Soon-Chul Ur

Effect of synthesizing method on the properties of LiFePO4/C composite for rechargeable lithium-ion batteries

Olivine-type LiFePO4/C cathode materials are fabricated with FePO4 powders that are pre-synthesized by two different processes from iron chloride solution. Process I is a modified precipitation method which is implemented by the pH control of a solution using NH4OH to form FePO4 precipitates at room temperature. Process II is a conventional precipitation method, of which H3PO4 (85%) solution is gradually added to a FeCl3 solution during the process to maintain a designated mole ratio. The solution is subsequently aged at 90°C in a water bath until FePO4 precipitates appear. In order to synthesize LiFePO4/C composites, each batch of FePO4 powders is then mixed with pre-milled lithium carbonate and glucose (8 wt. %) as a carbon source in a ball-mill. The structural characteristics of both LiFePO4/C composites fabricated using iron phospates from two different routes have been examined employing XRD and SEM. The modified precipitation process is considered to be a relatively simple and effective process for the preparation of LiFePO4/C composites owing to their excellent electrochemical properties and rate capabilities.

Phase transformation and thermoelectric properties of n-type Fe0.98Co0.02Si2 processed by mechanical alloying

Abstract n-type Fe0.98Co0.02Si2 powders have been produced by mechanical alloying process and consolidated by vacuum hot pressing. As-milled powders were of metastable state and fully transformed to β-FeSi2 phase by subsequent isothermal annealing. However, as-consolidated iron silicides consisted of untransformed mixture of α-Fe2Si5 and e-FeSi phases. Isothermal annealing has been carried out to induce the transformation to a thermoelectric semiconducting β-FeSi2 phase. The transformation behavior of β-FeSi2 was investigated by utilizing DTA, a modified TGA, SEM and XRD analyses. Isothermal annealing at 830 °C in vacuum led to the thermoelectric semiconducting β-FeSi2 phase transformation, but some residual metallic α- and a-phases were unavoidable even after 96 h of annealing. Microstructures of iron silicides were investigated using SEM and TEM. Thermoelectric properties of β-FeSi2 materials before and after isothermal annealing were evaluated in this study. It was shown that thermoelectric properties were remarkably enhanced by isothermal annealing due to the transformation from metallic α- and e-phases to semiconducting β-phases.

High temperature oxidation of an oxide-dispersion strengthened NiAl

The oxidation behavior of an oxide-dispersion strengthened (ODS) NiAl has been studied between 900 and 1100°C in air. The dispersoids of mostly Al2O3 in fine-grained β-NiAl were incorporated by mechanical alloying (MA) in an argon atmosphere and hot pressing. It was found that excessive amounts of dispersoids and voids within the matrix had serious negative effects on the oxidation resistance of β-NiAl, by allowing for a more rapid formation of oxide scales and by providing fast diffusion paths for oxygen. Below the thin surface oxide scales consisted of α-Al2O3, NiAl2O4 and Ni2O3, an internal oxidation zone was formed deep into the matrix. No metastable transient aluminas were formed during oxidation. The oxide ridge structure began to evolve after oxidation at 1100°C at the oxide–gas interface.

Electronic transport properties of Ni-doped CoSb3 prepared by encapsulated induction melting

Ni-doped CoSb3 skutterudites were prepared by encapsulated induction melting and their thermoelectric and electronic transport properties were investigated. The negative signs of Seebeck and Hal coefficients for all Ni-doped specimens revealed that Ni atoms successfully acted as n-type dopants by substituting Co atoms. The carrier concentration increased as the Ni doping content increased, and the Ni dopants could generate excess electrons. However, the carrier mobility decreased as the doping content increased, which indicates that the electron mean free path was reduced by the impurity scattering. The Seebeck coefficient and the electrical resistivity decreased as the carrier concentration increased, as the increase in carrier concentration by doping overcame the decrease in the carrier mobility by impurity scattering. The Seebeck coefficient showed a negative value at all temperatures examined and increased as the temperature increased. The temperature dependence of electrical resistivity suggested that Co1−xNixSb3 is a highly degenerate semiconducting material. Thermal conductivity was considerably reduced by Ni doping, and the lattice contribution was dominant in the Ni-doped CoSb3.

Dependence of material properties on piezoelectric microspeakers with AlN thin film

This paper reports the dependence of residual stress on the piezoelectric microspeakers that are audible in open air with high quality piezoelectric AlN thin film deposited onto Mo/Ti electrode. This successful achievement is followed by using a compressively stressed silicon nitride film as a supporting diaphragm and high quality AlN thin film with compressive residual stress (less than -100 MPa). The sound pressure level (SPL) is relatively small when we use a tensile stressed silicon nitride film, the SPL of the fabricated microspeakers that have compressively stressed composite diaphragm shows more than 60 dB from 100 Hz to 15 kHz and the highest SPL is about 100 dB at 9.3 kHz with 20 Vpeak-to-peak sinusoidal input biases and 10 mm distances from the fabricated microspeakers to the reference microphone. The packaging structure enhances the low frequency characteristics significantly.

MICROMACHINED PIEZOELECTRIC MICROSPEAKERS FABRICATED WITH HIGH QUALITY ALN THIN FILM

ABSTRACT This paper describes the micromachined piezoelectric microspeakers that can produce the audible signal with 20 V peak-to-peak input voltages. The diaphragm size is 4 × 4 mm2 and the thickness of diaphragm is around 1 micron meter except partially etched piezoelectric area. The maximum sound output pressure of the microspeaker is even higher than ever before with a small diaphragm in high frequency range around 10 kHz. This successful result bases upon using high quality AlN thin film. The deposited AlN thin film shows c-axis oriented columnar structure and very fine grains. The highest SPL (Sound Pressure Level) measured from 300 Hz to 12 kHz shows about 100 dB around 10 kHz in case of circular type microspeaker and about 76 dB in case of cross type, respectively.

Effect of high-energy milling process on microstructure and piezoelectric/dielectric properties of 0.99Pb(Zr0.53Ti0.47)O3-0.01BiYO3 ceramic for piezoelectric energy harvesting devices

A piezoelectric ceramic, 0.99Pb(Zr0.53Ti0.47)O3-0.01BiYO3 [PZT-BY], was fabricated via high-energy milling (HEM) and conventional ball milling methods. The HEM process improved the reaction activity and homogeneity of the materials used throughout the process and reduced the calcination temperature by 150°C compared with the conventional process. The effects of the HEM process on the microstructure, dielectric, and piezoelectric properties of the PZT-BY ceramic were systematically investigated. The modified method using HEM resulted in a high d33 of 315 pC/N and kp of 63%, with a low K33T of 684, whereas the conventional process had values of 280 pC/N, 56%, and 748 respectively. In addition, piezoelectric voltage constant (g33), and transduction coefficient (d33 × g33) have also been evaluated. The large (d33 × g33) of 16,568 × 10−15 m2/N indicates that the PZT-BY ceramic fabricated by the HEM process is a good candidate material for energy harvesting devices.

Thermoelectric properties of mechanically alloyed iron disilicides consolidated by various processes

Mn doped p-type iron disilicide powders have been produced by a mechanical alloying process. As-milled powders were of metastable state and mostly transformed to β-FeSi2 phase by subsequent isothermal annealing. As-milled powders were consolidated by various processes such as sintering of the cold compact in vacuum, vacuum hot pressing (VHP), and spray drying/atmospheric plasma thermal spraying. Phase transitions during the processes were investigated using XRD, EDS, and SEM. As-consolidated specimens consisted of a mixture of α-Fe2Si5 and ε-FeSi phases, which were gradually transformed into a thermoelectric semiconducting α-FeSi2 phase by subsequent isothermal annealing in the vicinity of 845°C in vacuum. However, some residual α and ε phases remained even after prolonged annealing. Thermoelectric properties were measured as a function of temperature and correlated with phase transformation. They showed optimum values in the vacuum hot pressed specimen due to a higher fraction of β phase and/or higher density.

The effects of residual stresses in the composite diaphragm on the performance of piezoelectric microspeakers

Piezoelectric microspeakers are fabricated with AlN film on circular and square Mo/Ti electrodes, and the acoustic pressures are measured with and without packaging structure as a function of frequency. As residual stresses of the composite diaphragm are decreased from +20 MPa to −20 MPa, the output pressure of the fabricated chip (square electrodes) is increased from 1.16 mPa to 96.2 mPa. The packaged microspeaker shows enhanced uniform sound pressure level (SPL) from 1 kHz to 15 kHz with more than 100 dB SPL at 2.50 kHz, with square electrodes in particular. For the circular electrodes, the resonant frequency is revealed to be 3.06 kHz, and the SPL at resonant frequency is measured at 107 dB, which is somewhat higher than for the square electrode. However, the output pressures of the microspeaker show lower values than that of the square electrodes at 1 kHz sinusoidal frequency.

Properties of Co-doped N-type FeSi2 processed by mechanical alloying

Iron-silicide was produced with a mechanical alloying process and consolidated through vacuum hot pressing. The as-milled powders were of metastable state and fully transformed into the ß-FeSi2 phase through subsequent isothermal annealing. The as-consolidated iron silicides consisted of an untransformed mixture of α-Fe2Si5 and ɛ-FeSi phases and a partially transformed β-FeSi2 phase was found in the low density compact. Isothermal annealing was carried out to induce transformation into a thermoelectric semiconducting β-FeSi2 phase. The transformation behavior of the β-FeSi2 was investigated utilizing DTA, SEM, and XRD analyses. Isothermal annealing at 830°C in vacuum led to a thermoelectric semiconducting β-FeSi2 phase transformation, but some residual metallic α and ɛ-phases were unavoidable even after 96 hours of annealing. The iron silicide microstructures were investigated using SEM and TEM. The mechanical and thermoelectric properties of the β-FeSi2 materials before and after isothermal annealing are characterized in this study.