• 한국자기학회지
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• 제19권1호
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• pp.12-16
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• 2009

Z-Type 바리움 페라이트($Ba_{3}Co_{0.8}Zn_{1.2}Fe_{24}O_{41}$)를 공침법으로 제조하고 결정구조와 자기적 성질을 조사하였다. $Ba_{3+{\delta}}Co_{0.8}Zn_{1.2}Fe_{24}O_{41}$(${\delta}$ = 3, 5, 7, 13 wt%)와 $Ba_{3}Co_{0.8}Zn_{1.2}Fe_{{24}+{\delta}}O_{41}$(${\delta}$ = 5, 7, 10 wt%)의 조성을 가진 분말의 ${\delta}$ 값에 따른 자기적 성질의 변화를 조사하였다. 제조된 모든 바리움 페라이트에는 Z-type으로 상전이 하지 않은 M-type이 상당량 잔류하고 있었다. 포화자화는 Ba와 Fe의 과잉 첨가량 ${\delta}$가 각각 7 wt%와 5 wt% 일 때 최대로 나타났고 보자력은 조사된 모든 분말에서 다소 높게 나타났다. 높은 보자력은 M-type이 잔류하기 때문이다. 과잉의 Ba와 Fe 첨가에 따른 결정구조의 변화는 없었다. Z-type 바리움 페라이트의 연자성 특성을 향상시키기 위해서는 M-type이 Z-type 형성온도에서 상전이가 일어나도록 해야 함을 알았다.

• 자원리싸이클링
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• 제7권4호
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• pp.43-49
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• 1998

다양한 hard ferrite용 국산 산화철(${\alpha}Fe_2O_3$)의 분체특성이 Sr-ferrite의 자기특성에 미치는 영향을 조사하였다. 산화철의 평균입도와 입도분호는 철강 냉연공장의 염산폐액 정제방법에 따라 서로 구분되었으며, Sr-ferrite의 자기특성과 미세조직에 큰 영향을 주었다. Ruthner 산화철의 응집현상은 Sr-ferrite의 포화자화를 감소시켰으며, Chemirite(CY) 산화철의 $0.80{\mu}\textrm{m}$이상의 조대한 입자는 Sr-ferrite의 고유보자력을 감소시켰다. 평균입도가 $0.14\mu\textrm{m}$로 가장 작은 Chemirite 포항2냉연(EP) 산화철을 사용하여, 5.8mol, $1150^{\circ}C$/1시간의 하소조건에서 제조된 Sr-ferrite는 포화자화가 68.2emu/g, 고유보자력이 4300Oe인 최적의 자기특성을 나타내었다.

• Journal of Magnetics
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• 제20권1호
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• pp.31-39
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• 2015

Macroscopic hysteresis loops and microscopic magnetic moment distributions have been determined by a three-dimensional (3D) model for exchange-coupled Sm-Co/${\alpha}-Fe$/Sm-Co trilayers with in-plane collinear easy axes. These results are carefully compared with the popular one-dimensional (1D) micromagnetic models and recent experimental data. It is found that the results obtained from the two methods match very well, especially for the remanence and coercivity, justifying the calculations. Both nucleation and coercive fields decrease monotonically as the soft layer thickness $L^s$ increases while the largest maximum energy product (roughly 50 MGOe) occurs when the thicknesses of hard and soft layers are 5 nm and 15 nm, respectively. Moreover, the calculated angular distributions in the thickness direction for the magnetic moments are similar. Nevertheless, the calculated nucleation and pinning fields as well as the energy products by 3D OOMMF are systematically smaller than those given by the 1D model, due mainly to the stray fields at the corners of the films. These demagnetization fields help the magnetic moments at the corners to deviate from the previous saturation state and facilitate the nucleation. Such an effect enhances as $L^s$ increases. When the thicknesses of hard and soft layers are 10 nm and 20 nm, respectively, the pinning field difference is as large as 30%, while the nucleation fields have opposite signs.

• 한국자기학회지
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• 제7권5호
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• pp.243-248
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• 1997

Ar+ $O_{2}$ 혼합가스 중에서 반응성 스퍼터링을 통해 직접 Fe-Hf-O계 초미세결정 연자성 박막을 제조하였으며, 이때 산소분압비의 변화가 Fe-Hf-O 박막의 미세구조 및 자기적 특성에 미치는 영향을 조사하였다. 산소분압이 증가함에 따라 박막의 포화자속밀도는 점차적으로 감소하며 연자기특성이 10%까지는 향상되다가 다시 열화되는 경향을 나타내었다. 최적조건인 10%의 산소분압에서 증착한 F $e_{82}$H $f_{3.4}$ $O_{14.6}$ 초미세결정 박막은 열처리 없이 증착한 상태에서 우수한 연자기 특성을 나타내었으며, 이때의 자기적 특성은 각각 포화자속밀도 17.7 kG, 보자력 0.7 Oe 및 실효투자율 2,500(100 MHz)의 값을 나타내었다. 산소분압이 증가함에 따라 결정립 크기가 감소하며 15% 이상의 산소분압에서는 F $e_{3}$ $O_{4}$가 생성되었다. 따라서 10%에서 가장 우수한 연자기 특성을 나타내는 것은 결정립 크기와 산화물 생성에 의해 설명될 수 있다. Fe-Hf-O계 초미세결정 박막의 전기비저항은 산소분압이 증가함에 따라 증가하는 경향을 나타내었다. 우수한 연자기 특성을 나타내는 F $e_{82}$H $f_{3.4}$ $O_{14.6}$ 박막의 경우, 약 150 .mu. .ohm.cm로 산소를 첨가하지 않은 경우의 30 .mu. .ohm. cm에 비하여 약 5배 증가된 값을 나타내었다. 따라서 F $e_{82}$H $f_{3.4}$ $O_{14.6}$초미세결정 박막이 고주파에서 우수한 연자기 특성을 나타내는 원인은 주로 높은 전기비저항과 미세하게 형성된 결정립에 기이한 것으로 생각된다.

• 한국분말재료학회지
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• 제28권4호
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• pp.293-300
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• 2021

We investigate the effect of phosphorous content on the microstructure and magnetic properties of Fe_83.2Si_5.33-0.33xB_10.67-0.67xP_xCu_0.8 (x = 1-4 at.%) nanocrystalline soft magnetic alloys. The simultaneous addition of Cu and P to nanocrystalline alloys reportedly decreases the nanocrystalline size significantly, to 10-20 nm. In the P-containing nanocrystalline alloy, P atoms are distributed in an amorphous residual matrix, which suppresses grain growth, increases permeability, and decreases coercivity. In this study, nanocrystalline ribbons with a composition of Fe_83.2Si_5.33-0.33xB_10.67-0.67xP_xCu_0.8 (x = 1-4 at.%) are fabricated by rapid quenching melt-spinning and thermal annealing. It is demonstrated that the addition of a small amount of P to the alloy improves the glass-forming ability and increases the resistance to undesirable Fe_x(B,P) crystallization. Among the alloys investigated in this work, an Fe_83.2Si₅B₁₀P₁Cu_0.8 nanocrystalline ribbon annealed at 460℃ exhibits excellent soft-magnetic properties including low coercivity, low core loss, and high saturation magnetization. The uniform nanocrystallization of the Fe_83.2Si₅B₁₀P₁Cu_0.8 alloy is confirmed by high-resolution transmission electron microscopy analysis.

• 설비공학논문집
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• 제16권12호
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• pp.1134-1139
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• 2004

The evaporative heat transfer coefficient of $CO_2$ (R-744) in a horizontal tube was investigated experimentally. The experiments were conducted without oil in a closed refrigerant loop which was driven by a magnetic gear pump. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and evaporator (test section). The test section consists of a smooth, horizontal stainless steel tube of inner diameter of 7.75 mm. The experiments were conducted at mass flux of 200 to 500 kg/m$^2$s, saturation temperature of -5 to 5$^{\circ}C$, and heat flux of 10 to 40kW/m$^2$. The test results showed the heat transfer of $CO_2$ has a greater effect on nucleate boiling more than convective boiling. Mass flux of $CO_2$ does not affect nucleate boiling too much, and the effect of mass flux on evaporative heat transfer of $CO_2$ is much smaller than that of refrigerant R-22 and R-134a. In comparison with test results and existing correlations, correlations failed to predict the evaporative heat transfer coefficient of $CO_2$, therefore, it is necessary to develope reliable and accurate predictions determining the evaporative heat transfer coefficient of $CO_2$ in a horizontal tube.

• 한국자기학회지
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• 제5권5호
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• pp.641-644
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• 1995

We have studied the effect of the nitrogen on the microstructure, thermomagnetic properties and corrosion resistance of Fe-Hf-C-N nanocrystalline thin films with high permeability and high saturation magnetization. These films were fabricated by reactive sputtering in $Ar+N_{2}$ plasma using an rf magnetron sputtering apparatus. As $P_{N2}$ increases, the microstructure changes from amorphous to crystalline $\alpha$-Fe phase and again returns to amorphous one. Spin wave stiffness constant increases with $P_{N2}$ until 5% $P_{N2}$, and then decreases with the further increase. This trend corresponds well with that of the microstructure with increasing $P_{N2}$. The Fe-Hf-C-N films with over 3% $P_{N2}$ show higher corrosion resistance than the N-free Fe-Hf-C films. The Fe-Hf-C-N films are considered to have high potentials for the head core materials suitable for high density recording systems, owing to their excellent soft magnetic properties and corrosion resistance.

• 조명전기설비학회논문지
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• 제21권10호
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• pp.29-34
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• 2007

본 논문에서는 사고각에 따른 자속구속형 초전도 사고전류제한기의 사고전류제한 특성을 분석하였다. 이를 위해 자화가지를 포함한 등가회로로부터 사고각에 따른 자속구속형 초전도 사고전류제한기의 철심내부자속을 유도하였으며 사고전류에 미치는 영향을 분석하였다. 사고전류제한 실험을 통해, 철심의 포화와 관련한 여자전류를 유도하였으며 사고각에 따른 변화를 분석하였다. 사고전에는 발생되지 않았지만 사고가 발생함과 동시에 여자전류크기가 증가하였으며 특히, 자속구속형 초전도 사고전류제한기를 가극결선한 경우가 감극결선한 경우보다 보다 크게 증가되는 것을 확인할 수 있었다. 등가회로로부터 분석한 결과와 실험결과는 일치되는 것을 확인할 수 있었다.

• 한국의학물리학회지:의학물리
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• 제30권4호
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• pp.139-149
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• 2019

Purpose: With neurodegeneration, the signal intensity of the cerebrospinal fluid (CSF) in the brain increases. The objective of this study was to evaluate chemical exchange saturation transfer (CEST) signals with and without the contribution of CSF signals in elderly human brains using two different 3T magnetic resonance imaging (MRI) sequences Methods: Full CEST signals were acquired in ten subjects (Group I) with a three-dimensional (3D)-segmented gradient-echo echo-planar imaging (EPI) sequence and in ten other subjects (Group II) with a 3D gradient and spin-echo (GRASE) sequence using two different 3T MRI systems. The segmented tissue compartments of gray and white matter were used to mask the CSF signals in the full CEST images. Two sets of magnetization transfer ratio asymmetry (MTR_asym) maps were obtained for each offset frequency in each subject with and without masking the CSF signals (masked and unmasked conditions, respectively) and later compared using paired t-tests. Results: The region-of-interest (ROI)-based analyses showed that the MTR_asym values for both the 3D-segmented gradient-echo EPI and 3D GRASE sequences were altered under the masked condition compared with the unmasked condition at several ROIs and offset frequencies. Conclusions: Depending on the imaging sequence, the MTR_asym values can be overestimated for some areas of the elderly human brain when CSF signals are unmasked. Therefore, it is necessary to develop a method to minimize this overestimation in the case of elderly patients.

• 한국마린엔지니어링학회:학술대회논문집
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• 한국마린엔지니어링학회 2005년도 전기학술대회논문집
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• pp.189-194
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• 2005

The evaporation heat transfer and pressure drop of $CO_2$ in a small diameter tube was investigated experimentally. The experiments were conducted without oil in a closed refrigerant loop which was driven by a magnetic gear pump. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and evaporator(test section). The test section was made of a horizontal stainless steel tube with the inner diameter of 4.57 mm, and length of 4 m. The experiments were conducted at mass flux of 200 to 700 $kg/m^2s$, saturation temperature of $0^{\circ}C$ to $20^{\circ}C$, and heat flux of 10 to 20 $kW/m^2$ . The test results showed the evaporation heat transfer of $CO_2$ has great effect on more nucleate boiling than convective boiling. The evaporation heat transfer coefficients of $CO_2$ are highly dependent on the vapor quality, heat flux and saturation temperature. The evaporation pressure drop of C02 are highly dependent on the mass flux. In comparison with test results and existing correlations, correlations failed to predict the evaporation heat transfer coefficient and pressure drop of $CO_2$, therefore, it is necessary to develop reliable and accurate predictions determining the evaporation heat transfer coefficient and friction pressure drop of $CO_2$ in a horizontal tube.