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Summary of the Workshop on Structural Nanomaterials Session 2: Fabrication and Production Session 2 examined the production of nanoparticulates on a large scale and the fabrication of engineering structures having nano-size grains. The discussion on fabrication looked at the densification of nanoparticulates of metals and ceramics and covered conventional and novel processes. It also considered a deformation processing technique known as equal channel angular pressing (ECAP). This technique uses severe plastic deformation to refine grain structures to the nano regime. The first presentation, by T.S.Sudarshan of Materials Modification Inc., was “Issues in Consolidation of Nanopowders.” Dr. Sudarshan discussed the challenges related to the consolidation of nanoparticulate materials. He highlighted the characteristics of nanoparticulates and emphasized that the much greater surface area and small diameter of the nanoparticles could be both a help and a hindrance in consolidation. Benefits in consolidation such as high driving forces, short diffusion distances, and increased sinterability at low temperatures were offset by agglomeration of the particles, high interparticle friction, absorbed gases, and low compressibility. Dr. Sudarshan discussed the use of external pressure as a method of overcoming the limitations in pressureless sintering and highlighted many of the existing pressure-assisted consolidation methods. The characteristics of a successful nanoparticulate consolidation technique were said to be as follows: One hundred percent consolidation (is this needed?) Minimal grain growth; Known powder characteristics (size, shape, size distribution, and so on); Known materials characteristics; and Fast, cheap, near-net-shape fabrication. The requirements for achieving well-consolidated material were said to be: High pressure, High temperature, and Short exposure time. Several of the techniques were discussed, with size capability and scale-up highlighted as the typical limitations. In nearly every case the processing technique was deficient in some way. Particular emphasis was given to Plasma Pressure Compaction (P2C™) and
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Summary of the Workshop on Structural Nanomaterials the capabilities of that system were presented. These included the ability to scale up and to implement automation, the rapidity of processing, and the shape-making capability. The presentation by Dr. Sudarshan also discussed wet processing methods, including osmotic consolidation, pressure filtration, and tape casting. Each of these methods was said to produce consolidated materials with limited thickness. One processing factor discussed in detail was the effect of powder size distribution on the green density of tungsten powder compacts, with the green density in turn having an effect on the final consolidated density. It was shown that finer particles typically had lower green densities than coarse-grained counterparts and that multimodal distributions had higher densities than single-mode distributions. There may be advantages to considering multiphase, multigrained, multistructured materials. Dr. Sudarshan indicated that powder chemistry was vital in determining the density during densification. He also said that oxygen content and other impurities may limit densification if one uses powder that is impure to start with. Dr. Sudarshan spoke of the need for measuring impurities, most importantly, oxygen. Finally, Dr. Sudarshan stated that the storage life of these nanopowders was an issue as they tend to agglomerate with time and create closed pores that compress and are hard remove. Dr. Sudarshan summarized his presentation as follows: Powder characteristics are critical in obtaining dense nanocrystalline materials. Powder sizing is very important as it determines the grain boundaries. The effect of orientation of boundaries is not well understood. Several techniques for fast consolidation are available, with the choice depending on the material and the user’s processing requirements. Densification mechanisms must be modeled (similar to densification maps available for sintering and HIPing). More mechanistic evaluations are needed—grain boundary sliding versus grain growth? Sensors are needed to understand what is happening at the nanolevel. The second presentation, by Merrilea Mayo of Pennsylvania State University, was “(Bulk) Processing of Nanocrystalline Ceramics.” It discussed the difficulties in taking an ultrafine ceramic material and producing a fully dense structure, without dopants or second phases, while maintaining the grain size below 100 nm. Professor Mayo outlined a series of surprises that can occur when ultrafine particulate ceramic materials are being processed. The first of the surprises is that the phase diagram can shift unexpectedly owing to shrinking particle size. This type of phase shift is unexpected since this shift is not seen in conventional materials. The shift in the phase diagram also means that critical material properties change along with the change in crystallographic structure. This change, in turn, affects the processing parameters. Without having prior knowledge of the nanoscale
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Summary of the Workshop on Structural Nanomaterials particulates, this sort of shift in the phase transition is unexpected, although in retrospect it could have been predicted by thermodynamic calculation. The second surprise is that very high compaction pressures are needed to attain fully dense, nanocrystalline ceramics when using dry pressing and pressureless sintering. The large number of particles in a given volume of nanoparticles lead to a large surface area and a large number of interparticle contacts, which in turn lead to high frictional forces and high elasticity in compacted powders. To be successful, dry pressing and pressureless sintering need to be accomplished in the 1- to 9-GPa regime. This typically means that the sample size will be small and may lack the mechanical properties expected of a ceramic nanomaterial. Professor Mayo reported that Kear et al. have used very high pressure compaction and have taken advantage of phase transitions at high pressure to develop a process for transformation-assisted consolidation. At these high pressures (and low temperatures) there is a high nucleation rate of the new phase, but the growth rate of this phase is very low. The grain size achieved is often smaller than the original grain size. The limitation is that the sample size is often very small. Professor Mayo reported on the use of high-temperature processing for nanocrystalline ceramics. The use of high temperatures allows diffusion processes to occur, which in turn allows pores to shrink. In nanoceramics the pores can close by plastic deformation, further enhancing densification. Hot consolidation processes are non-hydrostatic and are usually slow, batch processes that sometimes require canning. The non-hydrostatic characteristic means that density can vary with position in the specimen. During the follow-up question-and-answer portion of the session, Jim McCauley of the Army Research Laboratory commented that no one had mentioned microwave sintering. Professor Mayo replied that she hadn’t seen an advantage to microwave sintering in her work. Jim Rawers commented that microwaving of metal powders can be tricky owing to the possibility of localized surface melting. Professor Mayo believes that the best prospect for commercialization lies with wet processing. She stated that wet processing offers consolidation at low stresses, implying large samples and a high degree of homogeneity. Many processing issues can limit the use of wet processes. One of these is the dissolution of the very fine particles in water. The kinetics of this process are very fast, and dissolution cannot be avoided. Using a solution for which there is no intrinsic solubility of the nanoparticle avoids the problem altogether. A further problem in processing slurries and pastes is that the compacts sometimes never dry. The reason is that the vapor pressure above a pore is determined, in part, by the radius of the pore, and for nanopores the vapor pressure is very low. As a result, elevated temperatures or low humidity environments are needed to completely dry these materials. The last area that Professor Mayo addressed was cracking of compacts made by wet processing. The drying stress scales linearly with permeability or inversely with the
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Summary of the Workshop on Structural Nanomaterials square of pore size. The large internal stresses that may result can lead to severe cracking. The best method of avoiding this problem is to take control of the drying process and employ methods to limit the drying rates. Professor Mayo summarized her presentation with the following points: Phase diagrams shift with grain size: Unexpected phases arise in nanosystems. Dry pressing and sintering require monumental (impractical) pressures. High-temperature processing (hot pressing, HIPing, sinter-forging) is feasible but is currently limited, in part by geometry, in part by size, and in part by mass production. Wet processing offers the promise of large scale, low stresses, high throughput, and high homogeneity, but myriad science and engineering issues remain. The third presentation of the session, by Terence Langdon of the University of Southern California, was entitled “Processing of Ultrafine Grained Materials Through Severe Plastic Deformation: Potential for Achieving High Strength and a Superplastic Forming Capability.” It addressed the use of severe plastic deformation to attain ultrafine grain sizes. Professor Langdon began his presentation by stating that methods under investigation for producing ultrafine grained structures are not sufficient due to difficulty in eliminating porosity and the inability to produce large structures. He postulated that severe plastic deformation might be the best method. He cited three candidate processes: Reciprocating extrusion, High-pressure torsion, Equal channel angular pressing (ECAP). The reciprocating extrusion process is easy to apply and can be used for low-ductility materials, but it may not produce ultrafine grain sizes. The high-pressure torsion method results in nanocrystalline grains but is very limited in its size (scale-up issues). The ECAP process was said to give the best opportunity for producing the ultrafine grain size desired. It, too, is easy to use and can be used on a wide range of materials. Its main disadvantage is that the scale-up prospects are uncertain. Each pass of a workpiece through a 90° ECAP die applies a strain of 1, and with each additional pass the strain accumulates. The accumulated strain raises the strength of the worked material and also reduces the grain size substantially. Professor Langdon provided several examples of aluminum alloys strengthened by the ECAP process. Professor Langdon specifically cited the desire to use ultrafine-grained aluminum alloys for superplastic-forming applications. He showed results from a novel Al-3Mg-0.2 Sc alloy, with which the elongations to failure were many times greater than with conventionally processed alloy. These high elongations were also attained at greater strain rates, leading Professor Langdon to state that many benefits were to be had in superplastic processing.
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Summary of the Workshop on Structural Nanomaterials Professor Langdon commented that in order for the ECAP method to be commercialized on a large scale, several problems needed to be solved, including the following: Scaling up for larger samples, Simplifying the process (i.e., a multipass facility versus a single-pass one), More information on the ECAP process parameters and their relation to microstructure, Moving ECAP from a batch to a continuous process. James Rawers of the Department of Energy’s Albany Research Center gave the final presentation of session 2, “Large Scale Production of and Fabrication with Metallic Nanomaterials: Barriers to Commercial Development of Nanotechnology for Structural Material Products.” Dr. Rawers began his presentation by describing the benefits and advantages of using attrition milling to fabricate nanocrystalline powders. He said that large-scale milling production need not be labor intensive as it is possible for the attritor mill to run unattended (for example, over a weekend). He said that with attrition milling a slight decrease in quality, in terms of purity, might be observed but that the impurities might be beneficial in the long run. The attrition process was described as milling media (balls) impacting on powders. This cold works the powder, with a resulting buildup of dislocations that in turn lead to grain refinement. With milling, alloying is possible by introducing powders of the alloying elements. Dr. Rawers stated that the scale of production ranges from grams to kilograms and that the milling process is well understood from both theoretical and experimental points of view. The milling and the compaction of iron was cited as one example. The compaction could take place by any of the techniques, as discussed by Dr. Sudarshan in the first presentation of session 2. In fact the compaction requires no new technologies. Several choices are available, and all can produce fully dense structures with near-net-shape capability. Dr. Rawers compared the results from a variety of compaction/consolidation methods, examining the grain size and hardness achieved by each processing method. Mechanical property data were shown for the compacted iron, and the Hall-Petch effect was noted. Also noted was the significant decline in properties under tensile loading relative to compressive loading. Comparisons with conventional materials were very favorable. Dr. Rawers offered the following observations regarding the barriers to commercial development: The barriers are not new and it is not uncommon to see 25 to 30 years pass from discovery to application. Nanomaterials are not exempt from this development cycle. The nanomaterials field will move forward through the discovery of new materials that enhance existing performance, are more efficient, and more economical.
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Summary of the Workshop on Structural Nanomaterials He further cautioned that nano-X will not cure everything. It is best to combine the properties of nanomaterials with the application of design. Although there is considerable information available on nanomaterials, it appears to be scattered in separate papers and presentations. Dr. Rawers strongly suggested that some agency fund a program to gather nanomaterials information into a single database (see also Professor Berndt’s comments on the need for a nanomaterials database in the session 1 chapter). Dr. Rawers believes that there is considerable overlap between the different approaches and different materials that have been studied, but it is very hard to see what the different studies have in common and, more important, what is missing. The question-and-answer portion of the session highlighted the following observations: Ease of processing can be improved via multiphase, mixed-length scales (there is a tendency to think of nanomaterials as a single-phase, single-length-scale material). There are a number of consolidation techniques from which to choose depending on specific material. Simple oxides dominate the current market. Nanomaterials is still a relatively small market segment—$333 million in 2000. “Partial nano” might be not only good but also preferable in some applications. Nanoscience needs a big mission or grand challenge. Consistent characterization of powder is definitely lacking. Modeling studies at the atomistic level, as required for nano-science, are at the threshold of credibility. Challenges: Control powder sizing (size and size distribution) and characteristics; Solve the limitations of high-temperature processing; Solve the myriad science and engineering issues to fulfill the promise of wet processing in terms of its scalability, low stresses, high throughput, and high homogeneity; Control orientation of grain boundaries; Model densification mechanisms (e.g., what leads to grain growth?); Generate accurate phase diagrams at the nanoscale; and Pursue industrial innovation/automation. Barriers: Scale-up of processes to produce the larger samples required; Excessively high pressure needed for dry pressing and sintering; Complexity of the severe plastic deformation (SPD) process; Limited data on optimum SPD processing, with more information needed on: Development of texture, Determination of grain boundary orientations,
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Summary of the Workshop on Structural Nanomaterials Finite element modeling of die geometry, and Quantification of the effects of die temperature, friction, construction, and wear; Nonexistent nanomaterials information database; Safety and environmental issues; and Lack of standards (similar to those of ISO or ASTM) for measuring properties.
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Representative terms from entire chapter: