## Structural and electromagnetic effects caused by the addition of niobium pentoxide in the cobalt ferrite

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Abstract
08-041 Francisco Eduardo Carvalho Carvalho, F.E.(Instituto de Estudos Avançados); Lemos, L.V.(Instituto de Estudos Avançados); Migliano, A.C.(Instituto de Estudos Avançados); da Silva, M.R.(Universidade Federal de Itajuba); Pullar, R.C.(Universidade de Aveiro); Materials used in structures and components of aerospace systems must be lightweight and withstand high temperatures. In order to reduce the specific mass of cobalt ferrite developed to be applied in aerospace systems, niobium pentoxide (Nb2O5) was added to the cobalt spinel ferrite powders (CoFe2O4) in varying amounts of 5%, 10%, 15% and 20%. Samples were prepared by the conventional ceramic method with suitable geometry to evaluate if the crystalline phases, microstructure and electromagnetic behavior of these compositions were significantly affected by the addition of niobium oxide. The samples were sintered at 1200 °C, 1300 °C, 1400 °C and 1500 °C, since the potential applications of these materials imply that they must to survive at these temperatures, as in the case of radomes when subjected to atmospheric re-entry conditions. After observing a single crystalline phase (CoFe2O4), the presence of a non-crystalline phase was evidenced and identified as iron niobate (FeNbO4). It was responsible for filling the pores and causing the grain boundary pinning effect. This phenomenon increases with the amount of niobium added, and a more detailed analysis of the new phase using Raman spectroscopy indicates that it is composed by two sub phases. The evidence of a lamellar morphology motivated an investigation through AFM images to be associated with its electromagnetic evaluation. Complex measurements of permittivity ( $Ɛ$*) and permeability ($µ$*) were performed which indicated a reduction of their values with addition of niobia, although the values for each sample were relatively stable between 300 MHz and 10 GHz. From these results reflectivity graphs were obtained indicating an increase in electromagnetic losses. Finally, the hysteretic cycles of the studied compositions are shown, revealing a behavior with characteristics of a super paramagnetic material.
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