Browsing by Author "Wu, Hsin-Jay"

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Designing Environmentally Friendly High-zT Zn4Sb3 via Thermodynamic Routes
(2019-10-01)
Wu, Hsin-Jay
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Wei, Pai-Chun
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Su, Hui-Yi
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Wang, Kuang-Kuo
;
Yen, Wan-Ting
Zn4Sb3-based compounds constitute a lead-free material family with a best thermoelectric figure of merit (zT) in the midtemperature range. Unlike being a stoichiometric compound, the mutual solubility of Zn and Sb elicits rich microstructures and the structural instability of Zn4Sb3. The solubility limit and neighboring phase relations are crucial for the delicate balance between the thermoelectric performance and phase stability of Zn4Sb3. In this work, we constructed the isothermal section of the Zn-Sb-In ternary phase diagram, built the zT map near the beta-Zn4Sb3 phase region, and showed that the formation of multiscale microstructures has a profound impact on the electronic and phonon transport properties and phase stability. A high-zT zone was identified near the phase boundary between the two-phase InSb + Zn5Sb4In2 and the three-phase Zn4Sb4+InSb+ Zn5Sb4In2 regions. A sample with a nominal composition of Zn-3.8 In0.2Sb3 exhibited an ultralow kappa(L) of 0.2 (W m(-1) K-1), an enhanced PF of 1.75 (mW m(-1) K-2), and a remarkable zT value of 1.8 at 698 K. These state-of-the-art thermoelectric properties were attributed to the simultaneous enhancement in phonon scattering and the carrier energy-filtering effect in a unique hierarchical microstructure, in which InSb nanoprecipitates are dispersed in Zn5Sb4In2 coarse grains, and the latter are embedded in the host matrix In-Zn4Sb3. These results opened an avenue for environmentally friendly cost-effective midtemperature thermoelectric materials.
Development of Novel Lightweight Dual-Phase Al-Ti-Cr-Mn-V Medium-Entropy Alloys with High Strength and Ductility
(2020-01-01)
Liao, Yu-Chin
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Chen, Po-Sung
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Li, Chao-Hsiu
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Tsai, Pei-Hua
;
Jang, Jason S. C.
A novel lightweight Al-Ti-Cr-Mn-V medium-entropy alloy (MEA) system was developed using a nonequiatiomic approach and alloys were produced through arc melting and drop casting. These alloys comprised a body-centered cubic (BCC) and face-centered cubic (FCC) dual phase with a density of approximately 4.5 g/cm(3). However, the fraction of the BCC phase and morphology of the FCC phase can be controlled by incorporating other elements. The results of compression tests indicated that these Al-Ti-Cr-Mn-V alloys exhibited a prominent compression strength (1940 MPa) and ductility (30%). Moreover, homogenized samples maintained a high compression strength of 1900 MPa and similar ductility (30%). Due to the high specific compressive strength (0.433 GPag/cm(3)) and excellent combination of strength and ductility, the cast lightweight Al-Ti-Cr-Mn-V MEAs are a promising alloy system for application in transportation and energy industries.
Enhancing thermoelectric performance by Fermi level tuning and thermal conductivity degradation in (Ge1-xBix)Te crystals
(2019-06-13)
Wei, Pai-Chun
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Cai, Cheng-Xun
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Hsing, Cheng-Rong
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Wei, Ching-Ming
;
Yu, Shih-Hsun
In this work, a high thermoelectric figure of merit, zT of 1.9 at 740 K is achieved in Ge1-xBixTe crystals through the concurrent of Seebeck coefficient enhancement and thermal conductivity reduction with Bi dopants. The substitution of Bi for Ge not only compensates the superfluous hole carriers in pristine GeTe but also shifts the Fermi level (E-F) to an eligible region. Experimentally, with moderate 6-10% Bi dopants, the carrier concentration is drastically decreased from 8.7 x 10(20) cm(-3) to 3-5 x 10(20) cm(-3) and the Seebeck coefficient is boosted three times to 75 mu VK-1. In the meantime, based on the density functional theory (DFT) calculation, the Fermi level E-F starts to intersect with the pudding mold band at L point, where the band effective mass is enhanced. The enhanced Seebeck coefficient effectively compensates the decrease of electrical conductivity and thus successfully maintain the power factor as large as or even superior than that of the pristine GeTe. In addition, the Bi doping significantly reduces both thermal conductivities of carriers and lattices to an extremely low limit of 1.57W m(-1)K(-1) at 740 K with 10% Bi dopants, which is an about 63% reduction as compared with that of pristine GeTe. The elevated figure of merit observed in Ge1-xBixTe specimens is therefore realized by synergistically optimizing the power factor and downgrading the thermal conductivity of alloying effect and lattice anharmonicity caused by Bi doping.
From Dislocation to Nano-Precipitation: Evolution to Low Thermal Conductivity and High Thermoelectric Performance inn-Type PbTe
(1970-01-01)
Deng, Ping-Yuan
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Wang, Kuang-Kuo
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Du, Jia-Yu
;
Wu, Hsin-Jay
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材料科學與工程學系
PbTe-based alloys have been widely used as mid-temperature thermoelectric (TE) materials since the 1960s. Years of endeavor spurred the tremendous advances in their TE performance. The breakthroughs forn-type PbTe have been somewhat less impressive, which limits the overall conversion efficiency of a PbTe-based TE device. In light of this obstacle, ann-type Ga-doped PbTe via an alternative thermodynamic route that relies on the equilibrium phase diagram and microstructural evolution is revisited. Herein, a plateau ofzT= 1.2 is achieved in the best-performing Ga0.02Pb0.98Te in the temperature range of 550-673 K. Notably, an extremely high averagezT(ave)= 1.01 is obtained within 300 - 673 K. The addition of gallium optimizes the carrier concentration and boosts the power factorPF = S-2 rho(-1). Meanwhile, the kappa(L)of Ga-PbTe reveals a significantly decreasing tendency owing to the defect evolution that changes from dislocation loop to nano-precipitation with increasing Ga content. The pathway for both the kappa(L)reduction and defect evolution can be probed by an equilibrium phase diagram, which opens up a new avenue for locating highzTTE materials.
Thermodynamic Routes to Ultralow Thermal Conductivity and High Thermoelectric Performance
(1970-01-01)
Wei, Pai-Chun
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Liao, Chien-Neng
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Wu, Hsin-Jay
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Yang, Dongwang
;
He, Jian
Thermoelectric (TE) research is not only a course of materials by discovery but also a seedbed of novel concepts and methodologies. Herein, the focus is on recent advances in three emerging paradigms: entropy engineering, phase-boundary mapping, and liquid-like TE materials in the context of thermodynamic routes. Specifically, entropy engineering is underpinned by the core effects of high-entropy alloys; the extended solubility limit, the tendency to form a high-symmetry crystal structure, severe lattice distortions, and sluggish diffusion processes afford large phase space for performance optimization, high electronic-band degeneracy, rich multiscale microstructures, and low lattice thermal conductivity toward higher-performance TE materials. Entropy engineering is successfully implemented in half-Huesler and IV-VI compounds. In Zintl phases and skutterudites, the efficacy of phase-boundary mapping is demonstrated through unraveling the profound relations among chemical compositions, mutual solubilities of constituent elements, phase instability, microstructures, and resulting TE properties at the operation temperatures. Attention is also given to liquid-like TE materials that exhibit lattice thermal conductivity at lower than the amorphous limit due to intensive mobile ion disorder and reduced vibrational entropy. To conclude, an outlook on the development of next-generation TE materials in line with these thermodynamic routes is given.
Thermoelectric Figure-of-Merit of Fully Dense Single-Crystalline SnSe
(2019-03-01)
Wei, Pai-Chun
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Bhattacharya, Sriparna
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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.
Titanium-based thin film metallic glass as diffusion barrier layer for PbTe-based thermoelectric modules
(2019-01-01)
Yu, Chia-Chi
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Wu, Hsin-Jay
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Agne, Matthias T.
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Witting, Ian T.
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Deng, Ping-Yuan
The thin film metallic glass (TFMG) is an effective diffusion barrier layer for PbTe-based thermoelectric (TE) modules. Reaction couples structured with Cu/TFMG/PbTe are prepared via sputter-deposition and are annealed at 673 K for 8-96 h. The transmission line method is adopted for the assessment of electrical contact resistivity upon the PbTe/TFMG, and the value remains in the range of 3.3-2.5 x 10(-9) (Omega m(2)). The titanium-based TFMG remains amorphous upon annealing at 673 K for 48 h and effectively blocks the inter-diffusion by not having grain-boundaries, which only allows the bulk diffusion between the metal electrode and the TE substrate. (C) 2019 Author(s).