• Applied Microscopy
• /
• 제47권3호
• /
• pp.131-147
• /
• 2017

In this work further optical and electrical investigations of pure and Zr doped $Mn_3O_4$ (from 0 up to 20 at.%) thin films as a function of frequency. First, the refractive index, the extinction coefficient and the dielectric constants in terms of Zr content are reached from transmittance and reflectance data. The dispersion of the refractive index is discussed by means of Cauchy model and Wemple and DiDomenico single oscillator models. By exploiting these results, it was possible to estimate the plasma pulse ${\omega}_p$, the relaxation time ${\tau}$ and the dielectric constant ${\varepsilon}_{\infty}$. Second, we have performed original ac and dc conductivity studies inspired from Jonscher model and Arrhenius law. These studies helped establishing significant correlation between temperature, activation energy and Zr content. From the spectroscopy impedance analysis, we investigated the frequency relaxation phenomenon and hopping mechanisms of such thin films. Moreover, a special emphasis has been putted on the effect of the oxidation in air of hausmannite thin films to form $Mn_2O_3$ ones at $350^{\circ}C$. This intrigue phenomenon which occurred at such temperature is discussed along with this electrical study. Finally, all results have been discussed in terms of the thermal activation energies which were determined with two methods for both undoped and Zr doped $Mn_3O_4$ thin films in two temperature ranges.

• 전기학회논문지
• /
• 제57권3호
• /
• pp.398-402
• /
• 2008

Recently, the development of superconducting fault current limiters (SFCLs) has been required as power demands increase in the power system. A distribution-level prototype resistive SFCL using coated conductor (CC) has been developed by Hyundai Heavy Industries Co., Ltd. and Yonsei University for the first time in the world. The ratings of the SFCL are 13.2kV/630A at normal operating condition. A novel non-inductive winding method is used in fabricating coils so there is almost zero impedance during normal operation. The distribution SFCL is cooled by sub-cooled liquid nitrogen $(LN_2)$ of 65K and 3 bar to enhance cryo-dielectric performance, critical current density, and thermal conductivity. In order to make reliable operation of an SFCL in real power systems, we monitored and controled its operation conditions by using supervisory control and data acquisition (SCADA) method. Thus, a monitoring system for the SFCL employing information technology (IT) is proposed and developed to be on the lookout for the operation conditions such as inside temperature, inside pressure, $LN_2$ level, voltage and current. Since operation temperature should be kept constant, bang-bang control for temperature feedback with a heater attached to the cold head of cryo-cooler is applied to the system. Short-circuit tests with prospective fault current of 10kA and AC dielectric withstand voltage tests up to 143kV for 1 minute were successfully performed at Korea Electrotechnology Research Institute. This paper deals with the development of a distribution level SFCL and its monitoring system for reliable operation.

• Transactions on Electrical and Electronic Materials
• /
• 제8권1호
• /
• pp.41-45
• /
• 2007

Phospho-olivine $LiFePO_4$ cathode materials were prepared by hydrothermal reaction. Carbon black was added to enhance the electrical conductivity of $LiFePO_4$. The structural and morphological performance of $LiFePO_4$ and $LiFePO_4$-C powders were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). $LiFePO_4$/Li and $LiFePO_4$-C/Li cells were characterized electrochemically by cyclic voltammogram (CV), charge/discharge experiments and ac impedance spectroscopy. The results showed that the discharge capacity of $LiFePO_4$/Li cell was 147 mAh/g at the first cycle and 118 mAh/g after 30 cycles, respectively. The discharge capacity of $LiFePO_4$-C/Li cell with 5 wt% carbon black was the largest among $LiFePO_4$-C/Li cells, 133 mAh/g at the first cycle and 128 mAh/g after 30 cycles, respectively. It was demonstrated that cycling performance of $LiFePO_4$-C/Li cell with 5 wt% carbon black was better than that of $LiFePO_4$/Li cell.

• 한국자기학회지
• /
• 제6권4호
• /
• pp.264-271
• /
• 1996

본 연구에서는 SQUID를 이용한 생체자기 계측연구에 사용될 자기차폐실의 설치 및 차폐 특성에 대해 소개한다. 차폐실 내부공간의 크기는 $2m(길이){\times}2m(폭){\times}2.5m(높이)$이고 차폐재료는 높은 투자율을 가진 Mumetal과 전기 전도도가 높은 알루미늄으로 구성되어 있다. 직류 지구자장에 대한 차폐율은 차폐실 중심에서 60 dB 이상이며 교류자장에 대한 차폐율은 1 Hz에서는 60 dB, 10 Hz에서는 80~100 dB의 값을 가진다. 차폐실내에서의 자장 잡음은 1 Hz에서 $500\;fT/{\sqrt}Hz$, 10 Hz에서 $100\;fT/{\sqrt}Hz$의 값을 가지며 자장구배잡음은 1 Hz에서 $7\;fT/cm{\sqrt}Hz$ 이하로서 SQUID 자력계에 의한 심자도와 SQUID gradiometer에 의한 뇌자도측정이 가능한 수준이다. 자체 개발된 SQUID 센서를 이용하여 차폐실내에서 심장 및 노로부터 발생하는 자기신호를 검출하였다.

• 대한안전경영과학회지
• /
• 제16권2호
• /
• pp.237-246
• /
• 2014

Researches on the elimination of sulfur and nitrogen oxides with catalysts and absorbents reported many problems related with elimination efficiency and complex devices. In this study, decomposition efficiency of harmful gases was investigated. It was found that the efficiency rate can be increased by moving the harmful gases together with SPCP reactor and the catalysis reactor. Calcium hydroxide($Ca(OH)_2$), CaO, and $TiO_2$ were used as catalysts. Harmful air polluting gases such as $SO_2$ were measured for the analysis of decomposition efficiency, power consumption, and voltage according to changes to the process variables including frequency, concentration, electrode material, thickness of electrode, number of electrode winding, and additives to obtain optimal process conditions and the highest decomposition efficiency. The standard sample was sulfur oxide($SO_2$). Harmful gases were eliminated by moving them through the plasma generated in the SPCP reactor and the $Ca(OH)_2$ catalysis reactor. The elimination rate and products were analyzed with the gas analyzer (Ecom-AC,Germany), FT-IR(Nicolet, Magna-IR560), and GC-(Shimazu). The results of the experiment conducted to decompose and eliminate the harmful gas $SO_2$ with the $Ca(OH)_2$ catalysis reactor and SPCP reactor show 96% decomposition efficiency at the frequency of 10 kHz. The conductivity of the standard gas increased at the frequencies higher than 20 kHz. There was a partial flow of current along the surface. As a result, the decomposition efficiency decreased. The decomposition efficiency of harmful gas $SO_2$ by the $Ca(OH)_2$ catalysis reactor and SPCP reactor was 96.0% under 300 ppm concentration, 10 kHz frequency, and decomposition power of 20 W. It was 4% higher than the application of the SPCP reactor alone. The highest decomposition efficiency, 98.0% was achieved at the concentration of 100 ppm.

• 한국재료학회:학술대회논문집
• /
• 한국재료학회 2009년도 춘계학술발표대회
• /
• pp.56.1-56.1
• /
• 2009

The intermediate operating temperature of solid oxide fuel cells (IT-SOFCs) have achieved considerable importance in the area of power fabrication. This is because to improve materials compatibility, their long-term stability and cost saving potential. However, to conserve rational cell performance at reduced-temperature regime, cathode performance should be obtained without negotiating the internal resistance and the electrode kinetics of the cell. Recently, double perovskite structure cathodes have been studied with great attention as a potential material for IT-SOFCs. In this study, double-perovskite structured cathodes of $GdBaCoCuO_{5+\delta}$, $GdBaCo_{2/3}Cu_{2/3}Fe_{2/3}O_{5+\delta}$ compositions and $(1-x)GdBaCo_2O_{5+\delta}+xCe_{0.9}Gd_{0.1}O_{1.95}$ (x = 10, 20, 30 and 40 wt.%) composites were evaluated as the cathode for intermediate temperature solid oxide fuel cells(IT-SOFCs). Electrical conductivity of the cathodes were measured by DC 4-probe method, and the thermal expansion coefficient of each sample was measured up to $900^{\circ}C$ by a dilatometer study. Area specific resistances(ASR) of the $GdBaCo_{2/3}Cu_{2/3}Fe_{2/3}O_{5+\delta}$ cathode and 70 wt.% $GdBaCo_2O5+\delta$ + 30wt.% Ce0.9Gd0.1O1.95 composite cathode on CGO electrolyte substrate were analyzed using AC 3-probe impedance study. The obtained results demonstrate that double perovskite-based compositions are promising cathode materials for IT-SOFCs.

• 멤브레인
• /
• 제15권4호
• /
• pp.349-354
• /
• 2005

이트리아 안정화 지르코니아(yttria stabilized zirconia, YSZ)를 전해질로 선정하여 소결조건에 따른 열적 안정성과 전기적인 특성을 분석하였다. SEM사진으로 소결온도가 증가할수록 입자가 커지므로 상대적으로 기공은 줄어드는 것을 보였고 입자크기에 따른 영향을 확인하였다. 전기적 특성을 알아보고자 2단자법(2-probe method)으로 $800\~1000^{\circ}C$의 오도에서 교류 임피던스 측정을 통하여 전해질 내의 저항과 전기전도도 측정으로 입자 내부 저항 및 전기적 성능을 평가하였다. 소결온도가 $1400^{\circ}C$일 때 건식법과 습식법에서 밀도는 각각 6.13, 6.25 $g/cm^3$이며, 상대밀도는 각각 98, 99$\%$였다. 소결온도가 올라갈수록 저항은 낮아지고, 전도도는 커지는 것을 확인할 수 있으며, 건식 및 습식법으로 제작한 전해질의 전기전도도는 $10000^{\circ}C$에서 각각 $8.8\times10^{-2},\;11\times10^{-2}$ S/cm이었다.

• 한국전기전자재료학회:학술대회논문집
• /
• 한국전기전자재료학회 2006년도 추계학술대회 논문집 광주전남지부
• /
• pp.48-52
• /
• 2006

Phospho-olivine $LiFePO_4$ cathode materials were prepared by hydrothermal reaction. Carbon black was added to enhance the electrical conductivity' of $LiFePO_4$. The structural and morphological performance of $LiFePO_4$ and $LiFePO_4$-C powders were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). $LiFePO_4$/Li and $LiFePO_4-C$/Li cells were characterized electrochemically by cyclic voltammogram (CV), charge/discharge experiments and ac impedance spectroscopy. The results showed that the discharge capacity of $LiFePO_4$/Li cell was 147 mAh/g at the first cycle and 118 mAh/g after 30 cycles, respectively. The discharge capacity of $LiFePO_4-C$/Li cell with 5wt% carbon black was the largest among $LiFePO_4-C$/Li cells, 133 mAh/g at the first cycle and 128 mAh/g after 30 cycles, respectively. It was demonstrated that cycling performance of $LiFePO_4-C$/Li cell with 5wt% carbon black was better than that of $LiFePO_4$/Li cell.

• Korean Chemical Engineering Research
• /
• 제57권2호
• /
• pp.149-163
• /
• 2019

영구 또는 유도 쌍극자를 가지는 물질은 불균일한 전기장 하에서 전기장의 구배 방향을 따라 힘을 받게 되는데, 이 힘에 의한 물질의 이동을 유전영동(dielectrophoresis, DEP)이라 한다. DEP 힘의 크기와 방향은 입자와 매질의 유전율과 전도도, 그리고 가해지는 교류 전기장의 주파수에 의해 영향을 받게 되므로, 이러한 변수를 제어함으로써 입자의 이동을 정확하게 조작할 수 있다. 또한, 전기영동과는 달리 쌍극자가 유도되는 모든 입자에 적용이 가능하다는 장점이 있다. 이러한 DEP 기술은 미세 유체 공학은 물론 바이오 센서, 마이크로 칩 분야 등 다양한 분야에서 활용되고 있다. 본 논문은 먼저 DEP의 기본원리를 설명하고, DEP를 이용한 연구에서 주로 사용되는 대표적인 마이크로 전극의 구조에 대해 논의한다. 그리고, DEP의 대표적 응용분야인 입자의 분리 및 포집, 자기조립(self-assembly) 연구를 소개한다.

• Journal of Electrochemical Science and Technology
• /
• 제10권2호
• /
• pp.223-230
• /
• 2019

A facile method for surface coating with $Li_2TiF_6$ which has a high lithium-ion conductivity, on $LiMn_2O_4$ spinel cathode material for high performance lithium ion batteries. The surface coating is performed by using a co-precipitation method with $Li_2CO_3$ powder and $H_2TiF_6$ solution under room temperature and atmospheric pressure without special equipment. Total coating amount of $Li_2TiF_6$ is carefully controlled from 0 to 10 wt.% based on the active material of $LiMn_2O_4$. They are evaluated by a systematic combination of analyses comprising with XRD, SEM, TEM and ICP. It is found that the surface modification of $Li_2TiF_6$ is very beneficial to high cycle life and excellent rate capability by reducing surface failure and supporting lithium ions transportation on the surface. The best coating condition is found to have a high cycle life of $103mAh\;g^{-1}$ at the 100th cycle and a rate capability of $102.9mAh\;g^{-1}$ under 20 C. The detail electrochemical behaviors are investigated by AC impedance and galvanostatic charge and discharge test.