Fluorine-Free Fabrication of MXene via Photo-Fenton Approach for Advanced Lithium-Sulfur Batteries.

The mainstream synthesis method for MXene is using aqueous fluorine-containing acidic solutions to eliminate the A-element layers from their MAX phases. However, this strategy is environmentally hazardous and impairs the material performance (e.g., supercapacitor and Li-S batteries) owing to the presence of -F terminations. Herein, we exploit a low-temperature "soft chemistry" approach based on photo-Fenton (P.F.) reaction for the fabrication of F-free Ti3C2 (Ff-Ti3C2) with high purity of 95%. It is confirmed that the continuous generation of highly reactive oxygen species (HO• and O2•- radicals) during the P.F. reaction weakens the metallic Ti-Al bonds in the MAX phase and promotes the formation of high concentration OH- anions, which are conducive to the sequential topochemical deintercalation of Al layers. Moreover, the strengthened charge accumulation on the Ff-Ti3C2 surface creates rich electron "reservoirs" for actuating the Li-S chemistry, which not only strengthens the host-guest interactions but also propels the kinetics of the polysulfide conversion. Taking advantage of the superior mechanical robustness, better electrolyte wettability, and improved electrocatalytic activity, the resultant Ff-Ti3C2 can be used as an ideal sulfur host and Li-S chemistry mediator for advanced flexible Li-S batteries.

Hot Deformation Behavior of the 25CrMo4 Steel Using a Modified Arrhenius Model.

25CrMo4 steel is widely used in the manufacturing of high-speed train axles due to its excellent mechanical properties. The purpose of this study is to develop an accurate modified constitutive model to describe the hot deformation behavior of the steel. Isothermal compression experiments were performed at different strain rates (0.01, 0.1, 0.5, and 1 s-1) and different temperatures (950, 1000, 1050, and 1100 °C) using a Gleeble-3800 thermal simulator. The microstructure after hot deformation was observed by the electron backscatter diffraction (EBSD), and the effects of temperature and strain rate were analyzed. The results showed that the coupling effect of temperature and strain rate on the dislocation density led to the change in the shape of the true stress-strain curve and that dynamic recovery (DRV) and dynamic recrystallization (DRX) caused the macroscopic softening phenomenon, with DRX being the main mechanism. Based on the true stress-strain curves, the strain-compensated Arrhenius constitutive model was calibrated. To improve prediction ability, a modified Arrhenius constitutive model was proposed, in which the temperature and strain rate coupling correction functions were incorporated. The original, modified Arrhenius models were evaluated according to the absolute relative error (ARE), the average absolute relative error (AARE), and the correlation coefficient (R2). Compared with the original model, the modified Arrhenius model has a higher prediction accuracy, with the ARE value mostly below 4%, the AARE value of 1.91%, and the R2 value of 0.9958.