In this paper an assessment of the processing and properties of interpenetrating phase composites is presented. Such materials, defined as multiphase composites in which each phase is topologically interconnected throughout the microstructure, were the subject of a DOE Basic Energy Sciences sponsored workshop on research needs and opportunities held in July 1989 in Snowmass, Colorado. In place of a report representing the proceedings of the workshop, this paper provides a review of the field in its present, nascent state and identifies some of the critical scientific and technological issues posed by development of this class of material. The emphasis is on processing approaches and properties of interpenetrating phase composites. A list of the participants contributing to the workshop is given.

Interpenetrating Phase Composites

An analysis is presented that relates the crack spacing in a brittle film on an elastic substrate to the stress in the film, its thickness and fracture toughness. This analysis differs from an earlier one presented by one of the authors in that it considers the effect of a sequential, rather than a concerted, propagation of cracks, and predicts a larger crack spacing. The validity of the present analysis was confirmed by experimental results from a model system consisting of epitaxial PrBa2Cu3O7−x films on SrTiO3 substrates. The experimentally observed relationship between the crack spacing and the film thickness was in excellent agreement with the theory.

Cracking of brittle films on elastic substrates

A number of the important issues in the processing of dense, superconducting ceramics are addressed and illustrated with examples drawn from our studies of the processing of yttrium barium cuprate. Among the phenomena affecting the attainment of dense, fully superconducting materials are those common to the processing of other ceramics, namely, control of composition, prevention of microcracking, adequate time for the decomposition, and escape of intermediaries prior to surface densification (the so-called “binder burn-off” problem). Additional requirements include the necessity to ensure complete oxygenation throughout the microstructure for full volume superconductivity and the control of texture development for maximizing the attainable transport critical current density. On the basis of the processing issues described, it is concluded that the optimum microstructure is one having finescale, interconnected porosity (to ensure escape of carbonate-like species and subsequent, rapid oxygenation) with a fine grain size (∼1 μm) to avoid microcracking.

Issues in the Processing of Cuprate Ceramic Superconductors

By imaging the as-grown surfaces of sputtered and laserablated YBa2Cu3O7−δ films with scanning tunneling microscopy (STM), we have directly observed spiral-shaped growth terraces which emanate from screw dislocations. The density of screw dislocations was observed to decrease with increasing growth temperature and substrate misorientation. The surface structures observed by STM together with cross-sectional transmission electron microscope (TEM) images provide insights into the mechanisms of crystal growth operative during the formation of YBa2Cu3O7−δ films grown using these two widespread techniques.

Screw dislocation mediated growth of sputtered and laser-ablated YBa2Cu3O7−δ films

The stability of YBa2Cu3O7−x in water and 100% humidity has been investigated at three temperatures using pH measurements, x‐ray diffraction, and scanning electron microscopy. The oxide‐ceramic superconductor is highly unstable and reacts rapidly with water and also degrades in moisture. Dissolution of the oxide perovskite in water is highly incongruent. The corrosion products are found to be BaCO3, CuO, O2, etc. Barium hydroxide is first formed which further reacts with atmospheric CO2 to form needle‐shaped crystals of BaCO3. For any practical applications, devices made from these materials would have to be protected with an impermeable coating to prevent deterioration from atmosphere.

Chemical durability of high‐temperature superconductor YBa2Cu3O7−x in aqueous environments

We report fracture toughness measurements on single crystals of YBa2Cu3Ox, the phase responsible for superconductivity above liquid‐nitrogen temperatures. Indentation crack length measurements on the (010) orthorhombic crystal growth faces revealed the (100) and (001) planes as preferred fracture planes. The toughness of these planes is Kc=1.1±0.3 MPa m1/2, and the hardness H=8.7±2.4 GPa. The observed growth of both radial and lateral cracks in ambient air suggests that these crystals are susceptible to moisture‐enhanced nonequilibrium crack propagation.

Timothy R. Dinger

Papers

Fracture toughness measurements of YBa2Cu3Ox single crystals