Earticle

Coated conductor is an important candidate for power cable applications due to its high current density. Even for DC power cable transmission, we must study the transport properties of HTS tapes after slow and fast discharge. In order to evaluate relation of the magnetic field with applied current we developed a scanning magnetic field measurements system by employing a Hall probe. This work presents the measurements of the magnetic fields above a coated conductor by varying applied current pattern. In the work, a transport current of 100 A, less than the critical current, is applied to YBCO coated conductor. We measured the residual magnetic field distributions after cut off the transport current with slow and fast operations. The results show differences of the magnetic field profiles and the corresponding current profiles by an inverse solution from the magnetic field measurement between these two operations because of the hysteresis of coated conductor excited by the transport current.

This paper investigates the mechanism of thermal burn-out and contact welding failures occurring in the master power switches of tracked armored vehicles. Since these tactical vehicles employ a mechanical ON/OFF switching system directly coupled with high-capacity batteries, a transient electrical arc inevitably generates during the brief moment of contact engagement. This arcing causes initial physical and micro-structural damage to the mating upper and lower contact plates. Notably, field data reveals that this failure is highly concentrated in specific production batches, where minor degradation escalates into catastrophic thermal burn-out and contact adhesion (welding). To identify the root cause of this accelerated failure, this study focuses on the causal relationship between the contact area and localized current density. Macroscopic and comparative analysis demonstrates that the defective switches from the problematic production lot possess an effective contact area of only 30% compared to normal, non-defective components. This significant reduction in contact area forces the electrical current to constrict into narrow paths, leading to an sharp surge in local current density and subsequent Joule heating. The combination of initial arc erosion and severe thermal runaway ultimately triggers the melting and welding of the contacts. The findings of this study emphasize that ensuring a sufficient effective contact area is critical in the manufacturing stage of high-current military switches to prevent catastrophic thermal failures under harsh operational environments.

본 연구의 목적은 호소의 정체로 인한 발생되는 호소내부의 현상을 개선하고자, 호소에 밀도류를 유발하는 장치를 투입하여 호소의 수체를 순환함(윤정환 등, 2010)으로써 호소의 정체에 따르는 현상을 개선하고져 한다. 따라서 호소는 성층화나 부영양화현상을 예방하는 물리적 조건이 조성될 것으로 판단된다. 이를 위해 밀도류를 발생시킴으로써 물순환을 유도하는 장치인 DCG(density current generator)를 대상호소에 적용하였을 때 그 효과를 분석하고자 호소수의 용존산소, 수온, 염분의 시공간적 분포를 조사, 분석하였다.

The purpose of this study was to investigate on the improvement of water quality in lakes which are polluted by contaminants internal and external of lakes. For this DCG, a purifier using natural phenomenon called density current, was applied to target lake, and the change of water quality (DO, Temperature, Salinity) before and after the operation were compared and analyzed. Also, another purpose is to take advantage of this as a basic material for improving water quality of fresh water lakes, dams and reservoirs frequently experiencing similar problems like eutrophication.

The effects of MgB4 addition on the superconducting properties and the microstructure of in situ processed MgB2 bulk superconductors were studied. MgB4 powder of 1-20 wt.% was mixed with (Mg + 2B) powder and then pressed into pellets. The pellets of (Mg + 2B + xMgB4) were heat-treated at 650 ℃ for 1 h in flowing argon. The powder X-ray diffraction (XRD) analysis for the heat-treated samples showed that the major formed phase in all samples was MgB2 and the minor phases were MgB4 and MgO. The full width at half maximum (FWHM) values showed that the grain size of MgB2 decreased as the amount of MgB4 addition increased. MgB4 particles included in a MgB2 matrix is considered to suppress the grain growth of MgB2. The onset temperatures (Tc,onset) of MgB2 with MgB4 addition (0-10 wt.%) was between 37-38 K. The 20 wt.% MgB4 addition slightly reduced the Tc,onset of MgB2 to 36.5 K. This result indicates that MgB4 addition did not influence the superconducting transition temperature (Tc) of MgB2 significantly. On the other hand, the small additions of 1-5 wt.% MgB4 increased the critical current density (Jc) of MgB2. The Jc enhancement by MgB4 addition is attributed not only to the grain size refinement but also to the possible flux pinning of MgB4 particles dispersed in a MgB2 matrix.

In this review paper, we report the characteristics of the critical current density in FeAs based superconductors which is newly discovered by Hosono group of Tokyo Institute of Technology on 2008. Since the many specimens in present stage are not single crystals, there are two kinds of critical current density observed in the specimens which are so-called local and global critical current densities. Therefore, it is necessary to evaluate both kinds of critical current densities. The history effect in which the global critical current density shows different values in increasing and decreasing magnetic field is also observed when the specimens have the local and the global critical current densities. The wire which critical current is 180 A is successfully developed with using the knowledge of abovementioned characteristics of two kinds of critical current densities and the history effect.

The pure and carbon (C)-doped MgB2 thin films were fabricated on Al2O3 (0001) substrates at a temperature of 650 °C by using hot-filament-assisted hybrid physical–chemical vapor deposition technique. The Tc value for pure MgB2 film is 38.5 K, while it is between 30 and 35 K for carbon-doped MgB2 films. Expansion in c-axis lattice parameter was observed with increase in carbon doping concentration which is in contrast to carbon-doped MgB2 single crystals. Significant enhancement in the critical current density was obtained for C-doped MgB2 films as compared to the undoped MgB2 film. This enhancement is most probably due to the incorporation of C into MgB2 and the high density of grain boundaries, both help in the pinning of vortices and result in improved superconducting performance.

2G HTS wire with high engineering current density is desired for applications where compact, high power density superconducting equipment is important. We have succeeded in enhancing engineering current density of commercial SuperOx 2G HTS wire based on GdBCO by increasing the HTS layer thickness without fast degradation of the HTS film microstructure. This was possible after improving the temperature uniformity along the HTS film deposition zone. In particular, the wire engineering current density was increased from 700-770 A/mm2 (for a 65 μm-thick wire without stabilisation) or 430-480 A/mm2 (for a 105 μm-thick stabilised wire) at the beginning of this study to almost 1200 A/mm2 (for a 67 μm-thick wire without stabilisation) or 770 A/mm2 (for a 107 μm-thick stabilised wire) at completion of this study.

Relationship between reflection resonance and applied current density in Bragg photonic crystal has been investigated. Multiple bit encodes of distributed Bragg reflector features have been prepared by electrochemical etching of crystalline silicon by using various square wave current densities. Optical characterization of multi-encoding of distributed Bragg reflectors on porous silicon was achieved by Ocean optics 2000 spectrometer for the search of possible applications of multiple bit encoding of distributed Bragg reflectors such as multiplexed assays and chemical sensors. The morphology and cross-sectional structure of multi-encoded distributed Bragg reflectors was investigated by field emission scanning electron micrograph.

This study was carried out to investigate the effect of milling of boron (B), which is one of raw materials of MgB2, on the critical current density (Jc) of MgB2. B powder used in this study is semi-amorphous B (Pavezyum, Turkey, 97% purity, 1 micron). The size of B powder was reduced by planetary milling using ZrO2 balls (a diameter of 2 mm). The B powder and balls with a ratio of 1:20 were charged in a ceramic jar and then the jar was filled with toluene. The milling time was varied from 0 to 8 h. The milled B powders were mixed with Mg powder in the composition of (Mg+2B), and the powder mixtures were uniaxially pressed at 3 tons. The powder compacts were heat-treated at 700℃ for 1 h in flowing argon gas. Powder X-ray diffraction and FWHM (Full width at half maximum) were used to analyze the phase formation and crystallinity of MgB2. The superconducting transition temperature (Tc) and Jc of MgB2 were measured using a magnetic property measurement system (MPMS). It was found that B2O3 was formed by B milling and the subsequent drying process, and the volume fraction of B2O3 increased as milling time increased. The Tc of MgB2 decreased with increasing milling time, which was explained in terms of the decreased volume fraction of MgB2, the line broadening of MgB2 peaks and the formation of B2O3. The Jc at 5 K increased with increasing milling time. The Jc increase is more remarkable at the magnetic field higher than 3 T. The Jc at 5 K and 4 T was the highest as 4.37×104 A/cm2 when milling time was 2 h. The Jc at 20 K also increased with increasing milling time. However, The Jc of the samples with the prolonged milling for 6 and 8 h were lower than that of the non-milled sample.

Recently, cable conductors composed of numerous coated conductors have been developed to transport huge current for largescale applications, for example accelerators and fusion reactors. Various cable conductors such as CORC (Conductor on round core), Roebel Cable, and TSTC (Twisted stacked tape cable) have been designed and tested to apply for large-scale applications. But, these cable conductors cannot improve the engineering critical current density (Je) because they are made by simple stacking of coated conductors. In this study, multi-HTS (High temperature superconductor) layers on one substrate (MHOS) wire was fabricated to increase the engineering critical current density by using the exfoliation of superconducting layer from substrate and silver diffusion bonding method. By the repetition of these processes, the 10 m long 6-layer MHOS conductor was successfully fabricated without any intermediate layers like buffer or solder. 6-layer MHOS conductor exhibited a high critical current of 2,460A/12mm-w. and high engineering critical current density of 1,367A/mm2 at liquid nitrogen temperature.

In this study, the effects of the compaction method for (Mg+2B) powders on the apparent density and superconducting properties of MgB2 bulk superconductor were investigated. The raw powders used in this study were nano-sized boron (B) and spherical magnesium (Mg). A batch of a powder mixture of (Mg+2B) was put in a steel mold and uniaxially pressed at 1 ton or 3 tons into pellets. Another batch of the powder mixture was uniaxially pressed at 1 ton and then pressed isostatically at 1800 kg/cm2 in the water chamber. All pellets were heat-treated at 650℃ for 1 h in flowing argon gas for the formation of MgB2. The apparent density of powder compacts pressed at 3 ton was higher than that at 1 ton. The cold isostatic pressing (CIP) in a water chamber allowed further increase of the apparent density of powder compacts, which influenced the pellet density of the final products (MgB2). The compaction methods (uniaxial pressing and CIP) did not affect the formation of MgB2 and superconducting critical temperature (Tc) of MgB2, but affected the critical current density (Jc) of MgB2 significantly. The sample with the high apparent density showed high Jc at 5 K and 20 K at applied magnetic fields (0-5 T).

MgB2 superconductor is highly sensitive to the Mg content. Even if the samples are synthesized with the appropriate looking stoichiometric ratio, the heat treatment leads to the loss of Mg either to ambiance or to MgO. To avoid it, either excess Mg is added in the starting powder or sealed ampoule annealing is employed. In this paper the effect of open Mg sintering ambiance on the ex-situ MgB2 was studied to enhance its superconducting properties. The open Mg ambiance was created to avoid any overpressure of Mg by providing a hole in Fe tube used as sample holder. The decrease in resistivity of the synthesized sample was observed through the increased temperature dependence of electron-phonon interactions. A clear enhancement in the superconducting cross-sectional area and hence the in-field critical current density is obtained

The Lorentz force distribution of a high T c superconducting tape with increasing transport currents in magnetic field (Ha) was visualized. The external magnetic field was applied normally to the coated conductor tape surface after zero-field cooling, and the transport current (Ia) was increased stepwise from 0 to 90 % of the values of the critical current (Ic (Ha}) at applied filed, Ha. The field distribution (H(x}) near the sample surface across the tape width (2w) was measured using the scanning Hall probe method. Applying an inversion to the measured field distribution, we obtained the underlying current distribution (J(x}), from which the magnetic induction, B(x) was calculated with Biot-Savart law. Then Lorentz force per unit length was calculated using F(x}=J(x}xB(x}, which appears to be very inhomogeneous along the tape width due to the complicated distributions of J(x} and B(x}.

음부력을 갖는 유체가 소규모 저수지로 유입되어 일어나는 밀도류의 거동 및 순환이 차원해석과 수리실험을 이용하여 해석된다. 무차원침강점 및 밀도류전면속도, 밀도류전면이동거리, 머리 뒤의 밀도층내회석 등은 유입밀도후르드수, Fre의 영향을 받으며 밀도층의 두께, 흐름양상 및 저수지내 밀도의 변화는 밀도류전면이 하류단에 도달하여 반사되기 전과 후의 양상이 현저하게 다르다. 밀도층의 두께는 전자의 경우에는 Fre, 후자의 경우는 시간과 Fre의 영향을 받으며 이들은 지수식으로 표현할 수 있다.

The behavior of a negative buoyant flow flowing into a small reservoir is analysed by experiments and dimensional analysis. The nondimensional plunge point, head velocity, head travel distance and the dilution of density current are dependent on the inflow densimetric Froude number, Fre. The thickness of density current, flow pattern and density difference are different from two cases as follows; before the front of head arrive at the downstream end and after. Before the arrival, the thickness of density current is dependent on Fre and after the arrival, time and Fre. It was found that they can be expressed as power laws.

밀도가 큰 유체의 유입으로 인한 소규모저수지의 바닥밀도류에 의한 순환이 수치해에 의하여 해석된다. 밀도류전면이 하류경계에 도달하기전에는 밀도류전면의 이동에 따라 침강점 하류에 형성된 반시계방향 와의 이동에 의하여 저수지내 순환과 혼합이 이루어지며, 밀도류전면이 하류경계에 도달한 후에는 하류경계에서 밀도류의 상승으로 인한 일종의 내부파가 형성되고 이 파의 상 하류 반복 전파됨에 따라 저수지내 혼합이 이루어지면서 밀도층의 두께는 증가한다. 하류경계에서 유출되는 유체의 희석은 유입밀도후르드수 Fre, 저수지길이, 시간의 함수이며 유출유체의 밀도가 일정값에 도달되는 시간은 저수지길이가 길수록, Fre가 작을수록 증가하는 것으로 나타났다.

The ciculation due to bottom density current produced by a dense inflow into a small reservoir is analysed by numerical scheme. Before the front of the density current arrives at the downstream end, the mixing in the reservoir is mainly caused by the anticlockwise vortex formed at the downstream of plunging point along the movement of bottom density current. Upon the arrival of the front of the density current at the downstream end an internal surge is created through an internal hydraulic jump. With repeated propagation of the internal surge back and forth the mixing in the reservoir is progressed and the thickness of dense layer is increased upward. The dilution of the overflow at downstream end is found to depend on inflow densimetric Froude number, reservoir length and elapsed time. The time required for the overflow to attain a specified dilution increases as reservoir length increases and Fre decreases.

본 연구에서는 염분과 온도차에 의한 밀도류를 해석하기 위하여 기존의 3차원 파동장 모델(LES-WASS-3D ver. 1.0)을 토대로 염분과 온도에 관한 상태방정식 및 3차원 이류-확산 방정식을 도입하여 새로운 수치모델(LES-WASS-3D ver. 2.0)을 개발하였다. 새롭게 개발한 수치모델의 적용성을 검토하기 위하여 기존의 수리모형실험결과와 비교 분석하였다. 그 결과, 이류-확산하는 해수의 형태 및 연직 염분농도 뿐만 아니라 방출하는 온수의 유속 및 온도를 매우 잘 재현하는 것으로 나타났다. 이로써 본 연구에서 개발한 수치모델의 타당성 및 유효성이 검증되었다.

In order to analyze the density current due to salt and temperature difference, this study develops new numerical model (LES-WASS-3D ver. 2.0) by introducing state equation for salt and temperature and 3D advection-diffusion equation to existing 3D numerical wave tank (LES-WASS-3D ver. 1.0). To verify the applicability, the newly-developed numerical model is analyzed comparing to the experimental result of existing numerical model. In the result, it well implement the behavior and vertical salt concentration of advected and diffused seawater as well as flow velocity and temperature of the discharged warm water. This confirms the validity and effectiveness of the developed numerical model.

산업계에서는 희생양극의 원리를 이용한 아연도금이 사용되고 있다. 순수아연도금보다 내식성을 증가시키기 위한 방안의 하나로 Zn-Ni 합금도금이 개발되었다. 합금 도금층은 순 아연 도금층에 비하여 4-5배의 내식성을 가지고 있어서 도금 단가가 높음에도 불구하고 고내식성을 요구하는 자동차 부품 등에 적용이 증가되고 있다. Zn-Ni 합금도금액은 황산욕, 염화욕, 알칼리욕과 암모니아욕 등이 사용되고 있다. 여기에서는 염화욕에서 합금도금의 조성에 미치는 전해조건의 영향을 조사하였다. 그 결과는 음극 과전압 및 확산 계수에 의해 설명하였다. 일반적으로 음극의 과전압이 증가함에 따라 활성화분극보다 농도분극이 중요하게 된다. 농도분극은 확산 층 내의 원소 확산에 의해 결정된다. 즉 음극의 과전압이 증가함에 따라 확산계수가 큰 Zn 함량이 증가한다.

In the industry, galvanizing using the principle of sacrificial anode is used. Zn-Ni alloy plating was developed as one of the measures to increase the corrosion resistance rather than pure zinc plating. The alloy plating layer has a corrosion resistance of 4-5 times that of the pure zinc plating layer, so that it is applied to automotive parts requiring high corrosion resistance even though the plating cost is high. The amount of Zn-Ni alloy plating solution is a sulfuric acid bath, a chlorinated bath, an alkali bath, and an ammonia bath. Here, the influence of the electrolytic conditions on the composition of the alloy plating in the chloride bath was investigated. The results are explained by the cathode overvoltage and the diffusion coefficient. In general, as the overvoltage of the cathode increases, the concentration polarization becomes more important than the activation polarization. The concentration polarization is determined by element diffusion in the diffusion layer. That is, as the overvoltage of the cathode increases, the Zn content having a large diffusion coefficient increases.

For large scale power applications of HTS conductor, it is getting more important to have a stacked HTS coated conductor with low loss and large current capacity. But it was not easy to measure some electric properties. Stabilizer free YBCO CC for striated/ stacked conductors is easily burned out during the measurement of the critical current density because it has no stabilizer and it is difficult to set-up the current lead and voltage taps because it has many pieces of YBCO CC in a conductor. Instead of direct measuring the critical current of a stacked HTS coated conductor, indirect estimation from measuring a magnetization loss of HTS coated conductor could be useful for practical estimation of the critical current. The magnetization loss of a superconductor is supposed to be affected by a full penetrating magnetic field, and it tends to show an inflection point at the full penetrating magnetic field when we generate the graph of magnetization loss vs. external magnetic field. The full penetrating magnetic field depends on the shape of the conductor and its critical current density, so we can estimate the effective critical current density from measuring the magnetization loss. In this paper, to prove the effectiveness of this indirect estimation of the critical current, we prepared several different kinds of YBCO CC(coated conductor) including a stacked conductor short samples and measured the magnetization losses and the critical currents of each sample by using linked pick up coils and direct voltage measurement with transport current respectively.