K.W. Cho

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.

Mechanical Alloying and Thermoelectric Properties of CoSb3 Skutterudite

In an effort to synthesize homogenized single phase d-CoSb3, this study considers the mechanical alloying (MA) of elemental Co and Sb powders using a nominal stoichiometric composition followed by hot pressing. Single phase, undoped CoSb3 skutterudites were successfully produced by vacuum hot pressing using MA powders without subsequent annealing. Phase transformations during mechanical alloying, powder annealing, and hot pressing were systematically investigated using XRD and SEM. Thermoelectric properties were measured and compared with the results of similar studies. Temperature dependences were also evaluated, and their correlations to phase transformation were examined.

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.

The Effect of NiO Addition to the PNN-PZT Piezoelectric Ceramics on Piezoelectric Properties

Perovskite ceramics were synthesized by conventional ceramic processing technique. In order to modify piezoelectric properties for sensor application in this system, NiO addition was considered to provide as an acceptor, which was known to occupy with B site in the structure. The effect of NiO addition up to on the following piezoelectric properties as well as sintering properties was investigated. When NiO added more than , average grain size was decreased and second phase was found to form. Moreover, the second phase caused decrease in relative dielectric constant , electro-mechanical coupling factor , and piezoelectric charge constant , while increasing mechanical quality factor . When NiO was added, density, dielectric properties and piezoelectric properties were abruptly increased.

Thermoelectric Properties of Co1-xNbxSb3 Prepared by Induction Melting

Induction melting was attempted to prepare the undoped and Nb-doped CoSb3 compounds, and their thermoelectric properties were investigated. Single phase d-CoSb3 was successfully obtained by induction melting and subsequent annealing at 400°C for 2 hours in vacuum. The positive signs of Seebeck coefficients for all the specimens revealed that Nb atoms acted as p-type dopants by substituting Co atoms. Electrical conductivity decreased and then increased with increasing temperature, indicating mixed behaviors of metallic and semiconducting conductions. Electrical conductivity increased by Nb doping, and it was saturated at high temperature. Maximum value of the thermoelectric power factor was shifted to higher temperature with the increasing amount of Nb doping, mainly originated from the Seebeck coefficient variation.

Ferroelectric Properties of Lead-Free (Bi,La)4Ti3O12 Thin Film Deposited on MTP Cell Structure for High Density FeRAM Device

Ferroelectric properties of Pb-free (Bi,La)4Ti3O12 (BLT) films were optimized on a newly developed MTP cell structure. BLT films were coated on Pt/IrOx/Ir bottom electrode using sol-gel solutions. The composition of the optimized BLT film was about Bi3.25La0.75Ti3.0O12, which was analyzed by ICP-MS method. The switchable polarization obtained in a 100nm-thick BLT film was about 20 uC/cm2 at the 3 V applied voltage, and the optimized BLT film showed little fatigue loss about 10% up to 1×1011 cycles. The imprint properties of the BLT film were also characterized at 25 °C and 90 °C operating temperature after 125 °C data storage. Regardless of operating temperature, switchable polarization of BLT had a sufficiently large margin for device operation up to 10 years.

Thermoelectric Properties of Skutterudite CoSb 3 Prepared by Arc Melting

The arc melting was employed to prepare undoped compounds and their thermoelectric properties were investigated. Specimen annealed at for 24 hrs showed sound microstructure. However, considerable voids and cracks were found after annealing at above . It seems to be attributed to the phase dissociation and thermal expansion due to phase transitions during annealing and cooling. Single phase was successfully obtained by annealing at for 24 hrs. In the case of increasing annealing temperature, phase decompositions occurred. Undoped showed p-type conduction and intrinsic semiconducting behavior at all temperatures examined. Thermoelectric properties were remarkably improved by annealing and they were closely related to phase transitions.

Phase Transformation and Thermoelectric Properties of Fe 0.92 Mn 0.08 Si 2 Prepared by Mechanical Alloying

In an attempt to enhance phase transformation and homogenization of Mn-doped , mechanical alloying of elemental powders was applied. Cold pressing and sintering in vacuum were carried out to produce a dense microstructure, and then isothermal annealing was employed to induce a phase transformation to the -semiconductor. Phase transitions in this alloy system during the process were investigated by using XRD, EDS and SEM. As-milled powders after 100 h of milling were shown to be metastable state. As-sintered iron silicides consisted of untransformed mixture of -and -FeSi phases. -phase transformation was induced by subsequent isothermal annealing at , and near single phase of -was obtained after 24 h of annealing. Thermoelectric properties in terms of Seebeck coefficient, and electrical conductivity were evaluated and correlated with phase transformation. Seebeck coefficient electrical resistivity and hardness increased with increasing annealing time due to phase transformation.