Rajeev Kumar

In this work, EMI shielding behaviors in the X-band frequency have been investigated for flexible polyvinylidene fluoride (PVDF) composites containing globular- and tubular-shaped carbonaceous nanostructures embedded with mono-metallic (Ni) and bi-metallic (FeNi, CoNi, MnNi) alloy nanoparticles. Pyrolysis was carried out at two different temperatures (800 °C and 1000 °C) to synthesize carbonaceous materials with two different morphologies. Carbon nanotubes (CNTs) are predominantly seen in the samples synthesized at lower temperature (800 °C), whereas carbon globules (CGs) are observed for the samples synthesized at higher temperature (1000 °C). The PVDF-CNT composites show superior microwave shielding behavior than the PVDF-CG composites, which is attributed to the enhanced absorption of the microwave through Ohmic conduction and interfacial polarization loss. The 1-D structure of CNTs provides the required conduction path for the electrons and forms a network to trap the microwave within them via multiple scattering. The microwave absorption behavior of the composites predominantly results from the metallic nature of the embedded nanoparticles, the graphitic layer encapsulating them and the graphitic walls of the CNTs. We further demonstrate the direct correlation of the EMI shielding behavior of the nanocomposites with the morphology of carbonaceous nanomaterials and the conductivity of the embedded metallic nanoparticles.

The corrosion resistance behavior of graphene oxide (GO) sheets-coated carbonyl iron (CI) microspheres (GO/p-CI sample) was investigated and compared with that of bare CI particles. The GO coating on the CI particles was achieved by utilizing 4-aminobenzoic acid as grafting agent. The cyclic voltammetry of the electrode containing this GO/p-CI sample in 1 mol/L KCl solution does not show any oxidation-peak in contrast to that of the bare carbonyl iron (CI) containing electrode. The charge transfer resistance of GO/ p-CI sample was measured to be higher than that of bare CI. The corrosion-potential shifts towards the positive potential direction confirming higher passivity/less corrosive nature of the GO/p-CI sample. Furthermore, the corrosion-current of GO/p-CI sample was lower than that of the bare CI particles. Our results confirm the passivity and excellent corrosion protection behavior of the GO coating on iron structures.

• Cu-doped ZnO (Zn 1-x Cu x O) nano-ceramics were prepared by ball-milling of the CuO and ZnO powders. • Up to 3 at% of Cu could be doped in ZnO via ball milling, beyond which Cu precipitates as CuO during calcination. • The ac conductivity values decrease by Cu doping in ZnO, making it a better dielectric material. • Cu doping in ZnO via ball milling is cost effective and scalable to industrial level. Nanocrystalline Cu doped ZnO (Zn 1-x Cu x O, x = 0, 0.01, 0.02, 0.03 and 0.04) samples were synthesized by high energy ball milling technique (HEBM). The strain developed during ball milling and incorporation of Cu into the Zn-site in ZnO lattice is depicted as broadening of the full width at half maximum (FWHM) of the XRD. The X-ray diffraction peak-widths (FWHM) increases with increase in Cu-concentration. Furthermore, the mechanical impact and the heat produced during ball milling helps in the formation of aggregates. The size of these aggregates was observed to increase with Cu-concentration. Upon calcination, these aggregated structures form particle, resulting in bigger particles for higher concentration of Cu. The XRD results confirm that up to ∼3 at% of Cu can be doped in ZnO lattice, beyond which CuO precipitates. The impedance spectroscopy and the ac-conductivity results confirm the improvement in dielectric properties of ZnO by Cu doping. The decrease in magnitude of Z′ on increase in temperature confirms the negative temperature coefficient of resistance (NTCR) behaviour of the samples. The ac conductivity of ZnO decreases with Cu doping and follows the correlated barrier hopping (CBH) model in the investigated temperature and frequency range.

The structural and magnetic properties of Al substituted yttrium-iron garnet (Y 3 Al x Fe 5-x O 12 , x ¼ 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6 and 1.8) ceramic powders synthesized using solution combustion method were investigated. Post combustion, the samples were calcination at 1045 C for 6 h and subsequently at 1200 C for 6 h to obtain phase-pure garnets. X-ray diffraction (XRD) results confirm the formation of garnets with Ia3d structure. The occupancy of Y 3þ ions in the dodecahedral site and the distribution of Al 3þ and Fe 3þ ions in the tetrahedral and octahedral sites in the bcc structure of the garnet were confirmed by Rietveld refinement of XRD patterns, M€ ossbauer spectroscopy and 57 Fe internal field NMR spectroscopy. For low Al content, Al 3þ ions have preference to occupy tetrahedral (T d) sites than the octahedral (O h) sites. At higher Al content the distribution of Al tends towards a ratio of 3:2 at the tet-rahedral:octahedral site. Increase in Al doping results in the decrease in the lattice parameter due to smaller size of Al 3þ as compared to Fe 3þ ion. All the studied samples show coral-network-like surface morphology. The saturation magnetization (M S) values decrease from ~26.94 emu/g to ~ 0.17 emu/g with increase in Al content from 0.0 to 1.8. Further addition of Al makes the sample paramagnetic at RT. Substitution of non-magnetic Al 3þ reduces the saturation magnetization rapidly due to the decrease in the superexchange interaction in the crystal.

Aluminium substituted cobalt-nickel ferrite nanoparticles were synthesized by citrate gel auto-combustion method followed by annealing at 1000 °C for 1 h in air. Scanning electron micrographs of all the samples show crystalline particles of irregular morphology with a small variation in particle sizes (~ 110–160 nm). From the analysis of the X-ray diffraction results we observed that the unit cell parameter decreases linearly with increase in aluminium concentration due to the smaller ionic radius of the Al 3+ ions substituting the other cations such as Co 2+ , Ni 2+ and Fe 3+ ions in the compounds. The room temperature Mössbauer spectra of the samples show Zeeman split sextet patterns corresponding to the tetrahedral (Th) and octahedral (Oh) interstitial iron (Fe 3+) cations. The observed magnetic hyperfine field (B hf) decreases with increase in Al-concentration due to the distribution of diamagnetic Al 3+ in the environment of 57 Fe probe atoms. The saturation magnetization measured by Vibrating Sample Magnetometer (VSM) shows a similar trend like that of B hf. The distributions of the cations obtained from the Rietveld refinement and Mössbauer spectroscopy results indicate an increase in Fe 3+ (Th)/Fe 3+ (Oh) occupancy-ratio on increasing Al 3+ concentration, and Ni 2+ cations prefer the octahedral site, whereas Co 2+ and Al 3+ ions redistribute themselves in tetrahedral and octahedral sites, in the ratio 2:3.

Manganese ferrite (MnFe 2 O 4) nanoparticles were synthesized by sol-gel auto-combustion method using manganese nitrate and ferric nitrate as precursors and citric acid as a fuel. Scanning electron micrographs show irregularly shaped morphology of the particles. The as-prepared samples were annealed at 400, 500, 600 and 800 °C for 2 h in air. The phase identification and structural characterizations were performed using powder X-ray diffraction technique along with Mössbauer spectroscopy. Magnetization loops and 57 Fe Mössbauer spectra were measured at RT. After annealing the sample at or below $ 500 °C, we observed two different spinel phases corresponding to two different lattice parameters. This is originating due to the partial oxidation of Mn 2+ to Mn 3+. At high annealing temperatures ($ 600 °C or above) the spinel MnFe 2 O 4 phase decomposes into crystalline a-Mn 2 O 3 and a-Fe 2 O 3 phases, and amorphous FeO phase.