Xiaosong Zhou

Effects of carbon nanotubes on the dehydrogenation behavior of magnesium hydride at relatively low temperatures

Dehydrogenation in MgH2 was found to lead to a transition from an incubation stage controlled by nucleation to an accelerated stage controlled by the movement of the interface. Addition of carbon nanotubes destabilized Mg–H bonding and reduced the energy barrier for nucleation, which is mainly related to the electron affinity of their curved surface.

First-Principles Study of the Structural Stability and Electronic and Elastic Properties of Helium in α-Zirconium

First-principles calculations within density functional theory have been performed to investigate the behaviors of helium in α-zirconium. The most favorable interstitial site for He in -Zr is not an ordinary tetrahedral or octahedral site, but a basal octahedral site with a formation energy as low as 2.40 eV. The formation energy reduces to 1.25 eV in the presence of preexisting vacancies. The analysis on the density of states and the charge density has been carried out. In addition, the influences of He and small He-V complexes on the elastic properties have been studied. The He-V complexes have been found to greatly affect the elastic properties compared with He alone.

Ti3C2 MXene: a promising microwave absorbing material

In this work, we demonstrate the enhancement of microwave attenuation capability of Ti3C2 enabled microwave absorbing materials (MAMs) within a frequency range of 2–18 GHz. Ti3C2 nano-sheet/paraffin composites exhibit enhanced microwave absorbing performance with an effective absorbing bandwidth of 6.8 GHz (11.2–18 GHz) at 2 mm and an optimal reflection loss of −40 dB at 7.8 GHz. Moreover, mechanisms for the dielectric responses of the Ti3C2 MXene nanosheets are intensively discussed. Three typical electric polarizations of Ti3C2 are illustrated with the Cole–Cole diagram. The enhanced microwave absorbing properties can be ascribed to the high dielectric loss accompanied with the strong multi-reflections between MXene layers.

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.

Progress of Helium Evolution in Aging Titanium Tritide Film

Abstract Helium release from titanium tritide films at room temperature have been studied. The evolution of lattice defects in long-aged titanium tritide films is also investigated by X-ray diffraction (XRD) over a period of about 1600 days (>4 years). And the thermal desorption (TD) has been used to investigate the 3He release from titanium tritide film with 3He/Ti atom ratio from 0.006 to 0.325. Results of XRD, TD and helium release were synthesized. A continuum-scale evolutionary model of helium for aging titanium tritide film is described which accounts for major features of the tritide experiment data. The combined stress-assisted-block loop punching growth for random bubble arrays and an average ligament stress criterion predicts an onset of inter-bubble fracture in good agreement with the He/Ti ratio observed for rapid He release.

Analytical Bond-order Potential for hcp‐Y

The lattice parameters, elastic constants, cohesive energy, structural energy differences, as well as the properties of point defects and planar defects of hexagonal close‐packed yttrium (hcp‐Y) have been studied with ab initio density functional theory for constructing an extensive database. Based on an analytical bond-order potential scheme, empirical many‐body interatomic potential for hcp‐Y has been developed. The model is fitted to some properties of Y, e.g., the lattice parameters, elastic constants, bulk modulus, cohesive energy, vacancy formation energy, and the structural energy differences. The present potential has ability to reproduce defect properties including the self‐interstitial atoms formation energies, vacancy formation energy, divacancy binding energy, as well as the bulk properties and the thermal dynamic properties.

KD-S SiCf/SiC composites with BN interface fabricated by polymer infiltration and pyrolysis process

Continuous silicon carbide fiber reinforced silicon carbide matrix (SiCf/SiC) composites are attractive candidate materials for aerospace engine system and nuclear reactor system. In this paper, SiCf/SiC composites were fabricated by polymer infiltration and pyrolysis (PIP) process using KD-S fiber as the reinforcement and the LPVCS as the precursor, while the BN interface layer was introduced by chemical vapor deposition (CVD) process using borazine as the single precursor. The effect of the BN interface layer on the structure and properties of the SiCf/SiC composites was comprehensively investigated. The results showed that the BN interface layer significantly improved the mechanical properties of the KD-S SiCf/SiC composites. The flexure strength and fracture toughness of the KD-S SiCf/SiC composites were evidently improved from 314±44.8 to 818±39.6 MPa and 8.6± 0.5 to 23.0±2.2 MPa·m1/2, respectively. The observation of TEM analysis displayed a turbostratic structure of the CVD-BN interface layer that facilitated the improvement of the fracture toughness of the SiCf/SiC composites. The thermal conductivity of KD-S SiCf/SiC composites with BN interface layer was lower than that of KD-S SiCf/SiC composites without BN interface layer, which could be attributed to the relative low thermal conductivity of BN interface layer with low crystallinity.

Electromagnetic interference shielding performance of nano-layered Ti3SiC2 ceramics at high-temperatures

The X-band electromagnetic interference (EMI) shielding properties of nano-layered Ti3SiC2 ceramics were evaluated from room temperature up to 800°C in order to explore the feasibility of Ti3SiC2 as efficient high temperature EMI shielding material. It was found that Ti3SiC2 exhibits satisfactory EMI shielding effectiveness (SE) close to 30 dB at room temperature and the EMI SE shows good temperature stability. The remarkable EMI shielding properties of Ti3SiC2 can be mainly attributed to high electrical conductivity, high dielectric loss and more importantly the multiple reflections due to the layered structure.

Fabrication processing effects on the microstructure and morphology of erbium film

The effect of substrate temperature on the microstructure and the morphology of erbium film are systematically investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). All the erbium films are grown by electron-beam vapor deposition (EBVD). A novel preparation method for observing the cross-section morphology of the erbium film is developed. The films deposited at 200 °C have (002) preferred orientation, and the films deposited at 450 °C have a mixed (100) and (101) texture, due to the different growth mechanisms of surface energy minimization and recrystallization, respectively. The peak positions and the full widths at half maximum (FWHMs) of erbium diffraction lines (100), (002), and (101) shift towards higher angles and decrease with the increasing substrate temperature in a largely uniform manner, respectively. Also, the lattice constants decrease with increasing temperature. The transition in the film stresses can be used to interpret the changes in peak positions, FWHMs, and lattice constants. The stress is compressive for the as-growth films, and is counteracted by the tensile stress formed during the process of temperature cooling to room temperature. The tensile stress mainly originates from the difference in the coefficients of thermal expansion of the substrate-film couple.

Dependences of microstructure on electromagnetic interference shielding properties of nano-layered Ti 3 AlC 2 ceramics

The microstructure dependent electromagnetic interference (EMI) shielding properties of nano-layered Ti3AlC2 ceramics were presented in this study by comparing the shielding properties of various Ti3AlC2 ceramics with distinct microstructures. Results indicate that Ti3AlC2 ceramics with dense microstructure and coarse grains are more favourable for superior EMI shielding efficiency. High EMI shielding effectiveness over 40u2009dB at the whole Ku-band frequency range was achieved in Ti3AlC2 ceramics by microstructure optimization, and the high shielding effectiveness were well maintained up to 600u2009°C. A further investigation reveals that only the absorption loss displays variations upon modifying microstructure by allowing more extensive multiple reflections in coarse layered grains. Moreover, the absorption loss of Ti3AlC2 was found to be much higher than those of highly conductive TiC ceramics without layered structure. These results demonstrate that nano-layered MAX phase ceramics are promising candidates of high-temperature structural EMI shielding materials and provide insightful suggestions for achieving high EMI shielding efficiency in other ceramic-based shielding materials.