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Han, HyukSu,Park, Kyoung Ryeol,Hong, Yu-Rim,Shim, Kwangbo,Mhin, Sungwook Elsevier 2018 Journal of Alloys and Compounds Vol.732 No.-
<P><B>Abstract</B></P> <P>Effect of Fe incorporation on electrical properties of (Ni<SUB>x</SUB>Co<SUB>y</SUB>Mn<SUB>3-x-y</SUB>)O<SUB>4</SUB> (NMC) spinel compound is investigated for the application to a negative temperature coefficient (NTC) thermistor. Cation distribution of the Fe doped NMC (FNMC) is calculated based on the size of ions located at tetragonal A sites and octahedral B sites in spinel structure, which can be closely related with temperature dependent electrical properties of the FNMC. With change of Fe contents, the ratio of Mn<SUP>3+</SUP>/Mn<SUP>4+</SUP> in octahedral B site of FNMC is changed which can determine the temperature sensitivity factor (B-value). Hopping conduction mode relating with activation energy and hopping distance was also discussed depending on Fe contents, based on the small polaron hopping theory.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Hopping mode in FNMC occurs through nearest neighboring hopping (NNH) motion when Fe content increases above 0.5. </LI> <LI> Variable range hopping (VRH) motion was determined as hopping conduction mechanism for FNMC with Fe content lower than 0.5. </LI> <LI> Cation distribution in FNMC with different Fe content is proposed based on the theoretical model. </LI> </UL> </P>
Han, HyukSu,Nam, Hyun Nam,Eun, Youngkee,Lee, Su Yeon,Nam, Jeongho,Ryu, Jeong Ho,Lee, Sung Yoon,Kim, Jungin The Korea Association of Crystal Growth 2016 한국결정성장학회지 Vol.26 No.5
Foam reaction injection molding (FRIM) is a widely used process for manufacturing polyurethane foam with complex shapes. Numerical model for polyurethane foam forming reaction during FRIM process has been intensively investigated by a number of researchers to precisely predict final shapes of polyurethane foams. In this study, we have identified a problem related with a previous theoretical model for polyurethane foam forming reaction. Thus, previous theoretical model was modified based on experimental and computational results.
Han, HyukSu,Nam, Hyun Nam,Eun, Youngkee,Lee, Su Yeon,Nam, Jeongho,Ryu, Jeong Ho,Lee, Sung Yoon,Kim, Jungin The Korea Association of Crystal Growth 2016 韓國結晶成長學會誌 Vol.26 No.6
Foam reaction injection molding (FRIM) is a widely used process for manufacturing polyurethane foam with complex shapes. The modified theoretical model for polyurethane foam forming reaction during FRIM process was established in our previous work. In this study, using the modified model, parametric study for FRIM process was performed in order to optimize experimental conditions of FRIM process such as initial temperature of mold, thickness of mold, and injection amount of polymerizing mixture. In addition, we applied the modified model to real application of refrigerator cabinet to determine optimal manufacturing conditions for polyurethane FRIM process.
Han, HyukSu,Lee, Hanchan,Lim, Jiun,Kim, Kang Min,Hong, Yu-Rim,Lee, Jaeseok,Forrester, Jennifer,Ryu, Jeong Ho,Mhin, Sungwook Elsevier 2017 CERAMICS INTERNATIONAL Vol.43 No.18
<P><B>Abstract</B></P> <P>(Ni,Co,Mn)O<SUB>4</SUB> (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.</P>
Fe doped Ni-Mn-Co-O ceramics with varying Fe content as negative temperature coefficient sensors
Han, HyukSu,Mhin, Sungwook,Park, Kyoung Ryeol,Kim, Kang Min,Lee, Jung-Il,Ryu, Jeong Ho Elsevier 2017 CERAMICS INTERNATIONAL Vol.43 No.13
<P><B>Abstract</B></P> <P>Fe-doped Ni-Mn-Co-O (FNMC) ceramics with the formula, (Fe<SUB>x</SUB>Ni<SUB>0.3</SUB>Co<SUB>0.9</SUB>Mn<SUB>1.8−x</SUB>, x=0.1, 0.3, 0.5, and 0.7)O<SUB>4</SUB> 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.</P>
Han, HyukSu,Lee, Jae Seok,Ryu, Jeong Ho,Kim, Kang Min,Jones, Jacob L.,Lim, Jiun,Guillemet-Fritsch, Sophie,Lee, Han Chan,Mhin, Sungwook American Chemical Society 2016 The Journal of Physical Chemistry Part C Vol.120 No.25
<P>Hopping motions in cobalt manganese spinel oxides with high cobalt concentration (CoxMn3-xO4, 2.3 <= x <= 2.7) are investigated in order to clarify the origin of unusual electrical behaviors as negative temperature coefficient (NTC) thermistors. Based on the resistance versus temperature (R-T) characteristics, hopping conduction mechanisms in MCO compounds (x = 2.3 and 2.5) are attributed to variable range hopping (VRH) motion with a parabolic distribution of the density of states (DOS) near the Fermi level. However, when Co content increases up to 2.7, transition in the hopping motion occurs from VRH to the nearest neighboring hopping (NNH) motion, which can be responsible for a huge increase of the resistance accompanied by decrease of the factor of thermal sensitivity (B value) in MCO compounds (x = 2.7). Also, hopping distance and activation energies for MCO (x = 2.3 and 2.5) compounds following VRH conduction are calculated as a function of temperature, indicating that higher B value observed in MCO (x = 2.5) compound is due to the larger hopping distance compared to that of MCO (x = 2.3) compound.</P>