Girija S. Chaubey

Zinc ferrite nanoparticles as MRI contrast agents

Mixed spinel hydrophobic ZnxFe1-xO x Fe2O3 (up to x = 0.34) nanoparticles encapsulated in polymeric micelles exhibited increased T2 relaxivity and sensitivity of detection over clinically used Feridex.

High thermal stability of carbon-coated L10-FePt nanoparticles prepared by salt-matrix annealing

Department of Physics, University of Texas at Arlington. Ames Laboratory and Department of Materials Science and Engineering, Iowa State University. NanoTech Institute, University of Texas at Dallas.

Shape control of FePt nanocrystals

Preceedings of the 53rd Annual Conference on Magnetism and Magnetic Materials.Hard Magnetic Materials

Fabrication of nanopeapods: scrolling of niobate nanosheets for magnetic nanoparticle chain encapsulation.

Scrolling of niobate nanosheets (NSs) in the presence of magnetic nanoparticle (NP) chains can lead to peapodlike structures. Surface functional groups on both the NSs and NPs are important in directing the assembly and subsequent NS convolution. The dimensions of the peapods are typically dictated by the diameters of the NPs and the length of the NP chains.

Structural phase transition and ferromagnetism in monodisperse 3 nm FePt particles

Department of Physics, The University of Texas at Arlington. Center for Materials Research and Analysis, and Department of Physics and Astronomy, University of Nebraska. Ames Laboratory and Department of Materials Science and Engineering, Iowa State University

Synthesis and piezoelectric response of cubic and spherical LiNbO3 nanocrystals

Methods have been developed for the shape-selective synthesis of ferroelectric LiNbO3 nanoparticles. Decomposition of the single-source precursor, LiNb(O-Et)6, in the absence of surfactants, can reproducibly lead to either cube- or sphere-like nanoparticles. X-Ray diffraction shows that the LiNbO3 nanoparticles are rhombohedral (R3c). Sample properties were examined by piezoresponse force microscopy (PFM) and Raman where both sets of nanoparticles exhibit ferroelectricity. The longitudinal piezoelectric coefficients, d33, varied with shape where the largest value was exhibited in the nanocubes (17 pm V−1 for the cubes versus 12 pm V−1 for spheres).

Bimagnetic nanoparticles with enhanced exchange coupling and energy products

Bimagnetic FePt/Fe3O4 nanoparticles with core/shell or heterodimer structure have been prepared using a sequential synthetic method. The dimension of both FePt and Fe3O4 was tuned by varying the synthesis parameters. The as-synthesized bimagnetic nanoparticles were superparamagnetic at room temperature. After being annealed in a reducing atmosphere, the FePt/Fe3O4 bimagnetic nanoparticles were converted to a hard magnetic nanocomposite with enhanced energy products due to the exchange coupling between the hard and soft magnetic phases. It was found that the exchange coupling in nanocomposites made from the core/shell nanoparticles is stronger than that from the heterodimer nanoparticles. By tuning the dimensions of the FePt and Fe3O4 phases, the energy product up to 17.8 MGOe was achieved in the annealed nanocomposites, which is 36% higher than the isotropic single-phase FePt counterpart.

Synthesis and characterization of FeCo nanowires with high coercivity

Ferromagnetic FeCo nanocrystals with high coercivity have been synthesized using a reductive decomposition method. The sizes and shapes of the nanocrystals were found to be dependent on reaction parameters such as the surfactant ratio, the precursor concentration and the heating rate. Synthesized nanocrystals have a body-centered cubic crystal structure for both particles and nanowires and the (110) crystalline direction is along the long axis of the nanowires. The coercivity and magnetization of the FeCo nanocrystals are found to be dependent on morphology. Nanowires of Fe60Co40 with saturation magnetization of 92 emu g(-1) and coercive force of 1.2 kOe have been obtained in this study.

Synthesis of monodisperse FeCo nanoparticles by reductive salt-matrix annealing

We report here a novel synthetic method to prepare monodisperse air-stable FeCo nanoparticles with average sizes of 8, 12 and 20 nm. CoFe2O4 nanoparticles of different sizes were first synthesized by a chemical solution method. The as-synthesized CoFe2O4 nanoparticles were then mixed with ball-milled NaCl powders and heated to 400-500 ° C in forming gas (Ar 93%+H2 7%). The salt powder worked as a separating medium that prevents the CoFe2O4 nanoparticles from agglomerating during the heat treatment while the forming gas reduces the CoFe2O4 nanoparticles to FeCo nanoparticles. Monodisperse FeCo nanoparticles were recovered by dissolving the NaCl in water and subsequently washing with ethanol and acetone. Structural analyses confirmed that FeCo nanoparticles retained the same size as their oxide precursors. The size of the FeCo nanoparticles can be well tuned by controlling the size of the CoFe2O4 nanoparticles. The saturation magnetization of FeCo nanoparticles is size dependent and increases with size.

Microstructural and thermal investigations of HfO2 nanoparticles

Monodispersed HfO2 nanoparticles can be readily prepared at room temperature by the ammonia catalyzed hydrolysis and condensation of hafnium(IV) tert-butoxide in the presence of a surfactant. The nanoparticles are faceted with an average diameter of about 4 nm. The as-synthesized amorphous nanoparticles crystallize upon post-synthesis heat treatment. The crystallization temperature of the nanoparticles can be controlled by adjusting the annealing atmosphere. The HfO2 nanoparticles have a narrow size distribution, large specific surface area and the thermal conductivity of pressed pellets is drastically reduced compared to the bulk counterpart. The specific surface area was about 239 m2 g−1 on as-prepared samples while those annealed at 500 °C have a surface area of 221 m2 g−1 showing that the heat treatment produced no significant increase in particle size. Transmission electron microscopy (TEM) further confirmed that the nanoparticles annealed at different temperatures while X-ray diffraction studies of the crystallized nanoparticles revealed that HfO2 nanoparticles were monoclinic in structure. High density pellets of the as-synthesized HfO2 nanoparticles were obtained, using both spark plasma sintering and uniaxial hot pressing, and their thermal conductivity was measured in the temperature range from 300 to 775 K. A large reduction of the thermal conductivity was observed for HfO2 nanoparticles as compared to that of bulk HfO2. The decrease in thermal conductivity is discussed in terms of the microstructure of the compacted samples. The synthetic procedure used in this work can be readily modified for large scale production of monodispersed HfO2 nanoparticles.