Session 4: Structure, Properties, and Characterization

Julia Weertman of Northwestern University presented her brief, “Importance of the Characterization of Nanostructured Materials in Understanding Their Mechanical Behavior.” Two key factors influencing strength were emphasized: defects and microstructure.

Professor Weertman emphasized that in evaluating the mechanical properties of a nanocrystalline material, it is essential to quantify the defect content as well as to examine microstructural features such as grain size, grain size dispersion, distribution of misorientation angles between adjacent grains, and internal strains. All of these can greatly influence the material’s strength, she said. For example, defects such as pores and microcracks can completely mask the intrinsic strength of nanocrystalline material. Professor Weertman presented several cases of variations by a factor of more than two in the hardness of samples of the same nanocrystalline metal with the same average grain size but different porosity. Similarly, early workers in the field interpreted the anomalously low elastic modulus of nanocrystalline materials as resulting from an unusual grain boundary structure peculiar to this class of materials. However, subsequent measurements, which took into account porosity effects, showed that the unrelaxed moduli (obtained from high frequency measurements) in nanocrystalline metals are similar to those of their coarse-grain counterparts. She pointed out that as processing improves, so do mechanical properties. Calculations show that a nanocrystalline metal with a wide dispersion in grain size will yield at a much lower stress than a sample with the same average grain size but much tighter grain size distribution. Thermal history also strongly affects mechanical behavior, possibly through the relief of the high internal strains often present in nanocrystalline metals.

Professor Weertman observed that small-angle neutron scattering, positron annihilation spectroscopy, x-ray diffraction, precision density measurements, and TEM are currently among the most useful tools for characterizing nanocrystalline materials.

She listed the following challenges:

  • Develop rapid, easy-to-use methods to characterize relevant microstructural features and defect content in nanocrystalline metals.

  • While not strictly speaking part of the present brief, an overarching challenge is the development of nanocrystalline material that is free of porosity and microcracks and other defects that degrade mechanical behavior.



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OCR for page 23
Summary of the Workshop on Structural Nanomaterials Session 4: Structure, Properties, and Characterization Julia Weertman of Northwestern University presented her brief, “Importance of the Characterization of Nanostructured Materials in Understanding Their Mechanical Behavior.” Two key factors influencing strength were emphasized: defects and microstructure. Professor Weertman emphasized that in evaluating the mechanical properties of a nanocrystalline material, it is essential to quantify the defect content as well as to examine microstructural features such as grain size, grain size dispersion, distribution of misorientation angles between adjacent grains, and internal strains. All of these can greatly influence the material’s strength, she said. For example, defects such as pores and microcracks can completely mask the intrinsic strength of nanocrystalline material. Professor Weertman presented several cases of variations by a factor of more than two in the hardness of samples of the same nanocrystalline metal with the same average grain size but different porosity. Similarly, early workers in the field interpreted the anomalously low elastic modulus of nanocrystalline materials as resulting from an unusual grain boundary structure peculiar to this class of materials. However, subsequent measurements, which took into account porosity effects, showed that the unrelaxed moduli (obtained from high frequency measurements) in nanocrystalline metals are similar to those of their coarse-grain counterparts. She pointed out that as processing improves, so do mechanical properties. Calculations show that a nanocrystalline metal with a wide dispersion in grain size will yield at a much lower stress than a sample with the same average grain size but much tighter grain size distribution. Thermal history also strongly affects mechanical behavior, possibly through the relief of the high internal strains often present in nanocrystalline metals. Professor Weertman observed that small-angle neutron scattering, positron annihilation spectroscopy, x-ray diffraction, precision density measurements, and TEM are currently among the most useful tools for characterizing nanocrystalline materials. She listed the following challenges: Develop rapid, easy-to-use methods to characterize relevant microstructural features and defect content in nanocrystalline metals. While not strictly speaking part of the present brief, an overarching challenge is the development of nanocrystalline material that is free of porosity and microcracks and other defects that degrade mechanical behavior.

OCR for page 23
Summary of the Workshop on Structural Nanomaterials Ganesh Skandan of Nanopowder Enterprises followed Professor Weertman with his talk “Synthesis and Processing of Nanopowders: Overcoming Technical and Commercial Challenges.” He then discussed several applications of nanopowders and their characterization: The performance of Li-ion batteries is limited by the electrodes. With the use of nanopowders of lithiated V2O5, MnO2, tin alloys, and Li4Ti5O12, Li ion diffusion is faster and a 30 to 60 percent improvement in capacity is obtained. The basic science issues of cyclability, mixing, and electrolyte-electrode interface need to be investigated. The property enhancement in electrodes needs to be transferred to cells and then to batteries, which is a major leap. Dr. Skandan noted that Yt+3 ions are potentially superior to Li+1 ions. However, he felt that it would take 12 years to bring Yt-ion technology to market because it requires much more testing. Thermal spray powders for use by the U.S. Navy must work in the Navy’s spray guns. Otherwise they will not be considered. In the case of WC/Co powders, a mixture of coarse and nanosize powders gives better results. Further, the Co content can be reduced. Adding Cr to the powders increases corrosion resistance, a factor important to the Navy. Dr. Skandan’s company can produce tons of the powder. The challenge is cost. There is no commercially available tool that can characterize all the relevant features of a nanopowder. Existing methods are fooled by agglomeration in the powders. The size distribution of powders can be completely characterized by small-angle x-ray scattering (SAXS) and ultra-SAXS. The key is to develop a SAXS system that can be used by an engineer/scientist not skilled in SAXS.