6 results on '"Sungwook Mhin"'
Search Results
2. The effect of pH control on synthesis of Sr doped barium titanate nanopowder by oxalate precipitation method
- Author
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Yu Rim Hong, Kyeong Ryeol Park, Jae Eun Jeon, Sungwook Mhin, HyukSu Han, and Kwang Bo Shim
- Subjects
Materials science ,Inorganic chemistry ,02 engineering and technology ,010402 general chemistry ,01 natural sciences ,Oxalate ,law.invention ,chemistry.chemical_compound ,Impurity ,law ,Phase (matter) ,Materials Chemistry ,Calcination ,Perovskite (structure) ,Process Chemistry and Technology ,Doping ,021001 nanoscience & nanotechnology ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Amorphous solid ,chemistry ,Barium titanate ,Ceramics and Composites ,0210 nano-technology ,Nuclear chemistry - Abstract
Sr substituted BaTiO 3 (BST) nanopowders were prepared using oxalate co-precipitation methods. During synthesis, oxygen was applied to the oxalate solution in order to control the oxidation states of Ti. Control of pH in the solution results in different crystalline phases of the powders after calcination; pure perovskite BST was obtained in the powder prepared at pH 3 solution, while perovskite BaTiO 3 (BTO) and impurities were observed in powders prepared at pH 1 solution. Phase transformation from amorphous to crystalline BST during heat treatment was discussed by different analysis technique. Also, morphology and elemental distribution of the pure BST was investigated.
- Published
- 2018
3. Hopping conduction in (Ni,Co,Mn)O4 prepared by different synthetic routes: Conventional and spark plasma sintering
- Author
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Jennifer S. Forrester, Kang Min Kim, Hanchan Lee, Yu-Rim Hong, Jiun Lim, HyukSu Han, Jaeseok Lee, Sungwook Mhin, and Jeong Ho Ryu
- Subjects
010302 applied physics ,Materials science ,Scanning electron microscope ,Process Chemistry and Technology ,Metallurgy ,Analytical chemistry ,Spark plasma sintering ,Sintering ,02 engineering and technology ,Activation energy ,021001 nanoscience & nanotechnology ,Polaron ,01 natural sciences ,Variable-range hopping ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,X-ray photoelectron spectroscopy ,0103 physical sciences ,Materials Chemistry ,Ceramics and Composites ,0210 nano-technology ,Temperature coefficient - Abstract
(Ni,Co,Mn)O 4 (NMC) oxides were prepared by conventional sintering (CS) and spark plasma sintering (SPS) using micro and nanopowders. Small hoping polaron theory was used in order to investigate effect of processing routes on electrical properties of NMC oxides as negative temperature coefficient (NTC) thermistors. Also, X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) techniques were utilized to analyze compositional and structural effects on the electrical properties of NMC compounds. Hopping conduction in NMC prepared by SPS and CS using nanopowder occurs via variable range hopping (VRH) mechanism, however conduction in NMC prepared by CS using micropowder follows nearest neighboring hopping (NNH) mode. Hopping distance and activation energy for the VRH mode were calculated using corresponding physical model.
- Published
- 2017
4. Fe doped Ni-Mn-Co-O ceramics with varying Fe content as negative temperature coefficient sensors
- Author
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Sungwook Mhin, Jeong Ho Ryu, Kyoung Ryeol Park, Kang Min Kim, Jung-Il Lee, and HyukSu Han
- Subjects
Materials science ,Analytical chemistry ,Sintering ,02 engineering and technology ,engineering.material ,01 natural sciences ,Crystallinity ,X-ray photoelectron spectroscopy ,Electrical resistance and conductance ,0103 physical sciences ,Materials Chemistry ,Ceramic ,010302 applied physics ,Valence (chemistry) ,Process Chemistry and Technology ,Spinel ,Metallurgy ,021001 nanoscience & nanotechnology ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,visual_art ,Ceramics and Composites ,engineering ,visual_art.visual_art_medium ,0210 nano-technology ,Temperature coefficient - Abstract
Fe-doped Ni-Mn-Co-O (FNMC) ceramics with the formula, (Fe x Ni 0.3 Co 0.9 Mn 1.8−x , x=0.1, 0.3, 0.5, and 0.7)O 4 were synthesized using a spray drying process. Effect of Fe content on the phase evolution of FNMC compound during heat treatment was studied using X-ray diffraction. A single phase cubic spinel structure with improved crystallinity was observed when the Fe content increased over 0.5. Fe, Ni, Mn and Co ions were homogeneously distributed in the sintered the FNMC ceramics, implying no phase separation occurred during sintering. The valence state of each element was analyzed using X-ray photoelectron spectroscopy, which revealed that Ni, Co, and Fe might have a single valence state while Mn had mixed valence states in the FNMC spinel compounds. The temperature-dependent electrical resistance for the FNMC ceramics was measured. Room temperature resistance and the B-value substantially increased for the FNMC samples with Fe content higher than 0.5. The electrical properties of FNMC compounds can be optimized by controlling the Fe content, which is directly indicative of their potential role as negative temperature coefficient sensors.
- Published
- 2017
5. Synthesis of CoxMn1−xO4 (0.9≤x≤2.7) nanopowders with controlled phase and composition via a gel-combustion method
- Author
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Kang Min Kim, Sunghwan Yeo, HyukSu Han, Jiun Lim, Kyoung Ryeol Park, Sungwook Mhin, Jeong Ho Ryu, Jae Seok Lee, and Jennifer S. Forrester
- Subjects
Thermogravimetric analysis ,Valence (chemistry) ,Materials science ,Infrared ,Scanning electron microscope ,Process Chemistry and Technology ,Spinel ,Analytical chemistry ,02 engineering and technology ,Crystal structure ,engineering.material ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,law.invention ,X-ray photoelectron spectroscopy ,law ,Materials Chemistry ,Ceramics and Composites ,engineering ,Calcination ,0210 nano-technology - Abstract
Co x Mn 3−x O 4 (CMO) nanopowders with a wide compositional range (0.9 ≤ x ≤ 2.7) are synthesized using a gel combustion method. The effect of Co content and calcination temperature on the crystal structure of the CMO nanopowders is studied using X-ray diffraction. A highly crystalline cubic spinel structure can be obtained at a relatively low temperature (~700 °C) for CMO with a Co content of 1.8. Further increases in Co content to 2.7 leads to a higher formation temperature (~800 °C) required in order to form a crystalline cubic structure. X-ray photoelectron spectroscopy revealed that the Co and Mn cations have mixed valence states. Energy dispersive X-ray elemental mapping indicates that Co and Mn cations are homogeneously distributed in the CMO nanopowders. Thermogravimetric analysis, Fourier transform infrared, scanning electron microscopy, and X-ray fluorescence are utilized to investigate the formation of the CMO nanopowders, as well as the compositional and structural properties.
- Published
- 2016
6. Phase evolution of (Ni, Co, Mn)O4 during heat treatment with high temperature in situ X-ray diffraction
- Author
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Sunghwan Yeo, HyukSu Han, Sungwook Mhin, Jeong Ho Ryu, Dong-Hyun Kim, and Jung-Il Lee
- Subjects
010302 applied physics ,Thermogravimetric analysis ,Materials science ,Process Chemistry and Technology ,Non-blocking I/O ,Spinel ,Thermal decomposition ,Analytical chemistry ,02 engineering and technology ,engineering.material ,021001 nanoscience & nanotechnology ,01 natural sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Differential thermal analysis ,Phase (matter) ,0103 physical sciences ,X-ray crystallography ,Materials Chemistry ,Ceramics and Composites ,engineering ,Thermal stability ,0210 nano-technology - Abstract
Phase evolution of (Ni, Co, Mn)O 4 (NMC) for negative thermal coefficient (NTC) thermistor is investigated using high temperature X-ray diffraction. Diffraction patterns were collected during heating and cooling, allowing the observation of chemical reactions in the range of temperature between 25 and 1400 °C. NiO, Mn 3 O 4 and CO 3 O 4 as the reactant materials were prepared using spray drying. As increasing temperature, phase transformation of Mn 3 O 4 to Mn 2 O 3 was first observed prior to the formation of NMC spinel structure at 700 °C. Between 1000 and 1200 °C, only spinel NMC was observed without any other phases. Also, grain growth of NMC was observed. Phase transition from cubic to tetragonal was observed during cooling. With further increasing temperature up to 1400 °C, thermal stability of NMC decrease, which was confirmed by NiO separation due to the thermal decomposition. Thermogravimetric and differential thermal analysis are performed on NMC compound and the results support the mechanism of phase formation presented by high temperature XRD data. Also, electrical resistance of NMC is measured as a function of temperature (−40–80 °C) and the factor of thermal sensitivity ( B value) is calculated.
- Published
- 2016
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