Expanding sand into spacier materials.

Expanding sand into spacier materials

It would take about 20 gallons of paint to cover a surface area comparable to that in a sugarcube-sized chunk of some newly made microporous materials. Their labyrinthine interiors could potentially serve a cozy sites for catalyzing chemical reactions or as minuscule sieves penetrable only by molecules of certain shapes and sizes. They might also proved useful in efforts to design specialty glasses that, for instance, bend light to prespecified degrees.

The crystal structure of sand led chemists to the architectural principle behind the new materials. On the molecular scale, a grain of sand is primarily a nonporous, three-dimensional framework known mineralogically as silicate. It consists of alternating silicon and oxygen atoms with two additional oxygen atoms attached to each silicon atom.

Kenneth J. Shea and Douglas A. Loy of the University of California, Irvine, working with Owen W. Webster of the Du Pont Co. in Wilmington, Del., reasoned that they might create a huge variety of silicate-like structures, some with builtin networks of predesigned pores, by learning to insert molecules of specific lengths as spacing units between silicon atoms at regular intervals in the framework. To test their reasoning they constructed components, or monomers, for the frameworks by attaching silicon-based chemical groupss to either end of a single benzene ring or to a rigid string of several rings. Dissovling these monomers in a solvent such as ethanol and then adding an acidic water solution triggers the several-hour, framework-forming reaction.

The reaction initially produces soft, fragile and transparent gels. Removing most of the water during a careful, two-day drying process yields hard, glassy materials with an interior Shea likesns to a "rat's nest." Such sol-gel reactions are becoming increasingly important for making ultrapure glasses, ceramics, coatings and fibers, the researchers write in the November/December CHEMISTRY OF MATERIALS. So far, they report making amorphous materials that lack the long-range atomic or molecular order and predictability of crystals and of zeolites (an important class of molecular sieves used most notably for producing gasoline). The work illustrates scientists' increasing ability to dictate the properties of a macroscopic material by controlling its assembly on a molecular level, Shea says.

By doping the materials wit compounds that absorb and reemit specific wavelengths of light, researchers may someday design components for the forthcoming technology known as photonics, Shea says. Instead of manipulating and channeling the flow of electrons as do today's microelectronic devices, photonic devices would govern the flow of light.

At last November's meeting of the Materials Research Society, chemist Larry L. Hench of the Universtiy of Florida in Gainesville reported using related sol-gel techniques to create microporous glasses. Hench says he suspects the technique could allow engineers to make precision optics without time-consuming grinding and polishing steps. His group already has fashioned potential radiation-detecting glasses by filing the nanometer-scale pores in the glass with chemicals that emit visible light after absorbing either gamma rays or ultraviolet light.