Papers by Author: Sung Lim Ryu

Abstract: The (Na_0.52K_0.44)(Nb_0.9Sb_0.06)O₃-0.04LiTaO₃ (NKNS-LT) ceramics doped with various Cu₂O contents were prepared by the conventional solid state reaction method. The Cu₂O content was varied in the range of 0.1~0.4 wt%. The effects of Cu on microstructure, crystallographic phase transition, and piezoelectric properties were investigated. The material with perovskite structure had a tetragonal phase (T1) when Cu₂O concentration was less than 0.3 wt% and it was transformed to another tetragonal phase (T2) when the Cu₂O amount was greater than 0.3 wt%. The phase boundary between T1 and T2 phases was appeared at around 0.3 wt% of Cu₂O concentration. The piezoelectric properties were shown the maximum values at the composition of the phase boundary. The electro-mechanical coupling factor (k_p) was 0.42 and the piezoelectric charge constant (d₃₃) was 245 pC/N at the 0.3 wt% of Cu₂O concentration.

Abstract: Half-Heusler alloys are one of the potential thermoelectric materials for medium to high temperature range application. As a part of fundamental study to establish processing route and to observe thermoelectric properties in undoped state, ZrCoSb was selected, processed and evaluated. In an attempt to produce a half-Heusler thermoelectric materials having ultra fine grain structures, ZrCoSb was synthesized by mechanical alloying of stoichiometric elemental powder compositions, and consolidated by vacuum hot pressing. Phase transformations during mechanical alloying and hot consolidation were investigated using XRD, SEM and EDS. Single-phase, half-Heusler was successfully produced by vacuum hot pressing using as-milled powders without subsequent annealing. Thermoelectric properties as functions of temperature were evaluated in terms of Seebeek coefficient, electrical conductivity, thermal conductivity and the figure of merit for the hot pressed specimens. Mechanically alloyed half-Heusler phase, ZrCoSb, appeared to have a great potential as a thermoelectric materials in this study.

Abstract: Micro porous ceramics made out of volcanic rock have been considered as a pollution control material, since they show unique functional properties such as far infrared ray(IR) radiation, deodorization, anti bacterial function, and hygroscopic function. Here, it is well known that the ceramics have shown a mere adsorption function rather than the decomposition of absorbed substances. In an attempt to endow a sustained deodorization capability into the micro porous ceramics made out of volcanic rock, which has a normal pollution control function, TiO₂ slurry as a photo catalyst was prepared and spray coated on the surface of the ceramics. Specific heat treatment has been performed at 200°C for 30 minutes after spray coating of TiO₂ slurry and the subsequent deodorization properties were evaluated using the standard gas-bag method, which was established by Korea Photo catalyst Association and Japan Photo catalyst Association.

Abstract: Recent search for novel thermoelectric materials has revealed a new class of compounds with half-Heusler structure as good candidates for higher performance thermoelectric conversion [1-2], since the structure was first found [3]. The unit cell of the half-Heusler with a space group F4¯ 3m consists of 4 interpenetrating cubic lattices. The crystallographic sites (0,0,0) and (1/4, 1/4, 1/4) are occupied by two different transition metals, the (1/2, 1/2, 1/2) site is occupied by Sn, Sb, or Bi, and the site (3/4, 3/4, 3/4) is empty [4,5]. Candidates with this structure for the investigation would be categorized into the combination of (Ti/Zr/Hf)(Co/Ni/Pt)(Sb/Sn/Bi) [4-6].

Abstract: Ferroelectric properties of Lead-free (Bi,La)4Ti3O12 (BLT) films were evaluated on the newly developed MTP (Merged Top-electrode and Plate-line) cell structure. The BLT film was deposited by pulsed-DC sputtering method on a buried Pt/IrOx/Ir bottom electrode stack with W-plug. The BLT composition in the sintered sputtering target was Bi4.8La1.0Ti3.0O12. However, the deposited film composition was about Bi4.0La1.0Ti3.0O12 after the heat treatment of crystallization at 700°C/O2/30sec. And grains of the BLT film were randomly oriented and uniformly small ellipsoidal shape (long direction: ~100nm, short direction: ~20 nm). The remnant polarization (2Pr) and the leakage current density measured in the 100nm-thick BLT film were about 21 C/cm2 and 3 ×10-5 A/cm2 at 3 V, respectively. The fatigue loss was about 10% of the initial polarization value after 1×1011 fatigue cycles.

Abstract: 0.03Pb(Sb0.5Nb0.5)O3-0.03Pb(Mn1/3Nb2/3)O3-(0.94-x)PbTiO3-xPbZrO3 ceramics doped with Y2O3 were synthesized by conventional bulk ceramic processing technique. Phases analysis, microstructures and piezoelectric properties were investigated as a function of Y2O3 contents (0.03, 0.05, 0.1 0.3, 0.5 and 0.7 wt.%). Microstructures and phases information were characterized using a scanning electron microscope (SEM) and an X-ray diffractometer (XRD). Relative dielectric constant (K33 T) and coupling factor (kp) were obtained from the resonance measurement method. Both K33 T and kp were shown to reach to the maximum at 0.1wt.% Y2O3. In order to evaluate the stability of resonance frequency and effective electromechanical coupling factor (Keff) as a function of Y2O3 contents under strong electric field, the variation of resonance and anti-resonance frequency were also measured using a high voltage frequency response analyzer(FRA5096) under various alternating electric fields from 10V/mm to 80V/mm. It was shown that the effective electromechanical coupling factor was stabilized along with increasing Y2O3 contents.

Abstract: 1mm-thick BLT ceramics were sintered in accordance with a bulk ceramic fabrication process. All XRD peaks detected in the sintered ceramics were indexed as the Bi-layered perovskite structure without secondary phases. Density was increased with increasing the sintering temperature up to 1050°C and the maximum value was about 98% of the theoretical density. The remanent polarization (2Pr) value of BLT ceramic sintered at 1050°C was approximately 6.5 μC/cm2 at the applied voltage of 4.5kV. The calculated electromechanical couping factor (kt) of it was about 5% and the mechanical quality factor (Qm) was about 2200. From these results, a BLT ceramic target for plused laser deposition (PLD) system was successfully fabricated.

Abstract: Zn4Sb3 was successfully produced by a hot pressing technique, and its thermoelectric properties were investigated in the temperature range from 4K to 300K. The Seebeck coefficient, electrical conductivity, thermal conductivity, and thermoelectric figure of merit showed a discontinuity in variation at 242K, indicating the α-Zn4Sb3 to β-Zn4Sb3 phase transformation. Lattice thermal conductivity was found to be dominant in the total thermal conductivity of Zn4Sb3. Therefore, it is expected that thermoelectric properties can be improved by reducing the lattice thermal conductivity inducing phonon scattering centers.

Abstract: To enhance high power characteristics for piezoelectric transformer, an alloy design approach in PZT base ceramic system was considered. Various Zr/Ti ratio compositions in 0.03Pb(Sb0.5Nb0.5)O3- 0.03Pb(Mn1/3Nb2/3)O3-(0.94-x)PbTiO3-xPbZrO3 (0.445
x
0.475 mol)[PSN - PMN - PZT] system were synthesized by the conventional bulk ceramic processing technique. To improve high power characteristics at their morphotropic phase boundary (MPB) composition of PSN-PMN-PZT system, various spectra of Zr/Ti ratio were systematically experimented, and their effects on the subsequent piezoelectric properties and dielectric properties for high power piezoelectric transformer application were investigated using an impedance analyzer and a laser vibrometer. Microstructure and phase information were characterized using X-ray diffractometer (XRD) and a scanning electron microscope (SEM). When the Zr/Ti ratios were 0.475/0.465, the mechanical quality factor and electromechanical coupling factor were shown to reach the maximum, indicating that this alloy design can be a feasible composition for high power transformer.

Abstract: In an approach to acclimate ourselves to the recent ecological consciousness trends, a lead free piezoelectric material, bismuth sodium barium titanate (Bi0.5Na0.5)0.94Ba0.06TiO3 (BNBT), was considered as an environment-friendly alternative to the PZT system. A perovskite BNBT was synthesized by the conventional bulk ceramic processing technique.
La2O3 as a dopant was incorporated into the BNBT system up to 0.025 mol, and the doping effects on subsequent piezoelectric and dielectric properties were systematically investigated. In the case of La2O3 addition, the formation of grain boundary coherency was remarkably increased, and the sintered density was increased with increasing La2O3 contents. Piezoelectric and dielectric properties were shown to have the maximum value at 0.02 mol of La2O3 addition. La3+ ions were believed to act as a softener in the BNBT system and to enhance dielectric and piezoelectric properties in this study.