Summary of the Workshop on Structural Nanomaterials (2001)

Five presentations were given in this session on topics related to the synthesis, assembly, and processing of nanostructured materials. A panel of presenters then fielded questions from one another and from other participants. The nanomaterials discussed included carbon nanotubes, metal nanopowders and particles, and ceramic powders. The processes ranged from nanomilling and mechanical alloying, to thermal spray, powder consolidation, and other additive processes, to the processing of nanocomposites.

Joseph W.Piche of Eikos Inc. led off the session with his talk, “Nanoshield Composite Electromagnetic Shielding for Hardening to Electromagnetic Interference and Electrostatic Dissipation.” He gave an overview of what is currently being done or planned for the next 5 years in the processing of electromagnetic interference (EMI) shielding products. He reminded the participants that in industry, what is used is what is economical, such as the trays used to carry disks in semiconductor processing. While cost is a challenge, production rates of nanotubes are scaling up to meet the demand. Mr. Piche felt that the cost of carbon nanotubes at a quality level required for immediate application was not an issue. He quoted a current price of $29/lb in large quantity. The issue is producing high-quality carbon nanotubes for higher performance applications.

Mr. Piche anticipated the production of multiwall carbon nanotube (MWCNT) products within 1 to 5 years. Carbon nanotube suppliers to Eikos are already producing tons-per-day quantities of material, although the nanotubes are of low grade. Piche cited several current applications for single-wall and MWCNTs, such as EMI shielding, electrostatic dissipation protection, and capacitor dielectrics.

Mr. Piche identified the following opportunities, barriers, and future applications:

• Enhancing EMI shielding by “postprocessing” the tubes,

• Improving optical clarity without affecting other properties,

• Improving the reproducibility of product, and

• Shielding for Army mobile shelters (future application).

Evan Ma of Johns Hopkins University presented a talk entitled “Structural Nanomaterials Prepared by Mechanical Milling/Alloying.” He focused on the production of nanomaterials, including nanocomposite powders, by milling and mechanical alloying. He indicated that this solid-state processing method produced grain sizes of a few to 30 nm and could be used for ceramics, metals, polymers, and composites. The challenge is to consolidate these powders while maintaining the nanograin size. Pressure-assisted

processes such as hot isostatic pressing (HIPing) and rapid high-temperature sintering are being used to get centimeter-size samples.

Professor Ma noted that nanoscale metals, including consolidated nanomaterials, exhibit small, uniform tensile elongations. He believes the reasons are (1) porosity/processing defects in the sample, especially for consolidated samples and, more importantly, (2) the lack of strain hardening and strain rate hardening in these high-strength metals. Without hardening, necking instability sets in early. Professor Ma stated that nanometals are very strong and have significant ductility in compression. But if tested in tension, usually the ductility appears low. One needs to identify/provide mechanisms for strain and strain rate hardening if large tensile elongation is desired.

Professor Ma listed the following opportunities, barriers, and potential applications:

• Processing requires high pressure and/or fast consolidation with process control.

• Processing requires improved consolidation for better throughput and reduction of costs.

• The deformation behavior of materials such as pure Fe is significantly different at smaller scales—the tensile behavior of the nanoscale material exhibits extremely low uniform elongation.

• The fundamental mechanisms are not well understood.

• Contamination can occur, pointing to the need for “seasoned” tooling and inert or reducing atmospheres.

• Tank penetrators for Army applications are a potential application.

Next, Klaus Tomantschger of Integran Technologies gave his presentation, “Electrosynthesis of Nanocrystalline Metals, Alloys, and Metal-Matrix Composites: Achievements and Future Challenges.” He concentrated on electrosynthesis methods for thermomechanical processing of materials with original grain sizes of less than 100 nm. He provided several examples of this electrodeposition process that occurs at room temperature, producing either dense coatings or freestanding forms such as foils of varying thickness. Dr. Tomantschger gave an example of the effect of grain size (10 nm, 100 nm, conventional) on mechanical properties such as yield, ultimate tensile strength, and modulus of elasticity that indicated that nanoscale materials could exhibit better wear or fatigue resistance than their conventional counterparts, contrary to some other findings for high cycle fatigue. Examples of current technologies include the creation of nanofoils by rotating a 2-foot-wide drum in the electrolyte and peeling off the resulting foil continuously, at a production rate of 4 to 5 tons/machine/year.

Dr. Tomantschger presented multiple opportunities, barriers, and potential applications:

• Developing process modification to further improve the hardness of wear-resistant materials by process control and postprocess annealing,

• Gaining control over grain growth during processing,

• Creating structures in magnetic materials that have both high saturation and low coercivity, and

• Future applications: copper foil for high-density printed circuit boards, replacing hard Cr with nano Co, nanocoatings such as Ni-Mo that have reduced friction coefficient and increased hardness, nanocomposite coatings such as Ni-SiC with improved hardness and strength, metallization of plastic parts using nano Ni-Mo, and developing micromachines based on nanomaterials.

Chris Berndt of SUNY Stony Brook spoke on his topic, “The Unique Mechanical Properties of Thermal Spray Coatings Interpreted from a Nanostructural Viewpoint.” Professor Berndt gave a thorough update on thermal spray that included processing of thermal barrier coatings, control of the process, including an understanding of defect generation, and effect upon performance. The basic need is to understand the relationship between the processes, the selection and optimized use of feedstock, and the resulting performance of the structures. The application of nanomaterials in a production environment will require more process modeling and control efforts. A fundamental challenge is to take advantage of nanophenomena, for example by creating a framework of cracks at the nanoscale to accommodate thermal gradients so that as the temperature increases, the material can accommodate the temperature change. Professor Berndt provided a list of his top six opportunities, barriers, and potential applications:

• What is the vision for the next 5, 10, and 20 or more years? This has not been articulated.

• A big barrier is the current belief that if it is not 100 percent nano, then there is no technical advantage. Existing R&D has proven this a fallacy. The scientific and engineering approaches should be adjusted to recognize that partial nano might be not only good, but also preferable in some applications. There needs to be a cultural adjustment in the nano-community to recognize this potential. Engineering applications will require the integration of materials at all length scales.

• Block funding (i.e., centers and institutes) is not always the best approach. Such consolidation schemes have negative impacts on individual creativity, and the creation of large centers will not address core issues.

• Database activities are needed. Use the resources of the National Institute of Standards and Technology (NIST). Sharing of such resources is vital for intelligent and efficient growth.

• The current skepticism concerning the future of nanoscience needs to be addressed. Such cynicism is not really justified and must be negated. This is a major barrier within our conservative engineering culture that is impeding funding and progress. We need to focus on the positives, to address short-term versus long-term R&D.

• The “picking of low-lying fruit” needs to be reconciled with the ability to take on really challenging problems. It is agreed that picking the sure winners is good for program longevity. However, there need to be some high-risk, “out of this world” tasks as well.

Jackie Ying of the Massachusetts Institute of Technology wrapped up Session 1 with her presentation, “Synthesis and Application of Nanostructured Materials.” Professor Ying focused on the chemistry of nanostructured materials, specifically looking at applications that demanded a mix of chemical, physical, and biological properties. She presented

numerous nanoscale applications where, again, the challenges are to keep the cost down and to improve the processing efficiency.

Professor Ying first discussed hydroxyapatite (HAP)-biocompatible material (close resemblance to bone minerals). The challenge with conventional HAP is that it is difficult to sinter (it decomposes at temperatures greater than 1000 °C) and has poor mechanical properties. Nano-HAP and nano-HAP with 3 percent Zr (in the form of highly dispersed zirconia nanocrystals) have better bending and compressive strengths than conventional HAP. Nano-HAP also exhibits enhanced protein adsorption and cell attachment (increased osteoblast adhesion, proliferation, and mineralization).

The second application presented dealt with catalytic combustion using nanoparticles. Burning light hydrocarbons such as methane and natural gas could reduce the impact of CO₂ emissions on global warming. However, flame temperatures over 1300 °C also produce NO_x emissions, which are a major component of smog. Catalytic combustion can lower the flame temperature, reduce NO_x emissions, and provide the potential for burning ultralean mixtures. This requires a catalyst active from 400 °C to 1300 °C (for methane, light-off—defined as 10 percent conversion of the fuel stream—should ideally occur at about 400 °C).

Professor Ying’s research group has synthesized nanocrystalline barium hexaaluminate (BHA) catalyst materials using a reverse-microemulsion-mediated sol-gel technique. In this approach, a reverse microemulsion is used to confine the hydrolysis and polycondensation reactions to nanometer-size aqueous domains. Normally, BHA is difficult to synthesize as nanocrystals owing to the different reactivity of barium and aluminum precursors. The nanocrystalline BHA provides superior catalytic conversion. Nanocrystallization occurs at 1100 °C, with a surface area of 250 m²/g when calcined at 800 °C (10-nm grains). Methane light-off occurs at 590 °C. For nanocrystalline BHA plus a CeO₂ coating, methane combustion can be sustained down to 400 °C. In contrast, conventional BHA shows substantial grain growth (15 m²/g surface area) and a methane light-off at 720 °C.

Professor Ying’s third application was in the area of semiconductor gas sensors to detect toxic gases via changes in sensor resistivity. She presented results for a SnO₂-In₂O₃ nanocomposite semiconductor with ultrahigh sensitivity, high-temperature stability, and engineered gas selectivity. The nanocomposite showed improved surface area and reduced grain size. A 75:25 SnO₂-In₂O₃ material showed suppressed grain growth to <10 nm at 800 °C. Platinum doping enhanced sensitivity and selectivity for CO, while an Al₂O₃ coating improved sensitivity and selectivity for NO_X. By changing the dopant, the sensor can be made sensitive and selective for a specific gas.

Professor Ying’s fourth application involved a ligand-assisted templating process to produce transition metal molecular sieves. These sieves are attractive for applications in fine chemicals and pharmaceuticals synthesis.

The final application concerned bismuth nanowires produced in a dielectric matrix using a template-assisted fabrication process. Anodic alumina, which has a hexagonally packed

array of parallel nanometer-size channels, is used as the template. Bi nanowire arrays are then fabricated by pressure injection of the bismuth liquid melt into this porous template. By selectively etching away the alumina, freestanding, micron-length, single-crystal wires can be made with a high aspect ratio.

In summary, Professor Ying highlighted several challenges and potential applications:

• Preventing the composite oxides from reacting with one another under the reversemicroemulsion-mediated synthesis conditions so that the synthesis method can be applied to develop other complex-oxide catalysts;

• Detecting and distinguishing trace levels of toxic gases, rapid response, and response reproducibility; and

• Applications: biomaterials, catalysts, gas sensors, molecular sieves, and nanowires.

In the follow-up question-and-answer portion of the session, the audience and session speakers addressed scale-up, producibility, science versus technology, and personnel needs:

Thomas Gates of NASA Langley Research Center asked about impurities as a showstopper and how that issue was being addressed. Mr. Piche responded that process consistency may be more important than specific impurity level. It is necessary to know what you have so that you can develop processes that accommodate impurities. Dr. Tomantschger stated that impurities were not a problem for Integran Technologies. Process control is its issue. Professor Ying felt that the academic community should pay attention to scalability.

Maurice Gell of the University of Connecticut said that nanoscale devices have been around for years and asked whether they would ever become practical. Professor Berndt replied that coatings work but proprietary cases hold up knowledge transfer, stifling innovation. He felt that science lags engineering. Mr. Piche observed that venture capital will find the products that are ready for the marketplace. There is no shortage of venture capital, and money will be spent on advanced nanomaterials. Venture capitalists are concerned only with potential return.

Professor Ying mentioned her personal experience of research grants to highlight the differences between science and technology. She said that in some cases, the same proposals in nanomaterials had been turned down for being either too science-oriented or too technology-oriented, depending on the funding agency. She felt that agencies need to decide what is needed—long-term research or faster product development.

An audience member suggested that raising false expectations—for example, by promising what cannot be delivered—should be avoided. Instead, a toolbox should be created that industry will find ways to use. Ken Crelling from the Department of Defense reiterated that technology leads science. He felt that, in general, science has always lagged, but the rate at which it lags is greater than before. He also noted that other countries are emphasizing nanoscience over nanotechnology, and that these countries are not that far behind the United States in nanoscience. Jackie Ying disagreed with the

statement that technology leads science. Robert Price of the Department of Energy said that there is a need to set up a broad-range program, with university research, centers of research, and balanced programs that include chemistry, metallurgy, and physics. Several attendees then noted that nanotechnology has been too discipline-oriented, and instead needs a multidisciplinary approach.

Delcie Durham of the National Science Foundation (NSF) asked for specific barriers, in light of the goals of the workshop. Mr. Piche stated that the manpower shortage in the high-technology workforce has made it difficult for companies to hire talented people with some relevant expertise in nanomaterials. Professor Ying agreed with that assessment. She felt that the federal government devotes more funding to materials research than to university education programs that can address the multidisciplinary needs of nanotechnology. Mr. Piche noted that industry does not necessarily need Ph.D.s for scale-up operations. Mr. Piche also stated that his company looks for bright, versatile employees. Specific expertise is not as important as intellectual elasticity. Mr. Piche clarified that his company hires both U.S. and non-U.S. citizens, but that U.S. citizenship is required for government-funded contracts. Professor Berndt felt that multidisciplinary education should begin not in graduate school but at the undergraduate level. Professor Berndt also raised a concern about the proprietary nature of private industry research. Limited access to research findings may slow advances in nanotechnology or result in duplication of federal research efforts. Professors Mayo and Weertman both noted that the pool of available U.S. graduate students is small.