Catalytic carbon-bed sintering of CNTs/calcium aluminate cement and its effects on thermal mechanical properties of refractory castables

To improve the dispersion of carbon nanotubes in refractory castables and exert the strengthening effect of carbon nanotubes on castables, the core-shell CNTs/CAC (carbon nanotubes/calcium aluminate cement) was synthesized by the catalytic carbon-bed sintering method. The effects of catalyst content on the microstructure of CNTs/CAC powders were evaluated by Raman spectroscopy, SEM, and HRTEM. The results suggested that the optimal addition amount of catalyst was 0.75 wt%. The diameter of the bamboo-like carbon nanotubes was -50 nm and the length of the carbon nanotubes was -120 mu m. Carbon nanotubes and small calcium aluminate particles were interspersed with each other and wrapped in a compact calcium aluminate shell with a thickness of 3-5 mu m. Moreover, the DFT calculation was employed to investigate the growth mechanism of carbon nanotubes. Electrons were transferred from Ni to H, O, and C atoms, increasing bond length and weakening the strength of C-H and C---O bonds which was conducive to the growth of carbon nanotubes. The formation of the core-shell structure was in direct connection with the liquid-phase eutectic phase. Calcium carbonate and alumina reacted preferentially to form C12A7, and then C12A7 continued to react with alumina to form CA and CA2. With the increase of sintering temperature, a large amount of liquid phase was formed. Carbon nanotubes were wrapped by the liquid phase of calcium aluminate to form a core-shell structure. The CNTs/CAC cement bonded Al2O3-SiC castables had great mechanical properties and thermal shock resistance. Compared with Al2O3-SiC castables bonded with Secar71, the HMOR and thermal shock residual strength ratio of Al2O3-SiC castables bonded with CNTs/CAC cement were increased by 24.7% and 22.1%, respectively.