Soon Young Kweon

CHARACTERISTICS OF PIEZOELECTRIC MICRO-SPEAKER FABRICATED WITH ZNO THIN FILM

ABSTRACT A piezoelectric microspeaker was fabricated on the 5 × 5 mm2 diaphragm with highly c-axis oriented ZnO thin film on a heavily compressive-stressed silicon-nitride film. Four masks were used in the fabrication processes of the piezoelectric microspeaker. 0.5 μ m-thick piezoelectric ZnO film was deposited by RF-magnetron sputtering method on an Al bottom electrode. Then, 0.2 μ m-thick Al film was deposited for the top electrode. To enhance the ZnO film quality for improving the microspeaker performance, two-step deposition technique was adopted. The back-side silicon nitride was patterned, and silicon substrate was removed to make a diaphragm. The speaker output pressure level (SPL) of the fabricated microspeaker was measured with the variation of sinusoidal input frequency and compared to that of the commercial piezo-ceramic speaker. The largest SPL of the microspeaker was about 92 dB at the input voltage of 6Vpeak-to − peak, which was measured at the distance of 3 mm from the microspeaker in open field. The SPL of microspeaker was nearly same as that of a commercial piezo-ceramic speaker.

Energy efficiency alloy design in PSN-PMN-PZT ceramic system for piezoelectric transformer application

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 (0.445x0.475 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.

Effects of La2O3 on the Piezoelectric Properties of Lead-Free (Bi0.5Na0.5)0.94Ba0.06TiO3 Piezoelectric Ceramics

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.

Grain Size and Orientation Control in Lead-Free (Bi,La)4Ti3O12 Thin Film Deposited by Spin-On Method for High Density FeRAM Device

A 16Mb 1T1C FeRAM device was successfully fabricated with the lead-free BLT capacitors. The average value of the switchable polarization obtained in the 32k-array (unit capacitor size: 0.68 μm2) BLT capacitors was about 16 μC/cm2 at the applied voltage of 3V and the uniformity within an 8-inch wafer was about 2.8%. But random bit failures were detected during the measuring the bit-line signal of each cell. It was revealed that the grain size and orientation of the BLT thin film were severely non-uniform. Therefore, the grain size and orientation was optimized by varying the process conditions of nucleation step. The random bit failure issue was solved by adopting the optimized BLT film. The cell signal margin of the optimized FeRAM device was about 340 mV.

Microstructure Characteristics and Electrical Properties of Sintered (Bi,La)4Ti3O12 Ferroelectric Ceramics

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.

Fabrication of Novel Ag-Paste and Evaluation of its Electrical Properties in Bulk-Si Solar Cell

Two kinds of Ag-pastes were prepared for integrating the bulk Si solar cell. One is the Ag-paste mixed with Pb-based glass frit and the other is that mixed with Pb-free glass frit. The major components in the silver paste were 84 wt% Ag, 2 wt% glass frit, 11 wt% solvent of buthyl-cabitol acetate, and 2 wt% the other additives. After fabricating the Ag-pastes, they were coated on the SiNx/n+/p- stacks of a commercial mono-Si substrate. The solar cell efficiency was 17.6% in case of the silver paste mixed with Pb-based glass frit, which was one of the world-best records in the technology. By the way, the efficiency was 16.2% in the solar cell integrated with the silver paste mixed with Pb-free glass frit. The lower performance in the Pb-free Ag-paste was caused by the higher serial resistance and the lower shunt resistance in comparison with the Pb-containing Ag paste.

Cu2O Doping Effects on Microstructure and Piezoelectric Properties in (Na0.52K0.44)(Nb0.9Sb0.06)O3-0.04LiTaO3 Pb-Free Ceramics

The (Na0.52K0.44)(Nb0.9Sb0.06)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 (kp) was 0.42 and the piezoelectric charge constant (d33) was 245 pC/N at the 0.3 wt% of Cu2O concentration.

Microstructures and Electrical Properties of Random-Oriented (Bi,La)4Ti3O12 Thin Film Deposited by Pulsed-DC Sputtering Method

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.

Thermoelectric Properties of Nanostructured CoSb3 Synthesized by Mechanical Alloying

Undoped CoSb3 powders were synthesized by mechanical alloying of elemental powders using a nominal stoichiometric composition. Nanostructured, single-phase skutterudite CoSb3 was successfully produced by vacuum hot pressing using as-milled powders without subsequent annealing. Phase transformations during synthesis were investigated using XRD, and microstructure was observed using SEM and TEM. Thermoelectric properties in terms of Seebeck coefficient, electrical conductivity, thermal conductivity and figure of merit were systematically measured and compared with the results of analogous studies. Lattice thermal conductivity was reduced owing to increasing phonon scattering in nanostructured CoSb3, leading to enhancement in the thermoelectric figure of merit. Mechanical Alloying associated with vacuum hot pressing technique offers an alternative potential processing route for the production of skutterudites.

Hydrogen Absorption Kinetics of Ti-Nb-Cr Alloys

The Ti-Cr systems are known to consist of BCC solid solution, C36, C14, C15 Laves phase at high temperature. Among others, BCC solid solution phase has been reported to have a high hydrogen storage capacity. However, activation, wide range of hysteresis at hydrogenation/dehydrogenation, and degradation of hydrogen capacity due to hydriding/dehydriding cycles have to be improved for its application. In this study, for improving such a problem, we added on Nb. To obtain a target materials, Ti-1Nb-10Cr, Ti-3Nb-10Cr and Ti-5Nb-10Cr specimens were prepared by planetary ball mill. The milling process was carried out under nitrogen atmosphere. Specimens synthesized were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermo gravimetric analysis/differential scanning calorimetry(TG/DSC). In order to examine hydrogen absorption kinetics, Sieverts type automatic pressure-composition- isotherm (PCI) was performed at 293, 323 , 373 and 423K under 1, 5MPa hydrogen atmosphere.