236 results
Search results
Now showing 1 - 10 of 236
Publication An infiltration method for preparing single-wall nanotube/epoxy composites with improved thermal conductivity(2006-04-13) Du, Fangming; Guthy, Csaba; Fischer, John E; Kashiwagi, Takashi; Winey, Karen IRecent studies of SWNT/polymer nanocomposites identify the large interfacial thermal resistance at nanotube/nanotube junctions as a primary cause for the only modest increases in thermal conductivity relative to the polymer matrix. To reduce this interfacial thermal resistance, we prepared a freestanding nanotube framework by removing the polymer matrix from a 1 wt % SWNT/PMMA composite by nitrogen gasification and then infiltrated it with epoxy resin and cured. The SWNT/epoxy composite made by this infiltration method has a micron-scale, bicontinuous morphology and much improved thermal conductivity (220% relative to epoxy) due to the more effective heat transfer within the nanotube-rich phase. By applying a linear mixing rule to the bicontinuous composite, we conclude that even at high loadings the nanotube framework more effectively transports phonons than well-dispersed SWNT bundles. Contrary to the widely accepted approaches, these findings suggest that better thermal and electrical conductivities can be accomplished via heterogeneous distributions of SWNT in polymer matrices.Publication A Bond-Order Potential Incorporating Analytic Screening Functions for the Molybdenum Silicides(2004-11-29) Cawkwell, Marc J; Mrovec, Matous; Nguyen-Manh, Duc; Pettifor, David G; Vitek, VaclavThe intermetallic compound MoSi2, which adopts the C11b crystal structure, and related alloys exhibit an excellent corrosion resistance at high temperatures but tend to be brittle at room and even relatively high temperatures. The limited ductility of MoSi2 in ambient conditions along with the anomalous temperature dependence of the critical resolved shear stress (CRSS) of the {110)<111], {011)<100] and {010)<100] slip systems and departure from Schmid law behavior of the {013)<331] slip system can all be attributed to complex dislocation core structures. We have therefore developed a Bond-Order Potential (BOP) for MoSi2 for use in the atomistic simulation of dislocations and other extended defects. BOPs are a real-space, O(N), two-center orthogonal tight-binding formalism that are naturally able to describe systems with mixed metallic and covalent bonding. In this development novel analytic screening functions have been adopted to properly describe the environmental dependence of bond integrals in the open, bcc-based C11b crystal structure. A many-body repulsive term is included in the model that allows us to fit the elastic constants and negative Cauchy pressures of MoSi2. Due to the internal degree of freedom in the position of the Si atoms in the C11b structure which is a function of volume, it was necessary to adopt a self-consistent procedure in the fitting of the BOP. The constructed BOP is found to be an excellent description of cohesion in C11bMoSi2 and we have carefully assessed its transferability to other crystal structures and stoichiometries, notably C40, C49 and C54 MoSi2, A15 and D03 Mo3Si and D8m Mo5Si3 by comparing with ab initio structural optimizations.Publication Nanoparticle Networks Reduce the Flammability of Polymer Nanocomposites(2005-12-01) Kashiwagi, Takashi; Du, Fangming; Douglas, Jack F.; Winey, Karen I; Harris, Richard H.; Shields, John R.Synthetic polymer materials are rapidly replacing more traditional inorganic materials such as metals and natural polymeric materials such as wood. Since these novel materials are flammable, they require modifications to decrease their flammability through the addition of flame-retardant (FR) compounds. Recently, environmental regulation has restricted the use of some halogenated FR additives, initiating a search for alternative FR additives. Nanoparticle fillers are highly attractive for this purpose since they can simultaneously improve both the physical and flammability properties of the polymer nanocomposite. We show that carbon nanotubes can surpass nano-clays as effective FR additives if they form a jammed network structure within the polymer matrix, such that the material as a whole behaves rheologically like a gel. We find this kind of network formation for a variety of highly extended carbon-based nanoparticles: single and multi-walled nanotubes, as well as carbon nanofibers.Publication Two-Step Sintering of Ceramics with Constant Grain-Size, I. Y2O3(2006-02-01) Wang, Xiao-Hui; Chen, Pei-Lin; Chen, I-WeiIsothermal and constant-grain-size sintering have been carried out to full density in Y2O3 with and without dopants, at as low as 40% of the homologous temperature. The normalized densification rate follows Herring’s scaling law with a universal geometric factor that depends only on density. The frozen grain structure, however, prevents pore relocation commonly assumed in the conventional sintering models, which fail to describe our data. Suppression of grain growth but not densification is consistent with a grain boundary network pinned by triple-point junctions, which have a higher activation energy for migration than grain boundaries. Long transients in sintering and grain growth have provided further evidence of relaxation and threshold processes at the grain boundary/triple point.Publication Abnormal deformation behavior in Polysynthetically-twinned TiAl crystals with A and N orientations ---- an AFM study(2004-11-29) Chen, Yali; Pope, David PPolysynthetically-twinned TiAl crystals were deformed by compression with loading axis parallel and perpendicular to the lamellar interfaces. The deformation structures on the free surfaces were scanned using a dimension AFM with scan directions parallel and perpendicular to the lamellar interfaces. Abnormal deformation behaviors were observed to occur in both orientations. When the compression axis is parallel to the lamellar interfaces, the gamma and alpha lamellae deform primarily by shear in planes inclined with the lamellar interface, while the shear vectors lie in the interface. However, in-plane shear, shear in slip planes parallel to the lamellar interfaces, also occurs along the lamellar interfaces. When the loading axis is perpendicular to the lamellar interface, in-plane shear was found to be dominant at the beginning stage of plastic deformation and contributes more to the macroscopic strain. These behaviors are controversial to the Schmid’s Law since the applied resolved shear stress for these deformation systems is zero. The abnormal phenomenon was explained by the large coherency stresses along the lamellar interfaces.Publication Rationalization of the plastic flow behavior of Polysynthetically-twinned (PST) TiAl crystals based on slip mode observation using AFM and Schmid's law(2004-11-29) Chen, Yali; Pope, David PPST TiAl samples of different orientations were prepared and deformed by compression at room temperature. The deformation structures on the free surfaces were scanned using an AFM. It was found that when the angle between the lamellar interfaces and the loading axis is between 20 degree and 80 degree, PST samples deform primarily by shear in slip planes parallel to the lamellar interfaces. When the angle is below 20 degree, both the gamma phase and the alpha 2 phase deform by shear in slip planes inclined with the lamellar interfaces, but the shear vectors lie in the interface. When the angle is close to 90 degree, complex deformation behavior occurs. Shear in planes parallel to the lamellar interfaces contributes more to the overall strain in the directions perpendicular to the loading axis and the out-of-plane shear contributes to the strain in the compression direction. The characteristic U-shape curve of the yield stress versus the angle between the loading axis and the lamellar interfaces can be explained quite well using different C.R.S.S. for the three different deformation modes.Publication Tunable Microfluidic Optical Devices with Integrated Microlens Array(2006-08-01) Hong, Kuang-Sheng; Wang, Jing; Sharonov, Alexey; Chandra, Dinesh; Aizenberg, Joanna; Yang, ShuThe interest in dynamically tuning light has attracted great attention to the fabrication of tunable microlens arrays. Here we discuss the fabrication and characterization of a simple, robust, yet tunable microfluidic optical device with integrated microlens array. The microfuidic device with desired channel structure was micromachined on a polycarbonate plate with a resolution up to 100 µm, followed by thermal bonding two plates above their glass transition temperature. The microlens arrays were replica molded on a glass slide, which was then attached to the polycarbonate plates. By simply actuating the liquids with variable refractive index into the fluidic channel to immerse the lens arrays without moving or deformation of microlenses, a large change of focal length of more than 10 times (ƒ=0.74 to 8.53) was achieved. When a dye-containing liquid was pumped into the microfluidic channel to cover the lenses, the light transmission through the lenses was reduced from about 95% to 55% when the dye concentration was increased to 10 w/v %. The knowledge we gain from these studies will provide important insights to contruct new, adaptive, micro-scale optical devices with multiple functionalities.Publication Two-Step Sintering of Ceramics with Constant Grain-Size, II: BaTiO3and Ni–Cu–Zn Ferrite(2006-02-01) Wang, X.-H.; Deng, X.-Y.; Bai, Hai-Lin; Zhou, H.; Qu, Wei-Guo; Li, L.-T.; Chen, I-WeiWe investigated the preparation of bulk dense nanocrystalline BaTiO3 and Ni–Cu–Zn ferrite ceramics using an unconventional two-step sintering strategy, which offers the advantage of not having grain growth while increasing density from about 75% to above 96%. Using nanosized powders, dense ferrite ceramics with a grain size of 200 nm and BaTiO3 with a grain size of 35 nm were obtained by two-step sintering. Like the previous studies on Y2O3, the different kinetics between densification diffusion and grain boundary network mobility leaves a kinetic window that can be utilized in the second-step sintering. Evidence indicates that low symmetry, ferroelectric structures still exist in nanograin BaTiO3 ceramics, and that saturation magnetization is the same in nanograin and coarse grain ferrite ceramics.Publication Atomistic Studies of Dislocation Glide in gamma-TiAl(2002-12-02) Porizek, Radoslav; Znam, Stefan; Vitek, Vaclav; Nguyen-Manh, Duc; Pettifor, David G.Computer simulation of the core structure and glide of ordinary 1/2<110] dislocations and <101] superdislocations in L10 TiAl has been performed using the recently constructed Bond-Order Potentials. This description of atomic interactions includes explicitly, within the tightbinding approximation, the most important aspects of the directional bonding, namely d-d, p-p and d-p bonds. The ordinary dislocation in the screw orientation was found to have a non-planar core and, therefore, high Peierls stress. The superdislocation was found to possess in the screw orientation either a planar (glissile) or a non-planar (sessile) core structure. However, the glissile core transforms into the sessile one for certain orientations of the applied stress. This implies a strong asymmetry of the yield stress and the break down of the Schmid law when the plastic flow is mediated by superdislocations. At the same time, this may explain the orientation dependence of the dislocation substructure observed in the single-phase gamma-TiAl by electron microscopy.Publication Core-shell diamond-like silicon photonic crystals from 3D polymer templates created by holographic lithography(2006-06-26) Moon, Jun Hyuk; Yang, Shu; Dong, Wenting; Perry, Joseph W; Adibi, Ali; Yang, Seung-ManWe have fabricated diamond-like silicon photonic crystals through a sequential silica/silicon chemical vapor deposition (CVD) process from the corresponding polymer templates photopatterned by holographic lithography. Core-shell morphology is revealed due to the partial backfilling of the interstitial pores. To model the shell formation and investigate its effect to the bandgap properties, we developed a two-parameter level-set approach that closely approximated the core-shell morphology, and compare the bandgap simulation with the measured optical properties of the 3D crystals at each processing step. Both experimental and calculation results suggest that a complete filling is necessary to maximize the photonic bandgap in the diamond-like structures.