The effect of nano additives modified by functionalized groups on the physical and mechanical properties of alumina‑carbon-based refractories

Abstract

Alumina‑carbon-based refractories are widely used in the steelmaking industry due to their exceptional physical and mechanical properties. However, their primary drawback is poor mechanical strength, particularly at elevated temperatures. This study addresses this limitation by incorporating and comparing nano-alumina and its composite with functionalized multi-walled carbon nanotube (MWCNT) additives, which were dispersed in phenolic resin used as a binder. To enhance the dispersion of nano-additives within the alumina‑carbon matrix, the particles were functionalized with 3-aminopropyltriethoxysilane (APTES) groups. The modified additives were integrated into the matrix and mixed with coarse- and medium-sized alumina‑carbon particles, as determined by the modified Andriazen equation. The samples were uniaxially pressed at 150 MPa, tempered at 200 °C for 6 h, and sintered at 1450 °C for 2 h in a reducing coke-bed atmosphere. The physical and mechanical properties, including apparent density, apparent porosity, cold crushing strength (CCS), and hot modulus of rupture (HMOR), were evaluated according to DIN standards. Additionally, Weibull modulus analysis was conducted to assess material reliability and structural integrity. The results indicate that the reference sample (without nano-additives) exhibited the lowest CCS (110 MPa) and HMOR (8 MPa), whereas the sample containing 1 wt% of the alumina-MWCNT nanocomposite achieved the highest CCS (156 MPa) and HMOR (12 MPa). Weibull modulus analysis confirmed improved reliability, with reduced variability in mechanical performance.

Microstructural and phase analyses using SEM, TEM, FT-IR, and XRD revealed that the enhancement in mechanical properties was attributed not only to the increased formation of SiC via a vapor-solid mechanism but also to a morphological transformation from cubic to hexagonal SiC induced by silanization. Moreover, the presence of MWCNTs played a crucial role as templates for the hexagonal SiC phase. TEM analysis further demonstrated that MWCNTs acted as nucleation sites for SiC formation within the refractory matrix. These findings highlight the significant role of silane-functionalized nano-additives in improving dispersion, mechanical strength, and high-temperature stability in alumina‑carbon refractories, paving the way for enhanced industrial applications.