Study of Microstructure and Performance Evaluation of Zr-Sn-Nb Joints by Electron Beam Welding.

In this work, Zr-Sn-Nb alloy was joined by electron beam welding (EBW). A defect-free Zr-Sn-Nb joint with sound appearance was obtained. The grains in the weld zone (WZ) and heat-affected zone (HAZ) are significantly coarsened. The columnar grains with a maximum grain size of 0.5 mm are distributed in the upper region of the WZ, while the equiaxed grains are almost located in the bottom region of the WZ. The WZ is mainly composed of the dominant α-Zr, α'-Zr and a few ß phases. The grain orientation of WZ and HAZ is uniform, indicating that no obvious preferred orientation existed. Coarse grains and fine acicular α' phases increase the strength of the joint, but reduce the plasticity and toughness of the joint. The tensile strengths of the joints at room temperature (RT) and 375 °C were 438 MPa and 313 MPa, respectively. The RT impact energy of the joint is 18.5 J, which is only 58.3% of the BM. The high purity of the EBW process and unsignificant grain orientation minimizes damage to the corrosion resistance of Zr-Sn-Nb alloy joints. The corrosion weight gain of the joint specimen and the BM specimen were 12.91 mg/dm2 and 12.64 mg/dm2, respectively, and the thicknesses of the cross-section corrosion layer were 12-15 µm and 9-12 µm, respectively.

A mussel-pearl side chain interaction in mercury(II) and phenol removal by sulfur-functionalized covalent organic frameworks: A DFT study.

The covalent organic framework materials (COFs) with excellent chemical and physical characteristics have been rapidly developed as adsorbents in the application of environmental remediation. In the design of COFs, the selection of functional groups and side chains is of great significance. Herein, density function theory (DFT) method is used to illustrate the adsorption behavior and mechanism of three sulfur-functionalized COFs (S-COFs) for the adsorption of mercury(II) and phenol. According to the analysis of geometric configurations and electronic properties, it demonstrated that the side chains of S-COFs with high flexibility and concentrated sulfur-functional groups, acting like a closed mussel which tightly confined the contaminants, the highest adsorption was -24.32 kcal/mol. The adsorption mechanism of phenol and mercury(II) on S-COFs was elucidated. For phenol, hydrogen bonds and π-π stacking interaction played an important role in the adsorption process, while the coordination interaction was dominated for the adsorption of mercury(II). This research explains the importance of selecting appropriate functional groups and side chains for COFs in the removal of contaminants in the molecular scale, and reveals the great potential of COFs in environmental remediation applications.

Chelating effect between uranyl and pyridine N containing covalent organic frameworks: A combined experimental and DFT approach.

Covalent organic frameworks (COFs) are promising adsorbents for removing heavy metal ions, and have high crystallinity, a porous structure, and conjugated stability. N-containing functional groups are known to have great affinity for uranyl ions. In this work, to explore the peculiarity of the pyridine N structure as an efficient adsorbent, we chose 2,2'-dipyridine-5,5'-diamine (Bpy) and pyridine-2,5'-diamine (Py) as the core skeletons, and 1,3,5-triformylphloroglucinol (Tp) as the linker to synthesize two crystalline and stable N-containing COFs named TpBpy and TpPy, respectively, through a facile solvothermal method. Characterization results demonstrated that TpBpy and TpPy possessed regularly growing pore sizes, large specific surface areas and relatively strong thermal resistances. The results of batch experiments showed that both COF materials were capable of the effective removal of uranyl with uptake capacities of 115.45 mg g-1 and 291.79 mg g-1, respectively. In addition, density functional theory (DFT) simulations highlighted the beneficial chelation effect of the double N structure in pyridine monomers for removing uranyl ions. Combining systematic experimental and theoretical analyses, the adsorption process and interaction mode of porous COFs and UO22+ were revealed, to provide predictable support for the application of pyridine N-containing COFs in the field of environmental remediation.

Extremely stable amidoxime functionalized covalent organic frameworks for uranium extraction from seawater with high efficiency and selectivity.

Uranium extraction from seawater is of strategic significance for nuclear power generation. Amidoxime-based functional adsorbents play indispensable roles in the recovery of seawater uranium with high efficiency. Nevertheless, balancing the adsorption capacity and selectivity is challenging in the presence of complicated interfering ions especially vanadium. Herein, a polyarylether-based covalent organic framework functionalized with open-chain amidoxime (COF-HHTF-AO) was synthesized with remarkable chemical stability and excellent crystallinity. Impressively, the adsorption capacity of COF-HHTF-AO towards uranium in natural seawater reached up to 5.12 mg/g, which is 1.61 times higher than that for vanadium. Detailed computational calculations revealed that the higher selectivity for uranium over vanadium originated from the specific bonding nature and coordination pattern with amidoxime. Combining enhanced adsorption capacity, excellent selectivity and ultrahigh stability, COF-HHTF-AO serves as a promising adsorbent for uranium extraction from the natural seawater.

Adsorption Properties of Hydrated Cr3+ Ions on Schiff-base Covalent Organic Frameworks: A DFT Study.

Considering the superior physiochemical property, increasing efforts have been devoted to exploiting the covalent organic frameworks (COFs) materials on the environmental remediation of heavy metal ions. Water pollution caused by Cr3+ metal ions is of special concern for scientists and engineers. Notwithstanding all the former efforts made, it is surprising that very little is known about the interaction mechanisms between the hydrated Cr3+ metal ions and COF materials. In present context, density functional theory (DFT) method is used to elucidate geometric and electronic properties with the purpose of putting into theoretical perspective the application values and interaction mechanisms for COF materials on Cr3+ capture. The results showed that all the five selected Schiff-base COFs materials displayed good adsorption performance on Cr3+ removal while the phenazine-linked and imine-COFs possessed the most favorable adsorption capacity due to the optimal chemical units and frameworks. The hydration effect was found to play a two-side role in the adsorption process and interaction mechanisms, involving coordination, hydrogen bonds, as well as weak non-covalent interactions, have been illuminated to explain the observed different adsorption behaviors. This study provides a general guidance for the design and selection of efficient COF materials as high-capacity Cr3+ adsorbents.

Computational Study on the Excited-State Decay of 5-Methylcytosine and 5-Hydroxymethylcytosine: The Common Form of DNA Methylation and Its Oxidation Product.

5-Methylcytosine (5mC) is the predominant epigenetic modification of DNA. 5mC and its sequential oxidation product, 5-hydroxymethylcytosine (5hmC), are crucial epigenetic markers which have a profound impact on gene stability, expression, and regulation. In the present work, ab initio electronic structure computations were performed to investigate the excited-state decay pathways for 5mC and 5hmC in both the neutral and protonated forms. Based on the theoretical quantities, four nonradiative decay pathways via conical intersections (CIs) were identified: ring distortion, ring opening, N-H dissociation, and intersystem crossing (ISC) pathways. Additional calculated potential energy surfaces revealed that ring distortion and ISC pathways were the most effective routes for 5mC and 5hmC, respectively. The influence of environmental factors, such as the solution and an acidic environment, was also explored in this study. Our study demonstrated that excited-state decay pathways via CIs are indispensable for the photostability of DNA epigenetic modifications and may be involved in ingenome stability and mammalian development.

N, P, and S Codoped Graphene-Like Carbon Nanosheets for Ultrafast Uranium (VI) Capture with High Capacity.

The development of functional materials for the highly efficient capture of radionuclides, such as uranium from nuclear waste solutions, is an important and challenging topic. Here, few-layered N, P, and S codoped graphene-like carbon nanosheets (NPS-GLCs) that are fabricated in the 2D confined spacing of silicate RUB-15 and applied as sorbents to remove U(VI)ions from aqueous solutions are presented. The NPS-GLCs exhibit a large capacity, wide pH suitability, an ultrafast removal rate, stability at high ionic strengths, and excellent selectivity for U(VI) as compared to multiple competing metal ions. The 2D ultrathin structure of NPS-GLCs with large spacing of 1 nm not only assures the rapid mass diffusion, but also exposes a sufficient active site for the adsorption. Strong covalent bonds such as P-O-U and S-O-U are generated between the heteroatom (N, P, S) with UO2 2+ according to X-ray photoelectron spectroscopy analysis and density functional theory theoretical calculations. This work highlights the interaction mechanism of low oxidation state heteroatoms with UO2 2+, thereby shedding light on the material design of uranium immobilization in the pollution cleanup of radionuclides.