The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Sang Sub Kim - One of the best experts on this subject based on the ideXlab platform.
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bifunctional sensing mechanism of sno2 zno composite nanofibers for drastically enhancing the sensing behavior in h2 gas
ACS Applied Materials & Interfaces, 2015Co-Authors: Akash Katoch, Jae-hun Kim, Yong Jung Kwon, Hyoun Woo Kim, Sang Sub KimAbstract:SnO2–ZnO composite nanofibers fabricated using an electrospinning method exhibited exceptional hydrogen (H2) sensing behavior. The existence of tetragonal SnO2 and hexagonal ZnO Nanograins was confirmed by an analysis of the crystalline phase of the composite nanofibers. A bifunctional sensing mechanism of the composite nanofibers was proposed in which the combined effects of SnO2–SnO2 homointerfaces and ZnO–SnO2 heterointerfaces contributed to an improvement in the H2 sensing characteristics. The sensing process with respect to SnO2–ZnO heterojunctions is associated not only with the high barrier at the junctions, but also the semiconductor-to-metallic transition on the surface of the ZnO Nanograins upon the introduction of H2 gas.
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growth behavior and sensing properties of Nanograins in cuo nanofibers
Chemical Engineering Journal, 2011Co-Authors: Sun-woo Choi, Jae Young Park, Sang Sub KimAbstract:Abstract The growth behavior of Nanograins in CuO nanofibers was investigated. The apparent activation energy for the growth of Nanograins is estimated to be ∼18 kJ/mol, which is smaller by one order of magnitude compared with those of bulk oxide ceramics. The isothermal grain growth behavior exhibits the lattice diffusion in a pore control scheme as a main growth mechanism. The sensing properties of the sensors fabricated with the CuO nanofibers have been investigated in terms of CO and NO2. Importantly, the sensitivity of the sensor fabricated with CuO nanofibers having larger Nanograins is much higher than that of the sensor with smaller Nanograins. The results suggest not only that the electrospinning-synthesized p-type CuO nanofibers hold promise for realizing sensitive and reliable gas sensors, also that calcination conditions need to be optimized to obtain the best sensing properties.
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growth behavior of Nanograins in nio fibers
Materials Chemistry and Physics, 2011Co-Authors: Sun-woo Choi, Jae Young Park, Sang Sub KimAbstract:The growth kinetics of Nanograins in NiO nanofibers has been investigated. Individual NiO nanofiebrs synthesized by electrospinning consist of Nanograins. The Nanograins coalesce and grow at the expense of smaller ones under higher calcination temperatures and prolonged calcination times. The growth kinetics of Nanograins makes a transition during growth at a critical size of ∼30 nm in diameter. At the early stage of growth, the activation energy and the growth exponent are found to be 10.94 kJ mol−1 and 9.09, respectively. They change to 137.58 kJ mol−1 and 2.04 at the later stage of growth at the critical size of nanogrians. In that growth stage, bamboo-like grains start to evolve.
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growth kinetics of Nanograins in sno2 fibers and size dependent sensing properties
Sensors and Actuators B-chemical, 2011Co-Authors: Jae Young Park, Sun-woo Choi, K Asokan, Sang Sub KimAbstract:Abstract The present study investigates the growth kinetics of SnO 2 Nanograins and determines the activation energy and mechanism of the growth in nanofiber form. The activation energy for the growth of the SnO 2 Nanograins was estimated to be ∼28.28 kJ/mol, which is an order of magnitude smaller than that of bulk SnO 2 . The estimated m value suggests that the growth mechanism of the Nanograins is primarily through lattice diffusion in the pore control scheme. Precise control of the calcination temperature and time is necessary to maximize the efficiency of electrospinning-synthesized SnO 2 nanofibers for sensor applications. Importantly, the sensor fabricated with nanofibers of small Nanograins showed much better sensing properties to CO and NO 2 comparing with the sensor fabricated with nanofibers of large Nanograins. A mechanism to explain this finding is suggested.
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Growth kinetics of Nanograins in Co3O4 fibers
Ceramics International, 2011Co-Authors: Sun-woo Choi, Jae Young Park, Sang Sub KimAbstract:Abstract The growth kinetics of Nanograins in Co 3 O 4 nanofibers has been investigated. Individual fibers were made up of Nanograins. The Nanograins were observed to coalesce and grow at the expense of the smaller ones, similar to the phenomenon observed in the sintering process of bulk ceramics. The activation energy and the growth kinetics of Nanograins were estimated, showing the dominant growth mechanism of Nanograins to be likely related to a lattice diffusion process.
Jae Young Park - One of the best experts on this subject based on the ideXlab platform.
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growth behavior and sensing properties of Nanograins in cuo nanofibers
Chemical Engineering Journal, 2011Co-Authors: Sun-woo Choi, Jae Young Park, Sang Sub KimAbstract:Abstract The growth behavior of Nanograins in CuO nanofibers was investigated. The apparent activation energy for the growth of Nanograins is estimated to be ∼18 kJ/mol, which is smaller by one order of magnitude compared with those of bulk oxide ceramics. The isothermal grain growth behavior exhibits the lattice diffusion in a pore control scheme as a main growth mechanism. The sensing properties of the sensors fabricated with the CuO nanofibers have been investigated in terms of CO and NO2. Importantly, the sensitivity of the sensor fabricated with CuO nanofibers having larger Nanograins is much higher than that of the sensor with smaller Nanograins. The results suggest not only that the electrospinning-synthesized p-type CuO nanofibers hold promise for realizing sensitive and reliable gas sensors, also that calcination conditions need to be optimized to obtain the best sensing properties.
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growth behavior of Nanograins in nio fibers
Materials Chemistry and Physics, 2011Co-Authors: Sun-woo Choi, Jae Young Park, Sang Sub KimAbstract:The growth kinetics of Nanograins in NiO nanofibers has been investigated. Individual NiO nanofiebrs synthesized by electrospinning consist of Nanograins. The Nanograins coalesce and grow at the expense of smaller ones under higher calcination temperatures and prolonged calcination times. The growth kinetics of Nanograins makes a transition during growth at a critical size of ∼30 nm in diameter. At the early stage of growth, the activation energy and the growth exponent are found to be 10.94 kJ mol−1 and 9.09, respectively. They change to 137.58 kJ mol−1 and 2.04 at the later stage of growth at the critical size of nanogrians. In that growth stage, bamboo-like grains start to evolve.
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growth kinetics of Nanograins in sno2 fibers and size dependent sensing properties
Sensors and Actuators B-chemical, 2011Co-Authors: Jae Young Park, Sun-woo Choi, K Asokan, Sang Sub KimAbstract:Abstract The present study investigates the growth kinetics of SnO 2 Nanograins and determines the activation energy and mechanism of the growth in nanofiber form. The activation energy for the growth of the SnO 2 Nanograins was estimated to be ∼28.28 kJ/mol, which is an order of magnitude smaller than that of bulk SnO 2 . The estimated m value suggests that the growth mechanism of the Nanograins is primarily through lattice diffusion in the pore control scheme. Precise control of the calcination temperature and time is necessary to maximize the efficiency of electrospinning-synthesized SnO 2 nanofibers for sensor applications. Importantly, the sensor fabricated with nanofibers of small Nanograins showed much better sensing properties to CO and NO 2 comparing with the sensor fabricated with nanofibers of large Nanograins. A mechanism to explain this finding is suggested.
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Growth kinetics of Nanograins in Co3O4 fibers
Ceramics International, 2011Co-Authors: Sun-woo Choi, Jae Young Park, Sang Sub KimAbstract:Abstract The growth kinetics of Nanograins in Co 3 O 4 nanofibers has been investigated. Individual fibers were made up of Nanograins. The Nanograins were observed to coalesce and grow at the expense of the smaller ones, similar to the phenomenon observed in the sintering process of bulk ceramics. The activation energy and the growth kinetics of Nanograins were estimated, showing the dominant growth mechanism of Nanograins to be likely related to a lattice diffusion process.
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growth of Nanograins in tio2 nanofibers synthesized by electrospinning
Journal of Nanoscience and Nanotechnology, 2010Co-Authors: Jae Young Park, Sun-woo Choi, K Asokan, Sang Sub KimAbstract:Present study focuses the effect of calcination temperature and its duration on the morphology and growth of Nanograins in individual TiO2 nanofibers synthesized by electrospinning method. Polyvinyl acetate and titanium tetraisopropoxide were used as chemical precursors along with other standard solvents in the synthesis process. This study shows that synthesized TiO2 nanofibers are randomly arranged and spreads uniformly over the Si substrate and possess polycrystalline nature consisting of Nanograins. Similar to the sintering behavior generally observed in bulk ceramics, the Nanograins coalesce and grow under higher calcination temperature and longer calcination time. The activation energy for the growth of Nanograins is found to be 47.2 kJ/mol. The dominant growth mechanism changes depending on the stages of calcination.
Sun-woo Choi - One of the best experts on this subject based on the ideXlab platform.
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Nanograins in electrospun oxide nanofibers
Metals and Materials International, 2015Co-Authors: Akash Katoch, Sun-woo ChoiAbstract:Oxide nanofibers synthesized by the electrospinning method have received considerable attention owing to their potential applications in various fields. This paper provides an overview of the growth behavior and the importance of the presence of Nanograins in oxide nanofibers synthesized by the electrospinning method. The growth behavior of Nanograins in various oxide nanofibers is described in terms of its effect on activation energy and growth exponent, which are then compared with the bulk counterparts. The lower activation energy of Nanograins in nanofibers by an order of magnitude revealed that the active participation of Nanograins during grain growth is due to higher chemical potential of atoms presented in nanosized grains. In addition, the influences of Nanograins on the electrical, gas-sensing, magnetic, optical, and photocatalytic properties of nanofibers are discussed. It is shown that optimization of the nanograin size is essential to ensure that the advantages of oxide nanofibers are utilized in different applications.
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growth behavior and sensing properties of Nanograins in cuo nanofibers
Chemical Engineering Journal, 2011Co-Authors: Sun-woo Choi, Jae Young Park, Sang Sub KimAbstract:Abstract The growth behavior of Nanograins in CuO nanofibers was investigated. The apparent activation energy for the growth of Nanograins is estimated to be ∼18 kJ/mol, which is smaller by one order of magnitude compared with those of bulk oxide ceramics. The isothermal grain growth behavior exhibits the lattice diffusion in a pore control scheme as a main growth mechanism. The sensing properties of the sensors fabricated with the CuO nanofibers have been investigated in terms of CO and NO2. Importantly, the sensitivity of the sensor fabricated with CuO nanofibers having larger Nanograins is much higher than that of the sensor with smaller Nanograins. The results suggest not only that the electrospinning-synthesized p-type CuO nanofibers hold promise for realizing sensitive and reliable gas sensors, also that calcination conditions need to be optimized to obtain the best sensing properties.
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growth behavior of Nanograins in nio fibers
Materials Chemistry and Physics, 2011Co-Authors: Sun-woo Choi, Jae Young Park, Sang Sub KimAbstract:The growth kinetics of Nanograins in NiO nanofibers has been investigated. Individual NiO nanofiebrs synthesized by electrospinning consist of Nanograins. The Nanograins coalesce and grow at the expense of smaller ones under higher calcination temperatures and prolonged calcination times. The growth kinetics of Nanograins makes a transition during growth at a critical size of ∼30 nm in diameter. At the early stage of growth, the activation energy and the growth exponent are found to be 10.94 kJ mol−1 and 9.09, respectively. They change to 137.58 kJ mol−1 and 2.04 at the later stage of growth at the critical size of nanogrians. In that growth stage, bamboo-like grains start to evolve.
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growth kinetics of Nanograins in sno2 fibers and size dependent sensing properties
Sensors and Actuators B-chemical, 2011Co-Authors: Jae Young Park, Sun-woo Choi, K Asokan, Sang Sub KimAbstract:Abstract The present study investigates the growth kinetics of SnO 2 Nanograins and determines the activation energy and mechanism of the growth in nanofiber form. The activation energy for the growth of the SnO 2 Nanograins was estimated to be ∼28.28 kJ/mol, which is an order of magnitude smaller than that of bulk SnO 2 . The estimated m value suggests that the growth mechanism of the Nanograins is primarily through lattice diffusion in the pore control scheme. Precise control of the calcination temperature and time is necessary to maximize the efficiency of electrospinning-synthesized SnO 2 nanofibers for sensor applications. Importantly, the sensor fabricated with nanofibers of small Nanograins showed much better sensing properties to CO and NO 2 comparing with the sensor fabricated with nanofibers of large Nanograins. A mechanism to explain this finding is suggested.
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Growth kinetics of Nanograins in Co3O4 fibers
Ceramics International, 2011Co-Authors: Sun-woo Choi, Jae Young Park, Sang Sub KimAbstract:Abstract The growth kinetics of Nanograins in Co 3 O 4 nanofibers has been investigated. Individual fibers were made up of Nanograins. The Nanograins were observed to coalesce and grow at the expense of the smaller ones, similar to the phenomenon observed in the sintering process of bulk ceramics. The activation energy and the growth kinetics of Nanograins were estimated, showing the dominant growth mechanism of Nanograins to be likely related to a lattice diffusion process.
Zenji Horita - One of the best experts on this subject based on the ideXlab platform.
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optical properties of nanocrystalline monoclinic y2o3 stabilized by grain size and plastic strain effects via high pressure torsion
Inorganic Chemistry, 2017Co-Authors: Hadi Razavikhosroshahi, Zenji Horita, Kaveh Edalati, Hoda Emami, Etsuo Akiba, Masayoshi FujiAbstract:Yttrium oxide (yttria) with monoclinic structure exhibits unique optical properties; however, the monoclinic phase is thermodynamically stable only at pressures higher than ∼16 GPa. In this study, the effect of grain size and plastic strain on the stability of monoclinic phase is investigated by a high-pressure torsion (HPT) method. A cubic-to-monoclinic phase transition occurs at 6 GPa, which is ∼10 GPa below the theoretical transition pressure. Microstructure analysis shows that monoclinic phase forms in Nanograins smaller than ∼22 nm and its fraction increases with plastic strain, while larger grains have a cubic structure. The band gap decreases and the photoluminescence features change from electric dipole to mainly magnetic dipole without significant decrease in the photoluminescence intensity after formation of the monoclinic phase. It is also suggested that monoclinic phase formation is due to the enhancement of effective internal pressure in Nanograins.
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allotropic phase transformation and photoluminescence of germanium Nanograins processed by high pressure torsion
Journal of Materials Science, 2016Co-Authors: Yoshifumi Ikoma, Takamitsu Toyota, Yoshimasa Ejiri, Katsuhiko Saito, Qixin Guo, Zenji HoritaAbstract:We report on allotropic phase transformation and nanograin refinement of Ge by severe plastic deformation using high-pressure torsion (HPT) under a pressure of 24 GPa. No appreciable formation of metastable phases occurred under compression prior to torsion, while a diamond cubic Ge-I phase and a tetragonal Ge-III phase were observed in the HPT-processed samples. The formation of the Ge-III phase was enhanced by introduction of shear strain. TEM observations revealed that HPT-processed samples consisted of micro- and Nanograins. It was indicated that grain refinement occurred due to the introduction of high density of lattice defects in metallic Ge-II during HPT processing, and then Ge-II transformed not only back to Ge-I but also to metastable Ge-III upon unloading. The Ge-III phase reversely transformed to Ge-I by intense Ar-ion laser irradiation or by thermal annealing. No appreciable photoluminescence (PL) was observed from the HPT-processed sample, while a broad PL peak in the range of 600–800 nm appeared after intense laser irradiation. A similar PL peak was also observed from thermally annealed samples. These results suggest that the appearance of the PL peak arises from Ge-I Nanograins.
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phase transformation and nanograin refinement of silicon by processing through high pressure torsion
Applied Physics Letters, 2012Co-Authors: Yoshifumi Ikoma, Katsuhiko Saito, Kazunori Hayano, Kaveh Edalati, Zenji HoritaAbstract:Si(100) wafers were subjected to severe plastic deformation under a pressure of 24 GPa using high-pressure torsion (HPT). Si wafers were plastically deformed at room temperature. HPT-processed samples were composed of metastable body centered cubic Si-III and rhombohedral Si-XII phases in the initial cubic diamond Si-I. The volume fraction of metastable phases increased with increasing plastic strain. Successive annealing at 873 K led to the reverse transformation of metastable phases. A broad photoluminescence peak centered at about 650 nm appears due to the reverse transformation of Si-III/Si-XII Nanograins and the reduction of number of defects in Si-I Nanograins.
H P Karnthaler - One of the best experts on this subject based on the ideXlab platform.
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size effects on martensitic phase transformations in nanocrystalline niti shape memory alloys
Materials Science and Technology, 2008Co-Authors: T Waitz, Thomas Antretter, F D Fischer, H P KarnthalerAbstract:AbstractResults of a systematic study are presented to review various effects of crystal size on the martensitic phase transformations in nanocrystalline NiTi shape memory alloys. The transformation temperatures and the transformed volume fraction strongly decrease with decreasing grain size less than about 100 nm. Transformation to martensite is not observed in grains smaller than a critical grain size of about 50 nm. The Nanograins significantly impact the morphology of B19′ martensite composed of (001) compound twins that occur at an atomic scale and violate the well established theory of martensite formation. Self-accommodation occurs by a herringbone morphology of two twinned variants. Contrary to the martensite, grain size hardly impacts the transformation to the R-phase. The experimental results are explained by a size dependent transformation barrier that accounts for the suppression of the martensitic transformation, its thermal stability and unique morphology in the Nanograins.
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size effects on the martensitic phase transformation of niti Nanograins
Journal of The Mechanics and Physics of Solids, 2007Co-Authors: T Waitz, Thomas Antretter, F D Fischer, N K Simha, H P KarnthalerAbstract:Abstract The analysis of nanocrystalline NiTi by transmission electron microscopy (TEM) shows that the martensitic transformation proceeds by the formation of atomic-scale twins. Grains of a size less than about 50 nm do not transform to martensite even upon large undercooling. A systematic investigation of these phenomena was carried out elucidating the influence of the grain size on the energy barrier of the transformation. Based on the experiment, Nanograins were modeled as spherical inclusions containing (0 0 1) compound twinned martensite. Decomposition of the transformation strains of the inclusions into a shear eigenstrain and a normal eigenstrain facilitates the analytical calculation of shear and normal strain energies in dependence of grain size, twin layer width and elastic properties. Stresses were computed analytically for special cases, otherwise numerically. The shear stresses that alternate from twin layer to twin layer are concentrated at the grain boundaries causing a contribution to the strain energy scaling with the surface area of the inclusion, whereas the strain energy induced by the normal components of the transformation strain and the temperature dependent chemical free energy scale with the volume of the inclusion. In the Nanograins these different energy contributions were calculated which allow to predict a critical grain size below which the martensitic transformation becomes unlikely. Finally, the experimental result of the atomic-scale twinning can be explained by analytical calculations that account for the transformation-opposing contributions of the shear strain and the twin boundary energy of the twin-banded morphology of martensitic Nanograins.
