Browsing by Author "Wei, Pai-Chun"

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Designing Environmentally Friendly High-zT Zn4Sb3 via Thermodynamic Routes
(2019-10-01)
Wu, Hsin-Jay
;
Wei, Pai-Chun
;
Su, Hui-Yi
;
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.
Enhancing thermoelectric performance by Fermi level tuning and thermal conductivity degradation in (Ge1-xBix)Te crystals
(2019-06-13)
Wei, Pai-Chun
;
Cai, Cheng-Xun
;
Hsing, Cheng-Rong
;
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.
The reduction of antiphase boundary defects by the surfactant antimony and its application to the III-V multi-junction solar cells
(2016-01-01)
Tong, Shih-chang
;
Wang, Jyh-Shyang
;
Wu, Chii-Bin
;
Wu, Chih-Hung
;
Chang, Jung-Ting
The CuPtB ordering and the antiphase boundaries (APBs) in GaInP2 epifilms were found to be diminished by the addition of the surfactant antimony (Sb) during the epitaxy. This is evidenced by the disappearance of the APBs related deep level peak in the Raman and photoluminescence (PL) spectra, as well as the significantly increased lifetime of the minority carriers. The GaInP2/metamorphic (MM)-GaInAs/Ge multi-junction solar cells made by this method possess enhanced short circuit current density and show a narrower distribution of the open circuit voltage. We believe the use of surfactant Sb to control APBs during the growth of InGaP2 would be very useful for designing GaInP2-based devices, such as solar cells, laser diodes and light-emitting diodes. (C) 2015 Elsevier B.V. All rights reserved.
Thermodynamic Routes to Ultralow Thermal Conductivity and High Thermoelectric Performance
(1970-01-01)
Wei, Pai-Chun
;
Liao, Chien-Neng
;
Wu, Hsin-Jay
;
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
;
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.