Ho-Sueb Lee

SINGLE-PHASED AND EMISSION-TUNABLE CaLa2-xEuxZnO5 PHOSPHORS WITH BLUE LIGHT EXCITATION FOR WLEDS

Single-phased and emission-tunable CaLa2-xEuxZnO5 phosphors were synthesized by citric sol-gel method. Precisely adjusting the activator concentration in CaLa2ZnO5 (CLZ) phosphor, we could obtain blue-green (BG) to orange-red (OR) emissions from the 5D2, 1, 0 → 7FJ levels of Eu3+ with the blue excitation wavelength of 468 nm, which perfectly matches with the emission of blue LED chip. This fascinating result has provoked us to blend the green and red color tunable single phased phosphors with and without YAG:Ce to fabricate blue LED based white light emitting diodes (LED). The emission features thus interpreted are appropriately explained through their chromaticity coordinates and energy level diagram. Besides, its thermal stability is also investigated.

SiO2 effect on spectral and colorimetric properties of europium doped SrO2–MgO–xSiO2 (0.8 ⩽ x ⩽ 1.6) phosphor for white LEDs

Silicate phosphors with compositions 1.99 SrO2–1.0 MgO–xSiO2–0.01 Eu2O3 (x = 0.8, 1.0, 1.2, 1.4 and 1.6) were prepared in a reducing atmosphere via a solid state reaction. The resultant phosphors were examined by using x-ray diffraction and confirmed to be a mixture of monoclinic Sr2SiO4 and orthorhombic Mg2(Si2O4). The scanning electron microscope images revealed that SiO2 content does not influence the morphology of the resultant phosphors. It was also observed that the excitation spectra are dependent on the monitored emission wavelength, and the emission spectra are dependent on the excitation wavelength and the SiO2 content. The energy transfer between Eu2+ ions occupying different Sr2+ sites was discussed. The colour coordinates for these phosphors are tunable based on both the excitation wavelength and the SiO2 content.

Microstructure and Piezoelectric Properties of Lead-Free Niobate Ceramics

Recently perovskite niobates were paid much attention as lead-free piezoelectric materials to replace lead-based Pb(Zr,Ti)O3 or relaxor systems which cause much environmental problems. Piezoelectric and ferroelectric properties of electromechanical coupling coefficient and quality factor in (K0.5Na0.5)NbO3 ceramic were studied. Dielectric properties of niobates including Ag ion; AgNbO3 and (Ag0.5Na0.5)NbO3 were studied. Phase transitions and dielectric relaxations were studied from the frequency and temperature dependent dielectric constant measurements. The piezoelectric properties were hardly observed in silver niobate systems.

Enhancement of Dielectric and Piezoelectric Properties of Na0.53K0.47)(Nb0.55Ta0.45)O3 Ceramics with Na/K Ion Excess in A-site

Lead-free Ta-modified (Na0.53+xK0.47+x)(Nb0.55Ta0.45)O3 ceramics (NKNT, x = −0.01, −0.005, 0, 0.005 and 0.01) were prepared by conventional solid-state reaction. The structural and electrical properties were systematically investigated for Na/K ions non-stoichiometry. The coexistence of the orthorhombic and the tetragonal phases was confirmed near room temperature. An A-site Na/K excess improved the dielectric, piezoelectric and ferroelectric properties; in particular, the ceramic with x = 0.005 showed the highest piezoelectric constant of d33 = 340 pC/N among all the compositions studied.

Dielectric and Piezoelectric Properties of Nonstoichiometric Sr 1±x Bi 2±y Ta 2 O 9 and Sr 1±x Bi 2±y Nb 2 O 9 Ceramics

x/ y/ and y ceramics were prepared by a solid state reaction method. X-ray diffraction analysis indicated that single-phase of Bi-layered perovskite was obtained. According to Sr/Bi content ratio, Curie temperature( ), electromechanical factor( ) and mechanical quality factor( ) were measured. The Curie temperature of SBN(SBT) rose from (314) to (426) when Sr/Bi content ratio was increased. In the case of Sr/Bi content ratio

FREQUENCY AND TEMPERATURE DEPENDENCE OF ELECTRICAL PROPERTIES IN PURE AND Nb-DOPED Bi4Ti3O12 CERAMICS

ABSTRACT Pure and Nb-doped Bi4Ti3O12 ceramics were prepared by using the conventional solid state reaction method. The dielectric permittivity and electrical modulus in pure and Nb-doped Bi4Ti3O12 ceramics were measured in the temperature range of 30 ∼ 700 °C and the frequency range of 1 Hz∼1 MHz through an impedance spectroscopy method. The obtained results showed well-defined relaxation peaks. The conductivity relaxation times are determined from the peaks of the modulus. The activation energies associated with hopping condition are 0.42 eV and 0.53 eV in pure and Nb-doped Bi4Ti3O12 ceramics, respectively.

Microstructure and Ferroelectric Properties of (Bi,Nd)4Ti3O12 Thin Films Fabricated by a Sol-Gel Process

We report on the ferroelectric properties of Bi 3.54 Nd 0.46 Ti 3 O 12 (BNT) thin films fabricated on Pt(111)/Ti/SiO 2 /Si substrates by a sol-gel process. The measured XRD patterns and SEM and TEM micrographs revealed that the films showed only a Bi 4 Ti 3 O 12 -type phase with random orientations. The Au/BNT/Pt capacitors showed excellent ferroelectric properties. For the film capacitor annealed at 700°C, the remanent polarization (2P r ) and the coercive field (2E c ) were 67 μC/cm 2 and 183 kV/cm at applied electric field of 300 kV/cm, respectively. Furthermore, the Au/BNT/Pt capacitor did not show any significant fatigue up to 1.5 × 10 10 read/write switching cycles at a frequency of 1 MHz.

Ionic Doping Effect in Bi-layered Perovskite SrBi 2 Nb 2 O 9 Ferroelectrics

Doping effect of various ions in Bi-layered ferroelectric (SBN) ceramics was studied. Undoped SBN ceramic and SBN ceramics doped with , , , , , , , and ions were made by a solid state reaction. Dielectric constants were measured with temperature. Ferroelectric transition temperature decreased with , , doping, but the transition temperature increased with , , , , or ionic doping. These results show that the ion size plays an important role in the ferroelectricity of SBN ceramic.

Microstructures and Electrical Properties of Niobium-doped Bi 4 Ti 3 O 12 Thin Films Fabricated by a Sol-gel Route

Bismuth layered structure ferroelectric thin films, / (BTO) and Nb-doped BTO (BTN) were prepared on the Pt(111)/Ti//Si(100) substrates by a sol-gel route. We investigated the Nb-doping effect on the grain orientation and ferroelectric properties. ion substitution for ion in perovskite layers of BTO decreased the degree of c-axis orientation and increased the remanent polarization (2Pr). The fatigue resistance of Nb-doped BTO thin film was shown to be superior to that of BTO, and the leakage current of Nb-doped BTO thin film was decreased about 1 order of magnitude compared with BTO. The improvement of ferroelectric properties with doping in BTO could be attributed to the changes in space charge densities and grain orientation in the thin film.

Circuit Parameter Effects in Pulse Switching Responses of Ferroelectric Capacitors

We report the effects of circuit parameters such as load resistance and capacitor size on the pulse switching current responses of ferroelectric capacitors. Fully integrated (La,Sr)CoO 3 /Pb(Nb,Zr,Ti)O 3 /(La,Sr)CoO 3 thin film capacitors were used. The switching response is strongly dependent on load resistance and capacitor size. With increasing load resistance and capacitor size, switching time increased. These results were explained in terms of RC time constant of measuring circuit.