• Geomechanics and Engineering
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• 제38권2호
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• pp.205-214
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• 2024

Permafrost refers to the condition where the ground is frozen. It is crucial to review and evaluate the ground's characteristics before construction. In this study, electrical resistivity surveying is chosen as the investigative technique to apply and illustrate the results on the state of permafrost ground and to summarize its applicability. Field experiments are conducted in the Yeoncheon area of South Korea, which has a freezing index of 522.6°C·days. The target area is categorized into two ground conditions: the first where the original ground freezes, and the second involves excavating the original ground up to a depth of 3 meters, backfilling it, and then artificially injecting fluid. Thus, frozen ground conditions are simulated under both natural and artificial circumstances. Electrical resistivity surveys are performed under both above-freezing and sub-zero temperature conditions, with the experiments conducted at sub-zero temperatures revealing relatively more high-resistivity zones due to the temperature conditions. In this area, the distribution of soil moisture content is also investigated using the Time Domain Reflectometry (TDR) technique. It is observed that the ground into which water is artificially injected had a relatively higher moisture content, although the difference is minor. Finally, a 3D map of the target ground is constructed based on the measured electrical resistivity values, and through this, the distribution of porosity, a crucial design parameter, is also depicted. This research demonstrates that the electrical resistivity technique can effectively evaluate the state of frozen and unfrozen ground and further suggests that it can detailed extract the characteristics of the target ground.

• 한국결정학회:학술대회논문집
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• 한국결정학회 2003년도 춘계학술연구발표회
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• pp.8-8
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• 2003

Electron-phonon interaction plays a significant role in forming of colossal magnetoresistance effect (CMR). Polaron formation was observed by neutron diffraction and by extended X-ray absorption fine structure (EXAFS) analysis. Local probe as given by the EXAFS is a useful method to study the polaronic charge and its dependence on temperature and ions size. Here we present the EXAFS study of polaronic charge in La/sub 0.7/Ca/sub 0.3-X/Ba/sub X/MnO₃ compositions. The single phase La/sub 0.7/Ca/sub 0.3-X/Ba/sub X/MnO₃ manganites (x=0; 0.03; 0.06, ..., 0.3) were prepared by ceramic technology [1]. The Curie temperature was determined by extrapolation of the temperature dependence of the magnetization (down to zero magnetization). EXAFS experiments were carried out at the 7C EC beam line of the Pohang Light Source (PLS) in Korea. The atomic pair distribution functions (PDF) were obtained by re-regularization method [2] from filtered spectra. The PDF for the x=0.3 sample showed a single peak function and for x=0.0, 0.03, 0.06, 0.09, 0.12 compositions were asymmetric in agreement with a small Jahn-Teller elongation of two (short and long) bonds of the MnO/sub 6/ octahedron. Dispersion, σ/sub Min-O//sup 2/, and asymmetry, σ/sub Min-O//sup 3/, of the Mn-O bond distances varied significantly with x and showed a maximums at x=0.09. The maximum of σ/sub Min-O//sup 2/ is caused by increase of dynamic rms displacements of the Mn-O distances near the T/sub C/. The observed x dependence of σ/sub Min-O//sup 3/ reflects the reduction of charge carriers mobility at approaching to T/sub C/ from low as well as high temperatures.

• Journal of Magnetics
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• 제16권2호
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• pp.186-191
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• 2011

[ $YBa_2C_3O_{7-x}$ ]films were fabricated on a $SrTiO_3$ (100) substrate using a trimethylaceate propionic acid (TMAP)-based MOD process by controlling the precursor solution viscosity, firing temperature, and by using various coatings. The viscosity of the precursor solution was controlled by the addition of Xylenes. The films were heat treated with different temperatures from 750 to $800^{\circ}C$. c-axis oriented films were obtained. After adding 9 ml of Xylene into the precursor solution, the $T_c$ of the YBCO film, which was coated 2 times and heat treated at $800^{\circ}C$, was 86 K and the measured $J_c$ was above 2.5 MA/$cm^2$ at 77 K in a zero-field.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2001년도 추계학술대회 논문집
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• pp.430-433
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• 2001

The 0.96MgTiO$_3$-0.04SrTiO$_3$+xCe(x=0∼1.6 wt%) ceramics were fabricated by the conventional mixed oxide method. The sintering temperature and time were 1300$^{\circ}C$, 2hr., respectively. From the X-ray diffraction patterns, it was found that the perovskite SrTiO$_3$ and ilmenite MgTiO$_3$ structures were coexisted in the 0.96MgTiO$_3$-0.04SrTiO$_3$+xCe(x=0∼1.6 wt%) ceramics. The dielectric constant($\varepsilon$$\sub$r/) was increased with addition of Ce. The temperature coefficient of resonant frequency($\tau$$\sub$f/) was gradually varied from positive value to the negative value with increasing the Ce. The temperature coefficient of resonant frequency of the 0.96MgTiO$_3$-0.04SrTiO$_3$+0.2Ce ceramics was near zero, where the dielectric constant, quality factor, and $\tau$$\sub$f/ were 20.68, 50, 272 and -0.5pm/$^{\circ}C$, respectively.

• 한국재료학회지
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• 제33권10호
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• pp.414-421
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• 2023

Infrared radiation (IR) refers to the region of the electromagnetic radiation spectrum where wavelengths range from about 700 nm to 1 mm. Any object with a temperature above absolute zero (0 K) radiates in the infrared region, and a material that transmits radiant energy in the range of 0.74 to 1.4 um is referred to as a near-infrared optical material. Germanate-based glass is attracting attention as a glass material for infrared optical lenses because of its simple manufacturing process. With the recent development of the glass molding press (GMP) process, thermal imaging cameras using oxide-based infrared lenses can be easily mass-produced, expanding their uses. To improve the mechanical and optical properties of commercial materials consisting of ternary systems, germanate-based heavy metal oxide glasses were prepared using a melt-cooling method. The fabricated samples were evaluated for thermal, structural, and optical properties using DSC, XRD, and XRF, respectively. To derive a composition with high glass stability for lens applications, ZnO and Sb₂O₃ were substituted at 0, 1, 2, 3, and 4 mol%. The glass with 1 mol% added Sb₂O₃ was confirmed to have the optimal conditions, with an optical transmittance of 80 % or more, a glass transition temperature of 660 ℃, a refractive index of 1.810, and a Vickers hardness of 558. The possibility of its application as an alternative infrared lens material to existing commercial materials capable of GMP processing was confirmed.

• 한국자기학회지
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• 제17권1호
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• pp.34-37
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• 2007

[ $Fe_{0.9}Zn_{0.1}Cr_2S_4$ ]를 직접합성법으로 제조하여 X선 회절기(XRD), 진동 시료 자화율 측정기(VSM), $M\"{o}ssbauer$ 분광기를 이용하여 시료의 결정학적 및 자기적 특성을 연구하였다. X선 회절도 분석 결과, 결정구조는 입방정형 스피넬 구조이며, 공간 그룹은 Fd3m으로 격자 상수는 $a_0=9.9967\;{\AA}$로 결정되었다. 100 Oe 인가자장하의 자화 곡선(ZFC: Zero field cooling)에서는 77 K 근방에서 첨점 형태의 특이 현상이 관측되었다. VSM과 $M\"{o}ssbauer$ 스펙트럼 분석 결과 $N\'{e}el$ 온도($T_N$)는 153 K로 결정되었다. $M\"{o}ssbauer$ 스펙트럼은 4.2 K에서 커다란 전기 사중극자 상호작용에 의한 비대칭적인 8-line 형태를 나타내었으며 이때의 전기 사중극자 분열치는 2.22mm/s이었다. 77 K에서 전기 사중극자 분열치는 0.20mm/s로 급격히 감소하였고 온도 상승과 함께 $M\"{o}ssbauer$ 스펙트럼 또한 8-line에서 6-line 형태로 변하였다. 상온에서의 이성질체 이동 값은 0.48mm/s로 철의 이온 상태가 전 온도 영역에서 $Fe^{2+}$로 결정되어 진다.

• 대한전기학회논문지:전기기기및에너지변환시스템부문B
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• 제50권7호
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• pp.331-337
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• 2001

With the increasing reliability of analysis schemes and the dramatically increased calculating speed, the computer simulation has become and indispensable process to predict the interruption capacity of circuit breakers. Generally, circuit breakers have to possess both the small current and large current interruption abilities and the circuit breaker designers need to evaluate its capacities to save the time and the expense. The analysis of small current and the large current interruption performances have been considered separately because the phenomena occurring in a interrupter are quite different. To analyze the dielectric recovery after large current interruption many physical phenomena such as heat transfer, convection and arc radiation, the nozzle ablation, the ionization of high temperature SF(sub)6 gas, the electric and themagnetic forces and so forth mush be considered. However, in the analysis of small current interruption performance only the cold gas flow analysis needs to be carried out because the capacitive current is to small that the influence from the current can be neglected. In this paper, an empirical equation which is obtained from a series of tests to estimate the dielectric recovery strength has been applied to a real circuit breaker. The results of analysis have been compared with the test results and the reliability has been investigated.

• 한국수소및신에너지학회논문집
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• 제32권6호
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• pp.573-584
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• 2021

Hydrogen is evaluated as one of the new energy sources that can overcome the limitations and pollution problems of conventional fossil fuels. Although hydrogen is CO₂-free, attention is required in NO_x emission and flame stability in order to use hydrogen in existing gas fuel system. However, use of electric grids is an unrealistic strategy for decarbonization for residential and commercial heating. Instead, use of H₂ that utilizes city gas grid is suggested as a reasonable alternative in terms of compatibility with existing systems, economic feasibility, and accessibility. In this study, the thermal efficiency and NO_x performance of the boiler according to the H₂ mixture ratio and vapor humidified ratio are reviewed for a humidified NG-H₂ boiler that vapor humidity to combustion air. Mixed fuel with H₂ (20%) is almost similar to NG in terms of efficiency, flame temperature, and pollution performance. Thus, it is expected to be directly compatible with the existing NG system. If the exhaust temperature of the H₂ boiler is lowered to around 60℃ at a humidified ratio of 15-20%, the NO_x emission concentration can be suppressed to about 5-10 ppm. The level of efficiency reaches 87% of the rated load efficiency, which is equivalent to the highest grade achievable.

• 한국자기학회지
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• 제13권4호
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• pp.171-175
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• 2003

Czochralski방법으로 성장된 정비조성 LiTaO$_3$ 단결정내에 불순물로 첨가되어 있는 Fe$^{3+}$ 이온에 대하여 상온에서 전자 상자성 공명(EPR : Electron Paramagnetic Resonance) 실험을 실행하였다. X-band(9.447 ㎓) 스펙트로미터를 사용하여 Fe$^{3+}$ 이온에 대한 공명 흡수선을 결정학적 주평면에서 시료를 돌려가면서 자기장을 가하여 각도의존 데이터를 얻었다. 이렇게 얻은 공명 자기장을 유효 스핀하밀토니안을 사용하여 분광학적 분리인자 g 및 영자기장 갈라지기(ZFS: Zero Field Splitting) 상수 b$_{2}$sup 0/ (=D)를 계산하였다. 분석결과 정비조성으로 성장시킨 LiTaO$_3$ 단결정에서 얻은 Fe$^{3+}$ 공명중심은 기존에 발표된 congruent 조성으로 성장시킨 단결정의 것과는 다른 것으로 밝혀졌다. 또한 Fe$^{3+}$ 이온에 관한 온도의존 공명 흡수선을 분석한 결과 실험 온도 구간에서는 LiTaO$_3$ 단결정의 어떠한 상변화도 없는 것으로 나타났다.

• 한국초전도ㆍ저온공학회논문지
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• 제23권2호
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• pp.1-5
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• 2021

We have investigated the electrical transport properties of polycrystalline tantalum nitride (TaN_x) films. Various compositions of tantalum (nitride) thin films have been deposited on SiO₂ substrates by reactive DC magnetron sputtering while changing the ratio of nitrogen partial pressure. The substrate temperature was maintained at 283 K during deposition. X-ray diffraction analyses indicated the presence of α-Ta and β-Ta phases in the Ta film deposited in pure argon atmosphere, while fcc-TaN_x phases appeared in the sputtering gas mixture of argon and nitrogen. The N/Ta atomic ratio in the film increased ranging from 0.36 to 1.07 for nitrogen partial pressure from 7 to 20.7%. The superconducting transition temperatures of the TaN_x thin films were measured to be greater than 3.86 K with a maximum of 5.34 K. The electrical resistivity of TaN_x thin film was in the range of 177-577 𝜇Ωcm and increased with an increase in nitrogen content. The upper critical filed at zero temperature for a TaN_0.87 thin film was estimated to exceed 11.3 T, while it showed the lowest T_c = 3.86 K among the measured superconducting TaN_x thin films. We try to explain the behavior of the increase of the residual resistivity and the upper critical field for TaN_x thin films with the nitrogen content by using the combined role of the intergrain Coulomb effect and disorder effect by grain boundaries.