Thermodynamic Behaviors of Magnetic-Fluid in a Thin Channel with Magnetic Field and Aspect Ratio

Kim, Sung-Chul,Seo, Jae-Hyeong,Lee, Dong-Yeon,Hong, Dong-Pyo,Kang, Suk-Ju,Lee, Moo-Yeon Korean Society for Precision Engineering 2014 International Journal of Precision Engineering and Vol.15 No.7

The objective of this study is to numerically investigate the thermodynamic behaviors of a magnetic fluid with nano-sized magnetic particles in a thin channel. The numerical model developed using the FEM (finite element method) technique with mesh sensitivity analysis was validated with existing data and thermodynamic characteristics such as isotherms, streamlines and Nusselt numbers of the magnetic fluid were analyzed with variations of the composition of magnetite, the aspect ratio, the magnetic field intensity and the inclination angle of the channel. The numerical analysis showed the increase in the magnetic volume force in the thin channel with increasing magnetic field intensity and magnetic fluid viscosity. Under all magnetic field intensities, the mean Nusselt numbers of the magnetic fluid in the thin channel at the inclination angle of <TEX>$45^{\circ}$</TEX> were on average 77.9% higher than those at the inclination angle of <TEX>$15^{\circ}$</TEX> due to the increased convective heat transfer in the thin channel.

Effect of honeycomb-patterned structure on electrical and magnetic behaviors of poly(@?-caprolactone)/capped magnetic nanoparticle composite films

Kim, J.K.,Basavaraja, C.,Umashankar, M.,Huh, D.S. IPC Science and Technology Press 2016 Polymer Vol.87 No.-

Novel biocompatible poly(@?-caprolactone) (PCL) was synthesized via in situ ring-opening polymerization of @?-caprolactone under different concentrations (10, 20, and 30 wt%) of capped magnetic nanoparticles (CMPs). The prepared PCL/CMP composites were characterized using Fourier transform infrared, UV-vis spectra, and X-ray photoelectron spectroscopy to demonstrate the interaction between PCL and CMP. X-ray diffraction patterns, scanning electron microscopy with energy dispersive X-ray spectroscopy, and thermogravimetric analysis were used to understand the structural nature and thermal stability of the polymer composites. Honeycomb-patterned thin films were fabricated by casting the composite solutions under humid conditions. The I-V characteristics and magnetic behavior of the honeycomb-patterned films were observed and compared with those of the flat thin composite films. The honeycomb-patterned PCL/CMP composite films revealed higher conductivity than the flat films and showed a hysteresis loop in the magnetization plotted against the applied magnetic field for magnetic behavior.

Resting-state functional magnetic resonance imaging investigation of the neural correlates of cognitive-behavioral therapy for externalizing behavior problems in adolescent bullies

Kim, Johanna Inhyang,Kang, Yun-Hyong,Lee, Jong-Min,Cha, Jungho,Park, Yeong-Hun,Kweon, Kuk-Ju,Kim, Bung-Nyun Elsevier 2018 Progress in neuro-psychopharmacology & biological Vol.86 No.-

<P><B>Abstract</B></P> <P>The purpose of this study was to investigate the neural correlates of cognitive-behavioral therapy (CBT) for externalizing behavior problems in perpetrators of school bullying using assessments of brain activity and behavior. Twenty-five adolescent bullies participated in an 8-session intervention. Prior to and after participation, 24 adolescents were evaluated using the Child Behavior Checklist (CBCL) and 23 completed resting-state functional magnetic resonance imaging. Changes in the fractional amplitude of low-frequency fluctuations (fALFF) and scores on the CBCL were analyzed. We also compared the identified changes into 2 groups (low and high delinquency) differentiated by a cutoff of 65 points on the delinquency subscale of the CBCL. Following the intervention, participants exhibited improvement in the subscores of the CBCL and decreases in the fALFF of the inferior parietal lobule, lingual, interior frontal and middle occipital gyrus. A positive correlation was observed between changes in the CBCL externalizing behavior scores and fALFF of the inferior frontal gyrus. The high delinquency group showed a greater decrease in delinquency and externalizing CBCL subscores across time than did the low delinquency group. The high delinquency group had more areas that showed change in fALFF post-intervention than did the low delinquency group. A positive correlation was observed between changes in the CBCL delinquency scores and fALFF of the precentral gyrus in the high delinquency group. The results indicate that this CBT for externalizing behavior problems in bullies had more positive effects on delinquent behavior in adolescents with high levels of delinquency, and these changes were associated with functional changes in brain activity.</P> <P>Trial registration: ClinicalTrials.gov identifier: NCT02670876 </P> <P><B>Highlights</B></P> <P> <UL> <LI> School bullying is a universal phenomenon that has significant negative effects on mental health for both the perpetrator and victim. </LI> <LI> We conducted cognitive-behavioral therapy targeting cognitive distortions related to externalizing problems commonly found in bullies. </LI> <LI> The purpose of this study was to investigate the effects and neural correlates of this intervention on externalizing problems. </LI> <LI> We found a positive effect on externalizing problems and this was associated with changes in spontaneous neural activity in related brain regions. </LI> <LI> Further studies that include large homogeneous samples and well-controlled designs are warranted. </LI> </UL> </P>

First-to-second-order magnetic-phase transformation in La_0.7Ca_0.3-xBa_xMnO₃ exhibiting large magnetocaloric effect

Phan, T.L.,Dang, N.T.,Ho, T.A.,Manh, T.V.,Thanh, T.D.,Jung, C.U.,Lee, B.W.,Le, A.T.,Phan, A.D.,Yu, S.C. Elsevier Sequoia 2016 Journal of alloys and compounds Vol.657 No.-

We have prepared polycrystalline samples La<SUB>0.7</SUB>Ca<SUB>0.3-x</SUB>Ba<SUB>x</SUB>MnO<SUB>3</SUB> (x = 0, 0.025, 0.05, 0.075 and 0.1) by solid-state reaction, and then studied their magnetic properties and magnetocaloric (MC) effect based on magnetization versus temperature and magnetic-field (M-H-T) measurements. Experimental results reveal the easiness in tuning the Curie temperature (T<SUB>C</SUB>) from 260 to about 300 K by increasing Ba-doping concentration (x) from 0 to 0.1. Under an applied field H = 50 kOe, maximum magnetic-entropy changes around T<SUB>C</SUB> of the samples can be tuned in the range between 6 and 11 J kg<SUP>-1</SUP> K<SUP>-1</SUP>, corresponding to refrigerant-capacity values ranging from 190 to 250 J kg<SUP>-1</SUP>. These values are comparable to those of some conventional MC materials, and reveal the applicability of La<SUB>0.7</SUB>Ca<SUB>0.3-x</SUB>Ba<SUB>x</SUB>MnO<SUB>3</SUB> materials in magnetic refrigeration. Analyses of the critical behavior based on the Banerjee criteria, Arrott plots and scaling hypothesis for M-H-T data prove a magnetic-phase separation when Ba-doping concentration changes. In the doping region x = 0.05-0.075, the samples exhibits the crossover of first- and second-order phase transitions with the values of critical exponents β and γ close to those expected for the tricritical mean-field theory. The samples with x < 0.05 and x > 0.075 exhibit first- and second-order transitions, respectively. More detailed analyses related to the Griffiths singularity, the critical behavior for different magnetic-field intervals started from 10 kOe, and the magnetic-ordering parameter n = dLn|ΔS<SUB>m</SUB>|/dLnH (where ΔS<SUB>m</SUB> is the magnetic-entropy change) demonstrate magnetic inhomogeneities and multicritical phenomena existing in the samples.

Monte Carlo Investigation of Spatially Adaptable Magnetic Behavior in Stretchable Uniaxial Ferromagnetic Monolayer Film

Yongyut Laosiritaworn,Wimalin Laosiritaworn 한국자기학회 2015 Journal of Magnetics Vol.20 No.1

In this work, Monte Carlo simulation was employed to model the stretchable Ising monolayer film to investigate the effect of the spatial distance variation among magnetic atoms on magnetic behavior of the film. The exchange interaction was considered as functions of initial interatomic distance and the stretched distance (or the strain). Following Bethe-Slater picture, the magnetic exchange interaction took the Lennard-Jones potential-like function. Monte Carlo simulations via the Wolff and Metropolis algorithms were used to update the spin systems, where equilibrium and dynamic magnetic profiles were collected. From the results, the strain was found to have strong influences on magnetic behavior, especially the critical behavior. Specifically, the phase transition point was found to either increase or decrease depending on how the exchange interaction shifts (i.e. towards or away from the maximum value). In addition, empirical functions which predict how the critical temperatures scale with initial interatomic distance and the strain were proposed, which provides qualitatively view how to fine tune the magnetic critical point in monolayer film using the substrate modification induced strain.

Short-range magnetic order in La1-xBaxCoO3 cobaltites

Phan The Long,Dimitar N. Petrov,J. Ćwikd,N.T. Dang,Viet Dongquoc 한국물리학회 2018 Current Applied Physics Vol.18 No.11

Magnetization versus temperature and magnetic-field measurements, M(T, Ha), have been carried out to study the magnetic and critical properties of polycrystalline La1-xBaxCoO3 (x=0.3 and 0.5) cobaltites. These compounds with the density of ∼6.2 g/cm3 crystallized in the R3c rhombohedral and Pm3m cubic structures, respectively. With an applied field Ha=200 Oe, M(T) data have revealed that the samples with x=0.3 and 0.5 exhibit the ferromagnetic-paramagnetic (FM-PM) phase transition at the Curie temperature points TC=202 and 157 K, respectively. At 4.2 K, the saturation magnetization (Msat) decreases from 35.9 emu/g for x=0.3–26.1 emu/g for x=0.5. Particularly, the critical-behavior analyses in the vicinity of TC reveal all samples undergoing a second-order phase transition, with critical exponent values (β=0.328 and γ=1.251 for x=0.3, and β=0.331 and γ=1.246 for x=0.5) close to those expected for the 3D Ising model. This proves short-range magnetic order existing in La1-xBaxCoO3. We believe that magnetic inhomogeneities due to the mixture of holerich FM regions (confined in the trivalent-cobalt hole-poor anti-FM matrix) and uniaxial anisotropy prevent longrange order in La1-xBaxCoO3.

Reduced graphene oxide wrapped ZnFe2O4 nanospheres as selective magnetically recyclable Photocatalysts under visible light irradiation

Aruljothi C.,Manivel P.,Vasuki T. 한국탄소학회 2022 Carbon Letters Vol.32 No.7

Magnetically separable and reusable zinc ferrite/reduced graphene oxide (ZnFe2O4/rGO) nanocomposite has been prepared by hydrothermal method. The results illustrate that the construction of ZnFe2O4 and rGO occur concurrently in a hydrothermal reaction that initiates the formation of rGO-wrapped ZnFe2O4 nanospheres. The morphological and structural features of the ZnFe2O4/rGO nanocomposites reveal that the rGO nanosheets anchored to the ZnFe2O4 sphere act as a self-protective clamping layer to avoid the photo corrosion effect under photo irradiations. The nanocomposites express the soft magnetic behavior with high saturation magnetization under annealing temperature at 300 °C, which may attribute to the well-defined crystalline structure and surface defects. In addition, the GZF 300 nanocomposites exhibit the enhanced photocatalytic degradation over Rhodamine B dye which is 3.4, 1.15, and 1.32 times higher than that of ZF, GZF, and GZF 600 over under visible irradiation in 120 min. The GZF 300 nanocomposites demonstrate their ability to degrade RhB efficiently, even after several photocatalysis cycles with high catalyst recovery by its magnetically separable behavior. The high densities of oxygen defects improvise electron transfer from ZnFe2O4 to rGO and delay the recombination process of the nanocomposite, resulting in enhanced visible photocatalytic activity. The strong magnetic properties of rGO wrapped ZnFe2O4 nanocomposite catalysts the easy separation from the suspension system for multiple usages in water treatment.

Critical behavior and magnetic entropy change in La_0.7Ca_0.3Mn_0.9Zn_0.1O_3 perovskite manganite

판더롱,P.Q. Thanh,N.H. Sinh,K.W. Lee,유성초 한국물리학회 2011 Current Applied Physics Vol.11 No.3

We studied the critical behavior and magnetic entropy change in a perovskite-manganite compound of La_0.7Ca_0.3Mn_0.9Zn_0.1O_3 around its Curie temperature of T_C = 206.75 K. Experimental results revealed that the sample exhibited the second-order magnetic phase transition with the exponents β = 0.474 and γ = 1.152 close to those expected from the mean-field theory (β = 0.5 and γ = 1.0). In the vicinity of T_C,the magnetic entropy change DSM reached maximum values of 1.1, 1.7, and 2.7 J/kg K under magnetic-field variations of 10, 20, and 35 kOe, respectively. These ΔS_M values are much lower than those reported previously on the parent compound of La_0.7Ca_0.3MnO_3. The nature of this phenomenon is discussed by means of the characteristics of the magnetic phase transition, and critical exponents.

NMR을 이용한 구조 및 입도 차이에 따른 분말 포도당의 molecular relaxation behavior의 관찰

정명수,Chung, Myung-Soo 한국식품과학회 2002 한국식품과학회지 Vol.34 No.6

제조방법이 서로 다른 3가지 종류의 분말포도당을 표준체를 사용하여 입자크기별로 분류하여 각 시료에 대한 온도 변화에 따른 relaxation behavior, 즉 molecular mobility의 변화를 관찰하기 위하여 low resolution pulsed NMR로 각 온도에서 spin-spin relaxation time constant$(T_2)$을 측정하였다. 분말포도당 종류별로 $T_2-$온도 곡선을 작성하여 입도의 변화에 따른 각 시료의 glass transition 특성을 비교, 분석하였다. 또한 주사전자현미경으로 각 시료의 미세구조를 관찰함으로써 구조적 차이가 제조방법 및 입도의 변화에 따른 분말포도당의 relaxation behavior의 변화에 어떤 영향을 미치는지 조사하였다. 무수결정포도당은 본 연구에서의 측정 온도범위에서 입도에 관계없이 molecular mobility에 대한 측정값인 $T_2$의 변화가 거의 없는 것으로 나타나 온도변화에 매우 안정한 결정구조를 가진다는 사실을 확인할 수 있었다. 반면에 함수결정포도당은 결정구조를 가지지만 화학구조상 물분자를 함유하기 때문에 어느 온도$(65{\sim}70^{\circ}C$ 부근) 이상이 되면 결정구조가 깨어지면서 급격하게 caking이 시작되는 경향을 보여주었고, 무정형 구조를 가지는 정제포도당은 구조적 특성상 함수결정포도당보다 낮은 온도$40^{\circ}C$ 부근)에서부터 molecular mobility의 변화가 관찰되는 caking에 매우 민감한 물질임을 알 수 있었다. 입자크기의 차이는 제조방법의 차이에 비해 분말포도당의 molecular relaxation behavior에 그다지 큰 영향을 미치지는 않지만 입도가 커질수록 입자간의 접촉표면적이 줄어들어 caking에 대한 안정성이 다소 향상될 수 있음을 알 수 있었다..92에서 총 3개의 peak가 검출되었고, 이때의 검출파장은 약 $380{\sim}490\;nm$ 사이의 파장으로서 gradient mode로 한 이동상의 조건과 일치하였다. 또한 RT값이 3.27의 peak에서 가장 많은 색소가 검출되었다. 그러나 gradient mode로 한 이동상의 조건에서 7개의 peak가 검출되었고 isocratic mode로 한 이동상의 조건에서는 3개의 peak가 검출된 것으로 보아 정성적으로는 gradient mode로 한 이동상의 조건이 더 적합한 것으로 사료된다. 또한, PDA-HPLC를 이용한 참조기와 수조기의 색소성분을 비교한 결과, 수조기와 참조기에서 검출되는 peak중 각각의 3개의 peak는 유사한 RT값을 보였으나, 참조기 및 수조기의 각각 1개의 peak는 서로 다른 RT값을 보였으며 참조기 성분의 peak중 RT = 31.02는 수조기 성분에서는 발견되지 않아, 이 성분은 참조기 성분과 연관성이 클 것으로 사료된다.ne 함량은 일반꽃포류와 구운꽃포류에서 각각 $4{\sim}43\;mg$과 $18{\sim}74\;mg$이었다. t-검정 결과 일반꽃포류와 구운꽃포류 함량간에는 수분, glutamic acid, creatinine 등에서 유의차가 있었으나, 그 이외에 대부분의 성분간에는 유의차가 인정되지 않는 것으로 나타났다.갈근 추출액을 급여한 흰쥐군은 50 ppm의 카드뮴액만을 급여한 흰쥐군과 비교하여 신장내 카드뮴 함량과 GPT 및 LDH 활성도, renin 활성도가 유의적으로 감소되었고 신장 무게는 정상 흰쥐와 같은 수준으로 회복하였고 GOT 활성도 역시 정상 흰쥐와 같은 수준으로 감소를 보여 갈근이 카드뮴 중독 흰쥐에서 신장 등의 장기내 카드뮴 축적의 감소로 카드뮴 중독 작용의 경감 효과를 갖는 것으로 나타났다.실이나 최근 세계화는 곧 우리의 전통문화를 알리는 것이라는 목소리에 주부들이 전통음식에 대한 Molecular relaxation behaviors of crystalline glucose anhydrous, crystalline glucose monohydrate, and amorphous glucose with different particle sizes were observed by measuring spin-spin relaxation time constant $(T_2)$ at the temperature range of $-20\;to\;110^{\circ}C$ using temperature-controlled low field nuclear magnetic resonance spectroscopy. No change in $T_2$ values of crystalline glucose anhydrous was observed throughout the temperature range, whereas $T_2$ values of crystalline glucose monohydrate and amorphous glucose increased from around $45\;and\;65^{\circ}C$, respectively. These results indicate that molecular mobility of crystalline glucose anhydrous does not change even at temperature higher than $100^{\circ}C$ and that the stability of powdered glucose could be improved by increasing the particle size of materials.

Fractal Nature of Magnetic Colloidal Dispersion with Cobalt Iron Oxide and Metal Iron Particles

( Kwan Han Yoon ),( Young Sil Lee ) 한국화학공학회 2022 Korean Chemical Engineering Research(HWAHAK KONGHA Vol.60 No.1

The microstructure of highly aggregated colloidal dispersions was investigated by probing the rheological behavior of magnetic suspensions. The dynamic moduli as functions of frequency and strain amplitude are shown to closely resemble that of colloidal gels indicating the formation of network structure. The two types of characteristic critical strain amplitudes, γ_c and γ_y, were characterized in terms of the changing microstructure. The amplitude of γ_c indicates the transition from linear to nonlinear viscoelasticity and depends only on particle volume fraction not magnetic interactions. The study of scaling behavior suggests that it is related to the breakage of interfloc, i.e., floc-floc structure. However, yielding strain, γ_y, was found to be independent of particle volume fraction as well as magnetic interaction. It relates to extensive deformation resulting in yielding behavior. The scaling of elastic constant, G_e, implies that this yielding behavior and hence γ_y is due to the breakage of long-range interfloc interactions. Also, the deformation of flocs due to increase strain was indicated from the investigation of the fractal nature.