The long-term fire safety of steel structures necessitates fire-resistant (FR) steels with superior strength retention after prolonged exposure to elevated temperatures. This study investigates two FR steels with Mo contents of 0.25 wt.% and 0.50 wt.% (correspondingly adjusted Cr contents to maintain a nearly constant total Mo + Cr alloy level) after thermal exposure at 600 °C for up to 6 h. Both steels exhibit a polygonal ferrite (PF) and granular bainite (GB) microstructure, but with different phase fractions. Increasing Mo to 0.5 wt.% promotes bainite formation, resulting in a predominantly granular bainitic microstructure (88% GB and 12% PF). A multi-scale characterization reveals that the higher-Mo/lower-Cr design not only increases the fraction of GB but also dramatically enhances its thermal stability. Importantly, Mo suppresses the premature precipitation of coarse (Nb, Ti)(C, N) in the parent austenite, thereby retaining solutes for the subsequent precipitation of a high number density of fine carbides within the bainitic ferrite laths during thermal exposure. The combination of a high-density dislocation substructure within the stable GB and the strong precipitation strengthening from these dispersed nano-carbides is primarily responsible for the superior strength retention in the higher-Mo/lower-Cr FR3 steel. This steel maintains a high yield strength of 440 MPa after 6 h at 600 °C, which is 158 MPa higher than its 0.25 wt.% Mo counterpart and satisfies the international fire-resistance criterion (YS600 °C/YSRT > 2/3). This work provides a mechanistic basis for optimizing Mo–Cr alloy design to achieve exceptional long-term high-temperature performance in FR steels.
Perovskite-layer structured (PLS) piezoelectric ceramics, featuring ultrahigh Curie temperatures (TC), high electrical resistivity (ρ), and excellent thermal stability, are promising for sensing above 600 °C. However, poor densifiability, difficult poling, and low piezoelectric coefficients (d33) still limit their practical applications. Focusing mainly on advances since 2018, this review critically evaluates recent progress in the structural understanding, compositional design, and processing of PLS ceramics. Improved control of lattice distortion, polarization switching, densification, and grain orientation has increased d33 to approximately 3–4 pC/N in Sr2Nb2O7-based ceramics and approximately 9 pC/N in La2Ti2O7-based systems. Nevertheless, in situ high-temperature piezoelectric data, long-term stability studies, and device-level validation remain limited. Further progress requires synergistic composition-processing optimization and standardized high-temperature evaluation to advance PLS ceramics toward practical ultrahigh-temperature sensing applications.
In this study, a series of reducible A2B2O7 composite oxides was synthesized using a simple citric acid sol-gel method. Different characterization techniques were employed to reveal the essential factors affecting the redox properties and surface chemically adsorbed oxygen species of these compounds. The results indicate that the redox properties of ionic oxides of the A-site and B-site influence the redox behavior of the corresponding A2B2O7 compounds, while the rigidity and looseness of the sub-crystal phase structure, as well as the lattice disorder degree, of these compounds affect the amount of surface chemisorbed oxygen. The type of sub-crystalline structure and lattice disorder are essential factors influencing the amount of chemisorbed oxygen species, with the order being disordered solid solution/disordered defect fluorite > pyrochlore > layered perovskite. Furthermore, it was found that the lattice disorder degree of Pr2Zr2O7 and Tb2Zr2O7 increases due to the complete reduction of Pr4+ or Tb4+ to Pr3+ and Tb3+ in a high-temperature reducing atmosphere (>600 °C). Similarly, disordered defect fluorite structures, such as Pr2Ce2O7 and Tb2Ce2O7, transform into rare-earth C-type crystalline structures in the same reducing atmosphere.
MgO–CaO refractories exhibit significant potential in the cement industry owing to their excellent refractoriness, robust resistance to alkaline corrosion, superior coating adherence, and high thermodynamic stability. Nevertheless, the intricate interfacial interaction mechanisms between these refractories and cement clinker at high temperature remain poorly understood. In this study, ab initio molecular dynamics (AIMD) simulations were employed to investigate the interactions between various crystallographic surfaces of CaO and the two primary mineral phases of cement clinker (tricalcium silicate, C3S, and dicalcium silicate, C2S). The results demonstrate that the CaO(100) surface maintained interfacial structural stability through ordered coordination and minimal charge transfer. Conversely, the CaO(111) surface exhibits high interfacial reactivity and surface degradation, driven by its unstable surface structure and strong interfacial polarization effects. The interfacial reactivity of the CaO(110) surface is intermediate, suggesting that preferential exposure of the CaO(110) surface may improve the kiln coating performance of the refractory, nevertheless, this implication remains a theoretical prediction and requires further experimental validation. These findings offer fundamental atomic-scale insights for the optimization of MgO–CaO refractories in industrial cement production.
