Summary of the Workshop on Structural Nanomaterials (2001)

Five presentations were given in this session on topics related to innovative applications for nanostructured materials. The presenters addressed the development, implementation, and commercial applications of nanomaterials coatings, soft and hard magnetic materials, and microelectromechanical systems (MEMS) devices. Processing methods included thermal spray and plasma deposition processing, self-assembly, and some semiconductor processing methods.

Maurice Gell of the University of Connecticut led off the session with a talk entitled “Development and Implementation of Plasma Sprayed Nanostructured Ceramic Coatings.” The goal of the project was the development of cost-effective processes for the implementation of nanostructured ceramic materials for Navy applications. Professor Gell’s team looked at controlling plasma spray deposition to improve resistance to sea salt build-up, to enhance strain tolerance; to improve spallation, cracking, and chipping resistance; and to increase wear, corrosion, and erosion resistance. A fivefold improvement in wear and erosion resistance was the primary goal. Smaller improvements were sought in adhesion, flexure resistance and residual stresses. A critical plasma spray parameter (CPSP) related hardness to regimes of melting (none, partial, complete) for nano alumina-titania coatings from 50- to 70-nm powders, blended and mixed, and deposited under different processing conditions:

CPSP=(voltage×current)/gas flow rate.

Microstructural development was characterized, and an unexpected result was that partial melting, with fully melted gamma phase and partially melted alpha phase, appeared to produce the best set of properties. The conclusion was that for bimodal microstructures, microstructures that incorporate nanoscale features can produce acceptable results, and that there are cases where fully nanostructured materials are not required. Crack resistance is an example of the advantage of bimodal structures, where cracks stopped at nonmelted materials, improving the resistance to cracking and sliding wear over the commercial coatings and fully nanostructured coatings. Alumina-titania coatings were successfully transitioned to shipboard and submarine applications.

Opportunities, barriers, and potential applications were said to be as follows:

• A new solution spray process that may improve the durability of electron beamphysical vapor-deposition (EB-PVD) coatings at lower cost appears promising for yttria stabilized zirconia (YSZ) feedstock.

• From the Navy’s perspective, coatings are “low hanging fruit” in terms of opportunities for nanomaterials; nanocoatings require market pull; and there is a need for nanocoating processes that both repair facilities and suppliers can use.

• The critical need continues to be the toughening of ceramics without sacrificing strength.

• Multidisciplinary teams, university/industry/government partnerships, and concurrent engineering methods capturing advances in materials, design, fabrication, and characterization methods are needed.

Sara Majetich of Carnegie Mellon University addressed the audience next. Her topic was “Compacted Nanocomposites for Hard and Soft Magnet Applications.” Professor Majetich discussed issues related to hard and soft magnetic materials and the advantages of compacted nanocomposites, where complex three-dimensional shapes can be fabricated for a variety of materials. She provided information on synthesis steps for the preparation, dispersion, and compaction of the nanocomposites. The exchange spring method would reduce the cost of the magnetic material by requiring less of the expensive hard phase. Arrays of soft phase with hard magnetic phase have exchange lengths on the order of 20 nm. Compacted soft magnets with random anisotropy can exhibit perfect exchange coupling. Professor Majetich presented an example of a self-assembled nanoparticle array of 4.5-nm Fe particles with a 3.6-nm barrier spacing that produced a room-temperature ferromagnet. Her research has indicated that arrays can exhibit very different magnetic properties depending on the interactions. She saw the barriers and challenges to be as follows:

• Need to advance fundamental knowledge of effects of nanoparticle size/preparation/ condition and processing effects on coercivity;

• Control of contamination, which otherwise reduces exchange coupling across interfaces; and

• Control of grain structure and interfaces (exchange coupling) as the knowledge of magnetic coupling between grains at the nanoscale level continues to advance.

Next, Norman Wereley of the University of Maryland presented his talk, “Synthesis and Applications of Magnetorheological Nanofluids.” He discussed synthesis and applications of magnetorheological nanofluids, first describing a simple magnetorheological fluid as one cup hydraulic oil mixed with one cup iron particles. He focused on how the nanofluids may behave under different flow conditions, where smart (magnetorheological and electrorheological) fluids can change rheological properties under applied fields. He reported interesting phenomena when particles are added, such as raising the yield stress.

Professor Wereley identified the following opportunities, barriers, and potential applications:

• Will replacing micron-size powders in these fluids with nanoparticles produce improvements in properties? For magnetorheological fluids, there are some indications that there will be improvements: reduced wear of hydraulic seals; longer shelf life, without particle caking or settling; reduced solids loading without

sacrificing performance; potential to tailor damping characteristics or control flow direction; and faster switching from zero field to full field and vice versa.

• Applications include fluid shock absorbers (the challenge is that powders can cause sealing problems), helicopter gun recoil damping, vibration isolators, helicopter lag dampers, adaptive suspension, and hybrid actuators.

• Fundamental issues that need to be addressed are particle loading, particle shape, particle size distribution, nanomechanics, Brownian motion, and particle chain formation modeling.

• Commercial needs will drive development.

The presentation of Geoffrey Malafsky of Technology Intelligence International was entitled “Opportunities and Barriers for Innovative Nanotechnology Applications.” Dr. Malafsky listed a number of the National Nanotechnology Initiative grand challenges, including the vision of “nanostructured materials by design,” and showed several examples of molecular control, electronic tailoring, and innovative nanomanufacturing. He focused on a concern about unintended consequences, such as uncontrolled development of self-replicating nanoscale machines, and vision versus reality in terms of insurmountable scenarios based on current scientific understanding.

Dr. Malafsky spoke also on opportunities, barriers, and future applications:

• Operational design and use introduce many new interdependent factors.

• Producibility and reliability issues must be addressed for successful transition: When will the technology mature? Where are the emerging technical options? Example: A purportedly mature MEMS RF switch radar seeker failed prototype functionality as a result of a reliability issue.

• Science needs to be followed by process and technology to achieve transition.

Danny Xiao of Inframat ended session 3 with a talk entitled “Improved Coating Properties Based on Commercial Nanostructured Feedstock Products.” Dr. Xiao presented developments in thermal spray processing for several materials, including nanostructured (<100 nm) Al₂O₃/TiO₂ ceramics and nanoscale (~200 nm) WC/Co cermets. His focus was on property improvements gained by the use of nanopowder feedstock. Nanoscale particles cannot be successfully thermal sprayed, owing to their extremely small masses and their inability to be carried in a moving gas stream for deposition onto a substrate. Inframat has developed a patented process to reconstitute the extremely small particles for thermal spray applications. In the reconstitution process, nanoparticles are dispersed in water to form slurries using mechanical agitation. The slurry is then spray dried to form thermal spray feedstocks with excellent flow characteristics. Improvements in the nanostructured material were significant compared with the conventional materials in terms of bending strength, bond strength (6000 versus 1900 psi), and wear resistance. Production of these coatings has been Navy certified under a military standard.

In the follow-up question-and-answer portion of the session, Lawrence Kabacoff of the Office of Naval Research asked Professor Gell about the thermal stability of YSZ coatings. Professor Gell replied that thermal cycling life results for thin foils held for 400

hours of exposure at 1121 °C showed that the structure coarsened from 30 to 60 nm. This coarsening rate is greater than the bulk behavior described in the literature.

In response to Professor Mayo’s question about the phase of the structure, Gell said it was tetragonal.

Henry Rack of Clemson University asked the speakers for their thoughts about barriers to commercialization, whether technical or economic. Professor Majetich said her number one problem is how to build the structures to get exchange coupling—that is, how to make the idealized magnetic structures in compaction. Compaction processes have high pressures and high temperatures, with atomic diffusion, so the connection between processing and structures and properties (e.g., consolidation versus phase equilibria) must be understood—nonequilibrium structures result, and interfaces or surfaces can create changes in the developing structure. The interfaces are the barrier for exchange spring magnet performance (the spacing across grain boundaries needs to be controlled). Corrosion and brittleness are additional problems.

Dr. Malafsky said that it is easy to focus on a specific phenomenon and ignore the large amount of other chemical effects going on. At the nano scale, any change will affect chemical properties. Thus, interdisciplinary teams bringing together users, requirements, and science are needed to understand the wide range of phenomena.

Professor Gell noted that one barrier is the administrative approach. A 10- to 25-year perspective is needed, with a disciplined concurrent engineering approach to prevent wasting money on premature scale-up. A series of visions must be created regarding where we want to be 10 or 20 years from now (e.g., a vision of a tough ceramic) and the activity of people around the country must be focused to achieve that vision.

Dr. Xiao said that the hurdles to commercialization are twofold: scientific issues have to be solved first, and transition to commercial scale is tough. Integrated product teams are needed to solve fundamental problems. Each step must be solved before moving on to the next one.

Workshop sponsor David Barlow asked about the Japanese work to toughen ceramics. Professor Gell said that the Japanese have launched a new initiative in nanostructured materials and high temperature coatings, but the emphasis is unknown. He said that the Japanese will put money where the United States puts money, based on the belief that the United States has some special insight into research.

The question-and-answer portion of the session highlighted the following issues:

• How to build the structures to get exchange coupling and how to make the idealized magnetic structures in compaction must be investigated.

• An understanding of the connection between processing and structures/properties is needed (e.g., between consolidation and phase equilibria).

• Interfaces are the barrier for exchange spring magnet performance. Corrosion and brittleness are additional problems.

• A 10- to 25-year perspective is needed.

• The scientific issues have to be solved first.

• Integrated product teams are needed to solve fundamental problems associated with the transition to the commercial scale.

• Interdisciplinary teams are needed.