Papers by Author: Man Soon Yoon

Abstract: Manganese silicide is one of the potential thermoelectric materials for high temperature application. As a fundamental investigation on these materials group, manganese monosilicides were synthesized by the mechanical alloying of stoichiometric elemental powder compositions, and the as-milled powders were consolidate by vacuum hot pressing. Phase transformation and microstructure evolution during mechanical alloying and hot consolidation were investigated using XRD and SEM. Near single phase of monosilicides were successfully produced in this process. Thermoelectric properties as a function of temperature were evaluated in terms of Seebeek coefficient, electrical resistivity, thermal conductivity and the figure of merit for the hot pressed specimens. Mechanically alloyed manganese monosilicide, MnSi, appeared to have a potential as a thermoelectric material in this study.

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: 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: 0.03Pb(Sb0.5Nb0.5)O3-0.03Pb(Mn1/3Nb2/3)O3-(0.94-x)PbTiO3-xPbZrO3 ceramics doped with CeO2 were synthesized by conventional bulk ceramic processing technique. Phases analysis, microstructures and piezoelectric properties were investigated as a function of CeO2 contents (0.03, 0.05, 0.1 0.3, 0.5 and 0.7wt%). 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% CeO2. In order to evaluate the stability of resonance frequency and effective electromechanical coupling factor (Keff) as a function of CeO2 addition 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 effective electromechanical coupling factor was increased by increasing the CeO2 additions.

Abstract: Processing and properties of a dome-shaped piezoelectric transformer with a composition of 0.03Pb(Sb0.5Nb0.5)O3-0.03Pb(Mn1/3Nb2/3)O3-0.465PbTiO3-0.475PbZrO3 have been investigated. A dome-shaped sample was fabricated by powder injection molding. The dimension of the dome-shaped sample was a 28 mm in diameter and 2.1mm in thickness with a curvature radius of 18 mm. Finite element modeling for the complicated piezoelectric transformer was applied to simulate vibration mode in the sample. The high power characteristics of a dome-shaped piezoelectric transformer were examined by the lighting test for a 55W PL lamp. The 55W PL lamp was successfully driven by the dome-shaped piezoelectric transformer with sustaining efficiency higher than 98%. The transformer with ring/dot area ratio of 2.1 exhibited the maximum properties in terms of output power, efficiency and temperature stability.

Abstract: This paper describes a micromachined piezoelectric bimorph microphone, which is built on a low stress Parylene diaphragm with high quality ZnO films. The ZnO thin film has fine grain with the average grain size less than 1
and the grain orientation is perpendicular to the substrate. The highest sensitivity of piezoelectric bimorph microphone fabricated in this study is about 0.74 mV/Pa, which is twice higher than that of an individual segmented electrode. Based upon the experimental results, we proposed the equivalent electrical circuit model of piezoelectric bimorph microphone.

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: Processing and properties of a dome-shaped piezoelectric transformer with a composition of 0.03Pb(Sb0.5Nb0.5)O3-0.03Pb(Mn1/3Nb2/3)O3-0.465PbTiO3-0.475PbZrO3 have been investigated. A dome-shaped sample was fabricated by powder injection molding. The dimension of the domeshaped sample was a 28 mm in diameter and 2.1mm in thickness with a curvature radius of 18 mm. Finite element modeling for the complicated piezoelectric transformer was applied to simulate strains and vibration mode in the sample. The high power characteristics of a dome-shaped piezoelectric transformer were examined by the lighting test for 27 and 34 watt T5 circle lamps connected in series. The series connected T5 circle lamps were successfully driven by the domeshaped piezoelectric transformer with sustaining efficiency higher than 98.5%. The transformer with ring(input)/dot(output) electrode ratio of 2.5 exhibited the maximum properties in terms of output power, efficiency and temperature stability.

Abstract: The electromechanical properties of a newly proposed 3-dimensional piezoelectric transformer have been investigated. Especially, the effects of 3-dimensional geometry on the maximum tip displacement were carefully investigated. As a result, it was found that the maximum strain of the 3-dimensional piezoelectric device was significantly enhanced up to 4.5 times higher than that of a disk shape device. This data were in good agreement with the finite element model analysis of strains and vibration modes. Moreover, a very high voltage step-up ratio of 290 (10 times higher than the Rosen type), sustaining efficiency more than 96%, were achieved.

Abstract: The electromechanical properties of a newly proposed 3-dimensional piezoelectric actuator have been investigated. Especially, the effects of 3-dimensional geometry on the maximum tip displacement were carefully investigated. As a result, it was found that the maximum strain of the 3-dimensional piezoelectric device was significantly enhanced up to 4.5 times higher than that of a disk shape device. This data was in good agreement with the finite element model analysis of strains and vibration modes. Moreover, the field -induced displacement stability of dome-shaped 3- dimensional piezoelectric actuator at various ac freguencies was superior to Rainbow actuator.