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School of Materials Science and Engineering

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https://hdl.handle.net/1853/70762

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College of Engineering

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Now showing 1 - 10 of 23

Growth and field-emission property of tungsten oxide nanotip arrays

(Georgia Institute of Technology, 2005-11-21) Zhou, Jun ; Gong, Li ; Deng, Shao Zhi ; Chen, Jun ; She, Jun Cong ; Xu, Ning Sheng ; Yang, Rusen ; Wang, Z. L. (Zhong Lin)

Large-area, quasialigned nanotips of tungsten oxide have been grown by a two-step high-temperature, catalyst-free, physical evaporation deposition process. The tungsten oxide nanotips are single crystalline with growth direction of [010]. The tungsten oxide nanotips exhibit excellent field-emission properties with a low threshold field (for an emission current density of 10 mA/cm²) ~4.37 MV/m and uniform emission from the entire arrays, as well as high time stability. These results make tungsten oxide nanotip arrays a competitive candidate for field-emission displays.
Field emission of individual carbon nanotube with in situ tip image and real work function

(Georgia Institute of Technology, 2005-10-10) Xu, Zhi ; Bai, X. D. ; Wang, E. G. ; Wang, Z. L. (Zhong Lin)

The field emission properties of individual multiwalled carbon nanotubes have been measured simultaneously in correlation to the emitter images and their real work functions at tips by the in situ transmission electron microscopy method. The field emission of a single nanotube still follows the Fowler-Nordheim law. The field enhancement factor has been determined by the real work function rather than a given constant. In situ imaging and measurement show that the work function at the nanotube tip depends strongly on its structure and surface condition. This study provides an approach of direct linking field emission with the in situ emitter structure and the real work function at the emitter tip.
Interphase exchange coupling in Fe/Sm–Co bilayers with gradient Fe thickness

(Georgia Institute of Technology, 2005-09-30) Yu, Ming-hui ; Hattrick-Simpers, Jason ; Takeuchi, Ichiro ; Li, Jing ; Wang, Z. L. (Zhong Lin) ; Liu, J. Ping ; Lofland, S. E. ; Tyagi, Somdev ; Freeland, J. W. ; Giubertoni, D. ; Bersani, M. ; Anderle, M.

We have fabricated Fe/Sm–Co bilayers with gradient Fe thicknesses in order to systematically study the dependence of exchange coupling on the thickness of the Fe layer. The Fe layer was deposited at two different temperatures (150 and 300 °C) to study the effect of deposition temperature on the exchange coupling. Magneto-optical Kerr effect and x-ray magnetic circular dichroism (XMCD) have been employed as nondestructive rapid characterization tools to map the magnetic properties of the gradient samples. Systematic enhancement in exchange coupling between the soft layer and the hard layer is observed as the soft layer thickness is decreased. Separate exchange couplings of the Fe layer with Co and Sm in the hard layer are revealed through measuring the element-specific hysteresis curves using XMCD. The single-phase-like magnetization reversal critical thickness increases from 12 nm for Fe deposited at 150 °C to 24 nm for Fe deposited at 300 °C, indicating an important role of the state of the interface in the exchange coupling.
Synthesis and processing of nanocomposite permanent magnets approaches

(Georgia Institute of Technology, 2005-05-01) Thadhani, Naresh N. ; Wang, Z. L. (Zhong Lin)
Mechanical behavior and magnetic separation of quasi-one-dimensional SnO₂ nanostructures: A technique for achieving monosize nanobelts/nanowires

(Georgia Institute of Technology, 2005-03-25) Jin, Z. Q. ; Ding, Yong ; Wang, Z. L. (Zhong Lin)

The as-synthesized nanowires and nanobelts usually have a large size distribution. We demonstrate here a ball milling technique for narrowing the size distribution of oxide nanobelts and nanowires. High-resolution scanning and transmission electron microscopy reveals that the one-dimensional SnO₂ nanostructures with size >150 nm are sensitive to the milling effect and most of them were fractured into nanoparticles even after a short-time milling. These nanoparticles contain magnetic Fe components, which could be effectively separated from those nanobelts by employing a magnetic field. This feature promises a potentials application in the nanostructured materials separation. It was also found that the dominant size of the survived nanostructures is <100 nm. The good mechanical behavior of the nanostructures are not only related to the superior mechanical toughness due to small size, but also related to the low defect density.
Explosive shock processing of Pr₂Fe₁₄B/α–Fe exchange-coupled nanocomposite bulk magnets

(Georgia Institute of Technology, 2005-03) Jin, Z. Q. ; Thadhani, Naresh N. ; McGill, M. ; Ding, Yong ; Wang, Z. L. (Zhong Lin) ; Chen, M. ; Zeng, Hao ; Chakka, V. M. ; Liu, J. Ping

Explosive shock compaction was used to consolidate powders obtained from melt-spun Pr₂Fe₁₄B/α–Fe nanocomposite ribbons, to produce fully dense cylindrical compacts of 17–41-mm diameter and 120-mm length. Characterization of the compacts revealed refinement of the nanocomposite structure, with approximately 15 nm uniformly sized grains. The compact produced at a shock pressure of approximately 1 GPa maintained a high coercivity, and its remanent magnetization and maximum energy product were measured to be 0.98 T and 142 kJ/m³, respectively. The compact produced at 4–7 GPa showed a decrease in magnetic properties while that made at 12 GPa showed a magnetic softening behavior. However, in both of these cases, a smooth hysteresis loop implying exchange coupling and a coercivity of 533 kA/m were fully recovered after heat treatment. The results illustrate that the explosive compaction followed by post-shock heat treatment can be used to fabricate exchange-coupled nanocomposite bulk magnets with optimized magnetic properties.
Integration of metal oxide nanobelts with microsystems for nerve agent detection

(Georgia Institute of Technology, 2005-01-31) Yu, Choongho ; Hao, Qing ; Saha, Sanjoy ; Shi, Li ; Kong, Xiang Yang ; Wang, Z. L. (Zhong Lin)

We have assembled tin dioxide nanobelts with low-power microheaters for detecting dimethyl methylphosphonate (DMMP), a nerve agent simulant. The electrical conductance of a heated nanobelt increased for 5% upon exposure to 78 parts per billion DMMP in air. The nanobelt conductance recovered fully quickly after the DMMP was shut off, suggesting that the single-crystal nanobelt was not subject to poisoning often observed in polycrystalline metal oxide sensors. While the sensitivity can be improved via doping nanobelts with catalytic additives, directed assembly or growth of nanobelts on microsystems will potentially allow for the large-scale fabrication of nanosensor arrays.
Nanoarchitectures of semiconducting and piezoelectric zinc oxide

(Georgia Institute of Technology, 2005-01-21) Gao, Puxian ; Wang, Z. L. (Zhong Lin)

Semiconducting and piezoelectric zinc oxide has two important structure characteristics: the multiple and switchable growth directions: 010, 20, and 0001; and the {0001} polar surfaces. The fast growth directions create nanobelts of different crystallographic facets, and the polar surfaces result in bending of the nanobelt for minimizing the spontaneous polarization energy. A combination of these distinct growth characteristics results in a group of unique nanostructures, including several types of nanorings, nanobows, platelet circular structures, Y-shape split ribbons, and crossed ribbons. We present here the as-grown nanoarchitectures naturally created by combining some of the fundamental structure configurations of ZnO, which could be unique for many applications in nanotechnology.
Controlled synthesis and manipulation of ZnO nanorings and nanobows

(Georgia Institute of Technology, 2005-01-19) Hughes, William L. ; Wang, Z. L. (Zhong Lin)

An experimental procedure is presented for the controlled synthesis and manipulation of ZnO nanorings and nanobows at high purity and large yield. Atomic force microscopy manipulation of the nanostructures demonstrates their mechanical toughness and flexibility. Extensive bending of the nanorings and nanobows suggests an extremely high deformation limit with the potential for building ultrasensitive electromechanical coupled nanoscale sensors, transducers, and resonators.
Synthesis of Si-Ge Oxide Nanowires via the Transformation of of Si-Ge Thin Films with Self-Assembled Au Catalysts

(Georgia Institute of Technology, 2005) He, J. H. ; Wu, T. H. ; Hsin, C. L. ; Chen, L. J. ; Wang, Z. L. (Zhong Lin)

A technique has been developed to transform a Si-Ge thin film into Si-Ge oxide nanowires with the assistance of Au particles through a three-step annealing process. A honeycomb network of Au colloidal nanoparticles was self-assembled; 400°C annealing removes the surface surfactant; 800°C annealing forms hexagonally self-assembled Au particles on the thin-film surface; finally, a 1075°C annealing results in the growth of oxide nanowires on the surfaces of Au particles. Synthesized nanowires have an emission peak at 3.3 eV. This technique is useful for growing silicon oxide nanowires with a tunable amount of Ge doping.