Microwave absorption studies of Graphene (pristine, functionalized)- nano ferrites nanocomposite

Abstract

The use of electromagnetic absorbing / scattering materials have increased manyfolds due to the increased use of high tech electronics using broad electromagnetic spectrum. Electromagnetic absorption have widely been studied in many aspects including EMI suppression related to electronic housings, industrial test chambers, commercial scale infrastructural protection and radar absorbing materials. Electromagnetic scattering has been achieved by structural shaping besides others. These challenges associated with distributed loadings are generally described as spectral bandwidth, minimum weight budget, low thickness of absorbers, ease of manufacturing and electromagnetic flexibility. Moreover, the low frequency spectrum has been less explored considering the materials system and the challenges described above. Generally, stealth structures are designed for high frequency radars using structural deflection of radar as primary scattering mechanism. Newer anti stealth measures include low frequency radars (VHF/UHF/L bands) which are hard to be countered using conventional stealth features and materials. Ferrites have been used traditionally as electromagnetic absorber due to magnetic and dielectric loss. Graphene, being the one atom thick layer of honeycomb carbon atoms possesses the required set of properties like electronic and bonding and doping flexibility at the lowest set of dimension. Moreover, ferrites nanoparticles being the magnetic and dielectric in nature can be tailored to meet specific demands. The major goal of this study was to carry out studies about ferrites nano particles and its nanocomposites with graphene and rGO (reduced graphene oxide) for low frequency region, mostly upto 3GHz. Tuning the materials for low frequency microwave absorption involves the two major objectives: Firstly, synthesizing materials with high microwave absorption and secondly broadening the absorption bandwidth To meet these challenges, ferrites nanoparticles were prepared along with the nano composite with few and single layer graphene either in pristine and doped chemical formulations. These nanocomposites includes ZnFe2O4 -rGO , NiFe2O4 -rGO, CoFe2O4 -rGO, phosphorous doped single layer graphene- Ni0.5Zn0.5Fe2O4 and nitrogen doped single layer graphene- Ni0.5Zn0.5Fe2O4 nanocomposite. Solvothermal and hydrothermal synthesis reaction were carried out as one step facile reactions. For solvothermal synthesis and processing, ethanol and hexanol were used along with graphene oxide as rGO precursor in nanocomposites with pristine graphene. For doped graphene recipes, a environmentally friendly wet chemical scheme was used for the fabrication of nanocomposites. These nanocomposites powder were characterized for various tools including X-ray diffraction (XRD) for phase identification, scanning electron microscope (SEM) and transmission electron Microscope (TEM) for morphology and size determination , X-ray photoelectron spectroscopy (XPS) for compositional analysis, fourier transform infrared spectroscopy for functional attachments, UV-visible spectroscopy for optical response, electrical and dielectric measurements and microwave materials measurements for microwave dielectric properties and reflection loss which were characterized in low frequency region extending from 10 MHz to 1.5 GHz. These nanocomposites were synthesized with different weight percentages of rGO and doped graphene in various ferrites nano particle systems and were characterized. Graphene oxide was synthesized through improved Hummer method having high oxygen percentage (i.e. 45%) and was in-situ reduced under solvothermal condition. Nanocomposites series, namely ZnFe2O4 -rGO , NiFe2O4 -rGO and CoFe2O4 -rGO, were synthesized with 08-25%, 7-15% and 9-18% rGO. X-ray diffraction confirmed the formation of graphene oxide, ferrites nano particle and reduction of nanoparticles during nano composite synthesis. SEM and TEM were used to measure average nano particle sizes, and it lies around 27-27nm, 30-58nm and 15-27nm for ZnFe2O4 -rGO , NiFe2O4 -rGO, CoFe2O4 -rGO nanocomposite series, respectively. Phosphorous doped single layer graphene- Ni0.5Zn0.5Fe2O4 and nitrogen doped single layer graphene- Ni0.5Zn0.5Fe2O4 nanocomposites were synthesized with low doped-graphene content considering the minimum threshold limit of -10dB reflection loss with average ferrite nanoparticles size of 20nm. For microwave reflection loss, ZnFe2O4 -rGO showed reflection loss below -10dB both for nanopaticles and nanocomposite samples due to dielectric dissipation. NiFe2O4 -rGO synthesized through solvothermal method showed high microwave absorption in the entire UHF/L/S band instead of selective frequency resonances. CoFe2O4 -rGO nancomposites were prepared through Hexanol assisted solvothermal synthesis with CoFe2O4 nanparticles having ultra low dielectric constant of 3.21 (@1MHz) and nano composites with reflection loss around -10dB. The doped graphene nanocomposites like Phosphorous and Nitrogen doped graphene possessed considerable high reflection loss and wide frequency region with pristine Ni0.5Zn0.5Fe2O4 nanoparticles. The mechanism behind the microwave absorption was studied to explain the brief electronic, dielectric and magnetic aspects of electromagnetic scattering and absorption and their relation with reaction conditions, chemical formulations and electronic/magnetic modification.