make it a very promising material for creating a variety of films and coatings. The two main approaches to creating such coatings that exist today are growing graphene crystals by depositing carbon atoms from carbon-containing gases on a substrate and applying a suspension containing graphene flakes up to several micrometers in size to the surface. The first method allows you to get perfect single crystals, but it requires high temperatures, perfectly pure ingredients, special substrates. It is applicable primarily in microelectronics, to create graphene electronic components.

    The second method is much more unpretentious, but instead of single-crystal graphene, a layer of many overlapping graphene flakes with a structure that is far from ideal is formed on the surface - this is enough for many applications. To improve the quality of such a coating, different methods are used - annealing, plasma or chemical treatment. Scientists from the University of Illinois in Chicago, together with colleagues from South Korea, have developed a simple and well-scalable version of the second method for producing graphene coating without any post-processing.

    Glass plate with graphene sputtering and its surface under an electron microscope.

    Their method is that a suspension containing graphene oxide flakes is sprayed onto the product using a Laval nozzle. This type of nozzle is widely used in rocket and jet engines and allows to achieve supersonic gas flow rates. Thanks to this, the suspension is sprayed into the smallest droplets, allowing you to separate the individual flakes and apply them very evenly. At subsonic speeds, the flakes often fall to the surface in the form of clumping lumps. In addition, the smallest droplets blown by a supersonic gas stream dry out almost instantly.

    Another advantage is the kinetic energy with which the graphene hits the surface of the product, it is sufficient that the scales not only stick to it, but also smooth out, “healing” many structural defects. At supersonic speeds, graphene acquires a certain elasticity, stretches, and carbon atoms in the places of defects occupy the correct position, forming an ideal hexagonal lattice.

    Stretching due to kinetic energy of supersonic flow corrects graphene defects

    This method is very simple and scales well. By varying the concentration of the graphene oxide suspension, pressure and gas flow rate, it is easy to control the thickness and quality of the coating. The absence of the need for processing the coating expands the range of materials that can be coated with graphene spraying - polymers and plastics can be widely used that would not withstand post-treatment with plasma or high temperatures.

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