J.H. Wang

Growth of a single freestanding multiwall carbon nanotube on each nanonickel dot

Patterned growth of freestanding carbon nanotube(s) on submicron nickel dot(s) on silicon has been achieved by plasma-enhanced-hot-filament-chemical-vapor deposition (PE-HF-CVD). A thin film nickel grid was fabricated on a silicon wafer by standard microlithographic techniques, and the PE-HF-CVD was done using acetylene (C2H2) gas as the carbon source and ammonia (NH3) as a catalyst and dilution gas. Well separated, single carbon nanotubes were observed to grow on the grid. The structures had rounded base diameters of approximately 150 nm, heights ranging from 0.1 to 5 μm, and sharp pointed tips. Transmission electron microscopy cross-sectional image clearly showed that the structures are indeed hollow nanotubes. The diameter and height depend on the nickel dot size and growth time, respectively. This nanotube growth process is compatible with silicon integrated circuit processing. Using this method, devices requiring freestanding vertical carbon nanotube(s) such as scanning probe microscopy, field emissi...

Pairing Symmetry in Single-Layer Tetragonal Tl2Ba2CuOβ+δ Superconductors

A high-resolution scanning superconducting quantum interference device microscopy study of tetragonal single-layer Tl2Ba2CuOβ+δ films, deposited on tricrystal SrTiO3 substrates, demonstrates the effect of spontaneously generated half flux quanta. This observation shows that in addition to YBa2Cu3O7, the order parameter symmetry in Tl2Ba2CuOβ+δ is consistent with that of a dx2-y2 pair state. This result also rules out any bilayer or twinning effects and any pairing that is incompatible with the fourfold rotational symmetry as in the Tl2Ba2CuOβ+δ superconducting system.

Enhanced formation of 1223 phase by partial replacement of Bi for Tl in in-situ synthesized silver-sheathed superconducting tape of Tl1−xBixSr2−yCa2Cu3O9−δ

Abstract The in-situ synthesis method has been used in fabricating silver-sheathed superconducting tape of Tl 1− x Bi x Sr 2− y Ba y Ca 2 Cu 3 O 9−δ . It was surprisingly found that the partial replacement of Bi for Tl greatly enhanced the formation of superconducting 1223 phase, as compared to the partially Pb replaced superconductor Tl 0.5 Pb 0.5 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O 9−δ . The critical current densities of tapes with composition in the range of x=0.0-0.4 and y=0.0-2.0 have been determined by transport measurements. We found that the partial substitutions of both Tl by Bi and Sr by Ba can vary over a wide range without drastic reduction of the J c values, with the highest J c values obtained reproducibly at x =0.22 and y =0.4. A pellet of composition Tl 0.78 Bi 0.22 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O 9−δ was found to consist of a single 1223 phase according to the XRD patterns. At 77 K and in the absence of an external magnetic field, the observed J c is 1.33×10 4 A/cm 2 for the rolled tapes and 2.1×10 4 A/cm 2 for the rolled and pressed tapes. In the presence of the external magnetic field, the transport J c was limited by weak links.

Improvement of transport critical current density in in-situ synthesized silver-sheathed superconducting tape of Tl0.78Bi0.22Sr1.6Ba0.4Ca2Cu3O9−δ by intermediate annealing and pressing

Abstract Intermediate thermal annealing and static hydraulic pressing were applied to process in-situ synthesized silver-sheathed superconducting Tl 0.78 Bi 0.22 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O 9−δ tapes. It was found that this intermediate processing improved the tape performance as indicated by a 65% increase of transport critical current density ( J c ) at zero field and an even greater increase of 340% at 5.5 T. Scanning and transmission electron microscopy (SEM and TEM, respectively) and X-ray diffraction (XRD) were used to characterize the microstructural differences and phase formation of the tapes prepared by three different methods. It was found that a higher core density and more favorable pore distribution due to the intermediate processing were the main reasons for the improvement in J c . Field-cooled (FC, B = 50 G) magnetization measurements using a Superconducting Quantum Interference Device (SQUID) demonstrated a better flux pinning for the tape subjected to the intermediate processing, which is consistent with the greater improvement of the transport J c at 5.5 T than at zero field.

Relationship between grain connectivity, alignment, and critical current density in high-temperature superconducting bulk materials

Abstract Uniaxial hydraulic pressure was successfully employed to partially reorient the c -axes of the highly anisotropic superconducting grains in bulk Tl 0.78 Bi 0.22 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O 9−δ samples. The partial reorientation was confirmed by X-ray diffraction pattern, rocking curves, and SQUID magnetization measurements. Samples made from the superconducting powder with partial alignment in c -axis direction and samples made from reaction mixture (in-situ synthesis method) with random orientation should have a similar grain connectivity since both were made by the same fabrication procedure. Both types of samples were found to exhibit about the same J c value of 15 000–17 000 A/cm 2 at 77 K. By contrast, another pair of samples with the same c -axis alignment as shown by X-ray diffraction, rocking curve, and SEM measurements, but with different grain connectivity due to the different compression and heat treatment procedure used, showed a significant difference in J c . The just rolled tape had a J c of 13 300 A/cm 2 , whereas the rolled and subsequently compressed tape had a J c of 19 500 A/cm 2 at 77 K. Based on the results of these and other related experiments, we have to conclude that grain connectivity, not lattice alignment, is the decisive factor in determining the J c of Tl based 1223 bulk superconducting materials and silver-sheathed superconducting tapes made by the powder-in-tube method.

Superconducting thallium oxide films via an electro-deposited precursor

Abstract Electro-deposited precursor films of BaCaCuO (BCCO) doped with Ag were prepared on Ag foil. The electrodeposited films were processed using a two-zone anneal, which employs a Tl source zone and a separate sample zone, both within an O 2 ambient. The annealed films showed c -axis oriented 1223 as the major phase. At 77 K and no magnetic field, the critical current density was 4.42×10 4 A/cm 2 using the field criterion of 1 μV/cm. The field dependence for the Tl-1223 film was equally impressive, showing 8.2×10 3 A/cm 2 at 5.5 T (77 K) for the field oriented parallel to the ab plane ( H ‖ ab ) and parallel to the applied current.

Superconducting thallium oxide films by the electrodeposition method

Abstract A low cost electrodeposition method is tested for the fabricating high-temperature superconducting wire or tape. Recent studies on the development of biaxially textured electrodeposited (ED) films on Ag-coated, single-crystal substrates are reported. Pole-figure measurements of electrodeposited (Tl)–Ba–Ca–Cu–O and (Tl, Bi)–(Sr, Ba)–Ca–Cu–Ag–O films on 300 A Ag/LaAlO 3 , with thickness between 1 and 5 μm, clearly show strong biaxial texturing. The omega scan and phi scan of (TlBi) 0.9 Sr 1.6 Ba 0.4 Ca 2 Cu 3 Ag 0.2 O x indicate the full width half maximum (FWHM) of only 0.9° and 1.2°, respectively, which indicate a very high-quality film prepared by electrodeposition process. Transport measurements for the ED films show values of 3×10 5 A/cm 2 at 77 K in zero field. Single-layer thallium-oxide films, with Pb, Bi, and Sr substitution, offer the potential for operation at 77 K in practical magnetic fields of 3–5 T.

Microstructural development and critical currents in long-length (Tl, Bi)-(Sr, Ba)-Ca-Cu-O wires

Synthesis of long Tl−Ba−Ca−Cu−O wires has been achieved byin situ synthesis techniques. Substitutions of Bi for Tl and of Sr for Ba were found to greatly enhance formation of the superconducting phase. Detailed microstructural characterization was performed on wires processed under various conditions in order to elucidate the reaction mechanisms. In addition, microstructural studies were carried out to investigate the beneficial effects of uniaxial compression on critical current density. It was found that thein situ reaction follows a “natural” two-powder process with early formation of Tl-1212. This reaction pathway provides a mechanism for sintering and crack healing and protects existing Tl-1223 for regrowth. Intermediate heat treatment allows more complete advantage to be taken of the above processes, while subsequent pressing increases the density and promotes grain alignment with minimum damage. Further heat treatment sinters grains and allows completion of the transformation to Tl-1223. Elements of this process have been incorporated in a sandwich-type geometry using plate-like grains to promote additional grain alignment.

Continuous control of the superconducting transition temperature from overdoped to underdoped regimes in tetragonal Tl2Ba2CuO6+δ thin films

Epitaxial superconducting thin films Tl2Ba2CuO6+δ (Tl-2201) with a continuous range of doping levels have been synthesized. The films were made by rf magnetron sputtering and two-step postdeposition annealing, and their doping levels were controlled by varying the oxygen content through annealing in flowing argon. By annealing in flowing argon, the as-grown Tl-2201 thin films were changed continuously from the initially overdoped state, through the optimally doped state and then to the underdoped state. The normal state resistivity versus temperature curves of all the underdoped Tl-2201 thin films are convex, whereas those of the overdoped Tl-2201 thin films are all concave. Transmission electron microscopy bright-field images and selected area diffraction patterns of the 58 K underdoped Tl-2201 thin film showed neither twinning bands nor spot splitting associated with an orthorhombic crystal lattice. Therefore, it is concluded that underdoped Tl-2201 thin films are also tetragonal just like all the overd...

Superconducting epitaxial Tl2Ba2CuO6 + δ thin films on SrTiO3 with tetragonal lattice and continuously adjustable critical temperature

Abstract Epitaxial Tl 2 Ba 2 CuO 6 + ° (Tl-2201) thin films on single crytalline SrTiO 3 substrate have been synthesized by RF magnetron sputtering followed by a two-step annealing process. X-ray diffraction data show that the film is c -axis oriented with full width at half maximum of rocking curve (FWHM) of 0.265° and alignment in the ab -plane. The zero-resistance T c of the same film can be increased or decreased continually and reversibly in the 11–80 K range by annealing in argon or air, with no change in tetragonal lattice symmetry.