The formation of nano-scale clusters (nanoclusters) prior to the precipitation of the strengthening β" phase significantly influences the two-step aging behavior of Al-Mg-Si alloys. In this work, two types of nanoclusters are found to be formed at different temperatures. The characterization of these two nanoclusters has been performed from the viewpoints of composition and thermal stability using a three-dimensional atom probe (3DAP) and differential scanning calorimetry (DSC). Mg-Si co-clusters formed at room temperature (RT), Cluster(1), play a deleterious role in the subsequent formation of the β" phase because of the high thermal stability even at the bake-hardening (BH) temperature of 443K. In contrast, the nanoclusters formed by pre-aging at 373K, Cluster(2), are effective in the formation of the refined β", suggesting that Cluster(2) transforms more easily into the β" phase than Cluster(1). The quantitative estimation of the chemical compositions of the two nanoclusters suggests that the Mg/Si ratio is one of the key factors in addition to the internal structures consisting of Si, Mg and probably vacancies. The detailed two-step aging mechanism in Al-Mg-Si alloys is proposed based on the characteristics of the two types of nanoclusters.

Halogen-doped carbon nanoparticles (CNPs) were synthesized by a simple one-step solution plasma process at room temperature using a mixture of benzene (C₆H₆) and organics containing halogen atoms as the precursors (i.e., hexafluorobenzene (C₆F₆), hexachlorobenzene (C₆Cl₆), and hexabromobenzene (C₆Br₆)). The experimental results demonstrated that halogen doping, especially F and Cl, could lead to more efficient removal of residual hydrogen compared to carbon synthesized with pure benzene. This phenomenon was related to the different binding energies between hydrogen and halogens to form hydrogen halides. Their crystallinity and morphology did not change and remained the same as non-doped carbon. The electrochemical evaluation of oxygen reduction reaction (ORR) activity in an alkaline solution revealed that halogen doping did not play a significant role in shifting the onset potential for the ORR, while a slight enhancement in diffusion limited current density was observed at high overpotentials. Moreover, the electron transfer number involved in the ORR process determined from the Koutecky–Levich plot at −0.6 V was found to increase for halogen-doped carbons in the following order: F-CNPs > Br-CNPs > Cl-CNPs > CNPs. The improved ORR performance of F-CNPs could reasonably be attributed to the synergistic effects of specific bonding states between the halogen and carbon, structural defects and surface functional groups. Our results confirmed the validity of using halogen doping to improve the ORR catalytic activity of CNPs.