Shankar B. Dalavi

FTIR, magnetic and Mössbauer investigations of nano-crystalline FexCo1−x (0.4 ≤ x ≤ 0.8) alloys synthesized via a superhydride reduction route

The present investigation reveals the synthesis of capped nano-crystalline FexCo1−x (0.2 ≤ x ≤ 0.8) alloys via a superhydride reduction route using oleic acid and oleylamine as stabilizing agents. The synthesized nano-particles are stable against oxidation in an air atmosphere (air stable) at room temperature (298 K). The structure–property correlation in FeCo alloys has been achieved using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), magnetic and Mossbauer measurements. A FTIR study indicates the presence of organic content at the surface of the nanoparticles, which helps in the stabilization of the FeCo samples in an air atmosphere. FeCo alloys crystallize in a pure α-phase with the increase in the values of lattice parameters with the increase in Fe content, i.e. 2.836(±4) A, 2.852(±2) A, 2.859(±1) A and 2.868(±1) A for x = 0.2, 0.4, 0.6 and 0.8, respectively. Average crystallite sizes and TEM particle sizes were found to be in the range of ≈23–38 nm and 11–51 nm, respectively. The values of the saturation magnetization (Ms) for FeCo alloys range from 71.1–92.5 emu g−1 for heat treated materials and 93.1–142.2 emu g−1 after applying the corrections for organic wt% at the surface of the materials. The observed values of effective anisotropy constants (Keff) of FexCo1−x alloys from field cooled (FC) and zero field cooled (ZFC) studies (i.e. 1.5 kJ m−3, 4.6 kJ m−3 and 14.3 kJ m−3 for x = 0.4, 0.6 and 0.8, respectively) reveal the contribution from the reduced particle size and surface anisotropy. The maximum value of a hyperfine field for FexCo1−x alloys was found to be 34.9 T for Fe0.6Co0.4 composition and has been interpreted on the basis of enhancement of Fe moments in the disordered crystal lattice.

Synthesis of High Surface Area W2N and Co-W-N Nitrides by Chemical Routes

This study investigates the synthesis of high surface area W2N and Co–W–N nitrides by nitridation of various precursors obtained by chemical routes. For the synthesis of W2N nitride, WO3 precursors were obtained by acidifying Na2WO4·2H2O (acid route) and by thermal decomposition of the tungstate–citrate precursor. The solid-state reactivity, BET surface areas and pore structures of the nitride materials have been investigated in detail. Co–W–N nitride was obtained from CoWO4 synthesized by co-precipitation. W2N and Co–W–N nitrides crystallize in β-W2N structure. The single-point BET surface areas were estimated to be 58, 55 and 60 m2/g for the β-W2N nitride materials synthesized using commercial WO3 and WO3 obtained from acid and citrate precursor, respectively. The maximum surface areas (40 m2/g) are obtained for Co–W–N nitrides synthesized at 700 °C. We have investigated the change in pore volume and pore diameter when the synthesis conditions are changed. The thermogravimetric and differential thermal analysis studies corroborate the fine particle nature of the materials.

Magnetic Properties of Nanostructured Co and Ni Synthesized by Modified NaBH4 Reduction Route

Nanomaterials based on Co and Ni are technologically important because of their potential technological applications in recording media, catalysis, drug delivery systems, and so on. Recent research interests lie on the synthesis of Co and Ni nanomaterials by chemical synthesis, characterizations and studying for their interesting magnetic properties. In this investigation, we have focused on the synthesis of cobalt and nickel nanoparticles (NPs) in aqueous medium at ambient conditions by sodium borohydride reduction route. We have successfully stabilized the nanospheres comprising of Co and Ni by using polyethylene glycol (PEG) as capping agent. The Co and Ni nanomaterials were exhaustively characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and magnetic measurements. The phase purity and crystallite sizes were ascertained by using powder XRD method. Co and Ni NPs crystallize in face centered cubic (fcc) structure with lattice parameters (a) equal to 3.54 Å and 3.52 Å, respectively. The XRD lines were broad and indicate the fine particle nature of the materials. The estimated crystallite sizes were found to be 42 and 29 nm for Co and Ni, respectively. SEM micrograph studies show the particle sizes to be 80 and 70 nm, whereas TEM studies confirm the sizes to be 47 and 65 nm for Co and Ni, respectively. The electron micrograph studies indicate the appearance of agglomerates of the nanoparticles consisting of several crystallites. The specific magnetization versus field characteristics of Co and Ni nanoparticles shows the signature of the size and surface effects. The values of saturation magnetizations are found to be 122 and 47 emu/g, whereas the coercivity values are 111 Oe and 84 Oe for Co and Ni, respectively. In summary, we have synthesized high moment Co and Ni nanostructured materials with reduced coercivities, which may be useful for soft magnetic applications.

Chemical synthesis, characterizations and magnetic properties of nanocrystalline Fe50Co50 alloy

Nanocrystalline Fe50Co50 alloy has been synthesized successfully by chemical reduction route using superhydride as reducing agent and oleic acid and oleylamine as capping agents. Phase purity, crystallite size and lattice parameters of the synthesized NPs are determined by X-ray powder diffraction method. FeCo alloy crystallizes in body centered cubic (bcc) structure having crystallite size equal to 29 nm and lattice parameters equal to 2.8546 A. The size and shape morphologies of the material were studied by SEM analysis. SEM micrograph study shows the average particle size to be 60 nm and indicates the appearance of agglomerates of the nano-particles consisting of several crystallites. The room temperature magnetic hysteresis studies indicate ferromagnetic behavior of the materials. The values of saturation magnetization and coercivity were 65 emu/g and 460 Oe, respectively. Magnetic properties of the material were interpreted on the basis of fine particle magnetism.

Magnetic properties of Ni nanoparticles embedded in silica matrix (KIT-6) synthesized via novel chemical route

Thermally stable Ni nanoparticles have been embedded in mesoporous silica matrix (KIT-6) via novel chemical reduction method by using superhydride as reducing agent. X-ray diffraction (XRD) study confirms that pure and embedded Ni nanoparticles crystallize in face centered cubic (fcc) structure. Crystallite sizes of pure Ni, 4 wt% and 8 wt% Ni in silica were estimated to be 6.0 nm, 10.4 nm and 10.5 nm, respectively. Morphology and dispersion of Ni in silica matrix were studied by scanning electron microscopy (SEM). Magnetic study shows enhancement of magnetic moments of Ni nanoparticles embedded in silica matrix compared with that of pure Ni. The result has been interpreted on the basis of size reduction and magnetic exchange effects. Saturation magnetization values for pure Ni, 4 wt% and 8 wt% Ni in silica were found to be 15.77 emu/g, 5.08 emu/g and 2.00 emu/g whereas coercivity values were 33.72 Oe, 92.47 Oe and 64.70 Oe, respectively. We anticipate that the observed magnetic properties may find application as soft magnetic materials.