Dong-Chul Chung

High yield purification of multiwalled carbon nanotubes by selective oxidation during thermal annealing

Multiwalled carbon nanotubes were synthesized by electric arc discharge method in helium ambient with the pressure of 400 Torr and then purified by thermal annealing. During the annealing in air, the quartz tube in which the raw samples were placed was rotated in order to expose evenly the nanotubes and the carbonaceous particles to the air. The carbonaceous particles were presumably etched away by the selective oxidation with faster etching rate than nanotubes. This gives rise to very high yield of about 40%. It was found from Raman scattering measurements that the ratio of the intensity of G-line peak (1583 cm−1) to that of D-line peak (1285 cm−1) increased drastically by this purification process. Our density-functional tight-binding calculations clearly show that the desorption energy barrier of a C–O pair from the nanotube edge is 2.48 eV, higher than 0.3∼2.1 eV from an amorphous carbon, confirming the current approach of purification by the selective oxidation.

Effect of surface morphology of Ni thin film on the growth of aligned carbon nanotubes by microwave plasma-enhanced chemical vapor deposition

Aligned carbon nanotubes were synthesized on Ni-coated Si substrates using microwave plasma-enhanced chemical vapor deposition. The surface morphology of Ni thin films was varied with the rf power density during the rf magnetron sputtering process. It was found that the growth of carbon nanotubes was strongly influenced by the surface morphology of Ni thin film. Pure carbon nanotubes were synthesized on Ni thin film with uniformly distributed grain sizes, whereas large amounts of carbonaceous particles were produced in addition to the nanotubes, when the nanotubes were grown on Ni thin film with widely distributed grain sizes. With decreasing Ni-grain size, the diameter of nanotubes decreased and the length increased. High-resolution transmission electron microscope images clearly demonstrated the nanotubes to be multiwalled, and the graphitized structures were confirmed from the Raman spectra. Efficient field emission was observed from the diode structure with the nanotube tips.

Impedance variation of a flux-lock type SFCL dependent on winding direction between coil 1 and coil 2

The flux-lock type superconducting fault current limiter (SFCL) uses the magnetic coupling between two coils connected in parallel with each other. This SFCL has the merit that both the resistance of high-T/sub C/ superconducting (HTSC) element and the limiting current capacity can be simultaneously increased through the application of magnetic field into HTSC element. However, unlike other types SFCLs, the different fault current limiting characteristics can appear according to the winding direction between the two coils. In this paper, the relation of the impedance of the flux-lock type SFCL with the resistance of HTSC element was derived from its equivalent circuit using the current and the voltage relationship between the two coils. The limiting impedances of the flux-lock type SFCL due to the winding direction through the discrete fourier transform (DFT) analysis for the voltage and the current data obtained from the fault current limiting experiments were extracted and analyzed from the point of view of the limited line current and the resistance of HTSC element.

Responses of resistive superconducting-fault-current-limiters to unbalanced faults

We analyzed the unsymmetrical fault characteristics of resistive superconducting-fault-current-limiters (SFCL) based on YBCO thin films with the unbalanced faults such as a single line-to-ground fault, a double line-to-ground fault, and a line-to-line fault in a three-phase system. The unsymmetrical rates of fault phases were 6.4, 9.2, 8.8 at the fault onset, but decreased by 1.4, 1.5, 3.7 after 50 ms in the fault types, respectively. The positive sequence current I/sub 1/ was the highest in a double line-to-ground fault, immediately after the fault onset, but that of a line-to-line fault was the highest after 50 ms. This means the current limiting effect was the worst in a line-to-line fault, due to the unbalanced quench between the SFCL units. The negative sequence currents I/sub 2/ of a single and double line-to-ground faults were relatively low, except for the quench instant, because of the rapid interruption of fault currents by the SFCL. The zero sequence current I/sub 0/ was similar to the behavior of the negative sequence current. Finally, the positive sequence resistance Z/sub 1/ was reduced remarkably immediately after the fault but gradually approached the balanced positive sequence resistance prior to the system fault, except during a line-to-line fault. The simultaneous quench between the SFCL units was important for low line-to-line fault currents.

Circularly polarized HTS microstrip antenna array

A four-element, 11.67 GHz, high-T/sub c/ superconducting (HTS) microstrip antenna array with corporate feed network and circular polarization for direct broadcasting satellite (DBS) system was designed and built on a 0.5 mm thick MgO substrate. One antenna pattern was fabricated from gold thin film, and a second pattern was fabricated from YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) superconducting thin film. To improve the axial ratio of circularly polarized arrays, sequential rotation technique were used. Efficiency, radiation pattern, return loss and bandwidth were measured for both antennas at room temperature and at cryogenic temperatures. The array produced good circular polarization, and the gain of the array at 77 K, relative to a gold array at room temperature was approximately 1.54 dB. The measured return loss of our HTS antenna array was -35.79 dB at the resonant frequency of 11.67 GHz and the total effective bandwidth was about 3.4%. The results showed that high-temperature superconductors, when used in microstrip arrays, improved the efficiency of the circularly polarized HTS antenna array.

Current Limitation and Power Burden of a Flux-Coupling Type SFCL in the Three-Phase Power System According to Turn's Ratio and Fault Type

Most of the transmission system has a network structure to improve the reliability and stability of a power system. Fault current is continuously expected to increase by the increase of the power demand. If fault current exceeds the cutoff capacity of a circuit breaker, the circuit breaker is broken and the damage by fault current is expanded throughout the power system. Superconducting fault current limiter (SFCL) was designed to solve this problem in a power system. In this paper, we investigated the current limiting characteristics and power burden of superconducting elements of a flux-coupling type SFCL in three-phase power system. A Flux-coupling type SFCL is one of the resistive type SFCLs. The flux-coupling type SFCL was made by using a transformer. Reactors connected in each phase shared an iron core. When the superconducting elements were quenched in fault phase, the fault current flowed into the primary and secondary coils simultaneously. Thus, the current flowed into primary and secondary coils of sound phase by the magnetic coupling flux. Meanwhile, when the current of sound phase exceeded the critical current of the SFCL, superconducting elements connected in the sound phase were quenched. The value of the fault current tended to decrease as the first reactors ratio increased. Furthermore, the power burden of the superconducting element was reduced. The reduced power burden of the superconducting elements shortens the recovery time of the superconducting element, which is advantageous for cooperation with a reclosing system when the SFCL is applied to the system. As a result, we confirmed that the flux-coupling type SFCL operated effectively in the three-phase power system.

HTS microstrip antenna array for circular polarization with cryostat

We report the comparison results of an HTS antenna array for circular polarization with a self cryostat and comparison with a gold counterpart. The HTS antenna array designed in this work was fabricated using a YBCO superconducting thin film on an MgO substrate, and the designed resonant frequency was 11.85 GHz. We used square patches with truncated corners for circular polarization and employed a corporate feed network using sequential rotation techniques for enhancement of axial ratio. A cryostat was constructed for the measurement of each antenna, and liquid nitrogen was used as a coolant. Measurements from HTS antennas, including return loss, bandwidth, gain, efficiency and axial ratio are presented and compared with those of gold counterparts. The measured results show that useful antennas can be made using the proposed architecture for satellite communication systems.

Design and Characterization of the Integrated Matrix-Type SFCL

In this paper, we report the improved and integrated matrix-type superconducting fault current limiter (MFCL). The new MFCL proposed in this work has a structure that easily improves the quench characteristics of superconducting elements. This integrated MFCL is simply constructed by mounting a trigger element and superconducting elements in a single reactor, whereas older MFCL designs require several complex reactors for proper operation. Also this integrated type of structure provides a uniform and strong magnetic field with superconducting elements, compared with an old type MFCL. We designed and characterized the integrated MFCL with 1 times 3 superconducting modules and 2 times 3 superconducting modules. From experimental results, we confirmed that our integrated MFCL had advantages including a compact design, better quench characteristics and an easily adjustable increment of the capacity for fault current limiting.

Broadband HTS microstrip antennas for satellite communication

The major limitation of high-T/sub c/ superconducting (HTS) microstrip antennas is the narrow bandwidth due to the high Q and thin substrate. Defining bandwidth as the frequency range over which standing wave ratio (SWR) 2:1 or less, HTS antenna bandwidths are typically 0.85% to 1.1%. Thus considerable effort has been focused on developing antennas for broadband operation. In this work the HTS antenna, which consists of two triangle-radiation patches, was designed and fabricated using a YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) superconducting thin film on MgO substrate for broadband operation. The input impedance and other characteristics for this antenna have been calculated using the moment method. Experimental results for our HTS antenna were reported in terms of radiation patterns, return losses, bandwidths and other various characteristics. The bandwidth obtained was a significant 6.7%.

Discharge and current limiting characteristics of a superconducting fuse

We investigated the basic properties of a superconducting fuse (SF) based on YBCO/Au films. The SF consists of YBCO stripes covered with Au layers for current shunt. The fault current was limited to a designed value within 0.4 msec by resistance generated in YBCO/Au after its quenching. This enabled the SF to transfer small fault power and the suppressed current was sustained for more than 0.5 msec while Au layer melting and arcing. The arcing time was less than 2.5 msec, which is short enough to do self-interruption. Under the source voltage of 100 V/sub rms/, the longer the current limiting time was, the shorter its discharge time was. The current limiting time and its discharge time were reduced as the applied voltages increased in the range of 200-300 V/sub rms/. We thought that this was because the quench propagation was limited by local melting generated from higher voltage. The superconducting fuse could be designed in small size and compact structure using the characteristic of its high current density compared with the conventional fuse. The superconducting fuse was not harmful to the environment because it was immersed in liquid nitrogen.