We review the synthesis methods and properties of single crystal, powder, and thin film TlBaCaCuO high-temperature superconducting (Tl-HTS) materials. With transition temperatures ≥ 100 K for several compounds, Tl-HTS materials present real opportunities for applications above 77 K. Experiments using (1) single crystals: determined precise structural parameters and identified the complex Tl1+–Tl3+ equilibrium model; (2) powders: studied the complex thermodynamic phase diagram; and (3) epitaxial films: studied fundamental properties such as electron pair symmetry and the effect of controlled extrinsic defects on flux pinning strength, as well as providing the large-area surfaces required for device applications.

The subsolidus phase relationships of the high-Tc 2223 superconductor in the (Bi, Pb)–Sr–Ca–Cu–O (BSCCO) system have been examined at 810–820 °C. All experiments were carried out at ambient pressure in a 7.5% O2 (92.5% Ar) atmosphere. Eleven phases were found to exist in equilibrium with the 2223 phase. These 11 phases include CuO and 10 oxide solid solutions. From among these phases, a total of 48 five-phase combinations including the 2223 and 2212 phases were investigated experimentally, and 16 equilibrium assemblages were found which define a multicomponent compositional space corresponding to the 2223 + 2212 solid-state compatibility region. The subsolidus data form a partial basis for future investigation of the Pb-2223 primary phase field.

Nd1+xBa2−xCu3O6+d (Nd123) single crystals have been successfully grown by the top-seeded solution-growth method. Compositions of Nd123 could be controlled by applying two different methods: control of the oxygen partial pressure of the atmosphere and control of the liquid composition in air. The critical temperatures of Nd123 obtained by these two methods were 96 K (oxygen control) and 95 K (liquid composition control), respectively. The relationship between the peak effect in the Jc-H curve and heat treatment was investigated. The peak effect was found not to be an intrinsic property of Nd123; consequently it could be controlled by heat treatment.

Microstructure in melt-textured bulk RE1Ba2Cu3O6+d crystals (RE123; RE = Sm, Nd) was investigated, changing the initial composition from the tie-line composition of RE123–Sm2Ba1Cu1O5 (Sm211)/Nd4Ba2Cu2O10 (Nd422) to the Ba-enriched side. It was found that the Sm211/Nd422 particle size decreased in the liquid with increasing the Ba/Cu ratio of the initial composition, and this tendency was also found in the grown Sm123 crystals. Composition of the Sm123 grown crystal could be controlled by selecting the Ba-enriched initial composition to obtain an almost stoichiometric compound, which resulted in higher Tc values. Furthermore, the Jc values also increased under low magnetic fields due to the significant decrease of Sm211 particle size. Therefore, changing the initial composition toward the Ba-enriched side was found to be a new process to enhance both Jc and Tc values simultaneously.

A solidification model for Pr1+xBa2−xCu3O7−δ ternary oxides by the top seeded crystal pulling (SRL–CP: Solute Rich Liquid–Crystal Pulling) method is presented in which the composition of the grown single crystals is estimated from the starting composition in the crucible. This model involves the diffusion flux balance of each element at the growth interface in the liquid considering equilibrium tie-lines in the PrOy–BaO–CuO ternary phase diagram which have been obtained experimentally. The self-diffusion coefficient for Pr and the interdiffusivities for Ba and Cu in the liquid are used in this model because this liquid is a dilute solution for Pr. The calculated results are in good agreement with the experimental ones.

The quality and preparation of Sm–Ba–Cu–O (SmBCO) seeds, the level of Pt-doping, and the temperature distribution in the thermal processing system have been investigated in detail and identified as key processing factors in the fabrication of large melt processed Pt dope Y–Ba–Cu–O pseudocrystals. Optimization of these process variables and features has been performed to enable controlled processing of these materials up to several centimeters in diameter. Inhomogeneities and second phases present in the YBCO-Pt pseudocrystal microstructure have been analyzed and correlated with a variation of superconducting properties within the bulk of the sample. These observations have clear implications for the controlled processing of large YBCO-Pt pseudocrystals for electromagnetic applications.

Using classical molecular dynamics based on Lennard–Jones-like potentials, a mechanically stable YBa2Cu3O7 high Tc superconductor structure is generated. This process is controlled via interactive computer graphics. After doping atoms into or removing atoms from the sample using a recently implemented picking mechanism, the lattice oscillation energy is annihilated with a simulated annealing procedure. The remaining minimum ground state energy allows marking of the preferred doping location. Information on the doping mechanism is important because the magnetic and superconducting properties of these compounds depend very strongly on their oxygen content.

We report the results of measurements of resistivity up to 300 K and of thermoelectric power up to 400 K on ceramic samples of Nd1+xBa2−xCu3Oy with 0 ≤ x ≤ 0.65. The samples were fully oxygenated; they were characterized by x-ray diffraction and iodometric titration. The results are compared with data from literature reporting on effects of substitutions on the Ba site in 1–2–3 compounds and with our earlier experiments on substitution of Co for chain Cu. The focus is on the metal-nonmetal transition and on the nonmetallic regime in all these systems. Tc and the temperature dependence of S close to the metal-nonmetal transition and in the nonmetallic state seem to be determined by a single parameter, irrespective of the nature of the dopant. This result and its implications on the electronic structure of Rba2Cu3Oy (R = Y, or lanthanide ion) will be discussed.

Biaxially aligned YBa2Cu3O7−x (YBCO) films were fabricated on random Ni-based alloy tapes with yttrium stabilized-zirconia (YSZ) buffer layers deposited by ion-beam-assisted deposition (IBAD). Ar+ ion bombardment was found to have two significant effects on the crystalline structure of the YSZ buffer layers: to align a [100] axis with the substrate normal and a [111] axis with the bombarding beam axis. The resulting YSZ films were biaxially aligned on the random polycrystalline tapes, and the azimuthal distribution of the a- and b-axes of YBCO films on the top of the YSZ films was restricted to 10° FWHM. A critical current density (Jc) of 1.13 × 106 A/cm2 (77 K, 0 T) was obtained, and 1.1 × 105 A/cm2 was maintained at 5 T (77 K, B⊥c). The existence of both intrinsic and extrinsic pinning properties was clearly observed in the angular dependence of Jc with B⊥I. The longitudinal field effect on Jc was clearly observed, which indicated straight transport currents. This is evidence for strongly coupled current paths that demonstrate the bulk pinning properties of YBCO.

Much of the conductor development effort in the last decade has focused on optimizing the processing of (Bi, Pb)2Sr2Ca2Cu3Ox oxide-powder-in-tube conductors and (Bi, Pb)2Sr2CaCu2O8 (Bi-2212) and TlBa2Ca2Cu3Ox thick film conductors. It is demonstrated that in each of these conductors, critical current densities are dictated by the grain boundary misorientation distributions (GBMD's). Percolative networks of low-angle boundaries with fractions consistent with the active cross-sectional area of the conductor exist in each of these conductors. Further enhancements in the properties require increased numbers of small-angle grain boundaries. Given the processing methods used to fabricate these materials, no clear route employing a simple modification of the established processing method is apparent. To address this need, conductors with controlled or predetermined GBMD's are necessary. Development of biaxial texture appears to be the only possible way to increase the number of small-angle boundaries in a practical and controllable manner. We summarize in this paper recent results obtained on epitaxial superconducting films on rolling-assisted-biaxially-textured-substrates (RABiTS). This technique uses well established, industrially scalable, thermomechanical processes to impart a strong biaxial texture to a base metal. This is followed by vapor deposition of epitaxial buffer layers (metal and/or ceramic) to yield structurally and chemically compatible surfaces. Epitaxial YBa2Cu3O7–δ films grown using laser ablation on such substrates have critical current densities exceeding 106 A/cm2 at 77 K in zero field and have a field dependence similar to epitaxial films on single crystal ceramic substrates. Deposited conductors made using this technique offer a potential route for the fabrication of the next generation high temperature superconducting (HTS) wire capable of carrying high currents in high magnetic fields and at elevated temperatures.

To investigate potential limits to the rate at which high-quality YBa2Cu3O7–δ can be deposited, we have produced a series of 1 μm thick films by pulsed laser deposition on single-crystal SrTiO3 substrates at average rates ranging from 2 Å/s to 240 Å/s. The critical current density of low-rate films was over 2 MA/cm2 at 75 K, self field, but dropped linearly with rate to about 1 MA/cm2 at the upper end of the range. In addition, the superconducting transition temperature, resistivity above the transition, and performance in an applied magnetic field were all degraded by increasing the deposition rate. A change in c-axis lattice parameter suggests that possible causes for this degradation are oxygen deficiency or cation disorder with the latter being the more likely. Annealing high-rate films at 790 °C for as little as 20 min improved critical current density to within 20% of low-rate values, and resulted in dramatic improvements in other film properties as well.

For high temperature superconducting multichip modules and other related electronic applications, it is necessary to be able to fabricate several Y1Ba2Cu3O7–x (YBCO) layers separated by thick low dielectric constant dielectric layers. In this work, we report the successful fabrication of YBCO/YSZ/SiO2 (1–2 μm)/YSZ/YBCO multilayer structures on single crystal yttria stabilized zirconia (YSZ) substrates. In contrast to previously reported work, the top YBCO layer did not show any cracking. This is due to a technique that allows for stress relief in the SiO2 layer before the second YBCO layer is deposited. The top YBCO layer in our multilayer structure had Tc = 87 K and Jc = 105 A/cm2 (at 77 K), whereas the bottom YBCO layer had Tc = 90 K and Jc = 1.2 × 106 A/cm2 (at 77 K). We also showed that the quality of the bottom YBCO layer was preserved during the fabrication of the multilayer due to the annealing process during which O2 diffused into the YBCO, replacing the O2 lost during the deposition of the top YBCO layer.

Artificial barriers in Josephson junctions make it possible to change the height and width of the barrier independently. This technique can be realized in high-Tc Josephson junctions using the ramp technique. In this article the fabrication of ramp-type junctions is discussed and the importance of the morphology of the ramp is pointed out. Detailed investigations are described which address the surface roughness and the damage due to ion-beam structuring of ramps. It is shown that hard masks can significantly improve the ramp quality by reducing the ion impact angle. Furthermore, annealing of the so structured ramps leads to unit cell steps enforcing a step-flow growth mode.

Careful studies of the corrosion, redox, galvanic, and oxygen evolution/uptake reactions associated with YBa2Cu3O7–δ and related compounds have been completed. These studies have led to an understanding of the many factors that contribute to the poor material characteristics exhibited by these popular high-Tc phases. With knowledge of the structure-reactivity relationships, a powerful crystal engineering approach has been developed that is capable of producing cation substituted versions of YBa2Cu3O7–δ; the resulting compounds therefrom produced exhibit markedly improved processability, oxygen stability, and durability characteristics. These materials have been combined in thin film structures so as to make prototype SNS junctions and SQUID sensors which exhibit promising device performance characteristics.

A new class of complex perovskites REBa2ZrO5.5 (where RE = La, Ce, Eu, and Yb) have been synthesized and sintered as single phase materials by the solid state reaction method. The structure of these materials was studied by x-ray diffraction, and all of them were found to be isostructural, having a cubic perovskite structure. X-ray diffraction and resistivity measurements have shown that there is no detectable chemical reaction between YBa2Cu3O7–delta; and REBa2ZrO5.5 even under severe heat treatment at 950 °C, and that the addition of REBa2ZrO5.5 up to 20 vol.% in YBa2Cu3O7–δ shows no detrimental effect on the superconducting properties of YBa2Cu3O7-δ. Dielectric constants and loss factors are in the range suitable for their use as substrates for microwave applications. Thick films of YBa2Cu3O7–δ fabricated on polycrystalline REBa2ZrO5.5 substrates gave a zero resistance transition temperature Tc(0) ∼ 92 K, indicating the suitability of these materials as substrates for YBa2Cu3O7–δ.

A chemical approach to the formation of columnar defects involving the growth and incorporation of MgO nanorods into high temperature superconductors (HTS's) has been developed. MgO nanorods were incorporated into Bi2Sr2CaCu2Oz, Bi2Sr2Ca2Cu3Oz, and Tl2Ba2Ca2Cu3Oz superconductors at areal densities up to 2 × 1010/cm2. Microstructural analyses of the composites demonstrate that the MgO nanorods create a columnar defect structure in the HTS matrices, form a compositionally sharp interface with the matrix, and self-organize into orientations perpendicular and parallel to the copper oxide planes. Measurements of the critical current density demonstrate significant enhancements in the MgO nanorod/HTS composites at elevated temperatures and magnetic fields compared with reference samples.

The rate at which Ag-clad (Bi, Pb)2Sr2Ca2Cu3Ox tapes are cooled from their final reaction heat treatment influences both the intergranular connectivity and intragranular flux pinning strength of the polycrystalline filaments. As the cooling rate from 825 °C to 730 °C in 7.5% O2 was decreased over a range of 5 °C/min to 0.005 °C/min, Jc (77 K, 0 T) increased from ∼8 to ∼24 kA/cm2, and the irreversibility field increased from, ∼120 to, ∼200 mT. The Jc (4.2 K, 0 T) increased in a similar fashion. Cooling slowly also sharpened the critical temperature transition and increased the critical onset temperature from 107 K to 109 K. These improvements in the superconducting properties occurred despite partial decomposition of the (Bi, Pb)2Sr2Ca2Cu3Ox phase into non-superconducting impurity phases during the slow cooling. A microstructural basis for these multiple effects is described.

Two kinds of characteristic grain boundaries were observed in textured Bi2Sr2Can−1CunOx (n = 2 and 3) bulk material: one (P-type) is nearly parallel to the (001) plane, and the other (N-type) is approximately normal to the (001) plane. Low-angle tilt N-type boundaries are composed of arrays of dislocations. However, for a small c-axis misorientation, the regions between the dislocation cores are still well connected, providing “pathways” for supercurrents crossing the boundary plane. The P-type boundaries exhibit compositionally and structurally modulated faceting. Although we see local regions of the low Tc (2201) phase at low-angle tilt (<10°) P-type boundaries, there are also “pathways” crossing the boundary plane made up of the high Tc (2212) and (2223) phases. The characteristics of such low-angle tilt grain boundary structures can therefore be modeled to provide general insight into the correlation between high critical current densities and low-texture breadths. On the other hand, a weak link could be formed at high-angle (> 10°) boundaries where there are the low Tc (2201) or insulating phases.

This paper presents several key aspects of our approach to preparing artificially induced [001] tilt grain boundaries (GB's) with uniform, well-defined structures in YBa2Cu3Oy (YBCO) superconductors. GB structures formed in thin film and bulk bicrystals, respectively, will be compared. In YBCO thin film bicrystals, meandering rather than planar GB's are formed. Using a low film deposition rate has been demonstrated to reduce the magnitude of meander significantly, but complete elimination of the meander has not yet been accomplished. Thus, we have developed a dual-seeded-melt-texture process to produce uniform, planar GB's with controllable misorientation angles in YBCO bulk bicrystals. Transmission electron microscopy (TEM) studies reveal a remarkably planar and simple configuration on different length scales. Such a simple structure allows for an insightful interpretation of transport behavior across individual GB's.

Using a seeded melt growth (SMG) method, we have produced single-domain YBa2Cu3Ox with high levitation forces and trapped magnetic fields. A threshold temperature TL has been found above which extraneous nucleation does not occur. Surface nucleation has been suppressed when the top sample surface is coated with low melting compounds. The planar growth rates along the a- and c-axes have been found to be comparable within the undercooling range used in this study, and agree well with the current model. Major factors that strongly influence the levitation force have been studied in detail including domain geometry and orientation. Current physical models have been used to interpret the observed levitation force behaviors.