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정현성,박호식,Yongho Choa,Nosang V. Myung 대한금속·재료학회 2012 ELECTRONIC MATERIALS LETTERS Vol.8 No.2
Ternary silver antimony telluride (AgxSb1-xTey) thin films with tailored compositions were synthesized by a cationic exchange reaction of thermally evaporated antimony telluride thin films, as a simple and cost-effective approach. The composition of AgxSb1-xTey thin films was controlled by the reaction time. Temperature-dependent electrical properties of the AgxSb1-xTey thin films demonstrated phase transition behavior from 323 K to 343 K. The composition-dependent thermoelectric properties (i.e., electrical resistivity (ρ), Seebeck coefficient (S) and power factor (S2ρ)) of the as-deposited Sb54Te46, the transformed AgxSb1-xTey and the annealed AgxSb1-xTey thin films were investigated as a function of temperature.
( Hongbaek Cho ),( Tadachika Nakayama ),( Daeyong Jeong ),( Satoshi Tanaka ),( Hisayuki Suematsu ),( Koichi Niihara ),( Yongho Choa ) 한국복합재료학회 2015 Composites research Vol.28 No.5
Polyvinylidene fluoride (PVDF)-based nanocomposites are fabricated by incorporation of boron nitride (BN) nanosheets with anisotropic orientation for a potential high thermal conducting ferroelectric materials. The PVDF is dissolved in dimethylformamide (DMF) and homogeneously mixed with exfoliated BN nanosheets, which is then cast into a polyimide film under application of high magnetic fields (0.45~10 T), where the direction of the filler alignment was controlled. The BN nanosheets are exfoliated by a mixed way of solvothermal method and ultrasonication prior to incorporation into the PVDF-based polymer suspension. X-ray diffraction, scanning electron microscope and thermal diffusivity are measured for the characterization of the polymer nanocomposites. Analysis shows that BN nanosheets are exfoliated into the fewer layers, whose basal planes are oriented either perpendicular or parallel to the composite surfaces without necessitating the surface modification induced by high magnetic fields. Moreover, the nanocomposites show a dramatic thermal diffusivity enhancement of 1056% by BN nanosheets with perpendicular orientation in comparison with the pristine PVDF at 10 vol % of BN, which relies on the degree of filler orientation. The mechanism for the magnetic field-induced orientation of BN and enhancement of thermal property of PVDF-based composites by the BN assembly are elucidated.
Thermoelectric Properties of Ultralong Silver Telluride Hollow Nanofibers
Zhang, Miluo,Park, Hosik,Kim, Jiwon,Park, Hyounmyung,Wu, Tingjun,Kim, Seil,Park, Su-Dong,Choa, Yongho,Myung, Nosang V. American Chemical Society 2015 Chemistry of materials Vol.27 No.15
<P>Ultralong Ag<SUB><I>x</I></SUB>Te<SUB><I>y</I></SUB> nanofibers were synthesized for the first time by galvanically displacing electrospun Ni nanofibers. Control over the nanofiber morphology, composition, and crystal structure was obtained by tuning the Ag<SUP>+</SUP> concentrations in the electrolytes. While Te-rich branched p-type Ag<SUB><I>x</I></SUB>Te<SUB><I>y</I></SUB> nanofibers were synthesized at low Ag<SUP>+</SUP> concentrations, Ag-rich nodular Ag<SUB><I>x</I></SUB>Te<SUB><I>y</I></SUB> nanofibers were obtained at high Ag<SUP>+</SUP> concentrations. The Te-rich nanofibers consist of coexisting Te and Ag<SUB>7</SUB>Te<SUB>4</SUB> phases, and the Ag-rich fibers consist of coexisting Ag and Ag<SUB>2</SUB>Te phases. The energy barrier height at the phase interface is found to be a key factor affecting the thermoelectric power factor of the fibers. A high barrier height increases the Seebeck coefficient, <I>S</I>, but reduces the electrical conductivity, σ, due to the energy filter effect. The Ag<SUB>7</SUB>Te<SUB>4</SUB>/Te system was not competitive with the Ag<SUB>2</SUB>Te/Ag system due to its high barrier height where the increase in <I>S</I> could not overcome the severely diminished electrical conductivity. The highest power factor was found in the Ag<SUB>2</SUB>Te/Ag-rich nanofibers with an energy barrier height of 0.054 eV.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/cmatex/2015/cmatex.2015.27.issue-15/acs.chemmater.5b00960/production/images/medium/cm-2015-00960d_0010.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/cm5b00960'>ACS Electronic Supporting Info</A></P>
Zhang, Miluo,Kim, Jiwon,Kim, Seil,Park, Hosik,Jung, Hyunsung,Ndifor-Angwafor, N. George,Lim, Jaehong,Choa, Yongho,Myung, Nosang V. American Chemical Society 2014 Chemistry of materials Vol.26 No.8
<P>A cost-effective process that combines electrospinning and a galvanic displacement reaction was utilized to synthesize ultralong hollow Pb<SUB><I>x</I></SUB>Se<SUB><I>y</I></SUB>Ni<SUB><I>z</I></SUB> nanofibers with controlled dimensions, morphology, composition, and crystal structure. Ni nanofibers were electrospun with an average diameter of 150 nm and were used as the sacrificial material for the galvanic displacement reaction. The composition and morphology of the Pb<SUB><I>x</I></SUB>Se<SUB><I>y</I></SUB>Ni<SUB><I>z</I></SUB> nanofibers were controlled during the reaction by tuning the concentration of HSeO<SUB>2</SUB><SUP>+</SUP> in the electrolytes. Hollow Pb<SUB><I>x</I></SUB>Se<SUB><I>y</I></SUB>Ni<SUB><I>z</I></SUB> nanofibers with smooth surfaces were obtained from the low-concentration HSeO<SUB>2</SUB><SUP>+</SUP> solution (i.e., 0.01 and 0.05 mM), but the hollow nanofibers synthesized from the high-concentration HSeO<SUB>2</SUB><SUP>+</SUP> solution (i.e., 1 mM) have rough outer surfaces with nanocrystal protrusions. The Pb content of the nanofibers’ composition was varied from 3 to 42% by adjusting the HSeO<SUB>2</SUB><SUP>+</SUP> concentration. The thermoelectric properties of the nanofiber mats were characterized, and the highest Seebeck coefficient of approximately 449 μV/K at 300 K was found for the Pb<SUB>37</SUB>Se<SUB>59</SUB>Ni<SUB>4</SUB> nanofiber mat.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/cmatex/2014/cmatex.2014.26.issue-8/cm4041067/production/images/medium/cm-2013-041067_0012.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/cm4041067'>ACS Electronic Supporting Info</A></P>
Park, Hosik,Jung, Hyunsung,Zhang, Miluo,Chang, Chong Hyun,Ndifor-Angwafor, N George,Choa, Yongho,Myung, Nosang V RSC Pub 2013 Nanoscale Vol.5 No.7
<P>Electrospinning and galvanic displacement reaction were combined to synthesize ultra-long hollow tellurium (Te) nanofibers with controlled dimensions, morphology and crystallinity by simply tailoring the electrolyte concentration applied. Within different morphologies of nanofibers, the branched Te nanostructure shows the greatest sensing performance towards NO2 at room temperature.</P>
Electrodeposition of Compact Tellurium Thick Films from Alkaline Baths
Wu, Tingjun,Zhang, Miluo,Lee, Kyu-Hwan,Lee, Chang-Myoun,Lee, Hong-Kee,Choa, Yongho,Myung, Nosang V. Electrochemical Society 2017 Journal of the Electrochemical Society Vol.164 No.2
<P>Compact semiconducting tellurium thick films (i.e., upto 50 mu m) were electrodeposited at high rates (>100 mu m h(-1)) with great current efficiency (>85%) by optimizing the electrolyte composition (TeO32- concentration and pH) and deposition potential. The preferred orientation of as-deposited Te films changed from (001) to (101) direction as the deposition potential becomes more cathodic. Average grain size ranged from 66 to 135 nm depending the deposition parameters. Electrodeposited Te films were p-type semiconductors with the carrier concentration ranged from similar to 7.0 x 10(18) to similar to 3.1 x 10(19) cm(-3). The carrier concentration of Te films strongly dependent on the average grain size where larger average grain size resulted in a lower carrier concentration because of less structural defects. (C) 2017 The Electrochemical Society. All rights reserved.</P>