Graphene aerogel microspheres (GAMs) are unique materials that consisted by graphene porous network, and integrated various typical characters, such as electrical conductivity of graphene, high porosity of aerogel and monodispersed of porous microspheres. GAMs can be used as templates for drug delivery, superabsorbent, synthesizing porous inorganic catalysts or functional composite particles, etc. There are still great challenges towards well-controlled, mono-dispersed GAMs, such as fabrication of mono-disperse sol micro-droplets with resistance of amalgamation, and sol-gel transition of individual micro-droplets with thereafter capillary force reduction for drying. Furthermore, traditional strategies, e.g., emulsion polymerization as well as ultrasonic/electro spray, are not suitable for the fabrication of GAMs due to their long time aging process and poor control of droplet sizes.
To resolve the above challenges of GAMs, the Aerogel Team of the Chinese Academy of Sciences led by Prof. Zhang Xuetong at Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences introduced a programmable way, i.e., ink jetting - liquid marbling - supercritical fluid drying (ILS) coupling techniques to produce uniform GAMs. Specifically, ink-jetting technology was used to make uniform GO droplets; liquid-marbling technology was used to keep the GO droplets from amalgamation and to confine sol-gel transition of them, and supercritical fluid drying technique was used to reduce surface tension of gel precursors between solid framework and liquid medium which can guarantee smallest shrinkage of the resulting aerogels. The size of aerogel microspheres can be easily controlled by adjusting the ink-jetting program.
Such programmed GAMs have excellent properties (e.g., hydrophobicity, ultra-low density, low resistance, large specific surface area, monodisperse in diameter.) for electrical applications (tilting switch and liquid level switch) and phase change composites. 1) This GAM could be a qualified candidate for ball switch. The failure rate of commercial switch was 4 times higher than that of the GAM ball switch within 100 times tests. This may be assigned to the fact that the surface of ball in commercial switch was too smooth and easy to miss contact with the electrodes. 2) Benefit from the micro size, the GAM used in ball switch was 124 times smaller and 330 times lighter than the ball used in the commercial ball switch. The GAM in liquid level switch was 1.2 × 105 times smaller and 1.6 × 105 times lighter than the ball used in the commercial ones (EM15-2, ELECALL). This may point out the direction of the miniaturization and weight-lightening for both ball switches and liquid level switches. 3) the mono-dispersed graphene aerogel microsphere was used as a template, where paraffin was filled into the cavities. Such phase change microsphere was made into a heat flux buffer. The composite microsphere has stable thermal performance during phase change process. Because of its micro size, the heat contributed to the phase changing of paraffin was very small. It was found that only 0.027 J heat was enough to initial the phase change and trigger electrical resistance mutation.
This work was financially supported by the National Natural Science Foundation of China (51572285 and 21373024) and the National Key Research and Development Program of China (2016YFA0203301).
