This article briefly reviews ceramic superconductors from historical and materials perspectives. It describes the factors that distinguish high-temperature cuprate superconductors from most electronic ceramics and places them in the context of other families of superconducting materials. Finally, it describes some of the scientific issues presently being actively pursued in the search for the mechanism for high-temperature superconductivity and the directions of research into new superconducting ceramics in recent years.
DURING the long history of electronic oxide ceramics before 1986, even the most astute observer would have found few clues predicting the revolution in materials science and physics that those materials would eventually unleash. Oxide ceramics are an important part of electronics technologies because of their electrically insulating, optical, and dielectric properties, but very rarely for their ability to conduct electrical current. Insulators such as steatite, dielectrics such as SiO2 and Ta2 O5 , piezoelectrics such as lead zirconate titanate (PZT), optical materials such as LiNbO3 ,and the transparent semiconductor indium tin oxide (ITO) can be considered as representative of the large class of metal oxides with real and potential applications in electronics technologies. Al-though such materials are sophisticated and complex when con-sidered from any point of view, their technological usefulness in electronics generally results (with the exception of ITO and a handful of other materials) from their ability to block or highly resist the passage of electrical current and, as an important consequence in some cases, their transparency at optical wave-lengths. These important characteristics of most oxides are related to a fundamental chemical difference between the metallic elements and oxygen: the difference in their electronegativities. These differences in electronegativity result in semiconducting or insu-lating materials, with forbidden energy gaps between the highest occupied orbitals in the solid derived from oxygen electronic states and the lowest unoccupied orbitals derived from the metal elec-tronic states. The technical applications of electronic ceramics depend very highly on the materials scientist's manipulations of these energies, the introduction or elimination of defects that alter the distribution of energy states and/or their occupancy by charge carriers, and other factors such as the polarizabilities of the component atoms. Not all metal oxides are transparent. Transition-metal oxides have been used since ancient times as pigments because of strong optical-frequency localized electronic absorptions between differ-ent excited states of the electrons in the transition-metal d orbitals, which lead to color. As electronic conductors, however, i.e., as competitors for copper wire, with few exceptions, they have never been of significant technological interest. Their principal consid-eration as electrical conductors has come primarily from their stability under high temperature, chemically extreme conditions where metals are not expected to be thermodynamically stable, as, for example, is found in electrodes for magneto-hydrodynamic generators. This whole picture was stood up side down in late 1986 with the discovery of superconductivity at surprisingly high temperatures in an electronically conducting oxide based on lanthanum, barium, copper, and oxygen. Superconductivity is the passage of electrical current with zero resistance below a certain critical temperature (Tc). The best superconductors known at the time, to be described below, were metallic compounds, not ceramics, as one might expect as a consequence of the (no longer obvious) general fact that metals are good electrical conductors and oxides are not. Good superconductors do not have to be good metallic conductors above their superconducting transition temperature, however, and the fundamental characteristics that result in superconductivity at high temperatures in the copper oxides have little to do with any type of conventional understanding of the electronic properties of materials. In fact, more than a decade after their discovery, there continues to be no universally accepted theory for why these materials are superconductors. No one working in the field in the early days could possibly have foreseen that this would be the case after so much work and thought had been devoted to the problem.