Self-Assembly of Soot Nanoparticles on the Surface of Resistively Heated Carbon Microtubes in Near-Hexagonal Arrays of Micropyramids

Abstract

Almost regular hexagonal arrays of microscopic pyramids consisting of soot nanoparticles are formed on the surface of graphitized hollow filaments, which are resistively heated to ∼1800–2400 °C under an Ar atmosphere containing trace amounts of oxygen (∼300 ppm). At higher temperatures (T > 2300 °C, approximately) the soot particles are represented mainly by multishell carbon nano-onions. The height and width of the pyramids are strongly dependent on the temperature of the resistive heating, diminishing from 5 to 10 μm at T ≈ 1800 °C to ∼1 μm at 2300–2400 °C. Quasi-hexagonal arrays of the micropyramids are organized in the convex “craters” on the surface of the microtubes, which grow with the time of the thermal treatment. The pyramids always point normally to the surface of the craters, except at the boundaries between the craters, where the normal direction is not well-defined. The pyramids are soft and can be easily destroyed by touching them but can be hardened by heating them under an oxygen-free atmosphere. The pyramids are observed only on the exterior surface of the microtubes, not on their inner surface. This suggests that the thermophoretic force generated by a strong temperature gradient near the external surface of the tubes may be the cause of the micropyramid formation. Electrostatic charging of the soot nanoparticles due to thermionic emission may also be relevant to this phenomenon. The micropyramids can function as field emission point sources, as demonstrated with the use of a micronanoprobing station, mounted in a scanning electron microscope.

Room temperature 3D carbon microprinting

“…Reducing the environmental and energy footprints of carbon manufacturing processes is also important, given the considerable global market. However, carbon microstructures and nanostructures are commonly produced by pyrolyzing organic precursors of synthetic or biological origin 13,14 . Such an approach involves energy-intensive multiple-step processes, such as fiber spinning, stabilization, and carbonization at temperatures typically above 1200 °C13 .…”