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

Titanium metal greatly improves performance in many high-tech applications, from medical to military. It is also used to replace damaged human bones and as part of dental implants. Military and aerospace applications such as the SR-71 Blackbird, a high-speed reconnaissance aircraft that serves as an invaluable asset for scientific and intelligence efforts, make extensive use of titanium and titanium alloy structural components. Exploration vehicles used in the high-pressure environment of deep oceans also rely on the high strength and light weight of titanium structures.

The discovery and elucidation of the role of titanium in Ziegler-Natta catalysts stands as a milestone in the evolution of polymerization chemistry. The pioneering work that developed these heterogeneous systems led to the large-scale production of stereoregular polypropylene and other important polymers. The success of this work spurred development of other catalysts based on transition metals beyond titanium.

The optical properties of the common white titanium dioxide make it the most widely used opacifier in industry. Additionally, titanium dioxide, also known as titania, is the most stable of all known white pigments. DuPont has provided pigments based on the chemistry of titanium to the marketplace since the 1930s. DuPont's work began with the previously developed "sulfate" process, where titaniferous ores were digested in sulfuric acid and subsequently converted to either the anatase or rutile polymorph. As a result of a highly focused effort, DuPont developed the "chloride" process in the 1940s and commercialized it in the 1950s. This process--involving high temperatures, corrosive materials, solid-gas separations, and a variety of other related chemical and engineering challenges--was one of the most important developments for DuPont in the 20th century and is still used today.

The hiding power of TiO₂ pigment depends on optical properties generated at the particle level. Nanoparticles of titania are transparent to visible light but opaque to ultraviolet light. The performance and use of nanotitanium dioxide is becoming a hot commodity in many emerging and traditional markets, including wide use in the sunscreen and cosmetics industries. Other evolving applications for nanotitanium dioxide include thermal coatings, structural plastics, and environmental catalysts for water treatment or auto emissions. Future products could include self-cleaning and self-sanitizing countertops and paints.

Other advanced application areas for titania have recently emerged. One of the most promising directions is photochemistry. The surface reactivity of TiO₂ can be exploited for attachment of a variety of ligands, some of which--in conjunction with the host oxide--can exhibit extremely high light absorbance and photoactivity. In addition, titania can be a highly efficient semiconducting material--readily transporting photogenerated electrons into circuitry for practical harnessing of the electric output. The combination of these properties has been capitalized on in emerging photovoltaic device development.

As a result of this potential for new applications, DuPont has a strong interest in controlling the particle shape and size to optimize interactions with light, dependent upon the performance requirements of the end-use system. This goal is challenging because titanium dioxide particles have complex crystal shapes and because light scattering is affected by interactions between particles.

DuPont maintains its strong interest in titanium-based products. This ubiquitous oxide has created and continues to create new frontiers for scientists and new products for end-use consumers.

Thomas M. Connelly Jr. is a senior vice president and chief science and technology officer at DuPont. A chemical engineer by training, Connelly has been with DuPont since 1977 and has served as manager for a variety of DuPont businesses in the U.S., Europe, and Asia.

TITANIUM AT A GLANCE

Name: Named after the Titans of Greek mythology.

Atomic mass: 47.87

History: Discovered in 1791 by the British pastor Rev. William Gregor.

Occurrence: Primarily found in the minerals rutile, ilmenite, and sphene.

Appearance: Silvery metallic solid or dark gray powder.

Behavior: Very strong, light metal that is extraordinarily resistant to corrosion.

Uses: An important alloying agent with aluminum, molybdenum, manganese, iron, and other metals. Alloys (amalgams) of titanium are principally used for aircraft and missiles where lightweight strength and ability to withstand extreme temperatures are important.

ZIRCONIUM

NANCY B. JACKSON, SANDIA NATIONAL LABORATORIES

You always remember your first love; isn't that what they say? Zirconia, the oxide form of zirconium, was my constant companion for so many years. Before going on an airplane, I would have to empty my coat pockets of the many vials of white powder that had accumulated during my trips to the X-ray diffraction machine. Somehow, I didn't think that even in the 1980s airport security would shrug off small vials of white powder. What fascinated me was zirconium's relationship with oxygen. Although the chemistry between oxygen and zirconium has led to its use as an oxygen sensor and an oxygen conductor, it is the surface zirconium-oxygen bond that filled my days and nights for many years. Zirconia is an interesting catalyst, and it is the Zr–O bond that makes it interesting.