The inherently weak mechanical properties associated with monolithic high-temperature superconductors (HTS) can be improved by introducing properly selected strong ceramic whiskers into the HTS materials. In this research, processing and superconducting properties of monolithic Pb-doped Bi-2223 (BPSCCO) and MgO whisker-reinforced BPSCCO HTS composite materials have been systematically studied. A solid-state processing method is successfully developed to fabricate the (MgO) w ͞BPSCCO composite. The HTS composite contains a dense and highly pure BPSCCO matrix phase with a preferred grain orientation, which is reinforced by MgO whiskers randomly oriented in the plane perpendicular to the hot-pressing direction. The HTS composite material is shown to exhibit excellent superconducting properties. For example, a transport J c measured at 77 K in a zero field has been obtained to exceed 5000 A͞cm 2 in a (MgO) w ͞BPSCCO composite with 10% MgO whiskers by volume. Relationships among solid-state processing variables, HTS phase development, and superconducting properties of the monolithic BPSCCO and the HTS composite are established in the paper.8

The inherently weak mechanical properties of bulk monolithic high-temperature superconductors (HTS) have been a concern. Properly selected reinforcements in fiber and whisker forms have been introduced to the HTS ceramics to improve their mechanical properties. In this paper, mechanical behavior of a MgO-whisker reinforced Pb-doped Bi-2223 (BPSCCO) HTS composite fabricated by a solid-state processing method is studied. The (MgO)w/BPSCCO HTS composite has been shown to exhibit excellent superconducting properties. Elastic properties, strengths, and notched fracture toughnesses of both the monolithic BPSCCO and the (MgO)w/BPSCCO HTS composite are investigated. Detailed mechanical properties are reported for the first time for the (MgO)w/BPSCCO HTS composite. Mechanisms of strengthening and toughening in the MgO-whisker-reinforced HTS composite are also discussed.

In associated papers [Y. S. Yuan, M. S. Wong, and S. S. Wang, J. Mater. Res. 11, 8–17 (1996); J. Mater. Res. (1996, in press)] it has been shown that weak thermo-mechanical properties of a bulk monolithic high-Tc superconductor (HTS) can be improved by introducing strong ceramic whiskers into the HTS ceramic materials. In this paper, we report a further study of incorporating Ag particulates, (Ag)p, in a bulk monolithic BPSCCO and in the MgO-whisker reinforced BPSCCO composite. Effects of the (Ag)p addition on processing, microstructure, and superconducting and mechanical properties of the bulk monolithic BPSCCO and the (MgO)w/BPSCCO composite are investigated. The results indicate that the highly ductile Ag particulates promote densification of the BPSCCO matrix phase in the composite during hot pressing. The microstructure of the HTS composite with the (Ag)p addition is similar to that in the HTS material without the (Ag)p. The (MgO)w/BPSCCO composite with 10% (by weight) Ag particulates has been shown to possess excellent superconducting properties. The (Ag)p addition to both the monolithic BPSCCO and the (MgO)w/BPSCCO is found to increase appreciably their fracture toughnesses, but has little effects on mechanical strengths of the materials. Quantitative relationships have been established among solid-state processing variables, HTS phase developments, microstructures, and superconducting and mechanical properties of the (Ag)p/BPSCCO and the (MgO)w/(Ag)p/BPSCCO HTS composites.

Fracture behavior of a chopped-mat E-glass fiber reinforced hybrid resin composite knitted with continuous poly(ethylene terephthalate) (PET) fibers is investigated by a combined experimental and analytical study. In both opening-mode (mode-I) and shear-mode (mode-II) fracture studies of the composite, the macroscopic critical stress intensities or toughnesses of the material are found to be distinct along the warp and fill directions of the knitting PET fibers. Values of I4(xi~ ) and K(n w) are lower than those of K~f ) and K~f v), owing to different fracture mechanisms involved. In the mixed-mode fracture of the composite, a failure envelope in I~] and tfn is constructed. The different mechanisms involved in opening-mode, mode-II and mixed-mode fracture are studied with SEM observations.

A combined experimental and micromechanics investigation is conducted on elevated-temperature thermal expansion of PTFE/PEEK polymer-matrix composite reinforced with randomly oriented short carbon fibers (CF) and graphite flakes (Gr). In the experimental phase of the study, PTFE/PEEK polymer blends with different amounts of PTFE and four-phase CF/Gr/PTFE/PEEK composites with different volume fractions of graphite flakes were made from compression molding. Scanning electron microscopy was performed to evaluate the microstructure of the PTFE/PEEK matrix and the composite, especially the interface, and the size and dispersion of the particles. X-ray diffraction (XRD) was conducted to provide morphological information on the semi-crystalline PTFE/PEEK matrix of the composite. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were carried out to determine transition temperatures and thermomechanical properties of the composite and its constituent phases at the elevated temperature. Thermal expansions of neat PTFE and neat PEEK, the PTFE/PEEK polymer matrix, and the CF/Gr/PTFE/PEEK composite were obtained with a thermal–mechanical analyzer (TMA) in a dilatometric mode. Coefficients of thermal expansion (CTEs) of the PTFE/PEEK matrix and its CF/Gr/PTFE/PEEK composite were then determined from 25 °C up to an elevated temperature 240 °C. To augment the experimental study, micromechanics analyses are also conducted to determine thermal expansion coefficients of the PTFE/PEEK matrix and the CF/GR/PTFE/PEEK composite. The micromechanics solutions elucidate individual roles of different composite constituents, contributions of individual constituent materials’ temperature-dependent thermal and mechanical properties, the importance of composite microstructure and morphology, and the issue of thermal–mechanical coupling on the thermal expansion behavior of the complex CF/Gr/PTFE/PEEK composite at high temperature.