Sampat Raj Vadera - Academia.edu (original) (raw)

Papers by Sampat Raj Vadera

Aerospace Materials and Material Technologies, 2016

“Stealth” normally signifies “radar stealth”, but it actually means suppression of all the follow... more “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.

Integrated Ferroelectrics, 2010

A simple aqueous solution-based chemical methodology has been developed for the preparation of si... more A simple aqueous solution-based chemical methodology has been developed for the preparation of single-phase Ni1-xZnxFe2O4 nanopowder, where x = 0.2, 0.35.0.5 and 0.6. The average particle size of the synthesized powders was in the range of 30-50 nm. Precursor powders were ...

Advanced Science Letters, 2010

ABSTRACT Superparamagnetic iron oxide nanoparticles (SPIONs) were synthesized from Fe3+/Fe2+ salt... more ABSTRACT Superparamagnetic iron oxide nanoparticles (SPIONs) were synthesized from Fe3+/Fe2+ salts in the presence of different coating bio-materials such as glycine, gelatin, gum arabic and mixture of gelatin and gum arabic using NaOH as precipitating agent. The average diameter of crystallites for different samples is in the range of 7.8-13.5 nm deducted from X-ray diffraction (XRD). The size and size distribution of superparamagnetic iron oxide nanoparticles are also determined from the measured magnetization curve fitted by magnetic core and non-magnetic shell model based on the Langevin function with a Schulz size distribution. Particle size extracted from magnetization curve is consistent with crystallite size obtained from XRD. Further, non-magnetic shell is of the order of ∼0.7 nm thick. The thickness of biocompatible protective layer is found to be ∼4.5 nm. Fourier transform infrared (FTIR) spectra reveal the binding of coating materials on the surface of SPIONs. Atomic force microscope (AFM) images indicate the formation of colloidal cluster of size ∼120 nm. Coated outer shell of organic materials used in the present study not only provided the stability to the nanoparticles in solution but also can be highly useful for biomedical applications due to their known bio-compatibility.

Journal of Applied Physics, 2022

Note: This paper is part of the Special Topic on Microwave Absorption by Carbon-Based Materials a... more Note: This paper is part of the Special Topic on Microwave Absorption by Carbon-Based Materials and Structures.

International Journal of Green Nanotechnology, 2012

ABSTRACT Core-shell material with hollow cenospheres of low density as the core has been synthesi... more ABSTRACT Core-shell material with hollow cenospheres of low density as the core has been synthesized by depositing a layer of iron oxide shell on the surface of seed hollow cenospheres. The phase structure, morphology, particle size, chemical composition, and magnetic properties of the synthesized material have been characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), and vibrating sample magnetometer (VSM). Results confirm the coating of iron oxide on hollow cenospheres. Electromagnetic characterization of synthesized core-shell material was carried out in X band. Dilectric loss (tanδe) and magnetic loss (tanδm) were found in the range of ∼ 0.2 and 0.05, respectively, over 8–12 GHz. Simulation studies show the effective reflection loss (RL) is more than 10 dB over frequency range 9–10.5 GHz at a thickness of 2.2 mm. The low density iron oxide coated cenospheres have potential applications as microwave absorbing materials.

ABSTRACT A simple method for the preparation of magnetic nanocomposites consisting of magnetic na... more ABSTRACT A simple method for the preparation of magnetic nanocomposites consisting of magnetic nanoparticles (MNPs), polybenzoxazine (PB), linear low density polyethylene (LLDPE) and LLDPE-g-Maleic anhydride (LgM) is described. BaFe12O19 and Ni0.8Zn0.2Fe2O4 were used as MNPs. Composites were prepared by first forming benzoxazine-MNPs composite followed by melt blending with LLDPE and thermal curing of benzoxazine. The composites were characterized by X-Ray diffraction, thermo gravimetric analysis and differential scanning calorimeter, scanning electron microscopy, universal testing machine and vibrating sample magnetometer. Composite consisting of LLDPE, PB and LgM exhibited higher tensile strength (23.81 MPa) than that of pure LLDPE and more elongation at break (6.11%) than pure polybenzoxazine. Incorporation of MNPs in the composites did not affect much in the mechanical properties. Magnetic properties of the composites mostly were dependent on the nature of MNPs and loading level of MNPs in the composite. The composite films exhibited mechanical flexibility as well as magnetic properties.

Journal of Applied Polymer Science, 2013

A simple method for the preparation of magnetic nanocomposites consisting of cobalt ferrite (CF; ... more A simple method for the preparation of magnetic nanocomposites consisting of cobalt ferrite (CF; CoFe2O4) nanoparticles, polybenzoxazine (PB), linear low-density polyethylene (LLDPE), and linear low-density polyethylene-g-maleic anhydride (LgM) is described. The composites were prepared by the formation of benzoxazine (BA)–CF nanopowders followed by melt blending with LLDPE and the thermal curing of BA. The composites were characterized by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, scanning electron microscopy, universal testing machine measurement, and vibrating sample magnetometry. The composites consisting of LLDPE, PB, and LgM (47.5L–47.5PB–5LgM) exhibited a higher tensile strength (23.82 MPa) than pure LLDPE and a greater elongation at break (6.11%) than pure PB. The tensile strength of the composites decreased from 19.92 to 18.55 MPa with increasing CF loading (from 14.25 to 33.25 wt %). The saturation magnetization of the composites cont...

Materials Chemistry and Physics, 2003

A novel wet-chemical precipitation method is optimized for the synthesis of ZnS nanocrystals dope... more A novel wet-chemical precipitation method is optimized for the synthesis of ZnS nanocrystals doped with Cu+ and halogen. The nanoparticles were stabilized by capping with polyvinyl pyrrolidone (PVP). XRD studies show the phase singularity of ZnS particles having zinc-blende (cubic) structure. TEM as well as XRD line broadening indicate that the average crystallite size of undoped samples is similar to2 nm. The effects of change in stoichiometry and doping with Cu+ and halogen on the photoluminescence properties of ZnS nanophosphors have been investigated. Sulfur vacancy (Vs) related emission with peak maximum at 434 nm has been dominant in undoped ZnS nanoparticles. Unlike in the case of microcrystalline ZnS phosphor, incorporation of halogens in nanoparticles did not result V-Zn related self-activated emission. However, emission characteristics of nanophosphors have been changed with Cu+ activation due to energy transfer from vacancy centers to dopant centers. The use of halogen as...

Journal of Optoelectronics and Advanced Materials, 2008

Highly stable and optically transparent colloidal ZnO QDs have been synthesized at room temperatu... more Highly stable and optically transparent colloidal ZnO QDs have been synthesized at room temperature without using any surface capping agent. The as synthesized QDs show phase singularity of ZnO particles having wurzite (hexagonal) structure. TEM as well as AFM studies indicate that the average crystalline size of QDs is ∼ 7nm. Further, high resolution TEM image shows formation of hexagonal shape particles having lattice fringes along (001) plane of wurzite phase. Photoluminescence (PL) studies show bright luminescence with peak maximum at 530nm due to oxygen vacancy centers (V o ) present in QDs. The optical transmission spectrum of colloidal QDs of ZnO shows sharp absorption at 3.48eV which is blue shifted as compared to bulk ZnO (3.36eV) due to the quantum confinement effect. The band gap energy observed in QDs of ZnO is consistent with the energy calculated by using effective mass approximation model of Brus.

Journal of Optoelectronics and Advanced Materials, 2010

Ni 1-x Zn x Fe 2 O 4 (0 ≤ x ≤1) nanopowders (average particle size ∼ 30 nm) have been synthesized... more Ni 1-x Zn x Fe 2 O 4 (0 ≤ x ≤1) nanopowders (average particle size ∼ 30 nm) have been synthesized by using a ethylenediamine tetraacetic acid precursor based method. The synthesized nanopowders were obtained by calcining the precursors in air at 450°C. Microstructures of the nanopowders and samples obtained on sintering have been studied by using Scanning Electron Microscopy. DC electrical resistivity was found to be ∼10 7 Ωcm for the as-synthesized nanopowders and ∼10 6 Ωcm for sintered samples. Room temperature magnetization has also been measured for all compositions and it exhibits a decrease with increasing Zn content in the samples.

Polymer-Plastics Technology and Engineering, 2013

A simple method for the preparation of magnetic nanocomposites consisting of NiFe2O4 nanoparticle... more A simple method for the preparation of magnetic nanocomposites consisting of NiFe2O4 nanoparticles, polybenzoxazine (PB), linear low-density polyethylene (LLDPE) and LLDPE-g- Maleic anhydride (LgM) is described. Composites were prepared by forming benzoxazine- NiFe2O4 nanocomposites followed by melt blending with LLDPE and thermal curing of benzoxazine. The composites were characterized by XRD, DSC and TGA, FT-IR, SEM, UTM and VSM. The saturation magnetization of the composites containing 33.25 wt% NiFe2O4 was 7.72 emu/g and it decreased with decreasing amount of NiFe2O4 in the composite. The composite films exhibited mechanical flexibility as well as magnetic properties.

Journal of Nanoscience and Nanotechnology

Herein, we reports the application of various spinel ferrite nanoparticles, MFe2O4 (M = Co, Ni, C... more 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.

CrystEngComm

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

Applied Physics A

Nanostructured multiferroic BiFeO3 powder has been synthesized using sol–gel route followed by op... more 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.

Progress In Electromagnetics Research M

In this paper, studies on broadband microwave absorption and electromagnetic shielding effectiven... more 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 5 wt% 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.

Journal of the Magnetics Society of Japan

The paper describes the direct synthesis of magnetic nanocomposites of Y'-Fe 2 0) in presence of ... more The paper describes the direct synthesis of magnetic nanocomposites of Y'-Fe 2 0) in presence of varying concentrations of zinc ions. Infrared, Mossbauer and pH measurement studies on these materials reveal the effect of zinc ions on clustering of the iron oxide particles resulting in nanosize particles of critical size to give paramagnetic and/or ferrimagnetic phases.

Catalysis Letters, 2017

Here, synthesis and catalytic activity of a novel nanocatalyst (CuO@mTiO2@CF), consisting of CuO ... more 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.Graphical Abstract

Scientific Reports, 2017

The gel to carbonate precipitate route has been used for the synthesis of Ni 1−x Zn x Fe 2 O 4 (x... more The gel to carbonate precipitate route has been used for the synthesis of Ni 1−x Zn x Fe 2 O 4 (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 0.5 Zn 0.5 Fe 2 O 4-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 0.5 Zn 0.5 Fe 2 O 4 /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 0.5 Zn 0.5 Fe 2 O 4 /NBR composite samples for possible microwave absorption applications. Microwave absorbing materials are important in the present war scenario to reduce the Radar Cross Section (RCS) for strategic airborne objects 1. These materials also important for applications in civil sector viz. communication interference between electronic devices, antenna background clutters etc. Various dielectric, magnetic and carbonaceous materials, such as barium titanate 2 , core-shell materials 3 , carbon black 4 , carbonyl iron 5,6 , carbon nanotubes (CNTs) 7 , iron oxide 8 and ferrites 9,10 etc. are being investigated as the functional filler materials for microwave absorption applications. There are continuous efforts to achieve the enhanced wide band MW absorption properties by converting the materials in different geometries viz. ferrite nanofibers 11 , porous hollow microrods 12 , nested structure 13 etc. Further, among these, ferrite materials provide several advantages such as relatively smaller values of absorber thickness, particularly for lower frequencies in the MW region, tunability of ferromagnetic resonance (FMR) 14 , excellent environmental stability, etc. in comparison to the dielectric materials. Ni 1−x Zn x Fe 2 O 4 inverse spinel ferrite system attracted special attention because of possible tunable electrical 15,16 , magnetic 17,18 , and microwave absorption properties 19-22 by modulating the magnetic interaction at magnetic ion sites. Ni 1−x Zn x Fe 2 O 4 (at x = 0) having the inverse spinel structure of (Fe 3+) tet [Ni 2+ , Fe 3+ ] oct O 4 with cation's antiparallel spin arrangements at tetragonal (A) and orthogonal (B) sites gives rise to the magnetic properties due to the superexchange interaction between A and B sites mediated through O 2− ions. The net magnetization (M T) in Ni-Zn ferrite system is the result of the difference of magnetization at octahedral (M B) and tetrahedral sites (M A) giving rise to the ferrimagnetism. The spin arrangement in Ni-Zn system can be altered by substituting Zn 2+ ions, replacing Fe + 3 ions at the tetragonal sites (Zn x Fe 1−x 3+) tet [Ni 1−x 2+ , Fe 1+x 3+ ] oct O 4. This substitution results in the significant change of magnetic interaction and thus magnetic properties 23,24. The absorption frequencies of these

RSC Adv., 2016

Here, an ‘in situ’ co-precipitation reaction method has been reported for the preparation of CoFe... more Here, an ‘in situ’ co-precipitation reaction method has been reported for the preparation of CoFe2O4–RGO (CF–RGO) nanocomposites.

Journal of the American Ceramic Society, 2016

Tetragonal BaTiO3 bulk samples are prepared using the solid-state route in conjunction with inter... more 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.