Chemical Ordering Phase Transitions in Intermetallic Nanoparticles

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TECHNICAL SUMMARY:The proposed research will explore the fundamental questions concerning the chemical ordering phase transition in nanoparticles of intermetallic metal alloys. Specifically, modeling and measurement of the quantitative extent of chemical order of individual, sub-10nm nanoparticles as a function of particle size, particle composition, temperature, and process and local environment variations with a specific focus to probe the role of kinetics and equilibrium stability. The study will focus on L10 alloys, specifically NixFe1-xPt from the FePt family and CuAu. The ultimate goal is to understand phase transitions in nanoparticle systems and to optimize nanoparticle chemical ordering for magnetic storage applications. The understanding of chemical ordering in intermetallic nanoparticle alloys is limited. Equilibrium thermodynamic models gives a qualitative match to experiment, but the quantitative fits are poor. These shortcomings lead to the question: Is our understanding of nanoparticle ordering limited by the ordering kinetics or our incomplete understanding of the equilibrium thermodynamics? The order-disorder temperature and the order parameter through the phase transition will be measured as a function of particle size and processing conditions. Both in-situ TEM heating and ex-situ annealing studies will be compared. Single particle measurements and particle-to-particle variations will be integral to this work. The materials for study are specifically chosen to give a range of thermodynamic and kinetic properties useful for comparison. Computational modeling on these material systems will be closely coupled with experiment to explore the quantitative comparison to experiment and the relative role of possible kinetic and thermodynamic parameters. NON-TECHNICAL SUMMARY:A phase change in a material is seen as a significant change in structure or properties. Melting, a solid to liquid change, is one example. Chemical ordering/disordering of alloys is one technologically important example of a phase change. Future magnetic recording media will be made with nanometer sized particles of chemically ordered metal alloys. Particle size has been shown to have a big effect on melting phase changes and similarly this program will study the effect of size and composition variables on chemical ordering phase changes in metal alloys. A fundamental understanding of the factors influencing the chemical ordering phase change would eventually contribute to the societal benefit of high density magnetic data storage and enable further sophistication in nanotechnology applications. Both undergraduate and graduate students will be supported and trained to use specialized experimental tools. The proposed program will further develop the single nanoparticle analysis techniques developed for this effort.