In this work, Mn2+ substituted Sr-Ca-Al based hexaferrite nanoparticles with the nominal composition Ca0.2Sr0.8Fe10MnyAl2-yO19 (0 <= y <= 2) were synthesized by the sol-gel auto-combustion method. The impact of partial substitution of non magnetic Al3+ by magnetic Mn2+ on structural, magnetic and electrical properties were investigated. XRD confirmed the formation of phase-pure hexaferrite for low Mn content (y <= 1), with minor traces of secondary hematite phases appearing at higher Mn content (y = 1.5 and 2). FESEM revealed noticeable decrease in average particle size from 114.1 nm for y = 0 to 88 nm for y = 1 followed by an increase to 135 nm at y = 2 along with a morphological transition from platelet-like to irregular shaped particles. XPS analysis identified the oxidation states of Fe, Al, and Mn in the compositions indicating partial oxidation of Mn2+ to Mn3+ in the crystal beyond y > 1 to preserve the charge neutrality. Raman spectroscopy together with XRD and XPS analysis revealed the preferential cation site occupancy within the lattice due to substitution. Electrical conductivity of the samples increased with Mn substitution, which suggested an increased electron hopping further corroborating the structural analysis. Magnetic measurements showed that Ca0.2Sr0.8Fe10Mn1Al1O19 (y = 1) exhibits optimized magnetic properties with enhanced coercivity of 9.13 kOe compared to pure strontium hexaferrite and saturation magnetization of 46.38 emug(-1) accompanied by highest effective anisotropy constant of 2.57 & times; 10(5) Jm(-3). Tailoring of magnetic properties through structural and morphological modifications demonstrated a direct correlation between structure and functional properties of Mn substituted M-type Ca-Al-Sr hexaferrite, which is promising as a rare-earth free permanent magnet material.

In the present study, to the best of our knowledge, the microwave (MW) absorption properties of Ti3AlC2 MAX phase-based ceramic composites are reported for the first time to reduce the Radar Cross Section (RCS) of high temperature zones of airborne platforms. Ti3AlC2 powder was synthesized using a mechanically activated self-propagating high-temperature synthesis (MASHS) process while optimizing the precursor ratio to attain phase pure material as confirmed by X-ray diffraction (XRD) studies. The layered morphology of MAX phase grains is confirmed by Field Emission Scanning Electron microscope (FE-SEM), while estimating the elemental concentration. Raman spectroscopic analysis further confirms the formation of Ti3AlC2 MAX phase. The DSC/TGA investigation of Ti3AlC2 powder predicts the excellent thermal stability of the material system up to 1100 degrees C, which makes it a potential candidate for high temperature applications. The Ti3AlC2 based ceramic composites were prepared by impregnation of functional powder in an Alumina matrix with varying weight percentages. The Electromagnetic (EM) studies and Reflection Loss (RL) estimation suggest that 55 wt% Ti3AlC2 filled ceramic composite exhibits significant microwave absorption properties, achieving a RL of -44.16 dB at 10.86 GHz with a minimal thickness of 1.7 mm. Furthermore, a broad absorption bandwidth of similar to 2 GHz (9-11 GHz) with RL values <= -10 dB was achieved in the reported ceramic composite system. These results underscore the effectiveness of Ti3AlC2 based ceramic composites for stealth treatment of high temperature zones (similar to 1000 degrees C) of airborne platforms.

This work reports a simple yet unique way of tailoring the morphology of M-type Strontium Hexaferrite, SrFe12O19 (SFO) particles inside the nanofibers. The reported materials were synthesized by the polymer-sol-assisted electrospinning technique followed by calcination. Calcination temperature and rate of heating play significant role in controlling the morphology of SFO nanofibers in terms of shapes, size and distribution thereby having a direct bearing on the resultant magnetic properties. The synthesized materials were investigated using X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), High Resolution Transmission Electron Microscopy (HRTEM), Thermogravimetric-Differential Thermal Analysis (TG-DTA), and Magnetic Property Measurement System (MPMS). The XRD analysis shows formation of almost phase pure SFO with very minute quantities of secondary phase as hematite. The FESEM images show wide range of morphologies of SFO nanofibers and specifically the nanoparticles present within the nanofibers (from irregular to platelet-like shape) thereby demonstrating the tailoring of morphologies of not only of nanofibers but nanoparticles within the nanofibers simply through variations of two parameters viz. calcination temperature and rate of heating. The TG-DTA results show that complete decomposition of PVP and formation of the SFO crystallization takes place at a much lower temperature of 422 oC. The room temperature hysteresis measurement showed that the sample calcined at 800 oC at heating rate of 10 oC/min has shown enhanced magnetic properties (Ms, Mr, and Hc) than the samples calcined at 600 oC and 950 oC. The HRTEM studies show that the variation in heating rate also yields interesting variations in morphology of nanoparticles and their arrangement within nanofibers. This research work has resulted in a unique pathway of synthesizing nanofibers with controlled and variable morphology.

During the strategic military operations, the deployment of objects viz. tanks, rescue shelters, bunkers etc. need to masquerade from enemy sensors. The deployment of these objects with adaptive photochromic coatings with fast coloration & reversibility is highly effective way for their camouflaging. The present paper systematically reports the photochromic properties of W6+ transition metal doped TiO2 nanoparticles. Undoped as well as doped TiO2 nanoparticles have been synthesized by a one pot wet chemical method. The synthesized materials have been characterized by different techniques viz. XRD, AFM, Raman, SEM, EPR and UV–Vis spectroscopy. Photochromic behavior of functional material was studied by making its composite coatings in alkyd resin matrix. The effect of W6+ dopant ion concentration on photochromic properties has been explained. The coloration & reversibility characteristics of W6+: TiO2 is found superior as compared with V5+ & Mo6+ doped TiO2 nanomaterials, reported earlier.

A systematic study of the effects of the binder matrix on the microwave absorption properties of functional carbon nanoparticle-impregnated rubber composites is reported herein. Flexible rubber composites were fabricated by loading of conductive carbon black (CB) in three different elastomeric binder host matrices: silicone rubber, acrylonitrile butadiene rubber (NBR) and ethylene propylene diene monomer (EPDM) rubber. The DC conductivity, morphology and complex dielectric permittivity were investigated using the four-point probe contact method, scanning electron microscopy and vector network analyzer instrumentation, respectively. The percolation threshold was found to be lowest for CB-silicone rubber composites, i.e. 6 wt.

Silicone rubber based lightweight composites have been prepared using nano carbon black (CB) as fillers to achieve broadband microwave absorption properties and electromagnetic interference (EMI) shielding over the 8–18 GHz frequency range. The effect of the filler material characteristics on percolation threshold and resultant electromagnetic (EM) properties of composites has been investigated. The composites are prepared by loading different filler fractions (wt. %) of three nano size CB having different characteristics, viz., CB1 (∼5–10 nm), CB2 (∼15–20 nm), and CB3 (∼30–40 nm) in a silicone rubber matrix. The volume resistivity measurements suggest low value of percolation threshold, i.e., 2 wt. % for CB1-rubber composite, 3 wt. % for CB2-rubber composites, as compared to 15 wt. % for CB3-rubber composite. Filler concentration dependent EM properties, i.e., dielectric constant and dielectric loss tangent (tan δe), are evaluated for all the CB-rubber composites. Furthermore, calculated reflection loss (RL) values for these composites indicate that 3 wt. % CB1-rubber, 5 wt. % CB2-rubber, and 18 wt. % CB3-rubber composites can provide more than 90% microwave absorption (RL > −10 dB) in X (8–12 GHz) and Ku (12–18 GHz) bands with thickness ∼2.7 and ∼1.9 mm, respectively. Interestingly, the composites with higher loading (15 wt. %) of CB1 in the rubber matrix are found to give EMI shielding effectiveness values of ∼ 42 dB over the 8–18 GHz frequency range. In conclusion, CB1-silicone rubber composites have been found to give the best performance among the three studied composites. This composite provides >90% microwave absorption over the X band with 2.7 mm thickness and over the Ku band with thickness 1.9 mm with lowest concentration of fillers, i.e., 3 wt. % CB1 in the rubber matrix, and hence found potential for development of lightweight microwave absorbers for stealth applications.

Herein, we reports the application of various spinel ferrite nanoparticles, MFe2O4 (M = Co, Ni, Cu, Zn), as efficient catalyst for Biginelli reaction. All ferrite nanoparticles were synthesized using a novel aqueous solution based method. It was observed that, the catalytic activity of the ferrite nanoparticles followed the decreasing order of CoFe2O4 > CuFe2O4 > NiFe2O4 > ZnFe2O4. The most important feature of these ferrite nanocatalysts is that, these nanoparticles can directly be used as catalyst and no surface modification or functionalization is required. These ferrite nanoparticles are easily separable from reaction mixture after reaction by using a magnet externally. Easy synthesis methodology, high catalytic activity, easy magnetic separation and good reusability make these ferrite nanoparticles attractive catalysts for Biginelli reaction.

Graphitic carbon-coated Ni metal core nanoparticles with respective reflection loss for different thickness of Ni/C and rubber composites.

Here, we report a facile hydrothermal synthesis method to prepare BiFeO3 nanowire-reduced graphene oxide (BFO-RGO) nanocomposites. The unique properties of 2-D reduced graphene oxide (RGO) and 1-D BiFeO3 nanowires (BFO) were exploited to design nanocomposites to obtain high performing microwave absorber materials. The composite with 97 wt % BFO and 3 wt % RGO exhibited minimum reflection loss value of -28.68 dB at 10.68 GHz along with the effective absorption bandwidth (>= -10 dB) ranging from 9.6 to 11.7 GHz when the absorber thickness was only 1.55 mm. First-principles calculations based on density functional theory (DFT) of BFO, graphene, and BFO-RGO nanocomposites were performed to obtain information about their electronic structures to interpret their complex permittivity and its derived properties. To the best of our knowledge, this is the first time investigations on microwave absorption properties of the BiFeO3 nanowire and BFO-RGO nanocomposites have potential to be used as a lightweight, high performing microwave absorber in been reported, and this nanocomposite shows its the X-band region.

Here, synthesis and catalytic activity of a novel nanocatalyst (CuO@mTiO2@CF), consisting of CuO nanoparticles, mesoporous titanium oxide and Cobalt ferrite have been reported for the first time. The catalyst was synthesized using a simple aqueous solution based chemical methodology. Synthesized CuO@mTiO2@CF showed excellent catalytic activity towards various organic reactions such as (i) Epoxidation of styrene, (ii) Click reaction, (iii) Biginelli reaction, (iv) Reduction of 4-Nitrophenol and trifluralin in presence of excess NaBH4. Moreover, this novel nanocatalyst offered easy magnetic separation after the catalysis reaction and excellent reusability. Easy synthesis methodology, versatility, good reusability and easy separation make the nanocatalyst attractive in the field of heterogeneous catalysis.

The gel to carbonate precipitate route has been used for the synthesis of Ni1−xZnxFe2O4 (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 (tanδm) > −1.2 over the entire 2–10 GHz frequency range with an optimum value ~−1.75 at 6 GHz. Ni0.5Zn0.5Fe2O4- 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 Ni0.5Zn0.5Fe2O4/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 (fm1) and destructive interference at λm/4 matched thickness near ~10 GHz (fm2) in these composite systems. These studies suggest the potential of tuning the twin frequencies in Ni0.5Zn0.5Fe2O4/NBR composite samples for possible microwave absorption applications.

"Stealth" normally signifies "radar stealth", but it actually means suppression of all the following signatures: visual, radar, infrared, electromagnetic and sound. After a brief historical introduction, this chapter summarizes the basic stealth requirements for military assets, particularly airborne systems. Special sections are devoted to radar-absorbing materials and structures, plasma stealth (a means of active stealth), acoustic stealth and counter stealth.

Nanostructured multiferroic BiFeO3 powder has been synthesized using sol–gel route followed by optimized post-annealing treatment. The phase pure rhombohedral structure of prepared powder was confirmed by X-ray diffraction and Fourier transform infrared studies. The room temperature weak ferromagnetic nature (~ 0.15 emu/g) exhibited by the nanocrystalline BiFeO3 sample (~ 50 nm) is attributed to the canted spin ordering in the sample. The BiFeO3/NBR rubber composites, with 50–80 wt% filler loading fractions, show the dual band resonating microwave (MW) absorption behavior. The reflection loss (R.L.) values enhanced and required absorber thickness reduced simultaneously with increasing BiFeO3 loading fraction in composite samples. These results confirm that the ferroelectric properties of multiferroic BiFeO3 are contributing significantly for the observed MW absorption with respect to the magnetic contribution.

In this paper, studies on broadband microwave absorption and electromagnetic shielding effectiveness are reported in flexible rubber composites with low filler content of nanosize conducting carbon over 8–18 GHz frequency range of electromagnetic spectrum. Rubber based composites are prepared by loading of 1–15 wt% nanosize conducting Carbon Black (CB) in silicone rubber matrix. Effect of percentage loading of nanosize CB on DC conductivity, dielectric & microwave absorption properties and electromagnetic Shielding Effectiveness (SE) of silicone rubber composites is studied. The percolation threshold is achieved at low concentration (3 wt%) of CB in composites. The observed complex permittivity values revealed that composites with concentration of 5wt% CB can provide more than 90% microwave absorption (Reflection Loss > −10 dB) over 8–18 GHz at composite thickness of 1.9–2.7 mm. Further, composites with concentration of 15 wt% of CB shows −40 dB SE over the broad frequency range 8–18 GHz at thickness 2.8 mm. The effect of composite thickness on microwave absorption properties and shielding effectiveness is also analyzed. Thus, the prepared rubber composites with suitable concentration of nanosize CB as filler may be used as microwave absorber in stealth applications as well as for EMI shielding of electronic equipments in various civilian and military areas.

Here, an 'in situ' co-precipitation reaction method has been reported for the preparation of CoFe2O4-RGO (CF-RGO) nanocomposites. To the best of our knowledge, this is the first time a simple synthetic method is reported for the preparation of CoFe2O4-RGO nanocomposites where a hydrothermal technique was not used. The novelty of this technique lies in its simplicity, cost-effectiveness, and the capability of large scale production of CoFe2O4-RGO nanocomposites. The synthesized CoFe2O4-RGO nanocomposites possess excellent microwave absorbing properties as well as high photocatalytic activity towards the degradation of various dyes under visible light irradiation. 85CF-15RGO (85 wt% CF and 15 wt% RGO) showed excellent microwave absorption properties with a Reflection Loss (RL) of similar to 31.31 dB (similar to 99.94% absorption) at 9.05 GHz with an 8.2-10.92 GHz effective band width range. To the best of our knowledge 85CF-15RGO nanocomposite exhibited comparable and even superior microwave absorption properties in the X-band region than most of the ferrite based composites. 75CF-25RGO (75 wt% CF and 25 wt% RGO) acted as a very good magnetically separable photocatalyst for the degradation of various synthetic dyes (such as methyl orange, methylene blue, rhodamine B and a mixture of these dyes) under visible light irradiation emitted from a 100 W reading lamp. Moreover, CoFe2O4-RGO catalyst also showed easy magnetic separation with high reusability. The photocatalytic activity of 75CF-25RGO was found to be comparable and in some cases better than the various reported RGO-ferrite composites. The simple method of preparation and multifunctional character make CF-RGO nanocomposites attractive materials for application in the area of photocatalysis as well as microwave absorption.

A Ni0.8Zn0.2Fe2O4 reduced graphene oxide nanocomposite has been synthesized by a simple ‘in situ co-precipitation’ technique.

Tetragonal BaTiO3 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 BaTIO3 bulk samples. The 1100°C annealed BaTiO3 sample showed optimal tetragonality ~1.016 and the same is used for BaTiO3–acrylonitrile butadiene rubber (NBR) composites at different BaTiO3 loading fractions in parts per hundred (PHR). These BaTiO3/NBR composite systems exhibit dual band microwave resonance, widening the operating window for microwave absorption applications. Eighty PHR BaTiO3/NBR composite exhibits microwave reflection losses (RL) at 9.5 and 16.5 GHz with ~−9 and ~−18 dB reflection losses, respectively. The onset of dual band is attributed to the ferroelectric‐induced dipolar relaxation at 9.5 GHz and its second‐order resonance at 16.5 GHz in such composite systems.

CoFe2O4 nanoparticles, synthesized via a co-precipitation method at 120 °C, exhibited excellent microwave absorption properties, with minimum reflection loss of −55 dB (∼99.99%) at 9.25 GHz.