Sampat Raj Vadera

Graphitic carbon-coated Ni metal core nanoparticles are synthesized using controlled thermal treatment of Ni-hydroxide in an aniline–formaldehyde copolymer matrix under a N₂ atmosphere. The structural and microstructural studies substantiate the formation of a graphitic shell on the metallic nickel core. The 850 °C heat-treated core–shell structured material is ferromagnetic in nature with high saturation magnetization. The sample with the highest value of magnetic moment is impregnated in a rubber matrix to prepare composite samples with core–shell nanomaterials of different weight fractions. The sample with 70 wt% Ni/graphitic carbon core–shell powder in the elastomeric rubber matrix showed the optimal microwave absorption at a fairly low thickness of ∼1 mm. This is attributed to the optimized combination of the effective impedance matching and interfacial polarization losses. Further increase in the Ni/graphitic carbon filler material resulted in reduced microwave absorption due to the greater mismatch of impedance as compared to the desired free space impedance of ∼377 Ω. Thus, the optimized composite material is of great potential for microwave absorption in the Ku band.

The gel to carbonate precipitate route has been used for the synthesis of Ni_1−xZn_xFe₂O₄ (x = 0, 0.25, 0.5 and 0.75) bulk inverse spinel ferrite powder samples. The optimal zinc (50%) substitution has shown the maximum saturation magnetic moment and resulted into the maximum magnetic loss tangent (tanδ_m) > −1.2 over the entire 2–10 GHz frequency range with an optimum value ~−1.75 at 6 GHz. Ni_0.5Zn_0.5Fe₂O₄- Acrylo-Nitrile Butadiene Rubber (NBR) composite samples are prepared at different weight percentage (wt%) of ferrite loading fractions in rubber for microwave absorption evaluation. The 80 wt% loaded Ni_0.5Zn_0.5Fe₂O₄/NBR composite (FMAR80) sample has shown two reflection loss (RL) peaks at 5 and 10 GHz. Interestingly, a single peak at 10 GHz for 3.25 mm thickness, can be scaled down to 5 GHz by increasing the thickness up to 4.6 mm. The onset of such twin matching frequencies in FMAR80 composite sample is attributed to the spin resonance relaxation at ~5 GHz (f_m1) and destructive interference at λ_m/4 matched thickness near ~10 GHz (f_m2) in these composite systems. These studies suggest the potential of tuning the twin frequencies in Ni_0.5Zn_0.5Fe₂O₄/NBR composite samples for possible microwave absorption applications.

Here, an ‘ in situ ’ co-precipitation reaction method has been reported for the preparation of CoFe ₂ O ₄ –RGO (CF–RGO) nanocomposites.

Tetragonal BaTiO ₃ bulk samples are prepared using the solid‐state route in conjunction with intermediate high‐temperature annealing steps. The (002) and (200) X‐ray diffraction peaks near 2Ɵ~45° and 310, 520, and 720 cm ⁻¹ characteristic vibrational modes in Raman spectroscopic measurements confirm the tetragonal crystallographic structure of Ba TIO ₃ bulk samples. The 1100°C annealed BaTiO ₃ sample showed optimal tetragonality ~1.016 and the same is used for BaTiO ₃ –acrylonitrile butadiene rubber ( NBR ) composites at different BaTiO ₃ loading fractions in parts per hundred ( PHR ). These BaTiO ₃ / NBR composite systems exhibit dual band microwave resonance, widening the operating window for microwave absorption applications. Eighty PHR BaTiO ₃ / NBR composite exhibits microwave reflection losses ( RL ) at 9.5 and 16.5 GH z with ~−9 and ~−18 dB reflection losses, respectively. The onset of dual band is attributed to the ferroelectric‐induced dipolar relaxation at 9.5 GH z and its second‐order resonance at 16.5 GH z in such composite systems.

A Ni _0.8 Zn _0.2 Fe ₂ O ₄ reduced graphene oxide nanocomposite has been synthesized by a simple ‘ in situ co-precipitation’ technique.

In this paper, we report a simple but novel aqueous solution based ‘one-pot’ method for preparation of (NiFe₂O₄) _x –(SrFe₁₂O₁₉)_1−x nanocomposites consist of hard ferrite-soft ferrite phases. A physical mixing method has also been employed to prepare nanocomposites having same compositions. The effects of synthetic methodologies on the microstructures of the nanocomposites as well as their magnetic and microwave absorption properties have been evaluated. Crystal structures and microstructures of these composites have been investigated by using X-ray diffraction, transmission electron microscope and scanning electron microscope. In the nanocomposites, prepared by both methods, presence of nanocrystalline NiFe₂O₄ and SrFe₁₂O₁₉ phases was detected. However, nanocomposites, prepared by one-pot method, possessed better homogeneous distribution of hard and soft ferrite phases than the nanocomposites, prepared by physical mixing method. Nanocomposites, prepared by one-pot method, demonstrated significant spring exchange coupling interaction between hard and soft ferrite phases and exhibited magnetically single phase behaviour. The spring exchange coupling interaction enhanced the magnetic properties (high saturation magnetization and coercivity) and microwave absorption properties of the nanocomposites, prepared by one-pot method, in comparison with the nanocomposites prepared by physical mixing method as well as pure NiFe₂O₄ and SrFe₁₂O₁₉ nanoparticles. Minimum reflection loss of the composite was ∼−17 dB (i.e., 98% absorption) at 8.2 GHz for an absorber thickness of 3.2 mm.