Atomistic models of short chain branched (SCB) polyethylene melts containing up to 20-40 mol% of comonomer (1-butene, 1-hexene, 1-octene or 1-decene) have been equilibrated at 450 K using a connectivity altering Monte...
Atomistic models of short chain branched (SCB) polyethylene melts containing up to 20-40 mol% of comonomer (1-butene, 1-hexene, 1-octene or 1-decene) have been equilibrated at 450 K using a connectivity altering Monte Carlo method, and analyzed for topological constraints using Z1 and CReTA codes. The calculated tube diameters, 〈app〉, of SCB melts are found to scale with the backbone weight fraction, φ, as 〈app〉∼φ−0.46, close to the scaling predicted by the binary contact model, 〈app〉∼φ−0.5 and in disagreement with the packing model prediction 〈app〉∼φ−1.27. Similar scaling relationships are observed experimentally for polymer solutions, and reproduced by the present methods.
Dielectric metamaterials offer a potential low-loss alternative to plasmonic metamaterials at optical frequencies. However, demonstrations of dielectric metamaterials have so far been limited to microwave and mid-infr...
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Dielectric metamaterials offer a potential low-loss alternative to plasmonic metamaterials at optical frequencies. However, demonstrations of dielectric metamaterials have so far been limited to microwave and mid-infrared frequencies. In this work, we outline the development of purely dielectric zero-index metamaterials operating at optical frequencies. The metamaterial, formed from silicon rods, exhibits impedance matching with air, resulting in unity transmission at the zero-index point. Design and experimental realization of the metamaterials is presented. The metamaterials can potentially be used for a number of applications including compact lens systems, directional emitters, and transformation optics devices.
Graphene demonstrated potential for practical applications owing to its excellent electronic and thermal properties. Typical graphene field-effect transistors (FETs) and interconnects built on conventional SiO 2 /Si s...
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Graphene demonstrated potential for practical applications owing to its excellent electronic and thermal properties. Typical graphene field-effect transistors (FETs) and interconnects built on conventional SiO 2 /Si substrates reveal the breakdown current density on the order of 10 8 A/cm 2 , which is ~100× larger than the fundamental limit for the metals but still smaller than the maximum achieved in carbon nanotubes. It was discovered by some of us that graphene has excellent thermal conduction properties with the thermal conductivity K exceeding 2000 W/mK at room temperature [1]. Few-layer graphene largely preserves the heat conduction properties [2]. However, the thermally resistive SiO 2 , with the thermal conductivity in the range from 0.5 to 1.4 W/mK, creates a bottleneck for heat removal. The latter does not allow graphene to demonstrate its true current-carrying potential. We show that by replacing SiO 2 with synthetic diamond one can substantially increase the current-carrying capacity of graphene to as high as ~ 20×10 8 A/cm 2 under ambient conditions. The two-terminal and three-terminal top-gated graphene devices (see Figure 1) were fabricated on synthetic single-crystal diamond (SCD) and ultrananocrystalline diamond (UNCD). To ensure Si integration, the UNCD layers were grown at low temperatures compatible with Si CMOS technology [3]. Our results indicate that graphene's current-induced breakdown is thermally activated. It was found that the current carrying capacity of graphene can be improved not only on SCD but also on an inexpensive UNCD. The latter was attributed to the decreased thermal resistance of UNCD at elevated temperatures (see Figure 2). The obtained results are important for graphene's hetero-integration on Si substrates. The enhanced current-carrying capacity is beneficial for the proposed applications of graphene in interconnects and high-frequency transistors.
The crystal structure of a metal plays an important role in its relationship to its macroscopic properties as well as atomic mechanisms of structural change. As such, students need to have an ability to visualize plan...
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ISBN:
(纸本)9781467313537
The crystal structure of a metal plays an important role in its relationship to its macroscopic properties as well as atomic mechanisms of structural change. As such, students need to have an ability to visualize planar crystal atomic packing features, but often find it difficult. Thus, the research question here is, "How can a student's baseline knowledge and misconceptions of planar atomic packing features for different metal structures be measured and how well can instruction promote conceptual change and misconception repair." Answering this question will provide insight for developing more effective pedagogy for crystal structures. A multiple-choice survey with six items was developed using misconceptions from students' pencil and paper sketches of face-centered cubic (FCC) and body-centered cubic (BCC) atoms on (100), (110), and (111) planes. Pretests and posttests of the survey were administered to students in a Spring 2012 introductory materialsengineering course. Misconceptions that were revealed included: missing atoms, extra atoms, misplaced atoms, "non-touching atoms where they should touch" and "touching atoms that should not touch". Students' difficulty in solving increased from (100) to (110) to (111) planes for both BCC and FCC structures. Details of the survey instrument and results are described in the paper.
Atomic layer deposition (ALD) of an alumina overcoat can stabilize a base metal catalyst (e.g., copper) for liquid‐phase catalytic reactions (e.g., hydrogenation of biomass‐derived furfural in alcoholic solvents or ...
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Atomic layer deposition (ALD) of an alumina overcoat can stabilize a base metal catalyst (e.g., copper) for liquid‐phase catalytic reactions (e.g., hydrogenation of biomass‐derived furfural in alcoholic solvents or water), thereby eliminating the deactivation of conventional catalysts by sintering and leaching. This method of catalyst stabilization alleviates the need to employ precious metals (e.g., platinum) in liquid‐phase catalytic processing. The alumina overcoat initially covers the catalyst surface completely. By using solid state NMR spectroscopy, X‐ray diffraction, and electron microscopy, it was shown that high temperature treatment opens porosity in the overcoat by forming crystallites of γ‐Al 2 O 3 . Infrared spectroscopic measurements and scanning tunneling microscopy studies of trimethylaluminum ALD on copper show that the remarkable stability imparted to the nanoparticles arises from selective armoring of under‐coordinated copper atoms on the nanoparticle surface.
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