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  • 2005-2009  (1)
  • 1
    ISSN: 1662-0356
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Natural Sciences in General , Technology
    Notes: The sintering of RF plasma synthesized NiZn ferrite nanoparticles was studied. Theas-synthesized nanoparticles have been modeled as having a core-shell structure with richer Znconcentration on the surface. Most Zn cations occupy tetrahedral sites typical of zinc ferrites,while some of the Zn cations occupy tetrahedral sites in a (111) oriented surface layer in theform of ZnO. Ni and Fe cations show no evidence of such disorder and their positions areconsistent with the bulk spinel structure. This core-shell structure evolves by decomposition ofthe as-synthesized nanoparticles into Ni-and Zn-rich ferrites followed by the decomposition ofthe Zn-rich ferrites into ZnO and -Fe2O3 during sintering of the nanoparticles. Within the coreregion, sintering causes Ni to exit the ferrite structure and be reduced to a metallic form,possibly via a NiO intermediate. The miscibility gap in the pseudo-binary ZnFe2O4/NiFe2O4system was modeled using equilibrium solution data. Decomposition rates are interpretedconsidering inter-diffusion kinetics. Sintered nanoparticle compacts showed an evolution of a 4-phase mixture of ferrite + ZnO + -Fe2O3 + Ni with increasing sintering temperature.The average ferrite nanoparticle size is preserved up to very high sintering temperatures.These observations suggest that the ZnO shell contributes to the sintering process by surfacediffusion while acting as a barrier to the growth of the ferrite core. Metal edge EXAFS patternsof the sintered compacts confirm that Fe transforms from a single ferrite phase into a mixture of-Fe2O3 and ferrite; ZnO content progressively increases with sintering temperature andelemental Ni evolves from the ferrite with increasing sintering temperature. The saturationmagnetization and Curie temperature were observed to decrease as a function of sinteringtemperature, with an anomaly at the temperature where Ni starts to form. This is explained byZn diffusing from the core depleting the ferrite and increasing the amount of non-magnetic ZnOin the shell. AC magnetic measurements also vary systematically with the microstructuralevolution
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