• Progress in Superconductivity
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• v.9 no.1
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• pp.68-73
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• 2007

We fabricated the in-situ $MgB_2$ wires using the powder-in-tube method and investigated the effects of sintering temperature and SiC contents on the microstructure and superconducting properties. Pure $MgB_2$ wires and 5, 10, 20 wt.% SiC doped $MgB_2$ wires were sintered at $600-1000^{\circ}C$ for 30 minutes in Ar atmosphere. We found that $MgB_2$ phase was mostly formed at the sintering temperature of $700^{\circ}C$ and above, and the critical temperature ($T_c$) increased with increasing sintering temperature. For the $MgB_2$ sintered at $850^{\circ}C$, the highest critical current density ($J_c$) was obtained to be $3.7{\times}10^5\;A/cm^2$ at 5 K and 1.6 T by a magnetic properties measurement system (MPMS). The addition of SiC to the $MgB_2$ wires changed microstructure and critical properties. SEM observation showed that the $MgB_2$ core had considerable micro-cracks in undoped wire and the density of micro-cracks decreased with increasing SiC contents. The critical temperature decreased as the SiC contents increased, on the other hand, the critical current density of SiC doped $MgB_2$ wires in high magnetic field was enhanced compared to that of undoped $MgB_2$ wires.

• Korean Journal of Materials Research
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• v.22 no.11
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• pp.569-574
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• 2012

Large single grain $Gd_{1.5}Ba_2Cu_3O_{7-y}$ (Gd1.5) bulk superconductors were fabricated by a top-seeded melt growth (TSMG) process using an $NdBa_2Cu_3O_{7-y}$ seed. The seeded Gd1.5 powder compacts with a diameter of 50 mm were subjected to the heating cycles of a TSMG process. After the TSMG process, the diameter of the single grain Gd1.5 compact was reduced to 43 mm owing to the volume contraction during the heat treatment. The superconducting transition temperature ($T_c$) of the top surface of the single grain Gd1.5 sample was as high as 93.5 K. The critical current densities ($J_cs$) at 77 K and 1T and 1.5 T were in ranges of 25,200-43,900 $A/cm^2$ and 10,000-23,000 $A/cm^2$, respectively. The maximum attractive force at 77 K of the sample field-cooled using an Nd-B-Fe permanent magnet (surface magnetic field of 0. 527 T) was 108.3 N; the maximum repulsive force of the zero field-cooled sample was 262 N. The magnetic flux density of the sample field-cooled at 77 K was 0.311T, which is approximately 85% of the applied magnetic field of 0.375 T. Microstructure investigation showed that many $Gd_2BaCuO_5$ (Gd211) particles of a few ${\mu}m$ in size, which are flux pinning sites of Gd123, were trapped within the $GdBa_2Cu_3O_{7-y}$ (Gd123) grain; unreacted $Ba_3Cu_5O_8$ liquid and Gd211 particles were present near the edge regions of the single grain Gd1.5 bulk compact.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.218-218
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• 2012

Recently, hexagonal manganites have attracted much attention because of the coexistence of ferroelectricity and antiferromagnetic (AFM) order. The crystal structure of hexagonal manganites consists of $MnO_5$ polyhedra in which $Mn^{3+}$ ion is surrounded by three oxygen atoms in plane and two apical oxygen ions. The Mn ions within Mn-O plane form a triangular lattice and couple the spins through the AFM superexchange interaction. Due to incomplete AFM coupling between neighboring Mn ions in the triangular lattice, the system forms a geometrically-frustrated magnetic state. Among hexagonal manganites, $YMnO_3$, in particular, is the best known experimentally since the f states are empty. In addition, for applications, $YMnO_3$ thin films have been known as promising candidates for non-volatile ferroelectric random access memories. However, $YMnO_3$ has low magnetic order temperature (~70 K) and A-type AFM structure, which hinders its applications. We have synthesized $YMn1_{-x}Cr_xO_3$ (x = 0, 0.05 and 0.1) samples by the conventional solid-state reaction. The powders of stoichiometric proportions were mixed, and calcined at $900^{\circ}C$ for $YMn1_{-x}Cr_xO_3$ for 24 h. The obtained powders were ground, and pressed into 5-mm-thick disks of 1/2-inch diameter. The disks were directly put into the oven, and heated up to $1,300^{\circ}C$ and sintered in air for 24 h. The phase of samples was checked at room temperature by powder x-ray diffraction using a Rigaku Miniflex diffractometer with Cu $K{\alpha}$ radiation. All the magnetization measurements were carried out with a superconducting quantum-interference-device magnetometer. Our experiments point out that the Cr-doped samples show the characteristics of a spin-glass state at low temperatures.

• Journal of the Korean Magnetics Society
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• v.17 no.2
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• pp.71-75
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• 2007

Ultrasonic irradiation in a solution during the chemical reaction may accelerate the rate of the reaction and the crystallization at low temperature. We have synthesized nanometer sized zinc ferrite particles using chemical co-precipitation technique through a sonochemical method with surfactant such as oleic acid. The thermal behaviour of the zinc ferrite was determined by the thermoanalytical techniques (TGA-DSC). Powder X-ray diffraction measurements show that the samples have the spinel structure. Magnetic properties measurement were performed using a superconducting quantum interference device (SQUID) magnetometer.

• Progress in Superconductivity
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• v.6 no.2
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• pp.118-123
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• 2005

A low cost YBCO oxide powder was employed as a starting precursor for MOD process. YBCO oxide is advantageous over metal acetates or TFA salts which are popular starting precursors for conventional MOD-TFA process because that YBCO oxide precursor is cheap and easy to control molar ratio. YBCO thin films were prepared by this oxide-precursor-based MOD process and annealing condition was optimized. The YBCO thin film annealed at below $780^{\circ}C$ shows no transport $I_c$ and poor microstructure. Raman spectroscopic study of YBCO thin film indicates that YBCO thin film prepared at below $780^{\circ}C$ contains a number of imperfections such as non-superconducting $BaCuO_2$ phase, cation disorder, etc. However, the YBCO thin film treated at above $800^{\circ}C$ shows improvement in microstructure and current transport properties. This research was supported by a grant from Center for Applied Superconductivity Technology of the 21st Century Frontier R&D Program funded by the Ministry of Science and Technology, Republic of Korea.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.183-183
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• 2011

Multiferroic materials have been widely studied in recent years, because of their abundant physics and potential applications in the sensors, data storage, and spintronics. $BiFeO_3$ is one of the well-known single-phase multiferroic materials with $ABO_3$ structure and G-type antiferromagnetic behavior below the Neel temperature $T_N$ ~ 643 K, but the ferroelectric behavior below the Curie temperature $T_c$~1,103 K. In this study, the $BiFe_{1-x}Ni_xO_3$ (x=0 and 0.05) bulk ceramics were prepared by solid-state reaction and rapid sintering with high-purity $Bi_2O_32$, $Fe_3O_4$ and NiO powders. The powders of stoichiometric proportions were mixed, as in the previous investigations, and calcined at 450$^{\circ}C$ for $BiFe_{1-x}Ni_xO_3$ for 24 h. The obtained powders were grinded, and pressed into 5-mm-thick disks of 1/2-inch diameter. The disks were directly put into the oven, which has been heated up to 800$^{\circ}C$ and sintered in air for 20 min. The sintered disks were taken out from the oven and cooled to room temperature within several min. The phase of samples was checked at room temperature by powder x-ray diffraction using a Rigaku Miniflex diffractometer with Cu K${\alpha}$ radiation. The Raman measurements were carried out by employing a hand-made Raman spectrometer with 514.5-nm-excitation $Ar^+$ laser source under air ambient condition on a focused area of 1-${\mu}m$ diameter. The field-dependent magnetization measurements were performed with a superconducting quantum-interference-device magnetometer.

• Korean Journal of Materials Research
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• v.8 no.5
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• pp.394-398
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• 1998

We have studied the effect of Ag additions on the microstructure and the related mechanical and supercon¬ducting properties of $ YBa_{2}$$Cu_{3}$$O_{7-\delta}$ (YBCO) superconductors. A 5-15 vol.% of Ag was added to YBCO in the forms of Ag and $AgNO_{3}$, powder and the resultant microstructural evolution was evaluated. It was observed that the strength and fracture toughness of YBCO increased with increasing Ag content. These improvements in strength and fracture toughness are believed to be due to the strengthening mechanisms caused by the presence of Ag. In addition, YBCO-Ag composite superconductors showed higher values of strength and fracture toughness when Ag was added in the form of AgNO, than those of which was added Ag. The higher mechanical properties of YBCO- Ag resulting from $AgNO_{3}$, addition are probably due to the microstructure of more finely and uniformly distributed Ag particles. The addition of Ag also showed slightly improved critical current density of YBCO superconductors.

• Journal of the Korean Society for Heat Treatment
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• v.4 no.1
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• pp.13-18
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• 1991

The formation of high $T_c$ phase is very sluggish and c parameter of unit cell of high $T_c$ phase is about $37{\AA}$. High $T_c$ oxide superconductor with a $T_c$ above 100 K has been successfully prepared by solid state reaction method in BiSrCaCuO system by Pb adding. The microstructure related to the formation of the high $T_c$ phase has been investigated. As compared with YBCO compound, the formation reaction of the high $T_c$ requires long time heat treatment. It is due to the transformation from the low $T_c$ phase to high $T_c$ phase. The sintering just below the melting point of the calcined powder mixture is effective on the formation of the high $T_c$ phase in Pb-added BiSrCaCuO system. The growth of the high $T_c$ superconducting phase has a thin plate shape, which is characterized by the c parameter of $37{\AA}$. The formation of the high $T_c$ phases is delayed by the excessive addition of Pb. The lattice parameter(c) of the unit cell (both the low and high $T_c$ phases) is increased with increase of Pb.

• Journal of the Korean Physical Society
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• v.73 no.10
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• pp.1555-1560
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• 2018

$AlFe_2B_2$ produced by using a conventional arc melter has a ferromagnetic material with a Curie temperature ($T_C$) of around 300 K, but the arc-melt generates paramagnetic $Al_{13}Fe_4$ impurities during the synthesis of $AlFe_2B_2$. Impurities are brought to cause a decrease in magnetocaloric effects (MCEs). To investigate the effects of $Al_{13}Fe_4$ impurities on MCEs, we prepared and compared ascast and acid-treated samples, where the acid treatment was performed to remove the $Al_{13}Fe_4$ impurities. For the structural analysis, powder X-ray diffraction was carried out, and the measured data were subjected to a Rietveld refinement. The presence of $Al_{13}Fe_4$ impurities in the as-cast sample was observed in the phase analysis measurements. Magnetic properties were investigated by using Superconducting Quantum Interference Device (SQUID) measurements for the as-cast and the acid-treated $AlFe_2B_2$ samples. From isothermal magnetization measurements, Arrott plots were obtained showing that the transition of $AlFe_2B_2$ has a second-order magnetic phase transition (SOMT). The $T_C$ and the saturation magnetization increased for the acid-treated sample due to removal of the paramagnetic impurities. As a consequence, the magnetic entropy change ($-{\Delta}S$) increased in the pure $AlFe_2B_2$ samples, but the full width at half maximum in the plot of $-{\Delta}S$ vs. T decreased due to the absence of impurities.

• Progress in Superconductivity
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• v.3 no.1
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• pp.130-133
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• 2001

A multifilamentary Bi-2223 HTS tape for superconducting power applications was studied through the fabrication of 250-meter long tapes by the PIT(powder in tube) process. To fabricate continuous long wire, a drawing machine, a two-drum bull block and a rolled tape winding machine were developed. Especially, 250-meter long tapes were heat treated in the shape of pancake coil to reduce the heat affect zone and to achieve the high critical current. Engineering critical current density was improved through both the enhancements of critical current density by control of thermal process and the increase of filling factor by using thin Ag alloy sheath tubes less than 1.5 mm in thickness. We have made successfully 250-meter long 37 filamentary tapes with high filling factor up to 31 % employing the modified drawing and rolling technique. The critical current of 250-meter long tapes with pancake coil type was measured by transport method at self-field up to 250 gauss of center field. The measured values, based on the transport critical current at self-field, $I_{c}$ -B characteristics and magnetic field analysis, are 34 A of I$_{c}$ and 4.0 $kA/\textrm{cm}^2$ of $J_{e}$ at 250 m, 77 K, and 0 T. We also have achieved the 56 A of I$_{c}$ and 7.0 $0 kA/\textrm{cm}^2$ of$ J_{e}$ in short tapes at 77K, self-field, and 1$mutextrm{V}$/cm.