• 한국산학기술학회논문지
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• 제16권7호
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• pp.4835-4841
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• 2015

n형 FeSi2의 열전물성에 미치는 입자크기 및 성형압력의 영향에 대해 조사하였다. 입자크기가 다른 출발 분말을 각각 가압성형(성형압력; $70{\sim}220kg/cm^2$) 하였고, 제작한 성형체를 Ar 분위기 1473 K에서 7시간 소결한 후, 반도체상인 ${\beta}$상을 얻기 위해 1103 K에서 100시간 소둔처리 하였다. XRD, SEM 및 EDS를 이용해서 시편들의 미세구조 및 상분석을 행하였다. 동일 시료를 가지고 Ar 분위기 상온~1023 K에서 도전율과 Seebeck 계수를 동시에 측정하였다. 입자크기가 작을수록 소결밀도와 잔존 ${\varepsilon}$-FeSi 금속전도상 증가에 의해 도전율이 상승하였으며, Seebeck 계수는 700~800 K에서 최고값을 나타내었고, 입자크기가 작을수록 잔존 ${\varepsilon}$-FeSi 금속전도상 증가에 의해 감소하였다. 반면에 성형압력의 변화는 도전율 및 Seebeck 계수에 그다지 큰 영향을 미치지 않았다. 결과적으로 power factor는 성형압력 보다 입자크기에 큰 영향을 받았다.

• 한국재료학회지
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• 제15권4호
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• pp.217-223
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• 2005

Thermal, electrical and mechanical properties of high purity niobium and tantalum refractory rare metals were investigated tn evaluate the physical purity. Higher purity niobium and tantalum metals showed lower hardness due to smaller solution hardening effect. Temperature dependence of electrical resistivity showed a typical metallic behavior. Remarkable decrease in electrical resistivity was observed for a high purity specimen at low temperature. However, thermal conductivity increased for a high purity specimen, and abrupt increase in thermal conductivity was observed at very low temperature, indicating typical temperature dependence of thermal conductivity for high purity metals. It can be known that reduction of electron-phonon scattering leads to increase in thermal conductivity of high purity niobium and tantalum metals at low temperature.

• 대한기계학회논문집B
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• 제35권1호
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• pp.93-100
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• 2011

액체질소 온도부근에서 작동하는 고온초전도 케이블 시스템을 개발하기 위해서는 고온초전도 케이블 시스템에 사용되는 극저온 절연재료의 정확한 열물성 자료가 필요하다. 금속재료와 달리, 비금속 계열의 재료는 열저항이 크기 때문에 정확한 열 유입량을 측정하기 위해서는 특별한 주의가 요구된다. 본 연구에서는 극저온 냉동기를 이용하여 30K 부터 상온 사이에서 절연재료의 열전도도를 정확하게 측정할 수 있는 시스템을 개발하였다. 설계와 제작과정을 포함한 열전도도 측정 시스템을 자세하게 기술하였다. 또한 상온으로부터 침입하는 불가피한 열유입량을 최소화하기 위한 열전도도 측정 시스템의 최적화 과정을 소개한다.

• 한국세라믹학회지
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• 제40권11호
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• pp.1106-1112
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• 2003

$\beta$-FeSi$_2$의 열전물성에 있어서 산소의 역할을 규명하기 위해서, 고온상 ($\alpha$+$\varepsilon$)과 저온상 ($\beta$)-FeSi$_2$ 시료에 대해 산화처리에 따른 열전물성 측정 및 분석실험을 행하였다. 산화에 의해 소결밀도가 감소하였으며, 반도체상으로의 전이도 방해되었다. 모든 시료에서 도전율과 열전도율은 산화처리시간과 함께 감소하였다. 순수한 FeSi$_2$ 및 고온상 ($\alpha$+$\varepsilon$)을 산화처리한 시료 Seebeck 계수는 작은 양의 값을 나타낸 반면에, 저온상 ($\beta$)을 산화처리한 FeSi$_2$ 는 음의 값을 나타내었으며 약 500K 부근에서 최대값을 나타내었다. 또 산화시간과 함께 최대값도 증가하였다.

• Carbon letters
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• 제28권
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• pp.60-65
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• 2018

Linear carbon chains (LCCs) encapsulated inside the hollow cores of carbon nanotubes (CNTs) have been experimentally synthesized and structurally characterized by Raman spectroscopy and transmission electron microscopy. However, in terms of electronic conductivity, their transportation mechanism has not been investigated theoretically or experimentally. In this study, the density of states and quantum conductance spectra were simulated through density functional theory combined with the non-equilibrium Green function method. The encapsulated LCCs inside (5,5), (6,4), and (9,0) single-walled carbon nanotubes (SWCNTs) exhibited a drastic change from metallic to semiconducting or from semiconducting to metallic due to the strong charge transfer between them. On the other hand, the electronic change in the conductance value of LCCs encapsulated inside the (7,4) SWCNT were in good agreement with the superposition of the individual SWCNTs and the isolated LCCs owing to the weak charge transfer.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2009년도 춘계학술대회 논문집
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• pp.17-17
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• 2009

We report the invention of a direct growth method termed On-Film Formation of Nanowire (OFF-ON) for making high-quality single-crystal nanowires, i.e. Bi and $Bi_2Te_3$, without the use of conventional templates, catalysts, or starting materials. We have used the OFF-ON technique to grow single crystal semi-metallic Bi and compound semiconductor $Bi_2Te_3$ nanowires from sputtered Bi and BiTe films after thermal annealing, respectively. The mechanism for nanowire growth is stress-induced mass flow along grain boundaries in the polycrystalline films. OFF-ON is a simple but powerful method for growing perfect single-crystal semi-metallic and compound semiconductor nanowires of high aspect ratio with high crystallinity that distinguishes it from other competitive growth approaches that have been developed to date. Our results suggest that Bi and $Bi_2Te_3$ nanowires grown by OFF-ON can be an ideal material system for exploring their unique thermoelectric properties due to their high-quality single crystalline and high conductivity, which have consequence and relevance for high-efficiency thermoelectric devices.

• Nuclear Engineering and Technology
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• 제47권3호
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• pp.227-239
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• 2015

The thermal, mechanical, and neutronic performance of the metal alloy fast reactor fuel design complements the safety advantages of the liquid metal cooling and the pool-type primary system. Together, these features provide large safety margins in both normal operating modes and for a wide range of postulated accidents. In particular, they maximize the measures of safety associated with inherent reactor response to unprotected, doublefault accidents, and to minimize risk to the public and plant investment. High thermal conductivity and high gap conductance play the most significant role in safety advantages of the metallic fuel, resulting in a flatter radial temperature profile within the pin and much lower normal operation and transient temperatures in comparison to oxide fuel. Despite the big difference in melting point, both oxide and metal fuels have a relatively similar margin to melting during postulated accidents. When the metal fuel cladding fails, it typically occurs below the coolant boiling point and the damaged fuel pins remain coolable. Metal fuel is compatible with sodium coolant, eliminating the potential of energetic fuel-coolant reactions and flow blockages. All these, and the low retained heat leading to a longer grace period for operator action, are significant contributing factors to the inherently benign response of metallic fuel to postulated accidents. This paper summarizes the past analytical and experimental results obtained in past sodium-cooled fast reactor safety programs in the United States, and presents an overview of fuel safety performance as observed in laboratory and in-pile tests.

• 한국재료학회지
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• 제20권7호
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• pp.356-363
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• 2010

For this paper, we investigated the area specific resistance (ASR) of commercially available ferritic stainless steels with different chemical compositions for use as solid oxide fuel cells (SOFC) interconnect. After 430h of oxidation, the STS446M alloy demonstrated excellent oxidation resistance and low ASR, of approximately 40 $m{\Omega}cm^2$, of the thermally grown oxide scale, compared to those of other stainless steels. The reason for the low ASR is that the contact resistance between the Pt paste and the oxide scale is reduced due to the plate-like shape of the $Cr_2O_3$(s). However, the acceptable ASR level is considered to be below 100 $m{\Omega}cm^2$ after 40,000 h of use. To further improve the electrical conductivity of the thermally grown oxide on stainless steels, the Co layer was deposited on the stainless steel by means of an electroless deposition method; it was then thermally oxidized to obtain the $Co_3O_4$ layer, which is a highly conductive layer. With the increase of the Co coating thickness, the ASR value decreased. For Co deposited STS444 with 2 ${\mu}m$hickness, the measured ASR at $800^{\circ}$ after 300 h oxidation is around 10 $m{\Omega}cm^2$, which is lower than that of the STS446M, which alloy has a lower ASR value than that of the non-coated STS. The reason for this improved high temperature conductivity seems to be that the Mn is efficiently diffused into the coating layer, which diffusion formed the highly conductive (Mn,Co)$_3O_4$ spinel phases and the thickness of the $Cr_2O_3$(S), which is the rate controlling layer of the electrical conductivity in the SOFC environment and is very thin

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2005년도 춘계학술대회
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• pp.321-325
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• 2005

Bipolar plate is the main part with MEA in automotive PEMFC. It must have a good electrical conductivity and excellent corrosion resistance, be cost effective. Therefore, stainless steels have been studied by many researchers because of its corrosion resistance and cost benefits. But their properties are not sufficient for the application to bipolar plate for automotive PEMFC. In this work, we have performed stamping using various commercial stainless steels to select candidate material for biploar plate and to derive design parameters for stamping simulation. The results showed that a small curvature at the corner of flow field is more favorable due to easier a plastic deformation. Stamping process was simulated by changing surface condition, and the size and angle of channel. The optimum shape and spring back phenomena were evaluated. Surface coating was applied to increase the corrosion resistance and electrical conductivity of stainless steel. The electrical interfacial resistance was 10 to $15m{\Omega}cm^2$ under clamping force of 150psi. But corrosion resistance of coating on the stainless steel was not good due to the unstableness of microstructure.

• 한국세라믹학회지
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• 제54권5호
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• pp.349-365
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• 2017

Graphene has attracted a lot of attentions due to the unique electrical and optical properties. Compared with the noble metal plasmons in the visible and near-infrared frequencies, graphene can support surface plasmons in the lower frequencies of terahertz and mid-infrared and it demonstrates an extremely large confinement at the surface because of the particular electronic band structures. Especially, the surface conductivity of graphene can be tuned by either chemical doping or electrostatic gating. These features make graphene a promising candidate for plasmonics, biosensing and transformation optics. Furthermore, the combination of graphene and metasurfaces presents a powerful tunability for exotic electromagnetic properties, where the metasurfaces with the highly-localized fields offer a platform to enhance the interaction between the incident light and graphene and facilitate a deep modulation. In this paper, we provide an overview of the key properties of graphene, such as the surface conductivity, the propagating surface plasmon polaritons, and the localized surface plasmons, and the hybrid graphene/metasurfaces, either metallic and dielectric metasurfaces, from terahertz to near-infrared frequencies. Finally, there is a discussion for the current challenges and future goals.