@Article{CarvalhoLemMigMacPul:2018:StCoEl,
author = "Carvalho, F. E. and Lemos, L. V. and Migliano, A. C. C. and
Machado, Jo{\~a}o Paulo Barros and Pullar, R. C.",
affiliation = "{Instituto Tecnol{\'o}gico de Aeron{\'a}utica (ITA)} and
{Instituto Tecnol{\'o}gico de Aeron{\'a}utica (ITA)} and
{Instituto Tecnol{\'o}gico de Aeron{\'a}utica (ITA)} and
{Instituto Nacional de Pesquisas Espaciais (INPE)} and
{Universidade de Aveiro}",
title = "Structural and complex electromagnetic properties of cobalt
ferrite (CoFe2O4) with an addition of niobium pentoxide",
journal = "Ceramics International",
year = "2018",
volume = "44",
pages = "915--921",
keywords = "Cobalt ferriteNiobium oxideComplex permeabilityComplex
permittivityFeNbO4Radome materials.",
abstract = "Niobium pentoxide (Nb2O5) was added to cobalt spinel ferrite
(CoFe2O4) powders for the first time, at varying amounts of 0, 5,
10 and 15 wt%. The purpose was to evaluate the effect of niobia on
the crystalline phases, microstructure and complex electromagnetic
behaviour (complex permittivity and permeability between 300 MHz
and 10 GHz) of CoFe2O4. The samples were prepared by conventional
ceramic methods and sintered at 1475 °C, as potential applications
are as aerospace materials (radomes) which have to survive at such
temperatures upon re-entry. The only crystalline phase observed in
all samples was CoFe2O4, but microstructural evaluation showed
that a non-crystalline, niobium-rich intergranular region was
formed between the grains with niobium addition, and this apparent
liquid/glassy phase aided sintering as considerable grain growth
was also observed. It was shown by Raman spectroscopy that this
niobium-rich amorphous intergranular phase was FeNbO4. The
electromagnetic measurements of complex permittivity (\ε*)
and permeability (\μ*) measurements indicated a steady
decrease in both permittivity and permeability with increasing
niobium oxide addition, although the values for each sample were
relatively stable between 300 MHz and 10 GHz. The real
permittivity, \ε\′, decreased from ~ 12 in the pure
CoFe2O4 to ~ 5.5 with increasing addition, while its imaginary
part assumed values very close to zero. At the same time, the real
permeability, \μ\′, decreased from ~ 1.4 to ~ 1.1,
and a similar effect can be observed in the permeability curves.
The results of the complex measurements also allowed us to obtain
reflectivity graphs representing the energy loss of the incident
electromagnetic wave when crossing the layer of the evaluated
composition. The graphs are presented in the frequency domain, and
indicate that the reflection loss increases with the addition of
niobia.",
doi = "10.1016/j.ceramint.2017.10.023",
url = "http://dx.doi.org/10.1016/j.ceramint.2017.10.023",
issn = "0272-8842",
language = "en",
targetfile = "carvalho_structural.pdf",
urlaccessdate = "27 abr. 2024"
}