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

Abstract

Protective SiO2 coating deposited to iron microparticles is highly demanded both for the chemical and magnetic performance of the latter. Hydrolysis of tetraethoxysilane is the crucial method for SiO2 deposition from a solution. The capabilities of this technique have not been thoroughly studied yet. Here, two factors were tested to affect the chemical composition and the thickness of the SiO2 shell. It was found that an increase in the hydrolysis reaction time thickened the SiO2 shell from 100 to 200 nm. Moreover, a decrease in the acidity of the reaction mixture not only thickened the shell but also varied the chemical composition from SiO3.0 to SiO8.6. The thickness and composition of the dielectric layer were studied by scanning electron microscopy and energy-dispersive X-ray analysis. Microwave permeability and permittivity of the SiO2-coated iron particles mixed with a paraffin wax matrix were measured by the coaxial line technique. An increase in thickness of the silica layer decreased the real quasi-static permittivity. The changes observed were shown to agree with the Maxwell Garnett effective medium theory. The new method developed to fine-tune the chemical properties of the protective SiO2 shell may be helpful for new magnetic biosensor designs as it allows for biocompatibility adjustment.

Tuning Dielectric Loss of SiO2@CNTs for Electromagnetic Wave Absorption

“…They can be expressed separately by the formula ε r = ε − jε" and µ r = µ − jµ". (ε is the real part of permittivity, ε" is the imaginary part of permittivity, µ is the real part of permeability, and µ" is the imaginary part of permeability) [28][29][30]. As shown in Figure 4a-c, the ε values of the SiO 2 @Fe 3 C/Fe@NCNT-GT varied in a range of 16.63−9.81, and the ε" values of the SiO 2 @Fe 3 C/Fe@NCNT-GT varied in a range of 6.15−2.44.…”

Deposition of Thick SiO2 Coatings to Carbonyl Iron Microparticles for Thermal Stability and Microwave Performance

“…Since the maximum thickness of the shell is limited to 200 nm [ 42 ] in the one-stage Stöber synthesis, a multi-iterative deposition technique was studied. One “iteration” here was the full cycle of deposition including TEOS hydrolysis and air drying.…”

“…The filler volume in all cases was estimated as 20 vol.%. The samples were formed in the toroidal shape and placed inside a standard 7/3 mm coaxial transmission line [ 42 ]. S-parameters of the composite samples placed in the airline were measured in the frequency range of 0.1 to 20 GHz with a vector network analyzer (VNA).…”

“…The number of publications related to micro-sized iron particles and thick (>100 nm) SiO 2 shells is next to none, despite the fact that the topic is relevant in the scientific community. It has been shown previously, that it was not possible to obtain shells with a thickness of more than 200 nm either by varying the duration of the deposition reaction or by changing the concentrations of the reagents [ 42 ]. In the present work, a technique is shown to obtain thick SiO 2 shells of up to 475 nm as a result of a multi-iterative deposition of SiO 2 on iron particles.…”

Comparison of the microwave absorption performance of core–shell SiO2@C and hollow carbon nanospheres with different sizes

“…With its good stability and antioxidation features, SiO 2 has been frequently used in core-shell or yolk-shell structured microwave absorbing materials. [29][30][31][43][44][45][46][47][48] Meanwhile, several research studies have shown that the low permittivity of SiO 2 can reduce the complex permittivity of the composites and thus appropriate impedance matching can be achieved, which allows incident microwaves to enter the absorber as much as possible, and greatly improves the microwave absorption properties of the samples. [43][44][45][46][47][48][49][50][51] For example, Fan et al synthesized RGO-supported core-shell SiO 2 @SiO 2 /carbon hybrid microspheres and their minimum RL was found to be À24.1 dB with a thickness of 2.8 mm and a loading ratio 30 wt%.…”