Papers by Author: Bon Heun Koo

Abstract: We report the magnetic transition and large magnetic entropy change in Sr doped lanthanum manganites. Polycrystalline La_1-xSr_xMnO₃ (0.20 x 0.35) samples were prepared using the conventional solid-state reaction method. The results of X-ray diffraction indicates perovskite phase without any impurity. The magnetic study has revealed that the Curie temperature is influenced by Sr-concentration. The doping of Sr at La site affects the Mn-O bond length and Mn-O-Mn bond angle due to the difference in their ionic radii, consequently, the Curie temperature changed. A large magnetic entropy change has been observed for La_0.8Sr_0.2MnO₃ sample, the value of the maximum entropy change (SMmax) increases from 1.42 to 2.74 J/kgK as magnetic field increases from 1 to 2.5 T. This investigation suggests that La_1-xSr_xMnO₃ can be used as a potential magnetic refrigeration material.

Abstract: In the present work, we have prepared ZnO nanoparticles by a two-step mechanochemical synthesis method. The reaction was carried out in a paste state at room temperature with a short grinding time of 20 min. The prepared ZnO nanoparticles were characterized by using x-ray diffraction (XRD), transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis-differential thermal analysis (TGA/DTA). XRD and TEM results demonstrated that ZnO have a single phase nature with wurtzite structure with high crystallinity. The lattice parameters calculated from XRD pattern are a= 3.25 Å and c= 5.248 Å and the average grain size of the ZnO nanoparticles was found to be ~ 20 nm (TEM) or ~22 nm (XRD). FTIR spectra demonstrated the peak at ~455 cm^-1 which correspond to stretching mode of ZnO.

Abstract: AlON-Al₂O₃ coatings were prepared on Al2021 alloy by the electrolytic plasma processing (EPP) method. NaNO₂, NaNO₃ and NH₄NO₃ were chosen as nitrogen supply agents. The nitrogen inducing effect was studied by a combined composition and structure analysis of the coating layer carried out by X-ray diffractometer (XRD), scanning electron microscopy (SEM) for the specimens EPP-treated at room temperature for 15 min under a hybrid voltage of 260V DC plus 200V AC (50Hz) power. Microhardness tests and wear tests were carried out to correlate the evolution of microstructure and resulting mechanical properties.

Abstract: Al7075 alloy has typically good mechanical properties. Al7075 alloy has an ultimate tensile strength and yield strength, which is commonly used in the manufacturing of heavy-duty structures requiring good corrosion resistance and in applications requiring high strength to weight ratio, as well as good fatigue resistance. However, as compared to strength and corrosion of conventional steel material, it has some drawbacks. To overcome these disadvantages, Electrolytic plasma processing (EPP) was used. (EPP) is a novel electrochemical and physical surface treatment process for generating protective coatings on light metals. Because, these coating scan present high hardness and continuous barriers, it can offers good protection against abrasion, corrosion and heat as well as electrical insulation. The EPP treatments were carried out under a hybrid voltage of AC 200V (50Hz) combined with DC 260V power supply.

Abstract: The composites of (La_0.7Ca_0.3MnO₃)_0.97/(ZnO)_0.03 were fabricated with having different sintering temperature by conventional solid state reaction methods, and their magnetoresistance (MR) properties and electrical transport were investigated. The result of scanning electronic microscopy (SEM) and X-ray diffraction (XRD) indicated that no new phase appeared in the composites except LCMO and ZnO phases. specially, SEM results show that with increment of sintering temperature, the grain size of La_0.7Ca_0.3MnO₃ also increases, which plays a key on electrical and magnetic properties of the (La_0.7Ca_0.3MnO₃)_0.97/(ZnO)_0.03 composites. The electrical transport and magnetic was measured over a temperature range of 90 320K. tables etc. (2012H1B8A2026212), (NIPA-2013-H0301-13-2009)

Abstract: The ceramic coatings were prepared on the T6-tempered Al6061 alloy substrate under a hybrid voltage by PEO treatment in 5-30 min in a phosphate electrolyte. The X-Ray diffraction (XRD) and scanning electron microscopy (SEM) were used to investigate the coating microstructures. With increasing the PEO treating time, the different compositions of coatings eliminate the sharp slope of the microhardness profile. The coatings PEO-treated in 30min presents excellent abrasive resistance property.

Abstract: 2021 series aluminum alloy is used as the matrix material for its wide application in engineering to make AlON coating layers by the electrolytic plasma processing (EPP) method. The experiments were carried out on 2021 Al alloys in alkaline electrolytes which are eco-friendly and low-cost. The experimental electrolyte composition includes: 2g/L NaOH as the electrolytic conductive agent, 6~14g/L Na3PO4 as alumina formative agent, 0.5g/L NaNO3 as a nitrogen inducing agent. The effects of phosphate content variation are evaluated by a combined composition and structure analysis of the coating layer using with Philips-X’Pert X-ray diffractometer, JSM 5610 scanning electron microscopy for the specimens EPP-treated at room temperature in 10 min under a hybrid voltage (260V DC + 200V AC-50Hz). In addition, microhardness of the ceramic coatings was measured to correlate the evolution of microstructure and resulting mechanical properties. The wear tests show that a composite of AlON-Al2O3 high anti-abrasive coating formed as a result of a reactive process between Al in the alloy itself and O-N supplied by the electrolyte.

Abstract: The composites of (1-x)La0.7Ca0.3MnO3/xCoO and (1-x)La0.7Sr0.3MnO3/xCoO (x =0 ~ 0.09) were fabricated by conventional solid state reaction method, and their electrical transport and magnetoresistance (MR%) properties were investigated by physical property measurement system (PPMS). The result of x-ray diffraction (XRD) and scanning electronic microscopy (SEM) indicated that no new phase appeared in the composites except manganites matrix and CoO phases. CoO is mainly distributed at the grain boundaries and surfaces of the matrix. The resistivity LCMO and LSMO based composites were measured in the range 90-320K and 90-400K, respectively. The applied magnetic field is 0T and 0.5T. The observed variation of MR with varying Ca and Sr concentration has been qualitatively investigated.

Abstract: Composites with compositions La0.7Ca0.3MnO3(LCMO) and La0.7Sr0.3MnO3(LSMO)/SnO2 were prepared by a standard ceramic technique. The structure and morphology of the composites have been studied by the X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The XRD and SEM results indicate that no reaction occurs between La based maganite and SnO2 grains, and that SnO2 segregates mostly at the grain boundaries of La based maganite. The variation in resistivity with temperature has been studied and shows a semiconducting behavior, furthermore the composites exhibit metallic percolation. It is interesting to note that an enhanced magnetoresisitance (MR) effect for the composites is found over a wide temperature range from low temperature to room temperature in an applied magnetic field of 0.5 Tesla. The spin-polarized tunneling and the spin-polarized tunneling may be attributed to the enhanced low-field magnetoresistance (LFMR) effect.

Abstract: We indicated synthesization of LCMO by hydrothermal reaction. The results of transmission electron microscopy revealed that the LCMO particles had wide range in size and various in shapes. The LCMO particles had a perovskite-type crystal structure with some other phases. Magnetic property was measured by physical property measurement system. Their crystallinity and magnetization tended to increased with increasing reaction time.