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 
                  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 = "05 dez. 2020"