Enhanced strength and high reactivity of Al 2 O 3 ‒MgAl 2 O 4 ‒C ceramic filters with in situ β‐SiC whiskers: Role of nano‐Al 2 O 3

Abstract The role of nano‐Al 2 O 3 content on the phase composition, morphology, microstructure, mechanical properties, and purification performance for molten steel of Al 2 O 3 ‒MgAl 2 O 4 ‒C filters was investigated through X‐ray diffraction, scanning electron microscopy, energy dispersive spectrometry, and immersion test with molten steel. Results indicate that nano‐Al 2 O 3 improves slurry retention performance, increases filter strut thickness, and ensures uniform distribution without cracks. During the sintering process, nano‐Al 2 O 3 accelerates the mass transfer (Mg 2+ and Al 3+ ) between microporous Al 2 O 3 ‒MgAl 2 O 4 particle and nano‐Al 2 O 3 due to its high reactivity, thereby greatly enhancing the sintering driving force, which promotes the development and growth of neck connections between the microporous Al 2 O 3 ‒MgAl 2 O 4 particles, resulting in a substantial improvement in the mechanical properties. The filter N5, with 5 wt% nano‐Al 2 O 3 exhibits excellent mechanical properties with a cold compressive strength of 1.41 and 1.21 MPa after three thermal shock tests. Regarding molten steel purification, on the one hand, nano‐Al 2 O 3 enhanced the filter's resistance to molten steel erosion, protecting the internal graphene nanosheets from dissolving into the molten steel. On the other hand, its high reactivity significantly promoted the carbothermal reduction reaction, generating reductive gases that interact with and adsorb [Al], [O], and Al 2 O 3 inclusions in the molten steel, quickly forming a continuous and uniform Al 2 O 3 ‒MgAl 2 O 4 reaction layer at the interface, which contributed to the high molten steel purification efficiency, reducing inclusions by 44% and lowering the total oxygen content from 56.3 to 26.1 ppm. Finally, the role of nano‐Al 2 O 3 in sintering behavior and purification function as well as the comprehensive molten steel purification mechanism were proposed.