Sung-Wng Kim

Ammonia synthesis using a stable electride as an electron donor and reversible hydrogen store

Industrially, the artificial fixation of atmospheric nitrogen to ammonia is carried out using the Haber-Bosch process, but this process requires high temperatures and pressures, and consumes more than 1% of the worlds power production. Therefore the search is on for a more environmentally benign process that occurs under milder conditions. Here, we report that a Ru-loaded electride [Ca(24)Al(28)O(64)](4+)(e(-))(4) (Ru/C12A7:e(-)), which has high electron-donating power and chemical stability, works as an efficient catalyst for ammonia synthesis. Highly efficient ammonia synthesis is achieved with a catalytic activity that is an order of magnitude greater than those of other previously reported Ru-loaded catalysts and with almost half the reaction activation energy. Kinetic analysis with infrared spectroscopy reveals that C12A7:e(-) markedly enhances N(2) dissociation on Ru by the back donation of electrons and that the poisoning of ruthenium surfaces by hydrogen adatoms can be suppressed effectively because of the ability of C12A7:e(-) to store hydrogen reversibly.

Field-induced water electrolysis switches an oxide semiconductor from an insulator to a metal

Water is composed of two strong electrochemically active agents, H+ and OH− ions, but has not been used as an active electronic material in oxide semiconductors. In this study, we demonstrate that water-infiltrated nanoporous glass electrically switches an oxide semiconductor from insulator to metal. We fabricated a field-effect transistor structure on an oxide semiconductor, SrTiO3, using water-infiltrated nanoporous glass—amorphous 12CaO·7Al2O3—as the gate insulator. Positive gate voltage, electron accumulation, water electrolysis and electrochemical reduction occur successively on the SrTiO3 surface at room temperature. This leads to the formation of a thin (~3 nm) metal layer with an extremely high electron concentration (1015–1016 cm−2), which exhibits exotic thermoelectric behaviour. The electron activity of water as it infiltrates nanoporous glass may find many useful applications in electronics or in energy storage.

High temperature thermoelectric properties of p- and n-type β-FeSi2 with some dopants

Abstract Cr doped p-type and Co doped n-type β-FeSi2 alloys have been prepared using a powder metallurgy technique. Then the effects of Cu and Ge additions on their thermoelectric properties were investigated. Thermoelectric power α, electrical conductivity, σ, and thermal conductivity, κ were measured in a temperature range from 373 to 973 K. Their thermoelectric power is found to decrease with increasing an amount of both Cu and Ge in both type of β-FeSi2. In contrast, their electrical conductivity exhibit opposite tendency with respect to the amount of both dopants. Their thermal conductivity is more effectively reduced in the Ge addition to them than that in the Cu addition. The figure of merit, Z of binary β-FeSi2 is 0.19×10−4/K, and increases by the addition of Cr, Co, and Ge. The maximum figure of merit, Z of 1.3×10−4/K was obtained in Fe0.95Co0.05Si1.958Ge0.042 at 845 K.

Two-Dimensional Spin Density Wave State in LaFeAsO

The parent compound of the Fe oxypnictide superconductor, LaFeAsO, has been studied by pulsed neutron powder inelastic scattering measurement. The inelastic scattering intensity along the Q -axis at T =140 K exhibits a prominent asymmetric peak ascribed to the (π,π, l ) magnetic rod, suggesting the two-dimensionality of the spin density wave above the magnetic transition temperature. It persists even in the tetragonal phase up to room temperature, which is well above the magnetic transition temperature, corresponding to two antiferromagnetic sublattices or domains with a strong effective antiferromagnetic interaction J 2 between next-nearest-neighbour Fe magnetic moments in a single Fe square lattice.

Enhancement of high temperature thermoelectric properties of intermetallic compounds based on a Skutterudite IrSb3 and a half-Heusler TiNiSb

Abstract Phonon glass and electron crystal (PGEC) thermoelectric materials have been expected to be a new class of thermoelectric materials for high temperature applications. Among the efforts to optimize the high temperature thermoelectric properties of various PGEC thermoelectric materials, recent experimental works on the Skutterudite IrSb3 and half-Heusler TiNiSb intermetallic compounds are presented herein by which the material design concept for high energy conversion efficiency, i.e. a high figure of merit, is suggested. It is revealed that the thermoelectric efficiency of IrSb3 can be increased by the decrease of lattice thermal conductivity due to the rattling effect of La atoms filled in the structural vacancies of the Skutterudite crystal structure. In the half-Heusler TiNiSn, high temperature thermoelectric properties are improved by Hf substitution to the Ti sites by reducing lattice thermal conductivity and also by Sb doping to increase power factor. It is concluded that the proper alloy designing for controlling crystal structure and carrier concentration could enable these intermetallic compounds to exhibit a high potential for elevated temperature thermoelectric applications.

Effect of process conditions on the thermoelectric properties of CoSi

Abstract A CoSi n -type thermoelectric material is prepared using a powder metallurgy technique with two different starting materials. Measurements of thermoelectric power, α , electrical conductivity, σ , and thermal conductivity, κ , are carried out in a temperature range from room temperature to 973 K under 10 −1 torr vacuum. Thermoelectric power and electrical conductivity are found to decrease with increase in temperature in all cases. Thermal conductivity of CoSi fabricated from pulverized powders of cast materials is found to be lower than that from powders of reactive sintered materials, which result should come from the difference in porosity in specimens. It is shown that the thermoelectric figure of merit, Z , of the present CoSi currently fabricated by powder metallurgy is higher than that prepared by conventional ingot metallurgy methods.

Superconductivity in room-temperature stable electride and high-pressure phases of alkali metals.

S-band metals such as alkali and alkaline earth metals do not undergo a superconducting transition (SCT) at ambient pressure, but their high-pressure phases do. By contrast, room-temperature stable electride [Ca24Al28O64]4+⋅4e− (C12A7:e−) in which anionic electrons in the crystallographic sub-nanometer-size cages have high s-character exhibits SCT at 0.2–0.4u2009K at ambient pressure. In this paper, we report that crystal and electronic structures of C12A7:e− are close to those of the high-pressure superconducting phase of alkali and alkaline earth metals and the SCT of both materials is induced when electron nature at Fermi energy (EF) switches from s- to sd-hybridized state.

High-pressure synthesis of the indirectly electron-doped iron pnictide superconductor Sr 1 − x La x Fe 2 As 2 with maximum T c = 22 K

Compounds of Sr1-xLaxFe2As2 were synthesized by solid state reaction at 1273 K, under pressures of 2 or 3 GPa. The Sr1-xLaxFe2As2 phase was dominant up to x = 0.5, and superconductivity was observed at

Electromagnetic properties of undoped LaFePnO (Pn P, As)

x ge 0.2

High Pressure Synthesis of Indirectly electron-doped 122 Iron Superconductor Sr1-xLaxFe2As2 with a maximum Tc = 22 K

. A maximum critical temperature Tc of 22 K, and a maximum shielding volume fraction of ~70 % were obtained at x = 0.4. This is the first experimental demonstration of electron-doped superconductivity in 122-type iron pnictides, where electrons were doped through aliovalent substitution at the site of the alkaline earth metal. The optimal Tc was slightly higher than that for the Co-doped (directly electron-doped) samples (19 K), and much lower than that for the hole-doped case (37 K). The bulk superconductivity range was narrower than that for the Co-substituted case, and both ranges were much narrower than that for the hole-doped case. These observations revealed that differences in the electron-doping mode (direct or indirect) did not have a prominent effect on the optimal Tc or superconductivity range, compared with differences in carrier polarity.