This review article focuses on the sol-gel preparation of high temperature superconducting oxides wherein different classes of gel technologies were utilized. These involve: 1) the sol-gel route based upon hydrolysis-condensation of metal-alkoxides, 2) the gelation route based upon concentration of aqueous solutions involving metal-chelates, often called as “chelate gel” or “amorphous chelate” route, and 3) the organic polymeric gel route. This paper reviews the current status of these sol-gel processes, and illustrates the underlying chemistry involved in each sol-gel technology. It is demonstrated that the chemical homogeneity of the gel is often disturbed by the differences in the chemistries of the cations. Prior to gelation the starting precursor solution containing various forms of metal-complexes must be chemically modified to overcome this problem. Illustration of a variety of strategies for success in obtaining a homogeneous multicomponent gel with no precipitation is focal point of this review article.

Invited review “sol-gel” preparation of high temperature superconducting oxides

We have previously shown that Tc for Bi2 (Sr, Ca)n+1CunO2n+4+δ (n=1, 2 and 3) varies with oxygen stoichiometry δ determined by annealing in a variety of oxygen partial pressures and temperatures. Annealing results are now also presented for several Tl-superconductors in the series Tlm(Ba, Sr)2Can−1CunO2n+m+2+δ, for both m=1 and 2, revealing two general results for this class of Bi or TI superconductors: (1) effects on Tc due to labile oxygen occur for all m=2 compounds but are absent in m = 1 compounds with Sr. m=1 compounds with Ba do have variable oxygen but the effects on the c-axis are the opposite to m=2 compounds. This suggests for the latter that the labile oxygen resides in interstitial sites within the Bi2O2 or Tl2O2 bi layers; (2) the hole concentration per Cu decreases in progressing from n = 1 to 2 to 3 in either class so that the parent n = 3 and n = 2 compounds lie respectively on the low- and high-hole concentration sides of the peak in Tc while the n = 1 compounds extend out into the non-superconducting domain at very high hole concentration.

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General trends in oxygen stoichiometry effects on Tc in Bi and Tl superconductors

Recent advances in aerosol generation of materials are reviewed Gas-to-particle and spray processes (spray pyrolysis) for powder generation and various routes for film generation are discussed from the experimental and theoretical perspectives The range of materials generated by these routes has increased in recent years to include fullerenes and ceramic superconductors Many metals and various oxide and nonoxide ceramics have also been added to the list of materials generated by gas-phase routes Established aerosol routes such as vapor condensation have found widespread applications for generation of nanophase materials The formation of quantum dots via aerosol approaches has also been demonstrated The theoretical understanding of gas-to-particle conversion routes has advanced to include the finite rate of particle fusion or sintering occurring after collisions of particles The modeling of spray pyrolysis systems has provided insight into the control of particle morphology and reactor design In th

https://www.tandfonline.com/doi/pdf/10.1080/02786829308959650

Aerosol Processing of Materials

A comprehensive review of structure work on high-Tc oxides as reported during the years 1987 and 1988 is given. Thirteen structures are refined from X-ray single-crystal and/or neutron powder diffraction data:I. (Ba1−x,Kx)BiO3 (Tc=30 K),II. (La2−x, Srx)CuO4 (Tc=40 K),III. (Nd, Ce, Sr)2CuO4 (Tc=28 K),IV. (Nd2−x, Cex)CuO4 (Tc=24 K),V. YBa2Cu3O7 (Tc=90 K),VI. YBa2Cu4O8 (Tc=80 K),VII. Y2Ba4Cu7O14 (Tc=40 K),VIII. Pb2Sr2NdCu3O8 (Tc=70 K),IX. TlBa2CaCu2O7 (Tc=103 K),X. TlBa2Ca2Cu3O9 (Tc=120 K),XI. Tl2Ba2CuO6 (Tc=90 K),XII. Tl2Ba2CaCu2O8 (Tc=112 K),XIII. Tl2Ba2Ca2Cu3O10 (Tc=125 K). Except forI (perovskite type),II (K2NiF4 type) andIV (Nd2CuO4 type) they represent new structure types. Structure data, bond distances, structure drawings and calculated X-ray powder diffraction patterns are given for each compound. Structural features and correlations with superconductivity are discussed. The review contains 301 citations.

Crystal structures of high- T c oxides

We have prepared superconducting thin films of Y‐Ba‐Cu‐O with Tc onsets above 95 K by both diode and magnetron sputtering. Films with full superconductivity (R=0) at 85 K have been produced by dc magnetron sputtering. The compositions of the films are fairly uniform across an area 50 mm in diameter and through the film thickness. Structural properties of the films were studied by x‐ray diffraction. Critical current densities in the range of 3000 to 104 A/cm2 have been measured at 4.2 K.We have prepared superconducting thin films of Y‐Ba‐Cu‐O with Tc onsets above 95 K by both diode and magnetron sputtering. Films with full superconductivity (R=0) at 85 K have been produced by dc magnetron sputtering. The compositions of the films are fairly uniform across an area 50 mm in diameter and through the film thickness. Structural properties of the films were studied by x‐ray diffraction. Critical current densities in the range of 3000 to 104 A/cm2 have been measured at 4.2 K.

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Superconducting Y-Ba-Cu-O oxide fi lms by sputt ering

The crystal structure and superconducting properties of the Bi‐Pb‐Sr‐Ca‐Cu‐O system with Ca‐ and Cu‐rich nominal composition were investigated. A nearly single‐phased 110 K high Tc superconductor can be obtained with 852 °C/20 h sintering from the starting composition of Bi1.7Pb0.4Sr1.6Ca2.4Cu3.6Oy. X‐ray diffraction patterns, resistivity measurement, diamagnetic susceptibility results, and scanning electron micrographs all indicate that the Ca‐ and Cu‐rich nominal composition would result in better superconducting properties than those of Ca:Sr=1:1 Bi‐Pb‐Sr‐Ca‐Cu‐O compounds in a much shorter sintering time.

Accelerated formation of 110 K high Tc phase in the Ca‐ and Cu‐rich Bi‐Pb‐Sr‐Ca‐Cu‐O system

A family of high- T c superconductors with T c(zero) above 105 K is described. The compounds are seven-element blends with the general formula Tl 0.5 Pb 0.5 Ca 0.8 A 0.2 Sr 2 Cu 2 O x ( A = Y ), and rare-earth elements La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb). The results represent one of the first cases where chemical substitution has significantly raised T c in a known compound. The septenary compounds are tetragonal in structure with almost identical lattice parameters to those of now-famous Y 1 Ba 2 Cu 3 O y structure. The samples were highly reproducible, stable and remarkably homogeneous both in composition and structure. The preparative conditions were found to be much less stringent than those of other copper-based high- T c superconductors.

Superconductivity with Tc(zero) above 105 K in Tl-containing septenary oxides with Y1Ba2Cu3Oy-like structure

A preferentially oriented bulk YBa2Cu3Ox superconductor was prepared by the liquid phase method. A continuous dc current carrying capacity exceeding 120 A with current density higher than 37 300 A/cm2 at 77 K has been obtained. The high current effects causing fracture of the sample were studied in relation to the thermal diffusion properties. In magnetization measurements, flux jumps were observed for currents in the sample a‐b plane below the temperature of about 20 K. The jumps are more closely spaced with decreasing temperature or increasing field sweep rates.

High transport critical currents and flux jumps in bulk YBa2Cu3Ox superconductors

High-Tc superconducting Y-Ba-Cu-O, Bi-Ca-Sr-Cu-O, and Tl-Ca-Ba-Cu-O materials were synthesized using a modified citrate precursor process. Ethylenediamine was used to neutralize the metal citrate solution and facilitate the complex formation. Highly homogeneous and viscous gel without any precipitation was obtained. In the Tl-Ca-Ba-Cu-O system, the Ca-Ba-Cu-O precursor was first prepared by the sol-gel method, then mixed with thallium oxide for final sintering. The complex formation was monitored by UV/VIS spectrophotometry. The thermal decomposition of gels and the formation of superconducting phases had been studied using IR, DTA, TGA, and XRD. This process is highly reproducible and leads to powders with excellent homogeneity and small particle size for easy sinterability.

Synthesis and Characterization of High-Tc Superconducting Oxides by the Modified Citrate Gel Process

A Bi-Ca-Sr-Cu-O epitaxy layer has been successfully synthesized by the liquid-phase epitaxial (LPE) process on the MgO(001) substrate. X-ray diffraction patterns and SEM surface morphology analysis indicated a highly preferred orientation. This film showed superconductivity with onset and zero resistivity temperature of 110 K and 81 K, respectively.

P.T. Wu

Papers

Accelerated formation of 110 K high Tc phase in the Ca‐ and Cu‐rich Bi‐Pb‐Sr‐Ca‐Cu‐O system

Superconductivity with Tc(zero) above 105 K in Tl-containing septenary oxides with Y1Ba2Cu3Oy-like structure

High transport critical currents and flux jumps in bulk YBa2Cu3Ox superconductors

Synthesis and Characterization of High-Tc Superconducting Oxides by the Modified Citrate Gel Process

Epitaxial Growth of High-Tc Bi-Ca-Sr-Cu-O Superconducting Layer by LPE Process