Browsing by Author "Wei, X"

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    Hidden amorphous phase and reentrant supercooled liquid in Pd-Ni-P metallic glasses
    (International Conference on Neutron Scattering, 2017-07-12) Lan, S; Ren, Y; Wei, X; Wang, B; Gilbert, EP; Shibayama, T; Watanabe, S; Ohnuma, M; Wang, XL
    An anomaly in differential scanning calorimetry has been reported in a number of metallic glass materials in which a broad exothermal peak was observed between the glass and crystallization temperatures. The mystery surrounding this calorimetric anomaly is epitomized by half century long studies of Pd-Ni-P metallic glasses, arguably the best glass-forming alloys [1]. Here we show,using a suite of in-situ experimental techniques, including simultaneous small-angle neutron scattering-calorimetry, high-energy X-ray diffraction, and electron microscopy, that Pd-Ni-P alloys have a hidden amorphous phase in the supercooled liquid region. The anomalous exothermal peak is the consequence of a polyamorphous phase transition between two supercooled liquids, involving a change in the packing of atomic clusters over medium-range length scales as large as 18Å. With further temperature increase,the alloy reenters the supercooled liquid phase which forms the room-temperature glass phase upon quenching. The outcome of this study raises a possibility to manipulate the structure and hence the stability of metallic glasses through heat-treatment.
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    Simulation of competitive and cooperative egress movements on the crowd emergency evacuation
    (Elsevier, 2021-05-01) Cao, RF; Lee, EWM; Yuen, ACY; Chen, TBY; Cordeiro, IMDC; Shi, M; Wei, X; Yeoh, GH
    Empirical evidence suggests that evacuees tend to break down their well-coordinated motion and adopt competitive egress behaviour when they are confronting life-endangering situations (e.g., fire accident, earthquake). This phenomenon significantly influences crowd dynamics, and subsequently, the overall evacuation time. In this paper, a novel evacuation model has been developed, which takes the detailed cooperative and competitive egress movements of evacuees into consideration. The proposed model has been conducted to simulate crowd (i.e., with different proportion of competitive individuals within the crowd) evacuating from a single-exit compartment room (i.e., the exit width varies from 0.8 to 2.0m). Simulation results show that competitive evacuees exhibit an arc-shape formation in front of the bottleneck area (i.e., the exit). This is notably distinct from the cooperative evacuees generated shape formation. In addition, the evacuation performance (i.e., evacuation time, trajectories, density and flow rate) of competitive and cooperative evacuees over time are successfully captured for analysis. This study contributes to a better understanding of the crowd evacuation dynamics by considering the impact of competitive and cooperative egress movements. © 2021 Elsevier B.V.
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    Symmetry-mode analysis for intuitive observation of structure–property relationships in the lead-free antiferroelectric (1− x) AgNbO3–xLiTaO3
    (International Union of Crystallography, 2019-06-21) Lu, T; Tian, Y; Studer, AJ; Narayanan, N; Li, Q; Withers, RL; Jin, L; Mendez-Gonzalez, Y; Pelaiz-Barranco, A; Yu, DH; McIntyre, GJ; Xu, Z; Wei, X; Yan, H; Liu, Y
    Functional materials are of critical importance to electronic and smart devices. A deep understanding of the structure–property relationship is essential for designing new materials. In this work, instead of utilizing conventional atomic coordinates, a symmetry-mode approach is successfully used to conduct structure refinement of the neutron powder diffraction data of (1−x)AgNbO3–xLiTaO3 (0 ≤ x ≤ 0.09) ceramics. This provides rich structural information that not only clarifies the controversial symmetry assigned to pure AgNbO3 but also explains well the detailed structural evolution of (1−x)AgNbO3–xLiTaO3 (0 ≤ x ≤ 0.09) ceramics, and builds a comprehensive and straightforward relationship between structural distortion and electrical properties. It is concluded that there are four relatively large-amplitude major modes that dominate the distorted Pmc21 structure of pure AgNbO3, namely a Λ3 antiferroelectric mode, a T4+ a−a−c0 octahedral tilting mode, an H2 a0a0c+/a0a0c− octahedral tilting mode and a Γ4− ferroelectric mode. The H2 and Λ3 modes become progressively inactive with increasing x and their destabilization is the driving force behind the composition-driven phase transition between the Pmc21 and R3c phases. This structural variation is consistent with the trend observed in the measured temperature-dependent dielectric properties and polarization–electric field (P-E) hysteresis loops. The mode crystallography applied in this study provides a strategy for optimizing related properties by tuning the amplitudes of the corresponding modes in these novel AgNbO3-based (anti)ferroelectric materials. © International Union of Crystallography - Open Access
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    The symmetry-mode decomposition for better understanding of the structural evolution presented in polar functional materials
    (Society of Crystallographers in Australia and New Zealand, 2017-12-03) Lu, T; Tian, Y; Studer, AJ; Withers, RL; Wei, X; Yu, DH; Liu, Y
    The phase and structure evolution of the (1-x)AgNbO3-xLiTaO3 solid solution is investigated by the neutron diffraction, dielectric and ferroelectric measurements. The symmetry-mode decomposition of the distorted AgNbO3 structure defined on the experimental space group, Pmc21 has been conducted. The four main modes, T4+, H2, Λ3 and Γ4-, exhibit large distorted amplitude to stabilise the Pmc21 structure. The mode refinement with referring to the Pmc21 was adopted to (1-x)AgNbO3-xLiTaO3 material system. It is found that with the increasing LiTaO3 concentration, the orthorhombic phase partially transfers to the rhombohedral R3c phase and the fraction of the R3c phase gradually increases. Correspondingly, the mode amplitudes of the H2 and Λ3 drop abruptly. The hidden structural correlation between H2 and Λ3 modes facilitates the understanding of the antiferroelectric nature observed in the AgNbO3. The variation of the main modes rationally bridges the Pmc21 and R3c phases, revealing the underlying phase transition mechanism of these two phases. Additionally, the evolution of the R3c phase fraction and corresponded mode amplitude in both Pmc21 and R3c phases provides a clear picture to explain the additional peak observed in the temperature-dependent dielectric spectra and composition-dependent polarisation-electric field hysteresis loops.