Soon-Gul Lee

Conducting Polymer Nanotube and Nanowire Synthesized by Using Nanoporous Template: Synthesis, Characteristics, and Applications

We synthesized nanotubes and nanowires of conducting polypyrrole, poly(3,4-ethylenedioxythiophene), and polyaniline by using nanoporous template through electrochemical polymerization method. The DBSA, CSA, TBAPF 6 , or HClO 4 was used for dopant, and distilled water, acetonotrile, or NMP was used for solvent. From the SEM and TEM pictures, the formation of conducting polymer nanotube (CPNT) and nanowire (CPNW) was confirmed. The diameter and length of CPNT and CPNW were 200 nm and 10 40 μm. The length, nanotube, nanowire, and thickness of wall were determined from synthetic conditions such as polymerization time, current, and dopant. For structure, we investigated UV/Vis absorbance spectra and X-ray diffraction patterns. DC conductivity and I-V characteristic curve were measured for the systems prepared with various conditions of synthesis. For applications for CPNT and CPNW, we measured field emission characteristic curve for FED and capacitance.

Electrical, magnetic, and structural properties of lithium salt doped polyaniline

Abstract We report the results of temperature dependence of dc conductivity σ dc ( T ), electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS) experiments for the polyaniline (PAN) doped with various lithium (Li) salts such as LiPF 6 , LiBF 4 , LiClO 4 , LiAsF 6 , and LiCF 3 SO 3 . A quasi one-dimensional (1D) variable range hopping (VRH) model provide the best fitting for σ dc ( T ) the Li salt doped PAN samples, which is similar to that of protonic acid (HCl) doped polyaniline samples. The σ dc and its temperature dependence vary with the kind of Li salts used. Temperature dependence of thermoelectric power of PAN doped with LiPF 6 shows the metallic behavior. From EPR experiments, we obtain that χ P depends on the kind of Li salts. The results of XPS experiments are discussed to account for the doping procedure with Li salts. Temperature dependence of EPR linewidths and X-ray diffraction patterns are compared for the various Li salt doped PAN samples. The insulator–metal transition of PAN-LiPF 6 samples is also discussed.

Serial array high T/sub c/ SQUID magnetometer

SQUID magnetometers have been fabricated from series arrays of YBa/sub 2/Cu/sub 3/O/sub 7/ dc SQUIDs by pulsed laser film deposition and ion mill patterning techniques. The array was designed either in linear chain with flux focusers or in meander structure. The arrays consisted of 50-130 step-edge junction SQUIDs and each SQUID had inductance of 20-40 pH with 2-5 /spl mu/m junction width. A linear array of 50 SQUIDs showed a modulation amplitude of 150 /spl mu/V which corresponds to a gain of /spl sim/10. One of 80 SQUID meander arrays had a gain of as high as 50. These results provide feasibility of the series array of SQUIDs in the direct readout SQUID amplifier circuitry with wide bandwidth. Larger number SQUID arrays and processes to increase the gain to the theoretical value need to be studied.

High energy (MeV) ion-irradiated π-conjugated polyaniline: Transition from insulating state to carbonized conducting state

High energy (MeV)C2+,F2+, and Cl2+ ions were irradiated onto π-conjugated polyaniline emeraldine base (PAN-EB) samples. The energy of an ion beam was controlled to a range of 3–4.5MeV, with the ion dosage varying from 1×1012 to 1×1016ions∕cm2. The highest dc conductivity (σdc) at room temperature was measured to be ∼60S∕cm for 4.5MeV Cl2+ ion-irradiated PAN-EB samples with a dose of 1×1016ions∕cm2. We observed the transition of high energy ion-irradiated PAN-EB samples from insulating state to conducting state as a function of ion dosage based on σdc and its temperature dependence. The characteristic peaks of the Raman spectrum of the PAN-EB samples were reduced, while the D-peak (disordered peak) and the G peak (graphitic peak) appeared as the ion dose increased. From the analysis of the D and G peaks of the Raman spectra of the systems compared to multiwalled carbon nanotubes, ion-irradiated graphites, and annealed carbon films, the number of the clusters of hexagon rings with conducting sp2-bonded carb...

DIRECT-COUPLED SECOND-ORDER SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE GRADIOMETER FROM SINGLE LAYER OF HIGH TEMPERATURE SUPERCONDUCTOR

We developed a noble design of the planar-type single-layer second-order superconducting quantum interference device (SQUID) gradiometer, and demonstrated that the gradiometer did not respond to a uniform field or the first-order gradient of the field but responded sensitively to the second-order gradient. The device consisted of three parallel-connected pickup loops, each of which is directly coupled to the step-edge junction SQUID. The entire structure was made from a single layer of YBa2Cu3O7 patterned by photolithography with ion milling technique. Response of the device to the field was tested with three identical wire-wound coils coupled to each loop and balancing was achieved by using a small piece of superconductor placed in the center loop. Measured off balance was about 0.6% for the uniform field and 1.4% for the first-order gradient, which were believed not due to intrinsic imbalance but mostly due to slight difference in alignment of the test coils.

The test facility for the KSTAR superconducting magnets at SAIT

SSTF (Samsung Superconductor Test Facility) has been built with the primary goal of testing the KSTAR TF (Toroidal Field) and PF (Poloidal Field) magnets as well as CICC (Cable-in-Conduit Conductor) and superconducting strands in the most relevant manner. The facility is located at SAIT (Samsung Advanced Institute of Technology) near the KSTAR project home site. Two helium liquefiers of 120 liter/hr capacity have been utilized as refrigerators demonstrating simultaneous double mode operation of refrigeration and liquefaction. A forced flow supercritical helium cooling circuit allows the test facility to be operated at temperatures down to 4.5 K. Other major SSTF components are a large vacuum vessel (6 m diameter and 7.3 m height) with liquid nitrogen temperature shield, data acquisition and control system with EPICS (Experimental Physics and Industrial Control System), current leads, and 50 kA modular power supply with fast dump quench protection circuitry. SSTF has been used for the first test-phase of KSTAR CICC sample. The current status of SSTF as the KSTAR magnet test facility for components and qualification test is presented in detail.

MgB2 grain boundary nanobridges prepared by focused ion beam

We have fabricated MgB2 grain boundary nanobridges by focused-ion-beam etch and studied their transport properties. Nanobridges with a nominal width and length of 100 nm were patterned across naturally formed single grain boundaries in the microbridges, which were prepatterned by a standard argon ion milling technique. We have studied current-voltage (I-V) characteristics, the temperature-dependent critical current, and the normal-state resistance. The measured properties were interpreted based on a flux flow model. In the I-V curves, a typical resistively shunted-junction characteristic was observed near Tc, however, as temperature decreases, flux-flow behavior became dominant, in accordance with the crossover of the ratio of the bridge length to the coherence length from the single-phased regime to the flux-flow regime. The temperature-dependent critical current was Ic(T)∼(1−T/Tc)1–1.5, similar to that of a superconducting film. The normal-state resistance increased steeply as temperature approaches Tc,...

Josephson effects in weakly coupled MgB2 intergrain nanobridges prepared by focused ion beam

We have fabricated weakly coupled intergrain nanobridges from MgB2 films by a focused ion beam (FIB) patterning technique and studied their transition properties. The bridges were nominally 300 nm wide and 100 nm long, and crossed a single grain boundary perpendicularly. The temperature-dependent resistance data showed a two-step transition after FIB pattern with more-than-two-decade increase in the resistivity. Current-voltage curves showed the characteristics of an ideal Josephson junction. The measured data were perfectly matched with the theory of the resistively shunted junction model with thermal fluctuations at all measured temperatures. At 4.2 K, the measured data showed the effect of hysteresis in agreement with the estimated McCumber parameter βc>0.3. The hysteresis effect disappeared above 6 K. The critical currents obtained from fitting to the RSJ model were linearly dependent on temperature, implying that the grain boundary played an insulating barrier.

Tailoring of the preferred orientation and microstructure in the deposition of BaTiO3 thin films using pulsed laser deposition

Abstract In the deposition of BaTiO 3 thin films using pulsed laser deposition, the preferred orientations and microstructures were critically changed with ambient oxygen pressure. In order to tailor the preferred orientation, template layers which were deposited at different oxygen pressure were introduced. These template layers acted as seed layers, and the preferred orientation of entire films was the same as that of the template layer regardless of the oxygen pressure during deposition. Microstructure of the film was not related with the kind of the template layer. It was closely related with the oxygen pressure during the deposition. In the deposition at 10 −5 Torr, films had different preferred orientations according to the kind of a template layer. However, each film had a dense and smooth surface.

Fabrication and magnetocardiography application of the second-order superconducting quantum interference device gradiometer made from a single-layer of YBa2Cu3O7 film

We have fabricated single-layer, second-order high Tc superconducting quantum interference device (SQUID) gradiometers on SrTiO3 substrates and investigated their noise properties and performance in magnetocardiography. The gradiometer consists of three parallel pickup loops that are directly coupled to a step-edge junction SQUID, in such a way that the coupling polarity of two side loops is opposite to that of the center loop. The overall size of the device is 17.6 mm×6 mm with a baseline of 5.8 mm. The measured gradient noises are 0.45 and 0.84 pT/cm2/√Hz at 1 Hz for the shielded and the unshielded cases, respectively, which correspond to equivalent field noises of 150 and 280 fT/√Hz, respectively. In spite of the short baseline of 5.8 mm, the high common-mode rejection ratio of the gradiometer, 103, allowed us to record a magnetocardiogram of a human subject, which demonstrates the feasibility of the design in biomagnetic studies.