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Changwon, South Korea

Ramana C.V.,University of Texas at El Paso | Kolekar Y.D.,University of Texas at El Paso | Kamala Bharathi K.,University of Texas at El Paso | Sinha B.,Korea Institute of Material Science | Ghosh K.,Missouri State University
Journal of Applied Physics | Year: 2013

Manganese (Mn) substituted cobalt ferrites (CoFe2-xMn xO4, referred to CFMO) were synthesized and their structural, magnetic, and dielectric properties were evaluated. X-ray diffraction measurements coupled with Rietveld refinement indicate that the CFMO materials crystallize in the inverse cubic spinel phase. Temperature (T = 300 K and 10 K) dependent magnetization (M(H)) measurements indicate the long range ferromagnetic ordering in CoFe2-xMnxO4 (x = 0.00-0.15) ferrites. The cubic anisotropy constant (K1(T)) and saturation magnetization (Ms(T)) were derived by using the "law of approach" to saturation that describes the field dependence of M(H) for magnetic fields much higher than the coercive field (Hc). Saturation magnetization (Ms), obtained from the model, decreases with increasing temperature. For CoFe2O4, Ms decreases from 3.63 μB per formula unit (f.u.) to 3.47 μB/f.u. with increasing temperature from 10 to 300 K. CFMO (0.00-0.15) exhibit the similar trend while the magnitude of Ms is dependent on Mn-concentration. Ms-T functional relationship obeys the Bloch's law. The lattice parameter and magnetic moment calculated for CFMO reveals that Mn ions occupying the Fe and Co position at the octahedral site in the inverse cubic spinel phase. The structure and magnetism in CFMO are further corroborated by bond length and bond angle calculations. The dielectric constant dispersion of CFMO in the frequency range of 20 Hz-1 MHz fits to the modified Debye's function with more than one ion contributing to the relaxation. The relaxation time and spread factor derived from modeling the experimental data are ∼10-4 s and ∼0.35(±0.05), respectively. © 2013 AIP Publishing LLC.

Park Y.-S.,Kyung Hee University | Choi K.-H.,Kyung Hee University | Kim H.-K.,Kyung Hee University | Kang J.-W.,Korea Institute of Material Science
Electrochemical and Solid-State Letters | Year: 2010

We investigated the stacking sequence effect of indium zinc oxide (IZO)-Ag and IZO-Ag-IZO on the characteristics of multistacked flexible transparent electrodes for organic photovoltaics. In spite of the similar sheet resistances of the IZO-Ag and IZO-Ag-IZO stacked electrodes, the optical transparency of the latter is much higher than that of the former in the wavelength region of 400-800 nm due to the effective antireflection in the symmetric oxide-Ag-oxide structure. Furthermore, the flexible organic solar cells (OSCs) fabricated on the IZO-Ag-IZO-stacked multilayer electrode showed a higher power conversion efficiency than those fabricated on the IZO-Ag-stacked multilayer electrode due to the higher optical transparency of the former electrode in the main absorption region of the poly(3-hexylthiophere) and 1-(3-methoxycarbonyl)- propyl-1-phenyl-(6,6) C61 layers. This indicates that the multistacked electrode with the IZO-Ag-IZO sequence is more beneficial than that with the IZO-Ag sequence for low resistance and high transparency electrodes in flexible OSCs. © 2010 The Electrochemical Society.

Yoon J.,Korea Institute of Material Science | Cazacu O.,University of Florida | Mishra R.K.,General Motors
Materials Science and Engineering A | Year: 2013

In this paper an anisotropic model that accounts for the anisotropy and strong tension-compression asymmetry observed in AZ31 magnesium sheet is presented. Key in the formulation is the incorporation of distortional hardening, and consequently modeling of the effects of evolving anisotropy and evolving tension-compression asymmetry on the macroscopic response. Furthermore, application of the model to the simulation of axial crushing of a tube made of the same material reveals that strong differences between the response of AZ31 magnesium and that of usual steel qualities and aluminum should be expected. © 2012 Elsevier B.V..

Yoon J.,Korea Institute of Material Science | Park S.,Korea Institute of Material Science
Materials and Design | Year: 2014

Magnesium (Mg) alloys have been thoroughly researched to replace steel or aluminum parts in automotives for reducing weight without sacrificing their strength. The widespread use of Mg alloys has been limited by its insufficient formability, which results from a lack of active slip systems at room temperature. It leads to a hot forming process for Mg alloys to enhance the formability and plastic workability. In addition, forged or formed parts of Mg alloys should have the reliable initial yield and ultimate tensile strength after hot working processes since its material properties should be compatible with other parts thereby guaranteeing structural safety against external load and crash. In this research, an optimal warm forming condition for applying extruded Mg-Sn-Al-Zn (TAZ) Mg alloys into automotive parts is proposed based on T-shape forging tests and the feasibility of forged parts is evaluated by measuring the initial yield strength and investigating the grain size in orientation imaging microscopy (OIM) maps. © 2013 Elsevier Ltd.

Park S.H.,Korea Institute of Material Science | Yu H.,Korea Institute of Material Science | Yu H.,Shandong University | Kim H.S.,Korea Institute of Material Science | And 3 more authors.
Journal of Korean Institute of Metals and Materials | Year: 2013

The effects of copper (Cu) addition on the microstructure and mechanical properties of ZK60 alloy were investigated using an optical microscope (OM), a scanning electron microscope (SEM), and electron backscatter diffraction (EBSD) and by performing tensile tests of indirect-extruded ZK60 alloys with 0.5, 1.0, and 1.5 wt% Cu contents. The results revealed that the as-extruded ZK60 alloy had a bimodal grain structure composed of fine recrystallized grains and coarse unrecrystallized grains. The homogeneity of the microstructure was increased by Cu addition due to the promotion of dynamic recrystallization (DRX) by particle stimulated nucleation (PSN) at the Mg-Zn-Cu particles. This enhanced DRX behavior resulted In the reduction of the average grain size and weakening of the basal fiber texture of the as-extruded alloys. The yield and tensile strengths were improved by the Cu addition owing to the decreased grain size and increased number of particles, while the elongation was decreased due to the hard Mg-Zn-Cu particles. Copyright © The Korean Institute of Metals and Materials.

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