• Korean Journal of Materials Research
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• v.34 no.3
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• pp.138-143
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• 2024

Ni-CNT nanocomposites were synthesized via the electrical explosion of wire (EEW) in acetone and deionized (DI) water liquid conditions with different CNT compositions. The change in the shape and properties of the Ni-CNT nanopowders were determined based on the type of fluids and CNT compositions. In every case, the Ni nanopowder had a spherical shape and the CNT powder had a tube shape. However, the Ni-CNT nanopowders obtained in DI water exhibited irregular shapes due to the oxidation of Ni. Phase analysis also revealed the existence of nickel oxide when using DI water, as well as some unknown peaks with acetone, which may form due to the metastable phase of Ni. Magnetic properties were investigated using a Vibrating Sample Magnetometer (VSM) for all cases. Nanopowders prepared in DI water conditions had better magnetic properties than those in acetone, as evidenced by the simultaneous formation of super paramagnetic NiO peaks and ferromagnetic Ni peaks. The DI water (Ni:CNT = 1:0.3) sample revealed better magnetic results than the DI water (Ni-CNT = 1:0.5) because it had less CNT contents.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2009.11a
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• pp.263-263
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• 2009

The microtstructures and properties of alloys in the pseudo-binary $Bi_2Te_3-PbTe$ system were investigated as a first step towards the design of nanostructured materials with enhanced thermoelectric properties. The liquid alloys were cooled by water quenching method. Dendritic and lamellar structures were observed clearly by using environmental scanning electron microscope(eSEM) and electron probe micro analyzer(EPMA) take into account composition ratio between $Bi_2Te_3$ and PbTe. The compound $Pb_2Bi_6Te_{11}$ precipitated as a metastable phase under all conditions. The structure of those samples changed from dendritic to lamellar by increasing $Bi_2Te_3$ ratio of composition.

• Journal of Powder Materials
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• v.11 no.6 s.47
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• pp.467-471
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• 2004

In order to fabricate a high density sintered body of ITO, nano-sized ITO powders were synthesized by coprecipitation methods. Aqueous solutions of indium and tin salts were mixed and coprecipitated by changing their pH. Coprecipitated ITO powders possessed 20-30 nm crystallite size and a relatively high BET value ($35m^2/g$), however, aggregation of particles were occurred. Therefore, a novel recrystallization technique was applied in order to eliminate the aggregates. The recrystallized ITO material consists of a little bit larger needlelike crystals, $20nm{\times}80nm$, and it possesses a higher BET value $(57m^{2}/g)$ compared to the plain coprecipitated material $(35m^{2}/g)$. Metastable phase formation and higher content of aggregated particles were observed in the coprecipitated materials. Densification was 95% to 98% complete after 5 hour sintering at $1500^{\circ}C$ for the recrystallized powders while densities of the coprecipitated powders were below 75%.

• The Korean Journal of Ceramics
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• v.1 no.2
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• pp.75-80
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• 1995

Development of phases, evolution of microstructures, and dielectric response characteristics of amorphous lead iron tungstates during crystabllization were investiageted. A series of mircographs showing the evolution sequence of microstructures is presented. Crystallization was observed to initiate from inside of the amorphous material. A cubic perovskite phase developed fully at $760^{\circ}C$ from amorphous state via intermediate metastable crystalline structures. Dielectric constant of amorphous PFM was totally insensitive to the temperature change around the Curie temperature of crystalline material. Sintered pellet, with relative density of 96% and an almost pore-free dense internal microstructure, could be prepared from amorphous powder.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1996.10a
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• pp.157-165
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• 1996

Characteristics of microstructure, hardness and wear of two AI-20Si-5Fe-2Ni materials produced by powder metallurgy and spray forming respectively were studied at temperatures up to 560$^{\circ}C$. Above 300$^{\circ}C$, the microstructure of powder processed alloy became inhomogenous, whereas the spray formed alloy showed homogeneous microstructures. Hardness of both alloys was increased between 300$^{\circ}C$~490$^{\circ}C$ in both alloy. It was interpreted that increase in hardness was mainly related to the formation of stable and fine intermetallic phase from metastable one. It was showed that hoogenity of microstructure in the rapidliy soldified affected directly wear property.

• Journal of Magnetics
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• v.1 no.1
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• pp.42-45
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• 1996

We have studied the superconductivity of rapidly quenched $YPd_2B_2C$ ribbons. The superconductivity in bulk $YPd_2B_2C$ is completely suppressed by annealing as-cast ingots at hihger temperature, suggesting that the superconducting phase is metastable. The rapidly quenched sample, in the as-quenched state, shows the superconducting transition temperature of about 21 K. Form our magnetic data, the value for the upper critical field $H_c2$ (0) is estimated to be 10 T. The critical current density and the expected specific heat discontinuity $({\Delta}C/{\gamma}{T_c})$ were obtained to be about$10^8A/m^2$ at 10 Oe and 1.55, respectively.

• Bulletin of the Korean Chemical Society
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• v.31 no.11
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• pp.3225-3227
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• 2010

The thermal conductivity of layered metal chalcogenides such as $MT_2$ (M = Mo, W; T = S, Se) shows a marked decrease after exfoliation and subsequent restacking process. Random stacking of two-dimensional crystalline sheets circumvents thermal conduction pathways along a longitudinal direction, which results in a reduction in thermal conductivity. $WS_2$ and $WSe_2$ compounds retain p-type conducting behavior after exfoliation and restacking with decreased electrical conductivity due to the change in carrier concentration. $MoSe_2$ compound exhibits metallic behavior < $130^{\circ}C$ with a small Seebeck coefficient, which results from metastable 1T-$MoSe_2$ structure of the restacked phase.

• Journal of Welding and Joining
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• v.15 no.5
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• pp.25-35
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• 1997

현재, 표면개질에 주로 많이 사용되는 레이저는 세 종류로서, C $O_{2}$$laser(파장길이:10.6.\mum),$ Nd:YAG(neodymium-doped yettrium aluminum garnet) $laser(파장길이:1.06.\mum)$ 및 excimer laser(157~350nm) 등이다. 이 외에도 초기에는 ruby레이저빔이 사용되기도 하였으나, 현재는 많이 사용되고 있지 않다. 레이저 빔에 의한 표면개질에는 몇가지 장점이 있는데, 이러한 장점은 주로 급속가열과 급속냉각 효과에 기인하는 것이다. 즉, 1) 급냉효과에 의한 미세한 결정입자 형성, 2) 불안정상 (metastable phase) 또는 비정질 상 생성, 3) 열역학적 용해도 보다 높은 용해도. 4) 편석이 없는 균질한 미세조직, 5) 극히 낮은 기공도, 6) 좁은 열영향 부위, 7) 표면층과 모재 사이의 높은 결합력 등이다. 이 외에도 공정상의 장.단점들이 Ref.5, 6에 잘 요약 정리되어 있다. 지금까지 국내에서 레이저 표면개질에 대한 조사가 몇몇 있었으나, 본 조사에서는 보통 많이 다루어지지 않은 부분, 즉 충격경화 및 표면제어에 비중을 두었으며, 비용융 부분(I)과 용융부분(II)을 분리하여 정리하였다.

• Journal of the Korean Physical Society
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• v.73 no.10
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• pp.1561-1565
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• 2018

The dielectric properties of the uniaxial relaxor ferroelectric $Sr_xBa_{1-x}Nb_2O_6$ with x = 0.75 were investigated along the polar [001] direction as a function of temperature. The capacitance maximum showed the frequency dispersion commonly observed in relaxors. Additional weak dielectric anomalies were observed in the paraelectric phase; they were only seen during the heating process and disappeared upon subsequent cooling. These were attributed to the existence of large polar clusters strongly pinned at defects and/or to random fields and their metastable characters. Aligning the ferroelectric domains along the polar axis at room temperature removed the high-temperature dielectric anomalies. The dependences of the capacitance and the dielectric maximum temperature on the magnitude of the poling field were investigated.

• Korean Journal of Materials Research
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• v.14 no.3
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• pp.218-223
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• 2004

P-type $\beta$-FeSi$_2$ with a nominal composition of $Fe_{0.92}Mn_{0.08}Si_2$ powders has been produced by mechanical alloying process. As-milled powders were spray dried and consolidated by atmospheric plasma thermal spraying as a rapid sintering process. As-milled powders were of metastable state and fully transformed to $\beta$-$FeSi_2$ phase by subsequent isothermal annealing. However, as-thermal sprayed $Fe_{0.92}Mn_{0.08}Si_2$ consisted of untransformed mixture of $\alpha$-$Fe_2Si_{5}$ and $\varepsilon$-FeSi phases. Isothermal annealing has been carried out to induce transformation to the thermoelectric semiconducting $\beta$-$FeSi_2$ phase. Isothermal annealing at $845^{\circ}C$ in vacuum gradually led to the thermoelectric semiconducting $\beta$-$FeSi_2$ phase transformation, but some residual metallic $\alpha$ and $\varepsilon$ phases were unavoidable even after prolonged annealing. Thermoelectric properties of $\beta$-$FeSi_2$ materials before and after isothermal annealing were evaluated. Seebeck coefficient increased and electric conductivity decreased with increasing annealing time due to the phase transition from metallic phases to semiconducting phases. Thermoelectric properties showed gradual increment, but overall properties appeared to be inferior to those of vacuum hot pressed specimens.