Nanometer scale intermixing mechanism in Ag-based nanocomposites
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Nanometer scale intermixing mechanism in Ag-based nanocomposites Adrian D. Jianu1,2, Victor E. Kuncser1,2, Radu C. Nicula1, Petre Palade2, Manuela E. Stir1, Mihaela C. Bunescu2, Eberhard Burkel1 1 Universität Rostock, FB Physik, August-Bebel-Str. 55, D-18055 Rostock, Germany 2 National Institute for Materials Physics, P.O.Box MG-7, 76900, Bucharest-Magurele, Romania ABSTRACT A new Ag-Sm-Fe nanocomposite material was obtained by high-energy ball-milling of elemental α-Fe powder and melt-spun Ag-Sm flakes. Magnetic Fe nanocrystals were thus embedded in a paramagnetic Ag-based matrix. Synchrotron radiation powder diffraction and temperature dependent Mössbauer spectroscopy were used to study the nanometer scale Fe/Ag(Sm) intermixing mechanism. The evolution of the average grain size and microstrains in the Ag-based matrix vs. the milling time were obtained from the analysis of the X-ray diffraction linewidths. For reference Fe powder samples, a bimodal grain size distribution was evidenced by Mössbauer spectroscopy. The magnetic behaviour of the Fe nanocrystals was analysed with respect to the superparamagnetic relaxation effect and collective excitations. The nanometer scale intermixing of Fe nanocrystals in the Ag-based matrix is completed when the Fe and matrix grains have comparable dimensions.
INTRODUCTION High-energy ball-milling of elemental powders is a suitable technique for preparing solid solutions in systems with moderate positive enthalpies of mixing [1-4]. This method was used to embed Fe nanocrystals in a paramagnetic Ag-based matrix. High purity α-iron powder and Ag85Sm15 rapidly solidified brittle flakes were used as starting materials. The driving force of the nanometer scale intermixing of Fe into the Ag matrix is increased by the nanostructured nature of the Ag-Sm solid solution, induced by the non-equilibrium synthesis. Another favouring factor is the microstructure evolution during the mechanical alloying of this ductile/brittle system. The intermixing mechanism was found to strongly depend on the nanocrystal size of the components. The process was investigated by high-resolution X-ray diffraction and transmission 57Fe Mössbauer spectroscopy (TMS).
EXPERIMENTAL DETAILS Brittle flakes of rapidly-solidified Ag85Sm15 (at.%) were ball-milled together with an appropriate amount of Fe powder (Fluka, 99% purity) obeying an Fe:Sm atomic ratio of 2:1. Pure α-Fe powder was simultaneously ball-milled in a symmetrical vial in order to observe the relative evolution of the Fe grain dimensions. A brief description of the processing method was given elsewhere [7]. The samples were subsequently analysed by high-resolution synchrotron powder diffraction and temperature dependent TMS. The high-resolution powder diffraction experiments were performed at the B2 beamline of the HASYLAB synchrotron radiation laboratory, using
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monochromatic radiation ( λ = 0.12464 nm). The 57Fe Mössbauer spectra were collected with a constant acceleration spectrometer with symmetrical wave form and a 57Co source in a Rh mat
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