Jae Do Lee

Adsorption of NH3 and NO2 molecules on carbon nanotubes

Adsorption of NH3 and NO2 molecules on semiconducting single-walled carbon nanotubes is investigated using density functional theory. Both NH3 and NO2 molecules are found to bind to carbon nanotubes via physisorption. Electron charge transfer is found to be a major mechanism determining the conductivity change in carbon nanotubes upon exposure to NH3 and NO2 molecules. The calculated density of states is also considered to elucidate the differences in the NO2 and NH3 gas detection mechanism of carbon nanotubes.

Electronic Structure Simulation of Chromium Aluminum Oxynitride by Discrete Variational-Xα Method and X-Ray Photoelectron Spectroscopy

We use a first-principles discrete variational (DV)-Xα method to investigate the electronic structure of chromium aluminum oxynitride. When nitrogen is substituted for oxygen in the Cr–Al–O system, the N2p level appears in the energy range between O2p and Cr3d levels. Consequently, the valence band of chromium aluminum oxynitride becomes broader and the band gap becomes smaller than that of chromium aluminum oxide, which is consistent with the photoelectron spectra for the valence band using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). We expect that this valence band structure of chromium aluminum oxynitride will modify the transmittance slope which is a requirement for photomask application.

Atomic and electronic structures of doped grain boundaries in SrTiO3

The atomic and electronic structures of pristine, Mn- and Nb-doped grain boundaries in SrTiO3 are investigated by atomistic simulations and cluster calculations. The atomic structures of (310) symmetric tilt grain boundaries in SrTiO3 are determined by atomistic simulation using empirical potentials. The defect energies of Mn(Nb)-doped models are calculated and discussed in relation to the concentration profiles of Mn(Nb) in SrTiO3 grain boundaries. The local electronic structures near Mn(Nb)-doped grain boundaries in SrTiO3 are determined using embedded cluster calculations based on the density functional theory. The charge density of each system is calculated to elucidate the electronic structure of the grain boundary. The calculation results agree well with previous experimental observations of the atomic structures and grain boundary charges near the Mn(Nb)-doped grain boundary in SrTiO3.

Synthesis of Highly Concentrated Silver Nanoparticles Assisted Polymeric Dispersant

Silver nanoparticles are well known as novel materials as metal electrodes on ceramics, catalyst, antibacterial agent, and etc. The assisted role of polymeric ceramic dispersant on synthesis of highly concentrated silver nanoparticles was studied. The used ceramic dispersants with different kinds of counter ion of polyelectrolyte were polymethylmetacrylic ammonium salt (PMMA, i.e., Darvan C: R.T. Vanderbilt Co.), polyacrylic ammonium salt (PAA), polyacrylic amine salt (PAAm), and polyacrylic sodium salt (PAS). The formation of Ag(0) nanoparticles was followed by UV spectrometer, XRD and TEM. According to TEM investigations Ag(0) nanoparticles with the rage of 10 ~ 20 nm in diameter were produced. The possible concentration of batch synthesized silver nanoparticles was up to 10 wt%. Introduction Silver nanoparticles are recently attracting much interest from the viewpoints of both fundamental and actual appication[1-2], since they show the distinctive properties, such as high catalytic activity and excellent conductivity, and good antibacterial and optical porperties, and etc. In this work, we have carried out the synthesis of highly concentrated Ag nanoparticles by assisting polymeric dispersant. From these results, we intended to discuss the synthsis condition of nanoparticle and assisted role of polymeric dispersant on the synthesis. Experiment AgNO3 as Ag source materials and NaBH4 and Hydrazine as reducing agent were used. The polymeric dispersant was determined as best assisted material to prepared silver nanoparticles in preliminary experiment. We had been tried to use NP-10 (Polyoxyethylene Nonylphenyl Ether ; Nonionic), SDS (sodium dodecyl sulfate ; Anionic) , and AOT (Diisooctyl Sodium Sulfosuccinate ; Anionic) as another assisted surfactants. The colloidal silver sols was perepared as followed. Ice cold solution of AgNO3(5wt% calculated as Ag) containng various concentration of polymeric dispersant(Darvan C: R.T. Vanderbilt Co.). The used ceramic dispersants with different kinds of counter ion of polyelectrolyte were polymethylmetacrylic ammonium salt (PMMA, i.e., Darvan C), polyacrylic ammonium salt (PAA), polyacrylic amine salt (PAAm), and polyacrylic sodium salt (PAS). The formation of Ag(0) nanoparticles was followed by UV spectrometer, XRD(X-ray radiation equipped with monochrometer, model: D8 Discover, Cu Ka, Bruker Co,) and EF-TEM(EM912, Carl Zeiss, Germany, installed at Korea Basic Science Institute). Results and Discussions The assisted role of polymeric ceramic dispersant on synthesis of highly concentrated silver nanoparticles was studied. The size of Ag nanoparticle has been depended largely on the adding Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 141-142 doi:10.4028/www.scientific.net/KEM.264-268.141

Joining Method of Silicon Nitride to Carbon Steel

We investigated the method of joining silicon nitride having on its surface a thin layer of active silicon metal to carbon steel. The active silicon layer is formed through the thermal dissociation of silicon nitride (Si3N4) into silicon (Si) and nitrogen gas (N2). The active silicon layer is directly joined to carbon steel via an induced eutectic melting reaction between the silicon and iron (Fe) found in carbon steel, or via brazing of two materials using general brazing metal such as Ag-Cu alloys. This joining process does not require the use of expensive active brazing alloys such as Ag-Cu-Ti alloys or a sputtering method designed to coat the active metals on surface of silicon nitride.

Resonant Photoemission Spectroscopy and Theoretical Calculation of the Valence Band Structure in Chromium Aluminum Oxynitride

The electronic structure of chromium aluminum oxynitride has been investigated using resonant photoemission spectroscopy (RPES) and the discrete variational (DV)-Xα method. The RPES measurement of the electronic structure around the Cr 2p3/2 absorption edges exhibited significant resonant interference behavior for Cr 3d valence electrons, whereas it exhibited small resonant interference behavior for N 2p valence electrons. Therefore this RPES method can be useful for analyzing the valence band of chromium aluminum oxynitride film. The top of the valence band predominantly consists of Cr 3d and a small amount of N 2p. The difference between the measured photoemission spectra of the valence band and the DV-Xα calculation of chromium aluminum oxynitride ranges from 0.47 to 3.62 eV. This difference is probaly caused by the Coulomb interactions between the d electrons of chromium and the structure of the amorphous film. Through the experimental and theoretical studies, the valence band structure of chromium aluminum oxynitride came to be understand in detail.

Electronic structure of Ti4O7 using DV-Xα cluster calculation method

Abstract Ti 4 O 7 has been attracted, because it shows temperature-induced semiconductor-semiconductor-metal transitions. These transitions have been known to be induced by charge delocalization of Ti ions. We have initiated self-consistent local-density calculations on Ti 4 O 7 in order to investigate charge delocalization of Ti ions in metallic phase. Embedded cluster Discrete Variational (DV)-Xα method has been used to determine local charge distributions and density of states. Several model cluster calculations have been carried out for two different geometric structures in low-temperature semiconductor phase and high-temperature metallic phase of Ti 4 O 7 . The calculated projected density of states agreed well with the X-ray absorption spectra (XAS) for two different phases of Ti 4 O 7 . We have found that the charge delocalization within the cluster model is very small and a little change occurs in mainly Ti 3d across the metal-nonmetal transition.

The Structure Effect of Polymeric Dispersant on Rheological Behavior of Highly Concentrated Silicon Nitride Gelcasting Slip

Gelcasting has been expected as a promising approach to prepare near-net-shaped ceramic products. The preparation of highly concentrated gelcasting slip is one of key point in this process. The rheological behavior of concentrated silicon nitride gelcasting slip was investigated with the structure effect of polymeric dispersant and the ratio of dispersant to acrylamide monomer in order to prepare highly concentrated gelcasting slip. The slip was prepared by ball milling of silicon nitride batch composed of acrylamide monomer and polymeric dispersant after premixing them by an attritor. The slip mixed with initiator will be deaired in vacuum and cast into molds, and then polymerized. The consolidated green body will be obtained by drying the gelated slip. The polymeric dispersants with different kinds of polyelectrolyte were poly(methylmetacrylic ammonium salt), poly(acrylic ammonium salt), and poly(acrylic amine salt). Before the polymerization of gelcasting slip, the viscosity vs. shear rate was measured to evaluate the rheological behavior of the slip. In case of using the polyacrylic amine salt (PAAm), the high solid loading of silicon nitride slip was obtained up to 47vol% with a low viscosity.

Electronic structure of chromium aluminum oxide

Abstract We have investigated the local electronic structure of the CrAlO system using XPS and atomistic simulations and model cluster calculations. Our atomistic simulations show that no drastic change in atomic structure when Cr is replaced by Al. It is expected from that the ionic radius of Al 3+ (0.054 nm) is similar to that of Cr 3+ (0.062 nm) in the coordination number 6 environment. Therefore our cluster calculations, based on the structure obtained from atomistic simulation, reveal the characteristics of crystalline Cr 2 O 3 and Al 2 O 3 only. Unfortunately, our Cr-rich compounds are much out of stoichiometry of Cr 2 O 3 , rather close to Cr 3 O 2 . Thus we cannot expect 6-fold coordination of oxygens around Cr. Moreover, the discrepancy between O 2p PDOS and XPS results demonstrates that its local structure is much different from the crystalline structure of Cr 2 O 3 . On the contrary to Cr-rich compounds, the calculated O PDOS agrees well with XPS spectra of Al-rich compounds. It infers that the structure of Al-rich compounds seems to keep 6-fold coordination. Even the Al-excess compound of the sample (a) seems to keep 6-fold coordination locally. From our calculations and XPS results, we can conclude that the amorphous structure can be expected in Cr-rich compounds, while the crystalline structure is kept locally in Al-rich compounds.

Electronic structure of chromium aluminum oxynitride by DV-Xα method and photoelectron spectroscopy

Electronic structure of chromium aluminum oxynitride is calculated with the embedded cluster method within the framework of discrete variational (DV)-Xa method. Then calculation is verified by comparing with the experiments of photoelectron spectroscopy such as X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Electronic structure calculated theoretically is well consistent with photoelectron spectra under valence band. When nitrogen is substituted for oxygen in Cr-Al-O system, N2p level appears in the energy range between O2p and Cr3d level. So the valence band of chromium aluminum oxynitride becomes broader and the band gap becomes smaller than that of chromium aluminum oxide. We expect that this valence band structure of chromium aluminum oxynitride change the transmittance slope in mask application. We can properly elucidate the electronic structure of chromium aluminum oxynitride system through the theoretical calculation and experimental verification.