Chemical & Chemical Engineering News (80th Anniversary Issue), Vol. 81, No. 36, 2003, Sept. Edited by X. Lu Introduction

Nature has been kind to us. In 1911, she revealed to Kamerlingh Onnes that such an effect could also occur in a macroscopic object, known as a superconductor. A stream of electrons or an electrical current, once started, will flow forever in a ring made from a superconductor, the closest thing to a perpetual machine. It was superconductivity that first introduced me to yttrium and has kept me off the street for the past several decades.

The zero resistivity of a superconductor enables the transmission of an electrical current without energy loss. As a result, scientists recognized the technological promise that superconductors held immediately after discovery. One can envision a superconducting magnetic, levitated train gliding smoothly above a track at a speed faster than 500 km per hour; a superconducting generator three to six times smaller and lighter than its nonsuperconducting counterpart, producing the same amount of power without loss; a superconducting magnet generating a strong steady field that cannot be otherwise achieved for research and industry; superconducting sensors with unrivaled sensitivity; and electronic devices with ultrafast speed. Unfortunately, during my graduate school years, superconductivity occurred only at temperatures below 23 K close to absolute zero (0 K). To reach such low temperatures, one must use the rare, expensive, and difficult-to-handle liquid helium as a coolant, making application of superconductors impractical.

For decades after 1911, one main goal for scientists in the field of superconductivity was to look for materials that are superconducting at higher temperatures or that possess higher transition temperatures (T_c). Although yttrium is a metal that is not superconducting at ambient pressure, its carbon compound, Y₂C₃, doped with titanium has a T_c as high as 14.5 K. Until the mid-1980s, the compound was considered a high-temperature superconductor and attracted the attention of many scientists, including myself.

A new, record-high T_c of 35 K was discovered in La₂CuO₄ slightly doped with La by Alex Mueller and J. Georg Bednorz in 1986. My students and I detected superconductivity at 90 K in LaBa₂Cu₃O₇ (LBCO) in mid-January 1987. Unfortunately, the LBCO sample was unstable because of the impurity present, and thus the superconductivity observed disappeared the next day. Our high-pressure data suggested that a smaller trivalent element than La should alleviate the instability impasse. In late January 1987, my group at the University of Houston and the group led by my former student Maw-Kuen Wu at the University of Alabama observed superconductivity at 93 K in the stable compound YBa₂Cu₃O₇ (YBCO).

The discovery of superconductivity in YBCO above the temperature of liquid nitrogen has ushered in the new era of high-temperature superconductivity. It has made many superconductivity applications conceived decades ago more practical, since one can use plentiful, inexpensive, and easy-to-handle liquid nitrogen. It has opened up new frontiers for scientists to explore.

Who would have dreamed the wonderful world of high-temperature superconductivity would be initiated by yttrium?

Paul C. W. Chu is the T. L. L. Temple Chair of Science at the University of Houston, principal investigator at the Lawrence Berkeley National Laboratory, and president of the Hong Kong University of Science & Technology. Along with Maw-Kuen Wu, Chu discovered the first superconductor above liquid-nitrogen temperature. He received the National Medal of Science in 1988.

YTTRIUM AT A GLANCE

Name:Named after Ytterby, Sweden, which yielded many unusual minerals.

Atomic mass:88.91.

History: Discovered in 1789 by Finnish chemist Johann Gadolin.

Occurrence: Yttrium occurs in nearly all rare-earth mineral ores.

Appearance:Silvery white, soft metal.

Behavior: Yttrium is stable in air and is very reactive with the halogens. It is mildly toxic by ingestion.

Uses:Used in X-ray filters and superconductors. Yttrium oxide is combined with europium to give the red phosphor in color television tubes.

TITANIUM

THOMAS M. CONNELLY JR., DUPONT

The fascinating chemistry of titanium is closely linked to the development of several modern industries that have improved the quality of our lives.

Pure titanium metal does not occur in nature. It is derived primarily from ilmenite ore, a black mineral composed of FeTiO₃ and named for the Ilmen Lake and mountains of Russia. Ilmenite can be altered to a mixture of white to yellow titanium oxides known collectively as leucoxene ore, which is another source of titanium. Reduction to elemental titanium was not commercialized until the 1950s. Titanium's combination of high strength-to-weight and corrosion resistance, either alone or as an alloy, provides tremendous performance enhancements compared with more traditional metals used in structural applications.

BELOW THE SURFACE This crystalline atomic lattice of a TiO2 pigment particle has an essentially uniform amorphous nanolayer of silica coating. The inset shows the pigment morphology.