Xin Ju

High resolution characterization of modifications in fused silica after exposure to low fluence 355 nm laser at different repetition frequencies.

We report on the characterization of modifications in fused silica after exposure to low fluence (2 J/cm2) 355 nm laser at repetition frequencies of 1 Hz, 5 Hz and 10 Hz. Synchrotron based XRF spectroscopy is employed to study concentration variation of metal inclusions in the surface layer. Positron annihilation lifetime spectroscopy is used to probe atomic size defects variation in bulk silica. FT-IR is used to characterize changes of bond length and angle of Si-O-Si covalent bond of irradiated silica. Compared to the basic frequency, the big loss of cerium and iron concentration, the size enlargement of vacancy cluster and the decrease of Si-O-Si covalent bond length after 10 Hz laser irradiation are illustrated by our data. These tiny modifications provide important data to investigate laser damage mechanism.

Polymorphic crystals selected in the nucleation stage

Molecular dynamics simulations are used to explore the atomic mechanism of formation of polymorphic crystals. Cooling the Lennard-Jones systems, we observe that the system almost always evolves into a polymorphic crystal with either fivefold-symmetric stacking faults or single-direction stacking faults. The detailed analysis reveals that such an evolution depends on the configuration of fcc/hcp concomitance in the nucleation stage. A defect-induced model is then introduced to illustrate these two evolution routes. Through calculating the formation energies of the defective critical nuclei, we find that the polymorphic crystals seem to be determined by their critical nuclei, in which the relatively lower formation energy ensures the preponderance of the fivefold-symmetric cluster. Before the nucleation, we observe that thermal fluctuations prefer hcp-like particles over fcc-like ones while in the nucleation and growth stage this preference reverses. Notably, an extended step rule of Ostwald is seemingly suitable to characterise the growth process because of the temporary hcp layers appearing among fcc layers in the growth stage. Although the crystalline cluster with single-direction stacking faults has higher growth rate and structural order than its competitor, the component (fcc and hcp) proportion of the final crystals is almost always constant regardless of the polymorphic type. Our finding renews the understanding of the polymorphism of crystals, and possibly draws more attention of people intending to control the polymorphic structures through nucleation.

Rodlike CeO2/carbon nanocomposite derived from metal–organic frameworks for enhanced supercapacitor applications

The rod-shaped CeO2@C architectures have been synthesized by novel and facile methods initially from metal–organic frameworks (MOFs). The MOF-derived material showed a pseudocapacitance of 1102 F g−1 at 2 A g−1 and 418 F g−1 at a high current density of 20 A g−1 and had excellent cyclic stability of about 2.3% decrease after 5000 cycles. Results revealed that favorable effects of dispersed partially graphitic carbon in the CeO2@C composite could improve electronic conductivity, which made this product be an alternative high-performance electrode for supercapacitors.

Energetics of He and H Atoms in W–Ta Alloys: First-Principle Calculations

Properties of various defects of He and H atoms in W?Ta alloys are investigated based on density functional theory. The tetrahedral interstitial site is the most configured site for self-interstitial He and H in W and W?Ta alloys. Only a single He atom favors a substitutional site in the presence of a nearby vacancy. However, in the coexistence of He and H atoms in the presence of the vacancy, the single H atom favors the tetrahedral interstitial site (TIS) closest to the vacancy, and the He atom takes the vacancy center. The addition of Ta can reduce the formation energy of TIS He or H defects. The substituted Ta affects the charge density distribution in the vicinity of the He atom and decreases the valence electron density of the H atoms. A strong hybridization of the H s states and the nearest W d state s exists in W 53 He 1 H 1 structure. The sequence of the He p projected DOS at the Fermi energy level is in agreement with the order of the formation energy of the He?H pair in the systems.

Clustering and Dissolution of Small Helium Clusters in Bulk Tungsten

Density functional theory calculations are conducted to investigate the stability and interactions among small helium (He) clusters in bulk tungsten (W). The lowest energy structure of each cluster for sizes n = 1 to 6 is determined. With the formation of He clusters, He defects form in bulk W. The thermodynamics of the clusters are investigated in the temperature range of 1000–2300K using molecular dynamics. This study provides the information essential to understand small He cluster behavior in bulk W.

Synergistic effect of Li–Ti and K–Ti co-doping on the dehydrogenation properties of NaAlH4: an ab initio study

The synergistic effect of Li–Ti and K–Ti dopants on the energy, structure and electronic properties of NaAlH4 has been investigated using density functional theory and ab initio molecular dynamics. Pair distribution functional analyses show that the substitution of NaAlH4 by Li2Ti and K2Ti favor not only the release of H2 but also the reductions in the barrier energies of (AlH4)− units. Density of states analyses suggest that the weakening of Al–H bonds in Li2Ti- and K2Ti-doped NaAlH4 is attributed to the formation of Ti–Al and Ti–H phases. Bader atomic charges analyses imply that the strong interaction of Li–Al prevent the further formation of Ti–Al clusters. The results of H removal energies analyses reveal that both Li2Ti and K2Ti are favorable catalysts for NaAlH4. Thus, that a Li2Ti dopant, with lighter weight than K2Ti, can be applied as a favorable catalyst for NaAlH4 has been given for the first time.