The peak dimensionless thermoelectric figure-of-merit (ZT) of Bi2Te3-based n-type single crystals is about 085 in the ab plane at room temperature, which has not been improved over the last 50 years due to the high thermal conductivity of 165 W m−1 K−1 even though the power factor is 47 × 10−4 W m−1 K−2 In samples with random grain orientations, we found that the thermal conductivity can be decreased by making grain size smaller through ball milling and hot pressing, but the power factor decreased with a similar percentage, resulting in no gain in ZT Reorienting the ab planes of the small crystals by repressing the as-pressed samples enhanced the peak ZT from 085 to 104 at about 125 °C, a 22% improvement, mainly due to the more increase on power factor than on thermal conductivity Further improvement is expected when the ab plane of most of the small crystals is reoriented to the direction perpendicular to the press direction and grains are made even smaller

https://www.bc.edu/content/dam/files/schools/cas_sites/physics/pdf/Ren/225%20Nano%20Lett%2010%203373-3378%202010.pdf

Experimental Studies on Anisotropic Thermoelectric Properties and Structures of n-Type Bi2Te2.7Se0.3

Emerging electrosynthetic techniques such as electrogeneration of base, anodic oxidation, and ac (alternating current) synthesis provide simple and inexpensive alternative routes to the synthesis of ceramic thin films and coatings, nanoparticulate materials, and metastable phases. In this review, we survey illustrative examples to highlight the potential application of these techniques in meeting the goals of inorganic solid-state synthesis.

Electrochemical Synthesis of Metal Oxides and Hydroxides

The growth and properties of electronic oxide thin films are reviewed. In particular, the synthesis and properties of superconducting, insulating, conducting, magnetic, and semiconducting epitaxial oxide structures are discussed. Crystal structures and functional properties common to many oxide materials are briefly reviewed. A description of film-growth techniques follows. Finally, an extensive overview of the epitaxial growth for specific oxide material systems is given. This includes the epitaxial growth of electronic oxide thin films on oxide and non-oxide substrates.

Synthesis and properties of epitaxial electronic oxide thin-film materials

Deformation-textured, non-silver substrates, and solution-based deposition of buffer and superconductor layers offer routes to a low-cost YBCO coated-conductor technology for high-temperature superconducting wire Several significant steps towards such a technology are reported here: a solution-based Gd2O3 seed buffer layer was deposited by a web-coating technique over a metre-length tape of deformation-textured nickel with excellent texture and uniformity Also, short full-stack samples with YBCO performance up to 08 MA cm-2 at 77 K were prepared at Oak Ridge National Laboratory (ORNL) and American Superconductor (ASC) using a CeO2/YSZ/CeO2 buffer sequence on textured nickel and a trifluoroacetate (TFA) precursor YBCO process; in this case the buffers are deposited by e-beam and magnetron sputtering

https://iopscience.iop.org/article/10.1088/0953-2048/13/5/308/pdf

Low-cost YBCO coated conductor technology

Crucial to the successful development of a theoretical model for high-temperature superconductivity is knowledge of the symmetry of the order parameter (or wavefunction) that describes the pairing of electrons in the superconducting state. Several experimental studies1–8provide convincing evidence for an anisotropic order parameter, consistent with a symmetry. But none of these earlier experiments could rule out unambiguously an additional contribution from isotropic s-wave pairing; these experiments either involved superconductors with an orthorhombic crystal structure (for which a mixed s + d state is becoming increasingly recognized as a likely consequence9,10), or their interpretation required detailed modelling of the uncertain effects of disorder and defects. Here we report the results of an experiment designed to circumvent these difficulties: the material studied is the single-layer tetragonal superconductor Tl2Ba2CuO6+δ, and the experimental configuration is such that the interpretation of the results relies solely on symmetry considerations. Our results indicate that this material has pure pairing symmetry, so providing a starting point for understanding the more complex mixed s + d state that appears to characterize other high-temperature superconductors.

Pure d x 2 - y 2 order-parameter symmetry in the tetragonal superconductor TI 2 Ba 2 CuO 6+δ

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.

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

Intermediate thermal annealing and static hydraulic pressing were applied to process in-situ synthesized silver-sheathed superconducting Tl 078 Bi 022 Sr 16 Ba 04 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 55 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 55 T than at zero field

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

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.

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

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.

J.H. Wang

Papers

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

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

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

Superconducting thallium oxide films via an electro-deposited precursor

Pairing symmetry from in-plane torque anisotropy in Tl 2 Ba 2 CuO 6+δ thin films