Soonil Lee

Some fungal endophytes from vegetable crops and their anti‐oomycete activities against tomato late blight

Aims:u2002 To isolate endophytic fungi from vegetable plants and examine their in vivo anti‐oomycete activity against Phytophthora infestans in tomato plants.

Multicomponent phenol hydroxylase‐catalysed formation of hydroxyindoles and dyestuffs from indole and its derivatives

Aims:u2002 To establish multicomponent phenol hydroxylases (mPHs) as novel biocatalysts for producing dyestuffs and hydroxyindoles such as 7‐hydroxyindole (7‐HI) from indole and its derivatives.

Power-Generation Characteristics After Vibration and Thermal Stresses of Thermoelectric Unicouples with CoSb3/Ti/Mo(Cu) Interfaces

Reliability tests for thermoelectric unicouples were carried out to investigate the adhesion properties of CoSb3/Ti/Mo(Cu) interfaces. The n-type In0.25 Co3.95Ni0.05Sb12 and p-type In0.25Co3FeSb12 bulks were prepared for fabricating a thermoelectric unicouple (one p–n couple) by an induction melting and a spark plasma sintering process. Mo-Cu alloy was selected as an electrode for the unicouples due to its high melting temperature and proper work function value. Many thermoelectric unicouples with the CoSb3/Ti/Mo(Cu) interfaces were fabricated with the proper brazing materials by means of a repeated firing process. Reliability of the unicouples with the interfaces was evaluated by a vibration test and a thermal cycling test. After the thermal cycling and vibration tests, the power-generation characteristics of the unicouples were compared with the unicouples before the tests. Even after the vibration test, electrical power with a power density of 0.5xa0W/cm2 was generated. The Ti-interlayer is considered as a possible candidate for making a reliable unicouple with high adhesion strength. With the thermal cycling test, the resistance of the unicouple increased and the electrical power from the unicouple decreased. A failure mode by the thermal cycling test was ascribed to a complex effect of micro-cracks originated from the thermal stress and oxidation problem of the thermoelectric materials; that is, a thick oxide layer more than 300xa0μm was detected after a high-temperature durability test of n-type In0.25Co3.95Ni0.05Sb12 material at 773xa0K in air for 7xa0days.

An optimization of composition ratio among triple-filled atoms in In 0.3-x-y Ba x Ce y Co 4 Sb 12 system

Bulk nanostructured materials are important as energy materials. Among thermoelectric materials, the skutterudite system of CoSb3 is a representative material of bulk nanostructured materials. Filling a skutterudite structure with atoms that have different localized frequencies (also known as triple filling) was reported to be effective for lowering thermal conductivity. Among studies representing superior power factors, In-filled skutterudite systems showed higher Seebeck coefficients. This study sought to optimize the composition ratio among the triple-filled atoms in an In0.3-x-yBaxCeyCo4Sb12 system. The composition dependence of the thermoelectric properties was investigated for specimens with different ratios among the three kinds of filler atoms in the In0.3-x-yBaxCeyCo4Sb12 system. In addition, the process variables were carefully optimized for filled skutterudite systems to obtain a maximum ZT value.

Colligative thermoelectric transport properties in n-type filled CoSb3 determined by guest electrons in a host lattice

Among many kinds of thermoelectric materials, CoSb3 has received exceptional attention for automotive waste heat recovery. Its cage structure provides an ideal framework for the realization of phonon-glass electron-crystal strategy, and there have been numerous reports on the enhanced thermoelectric performance through the independent control of the thermal and electrical conductivity by introducing fillers into its cage sites. Herein, we report colligative thermoelectric transport properties in n-type CoSb3 from the viewpoint of “guest electrons in a host lattice.” Both the Seebeck coefficient and the charge transport properties are fundamentally determined by the concentration of the guest electrons, which are mostly donated by the fillers, in the conduction band of the host CoSb3. Comparing this observation to our previous results, colligative relations for both the Seebeck coefficient and the mobility were deduced as functions of the carrier concentration, and thermoelectric transport constants were d...

Composition-dependent charge transport and temperature-dependent density of state effective mass interpreted by temperature-normalized Pisarenko plot in Bi2−xSbxTe3 compounds

Composition-dependent charge transport and temperature-dependent density of state effective mass-dependent Seebeck coefficient were investigated in Bi2−xSbxTe3 (x = 1.56-1.68) compounds. The compounds were prepared by the spark plasma sintering of high-energy ball-milled powder. High-temperature Hall measurements revealed that the charge transport in the compounds was governed dominantly by phonon scattering and influenced additionally by alloy scattering depending on the amount of Sb. Contrary effects of Sb content on the Seebeck coefficient were discussed in terms of carrier concentration and density of state effective mass, and it was elucidated by temperature-normalized Pisarenko plot for the first time.

Thermal Stability of La0.9Fe3CoSb12 Skutterudite

The thermal stability of the La0.9Fe3CoSb12 skutterudite was examined by varying the aging conditions of temperature, time, and atmosphere (air and vacuum). Thermogravimetry, differential scanning calorimetry and x-ray diffraction analyses revealed that the La0.9Fe3CoSb12 was significantly oxidized at temperatures above 673xa0K in air. The Sb-based oxides were preferably formed when aged at high temperatures in air. The thickness of the oxide layers was found to increase with increasing aging temperature and time, while the activation energy for the oxidation of the Fe-Sb-based skutterudite was lower than that of the Co-Sb-based skutterudites. However, when the analysis was performed in a vacuum, the La0.9Fe3CoSb12 skutterudite was stable, and no oxide layers were observed after aging at 823xa0K for 100xa0h. These results suggest that a protective coating for the skutterudite materials or the encapsulated packaging of the skutterudite modules is required for high-temperature applications.

The Synthesis and Thermoelectric Properties of p-Type Li1-x NbO2-Based Compounds

We investigated the thermoelectric (TE) properties of a p-type oxide material (Li1−xNbO2, with xxa0=xa00–0.6). The composition was synthesized via a solid-state reaction method under a reducing atmosphere. The charge transport properties were determined through the electrical conductivity and Seebeck coefficient measurements. The electrical conductivity was non-monotonically varied with x value and showed metallic behavior with increased temperature and above 650xa0K temperature independent behavior dominated by extrinsic conduction. On the other hand, the Seebeck coefficient was increased with an increase in the temperature, and decreased gradually with an increase in the Li vacancy concentration by both synthesis and gradual phase transition to a Li-rich Li3NbO4 phase with temperature and appeared as an n-type TE at xxa0=xa00.6 under high temperatures, which was attributed to an Nb substitution into the Li site. The thermal conductivity was monotonically reduced with the increase in temperature due to the cation disorder defects and second phases. The Li vacancy induced Li1−xNbO2-based compounds under low oxygen partial pressure show promise as a candidate p-type material for thermoelectric applications, particularly for co-processing with n-type oxide thermoelectric materials fabricated under conditions of low oxygen partial pressure.

Charge transport and thermoelectric properties of double-filled Nd1–zYbzFe4–xCoxSb12 skutterudites

Abstractp-Type Nd1–zYbzFe4–xCoxSb12 (z = 0.25, 0.5, 0.75 and x = 0, 0.5, 1.0) skutterudites were synthesized by using encapsulated melting and hot pressing. The effects of Nd/Yb double filling and Co substitution for Fe (charge compensation) on the microstructure, the charge transport, and the thermoelectric properties of the skutterudite specimens were investigated. All specimens were transformed to the skutterudite phase by the annealing process, and a few secondary phases such as marcasite FeSb2 were formed together with the skutterudite phase, although their formation was suppressed with increasing Co content. The lattice constant changed with the filling ratio of Nd/Yb and Fe/Co substitution, which meant that the double filling of Nd/Yb and the substitution of Co for Fe were successfully performed. All specimens showed p-type conduction at temperatures ranging from 323 K to 823 K and exhibited degenerate semiconductor characteristics, in which the electrical conductivity decreased and the Seebeck coefficient increased with increasing temperature. The Seebeck coefficient increased with increasing Nd and Co contents due to the decreased carrier concentration while the electrical conductivity and the thermal conductivity decreased. The maximum Seebeck coefficient was obtained at temperatures in the range from 723 K to 823 K, and the thermal conductivity significantly increased at temperatures above 623 K due to bipolar conduction. The dimensionless figure of merit, ZT, showed maximum values at temperatures ranging from 723 K to 823 K due to the decrease in the Seebeck coefficient (or the decrease in the power factor) and the increase in the thermal conductivity at high temperatures. The maximum ZT = 0.81 was obtained for Nd0.75Yb0.25Fe4Sb12 at 823 K and for Nd0.75Yb0.25Fe3CoSb12 at 723 K.

Correlations among defect type, photoconductivity and photoreactivity of doped TiO2

The electrical conductivity (σ), photoconductivity and photocatalytic reactivity in doped crystalline TiO2 were measured as a function of the oxygen partial pressure (Po2), temperature, doping type and UV irradiation. The Po2 dependence of σ suggests that the predominant atomic defects in pure TiO2 are oxygen vacancies (Vo...) and interstitial titanium ions (Tii...), but the dominant defect is changed with Po2 and temperature. The photoexcited electrons in reduced and/or n-type doped TiO2 enhance both the photoconductivity and the photocatalytic reactivity by the reduction process. Therefore, these behaviors are strongly dependent on the electron concentration.