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

The properties of the Y1−xPrxBa2Cu3O7 (YPrBCO) system are reviewed. These include superconducting, normal state, structural, chemical, optical, magnetic, and thermal properties. The destruction of superconductivity with Pr doping is discussed in view of possible models such as hole filling, localization, magnetic pair-breaking, and the role of hybridization. Applications to electronic devices using YBCO/PrBCO/YBCO multilayers are also reviewed.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400017489

A review of the superconducting and normal state properties of Y1−xPrxBa2Cu3O7

Sol—Gel preparation of supported metal catalysts

Raman and IR-measurements on the high- T c material series La 2 CuO 4 and YBa 2 Cu 3 O 7 are reviewed. This summary focuses on lattice vibrations in these superconducting systems, on impurity phases, the influence of oxygen stoichiometry and element substitution. The determination of the superconducting gap, the phonon renormalization in the superconducting state, and the isotope effect with its implications on the theory of superconductivity in these materials are discussed. In addition, the measured lattice vibrations are compared with the results of lattice dynamics calculations.

Lattice vibrations in high-Tc superconductors: Optical spectroscopy and lattice dynamics☆

The density and the structure (fractal and non-fractal) of aerogels are modified either by the adjustment of the gelifying concentration, by a precise control of the viscous flow sintering process or by an isostatic pressure deformation. These aerogels have porosities ranging from 98% to 0%. The mechanical properties of the different aerogels (elastic modulus and strength) measured by 3 point bending, are dependent on their structure; they vary by five orders of magnitude as a function of density and follow power law evolution. However for the same relative density the elastic modulus and strength can increase by one order of magnitude due to a change in connectivity. These structural differences have been observed by SAXS experiments. The effects of the sintering process compared to that of the plastic transformation on the mechanical properties are explained by the associated structural changes. Sintering increases the network connectivity and the densification by compression leads to a new spatial arrangement of the clusters but their internal structure is not affected. In addition, relationships between structural and porous features and the mechanical properties are discussed in terms of percolation theory and the fractal approach. We show that the exponent of the power law does not depend on the fractal feature and percolation is only an approximation which cannot describe results.

Different kinds of structure in aerogels: relationships with the mechanical properties

An extensive study of magnetic, thermal, transport, and structural properties of the alloy Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}} is presented. The endpoints of the alloy series are a high-temperature superconductor Y-Ba-Cu-O and an antiferromagnetic semiconductor (Pr-Ba-Cu-O, {ital T}{sub {ital N}}=17 K). The superconducting transition temperature is reduced with increasing Pr concentration following the Abrikosov-Gorkov pair-breaking curve with critical concentration {ital x}{sub cr}=0.62. Alloying also reduces the Neel temperature approximately linearly with the Y content, and there is a concentration region (0.4{lt}{ital x}{lt}0.6) where antiferromagnetism and superconductivity is suggested to coexist. The specific heat, the susceptibility, the magnetic ordering, and the metal-to-semiconductor transition seen in the resistivity are consistent with the picture that Y{sup 3+} is replaced by Pr{sup 4+} with some degree of valence admixture of the Pr{sup 3+} configuration.

Magnetic ordering and superconductivity in Y1-xPrxBa2Cu3O7-y.

Depression of superconductivity, heavy fermion behavior and valence fluctuations in Y1−xPrxBa2Cu3O7

High-quality bulk REBa2Cu3O7−x (RE = Y,Eu,Gd,Nd,La) was synthesized by a solution method. Stoichiometric amounts of yttrium, barium, and copper acetates were dissolved in glacial acetic acid. The acid was boiled away leaving a glassy acetate precursor. This precursor has shown amorphous scattering in XRD, a phenomenon consistent with atomic level mixing of reactants. The glassy precursor was subsequently heat-treated with a 5 °C/min ramp to 900 °C and a 2 h soak at 900 °C in air. The final product was obtained after heat treatment under oxygen at 550 °C with slow cooling to room temperature. Final products were analyzed by XRD, SEM, and four probe de-resistivity measurements. The mechanism of both precursor and product formation was examined through substitution studies and XRD. It was found that a combination of a rare earth acetate, barium acetate, and acetic acid was necessary for the formation of an amorphous precursor.

Simplified preparation of REBa2Cu3O7−x via the acetate method

The temperature and Zn concentration dependence of the electrical resistivity, specific heat, magnetic susceptibility, and electron paramagnetic resonance (EPR) spectra of YBa2(Cu1−x
Zn
 x
 )3O7−y
 withy∼0.1 has been measured forx≤0.16. In addition, the temperature and field dependence of the magnetization has been measured for 2<T<300K and 0<H<9.0T, along with the temperature and quasihydrostatic pressure dependence of the electrical resistivity for selected samples for 0<P<13 GPa. The substitution of Zn for Cu in YBa2Cu3O7−y
 causes a rapid and nearly linear depression of the superconducting transition temperature,T

 c
 , withT

 c
 going to 0 K forx≥ 0.10. YBa2(Cu1−x
Zn
 x
 )3O7−y
 retains the YBa2Cu3O7-y
 orthorhombic structure forx≤0.16 for both the superconducting and nonsuperconducting samples. Initially, the unit cell volume increases nearly linearly with Zn content; however, an abrupt change occurs in the vicinityx=0.8–0.10. Forx 0) andρ increases gradually with increasing Zn content. However, forx≥ 0.10,ρ(T) becomes semiconductor-like, with a very rapid increase of the resistivity with increasingx. The electrical resistivity, magnetic susceptibility, EPR spectra, and specific heat all indicate that thed-holes associated with the Cu ions become localized in the nonsuperconducting phase,x>-0.10.

d-Hole localization and the suppression of superconductivity in YBa2(Cu1-xZnx)3O7−y

The conductivity of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}y}$ in the normal state has been obtained from a measurement of the reflectivity over a wide frequency range. The product of the Drude model parameters ${\ensuremath{\omega}}_{p}\ensuremath{\tau}l1$ indicates that the plasma oscillations are overdamped. The free-carrier electrodynamics are well described by a frequency-dependent scattering rate which rises linearly in frequency from the dc value and saturates at a value consistent with scattering at the unitarity limit with a concomitant frequency-dependent mass enhancement.

/pdf/free-carrier-relaxation-dynamics-in-the-normal-state-of-4tp17e2y7s.pdf

G.H. Myer

Papers

Magnetic ordering and superconductivity in Y1-xPrxBa2Cu3O7-y.

Depression of superconductivity, heavy fermion behavior and valence fluctuations in Y1−xPrxBa2Cu3O7

Simplified preparation of REBa2Cu3O7−x via the acetate method

d-Hole localization and the suppression of superconductivity in YBa2(Cu1-xZnx)3O7−y

Free-carrier relaxation dynamics in the normal state of sintered YBa2Cu3O7-y.