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Jayasimhadri, M.,Jha, Kaushal,Ratnam, B.V.,Woo, Hyun-Joo,Jang, Kiwan,Rao, A.S.,Haranath, D. ELSEVIER SCIENCE 2017 JOURNAL OF ALLOYS AND COMPOUNDS Vol.711 No.-
<P><B>Abstract</B></P> <P>In this work, Tb<SUP>3+</SUP> ions doped lead-germanate-tellurite (LGT) glasses were prepared by conventional melt quenching technique with different dopant concentrations ranging from 0.5 to 3.5 mol %. X-ray diffraction (XRD) and FT-IR analysis were carried out to analyze the structural properties of LGT glass. The excitation spectra revealed a single band centered in the NUV region at 380 nm by monitoring emission at 545 nm. The emission spectra consist of four bands, which are attributed to the <SUP>5</SUP>D<SUB>4</SUB>→<SUP>7</SUP>F<SUB>J</SUB> (J = 3–6) transitions. Among these transitions, the strong emission band was observed at 545 nm corresponding to the <SUP>5</SUP>D<SUB>4</SUB>→<SUP>7</SUP>F<SUB>5</SUB> transition and the optimized doping concentration of Tb<SUP>3+</SUP> ions was 2 mol %. The Huang's theory and I-H model indicate the possibility of energy transfer via electric dipole-dipole interaction between Tb<SUP>3+</SUP> ions. The CIE chromaticity coordinates were (<I>x</I> = 0.282 and <I>y</I> = 0.614) and emits intense yellowish green light. The decay curves measured for <SUP>5</SUP>D<SUB>4</SUB> level for the samples with different doping concentrations and the lifetime for the optimized sample was 548 μs. The results indicate that these glasses have potential applications in solid state lighting and display devices.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Optically transparent Tb<SUP>3+</SUP> doped PbO-GeO<SUB>2</SUB>-TeO<SUB>2</SUB> (LGT) glasses have been prepared. </LI> <LI> Excitation spectrum consists of single band in the NUV region at 380 nm. </LI> <LI> CIE coordinates indicate that Tb<SUP>3+</SUP> doped LGT glass emits yellowish green light. </LI> <LI> Huang's theory applied to reveal the energy transfer process between Tb<SUP>3+</SUP> ions. </LI> <LI> This glass may be potential to use in fabricating GaN based WLEDs. </LI> </UL> </P>
Ratnam, B.V.,Jayasimhadri, M.,Bhaskar Kumar, G.,Jang, K.,Kim, S.S.,Lee, Y.I.,Lim, J.M.,Shin, D.S.,Song, T.K. Elsevier Sequoia 2013 JOURNAL OF ALLOYS AND COMPOUNDS Vol.564 No.-
<P>An efficient green emitting Tb3+ doped NaCaPO4 (NCP) phosphor was synthesized by using conventional solid-state reaction for solid-state lighting applications. X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), FT-IR, emission and excitation properties were extensively investigated for NCP phosphors. X-ray diffraction analysis confirmed the formation of NaCaPO4 with orthorhombic structure. The excitation spectrum consists of strong 4f-4f transition at around 370 nm, which has higher intensity than the f-d transition. Emission spectra indicated that this phosphor can be efficiently excited by UV light in the range from 250 to 400 nm, and shows strong emission band centered at 547 nm. Analysis of the emission spectra with different Tb3+ concentrations revealed that the optimum dopant concentration for these NCP phosphors is about 5 mol% of Tb3+. Diminishing of D-5(3) level and increasing of D-5(4) level emission intensity with the Tb3+ concentration explained successfully. The emission color was analyzed and confirmed with the help of chromaticity coordinates and color temperature. The excellent luminescent properties of NaCaPO4: Tb3+ phosphor makes it as a potential green phosphor upon near-UV LED excitation. (C) 2013 Elsevier B.V. All rights reserved.</P>
Eswaraiah, S.,Kim, Yong Ha,Liu, Huixin,Ratnam, M. Venkat,Lee, Jaewook TERRA SCIENTIFIC PUBLISHING COMPANY 2017 Earth, Planets and Space Vol.69 No.1
<P>We have investigated the coupling between the stratosphere and mesosphere-lower thermosphere (MLT) in the Southern Hemisphere (SH) during 2010 minor sudden stratospheric warmings (SSWs). Three episodic SSWs were noticed in 2010. Mesospheric zonal winds between 82 and 92 km obtained from King Sejong Station (62.22 degrees S, 58.78 degrees W) meteor radar showed the significant difference from usual trend. The zonal wind reversal in the mesosphere is noticed a week before the associated SSW similar to 2002 major SSW. The mesosphere wind reversal is also noticed in 'Specified Dynamics' version of Whole Atmosphere Community Climate Model (SD-WACCM) and Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) simulations. The similar zonal wind weakening/reversal in the lower thermosphere between 100 and 140 km is simulated by GAIA. Further, we observed the mesospheric cooling in consistency with SSWs using Microwave Limb Sounder data. However, the GAIA simulations showed warming between 130 and 140 km after few days of SSW. Thus, the observation and model simulation indicate for the first time that the 2010 minor SSW also affects dynamics of the MLT region over SH in a manner similar to 2002 major SSW.</P>
Eswaraiah, S.,Kim, Yong Ha,Hong, Junseok,Kim, Jeong-Han,Ratnam, M. Venkat,Chandran, A.,Rao, S.V.B.,Riggin, Dennis Elsevier 2016 Journal of Atmospheric and Solar-Terrestrial Physi Vol.140 No.-
<P><B>Abstract</B></P> <P>A minor stratospheric sudden warming (SSW) event was noticed in the southern hemisphere (SH) during September (day 259) 2010 along with two episodic warmings in early August (day 212) and late October (day 300) 2010. Among the three warming events, the signature of mesosphere response was detected only for the September event in the mesospheric wind dataset from both meteor radar and MF radar located at King Sejong Station (62°S, 59°W) and Rothera (68°S, 68°W), Antarctica, respectively. The zonal winds in the mesosphere reversed approximately a week before the September SSW event, as has been observed in the 2002 major SSW. Signatures of mesospheric cooling (MC) in association with stratospheric warmings are found in temperatures measured by the Microwave Limb Sounder (MLS). Simulations of specified dynamics version of Whole Atmosphere Community Climate Model (SD-WACCM) are able to reproduce these observed features. The mesospheric wind field was found to differ significantly from that of normal years probably due to enhanced planetary wave (PW) activity before the SSW. From the wavelet analysis of wind data of both stations, we find that strong 14–16 day PWs prevailed prior to the SSW and disappeared suddenly after the SSW in the mesosphere. Our study provides evidence that minor SSWs in SH can result in significant effects on the mesospheric dynamics as in the northern hemisphere.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The mesosphere response to 2010 minor SSW over the Antarctica was observed. </LI> <LI> The mesospheric wind field is found to significantly differ from normal years. </LI> <LI> Mesospheric cooling (MC) in association with SSWs are found. </LI> <LI> 14–16 day planetary waves are noticed in mesosphere before the onset of SSW. </LI> <LI> SD-WACCM simulations are able to reproduce observed MC and wind reversal. </LI> </UL> </P>