Investigators: R. D. Shull, R. D. McMichael, L. H. Bennett, D. E. Mathews, R. V. Drew, H. J. Brown, L. J. Swartzendruber, U. Atzmony, J. W. Weissmueller

Technical Description:

The project aims at promoting the understanding of microstructure - property relations for materials with a nanometer-scale granular microstructure. The motivation for this is threefold: first, enable the creation of materials with optimized properties; second, allow the determination of intrinsic interface properties from measurement of the macroscopic averages of the materials properties; third, provide a better understanding of magnetic interactions in materials near defects like grain boundaries.

Technical Objectives:

• From measurements of the magnetization of nanocrystalline Y-Fe alloys and from Mössbauer data for the material, characterize the magnetic order of the Fe-rich grain boundary segregation layers.

• From measurements of the temperature dependence of the magnetization and high temperature x-ray measurements determine whether the alpha/gamma allotropic transformation of Fe in Fe-Cu alloys was reduced by a factor of 2 when nanometer grain sizes were formed by ball milling.

• Attempt to produce a fine dispersion of nanometer-sized magnetic "compounds" in an oxide by a sol gel process.

• From magnetization and M�ssbauer measurements, determine whether nanocrystalline Fe prepared by vapor condensation and by ball milling was magnetically any different than bulk Fe.

• Determine the excess free volume of the grain boundaries in a high density (i.e. nearly pore-free) nanocrystalline solid by a combination of macroscopic density and small- angle scattering measurements.

• Determine whether there was an enhancement of the magnetocaloric effect at a ferromagnetic/antiferromagnetic transition, as occurs in Fe_2-x(Hf_.83Ta_.17) alloys.

• Determine whether there was an enhanced magnetocaloric effect in Dy-Al-Fe garnet nanocomposites.

• Interest industry to apply magnetic nanocomposites as refrigerants in magnetic refrigerators.

• Determine the magnetic microstructure in a nanocrystalline ferromagnet by small-angle neutron scattering combined with magnetization data.

• Establish a relation between the intrinsic interface property interface stress on the one hand and measurable characteristics of the microstructure and of the internal stress distribution on the other hand.

• Optimize the processing of gamma-Fe₂O₃/polymer nanocomposites for application as advanced toner materials for high quality color copiers and printers.

• Organize symposia and workshops in the area to acquaint industry with the new observations being made on nanocrystalline materials.

Anticipated Outcome:

• Improved understanding of the intrinsic interface characteristics (magnetic order, thermodynamic properties) in fine-scaled granular solids.

• Answer to the question: how (if at all) is the grain boundary atomic structure at very fine grain sizes different from that in coarse-grained polycrystals.

• An understanding of the role grain boundaries play in determining the magnetic moments of ferromagnetic elements.

• Evidence of the enhanced magnetocaloric effects in magnetic nanocomposites predicted by NIST several years ago.

• Development of a method for preparing magnetic compounds in a nanocomposite.

• Answer to the questions: - What are the relative importance of different microstructural elements / defects (variations of the crystal lattice orientation; interfaces; strain fields; nonmagnetic inclusions) on the magnetic microstructure? - Is one of the above microstructural elements related to the coercivity (a technically important parameter) of the samples?

• Identification of suitable, i.e. relevant and measurable, characteristics of the microstructure.

Accomplishments for FY 1995:

• Showed that Fe-rich grain boundary segregation layers can order ferromagnetically.

• Performed scattering, magnetization and density measurements on electrodeposited nanocrystalline Ni.

• Scattering data will yield a new upper limit on grain boundary excess free volume in a nanocrystalline metal. Preliminary evaluation indicates good agreement of scattering data with a micromagnetic model developed at NIST.

• Prepared a fine dispersion of nanometer-sized ferromagnetic Fe₂P in silica by a sol gel process.

• Found an unusually high, 250 K, blocking temperature in a superparamagnetic Fe₂P/silica nanocomposite.

• Found an unusual time dependence in the magnetization of nanocrystalline Fe prepared by ball milling.

• Showed the ease with which contamination can occur in nanocrystalline Fe-Cu alloys, thereby causing erroneous conclusions by investigators in the field.

• Found an enhanced magnetocaloric effect in Dy-Fe-Al garnet nanocomposites.

• Showed how the averages of the internal elastic stress can be related to measurable stereological quantities, the moments of the interface orientation distribution function.

• Awarded an Small Business Innovation Research (SBIR) to a company for designing and constructing a magnetic refrigerator to show reasonable cooling at room temperature.

• Organized 3 symposia on Nanocrystalline Materials and one symposium on sol gel materials.

• Awarded a patent for Magnetic Nanocomposite Refrigerants.

Impacts and Technical Highlights:

• Xerox Corporation has started a pilot plant for the preparation of large quantities of gamma-Fe₂O₃/polymer nanocomposites for potential use in high quality copiers.

• Combined work has resulted in over 20 invited talks in FY95 to industry, universities, and professional societies.

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Last modified: Mon Jan 06 09:46:15 1997 Metallurgy Webmeister