Enhanced electromagnetic properties of carbon nanotubes and SiO2-coated carbonyl iron microwave absorber

Earthicle and Its Discontents: A Historical Critical Review of Iron (Oxide) Particles Singly and Doubly Shelled with Silica and/or Carbon

“…It was noted earlier that both silica-coated and carbon-coated iron (oxides) had been tested as microwave absorbers and they both had been able to reach minimum reflection loss values of up to −50 dB at absorber thicknesses of 2–3 mm. Albeit relying on simple mechanical mixing of carbon nanotubes and silica-coated carbonyl iron components rather than on more sophisticated particle coating procedures, a study from 2018 reported minimum reflection loss values of −51.5 dB at 15.4 GHz and the matching thickness of 1.5 mm in composites with 2 wt % carbon nanotubes . In 2019, three reports continuing the studies by Uskoković et al on the concept of the earthicle were released, the most comprehensive of which reported on the variation in the protocol for the synthesis of the earthicle from the one involving the pyrolysis of citrate-containing colloidal dispersions by thermal annealing to the hydrothermal synthesis that preserved that colloidal properties of the reagent, while creating an atomically thin carbon layer around the silica-coated nanoparticles and rendering them stable in aqueous suspensions with no added surfactants and capable of acting as ferrofluids thanks to the high surface charge of the carboxylated carbon crust (Figure ).…”

“…Albeit relying on simple mechanical mixing of carbon nanotubes and silica-coated carbonyl iron components rather than on more sophisticated particle coating procedures, a study from 2018 reported minimum reflection loss values of −51.5 dB at 15.4 GHz and the matching thickness of 1.5 mm in composites with 2 wt % carbon nanotubes. 319 In 2019, three reports continuing the studies by Uskokovićet al on the concept of the earthicle were released, the most comprehensive of which reported on the creating an atomically thin carbon layer around the silica-coated nanoparticles and rendering them stable in aqueous suspensions with no added surfactants and capable of acting as ferrofluids thanks to the high surface charge of the carboxylated carbon crust (Figure 17). Compared to the pyrolysis in air, which led to the formation of intermetallic FeSi compounds in the mesolayer and the diffusion of iron to the surface, the hydrothermal method proved milder in intensity, thus limiting the diffusion of atoms across the core/shell and shell/crust interfaces and also delamination and cracking issues stemming from the inevitably present thermal expansion coefficient mismatches between the individual phases comprising the earthicle, a problem otherwise common to SiC coatings.…”

Deposition of a SiO2 Shell of Variable Thickness and Chemical Composition to Carbonyl Iron: Synthesis and Microwave Measurements

“…Iron powders are widely applied in power transformers, inductors, sensors [ 1 ], electromagnetic compatibility solutions [ 2 , 3 ], and materials designed to decrease electromagnetic pollution [ 4 , 5 ]. Commonly used is the carbonyl iron with spherical particles of 2 [ 5 , 6 ]–10 [ 4 ] micron in the mean diameter.…”

“…Iron powders are widely applied in power transformers, inductors, sensors [ 1 ], electromagnetic compatibility solutions [ 2 , 3 ], and materials designed to decrease electromagnetic pollution [ 4 , 5 ]. Commonly used is the carbonyl iron with spherical particles of 2 [ 5 , 6 ]–10 [ 4 ] micron in the mean diameter. Carbonyl iron possesses remarkable magnetic properties, although chemical stability and electromagnetic performance are still to be improved when embedded into composite materials.…”

“…For example, two different final-stage thermal treatments may be applied for the Fe@SiO 2 drying. One is simply to keep the powder at 50–60 °C for a certain period [ 5 , 6 , 17 ]. The other is to anneal in an inert (N 2 , [ 8 ]) or reducing atmosphere (H 2 , [ 1 ]) at 500–800 °C for several hours.…”