Sanjay Kashyap

Synthesis of pure iron magnetic nanoparticles in large quantity

Free nanoparticles of iron (Fe) and their colloids with high saturation magnetization are in demand for medical and microfluidic applications. However, the oxide layer that forms during processing has made such synthesis a formidable challenge. Lowering the synthesis temperature decreases rate of oxidation and hence provides a new way of producing pure metallic nanoparticles prone to oxidation in bulk amount (large quantity). In this paper we have proposed a methodology that is designed with the knowledge of thermodynamic imperatives of oxidation to obtain almost oxygen-free iron nanoparticles, with or without any organic capping by controlled milling at low temperatures in a specially designed high-energy ball mill with the possibility of bulk production. The particles can be ultrasonicated to produce colloids and can be bio-capped to produce transparent solution. The magnetic properties of these nanoparticles confirm their superiority for possible biomedical and other applications.

Effect of Ni on growth kinetics, microstructural evolution and crystal structure in the Cu(Ni)–Sn system

Abstract The role of Ni addition in Cu on the growth of intermetallic compounds in the Cu–Sn system is studied based on microstructure, crystal structure and quantitative diffusion analysis. The diffraction pattern analysis of intermetallic compounds indicates that the presence of Ni does not change their crystal structure. However, it strongly affects the microstructural evolution and diffusion rates of components. The growth rate of (Cu,Ni)3Sn decreases without changing the diffusion coefficient because of the increase in growth rate of (Cu,Ni)6Sn5. For 3 at.% or higher Ni addition in Cu, only the (Cu,Ni)6Sn5 phase grows in the interdiffusion zone. The elongated grains of (Cu,Ni)6Sn5 are found when it is grown from (Cu,Ni)3Sn. This indicates that the newly formed intermetallic compound joins with the existing grains of the phase. On the other hand, smaller grains are found when this phase grows directly from Cu in the absence of (Cu,Ni)3Sn indicating the ease of repeated nucleation. Grain size of (Cu,Ni)6Sn5 decreases with further increase in Ni content, which indicates a further reduction of activation barrier for nucleation. The relations for the estimation of relevant diffusion parameters are established considering the diffusion mechanism in the Cu(Ni)–Sn system, which is otherwise impossible in the phases with narrow homogeneity range in a ternary system. The flux of Sn increases, whereas the flux of Cu decreases drastically with the addition of very small amount of Ni, such as 0.5 at.% Ni, in Cu. Analysis of the atomic mechanism of diffusion indicates the contribution from both lattice and grain boundary for the growth of (Cu,Ni)6Sn5 phase.

Densification mechanisms during reactive spark plasma sintering of Titanium diboride and Zirconium diboride

Abstract In this study, dense fine-grained ZrB2 and TiB2 were fabricated using reactive spark plasma sintering (RSPS) of ball-milled Zr/B and Ti/B mixtures. Systematic investigations were carried out to understand the mechanisms of reactive sintering. Two densification mechanisms were found to be operating during RSPS. The first stage of densification was due to self-propagating high temperature synthesis reaction leading to formation of ZrB2 and TiB2 compacts having relative density of ~48 and ~65%, respectively. The second stage of densification occurred at temperatures more than 1100 °C and resulted in final relative density of more than 98%. Electron backscatter diffraction and electron microscopy studies on interrupted RSPS samples as well as dense samples showed deformed grains and presence of slip steps while grain orientation spread map and pole figure analysis confirmed plastic flow. Plastic flow-aided pore closure is shown as major mechanism during reactive sintering.

Effect of indium addition on microstructural, mechanical and oxidation properties of suction cast Nb–Si eutectic alloy

Abstract The paper reports effect of small ternary addition of In on the microstructure, mechanical property and oxidation behaviour of a near eutectic suction cast Nb–19·1 at-%Si–1·5 at-%In alloy. The observed microstructure consists of a combination of two kinds of lamellar structure. They are metal–intermetallic combinations of Nbss–β-Nb5Si3 and Nbss–α-Nb5Si3 respectively having 40–60 nm lamellar spacings. The alloy gives compressive strength of 3 GPa and engineering strain of ∼3% at room temperature. The composite structure also exhibits a large improvement in oxidation resistance at high temperature (1000°C).

PREPARATION AND CHARACTERIZATION OF INDIUM OXIDE AND INDIUM TIN OXIDE FILMS BY ACTIVATED REACTIVE EVAPORATION

Transparent and conducting oxide films find many applications because of their excellent properties such as high optical transparency, low surface resistance, high infrared reflectance, etc. Realization of these properties depend upon the choice of the deposition technique and the control of deposition parameters. In this paper, we report the preparation of highly transparent and conducting films of indium oxide (In2O3) and indium tin oxide (ITO) by activated reactive evaporation on glass substrates. These films were deposited by evaporating pure indium and 90% In + 10% Sn alloy using an electron gun in the presence of oxygen ions at ambient temperature. Films of different thickness have been prepared and their optical, electrical and structural properties are studied. In2O3 films showed higher transparency (90%) compared to ITO films (85%) but the electrical resistivity was observed to be little higher (2.5 × 10-3 Ω cm) compared to ITO films (6 × 10-4 Ωcm). Hall measurements on aged ITO films gave the charge density of 3 × 1020 per cm3 and mobility 35.6 cm2/V-s. The refractive index and extinction coefficient were found to be around 2.0 and 0.005 for ITO films and 2.10 and 0.001 for In2O3 films at 550 nm respectively. ITO and In2O3 films were amorphous in nature for lesser thickness, but for thicker films, the partial crystallinity was observed.

Solid–state diffusion–controlled growth of the phases in the Au–Sn system

Abstract The solid state diffusion-controlled growth of the phases is studied for the Au–Sn system in the range of room temperature to 200 °C using bulk and electroplated diffusion couples. The number of product phases in the interdiffusion zone decreases with the decrease in annealing temperature. These phases grow with significantly high rates even at the room temperature. The growth rate of the AuSn4 phase is observed to be higher in the case of electroplated diffusion couple because of the relatively small grains and hence high contribution of the grain boundary diffusion when compared to the bulk diffusion couple. The diffraction pattern analysis indicates the same equilibrium crystal structure of the phases in these two types of diffusion couples. The analysis in the AuSn4 phase relating the estimated tracer diffusion coefficients with grain size, crystal structure, the homologous temperature of experiments and the concept of the sublattice diffusion mechanism in the intermetallic compounds indicate that Au diffuses mainly via the grain boundaries, whereas Sn diffuses via both the grain boundaries and the lattice.

Magnetic iron nanoparticles for in vivo targeted delivery and as biocompatible contrast agents

Iron nanoparticles (NPs) of size less than 20 nm were synthesized using an in-house developed cryomill. These NPs exhibit values of saturation magnetisation (∼180 emu g−1) close to that of pure iron. The particles were found to be nontoxic at concentrations required for MRI imaging as indicated by MTT assay. In vivo studies demonstrated the suitability of using these particles as contrast agents for MRI. The iron NPs were bio-capped with TRITC–dextran and injected into mice to study the transport behavior of the NPs under the influence of an external magnetic field. The iron NPs showed enhanced aggregation and contrast when a bar magnet was placed on the mice as observed by whole body fluorescence imaging.

Antioxidant efficacy of chitosan/graphene functionalized superparamagnetic iron oxide nanoparticles

The antioxidant potential of superparamagnetic iron oxide nanoparticles functionalized with chitosan and graphene were examined in the present work. Coprecipitation technique was followed for the synthesis of iron oxide nanoparticles. Graphene-iron oxide nanocomposites were synthesized by mechanical mixing followed by the heat treatment at moderate temperature. The chitosan coated iron oxide nanoparticles were prepared by dispersing nanoparticles in chitosan solution. The nanoparticles/nanocomposites were characterized using XRD, SEM, TEM and HAADF-STEM for phase structure, morphology and elemental analysis. The superparamagnetic behavior of nanoparticles/nanocomposites were confirmed by magnetic measurements using vibrating sample magnetometry. Antioxidant efficacy of these nanoparticles/nanocomposites were investigated in terms of free radical scavenging and reducing potential using an array of in vitro assay system. Ferric reducing antioxidant power (FRAP) and 2,2′-diphenyl-1-picrylhydrazyl (DPPH) were used for the antioxidant capacity. The investigation suggests that the graphene improves the antiradical response of iron oxide nanoparticles at higher concentration which is almost comparable to the ascorbic acid used as standard.

Metal Immiscibility Route to Synthesis of Ultrathin Carbides, Borides, and Nitrides

Ultrathin ceramic coatings are of high interest as protective coatings from aviation to biomedical applications. Here, a generic approach of making scalable ultrathin transition metal-carbide/boride/nitride using immiscibility of two metals is demonstrated. Ultrathin tantalum carbide, nitride, and boride are grown using chemical vapor deposition by heating a tantalum-copper bilayer with corresponding precursor (C2 H2 , B powder, and NH3 ). The ultrathin crystals are found on the copper surface (opposite of the metal-metal junction). A detailed microscopy analysis followed by density functional theory based calculation demonstrates the migration mechanism, where Ta atoms prefer to stay in clusters in the Cu matrix. These ultrathin materials have good interface attachment with Cu, improving the scratch resistance and oxidation resistance of Cu. This metal-metal immiscibility system can be extended to other metals to synthesize metal carbide, boride, and nitride coatings.