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School of Materials Science and Engineering

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College of Engineering

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Now showing 1 - 10 of 163

Reconciling Fracture Toughness Parameter Contradictions in Thin Ductile Metal Sheets in Tension

(Georgia Institute of Technology, 2017) Lanning, Wade R. ; Javaid, Syed S. ; Muhlstein, Christopher L.

The well-known trade-off between strength and fracture toughness in bulk specimens is often used to explain the low fracture toughness of very thin ductile face-centered cubic (FCC) metal specimens, but this interpretation contradicts the relative length scales of thickness-dependent strength and thickness-dependent fracture toughness. This study uses the concept of similitude to demonstrate that linear elastic fracture mechanics (LEFM) analysis of 25:4 μm thick annealed aluminum is invalid, though the resulting fracture toughness measurements fit well with the existing literature and idea of a strength/fracture toughness trade-off. Similarly, a elastic plastic fracture mechanics (EPFM) analysis is sensitive to out-of-plane deformation which cannot be practically eliminated or corrected for with a model. However a plastic collapse analysis using a critical net section stress criterion is demonstrably valid by the concept of similitude, is insensitive to out of plane deformation, and agrees with the evidence of extensive plasticity in the fracture surfaces.
A thin film triode type carbon nanotube field emission cathode

(Springer, 2013-08) Sanborn, Graham ; Turano, Stephan ; Collins, Peter ; Ready, W. Jud

The field electron emission of carbon nanotubes has been heavily studied over the past two decades for various applications, such as in display technologies, microwave amplifiers, and spacecraft propulsion. However, a commercializable lightweight and internally gated electron source has yet to be realized. This work presents the fabrication and testing of a novel internally gated carbon nanotube field electron emitter. Several specific methods are used to prevent electrical shorting of the gate layer, a common failure for internally gated devices. A unique design is explored where the etch pits extend into the Si substrate and isotropic etching is used to create a lateral buffer zone between the gate and carbon nanotubes. Carbon nanotubes are self-aligned to and within 10 microns from the gate, which creates large electric fields at low potential inputs. Initial tests confirm high field emission performance with an anode current density (based on total area of the device) of 293 μA cm-2 and a gate current density of 1.68 mA cm-2 at 250 V.
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.
In vitro Biomimetic Construction of Hydroxyapatite–Porcine Acellular Dermal Matrix Composite Scaffold for MC3T3-E1 Preosteoblast Culture

(Georgia Institute of Technology, 2011-02-26) Zhao, Hongshi ; Wang, Guancong ; Hu, Shunpeng ; Cui, Jingjie ; Ren, Na ; Liu, Duo ; Liu, Hong ; Cao, Chengbo ; Wang, Jiyang ; Wang, Z. L. (Zhong Lin)

The application of porous hydroxyapatite–collagen (HAp-Collagen) as a bone tissue engineering scaffold is hindered by two main problems: its high cost and low initial strength. As a native 3-dimenssional collagen framework, purified porcine acellular dermal matrix (PADM) has been successfully used as a skin tissue engineering scaffold. Here we report its application as a matrix for the preparation of HAp to produce a bone tissue scaffold through a biomimetic chemical process. The HAp-PADMscaffold has two-level pore structure, with large channels (*100 mm in diameter) inherited from the purified PADM microstructure and small pores (<100 nm in diameter) formed by self-assembled HAp on the channel surfaces. The obtained HAp-PADM scaffold (S15D) has a compressive elastic modulus as high as 600 kPa. The presence of HAp in sample S15D reduces the degradation rate of PADM in collagenase solution at 378C. After 7 day culture of MC3T3-E1 pre-osteroblasts, MTT data show no statistically significant difference on pure PADM framework and HAp-PADM scaffold ( p>0.05). Because of its high strength and nontoxicity, its simple preparation method, and designable and tailorable properties, the HAp- PADM scaffold is expected to have great potential applications in medical treatment of bone defects.
Tracking the catalyzed growth process of nanowires by in situ x-ray diffraction

(Georgia Institute of Technology, 2010-07-06) Kirkham, Melanie ; Wang, Z. L. (Zhong Lin) ; Snyder, Robert L.

Quasi-one-dimensional nanostructures of silicon, oxides, and other materials show great promise for a variety of applications. These nanostructures are commonly grown using metal catalyst nanoparticles. This paper investigates the growth mechanism of Au-catalyzed Si nanowires through in situ x-ray diffraction, and the results are compared to previously studied Au-catalyzed ZnO nanorods. The Si nanowires were found to grow from molten catalyst particles, however, the ZnO nanorods were found to grow from solid catalyst particles through a surface diffusion process. From this comparison, the relative bonding types of the catalyst and source material are determined to have a significant effect on the growth mechanism.
Synthesis of High-Quality Vertically Aligned Carbon Nanotubes on Bulk Copper Substrate for Thermal Management

(Georgia Institute of Technology, 2010-05) Lin, Wei ; Zhang, Rongwei ; Moon, Kyoung-Sik ; Wong, C. P.

Vertically aligned carbon nanotubes (VACNTs) grown on bulk copper substrate are of great importance for real-life commercial applications of carbon nanotubes (CNTs) as thermal interface materials in microelectronic packaging. However, their reproducible syntheses have been a great challenge so far. In this study, by introducing a well-controlled conformal Al₂O₃ support layer on the bulk copper substrate by atomic layer deposition, we reproducibly synthesized VACNTs of good alignment and high quality on the copper substrate. The alignment and the quality were characterized by scanning electron microscope, transmission electron microscope, and Raman spectroscopy. The key roles of the conformal Al₂O₃ support layer by atomic layer deposition are discussed. This progress may provide a real-life VACNT application for thermal management.
Growth direction and morphology of ZnO nanobelts revealed by combining in situ atomic force microscopy and polarized Raman spectroscopy

(Georgia Institute of Technology, 2010-01-14) Lucas, Marcel ; Wang, Z. L. (Zhong Lin) ; Riedo, Elisa

Control over the morphology and structure of nanostructures is essential for their technological applications, since their physical properties depend significantly on their dimensions, crystallographic structure, and growth direction. A combination of polarized Raman (PR) spectroscopy and atomic force microscopy (AFM) is used to characterize the growth direction, the presence of point defects and the morphology of individual ZnO nanobelts. PR-AFM data reveal two growth modes during the synthesis of ZnO nanobelts by physical vapor deposition. In the thermodynamics-controlled growth mode, nanobelts grow along a direction close to [0001], their morphology is growth-direction dependent, and they exhibit no point defects. In the kinetics-controlled growth mode, nanobelts grow along directions almost perpendicular to [0001], and they exhibit point defects.
Large enhancement in photon detection sensitivity via Schottky-gated CdS nanowire nanosensors

(Georgia Institute of Technology, 2010-01-06) Wei, Te-Yu ; Huang, Chi-Te ; Hansen, Benjamin J. ; Lin, Yi-Feng ; Chen, Lih-Juann ; Lu, Shih-Yuan ; Wang, Z. L. (Zhong Lin)

The Schottky contact based photon detection was demonstrated using CdS (visible light responsive), silicon (indirect n-type oxygen-non-adsorbing), and CuO (indirect p-type oxygen-adsorbing) nanowire nanosensors. With changing one of the two nanowire-electrode contacts from ohmic to Schottky, detection sensitivities as high as 105% were achieved by the CdS nanowire nanosensor operated at the reverse bias mode of −8 V, which was 58 times higher than that of the corresponding ohmic contact device. The reset time was also significantly reduced. In addition, originally light nonresponsive silicon and CuO nanowires became light responsive when fabricated as a Schottky contact device. These improvements in photon detection can be attributed to the Schottky gating effect realized in the present nanosensor system by introducing a Schottky contact.
Depth resolved luminescence from oriented ZnO nanowires

(Georgia Institute of Technology, 2009-12-14) Rosenberg, R. A. ; Haija, M. Abu ; Vijayalakshmi, K. ; Zhou, Jun ; Xu, Sheng ; Wang, Z. L. (Zhong Lin)

We have utilized the limited penetration depth of x-rays to study the near-surface properties of vertically aligned ZnO nanowires. For an energy of 600 eV the penetration depth varies between 3 and 132 nm as the incidence angle changes from 2° to 33°. Thus, by obtaining optical luminescence spectra as a function of incidence angle, it is possible to probe the near-surface region with nanometer-scale resolution. We will present angle dependent optical luminescence data from oriented ZnO nanowires. By fitting the results to a simple model, we extract a depth for the surface defect regions of ~14 nm.
Effect of Permittivity and Permeability of a Flexible Magnetic Composite Material on the Performance and Miniaturization Capability of Planar Antennas for RFID and Wearable Wireless Applications

(Georgia Institute of Technology, 2009-12) Martin, Lara J. ; Ooi, Sooliam ; Staiculescu, Daniela ; Hill, Michael D. ; Wong, C. P. ; Tentzeris, Emmanouil M.

This paper is an investigation of the feasibility of applying a mechanically flexible magnetic composite material to radio frequency identification (RFID) planar antennas operating in the lower ultrahigh-frequency (UHF) spectrum (∼300– 500 MHz). A key challenge is that the magnetic loss introduced by the magnetic composite must be sufficiently low for successful application at the targeted operating frequency. A flexible magnetic composite comprised of particles of Z-phase Co hexaferrite, also known as Co₂Z, in a silicone matrix was developed. To the authors’ knowledge, this is the first flexible magnetic composite demonstrated to work at these frequencies. The benchmarking structure was a quarter-wavelength microstrip patch antenna. Antennas on the developed magnetic composite and pure silicone substrates were electromagnetically modeled in Ansoft High- Frequency Sounder System full wave electromagnetic software. A prototype of the antenna on the magnetic composite was fabricated, and good agreement between the simulated and measured results was found. Comparison of the antennas on the magnetic composite versus the pure silicone substrate showed miniaturization capability of 2.4× and performance differences of increased bandwidth and reduced gain, both of which were attributed in part to the increase in the dielectric and magnetic losses. A key finding of this paper is that a small amount of permeability (μr∼2.5) can provide a substantial capability for miniaturization, while sufficiently low-magnetic loss can be introduced for successful application at the targeted operating frequency. This magnetic composite shows the capability to fulfill this balance and to be a feasible option for RFID, flexible wearable, and conformal applications in the lower UHF spectrum.