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I will discuss the application of local probe techniques such as EXAFS, atom probe tomography and electronmicroscopy to elucidate the rather complicated microstructure of novel nano-nanocomposites (NNC) systems. Wedefine a NNC as a random, bi-phasic nanodispersion, in which the characteristic sizes of both the constituent phasesare in nanometers [1]. It is possible to tune their physico-chemical properties via the nature of the components, thecomposition, and the size and morphology of both the phases. The properties of such systems are often not simplelinear superpositions of those of the components. I will discuss the role played by NNCs in different areas of materialsphysics, and attempt to understand and rationalize their observed physical properties in terms of their microstructure.There are certain apparently amorphous alloys such as Ag-Ni, Cu-Nb, Ag-Cu, and Ag-Fe, whose local structure iscontroversial as they are formed from metals with a large positive enthalpy of mixing, and should therefore havebeen immiscible. Using 3-D atom probe tomography and EXAFS analysis, we have established that there is indeednanoscale phase separation (or clustering) within the amorphous phase in immiscible alloys such as Cu-Nb, whichsubstantially reduces the Gibbs free energy of the undercooled liquid and stabilizes the amorphous phase in the formof a NNC [2, 3]. However, detailed EXAFS measurements conducted at both Cu (8,979 eV) and Nb edges (18,986 eV)clearly show that the near neighbor atomic coordinations of Cu and Nb are distinctly different, with the Nb-rich clustersexhibiting a larger degree of medium range topological order [4].
We have also found that the transport properties of a NNC consisting of a superconductor and a normal metal arepredominantly controlled by the superconducting proximity effect, which plays a significant role in such systems whenthe size of the superconducting particles is less than the coherence length. We have studied the following two typesof systems. (i) A sputter-deposited NNC comprising of inter-dispersed superconductor (Pb) and normal metal (Ag)particles [5]. (ii) Melt-quenched, nm-scale superconducting composite (bi-phasic) particles (Pb and Sn) embedded in ametallic matrix (Al) [6].
REFERENCES:
[1] P. Ayyub: Int. J. Nanotechnology 6 (2009) 530
[2] R. Banerjee, A. Puthucode, S. Bose and P. Ayyub: Appl. Phys. Lett. 90 (2007) 021904.
[3] R. Banerjee, S. Bose, A. Genc and P. Ayyub: J. Appl. Phys. 103 (2008) 033511.
[4] S. Chattopadhyay, S. G. Srinivasan, J. Du, S. D. Kelly, T. Shibata, P. Ayyub, and R. Banerjee; submitted
[5] S. Bose and P. Ayyub: Phys. Rev. B 76 (2007) 144510.
[6] S. Bose, V. Bhattacharya, K. Chattopadhyay and P. Ayyub, Acta Mater. 56 (2008) 4522.
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