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- 학과정보 : 물리학과 프런티어융합 (검색결과 3 건)
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HIDAYATI RAHMATUL 경희대학교 대학원 2024 국내박사
Superconducting materials are a unique class of functional materials that exhibit no electrical resistance when they are cooled below a specific critical transition temperature. The exploration and development of High-Entropy Alloys (HEAs) as innovative functional materials represent a rapidly evolving research area within the field of materials science. A number of HEA superconductors have been discovered so far, each demonstrating a range of intriguing properties. The field of HEA superconductors has been a hotbed of research activity since the first superconducting HEA was discovered in 2014. This dissertation delves into the synthesis and superconducting properties of High-Entropy Alloys (HEAs), specifically the Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 compound, using planetary ball milling and the Spark Plasma Sintering (SPS) method. The sintered HEA sample exhibited a BCC crystal structure with a small amount of secondary phases (Fe- based impurities). The superconducting phase transition was observed in the electrical resistivity and magnetic susceptibility measurements, with a Tc similar to that of an arc- melted (AM) sample. The study also revealed that the zero-temperature limit of the upper critical magnetic fields and coherence length were slightly decreased or comparable to the AM HEA sample. The field-dependent isothermal magnetization hysteresis exhibited typical type-II superconducting behavior with magnetic flux avalanches in the low magnetic field region. Notably, the calculated critical current density (Jc) was significantly increased compared to the AM HEA sample, attributed to enhanced pinning force due to point pinning as well as surface pinning effects. The vortex relaxation measurements showed stable remanent magnetization for over 10,000 s after magnetic fields were turned off, which is unlike conventional superconducting vortex relaxation behavior. The strong pinning force in the SPS sintered Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 compound is beneficial for practical applications because of the significantly enhanced Jc with stable remanent magnetization. The dissertation also explores the superconducting properties of Medium-Entropy Alloy (MEA), specifically Nb2/5Hf1/5Zr1/5Ti1/5 , synthesized by AM and powder metallurgical SPS methods. The SPS-MEA demonstrates a substantial improvement in vortex pinning, leading to an increase in the Jc when contrasted with AM-MEA. The research suggests that powder metallurgical processes for HEA synthesis enhance Jc and vortex pinning force while maintaining high mechanical strength, which is beneficial for practical applications in superconducting magnets. The study concludes that HEA and MEA have emerged as promising classes of materials due to their versatility and potential for various applications. In this study, we also discovered an unprecedented phenomenon of diamagnetically aligned spin-triplet superconductivity in an Fe-based high entropy alloy compound, NbTaTiZrFe. A significant diamagnetic signal was observed below 42 K, indicating a superconducting Tc of 7 K under zero-field-cooled magnetic susceptibility conditions. However, this transitioned to a ferromagnetic state under field-cooled conditions. The diamagnetic spin-triplet pairing is substantiated by a variety of experimental and theoretical evidence. This includes a significant diamagnetic signal at low fields coupled with a strong ferromagnetic coercive force (Hco) of 1800 Oe, a ferromagnetic spin flip signal exclusive to the diamagnetic state, and metallic ferromagnetism as confirmed by scanning magnetic force microscopy and spin-resolved density of states. Interestingly, the spin-triplet diamagnetic/ferromagnetic superconductivity can be manipulated by offset field conditions (direct- and oscillation- off) to eliminate a residual magnetic field at room temperature. This discovery of diamagnetically aligned spin-triplet superconductivity is novel and provides an excellent platform for exploring Majorana fermions and their potential application in quantum computation. Keywords High entropy alloy, superconductivity , critical current densities, pinning force, flux jump, vortex pinning, spin-triplet, ferromagnetic superconductivity, diamagnetic superconductivity, itinerant ferromagnet superconductor.
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The Eu4Ga8Ge16 is a homologous structure compound comparable with a cage structured clathrate Eu8Ga16Ge30 which investigated in respect of magnetocaloric effect (MCE). The Eu4Ga8Ge16 has an orthorhombic structure (Cmcm) with the charge-balancing Ga/Ge framework encompasses the Eu2+ atomic chains, resulting in the one-dimensional caged spin chain structure. It shows metamagnetic transition from antiferromagnetic state by low magnetic field which is thought to be beneficial to MCE. We examined the magnetic, electrical properties of single and polycrystalline Eu4Ga8Ge16 and its MCE. Also, hydrogen storage capacity of Eu4Ga8Ge16 is investigated since caged structure is beneficial to storing gas molecules. We found its unique properties that may come from its one-dimensional spin chain structure such as inverse magnetic hysteresis at T = 2 K, non-normal behaviors in electrical resistivity such as the Kondo-like effect, strong electron-phonon coupling. The magnetic entropy change of Eu4Ga8Ge16 is evaluated as -ΔSM = 14.6 J kg-1 K-1 and adiabatic temperature change is evaluated as ΔTad = 7.8 K at TN along the [100] direction, which are superior to previously reported type-I clathrate Eu8Ga16Ge30 compound. We also observed hydrogen is stored in the Eu4Ga8Ge16. Therefore, the giant magnetocaloric materials can be extendable to the antiferromagnets with low-magnetic field metamagnetic transition and caged magnetic materials such as Eu4Ga8Ge16 can be the multi-functional materials for magnetocaloric and hydrogen storage application. Moreover, we suggest that unique properties of Eu4Ga8Ge16 are worthy of further magnetic and electrical investigations.
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페로브스카이트 LaVO3 박막의 광학적, 구조적 특성 및 응용 연구
페로브스카이트 구조를 가진 LaVO3 박막은 태양광 발전 소자에서 활성층의 재료로 제안되었다. 본 연구에서는 LaVO3 박막의 여러 측정 장비를 이용하여 광학적, 구조적 특성들을 분석하였다. 먼저 AFM, SEM 그리고 XRD 측정을 통해 박막의 표면 및 단면 그리고 구조적 특성을 분석하였다. 또한 Ellpsometry 를 통해 1.41eV 의 LaVO3 밴드갭 에너지를 구하였으며, 흡광도 및 투과율을 측정하였다. 이러한 결과를 바탕으로 LaVO3를 활성층으로 사용한 자체 전력 광검출기와 반투명 태양전지를 제작하였다. WS2/h-BN/LaVO3 구조의 광검출기의 광전류/암전류 비율은 0V 에서 델타 함수 특성을 나타내어 ‘자체 구동’이 가능함을 보여주었다. 광검출기는 최대 0.27AW-1 반응성(R), 730nm 에서 4.6×1010 cm Hz1/2 W-1 검출성(D*)을 보여줬다. 또한, 온도 25℃, 상대습도 30%의 조건에서 2000 시간 동안 초기 R 의 87%를 유지하였다. 다음으로, TETA-Gr/WS2/LaVO3 구조의 태양전지는 LaVO3 박막의 두께가 200nm 일 때 5.07%의 전력 변환 효율(PCE)와 35%의 AVT 를 보였다. 소자의 반투명 특성을 극대화하기 위해 Al 반사 거울을 사용하여 소자의 PCE 를 5.64%로 더욱 개선하였다. 온도 60℃, 상대습도 30% 및 온도 80℃, 상대습도 50%의 두 조건에서 1000 시간동안 원래 PCE 의 91 및 84%를 유지하였다.
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