• Journal of Sensor Science and Technology
• /
• v.11 no.1
• /
• pp.48-53
• /
• 2002

Ferroelectric $Pb_{0.99}[(Zr_{0.6}Sn_{0.4})_{1-x}Ti_x]_{0.98}Nb_{0.02}O_3$(PNZST) thin films were deposited by a RF magnetron sputtering on $(La_{0.5}Sr_{0.5})CoO_3$(LSCO)/Pt/Ti/$SiO_2$/Si substrate using a PNZST target with excess PbO of 10 mole%. The crystallinity and electrical properties of the thin films with various composition ratio were investigated. The thin films deposited at the substrate temperature of $500^{\circ}C$ and the power of 80 W were crystallized to a perovskite phase after rapid thermal annealing(RTA) at $650^{\circ}C$ for 10 seconds in air. A PNZST thin films with Ti of 10 mole% showed the good crystallinity and ferroelectric properties. The remanent polarization and coercive field of the PNZST capacitor were about $20\;{\mu}C/cm^2$ and 50 kV/cm, respectively. The reduction of the polarization after $2.2{\times}10^9$ switching cycles was less than 10%.

• Applied Chemistry for Engineering
• /
• v.10 no.2
• /
• pp.293-298
• /
• 1999

Reaction mechanism was elucidated and reaction condition were optimized for the catalytic reaction synthesizing 2-methyl-4-methoxy-diphenylamine (MMDPA) which is an intermediate of Fluoran heat-sensitive dyestuff. Reactants consisted of 2-methyl-4-methoxyaniline (MMA), 3-methyl-4-nitroanisole (MNA), and cyclohexanone, and 5 wt % Pd/C was used as a catalyst. Experiments were run in an open slurry reactor equipped with reflux condenser, and products were analyzed by means of GC/MS and NMR. MMDPA yield of 90 mole % could be obtained after reaction time of 8~10 hours under the optimal reaction conditions comprising the reaction mass composition of MMA : MNA : cyclohexanone = 1 : 2 : 150 based on MMA input of 0.01 gmoles in xylene solvent, reaction temperature of $160^{\circ}C$, and catalyst amount of 0.5 g. It was found that the rate-determining step of overall reaction was dehydrogenation of the intermediate product obtained from condensation of MMA and cyclohexanone. Overall reaction rate and MMDPA yield were enhanced owing to hydrogen transfer reaction by introducing MNA together with MMA in the reaction mass. Excess cyclohexanone in the reaction mass played an important role of promoting the condensation of MMA and cyclohexanone.

• KSBB Journal
• /
• v.18 no.3
• /
• pp.190-196
• /
• 2003

An analytical system detecting DNA particularly utilizing a concept of membrane strip chromatography initially applied to home-version tests for, such as, pregnancy and ovulation has been developed. We have chosen S. typhimurium as model analyte among food-contaminating microorganisms that occurred in high frequencies, and invA gene, as a detection target, specific to Salmonella species. This gene was able to be amplified by PCR under optimal conditions employing newly designed primers in our laboratory. The PCR product was specifically measured via hybridization between the analyte and a DNA probe, which was a totally different feature from the conventional gel electrophoresis detecting the products based only on the molecular size. It is notable thar the DNA probe sequence was specially designed such that no separation of excess primers present after PCR was required. This was immobilized on a nitrocellulose (NC) membrane via streptavidin-biotin linkage minimizing a steric effect when the hybridization with the amplified DNA took place. The analyrical system detected the microorganism in a concentration of minimum $10^3$ cfu/mL (i.e., 10 cells per system), estimated from the standard curve, 20 to 40 minutes after adding the sample. This sneitivity was approximately 10 times higher than that of gel electrophoresis as an analytical tool conventionally used. Furthermore, the assay was able to be run at room temperature, which would ofter an extra advantage to users.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2013.02a
• /
• pp.183-184
• /
• 2013

Two main MBE growth techniques have been used: plasma-assisted MBE (PA-MBE), which utilizes a rf plasma to supply active nitrogen, and ammonia MBE, in which nitrogen is supplied by pyrolysis of NH3 on the sample surface during growth. PA-MBE is typically performed under metal-rich growth conditions, which results in the formation of gallium droplets on the sample surface and a narrow range of conditions for optimal growth. In contrast, high-quality GaN films can be grown by ammonia MBE under an excess nitrogen flux, which in principle should result in improved device uniformity due to the elimination of droplets and wider range of stable growth conditions. A drawback of ammonia MBE, on the other hand, is a serious memory effect of NH3 condensed on the cryo-panels and the vicinity of heaters, which ruins the control of critical growth stages, i.e. the native oxide desorption and the surface reconstruction, and the accurate control of V/III ratio, especially in the initial stage of seed layer growth. In this paper, we demonstrate that the reliable and reproducible growth of GaN on Si (110) substrates is successfully achieved by combining two MBE growth technologies using rf plasma and ammonia and setting a proper growth protocol. Samples were grown in a MBE system equipped with both a nitrogen rf plasma source (SVT) and an ammonia source. The ammonia gas purity was ＞99.9999% and further purified by using a getter filter. The custom-made injector designed to focus the ammonia flux onto the substrate was used for the gas delivery, while aluminum and gallium were provided via conventional effusion cells. The growth sequence to minimize the residual ammonia and subsequent memory effects is the following: (1) Native oxides are desorbed at $750^{\circ}C$ (Fig. (a) for [$1^-10$] and [001] azimuth) (2) 40 nm thick AlN is first grown using nitrogen rf plasma source at $900^{\circ}C$ nder the optimized condition to maintain the layer by layer growth of AlN buffer layer and slightly Al-rich condition. (Fig. (b)) (3) After switching to ammonia source, GaN growth is initiated with different V/III ratio and temperature conditions. A streaky RHEED pattern with an appearance of a weak ($2{\times}2$) reconstruction characteristic of Ga-polarity is observed all along the growth of subsequent GaN layer under optimized conditions. (Fig. (c)) The structural properties as well as dislocation densities as a function of growth conditions have been investigated using symmetrical and asymmetrical x-ray rocking curves. The electrical characteristics as a function of buffer and GaN layer growth conditions as well as the growth sequence will be also discussed. Figure: (a) RHEED pattern after oxide desorption (b) after 40 nm thick AlN growth using nitrogen rf plasma source and (c) after 600 nm thick GaN growth using ammonia source for (upper) [110] and (lower) [001] azimuth.

• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.30 no.3
• /
• pp.103-109
• /
• 2020

The effect of La₂O₃ addition on the crystalline phase, microstructure, and dielectric properties of BaTiO₃ has been studied as a function of the amounts of La₂O₃. 0.3 mol% TiO₂-excess BaTiO₃ powder was synthesized by solid-state reaction, and then the powder compacts with various amounts of La₂O₃ were sintered at 1250℃ for 2 hours. Room temperature XRD showed changes in the lattice parameters and a decrease of tetragonality (c/a) as the amounts of La₂O₃ increased. It can be explained that the phase transition from tetragonal to cubic phase occurred because La³⁺ replaced Ba²⁺ site, which increased the instability of the tetragonal phase. As La₂O₃ was added over 0.1 mol%, the critical driving force for growth (Δg_c) increased over maximum driving force (Δg_max). As the result, the grain size decreased with La₂O₃ addition. Dielectric constant decreased as the amounts of La₂O₃ increased, which was analyzed with crystal structure and microstructure.

• Journal of Radiation Protection and Research
• /
• v.17 no.2
• /
• pp.25-35
• /
• 1992

To evaluate tritium level in some environmental samples, tissue-free water tritium (TFWT) and tissue-bound tritium (TBT) were analyzed in rices, chinese cabbages and pine needles collected at 12 locations in Korea. The TFWT was recovered by freeze-drying of the samples and the TBT was obtained in the form of water by combustion of the dried samples. Tritium was measured by liquid scintillation counter. The concentrations of TFWT were in the range of $0.96{\sim}3.96 Bq/1,\; 0.83{\sim}3.40 Bq/1\;and\;1.02{\sim}3.01 Bq/1$ in rices, chinese cabbages and pine needles, respectively. The mean specific activity ratios (TBT/TFWT) were 0.94, 1.71 and 1.39 in rices, chinese cabbages and pine needles, respectiviely. This excess TBT in the samples may be attributed to the fact that the residence time of TBT in the plant is longer than that of TFWT. The specific activity ratio depends on the plant species, the exposed time to tritiated atmosphere, atmospheric moisture, temperature and diffusion factor.

• Nuclear Engineering and Technology
• /
• v.47 no.6
• /
• pp.678-699
• /
• 2015

Although the harsh space environment imposes many severe challenges to space pioneers, space exploration is a realistic and profitable goal for long-term humanity survival. One of the viable and promising options to overcome the harsh environment of space is nuclear propulsion. Particularly, the Nuclear Thermal Rocket (NTR) is a leading candidate for nearterm human missions to Mars and beyond due to its relatively high thrust and efficiency. Traditional NTR designs use typically high power reactors with fast or epithermal neutron spectrums to simplify core design and to maximize thrust. In parallel there are a series of new NTR designs with lower thrust and higher efficiency, designed to enhance mission versatility and safety through the use of redundant engines (when used in a clustered engine arrangement) for future commercialization. This paper proposes a new NTR design of the second design philosophy, Korea Advanced NUclear Thermal Engine Rocket (KANUTER), for future space applications. The KANUTER consists of an Extremely High Temperature Gas cooled Reactor (EHTGR) utilizing hydrogen propellant, a propulsion system, and an optional electricity generation system to provide propulsion as well as electricity generation. The innovatively small engine has the characteristics of high efficiency, being compact and lightweight, and bimodal capability. The notable characteristics result from the moderated EHTGR design, uniquely utilizing the integrated fuel element with an ultra heat-resistant carbide fuel, an efficient metal hydride moderator, protectively cooling channels and an individual pressure tube in an all-in-one package. The EHTGR can be bimodally operated in a propulsion mode of $100MW_{th}$ and an electricity generation mode of $100MW_{th}$, equipped with a dynamic energy conversion system. To investigate the design features of the new reactor and to estimate referential engine performance, a preliminary design study in terms of neutronics and thermohydraulics was carried out. The result indicates that the innovative design has great potential for high propellant efficiency and thrust-to-weight of engine ratio, compared with the existing NTR designs. However, the build-up of fission products in fuel has a significant impact on the bimodal operation of the moderated reactor such as xenon-induced dead time. This issue can be overcome by building in excess reactivity and control margin for the reactor design.

• Korean Journal of Materials Research
• /
• v.11 no.3
• /
• pp.178-184
• /
• 2001

Transparent and highly oriented KLN thin films have been grown by an rf- magnetron sputtering deposition method. A homogeneous and stable KLN target was prepared by calcine and sintering process. For KLN target, stoichiometry and composition excess with K of 30% and 60%, and Li of 15% and 30% respectively, was prepared. The targets were sintered at low temperature to prevent vaporization of K and Li. KLN thin films were fabricated by rf-magnetron sputtering method using those targets. In this experiment, using the target of composition excessed with K of 60% and Li of 30%, single phase KLN thin film was produced. KLN thin film has excellent crystallinity and highly c-axis oriented on Corning 1737 substrate. Transmittance of thin film in visible range was 90%, absorption edge is 333 nm and refractive index at 632.8 nm was 1.93.

• Korean Journal of Materials Research
• /
• v.12 no.9
• /
• pp.761-766
• /
• 2002

Fine $LaPO_4$:Tb phosphor particles with spherical shape were prepared by spray pyrolysis. The influence of the precursor type of phosphorous such as ($NH_4$)$_2$$HPO_4$, $NH_4$$H_2$$PO_4$, ($NH_4$)$_3$$PO_4$ and $H_3$$PO_4$ on the morphology and brightness of particles was investigated. As-prepared particles by spray pyrolysis had spherical shape when ($NH_4$)$_2$ $HPO_4$ and $NH_4$$H_2$$PO_4$ were used as the precursor of phosphorous. The precursor type of phosphorous affected the photoluminescence intensity of $LaPO_4$:Tb phosphor particles, but not significant. With changing the content of activator(Tb) and excess of phosphorous, the optimal composition giving the highest photoluminescence intensity was found. The spherical morphology of prepared $LaPO_4$:Tb particles was completely maintained even after the posttreatment up to $1050^{\circ}C$. When the posttreatment temperature was over $1100^{\circ}C$, the particles did not have the spherical shape anymore. However, the highest photoluminescence intensity of prepared $LaPO_4$:Tb particles was obtained at $1050^{\circ}C$. The photoluminescence characteristics of prepared $_LaPO4$:Tb under the vacuum ultraviolet(VUV) illumination was comparable with that of the commercial $Zn_2$$V_4$:Mn and (La,Ce)PO$_4$:Tb phosphor particles. At the optimal condition, the decay time of prepared spherical $LaPO_4$:Tb phosphor particles was about 6.8ms.

• Journal of the Korean Ceramic Society
• /
• v.39 no.9
• /
• pp.846-850
• /
• 2002

A PMN-PT-BT/Ag composite was prepared by surface modification with MgO sol with hoping to suppress silver's migration during sintering. The mixture of PbO, $N_2O_5,\;TiO_2\;with\;Mg(NO_3)_2$ instead of MgO was ball milled, the solvent was removed and then the dried powders were calcined at 950$^{\circ}C$/1h. The calcined powder were treated with 3.0 mol% $Ag_2O$ and 1.0 wt% MgO sol and calcined at 550$^{\circ}C$/1h. The dielectrics sintered at 1000$^{\circ}C$/4h under a flowing oxygen showed the density of 7.84g/$cm^3$, the room temperature dielectric constant of 18400, the dielectric loss of 2.4%, the specific resistivity of $0.24{\times}10^{12}{\Omega}{\cdot}cm$. It also showed the bending strength of $120.7{\pm}11.26$ MPa and the fracture toughness of $0.87{\pm}0.002\;MPam^{1/2}$ which were comparable to commercial PZT. The microstructure sonsisted of grains of ∼4${\mu}m$. SEM and SIMS analysis showed that Ag grew as ∼1${\mu}m$ and excess MgO as ∼0.5${\mu}m$.