Direct inkjet printing of mullite nano-ribbons from the sol–gel precursor

Bio-inspired ceramics, that can overcome the brittleness of ceramics, require building blocks with thickness of a few hundred nanometers. It is extremely challenging to precisely fabricate the building blocks for the bio-inspired ceramics. In this paper, we demonstrate the processing of dense mullite nano-ribbons, using the sol–gel inkjet printing method with post-heat treatment. These nano-ribbons had precisely controllable dimensions of sub-micrometer thickness and width of a few hundred of micrometers. A novel single-phase ink from the water-based mullite sol–gel precursor was developed that ensured inkjet printability and low-temperature formation of pure mullite phase. One of the greatest challenges was how to achieve uniform track from such inks, because the sol–gel inks had non-zero receding contact angles with the substrate. According to previous theoretical studies, the ink track would eventually become discrete beads. However, we found that solvent-evaporation-induced gelation played an important role in the stability of the printed lines. Taking advantage of the solvent evaporation and sol–gel transition upon substrate heating, we were able to print stable and continuous gel lines. After firing these printed lines at 1000 °C, the pure mullite phase without any undesirable intermediate phases was achieved. The printed lines and dots retained their shapes during firing. It is shown that the crack-free mullite nano-ribbons of the thickness ranging between ~90 and ~200 nm can be printed directly on substrates.