Synthesis and characterization of a geopolymer/hexagonal‑boron nitride composite for free forming 3D extrusion-based printing

This paper presents the development of a metakaolin based geopolymer suspension containing surface modified hexagonal boron nitride (SMh-BN) that is suitable for direct ink writing (DIW), enabling to obtain a new composite for assembling 3D structures with complex shapes. Using ball-milling, SMh-BN was successfully exfoliated to nanosheets and surface hydroxylated without damaging its crystal structure. SMh-BN nanosheets were homogeneously dispersed in a geopolymer matrix without microdefects or agglomerations due to their thermal conductivity and hydroxylated surface. The rheological properties of the composites firstly decreased and then increased along with the increasing of SMh-BN loading. Geopolymer composites with the addition of 1.0 and 3.0 wt% SMh-BN had lower viscosities, which effectively delayed geopolymerization time and increased operating time, while composites with the addition of 5.0 and 7.0 wt% SMh-BN had higher viscosities, and the 3D structures obtained by DIW had a higher precision and shape retention ability. The applied strategy made it possible to use SMh-BN as rheology property modifier. In particular, the strategy explored the interaction and rheology between h-BN and geopolymer, and offered the possibility of using DIW to optimize and fabricate mesosized objects with a variety of functionalities needed for different future applications.