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SUMMARY AND RECOMMENDATIONS 99 7 Summary and Recommendations Although the study of nanophase materials is still in its infancy, it is already quite clear that an exciting new area of research has opened up and that new materials with novel and useful properties will emerge. Just which avenues of endeavor will be most profitable are not clear at this writing. However, the demonstrated success in synthesizing nanophase metals, single-phase and multiphase alloys, and ceramics with different and sometimes improved properties over those previously available indicates that these avenues will be widespread. Furthermore, the possibilities for synthesizing nanophase composites appear highly encouraging. The report reflects the many different avenues of arriving at nanophase structures. Since the field is new the committee is recommending a broad approach to the making of such structures without waiting to prioritize the research. Priorities will vary depending on specific materials and applications. Much work needs to be done to realize the full potential of nanophase materials. Some examples are these: ⢠Macromolecular synthesis of polymers continues to be one of the most exciting and innovative areas of materials research. Recent advances include group transfer polymerization, aluminum catalyzed ring- opening polymerization, and cationic polymerization of vinyl ethers. These techniques provide efficient routes to a rich variety of molecular architectures, exhibiting a wide range of self-assembled structures and interesting properties. Further research in these and related areas will almost certainly yield additional payoffs, but this pales in comparison to what could be achieved by a concerted effort to replicate what is routinely accomplished in nature (i.e., through biotechnological approaches).
SUMMARY AND RECOMMENDATIONS 100 ⢠Reductive pyrolysis is a promising new initiative for the synthesis of nanophase cermets. It represents a radical departure from traditional powder metallurgy methods of making such materials and offers the potential for obtaining improved properties at lower manufacturing cost. Scale-up of the process needs to be addressed, along with its applicability to other cermet systems. ⢠Gel synthesis methods present opportunities for fabricating novel bicontinuous composites. Although sol-gel methods have been devised for making porous ceramic preforms with controlled interconnected porosity and pore size, scant attention has been given to the difficult problem of infiltrating the very fine pores with matrix materials. The properties of such bicontinuous nanophase composites also need to be investigated. ⢠The versatility of the laser pyrolysis, colloidal synthesis, and cryochemical synthesis methods for producing nanophase materials has been demonstrated. It remains to address the challenge of scale-up of these processes and to demonstrate that lowered sintering temperatures offer some real payoffs in terms of cost and performance. ⢠The new technique of cryomilling is a significant advance on the state of the art for producing dispersion strengthened materials. A new generation of oxide-dispersion strengthened (ODS) alloys can be expected from this process, which will have a major impact on advanced aerospace systems. Techniques have already been worked out for making bulk parts with improved high-temperature properties in aluminum- and iron-based alloys. It remains now to extend the technology to the new generation of intermetallic matrix materials, particularly the nickel and titanium aluminides. ⢠Surface modification by ion implantation is a proven technology that has gained much prominence in the thin-film-device industry. It remains now to exploit this technology for structural applications (e.g., to enhance wear and corrosion resistance). ⢠The gas-condensation method will require a number of modifications to make it suitable for the production of nanophase materials of practical sizes and dimensions. An important innovation would be to develop the capability for synthesizing and consolidating powders on a large scale in an ultrahigh- vacuum system, to avoid contaminating the final product. It appears that there are no impediments, at least in principle, to achieving this goal. Particle sizes and resulting nanophase grain sizes (less than 5 nm in scale) also need to be investigated for their effects on materials properties. ⢠Nanophase materials have been produced by novel rapid solidification methods, such as electrohydrodynamic atomization. Further work along these lines is needed, as well as the synthesis of powders and thin films by laser ablation methods.
SUMMARY AND RECOMMENDATIONS 101 ⢠Much progress has been made in the synthesis of ultrafine carbon and SiC filaments by catalytic methods. It remains now to fabricate filamentary reinforced composites from these high-specific-strength filaments, making use of polymeric, ceramic, or metallic-matrix materials. Another opportunity area is to devise new methods for the catalytic growth of other useful filamentary materials (e.g., TiC and AlN). ⢠Multilayer structures produced by MBE, electrodeposition, and cluster beam deposition are becoming important in structural materials. Opportunities exist to exploit these technologies to fabricate thin-film coatings that exhibit the supermodulus effect. Applications of such coatings to tool bits and high- performance fibers are potential payoff areas. ⢠Many different dispersed phase composites have been synthesized by CVD methods. Useful applications have been found for pyrolytic carbon that is reinforced with submicrometer-sized SiC particles. Promising new ceramic systems for high-temperature structural applications have been identified. Further research is needed to optimize their mechanical and physical properties. ⢠Although major advances have been made in the synthesis of polymer blends, self-reinforcing polymers, and block copolymers, further work is needed to optimize their materials properties. Efforts should also be made to capitalize on the recent breakthrough in understanding of the three-dimensional morphologies of bicontinuous block copolymers. ⢠Many different methods have been devised for making high-surface-area catalysts, including impregnation, ion exchange, thermal decomposition, vapor deposition, and intercalation methods. Further research is needed to enhance catalytic activity and selectivity for specific chemical processes. Complex multicomponent, multiphase ceramic materials offer the potential for multifunctional capabilities. ⢠Membrane research continues to be driven by diverse industrial needs (e.g., desalinization, pollutant removal, and chemical separations). Important areas of research include sintered particle membranes, solution-cast polymer filters, and molecular sieves. Microphase separation of block copolymers appears to be a particularly promising area of current research, with potential for control of pore size, morphology, and interconnectivity down to nanoscale dimensions. ⢠Traditional tools, such as transmission electron microscopy and x-ray and neutron scattering have been used successfully to characterize nanoscale materials. However, little work has been done on chemical mapping at the requisite fine scale. There is an urgent need to perform such studies, utilizing FIM atom- probe and scanning tunneling microscopy techniques. Analytical electron spectroscopy methods utilizing finer electron probe sizes should also be investigated. Other useful techniques include RBS, EXAFS, XPS,
SUMMARY AND RECOMMENDATIONS 102 NMR, Raman, infrared, Mössbauer, and positron annihilation spectroscopies, calorimetry, and chemisorption. ⢠The properties of nanophase materials are significantly different from those found in their conventional polycrystalline counterparts. Major changes are observed in sinterability, saturation magnetization, magnetic susceptibility, yield strength, fracture strength, density, thermal expansion, hardness, elastic modulus, and diffusivity. So far all measurements have been performed on relatively small quantities of material. There is a need to demonstrate that such remarkable property changes can be realized in bulk materials produced by scaled-up commercially viable processes. The enhanced elastic and plastic properties of nanophase materials are particularly intriguing. The supermodulus effect in particular is already beginning to find useful application in hard coatings. Again, making brittle ceramics ductile by exploiting enhanced diffusional creep at low temperatures in nanophase materials is clearly of crucial importance to the structural ceramics field and should be given high priority for future research. ⢠Nano-dispersed phases have been exploited in catalysis for decades. In fact, it is in this area that the nanostructure field has reached its highest level of maturity, both from the scientific and technological perspectives. Nevertheless, much work remains to be done to optimize catalyst systems to achieve the desired conversion efficiency, activity, and selectivity. In addition to addressing problems associated with control of particle size, stability, morphology, interactions, and pretreatments, there is a need to develop bifunctional and multifunctional catalyst systems for advanced processes. ⢠Ceramics frequently contain voids and inhomogeneities. While grain growth would likely result from consolidation of nanoscale ceramics, the probable benefits of relatively fine grain and uniform composition resulting from nanoscale starting material should be explored. ⢠Many applications for nanophase materials have been identified, but progress has been hampered by lack of sufficient quantities of material for performance evaluation studies and field testing. Thus, in addition to significantly augmenting the current level of support for basic research in the field, there is also a critical need to support work dedicated to the scale-up and manufacture of nanophase materials. The processing of catalytic materials on a very large scale is routine industrial practice. Rapid progress is also being made in the commercialization of Fe-Nd-B alloys by rapid solidification technology. These successes leave one optimistic that ways will be found to scale-up many of the synthesis and processing methods described in this report, which today are only laboratory-scale techniques. A case in point is the synthesis of Co/WC by reductive pyrolysis. A potential solution to this problem is to conduct
SUMMARY AND RECOMMENDATIONS 103 the whole operation in a fluid-bed reactor, where the reactive gas species are added to the fluidizing medium. Another example is the gas-condensation method, where scalability seems possible by utilizing high-vacuum electron-beam technology to produce the powders. The continuous collection of these powders for in situ consolidation and subsequent processing by means of gas-flow techniques can replace the static collection methods currently used in the laboratory.
SUMMARY AND RECOMMENDATIONS 104
