SCIENCE/TECHNOLOGY
Nanostructured Materials Promise To Advance Range of Technologies • Novel materials made from ultrasmall building blocks offer unusual mechanical, optical, magnetic properties Ron Dagani, C&EN Washington ore than 30 years ago, Nobel Prize-winning physicist Rich ard P. Feynman mused that in the future, when scientists have learned how to control the arrange ment of matter on a very small scale, they would see materials take on an enormously richer variety of proper ties. In the past few years, materials sci entists have begun to gain that kind of exquisite control. And for them, the fu ture that Feynman spoke of looks to be quite exciting. The new field that has begun to take shape is called nanostructured materi als. Its emergence was signaled earlier this year by the debut of a new journal and the first in a series of open, inter national conferences, both of which re fer to themselves by using that term. Nanostructured (or nanophase) ma terials are called that because the size of their building blocks is on the order of nanometers (10~9 meter) or tens of nanometers. Generally speaking, any material that contains grains or parti cles 1 to 100 nm across, or layers or fil aments of that thickness, can be consid ered a nanostructured material. It is a new realm for materials scientists, who once were limited to talking about structures whose dimensions are mea sured in micrometers (10"6 meter). "At the nano level, it's a really new ball game," remarks materials scientist Rustum Roy of Pennsylvania State University, University Park. The ultrasmall size of the building blocks (in at least one dimension) leads to dramati cally improved—or different—proper
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NOVEMBER 23,1992 C&EN
ties. Thus, for example, nanophase ce ramics can be stretched like chewing gum at 1600 °C, making them easier to fabricate into objects. At room temper ature, those ceramics also are tougher and stronger than their coarser grained cousins. Another ceramic—nanophase titania (Ti02)—is a more efficient cata lyst for sulfur removal than are con ventional forms of titania. Nanophase metals are significantly harder than coarser grained metals. A polymer composite containing nanosize iron ox ide particles is transparent and mag netic—and can be converted into a magnetic liquid. Still other nanostruc tured materials show combinations of electrical, magnetic, and optical proper ties that aren't available in convention al materials. The properties of nanostructured materials are determined by a complex interplay among the building blocks and the interfaces between them. In nanophase ceramics and metals, for in stance, the grains are so small that a relatively large volume of the solid consists of grain boundaries or interfac es. Strains and other forces acting
Siegel: searching for new properties
through these grain boundaries can strongly influence the material's me chanical properties. Multilayered sandwichlike materials also have special properties because of frequent, periodic interlayer bound aries. In a multilayer where each layer is, say, only four atoms thick, 50% of the atoms will lie at the interface be tween two different compositions. Thus, the chemistry and crystal struc ture in one layer can be quite different from those a few atoms away. Furthermore, the interactions of such finely divided structures with photons or electrons, for example, can lead to unusual optical and electronic proper ties. The mechanism of these interac tions is not clearly understood. The search for improved or unique properties and the desire to learn how to control them is what is driving re search into new nanostructured materi als, says materials scientist Richard W. Siegel of Argonne National Laboratory in Illinois. Just as properties are im proved when the grains are reduced in size from 100 μιη to 100 nm, they are further improved by going to 1 nm. So the trend is in the direction of smaller building blocks. (However, "nano is the limit," says Roy. Because the as semblage of atoms that constitutes a molecule or a unit cell is in the low na nometer range, it doesn't make much sense to talk about picostructured ma terials.) Scientists' goal of controlling proper ties also involves being able to tailor the properties to fit the requirements of specific applications. The most flexible way to do this is to make a nanocomposite—a mixture of two or more phas es having different compositions or structures, where at least one of the phases is nanoscale. The different properties of each phase can then be blended and adjusted at will. In some nanocomposites, scientists have already gained some mastery over the properties. Take, for example,
wave of the future, having major implica tions for industry and' technology. Already, a Liquid nitrogen number of companies around the world (par Scraper ticularly in Japan) are producing nanostructured materials for Main vacuum commercial applica chamber tions. In the U.S., two of the newest entrants in the field—NanoGas Evaporation inlet dyne, of New Bruns sources wick, N.J., and Nano Funnel phase Technologies Vacuum Corp., of Darien, 111.— pumps Bellows are producing bulk Fixed Lowquantities of nano piston pressure Anvil phase powders. Nanocompaction Slide unit dyne is concentrating on c o b a l t / t u n g s t e n Sleeve Highpressure carbide, a metal/ce compaction Piston ramic nanocomposite unit that is used to make Schematic drawing illustrates the classic gas-phase cutting tools and other condensation route to nanophase materials. The wear-resistant devices. precursor material evaporated from sources A and/ Nanophase Technolo or Β condenses in the high-purity inert gas and is gies is focusing on sin transported via convection to the liquid-nitrogengle-phase oxide ceram filled cold finger. The powdery material is then ics such as alumina, scraped from the cold finger, collected via the fun zirconia, and titania, nel, and consolidated, first in a low-pressure com which have numerous paction device and then in a high-pressure unit, all in vacuum industrial uses. The properties of these and other nano the iron/silica system studied by phys phase materials arise from their nanoicist Chia-Ling Chien of Johns Hopkins structure, and that, in turn, is deter University, Baltimore. This system con mined by how they are prepared and sists of iron nanoparticles dispersed in processed. There are many ways to silica. By changing only two parame make nanophase materials, and newer ters—the size of the iron particles and methods are being developed all the the volume fraction occupied by iron— time. Chien can vary many properties in a One of the classic synthetic routes, continuous fashion. For instance, by used by Nanophase Technologies, is changing the volume fraction alone, he known as gas-phase condensation. In says he can change the electrical con this method, a metal is evaporated (by ductivity of the composite by 14 orders heating or other techniques) in an inert of magnitude. atmosphere at reduced pressure and Nanocomposites, says Roy, offer "the then condensed in the gas phase to major opportunity" for systematically form metal clusters or nanocrystals. creating a nearly infinite array of new These collect on a cold finger and can materials that offer new, potentially use be oxidized to fine ceramic powder, ful combinations of properties. The types which is then scraped off the cold fin of nanocomposites being studied can ger and compacted inside the appara be classified in several broad catego tus. The compact may be sintered to ries, such as ceramic/ceramic, metal/ yield the final polycrystalline ceramic ceramic, metal/metal, and ceramic/ shape. To make nanocomposites, two polymer. Also attracting attention are different metals can be evaporated and combinations of other organic and in condensed at the same time. organic materials. In this method, Siegel explains, the Nanocomposites promise to be the starting material is converted into at
Nanophase materials can be prepared in the gas phase
oms, these are assembled into clusters, and then the clusters are squeezed to gether and consolidated into an ultrafine-textured solid. Nanophase Technologies, which was founded by Siegel and others in No vember 1989, is now scaling up this process. Each production unit at the company can churn out 50 to 100 g of nanophase material per hour, he says. And the company hopes to boost that to about 1 kg per hour by the end of this year. The oxide powders are sold to industrial users interested in explor ing the properties and potential appli cations of these materials. At Nanodyne, scientists are using a solution chemical route called spray conversion processing to produce bulk quantities of cobalt/tungsten carbide, which consists of tungsten carbide par ticles embedded in cobalt grains. The traditional way of making this engi neering composite requires that the tungsten carbide and cobalt powders be crushed, ground, blended, and then consolidated. With great effort, the grain size of the final product can, at best, be reduced to about 0.5 μιη (500 nm). Spray conversion processing, on
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