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School of Materials Science and Engineering

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College of Engineering

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Temperature driven in-situ phase transformation of PbWO₄ nanobelts

(Georgia Institute of Technology, 2011-06-23) Wang, Xue ; Ding, Yong ; Wang, Z. L. (Zhong Lin) ; Hu, Chenguo

Monoclinic raspite PbWO₄ nanobelts were synthesized by a facile composite-salt-mediated method. By in situ heating to above 538 °C inside the chamber of a transmission electron microscope, the raspite nanobelts transformed irreversibly to tetragonal scheelite phase. By analyzing the experimental data, three possible topotactic transformation relationships between raspite and scheelite phases have been proposed. With further increasing the temperature up to 618 °C, part of the PbWO₄ nanobelts reduced to tetragonal WO₃ nanorods owing to the evaporation of Pb.
Structural colors from Morpho peleides butterfly wing scales

(Georgia Institute of Technology, 2009-10-12) Ding, Yong ; Xu, Sheng ; Wang, Z. L. (Zhong Lin)

A male Morpho peleides butterfly wing is decorated by two types of scales, cover and ground scales. We have studied the optical properties of each type of scales in conjunction with the structural information provided by cross-sectional transmission electron microscopy and computer simulation. The shining blue color is mainly from the Bragg reflection of the one-dimensional photonic structure, e.g., the shelf structure packed regularly in each ridges on cover scales. A thin-film-like interference effect from the base plate of the cover scale enhances such blue color and further gives extra reflection peaks in the infrared and ultraviolet regions. The analogy in the spectra acquired from the original wing and that from the cover scales suggests that the cover scales take a dominant role in its structural color. This study provides insight of using the biotemplates for fabricating smart photonic structures.
Piezoelectric nanogenerator using CdS nanowires

(Georgia Institute of Technology, 2008-01-14) Lin, Yi-Feng ; Song, Jinhui ; Ding, Yong ; Lu, Shih-Yuan ; Wang, Z. L. (Zhong Lin)

Vertically grown cadmium sulfide (CdS) nanowire (NW) arrays were prepared using two different processes: hydrothermal and physical vapor deposition (PVD). The NWs obtained from the hydrothermal process were composed of alternating hexagonal wurtzite (WZ) and cubic zinc blende (ZB) phases with growth direction along WZ 0001 and ZB [111]. The NWs produced by PVD process are single crystalline WZ phase with growth direction along 0001. These vertically grown CdS NW arrays have been used to converting mechanical energy into electricity following a developed procedure [Z. L. Wang and J. Song Science 312, 242 (2006)]. The basic principle of the CdS NW nanogenerator relies on the coupled piezoelectric and semiconducting properties of CdS, and the data fully support the mechanism previously proposed for ZnO NW nanogenerators and nanopiezotronics.
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.
Doping and planar defects in the formation of single-crystal ZnO nanorings

(Georgia Institute of Technology, 2004-12-06) Ding, Yong ; Kong, Xiang Yang ; Wang, Z. L. (Zhong Lin)

We have recently reported the growth of freestanding, single-crystal, seamless nanorings of zinc oxide via a spontaneous self-coiling process during the growth of polar-nanobelts [X.Y. Kong et al., Science 303, 1348 (2004)]. The nanoring is made by coaxial and uniradius loop-by-loop winding of a fine ZnO nanobelt. An important fact is that each and every nanoring is made of a nanobelt that contains basal-plane planar defects, which are suggested to be important for leading the fastest growth of the nanobelt as well as lowering its elastic deformation energy. In this paper, high-resolution transmission electron microscopy is applied to investigate the nature of the planar defects in the nanobelts and in nanorings. The planar defects were initiated and formed by single-layer segregation of the doping element, such as indium, which was introduced in the growth process. The accumulation of impurity ions forms two vicinal InuO octahedral layers parallel to the basal plane. They form “head-to-head” and “tail-to-tail” polar-inversion domain boundaries. For a nanobelt that self-coils into a nanoring, we found that the head-to-head and tail-to-tail polar-inversion domain boundaries are paired, thus, the polarity of the nanobelt is unchanged. Therefore, our data support the proposed model [X.Y. Kong et al., Science 303, 1348 (2004)] that the nanoring is initiated by circularly folding a nanobelt due to long-range electrostatic interaction between the surface polar charges on the two sides, and a loop-by-loop winding of the nanobelt forms a complete ring.
Grain size dependence of magnetic properties in shock synthesized bulk Pr₂Fe₁₄B/α-Fe nanocomposites

(Georgia Institute of Technology, 2004-09-15) Jin, Z. Q. ; Thadhani, Naresh N. ; McGill, M. ; Li, Jing ; Ding, Yong ; Wang, Z. L. (Zhong Lin) ; Zeng, Hao ; Chen, M. ; Cheng, S.-F. ; Liu, J. Ping

The structural and magnetic properties of the melt-spun Pr₂Fe₁₄B/α-Fe nanocomposite powders consolidated via shock-wave compression and subjected to postshock thermal treatment were investigated. Shock compression results in grain refinement, which leads to a reduction of an effective anisotropy and therefore an increase in the ferromagnetic exchange length, resulting in an enhanced exchange coupling in fully consolidated bulk magnets. A small amount of amorphous phase formed during the shock compression were observed to crystallize into Pr₂Fe₁₄B upon annealing above 600 °C. The heat treatment also results in the recovery of coercivity partially lost during the consolidation, which can be related directly to the dependence of the effective anisotropy on the grain size, as illustrated by the transmission electron microscopy observation of grain refinement in the shock-consolidated bulk samples. A uniform grain morphology is suggested as a means for further increasing the magnetic properties of bulk nanocomposites.
Controlling the crystallization and magnetic properties of melt-spun Pr₂Fe₁₄B/α Fe nanocomposites by Joule heating

(Georgia Institute of Technology, 2004-05-12) Jin, Z. Q. ; Cui, B. Z. ; Liu, J. Ping ; Ding, Yong ; Wang, Z. L. (Zhong Lin) ; Thadhani, Naresh N.

Pr₂Fe₁₄B/α Fe based nanocomposites have been prepared through crystallization of melt-spun amorphous Pr₇Tb₁Fe₈₅Nb ₀.₅ Zr ₀.₅ B₆ ribbons by means of ac Joule heating while simultaneously monitoring room-temperature electrical resistance R. The R value shows a strong variation with respect to applied current I, and is closely related to the amorphous-to-nanocrystalline phase transformation. The curve of R versus I allows one to control the crystallization behavior during Joule heating and to identify the heat-treatment conditions for optimum magnetic properties. A coercivity of 550 kA/m and a maximum energy product of 128 kJ/m³ have been obtained upon heating the amorphous ribbons at a current of 2.0 A. These properties are around 30% higher than the values of samples prepared by conventionally (furnace) annealed amorphous ribbons.
Manganese-doped ZnO nanobelts for spintronics

(Georgia Institute of Technology, 2004-02-02) Ronning, C. ; Gao, Puxian ; Ding, Yong ; Wang, Z. L. (Zhong Lin) ; Schwen, D.

Zinc oxide (ZnO) nanobelts synthesized by thermal evaporation have been ion implanted with 30 keV Mn+ ions. Both transmission electron microscopy and photoluminescence investigations show highly defective material directly after the implantation process. Upon annealing to 800 °C, the implanted Mn remains in the ZnO nanobelts and the matrix recovers both in structure and luminescence. The produced high-quality ZnO:Mn nanobelts are potentially useful for spintronics.
Interface and defect structures of Zn–ZnO core–shell heteronanobelts

(Georgia Institute of Technology, 2004-01-01) Ding, Yong ; Kong, X. Y. ; Wang, Z. L. (Zhong Lin)

Interface and defect structures of Zn–ZnO core–shell nanobelts have been investigated using high-resolution transmission electron microscopy. Most of the nanobelts can be classified into two types from their growth directions: [20] and [0001], with the top/bottom surfaces being (0001) and (20), respectively. The Zn core and ZnO shell overlapped areas display a two-dimensional moiré pattern resulting from the lattice mismatch. In the 20 growth nanobelts, a network of three sets of misfit dislocations relaxes the mismatch strain in the top/bottom interfaces, and every set rotates 60° with respect to the other; there are two types of grains oriented in specific orientations that compose the side wall of the ZnO shell. In the [0001] growth nanobelts, a network containing a set of stacking faults in (0001) planes and a set of misfit dislocations in (010) planes takes the main role in the misfit relaxation. Threading dislocations indicated by terminating moiré fringes are present in both of them, which are located at the small angle rotated boundary between adjacent misoriented ZnO grains.