• Journal of the Korean Wood Science and Technology
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• v.41 no.4
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• pp.302-309
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• 2013

The highly porous cellulosic aerogels were prepared by freeze-drying method using sodium hydroxide-urea aqueous solution in the process of dissolution, gelation, regeneration and organic solvent substitution. The structural characteristics of porous aerogel were analyzed using scanning electron microscopy and nitrogen adsorption apparatus. As a result, the dissolving pulp was completely dissolved, but filter papers and holocellulose were divided into two layers (dissolved and undissolved parts) in the process of centrifugation. The structure of aerogel from dissolved pulp showed porous pores in the surface and net-shaped network in the inner part. Aerogels from filter paper and holocellulose had the condensed porous network surface and the open-pore nano-fibril network inner structure. Undissolved form of fibers was observed in the aqueous solution of aerogel from holocellulose. The BET value ($S_{BET}$) of aerogel from dissolved pulp was ranged in 260~326 $m^2/g$, and it was decreased with the increase of concentration. Whereas, the $S_{BET}$ value of aerogel from filter paper (198~418 $m^2/g$) was increased with the increase of concentration. The $S_{BET}$ value of aerogel from holocellulose were 137 $m^2/g$ at 2% (w/w) of cellulose, and it was increased to maximum 401 $m^2/g$ at 4% (w/w) of cellulose. Then, it was decreased at 5% (w/w) of cellulose.

• Korean Chemical Engineering Research
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• v.48 no.1
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• pp.75-79
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• 2010

The precursor powders with thin wall structure were prepared by spray pyrolysis from the spray solution with ethylenediaminetetraacetic acid, citric acid and $NH_4Cl$ flux. The $BaMgAl_{10}O_{17}:Eu$ phosphor powders formed from the spray solution without organic additives and flux material had sizes of $1{\sim}5{\mu}m$ and hollow structure with high thickness at post-treatment temperature of $1,200^{\circ}C$. However, $BaMgAl_{10}O_{17}:Eu$ phosphor powders formed from the spray solution with ethylenediaminetetraacetic acid, citric acid and $NH_4Cl$ flux had fine size and plate-like shape. The mean crystallite sizes of the phosphor powders with fine sizes were 23, 35, and 33 nm when the content of $NH_4Cl$ flux were 0, 6, 35 wt% of phosphor. The photoluminescence intensity of the phosphor powders formed from the spray solution with the optimum amount of $NH_4Cl$ flux as 35 wt% was 215% of that of the phosphor powders formed from the spray solution without flux material.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.23 no.1
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• pp.14-19
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• 2013

We have successfully synthesized $Bi_{4-x}La_xTi_3O_{12}$ (x~2) having Aurivillius-type layered perovskite structure from exfoliated layered perovskite oxide of $K_2La_2Ti_3O_{10}$ with Ruddlesden-Popper structure. The reaction between the exfoliated lanthanum titanate nanosheets and BiOCl nanocrystal resulted in the formation of polycrystalline $Bi_{4-x}La_xTi_3O_{12}$ (x~2) after heating above $700^{\circ}C$. Colloidal suspension of the nanosheets could be obtained by intercalating ethylamine (EA) into the protonated lanthanum titanate, $H_2La_2Ti_3O_{10}$, derived from $K_2La_2Ti_3O_{10}$. Transmission electron microscopic (TEM) analysis show that the exfoliated lanthanium titanate nanosheets have a thickness of a few nano meters. According to X-ray diffraction (XRD) analysis, the exfoliated lanthanium titanate was found to be transformed into $Bi_{4-x}La_xTi_3O_{12}$ (x~2) after restacking with BiOCl and subsequent thermal treatment at > $700^{\circ}C$.

• Investigative Magnetic Resonance Imaging
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• v.12 no.2
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• pp.142-147
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• 2008

Purpose : By using the micro-imaging unit modified from NMR spectrometer, the high resolution MRI protocols of finer than 100 micron in 5 minutes, is sought for mouse, which plays a central role in animal studies Materials and Methods : C57BL/6 mouse, lighter than 50 gram, is used for the experiments. The superconducting magnet is vertical type with 89 mm inner diameter at 4.9 Tesla. The diameter of rf-coil is 30 mm. Mostly used techniques are the fast spin echo and the gradient echo pulse sequence. Results : For 2D images, proton density and T2 weighted images are obtained and their optimum experimental variables were sought. Minute structure of mouse brain can be recognized and 3D brain image is also obtained additionally. 3D image will be useful particularly for the dynamic contrast study using various contrast agents. Conclusion : Like the case of human and other small animals, the high resolution of mouse brain is enough to recognize the minute structure of it. Recently, similar studies are reported domestically, but it seems only a beginning stage. Due to easiness of breeding/control, mouse MRI study will soon play a vital part in brain study.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.183-183
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• 2014

Recently, multiferroic materials gain much attention due to their fascinating fundamental physical properties. These materials offer wide range of potential applications such as data storage, spintronic devices and sensors, where both electronic and magnetic polarizations can be coupled. Among single-phase multiferroic materials, $BiFeO_3$ is typical because of the room-temperature magnetoelectric coupling in view of long-range magnetic- and ferroelectric-ordering temperatures. However, $BiFeO_3$ is well known to have large leakage current and small spontaneous polarization due to the existence of oxygen vacancies and other defects. Furthermore the magnetic moment of pure $BiFeO_3$ is very weak owing to its antiferromagnetic nature. Recently, various attempts have been performed to improve the multiferroic properties of $BiFeO_3$ through the co-doping at the A and the B sites, by making use of the fact that the intrinsic polarization and magnetization are associated with the lone pair of $Bi^{3+}$ ions at the A sites and the partially-filled 3d orbitals of $Fe^{3+}$ ions at the B sites, respectively. In this study, $BiFeO_3$, $Bi_{0.9}Ho_{0.1}FeO_3$, $BiFe_{0.97}Ni_{0.03}O_3$ and $Bi_{0.9}Ho_{0.1}Fe_{0.97}Ni_{0.03}O_3$ bulk compounds were prepared by solid-state reaction and rapid sintering. High-purity $Bi_2O_3$, $Ho_2O_3$, $Fe_2O_3$ and $NiO_2$ powders with the stoichiometric proportions were mixed, and calcined at $500^{\circ}C$ for 24 h to produce the samples. The samples were immediately put into an oven, which was heated up to $800^{\circ}C$ and sintered in air for 1 h. The crystalline structure of samples was investigated at room temperature by using a Rigaku Miniflex powder diffractometer. The field-dependent and temperature-dependent magnetization measurements were performed with a vibrating-sample magnetometer and superconducting quantum-interference device.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.250-250
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• 2012

Recently, multiferroic materials have attracted much attention due to their fascinating fundamental physical properties and potential technological applications in magnetic/ferroelectric data storage systems, quantum electromagnets, spintronics, and sensor devices. Among single-phase multiferroic materials, $BiFeO_3$, in particular, has received considerable attention because of its very interesting magnetoelectric properties for application to spintronics. Enhanced ferromagnetism was found by Fe-site ion substitution with magnetic ions. In this study, $BiFe_{1-x}Ni_xO_3$ (x=0 and 0.05) bulk ceramic compounds were prepared by solid-state reaction and rapid sintering. High-purity $Bi_2O_3$, $Fe_3O_4$ and NiO powders were mixed with the stoichiometric proportions, and calcined at $450^{\circ}C$ for 24 h to produce $BiFe_{1-x}Ni_xO_3$. Then, the samples were directly put into the oven, which was heated up to $800^{\circ}C$ and sintered in air for 20 min. The crystalline structure of samples was investigated at room temperature by using a Rigaku Miniflex powder diffractometer. The Raman measurements were carried out with a Raman spectrometer with 514.5-nm-excitation Ar+-laser source under air ambient condition on a focused area of $1-{\mu}m$ diameter. The field-dependent magnetization and the temperature-dependent magnetization measurements were performed with a vibrating-sample magnetometer. The x-ray diffraction study demonstrates the compressive stress due to Ni substitution at the Fe site. $BiFe_{0.95}Ni_{0.05}O_3$ exhibits the rhombohedral perovskite structure R3c, similar to $BiFeO_3$. The lattice constant of $BiFe_{0.95}Ni_{0.05}O_3$ is smaller than of $BiFeO_3$ because of the smaller ionic radius of Ni3+ than that of Fe3+. The field-dependent magnetization of $BiFe_{0.95}Ni_{0.05}O_3$ exhibits a clear hysteresis loop at 300 K. The magnetic properties of $BiFe_{0.95}Ni_{0.05}O_3$ were improved at room temperature because of the existence of structurally compressive stress.

• Applied Chemistry for Engineering
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• v.31 no.5
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• pp.539-544
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• 2020

A three-dimensional (3D) porous polyaniline (PANI) film was fabricated by a combined photo-and soft-lithography technique based on a large-area nanopillar array, followed by a controlled chemical dilute polymerization. The as-obtained 3D PANI film consisted of hierarchically interconnected PANI nanofibers, resulting in a 3D hierarchical nanoweb film with a large surface and open porous structure. Using electrochemical measurements, the resulting 3D PANI film was demonstrated as a flexible pH sensor electrode, exhibiting a high sensitivity of 60.3 mV/pH, which is close to the ideal Nernstian behavior. In addition, the 3D PANI electrode showed a fast response time of 10 s, good repeatability, and good selectivity. When the 3D PANI electrode was measured under a mechanically bent state, the electrode exhibited a high sensitivity of 60.4 mV/pH, demonstrating flexible pH sensor performance.

• Korean Journal of Materials Research
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• v.22 no.11
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• pp.631-635
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• 2012

Nickel oxide was doped with a wide range of concentrations (mol%) of Aluminum (Al) by solvothermal synthesis; single-phased nano powder of nickel oxide was generated after calcination at$900^{\circ}C$. When the concentration of Al dopant was increased, the reduced intensity was confirmed through XRD analysis. Lattice parameters of the synthesized NiO powder were decreased after treatment of the dopant; parameters were increased when the concentration of Al was over the doping limit (5 mol% Al). The binding energy of $Ni^{2+}$ was chemically shifted to $Ni^{3+}$ by doping $Al^{3+}$ ion, as confirmed by the XPS analysis. The tilted structure of the synthesized NiO with 5 mol% Al dopant and the polycrystalline structure of the $Ni_{0.75}Al_{0.25}O$ were observed by HR-TEM analysis. The electrical conductivity of the newly synthesized NiO was highly improved by Al doping in the conductivity test. The electrical conductivity values of the commercial NiO and the synthesized NiO with 5 mol% Al dopant ($Ni_{0.95}Al_{0.05}O$) were 1,400 s/cm and 2,230 s/cm at $750^{\circ}C$, respectively. However, the electrical conductivity of the synthesized NiO with 10 mol% Al dopant ($Ni_{0.9}Al_{0.1}O$) decreased due to the scattering of free-electrons caused by the large number of impurity atoms; the electrical conductivity of $Ni_{0.9}Al_{0.1}O$ was 545 s/cm at $750^{\circ}C$.

• Journal of the Korean Magnetics Society
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• v.23 no.1
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• pp.1-6
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• 2013

Spin-orbit coupling (SOC) effect is known to be the physical origin for various exotic magnetic phenomena in the low-dimensional systems. Recently, SOC also draws lots of attention in the study on magnetically doped thermoelectric alloys to determine their properties as the thermoelectric application as well as the topological insulator via the exact electronic structures determination near the Fermi level. In this research, aiming to investigate the spin-orbit coupling effect on the structural properties such as the lattice constants and the bulk modulus of the most widely investigated thermoelectric host material, $Bi_2Te_3$, we carried out the first-principles electronic structure calculation using the all-electron FLAPW (full-potential linearized augmented plane-wave) method. Employing both the local density approximation (LDA) and the generalized gradient approximation (GGA), the structural optimization is achieved by varying the in-plane lattice constant fixing the perpendicular lattice constant and vice versa, to find that the SOC effect increases the equilibrium lattices slightly in both directions while it markedly reduces the bulk modulus value implying the strong orientational dependence, which are attributed to the material's intrinsic structural anisotropy.

• Journal of the Korean Electrochemical Society
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• v.8 no.2
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• pp.82-87
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• 2005

The cathode material, $LiNi_{0.5}Mn_{1.5}O_4$, for high voltage applications of Li-ion batteries exhibits impurity phases due to oxygen deficiency during the high temperature heat treatment. The impurity phase reduces the electrochemical properties of the electrode since the deficiency spinel structure disturbs the lithium ion intercalation and deintercalation. In this study, Cr-substituted $LiNi_{0.5-x}Mn_{1.5}Cr_xO_4(0{\leq}x{\leq}0.05)$ powders are synthesized by a sol-gel method in order to reduce the amount of the impurity phases in the $LiNi_{0.5-x}Mn_{1.5}Cr_xO_4$. Thermal analysis of the cathode material shows that the $LiNi_{0.5}Mn_{1.5}O_4$ without Cr substitution looses $2\%$ of its weight due to oxygen deficiency but the amount of weight loss is diminished when Cr is substituted. XRD analysis also supports the reduction of the impurity phases in the cathode after chromium substitution, suggesting that the improvement of the electrochemical properties such as the capacity retention and electrochemical stability are attributed to the low content of impurity phases in the Cr-substituted $LiNi_{0.5-x}Mn_{1.5}Cr_xO_4.$