• Journal of the Korean GEO-environmental Society
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• v.13 no.3
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• pp.21-27
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• 2012

Estimating the thermal conductivity of clayey soils is important for enhancing the performance of geoengineering structures in cold regions and clay barriers for nuclear waste repositories, but specimen disturbance, saturation, and heat boundary conditions of the test apparatus hinder reliable measurements of the thermal conductivity of saturated clayey soils. This paper presents the results of an experimental study carried out using modified consolidation tests with the needle probe method to measure thermal conductivity. Experimental consolidation tests with saturated kaolinite were performed to investigate the effect of effective stress and dry density on thermal conductivity for saturated kaolinite. In addition, thermal conductivity of soil particles were thoroughly investigated and experimental results were used to evaluate the accuracy of the models to predict thermal conductivity.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.444-444
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• 2011

Recently, patterned magnetic films and elements attract a wide interest due to their technological potentials in ultrahigh-density magnetic recording and spintronic devices. Among those patterned magnetic structures, magnetic anti-dot patterning induces a strong shape anisotropy in the film, which can control the magnetic properties such as coercivity, permeability, magnetization reversal process, and magneto-resistance. While majority of the previous works have been concentrated on anti-dot arrays with a single magnetic layer, there has been little work on multilayered anti-dot arrays. In this work, we report on study of the magnetic properties of bilayered anti-dot system consisting of upper perforated Co layer of 40 nm and lower continuous Ni layer of 5 nm thick, fabricated by photolithography and wet-etching processes. The magnetic hysteresis (M-H) loops were measured with a superconducting-quantum-interference-device (SQUID) magnetometer (Quantum Design: MPMS). For comparison, investigations on continuous Co thin film and single-layer Co anti-dot arrays were also performed. The magnetic-domain configuration has been measured by using a magnetic force microscope (PSIA: XE-100) equipped with magnetic tips (Nanosensors). An external electromagnet was employed while obtaining the MFM images. The MFM images revealed well-defined periodic domain networks which arise owing to the anisotropies such as magnetic uniaxial anisotropy, configurational anisotropy, etc. The inclusion of holes in a uniform magnetic film and the insertion of a uniform thin Ni layer, drastically affected the coercivity as compared with single Co anti-dot array, without severely affecting the saturation magnetization ($M_s$). The observed changes in the magnetic properties are closely related to the patterning that hinders the domain-wall motion as well as to the magneto-anisotropic bilayer structure.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.42 no.10 s.340
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• pp.39-50
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• 2005

In this paper, we propose a performance improving MAC algerian for the IEEE 802.11 DCF. WLAN based IEEE 802.11 uses two control methods called 'Distributed Coordination Function(UF)' and 'Point Coordination Function(PCF)'. The nF controls the Urnsmission based on carrier sense multiple access with collision detection(CSMA/CA), that decides a random backoff time with the range of contention window for each terminal. Normally, each terminal the CW double after collision, and reduces the CW to the minimum after successful transmission. This paper proposes an enhanced DCF algorithm that decreases the CW smoothly after successful transmission in order to reduce the collision Probability by utilizing the current status information of WLAN. We also analyze the throughput and delay performance for the unsaturated case mathematically. Simulation results show that our algorithm enhances the saturation throughput of WLAN. They also coincide well with the analytical results.

• Journal of Magnetics
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• v.10 no.1
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• pp.10-13
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• 2005

We have studied effects of the partial substitution of $La^{3+}$ for $A^{2+}$ on the magnetic properties of double perovskites $A_2FeMoO_6$ (A=Ca, Sr and Ba). Polycrystalline $A_{2-x}La_xFeMoO_6(0{\leq}x{\leq}0.2)$ samples have been prepared by the conventional solid-state reaction in a stream of 5% $H_2$/Ar gas. The x-ray data indicate that A=Ca is monoclinic with the space group P$2_1$/n, A=Sr is tetragonal with the space group I4/mmm, and A=Ba is cubic with the space group Fm3m. The substitution of $La^{3+}$ for $A^{2+}$ results in a cell volume increase for A=Ca and a cell volume reduction for A=Ba. The decrease of saturation magnetization with increasing x arises from the reduction of magnetic moment associated with the electron doping and the disorder at the Fe and Mo sites. The partial substitution of magnetic $La^{3+}$ for $A^{2+}$ considerably enhances the Curie temperature $T_c$ from 316 K for x = 0 to 334 K for x = 0.2. This enhancement of $T_c$ with $La^{3+}$ doping originates from electron doping effects in addition to ionic size ones.

• Asian Journal of Turfgrass Science
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• v.9 no.1
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• pp.81-89
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• 1995

The variations of the soil texture, $SiO_2$ /$Al_2$$O_3$ ratio, buffer action, exchangeable base, ex-changeable hydrogen, and mineral nutrients were investigated to estimate the grade of the soil fertility of the soil derived from the different parent rocks such as the granite in Kwangnung and the basalt in Chejudo. The results investigated were showed as follows : Basalt soils in Chejudo belong to sandy clay, light clay and sandy clay loam, while gramite soils in Kwangnung sandy loam. The $SiO_2$ /$AI_2$$O_3$ ratio of the grassland in Chejudo was 1.11 and that of the oak forest soils was 1.24, while granite soils in Kwangnung 1.54 and 1.46, respectively. The buffer actions of ba-salt soils against the N /10 HCI and $Ca(OH)_2$ were stronger than those of granite soils. The $SiO_2$/$Al_2$$O_3$ + $Fe_2$$O_3$ ratios of grassland and oak forest soils of basalt in Chejudo showed 1.10 and 1.24 respectively, while those of the grassland and oak forest of Kwangnung 1.44 and 1.33. The base exchange capacity of basalt soils which has higher value of exchangeable hydrogen was stronger than that of granite soils. But the base saturation of granite soils showed higher value than that of basalt soils. Water contents of basalt soils in Chejudo was lower than that of granite soils fo Kwangnung Basalt soils in Chejudo contain still more humus and total nitrogen than gran-ite soils in kwangnung, The amount of available nitrogen, available phosphorus and exchangeable calcium of granite soils were more than that of basalt soils, Therefore, estimating the soil fertility, granite soils in Kwangnung is higher than that of basalt soils in Chejudo.

• Journal of the Korean Magnetics Society
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• v.8 no.5
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• pp.249-254
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• 1998

In relation to the application of magnetic method to the evaluation of irradiation damage (embrittlement) changes in the magnetic parameters(hysteresis loop and Barkhausen noise) and Vickers microhardness due to neutron irradiation and heat treatment were measured and compared. In the case of irradiation $(2.3{\times}10^{19}\;n/cm^2,\; E{\ge}1\;Mev,\; 288{\circ}C)$ hysteresis loop measurements show that susceptibility decreases as coercivity increase. Saturation magnetization do not show any change. Barkhausen noise amplitude and Barkhausen noise energy have decreased while Vickers microhardness has increased. For isothermally heat treated condition of irradiated specimen at 470 $^{\circ}C$ and 540 $^{\circ}C$, Barkhausen noise energy has increased while Vickers microhardness has decreased. Results of BNE and Vickers microhardness are reversed to the results on irradiated condition. All these consistent changes in magnetic parameter and Vickers microhardness measurement, which are thought to be resulted from the interaction between irradiation-induced defects and dislocation, and magnetic domain, respectively, show a possibility that magnetic measurement may be used to the evaluation of material degradation and recovery due to neutron irradiation and heat treatment, respectively, if a relevant large database in prepared.

• Journal of the Korean Magnetics Society
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• v.14 no.4
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• pp.120-126
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• 2004

In order to understand the temperature dependence of magnetic properties of $F_{84}Nb_{16}$(wt.%) thin films, ferromagnetic resonance experiments have been carried out. The ferromagnetic resonance spectra for all temperatures consist of several volume modes and one (or two) surface modes. It is suggested that both surface of the film have a perpendicular hard axis to the film plane (negative surface magnetic anisotropy). Saturation magnetization coincides with the Block's T$\^$2/3/ and spectroscopic splitting factor is almost constant in the temperature range from 113 K to 293 K. The surface magnetic anisotropy constant K$\_$s2/ of the film-substrate interface increased with decreasing temperature in the temperature range from 233 K to 293 K. The surface magnetic anisotropy constant K$\_$s1/ of the air-substrate interface decreased from -0.322 erg/$\textrm{cm}^2$ to -0.394 erg/$\textrm{cm}^2$ as the temperature decreased to 253 K and was almost constant below 233 K.233 K.

• Journal of Power Electronics
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• v.13 no.3
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• pp.369-380
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• 2013

Direct torque control based on space vector modulation (SVM-DTC) protects the DTC transient merits. Furthermore, it creates better quality steady-state performance in a wide speed range. The modified method of DTC using SVM improves the electrical magnitudes of asynchronous machines, such as minimizing the stator current distortions, the stator flux with electromagnetic torque without ripple, the fast response of the rotor speed, and the constant switching frequency. In this paper, the proposed method is based on two new control strategies for direct torque control with space vector modulation. First, fuzzy logic control is used instead of the PI torque and a PI flux controller to minimizing the torque error and to achieve a constant switching frequency. The voltages in the direct and quadratic reference frame ($V_d$, $V_q$) are achieved by fuzzy logic control. In this scheme, the switching capability of the inverter is fully utilized, which improves the system performance. Second, the close loop of stator flux estimation based on the voltage model and a low pass filter is used to counteract the drawbacks in the open loop of the stator flux such as the problems saturation and dc drift. The response of this new control strategy is compared with DTC-SVM. The experimental and simulation results demonstrate that the proposed control topology outperforms the conventional DTC-SVM in terms of system robustness and eliminating the bad outcome of dc-offset.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.37-37
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• 2010

Silicate-silsesquioxane or siloxane-silsesquioxane hybrid thin films are strong candidates as matrix materials for ultra low dielectric constant (low-k) thin films. We synthesized the silicate-silsesquioxane hybrid resins from tetraethoxyorthosilicate (TEOS) and methyltrimethoxysilane (MTMS) through hydrolysis and condensation polymerization by changing their molar ratios ([TEOS]:[MTMS] = 7:3, 5:5, and 3:7), spin-coating on Si(100) wafers. In the case of [TEOS]:[MTMS] 7:3, the dielectric permittivity value of the resultant thin film was measured at 4.30, exceeding that of the thermal oxide (3.9). This high value was thought to be due to Si-OH groups inside the film and more extensive studies were performed in terms of electronic, ionic, and orientational polarizations using Debye equation. The relationship between the mechanical properties and the synthetic conditions of the silicate-silsesquioxane precursors was also investigated. The synthetic conditions of the low-k films have to be chosen to meet both the low orientational polarization and high mechanical properties requirements. In addition, we have investigated a new solution-based approach to the synthesis of semiconducting chalcogenide films for use in thin-film transistor (TFT) devices, in an attempt to develop a simple and robust solution process for the synthesis of inorganic semiconductors. Our material design strategy is to use a sol-gel reaction to carry out the deposition of a spin-coated CdS film, which can then be converted to a xerogel material. These devices were found to exhibit n-channel TFT characteristics with an excellent field-effect mobility (a saturation mobility of ${\sim}\;48\;cm^2V^{-1}s^{-1}$) and low voltage operation (< 5 V). These results show that these semiconducting thin film materials can be used in low-cost and high-performance printable electronics.

• Journal of the Korean Vacuum Society
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• v.18 no.4
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• pp.266-271
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• 2009

We have investigated the lasing characteristics of GaAs-based 1300 nm wavelength region InAs Quantum Dot Laser Diode grown by Migration Enhanced Molecular Beam Epitaxy. Under a pulsed and CW operation, we observed the state switching of lasing wavelength from ground state (1302 nm) to excited state (1206 nm) due to the gain saturation of ground state. Under a pulsed operation, $J_{th}=92A/cm^2$, $\lambda_L=1311\;nm$ and under a CW operation, $J_{th}=247A/cm^2$, $\lambda_L=1320\;nm$.