Yong Suk Yang

Comparative analysis for the crystalline and ferroelectric properties of Pb(Zr, Ti)O3 thin films deposited on metallic LaNiO3 and Pt electrodes

The crystalline quality and ferroelectricity of the Pb(Zr,Ti)O3 (PZT) films deposited on the metallic LaNiO3 (LNO) and Pt electrodes were comparatively analyzed to investigate the possibility for their application to non-volatile memory devices. LNO thin films were successfully deposited on various substrates by using r.f. magnetron sputtering even at a low temperature ranging from 250 to 500 °C, and the ferroelectric PZT thin films were spin-coated onto the LNO and Pt bottom electrodes. Metallic LNO thin films exhibited [100] orientation irrespective of the substrate species and PZT films coated onto LNO films had highly a- and c-axis orientations, while those with Pt bottom electrode were polycrystalline. PZT films with LNO bottom electrode had smaller grain size and larger dielectric constant compared to those grown on the Pt electrode. The ferroelectric thin films fabricated on LNO bottom electrode displayed an asymmetric D–E hysteresis loop, which was explained by the defect effects formed at the interface. Especially, the LNO/PZT/LNO capacitor was found to significantly improve the polarization fatigue and the effects of the LNO electrodes to the fatigue were discussed.

Schottky barrier effects in the photocurrent of sol–gel derived lead zirconate titanate thin film capacitors

We studied the photoresponse of Pb(Zr0.53Ti0.47)O3 (PZT) thin films by measuring the current–voltage (I–V) curve at several ferroelectric polarization states illuminated by a monochromatic 3.5 eV UV light. The photocurrent in Pt/PZT/Pt capacitors was sensitive to the polarization state, and the poling voltage-dependent photocurrent showed very asymmetric hysteresis behavior. The capacitance that is dependent upon the thickness of the samples was first measured. Then, the capacitance of the interfacial layer at a state with no interdiffusion between Pt and PZT film was extrapolated by using an equivalent circuit model. The result of the extrapolation was 28.1u200aμF/cm2.

Schottky barrier effects in the electronic conduction of sol–gel derived lead zirconate titanate thin film capacitors

Pure and Fe-doped Pb(Zr0.53Ti0.47)O3 (PZT) thin film capacitors were fabricated by the sol–gel method and the leakage current versus voltage characteristics of these films were investigated at several temperatures and initial polarization states. After the initial poling of ferroelectric thin films, we measured two kinds of leakage current: (i) the full-switching current measured against an initial polarization direction and (ii) the nonswitching current measured toward an initial polarization direction. In the case of the full-switching current measurement, the anomaly of leakage current due to the switching of space charges was observed, which we suggest to be accumulated at a Schottky barrier region near an electrode or a gain boundary. In the case of the nonswitching current measurement, the Schottky diode rectifier current and the field-enhanced Schottky (ES) emission current came out without the switching current. The Poole–Frenkel emission process was dominant over ES emission for a Fe-doped PZT th...

ASYMMETRY IN FATIGUE AND RECOVERY IN FERROELECTRIC PB(ZR, TI)O3 THIN-FILM CAPACITORS

We investigate the fatigue and refreshment by dc-electrical field of the electrical properties of Pt/Pb(Ti,Zr)O3/Pt ferroelectric capacitors. We find an asymmetry in the refreshment, that is, the fatigued state can be refreshed by application of negative high dc voltage to the top electrode, but no refreshment is measured by positive dc-voltage application. We also find that the fatigue can be prevented by driving the capacitor asymmetrically.

Ionic transport in mixed-alkali glasses: hop through the distinctly different conduction pathways of low dimensionality

A feature common to various solids, including single-alkali (SA) and mixed-alkali (MA) glasses, is a frequency-dependent ionic conductivity that shows the power law and the linear behavior with frequency. In spite of the advances made, the origin of this behavior continues to be controversial. We report our measurements of the conductivity of a series of MA borate glasses (Li1?xAx)2B4O7 (A?=?Na, K, Rb, Cs; 0???x???1.0) in the frequency range of 100?Hz?15?MHz and in the temperature range from 300?K to less than the glass transition temperature Tg . Using a self-similar spatial structure model, we show that the real process of ionic transport in the SA and the MA glass systems can be described by the fractional kinetic equations containing non-integer integration/differentiation operators. In the procedure of a systematic deduction of the ionic transport in glass systems, we obtained two important insights. Firstly, the time-dependent conductivity ?(t)???exp?(t/? c)? reproduces the empirical expression of mean square displacement of the mobile ions ?r2 (t)? ?t? as a first approximation of ions moving through the fractal pathway and leads to the universal power-law behavior at frequency scales. Secondly, the modified fractional Rayleigh equation with a repulsive interaction provides a quantitative explanation for experimental findings on the SA and the MA glasses. Investigations on the power-law exponent ? in the SA and the MA borate glasses indicate that the ions move through the different branches of the fractal structured conduction pathways due to the structural character, associated with a site mismatch effect, and Coulomb blockade by the randomly distributed ions.

The Effect of Sintering Parameters on the Microstructure and Properties of Mg-6% Al Alloy in Spark Plasma Sintering

In this study, the effect of sintering parameters such as sintering temperature, pressure, and degassing treatment on the microstructural evolution and properties of a Mg-6%Al alloy consolidated by spark plasma sintering (SPS) was investigated. Experimental results revealed that degassing treatment prior to sintering was a critical step in the spark plasma sintering process. We concluded that the impurity concentration on the surface of the sintered material was remarkably reduced by degassing treatment and consequently bonding strength among particles was enhanced. A magnesium alloy of high ductility and strength was obtained when a sintering temperature and pressure of 0.82 Tm and 130 MPa, respectively, was employed combined with degassing treatment. †(Received January 11, 2013)

Temperature-dependent oxygen behavior of LixNi0.5Mn1.5O4 cathode material for lithium battery

We have investigated the temperature-dependent oxygen behavior in the lithium battery cathode LixNi0.5Mn1.5O4 (LNMO) materials in the temperature range 30-1000 °C. As the temperature increases, oxygen release occurs and the change of crystal structures from the face centered cubic spinel at 30 °C to other phases follows. The amount of released oxygen and the changed crystalline phases are dependent on Li content and temperature. These phenomena are reversible against temperature in air, but not in vacuum and argon gas environments. This study illustrates the important role of temperature and atmospheric environments in synthesizing the LNMO battery materials.

Study of Glass Transition Temperatures in Sugar Mixtures by DSC

We studied the glass transition temperatures of mono‐monosaccharide and mono‐disaccharide mixtures by differential scanning calorimetry. We found that glass transition temperatures of mono‐monosaccharide mixtures can be described by the Gordon‐Taylor equation. However the glass transition temperatures of mono‐disaccharide showed a deviation form the Gordon‐Taylor equation, indicating that the molecule size has an effect on the glass transition temperature of sugar mixtures.

Lithium disilicate 유리의 입자크기에 따른 결정화 기구

We have investigated the crystallization mechanism of the lithium disilicate (, LSO) glass particles with different sizes by isothermal and non-isothermal processes. The LSO glass was fabricated by rapid quenching of melt. X-ray diffraction and differential scanning calorimetry measurements were performed. Different crystallization models of Johnson-Mehl-Avrami, modified Ozawa and Arrhenius were adopted to analyze the thermal measurements. The activation energy E and the Avrami exponent n, which describe a crystallization mechanism, were obtained for three different glass particle sizes. Values of E and n for the glass particle with size under , , and , were 2.28 eV, 2.21 eV, 2.19 eV, and ~1.5 for the isothermal process, respectively. Those values for the non-isothermal process were 2.4 eV, 2.3 eV, 2.2 eV, and ~1.3, for the isothermal process, respectively. The obtained values of the crystallization parameters indicate that the crystallization occurs through the decreasing nucleation rate with a diffusion controlled growth, irrespective to the particle sizes. It is also concluded that the smaller glass particles require the higher heat absorption to be crystallized.