Unveiling the Amphoteric Behavior of TiO₂ in Fused CaF₂-TiO₂-MgO-SiO₂ Submerged Arc Welding Fluxes

A comprehensive understanding of structural features is essential for adequately addressing challenges towards the development of high-quality welding fluxes. In the present study, the amphoteric nature of TiO₂ in the designed CaF₂-TiO₂-MgO-SiO₂ fluxes has been fully demonstrated. The coexistence of ^[IV]Ti, ^[V]Ti, and ^[VI]Ti in the polymeric network has been verified via Ti2p X-ray photoelectron spectroscopy (XPS). Furthermore, it has been revealed that the ^[IV]Ti/^[VI]Ti ratio is significantly enhanced from 0.45 to 1.99 as the MgO/SiO₂ ratio increases from 0.75 to 1.30. Molecular dynamics (MD) simulations indicate that the value of NBO/T first increases and then decreases as the ^[IV]Ti/^[VI]Ti ratio continues to grow. In addition, through ²⁹Si magic angle spinning-nuclear magnetic resonance (MAS-NMR), it has been demonstrated that Q²(Si) increases at the expense of Q³(Si). Quantitative descriptions of bonding configurations further demonstrate that the presence of ^[IV]Ti–O–Si bonds becomes more abundant with enhanced ^[IV]Ti/^[VI]Ti ratio, thereby facilitating the transformation of Si–O_nb (non-bridging oxygen) into Si–O_b–Ti (O_b denotes bridging oxygen). Such phenomenon is likely rendered by the dominating presence of ^[IV]Ti acting as network former. Consequently, the amphoteric behavior of TiO₂ has been postulated, potentially offering insights into rational design and performance optimization towards TiO₂-bearing welding fluxes.