• Electronic Materials Letters
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• 제14권6호
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• pp.774-783
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• 2018

Full maximum-entropy mobility-spectrum analysis (FMEMSA) is the best algorithm among mobility spectrum analyses by which we can obtain a set of partial-conductivities associated with mobility values (mobility spectrum) by analyzing magnetic-field-dependent conductivity-tensors. However, it is restricted to a direct band-gap semiconductor and should be modified for materials with other band structures. We developed the modified version of FMEMSA which is appropriate for a material with a single anisotropic valley, or an indirect-band-gap semiconductor quantum-well with a single non-degenerate conduction-band valley e.g., (110)-oriented AlAs quantum wells with a single anisotropic valley. To demonstrate the reliability of the modified version, we applied it to several sets of synthetic measurement datasets. The results demonstrated that, unlike existing FMEMSA, the modified version could produce accurate mobility spectra of materials with a single anisotropic valley.

• Journal of Information Processing Systems
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• 제15권2호
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• pp.288-304
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• 2019

The growth of telemedicine-based wireless communication for images-magnetic resonance imaging (MRI) and computed tomography (CT)-leads to the necessity of learning the concept of image compression. Over the years, the transform based and spatial based compression techniques have attracted many types of researches and achieve better results at the cost of high computational complexity. In order to overcome this, the optimization techniques are considered with the existing image compression techniques. However, it fails to preserve the original content of the diagnostic information and cause artifacts at high compression ratio. In this paper, the concept of histogram based multilevel thresholding (HMT) using entropy is appended with the optimization algorithm to compress the medical images effectively. However, the method becomes time consuming during the measurement of the randomness from the image pixel group and not suitable for medical applications. Hence, an attempt has been made in this paper to develop an HMT based image compression by utilizing the opposition based improved harmony search algorithm (OIHSA) as an optimization technique along with the entropy. Further, the enhancement of the significant information present in the medical images are improved by the proper selection of entropy and the number of thresholds chosen to reconstruct the compressed image.

• 한국수소및신에너지학회논문집
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• 제25권6호
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• pp.594-601
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• 2014

Rare-earth (RE) nitrides can be used as magnetocaloric materials in low temperature. They exhibit ferromagnetism and have Curie temperature in the region from 6 to 70 K. In this study, Holmium nitride (HoN) nano particles were prepared through plasma arc discharge technique and their magnetocaloric properties were studied. Nitrogen gas ($N_2$) was employed as an active element for arc discharge between two electrodes maintained at a constant current. Also, it played an important role not only as a reducing agent but also as an inevitable source of excited nitrogen molecules and nitrogen ions for the formation of HoN phase. Partial pressure of $N_2$ was systematically varied from 0 to 28,000 Pa in order to obtain single phase of HoN with minimal impurities. Magnetic entropy change (${\Delta}S_m$) was calculated with data set measured by PPMS (Physical Property Measurement System). The as-synthesized HoN particles have shown a magnetic entropy change ${\Delta}S_m$) of 27.5 J/kgK in applied field of 50,000 Oe at 14.2 K thereby demonstrating its ability to be applied as an effective magnetic refrigerant towards the re-liquefaction of hydrogen.

• Journal of Magnetics
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• 제18권1호
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• pp.34-38
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• 2013

This paper studies the effects of the Mn-site substitution by nickel on the magnetic properties and the magnetocaloric properties of $La_{0.7}Ca_{0.3}Mn_{1-x}Ni_xO_3$ (x = 0, 0.05 and 0.1). The orthorhombic crystal structures of the samples are confirmed by the room temperature X-ray diffraction. The dependence of the Curie temperature ($T_C$) and the magnetic entropy change (${\Delta}S_M$) on the Ni doping content was investigated. The samples with x = 0 had the first order phase transition, while the samples with x = 0.05 and 0.1 had the second order phase transition. As the concentration of Ni increased, the maximum entropy change (${\mid}{\Delta}S_M{\mid}_{max}$) decreased gradually, from 2.78 $J{\cdot}kg^{-1}{\cdot}K^{-1}$ (x = 0) to 1.02 $J{\cdot}kg^{-1}{\cdot}K^{-1}$ (x = 0.1), in a magnetic field change of 15 kOe. The measured value of $T_C$ was 185 K, 150 K and 145 K for x = 0, 0.05 and 0.1, respectively. The phase transition temperatures became wider as x increased. It indicates that the Mn-site substitution by Ni may be used to tailor the Curie temperature in $La_{0.7}Ca_{0.3}Mn_{1-x}Ni_xO_3$.

• 설비공학논문집
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• 제16권4호
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• pp.383-389
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• 2004

Magnetic refrigeration is based on the magnetocaloric effect (MCE) - the ability of some materials to heat up when magnetized and cool down when removed from the magnetic field. The available techniques for studying the MCE we: (1) direct measurements by monitoring the change in the material's temperature during the application or removal of the magnetic field; and (2) indirect calculations from the experimental data of magnetization and/or specific heat as a function of the temperature and magnetic field. The MCE of gadolinium (Gd) has been demonstrated by direct measurements of temperature change, and isothermal magnetic entropy changes and adiabatic temperature changes have been calculated.

• Journal of Magnetics
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• 제18권3호
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• pp.331-338
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• 2013

We present a few significant advances in methods and concepts of magnetic measurements, aimed both at providing novel routes in the characterization of hard and soft magnetic materials and at improving our basic knowledge of the magnetization process. We discuss, in particular, investigation methods and experimental arrangements that have been developed in recent times for: 1) Hysteresis loop determination in extra-hard magnets by means of Pulsed Field Magnetometry; 2) Broadband observation of domain wall dynamics by highspeed stroboscopical Kerr techniques; 3) Entropy measurements in magnetocaloric materials by calorimetry in magnetic field. While pertaining to somewhat independent fields of investigation, all these measuring techniques have in common a solid approach to the underlying physical phenomenology and have a potential for further developments.

• 한국재료학회지
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• 제30권3호
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• pp.136-141
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• 2020

The magnetocaloric effect (MCE), which is the reversible temperature change of magnetic materials due to an applied magnetic field, occurs largely in the vicinity of the magnetic phase transition temperature. This phenomenon can be used to induce magnetic refrigeration, a viable, energy-efficient solid-state cooling technology. Recently, Metal-organic frameworks (MOFs), due to their structural diversity of tunable crystalline pore structure and chemical functionality, have been studied as good candidates for magnetic refrigeration materials in the cryogenic region. In cryogenic cooling applications, MCE using MOF can have great potential, and is even considered comparable to conventional lanthanum alloys and magnetic nanoparticles. Owing to the presence of large internal pores, however, MOF also exhibits the drawback of low magnetic density. To overcome this problem, therefore, recent reports in literature that achieve high magnetic entropy change using a dense structure formation and ligand tuning are introduced.

• 한국자기학회:학술대회 개요집
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• 한국자기학회 2008년도 Asian Magnetics Conference
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• pp.162.1-162.1
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• 2008

• Investigative Magnetic Resonance Imaging
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• 제25권2호
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• pp.118-128
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• 2021

Purpose: To investigate the value of MR textural analysis, including use of diffusion-weighted imaging (DWI) to differentiate malignant from benign soft-tissue tumors on 3T MRI. Materials and Methods: We enrolled 69 patients (25 men, 44 women, ages 18 to 84 years) with pathologically confirmed soft-tissue tumors (29 benign, 40 malignant) who underwent pre-treatment 3T-MRI. We calculated MR texture, including mean, standard deviation (SD), skewness, kurtosis, mean of positive pixels (MPP), and entropy, according to different spatial-scale factors (SSF, 0, 2, 4, 6) on axial T1- and T2-weighted images (T1WI, T2WI), contrast-enhanced T1WI (CE-T1WI), high b-value DWI (800 sec/mm²), and apparent diffusion coefficient (ADC) map. We used the Mann-Whitney U test, logistic regression, and area under the receiver operating characteristic curve (AUC) for statistical analysis. Results: Malignant soft-tissue tumors had significantly lower mean values of DWI, ADC, T2WI and CE-T1WI, MPP of ADC, and CE-T1WI, but significantly higher kurtosis of DWI, T1WI, and CE-T1WI, and entropy of DWI, ADC, and T2WI than did benign tumors (P < 0.050). In multivariate logistic regression, the mean ADC value (SSF, 6) and kurtosis of CE-T1WI (SSF, 4) were independently associated with malignancy (P ≤ 0.009). A multivariate model of MR features worked well for diagnosis of malignant soft-tissue tumors (AUC, 0.909). Conclusion: Accurate diagnosis could be obtained using MR textural analysis with DWI and CE-T1WI in differentiating benign from malignant soft-tissue tumors.

• Journal of Magnetics
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• 제16권4호
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• pp.440-443
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• 2011

In this study, we investigated the glass-forming ability (GFA) and magnetocaloric effect (MCE) of $Gd_{55}Co_{20}Al_{23}Si_2$ bulk glassy alloy. It is found that the addition of 2 at% Si is effective for extension of the supercooled liquid region (${\Delta}T_x$), the ${\Delta}T_x$ is 55 K for the $Gd_{55}Co_{20}Al_{25}$ glassy alloy, and increases to 79 K for the $Gd_{55}Co_{20}Al_{23}Si_2$ alloy. As a result, $Gd_{55}Co_{20}Al_{23}Si_2$ glassy alloys with diameters up to 5 mm were successfully synthesized. The alloys also exhibit large MCE, i.e., the magnetic entropy change (${\Delta}S_m$) of 8.9 J $kg^{-1}\;K^{-1}$, the full width at half maximum of the ${\Delta}S_m$ (${\delta}T_{FWHM}$) of 87 K, and the refrigerant capacity (RC) of 774 J $kg^{-1}$.