Browsing by Author "Li, Wen-Hsien"

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Confined martensitic phase transformation kinetics and lattice dynamics in Ni-Co-Fe-Ga shape memory alloys
(2016-05-15)
Cong, Daoyong
;
Rule, Kirrily Clair
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Li, Wen-Hsien
;
Lee, Chi-Hung
;
Zhang, Qinghua
Insights into the phase transformation kinetics and lattice dynamics associated with the newly discovered confined martensitic transformation are of great significance to the in-depth understanding of the phase transformation behavior responsible for the rich new physical phenomena in shape memory alloys and could shed light on the design of novel multifunctional properties through tuning the confined martensitic transformation. Here, we employ in-situ elastic and inelastic neutron scattering techniques to reveal the characteristics of transformation kinetics and lattice dynamics in two Ni55-xCoxFe18Ga27 magnetic shape memory alloys with x = 5.5 and 6.5. With a remarkable difference in transformation temperature, these two alloys show similar vibrational properties. Phonon softening of the TA(2)[xi xi 0] phonon branch, manifested as a dip at xi similar to 033 on the phonon dispersion curves, was disclosed by inelastic neutron scattering experiments; but this phonon softening is much less pronounced than that observed in stoichiometric Ni2MnGa or near-stoichiometric Ni-Fe-Ga Heusler alloys. The dispersion curves for the TA(2)[xi xi 0] phonon branch are only weakly dependent on temperature. The less pronounced phonon softening and very weak temperature dependence of the phonon dispersion curves could be attributed to the martensitic transformation which is confined by local inhomogeneities. This confined martensitic transformation shows a sluggish nature and occurs in a broad temperature range. The sluggish transformation was evidenced by the existence of residual austenite at low temperatures far below the martensitic transformation start temperature. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Evidence of High-Temperature Superconductivity at 18 K in Nanosized Rhombohedral Bi Enhanced by Ni-Doping
(2019-03-01)
Lee, Chi-Hung
;
Lin, Ken-Ming
;
Tang, Yu-Hui
;
Wu, Bo-Yong
;
Ma, Ma-Hsuan
Superconductivity in bulk rhombohedral Bi has recently been detected to appear below 0.53 mK and 5.2 mu T. Here, we unambiguously demonstrate that superconductivity in rhombohedral Bi can be greatly enhanced by incorporating Ni ions onto the Bi sites and reducing the size to the nanometer scale. The superconducting transition temperature T-C of 12 nm rhombohedral Bi nanoparticles (NPs) reaches 4 K at ambient pressure. T-C is significantly enhanced to reach 7, 12, and 18 K in 6, 8, and 10% Ni-doped Bi NPs, respectively, where superconductivity is found to coexist with ferromagnetism. Ni-doping causes a significant amount of electronic charges to shift toward the interconnecting regions between neighboring Bi ions. First-principles calculations reveal that the Ni ions serve as charge and spin suppliers for the developments of superconductivity and ferromagnetism.
Thermoelectric Figure-of-Merit of Fully Dense Single-Crystalline SnSe
(2019-03-01)
Wei, Pai-Chun
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Bhattacharya, Sriparna
;
Liu, Yu-Fei
;
Liu, Fengjiao
;
He, Jian
Single-crystalline SnSe has attracted much attention because of its record high figure-of-merit ZT approximate to 2.6; however, this high ZT has been associated with the low mass density of samples which leaves the intrinsic ZT of fully dense pristine SnSe in question. To this end, we prepared high-quality fully dense SnSe single crystals and performed detailed structural, electrical, and thermal transport measurements over a wide temperature range along the major crystallographic directions. Our single crystals were fully dense and of high purity as confirmed via high statistics Sn-119 Mossbauer spectroscopy that revealed < 0.35 at. % Sn(IV) in pristine SnSe. The temperature-dependent heat capacity (C-p) provided evidence for the displacive second-order phase transition from Pnma to Cmcm phase at T-c approximate to 800 K and a small but finite Sommerfeld coefficient gamma(0) which implied the presence of a finite Fermi surface. Interestingly, despite its strongly temperature-dependent band gap inferred from density functional theory calculations, SnSe behaves like a low-carrier-concentration multiband metal below 600 K, above which it exhibits a semiconducting behavior. Notably, our high-quality single-crystalline SnSe exhibits a thermoelectric figure-of-merit ZT similar to 1.0, similar to 0.8, and similar to 0.25 at 850 K along the b, c, and a directions, respectively.