The modeling of the electrical conductivity of polymer composites reinforced with conductive fibers is investigated. Existing models generally can be divided into percolation theories and non-percolation theories. The basis of the percolation theory is the fact that the conductivity of the composite increases dramatically at a certain fiber concentration called the percolation threshold. This theory can be used to model the behavior of the composite or to predict the percolation threshold itself. Non-percolation theories include terms, which account for microstructural data such as fiber orientation, length, and packing arrangement. A comparison of experimental data with predictions from the various models reveals that only the percolation theory is able to accurately model the conductive behavior of an actual composite. Two alternative new models, which predict the volume resistivity of a composite using microstructural data, are evaluated. The first model relates resistivity to the concentration and orientation of the fibers, while assuming perfect fiber-fiber contact. The relationship between resistivity and fiber concentration predicted by the model is in qualitative agreement with actual data, and predictions of the anisotropy in volume resistivity compare well with experimental results. The second model accounts for the effect of fiber-fiber contact and fiber length on composite resistivity. Predictions are in excellent agreement with experimental data for polypropylene composites reinforced with nickel-coated graphite fibers.

Estimation of the volume resistivity of electrically conductive composites

Raman spectroscopy was employed to evaluate the crystal structure and phonon modes of chemically substituted Ba(RE1/2Nb1/2)O3 microwave ceramics (RE = La, Nd, Sm, Gd, Tb, and Y). It was verified that these materials could be divided into tetragonal (ceramics with RE = Y, Tb, and Gd) and orthorhombic (RE = Sm, Nd, and La) structures. Lorentzian lines were used to fit the spectra, which presented 9 bands for the first group and 23 bands for the second group of ceramics. The position and width of the phonon modes were determined, and were correlated to the ionic radii and tolerance factors for the different atoms substituted in the B‘-site. It is believed that simple rotational distortions of the oxygen octahedra led to the occurrence of structures other than cubic, which is very difficult to detect by X-ray diffraction or even spectroscopic techniques.

Chemical Substitution in Ba(RE1/2Nb1/2)O3 (RE = La, Nd, Sm, Gd, Tb, and Y) Microwave Ceramics and Its Influence on the Crystal Structure and Phonon Modes

IR-spectroscopic characterization of A2BB′O6 perovskites

High-temperature superconducting (HTS) thick films have undergone a period of rapid progress since their discovery in 1986. In this review thick-film materials are described with special reference to their processing, their properties and the development of devices. The devices include HTS magnetic shields, resonators for microwave filters, antennas and wire. The translation of science into applications has taken place in a relatively short time and already impressive performance is seen in HTS prototype devices.

https://iopscience.iop.org/article/10.1088/0953-2048/10/4/001/pdf

High-temperature superconducting thick films

An article with an improved buffer layer architecture includes a substrate having a textured metal surface, and an electrically conductive lanthanum metal oxide epitaxial buffer layer on the surface of the substrate. The article can also include an epitaxial superconducting layer deposited on the epitaxial buffer layer. An epitaxial capping layer can be placed between the epitaxial buffer layer and the superconducting layer. A method for preparing an epitaxial article includes providing a substrate with a metal surface and depositing on the metal surface a lanthanum metal oxide epitaxial buffer layer. The method can further include depositing a superconducting layer on the epitaxial buffer layer, and depositing an epitaxial capping layer between the epitaxial buffer layer and the superconducting layer.

Method of depositing an electrically conductive oxide buffer layer on a textured substrate and articles formed therefrom

Percolation behavior of the normal state resistivity and superconductivity of YBa2Cu3O7−δ‐YBa2HfO5.5 composites has been studied by x‐ray diffraction and temperature‐ resistivity measurements. No detectable chemical reactivity was observed between YBa2Cu3O7−δ and a ceramic insulator YBa2HfO5.5 even when the two materials were mixed thoroughly and sintered at 1020 °C. Normal state and superconducting percolation threshold values are ≊23 vol % of YBa2Cu3O7−δ in the composite. The values obtained for the critical exponents describing the normal state transport behavior of the system matched with theoretically expected values for an ideal conductor‐insulator percolation system. The implications are discussed.

Electrical transport and superconductivity in YBa2Cu3O7−δ ‐YBa2HfO5.5 percolation system

YBa2HfO5.5: a new perovskite potentially suitable as a substrate for YBCO films

The processing and development of SmBa 2 NbO 6 as a non-reacting substrate for YBa 2 Cu 3 O 7−δ films have been reported for the first time. The dielectric constant and loss factor of this material are in a range suitable for its use as a substrate for microwave applications. A YBa 2 Cu 3 O 7−δ superconducting thick film screen-printed on SmBa 2 NbO 6 substrate gave a T c (0) =92 K and current density 2 × 10 5 A/cm 2 . This may be highest T c(0) reported in a YBa 2 Cu 3 O 7−δ thick film screen-printed directly on any substrate.

Superconducting YBa2Cu3O7−δ thick film (Tc(0) = 92 K) on SmBa2NbO6: a newly developed perovskite ceramic substrate

Development and characterization of GdBa2NbO6, a new ceramic substrate for YBCO thick films

A novel ceramic substrate useful for the preparation of superconducting films, said substrate having the formula REBa 2 MO 6 where RE represents rare earth metals--Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and M represents metals Nb, Sb, Sn, Hf, Zr; and a process for the preparation of superconducting YBa 2 Cu 3 O 7- δ thick films on new ceramic substrate.

J.K. Thomas

Papers

Electrical transport and superconductivity in YBa2Cu3O7−δ ‐YBa2HfO5.5 percolation system

YBa2HfO5.5: a new perovskite potentially suitable as a substrate for YBCO films

Superconducting YBa2Cu3O7−δ thick film (Tc(0) = 92 K) on SmBa2NbO6: a newly developed perovskite ceramic substrate

Development and characterization of GdBa2NbO6, a new ceramic substrate for YBCO thick films

Ceramic substrate useful for the preparation of superconducting films and a process for preparing the films