SCIENCE/TECHNOLOGY
New Material is Optically Transparent, Magnetic at Room Temperature • Nanocrystals of fFe203 are magnetic only in a magnetic field; hold promise for color imaging, refrigeration, other uses
magnetization at room temperature is more than 10 times greater than that of FeB03 or FeF3. The y-Fe 2 0 3 nanocrystals are prepared using a commercial ion-exchange resin containing sulfonate groups. The Na+ or FT ions associated with the sulfonates are easily exchanged with Fe2+ or Fe3+. Addition of concentrated sodium hydroxide produces iron hydroxides, which are then esearchers at Xerox Corp. have converted to y-Fe203 using hydrogen synthesized a new room-temperoxide or hydrazine. The oxide partiperature magnetic material that, cles are uniformly and densely distribunlike ordinary magnetic materials, is uted throughout the polymer matrix, optically transparent. Of the handful of the researchers find. The matrix, in fact, transparent magnets known, the new plays a critical role in controlling the material is the most strongly magnetic formation of the nanocrystals. at room temperature. Thus, it promises to have a broad range of potential apThe Xerox scientists and their collabplications. orators, including materials scientist Emmanuel P. Giannelis of Cornell UniThe material was discovered by physversity, Ithaca, N.Y., and physicist Berical inorganic chemist Ronald F. Ziolo nard A. Weinstein of the State Univerand coworkers at the Xerox Webster R e sity of New York, Buffalo, reported search Center in Webster, N.Y. It is a their results in the July 10 issue of Scicomposite of nanometer-size ferric oxide ence [257, 219 (1992)]. particles in a polymer matrix. The ferric oxide in this case is not oc-Fe203 Ziolo came upon the y-Fe 2 0 3 / (ordinary rust), but rather y-Fe203 polymer nanocomposite in his (maghemite), which has been used quest for a colorless magnetic mafor decades as the magnetic coating terial that could be dyed to make in audio and video recording tape. magnetic pigments for color copiThe oxide particles in the new comers. Such a colorless magnetic maposite are 2 to 10 nm across—much terial is unknown to science. Alsmaller than those in conventional though the transparent nanocommagnetic tape. posite doesn't fit the bill—it is amber-colored—"it is much more Because of their small size, the transparent than one would expect y-Fe203 nanocrystals are no longer [such a magnetic material] to be," ferrimagnetic ("hard magnets") Ziolo tells C&EN. And that transbut have become superparamagparency makes for brighter colors if netic, "a state in which the crysone is trying to make magnetic pigtals will stick to a magnet but not ments. to each other," Ziolo explains. In other words, they have become To magnetize a pigment, Ziolo "soft magnets"—magnetic only in explains, one typically mixes the the presence of a magnetic field. pigment with magnetic materials like magnetite (Fe304), which is In this new state, the nanocrysblack, or bulk y-Fe203, which is tals have potential applications in brown. The product tends to be a color imaging, magnetic memo- Transparent, spherical beads of the magnetic y-Fe203/ dark or muddy-looking pigment, ries for computers, magnetic flu- polymer nanocomposite look like disks in this and the color is highly distorted. ids, microwave and radar technol- photomicrograph. The large beads measure more Mixing the Xerox nanocomposite ogy, and magnetic refrigeration. than 100 \\m in diameter
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"The transparency of the new material is an added bonus, not usually found in magnetic material at room temperature," Ziolo notes. Why the nanocrystals are more transparent than larger crystals of y-Fe203 isn't yet clear, "but it appears to relate to the size of the particles." As the particle size of a material shrinks into the nanometer range, its optical, magnetic, electronic, and other properties can change in dramatic and surprising ways. That is why materials scientists are so enamored of nanostructured materials these days. Most magnetic materials are opaque at room temperature, Ziolo says. Two exceptions are FeB0 3 and FeF3, until now the strongest known optically transparent magnets at room temperature. However, their magnetic properties are too weak to be useful. Other transparent materials, such as K2CrCl4 and EuSe, have better magnetic properties, but only at temperatures near absolute zero. The new nanocomposite is a compromise between these extremes. Its
with a pigment produces some color alteration, he says, "but nowhere near what you would get with the typical black or brown magnetic materials." So the nanocomposite, at the very least, appears to be a step in the right direc tion. Several independent scientists familiar with Ziolo's paper say they are intrigued by the novel properties and potential uses of his new material. For example, Benjamin Abeles, a physicist at Exxon Research & Engineering Co. in Annandale, N.J., suggests that each magnetic particle in the nanocomposite potentially could be used as an element in a mag netic memory to record information. The trick, of course, "is you would have to find a way of addressing [each ele ment]," he says. And physicist Shahab Etemad of Bell Communications Re search in Red Bank, N.J., is impressed with the nanocomposite's processibility. If the material can be fashioned into magnetic fibers, he says, it might replace garnets in expensive devices called isola tors, which are key components in fiber optic amplifiers, used in fiber-optic com munications. If s a big "if," Etemad con cedes, but the possibility is worth pursuing. Interest in the nanocomposite also has surfaced at the National Institute of Standards & Technology (NIST) in Gaithersburg, Md. Researchers there are beginning a collaboration with Ziolo and his coworkers to explore the new material's potential in magnetic refrigeration. When a magnetic materi al is moved into, and then out of, a magnetic field, it first heats up and then cools down. This magnetocaloric effect can be harnessed in a magnetic refrigerator. Presently, such devices are useful only at temperatures below 30 Κ because the materials they are based on have relatively small magnetocaloric effects. Scientists would like to extend the refrigerators' operating range all the way up to room temperature. Materials composed of very small magnetic particles suspended in a non magnetic matrix are of interest in this regard because their magnetocaloric ef fect potentially could be much greater than that of conventional materials at higher temperatures and lower fields, says Robert D. Shull, a materials scien tist at NIST. In nanocomposites, he ex plains, "you can align the spins of small magnetic clusters much more ef ficiently with a given applied field than
improved magnetocaloric properties in certain all-inorganic composites. Ziolo's polymer matrix technique, Shull suggests, may allow them to explore a wider range of material classes. The technology is important, Ziolo points out, because "people want to get away from chlorofluorocarbons. If you can find a magnetic refrigerant that works at or near room temperature, you've got something really hot." Ron Dagani
if the atomic magnetic species were in dependent of each other," as in the paramagnetic refrigerants now in use. This could potentially increase the op erating temperature of a magnetic re frigerator or allow the use of lower ap plied fields, which would cut operating costs. "I don't know if the Xerox mate rial will indeed be practical for roomtemperature use," Shull says. "But it's conceivable that it would." So far, Shull's group has measured
Polyaspartic add process developed A commercial process to make polyaspartic acid hoPolyimide intermediate forms mopolymers and copoly during polymerization process mers has been developed by chemists at Donlar Corp., Harvey, ΠΙ. The com H,N—CHC0 2 H 200-300 °C pany expects to develop the biodegradable resins to replace polyacrylic acid in CH 2 C0 2 H certain applications. D,L-Aspartic acid Polyimide Donlar president Larry Koskan says initial efforts CH—CONH A-AcH—C02H will be for use as deter gent builders and in wa CH 2 —C0 2 H / \ C H 2 — C O N H ter treatment. Polyaspartic Polyaspartic acid acid will have to show clear advantages in per formance and biodegradability, however, because its cost will weights are controlled in a range of be $2.00 to $10 per lb, in comparison 25,000 to 30,000, Koskan says molecular with 90 cents to $1.50 per lb for poly weights of 100,000 have been achieved. acrylic acid. Alternatively, the polysuccinimide The process begins with heating of can be treated with other amino com crystalline racemic aspartic acid in a pro pounds to form graft coploymers. Kos file of temperatures from 200 to 300 °C kan says this approach can make re to split out water and form poly-Ν,α- sulting resins either more hydrophobic succinimide. Base hydrolysis then opens or less so. And the initial solid-phase the imide ring to give the resin poly polymerization can be carried out with merized 70 to 80% through the β-car- one or more other amino acids to make boxyl group and 20 to 30% through the copolymers and terpolymers. α-carboxyl group. Although molecular Stephen Stinson
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Proposals for dean-coal program enterfinalphase The fifth and final round of the Depart ment of Energy's Clean Coal Technolo gy (CCT) demonstration program is under way. DOE is soliciting proposals for the round, putting emphasis on superclean, high-efficiency, coal-based systems. DOE notes that improved environ mental performance is likely to become increasingly important as utilities and other energy companies seek ways to
meet expected growth in energy de mand while complying with the more stringent emissions standards going into effect. For example, the 1990 Clean Air Act permanently caps electric utili ty sulfur dioxide emissions nationwide in the year 2000. "In stressing these ultraclean power systems, we are setting our sights more on the long-range future of coal," says James G. Randolph, DOE assistant secJULY 20,1992 C&EN 21
