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The Role of Materials in Future Industries Session ChairLee Magid, University of Tennessee, Knoxville Nanomaterials, Julia Weertman, Northwestern University Biomaterials, David Tirrell, California Institute of Technology Optical Materials, Rod Alferness, Lucent Technologies Computational Materials Science, Sharon GIotzer, University of Michigan 1 Progress in industry and manufacturing and the nationls economic health rely on the continual discovery, development, and use of new materials for new products. Some of today's most exciting and groundbreaking science is focused on nanomaterials, biomaterials, and materials for optical communications. As in all areas of cutting-edge research, computation is playing an increasingly central role in the development and understanding of materials systems. NANOMATERIALS Nanomaterials are a relatively newly recognized class of materials. They were defined in this workshop as any material having at least one dimension less than 100 nanometers. While some naturally nanostructured materials are in use in commercial applications, artificially manufactured nanomaterials have demonstrated some particularly interesting and promising properties. Some examples of nanomaterial structures are shown in Figure 3-~. Nanolayer materials, such as those composed of thin layers of copper and nickel, can be much stronger and harder than either of the two components alone. This strengthening results from the disruption of some of the stress relief mechanisms in the crystalline structure. Layered materials can also exhibit a high resistance to corrosion, and there is evidence that the use of nanolayers can improve the fatigue life of ~~_~ ITS 1 ~ 1 _ · ~ , . - ~;~1~1~. ~~ano~ayerea semiconductor materials are being used to produce efficient electronic and optoelectronic devices such as diode lasers. Nanoparticle materials are attractive for nanoabrasive polishing, for highly targeted drug delivery, and even for cosmetics such as sun block. Nanohole materials find application as molecular sieves and nanograined materials, i.e., small-grain-size, three-dimensional materials that exhibit significant increases in strength. Nanotube materials consist of sheets of carbon atoms seamIessly wrapped in cylinders only a few nanometers in diameter but up to a millimeter long. The number of both specialized and large-scale applications of nanotubes is ~rc~win~ const~ntiv Their ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ is _ _ 1 1 ~ 1 1 , 1 plopcl~l~b Ud11 UC COllLI-OllCU Dy changing Ine orientation of the carbon sheet. Nanotubes 13
MATERIALS AND SOCIETY can be macie to be semiconclucting or conducting ant] have been user! to strengthen polymers. They can also be used as tips or probes for microscopy. Nanomaterials show great promise in new applications and new manufacturing processes, but many problems ant] challenges remain. The development and application of nanomaterials will neec! techniques to produce powders of high quality in sufficient quantities and at a low enough cost to improve the low fracture toughness and poor ductility of three- climensional materials, to address the difficulty of assembling nanocomponents, and to improve the thermal stability of nanostructures. BIOMATERIALS Biomaterials include, among many things, the medical implants carrier! by millions of Americans. Some biomaterials are made from components whose usefulness has been discovered by serendipity. An example is polymethy] methacrylate, or superglue, which is used in intraocular lenses. Another is shape-memory alloys, once a curiosity but now used to keep aortal stents round. The potential for newly engineered materials to solve many of the challenges for new implanted crevices is enormous ~ 1 1 · . ~ Particles, diameter<] OOnm Material with small grain size, <1OOnm Nanoscaie naultilayers Material containing nanoscale features Carbon structures, e.g., C60, nanotubes, etc. . .. . FIGURE 3-1 Examples of some basic nanomaterial structures. SOURCE: Julia Weertman, Northwestern University. Drug Every systems have great commercial and societal impact. Traditional internal drug delivery methods use polymeric materials that dissolve or diffuse the drugs. One new approach is the use of microneedIes, currently mace from silicon, that can inject drugs painlessly. Another approach is the "pharmacy on a chip," where conventional lithography makes many reservoirs on a chip and electrical impulses selectively rupture a membrane ant! deliver the required close of the chosen drug. Materials for implementing the array technologies needled for genomic and proteomic studies present new challenges. Making arrays of genes is relatively straightforward, because all genes have very similar chemistries. However, forming protein arrays is much more difficult, because proteins vary so widely. Moreover, simple polymer substrates cannot provide the amount of information required. New materials with new surface properties are needled. 14
MATERIALS IN FUTURE INDUSTRIES Microfluidic devices can move fluids through a maze of microscopic channels and chambers that have been fabricated with the same lithographic techniques used today for microelectronics production. They can be used, for instance, to separate cells, and they can have input channels as small as 10 micrometers. Although microfluidic devices are cost competitive with conventional cell sorters, materials challens,P.~ still limit the speed and capacity of the microfluidic approach. ~ ~ a.=, ~ v ... ...,.. ~ Ace_ The assembly of hybrid organic and inorganic materials exploits the ability of a small amount of organic material to organize the inorganic component of a hybrid material. An exciting example of an application for this type of structure is the use of peptides to distinguish gallium arsenide from silicon and silicate. 1 MATERIALS FOR OPTICAL COMMUNICATIONS Materials advances have enabled some of the great leaps in electronic switching and communication over the past 30 years, as shown in Figure 3-2. In order to continue this daunting trend, today's devices are moving steadily toward materials that use optical characteristics to increase their functionality. . Optical materials that are already being implemented in communication systems Include erbium-doped fibers that permit optical signals to be amplified without the use of electronics. Implementation of this technology can lead to increases in capacity by a factor of close to 1,000 when combined with wavelength-dispersive multiplexing, which uses up to 80 or so multiple wavelengths, with each wavelength carrying a separate communications channel. The resultant increase in power can mean that signal boosters are needed only every 50 miles, allowing telephone calls and e-mails to be transmitted optically across oceans or continents without going through an electronic system. These and other innovative fiber materials are being used today. However, over long distances, intrinsic fiber material nonlinearities fundamentally limit the system performance. New materials and approaches to overcome these limitations and provide linearity include photonic band gap materials and "holey" fibers. Fabrication of holey fibers fibers designed with tiny holes along their length is difficult. but the nav~ff of being able to engineer the fiber properties is considerable. New systems are increasingly needed for routing the communications along these high-capacity networks. These systems now resemble highways with complex interchanges where signals are split and rerouted toward their final destination. One enabling technology is MEMS, which can make such high-capacity switches with very low losses and switching densities unmatched by electronic switches. The arrays of mirrors can easily be scaled up, and switch times of milliseconds, while slow, are sufficient. Optical signal devices hold great promise for replacing electronic switching devices. Optical-controlled switching will require a nonlinear gate materin1 ~ mntP.rinl~ . 1 1 , 1 , · ~ ~ : _ _ A ~ _ ~ ~ ^~* ~~ ~ A$~~ B ~~ ~ecnnology Inal remains Immature. because optical-optical nonlinear interactions are weak, such devices use proportionally more power. Advances in gate materials will be needed to increase their efficiency. 15
MATERIALS AND SOCIETY ODO ~ 1 0~3 I CD ~~ ~0 _ ~ .. v -~e ,.,... it, ~ . ~ ~ - .... ~ .. ~ .... , ~ O.001 ~-'~'"' -' -~-:~ :~ ~~ 'I':: ~~.~'- ' ~.~.'.~ ~ ~ .' ' '.' ~: ~ -.~ ~ ~ ~'~ '.'. ~ 970 ~ 975 t 980 1985 ~ 990 ~ 995 2~)0Q 20~)5 FIGURE 3-2 Technology roadmap for lightwave electronics, showing the data rate capabilities enabled over the past 30 years. The materials used for electronics have ranged from silicon to gallium arsenide to indium phosphide. The devices these materials are used in are also many: They include bipolar transistors, metal oxide semiconductor field-effect transistors (MOSFETs), metal electron semiconductor field-effect transistors (MESFETs), heterojunction bipolar transistors (HBTs), and high-electron-mobility transistors (HEMTs). The transmission characteristics these materials have enabled in optical carriers (OCs) have improved from less than 500 million bits per second (Mbps) to over 40 billion bits per second (Gbps). SOURCE: Cherry Murray, Lucent Technologies. COMPUTATIONAL MATERIALS SCIENCE Computational approaches have ma(le tremendous progress at all length scales relevant to materials. These inclucle the macroscale (human dimensions), the microscale (atomic climensions), and the mesoscale, which bridges the two regimes. At the macroscale, researchers have long used such traditional methods as finite element analysis to predict mechanical and other properties of materials. These methods are now being coupled with image processing approaches to impart aclclitional physical cletail to the mociels. At the smallest scales, quantum mechanical ah initio (or electronic structure) calculations have predicted optical spectra in nanoscopic quantum clots. These models also predict the great mechanical strength of carbon nanotubes. Despite the excitement surrounding new materials, the simulation capabilities of these relatively new methods are currently limited to small calculations, typically for less than a thousand atoms. Improver! algorithms and computer speeds will be needled to mode! larger systems. Bridging the considerable gap between quantum mechanical simulations of small collections of atoms ant! macroscale calculations of materials properties are the molecular and mesoscale simulation methods. These may include tens of thousands to a billion atoms simulatecl using particle-basecl or field-based methocls. Such methods can provide insight into materials phenomena on the scale of several nanometers to several hundreds 16
MATERIALS IN FUTURE INDUSTRIES of microns, and on time scales from picoseconds to seconds or even hours, depending on the material or process modeled. Dendritic growth during solidification and polymer phase separation, for example, is often modeled with these methods. A goal for computational materials science is to play the same role in materials as molecular modeling does in the pharmaceutical industry. Challenges to reaching this goal include developing approaches to seamlessly integrate multiscale simulation methods and techniques to handle large quantities of data, training researchers, and sustaining the multidisciplinary infrastructure needed to attack and solve the problems. In general, the development of reliable mesoscale theory and methods will aid the better understanding and development of complex materials such as self-assembled nanotubes and quantum dots, bioinspired and biological materials, and nanoengineered materials designed molecule by molecule. Over the next decade, the use of computational methods to design, discover, and optimize nanomaterials, biomaterials, and optical materials will become increasingly prevalent. One workshop participant cited the view that the United States economy was built on materials: steel, aluminum, glass, cast iron, and plastics. Although these industries are still responsible for much of our growth, the new materials that are the subject of this report will play an increasingly important role. Comments from the Speakers "Computational approaches to simulate materials and anticipate behavior enable a modern approach to materials design, discovery, and optimization." Sharon Glotzer, University of Michigan "The enormous advances in optical communication systems over the past 5 years are largely due to materials advances on many fronts." Rod Alferness, Lucent Technologies "Nanomaterials are an exciting but very disparate class of materials which come in an ever increasing array of forms with a wide variety of applications." Julia Weertman, Northwestern University "Biomaterials encompass the traditional bioengineering of medical implants to newer areas of drug delivery, tissue engineering, materials for array technologies, microfluidics, and hybrid materials." David Tirrell, California Institute of Technology 17