• Bulletin of the Korean Chemical Society
• /
• v.23 no.5
• /
• pp.749-757
• /
• 2002

Chiral oxazolidin-2-one is easily prepared from D-mannitol and demonstrated to undergo highly diastereoselective alkylation reactions via lithium imide Z-enolates of its N-acyl derivatives to afford ${\alpha}-branched$ products. Evans syn and non-Evans sy n aldol products were also selectively obtained using this new auxiliary in high diastereomeric purity by simply changing the stoichiometry of TiCl4 and the nature of the amine base. Also, this new auxiliary is employed in diastereoselective Staudinger-type ${\beta}-lactam$ syntheses. Using 2-chloro-1-methylpyridinium iodide as the dehydrating agent, the reaction of auxiliary tethered acetic acid with trans imines gave the desired ${\beta}-lactams$ with cis-selectivity.

• Journal of Surface Science and Engineering
• /
• v.40 no.5
• /
• pp.203-208
• /
• 2007

Preliminary studies were conducted to develop a dimensionally stable anode (DSA)electrode prepared by reactive sputtering method. The microstructure, surface morphology and electrochemical properties of iridium oxide $(IrO_2)$ coatings synthesized by unbalanced magnetron sputtering (UBMS) and conventional DSA electrode were compared. In addition, the possibilities of $IrO_2$ films synthesized by UMB on a real DSA electrode were investigated by electro-chemical application test. The degree of non-stoichiometry and surface area were closely related to the electro-chemical activity of the $IrO_2$ electrode. The feasibility of making a DSA electrode prepared by PVD technique was demonstrated through the present work.

• The Korean Journal of Ceramics
• /
• v.6 no.4
• /
• pp.370-379
• /
• 2000

Simple ways to build up mixed-metal molecules which can act as potential single-source precursors to multimetallic oxides are reviewed. Emphasis is given to Lewis acid-base reactions between metal alkoxides M(OR)/sub n/, and between metal alkoxides and more accessible oxide precursors, carboxylates M(O₂CR)/sub n/ and β-diketonates M(β-dik)/sub n/. Characterization of the precursors is achieved in the solid state (single crystal X-ray diffraction, FT-IR) and by multinuclear NMR in solution. The reactions proceed toward the formation of aggregates in which the different metals display their usual coordinations numbers, often six for transition metals, as shown. Strategies for fixing the stoichiometry between the metals are developed. The reactivity of the MM species (dissociation, effects of chemical modifiers, of other metallic species, hydrolytic or non-hydrolytic condensation, etc.) will be indicated. Transformations into oxides are illustrated on precursors for titanates or niobates.

• Ceramist
• /
• v.22 no.2
• /
• pp.182-188
• /
• 2019

Alternative energy sources to the systems using hydrocarbon fuels have been actively developed due to exhaustion of fossil fuels and issue of global warming by CO₂. Fuel cells have attracted great attentions to solve these issues as electricity can be produced with product of clean H₂O by using H₂-O₂ as a fuel. Besides, using reverse reactions make it possible to produce H₂ and O₂ gas from electrolysis of water. There are various fuel cells systems depending on the types of electrolyte, and in this mini-reviews, the main aim is to focus on perovskite oxides as a catalyst for oxygen-evolution reactions in alkaline electrolysis and its potential to application of alkaline electrolysis systems.

• Current Photovoltaic Research
• /
• v.6 no.1
• /
• pp.12-16
• /
• 2018

The interest in perovskite solar cells has been skyrocketed owing to their rapid progress in efficiency in recent years. Here, we report the effect of non-stoichiometry in the methylammonium lead trihalide ($MAPbI_3$) precursors used in a solution process with different MAI : $PbI_2$ ratios of 1 : 0.96, 1 : 1.10, 1 : 1.15, and 1:1.20. With an increase in the $PbI_2$ content, the $PbI_2$ secondary phase was found to form at grain boundary region of perovskite thin films, as evidenced by X-ray diffraction (XRD) and scanning electron microscopy (SEM). In terms of device performance, open-circuit voltage in particular is significantly improved with increasing the molar ratio of $PbI_2$, which is possibly ascribed to the reduction in recombination sites at grain boundary of perovskite and hence the prolonged life time of light-generated carriers according to the reported. As a result, the $PbI_2-excess$ devices exhibited a higher power conversion efficiency compared to the MAI-excess ones.

• Journal of the Korean Magnetics Society
• /
• v.5 no.1
• /
• pp.21-26
• /
• 1995

The systematic variation of complex permeability and complex permittivity and their relationship with micro-wave absorbing properties are investigated in sintered Ni-Zn ferrites of non-stoichiometric composition. The specirrens of ${(Ni_{0.5}Zn_{0.5}O)}_{1-x}(Fe_{2}O_{3})_{1+x}$ spinels were prepared by a conventional ceramic processing technique. In the present study. complex permeability and permittivity can be controlled by the variation of ${\alpha}-Fe_{2}O_{3}$ contents in the spinel lattice. The primary effect of the excess ${\alpha}-Fe_{2}O_{3}$ is to increase the dielectric constant. while the notable decrease of magnetic loss is observed in the iron-deficient ferrites. The results suggest that the matching fre-queocyand matching thickness could be controlled by the variation of ${\alpha}-Fe_{2}O_{3}$ contents in the Ni-Zn ferrite.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
• /
• 2002.10b
• /
• pp.157-158
• /
• 2002

A surface hardenable low alloy carbon steel was implanted with medium energy (20 - 50KeV) $N_2^+$ ions to produced a modified hardened surface. The implantation conditions were varied and are given in several doses. The surface hardness of treated and untreated steels were measured using depth sensing ultra micro indentation system (UMIS). It is shown that the hardness of nitrogen ion implanted steels varied from 20 to 50GPa depending on the implantation conditions and the doses of implantation. The structure of the modified surfaces was examined by X-ray photoelectron spectroscopy (XPS). It was found that the high hardness on the implanted surfaces was as a result of formation of non-equilibrium nitrides. High-resolution XPS studies indicated that the nitride formers were essentially C and Si from the alloy steel. The result suggests that the ion implantation provided the conditions for a preferential formation of C and Si nitrides. The combination of evidences from nano-indentation and XPS, provided a strong evidence for the existence of $sp^3$ type of bonding in a suspected $(C,Si)_xN_y$ stoichiometry. The formation of ultra hard surface from relatively cheap low alloy steel has significant implication for wear resistance implanted low alloy steels.

• Journal of the Korean Vacuum Society
• /
• v.4 no.2
• /
• pp.194-200
• /
• 1995

2-3nm의 공간분해능을 갖는 전자에너지손실분광법(Electron Energy Loss Spectroscopy, EELS)을 이용하여, 전기적 특성조사된 YBa2Cu3O7-$\delta$ 고온초전도체 쌍결정의 결정경계(grain boundary)에서 산소의 조성변화를 조사하였다. Misorientation angle이 $14^{\circ}$, $28^{\circ}$,$ 30^{\circ}$인 3개의 쌍결정 중에서 Josephson junction 특성을 보인 $28^{\circ}$ 및 $30^{\circ}$결정경계에서의 oxygen 1s absorption edge는 결정내부에서의 oxygen 1s absorption edge와 매우 다름을 알 수 있었다. 이는 결정경계에서의 산소조성이 결정내부에 비해 낮음을 의미하며, 그 영역이 결정경계 부근 20-40nm로, coherence length에 비해 큼을 알 수 있다. 반면에, flux pinningxmr성을 보인 $14^{\circ}$결정경계에서의 oxygen 1s absorption edge는 결정내부에서와 별 차이를 보이지 않았다. 따라서 일반적으로 관찰되어온, misorientation angle이 큰 결정경계에서의 Josephson junction 특성은 결정경계 부근에서 산소의 조성이 낮아지는데에 기인하며, 그 원인은 결정경계면을 통해 산소가 out-diffusion되기 때문인 것으로 생각된다.

• Journal of the Korean Ceramic Society
• /
• v.42 no.5 s.276
• /
• pp.359-365
• /
• 2005

Silicon nitride coating films with various ratios of nitrogen to silicon contents were prepared and characterized. The film was coated on silicon substrate by sputtering method with changing nitrogen gas flow rate in a chamber. The nitrogen to silicon ratio was found to have values in a range from 0 to 1.4. Coated film was characterized with scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, nanoindentation scanning probe microscopy, x-ray photon spectrometry, and Raman spectrometry. Silicon nitride phase in all samples showed amorphous nature regardless of N/Si ratio. When N/Si ratio was 1.25, hardness and elastic modulus of silicon nitride film showed maximum with 22 GPa and 210 GPa, respectively. Those values decreased, when N/Si ratio was higher than 1.25. Raman spectrum showed that no silicon phase exist in the film. XPS result showed that the silicon-nitrogen bond was dominant way for atomic bonding in the film. The structure and property was explained with Random Bonding Model(RBM) which was consistent with the microstructure and chemistry analysis for the coating films.

• Journal of Powder Materials
• /
• v.18 no.3
• /
• pp.250-255
• /
• 2011

The potential application of ultrafine cerium oxide (ceria, $CeO_2$) as an oxygen gas sensor has been investigated. Ceria was synthesized by a thermochemical process: first, a precursor powder was prepared by spray drying cerium-nitrate solution. Heat treatment in air was then performed to evaporate the volatile components in the precursor, thereby forming nanostructured $CeO_2$ having a size of approximately 20 nm and specific surface area of 100 $m^2/g$. After sintering with loosely compacted samples, hydrogen-reduction heat treatment was performed at 773K to increase the degree of non-stoichiometry, x, in $CeO_{2-x}$. In this manner, the electrical conductivity and oxygen-response ability could be enhanced by increasing the number of oxygen vacancies. After the hydrogen reduction at 773K, $CeO_{1.5}$ was obtained with nearly the same initial crystalline size and surface. The response time $t_{90}$ measured at room temperature was extremely short at 4 s as compared to 14 s for normally sintered $CeO_2$. We believe that this hydrogen-reduced ceria can perform capably as a high-performance oxygen sensor with good response abilities even at room temperature.