We analyze the geometric structure and mechanical stability of a complete set of isostatic and hyperstatic sphere packings obtained via exact enumeration. The number of nonisomorphic isostatic packings grows exponenti...
详细信息
We analyze the geometric structure and mechanical stability of a complete set of isostatic and hyperstatic sphere packings obtained via exact enumeration. The number of nonisomorphic isostatic packings grows exponentially with the number of spheres N, and their diversity of structure and symmetry increases with increasing N and decreases with increasing hyperstaticity H≡Nc−NISO, where Nc is the number of pair contacts and NISO=3N−6. Maximally contacting packings are in general neither the densest nor the most symmetric. Analyses of local structure show that the fraction f of nuclei with order compatible with the bulk (rhcp) crystal decreases sharply with increasing N due to a high propensity for stacking faults, five- and near-fivefold symmetric structures, and other motifs that preclude rhcp order. While f increases with increasing H, a significant fraction of hyperstatic nuclei for N as small as 11 retain non-rhcp structure. Classical theories of nucleation that consider only spherical nuclei, or only nuclei with the same ordering as the bulk crystal, cannot capture such effects. Our results provide an explanation for the failure of classical nucleation theory for hard-sphere systems of N≲10 particles; we argue that in this size regime, it is essential to consider nuclei of unconstrained geometry. Our results are also applicable to understanding kinetic arrest and jamming in systems that interact via hard-core-like repulsive and short-ranged attractive interactions.
Continuous scaling of Si CMOS devices and circuits, increased speed and integration densities resulted in problems with thermal management of nanoscale device and computer chips [1]. Further progress in information, c...
详细信息
ISBN:
(纸本)9781467309943
Continuous scaling of Si CMOS devices and circuits, increased speed and integration densities resulted in problems with thermal management of nanoscale device and computer chips [1]. Further progress in information, communication and energy storage technologies requires more efficient heat removal methods and stimulates the search for thermal interface material (TIMs) with enhanced thermal conductivity. The commonly used TIMs are filled with the particles such as silver or silica. The conventional TIMs require high volume fractions of the filler (∼70%) to achieve thermal conductivity of ∼1-5 W/mK. Recently, some of us discovered that graphene has extremely high intrinsic thermal conductivity, which exceeds that of carbon nanotubes [2-4]. To use this property for thermal management of nanoscale electronic devices, we utilized the inexpensive liquid-phase exfoliated graphene and multi-layer graphene (MLG) as filler materials in TIMs (Figure 1). The thermal properties of the obtained graphene-epoxy composites were measured using the "laser flash" technique (Figure 2). It was found that the thermal conductivity enhancement factor exceeded a factor of 23 at 10% of the graphene volume loading fraction [5]. This enhancement is larger than anything that has been achieved using other fillers. We have also tested graphene flakes in the electrically-conductive hybrid graphene-metal particle TIMs. The thermal conductivity of resulting composites was increased by a factor of ∼5 in a temperature range from 300 K to 400 K at a small graphene loading fraction of 5-vol.-% [6]. The unusually strong enhancement of thermal properties was attributed to the high thermal conductivity of graphene, strong graphene coupling to matrix materials and the large range of the length-scale - from nanometers to micrometers - of the graphene and silver particle fillers. Graphene-based TIMs have a number of other advantages related to their viscosity and adhesion, which meet the industry requirements.
Although the basic laws of friction are simple enough to be taught in elementary physics classes and although friction has been widely studied for centuries, in the current state of knowledge it is still not possible ...
详细信息
Although the basic laws of friction are simple enough to be taught in elementary physics classes and although friction has been widely studied for centuries, in the current state of knowledge it is still not possible to predict a friction force from fundamental principles. One of the highly debated topics in this field is the origin of static friction. For most macroscopic contacts between two solids, static friction will increase logarithmically with time, a phenomenon that is referred to as aging of the interface. One known reason for the logarithmic growth of static friction is the deformation creep in plastic contacts. However, this mechanism cannot explain frictional aging observed in the absence of roughness and plasticity. Here, we discover molecular mechanisms that can lead to a logarithmic increase of friction based purely on interfacial chemistry. Predictions of our model are consistent with published experimental data on the friction of silica.
Heparin has been considered to be a potentially useful ligand for low-density lipoprotein（LDL） detection and analysis in a clinical *** order to construct an affinity surface for preferential adsorption of LDL,hepar...
详细信息
Heparin has been considered to be a potentially useful ligand for low-density lipoprotein（LDL） detection and analysis in a clinical *** order to construct an affinity surface for preferential adsorption of LDL,heparin-modified gold surface（GS-Hep） was fabricated by a self-assembling method and hydrophobic-modified gold surfaces（GS-Hydro） was used as a *** morphologies of the modified gold surfaces were investigated by atomic force microscopy（AFM） and the quantity of heparin bound to gold surface was assayed by the toluidine blue（TB） colorimetric *** contact angles were determined to investigate wettability on GS-Hep and *** plasmon resonance（SPR） technique was used subsequently to detect the selective binding of LDL with *** the investigation on the effect of pH on LDL adsorption suggests that lower pH lead to higher quantities of LDL adsorption on *** with GS-Hydro,GS-Hep is selective for LDL from both single and binary protein ***,adsorbed LDL on GS-Hep could be washed off by injecting elution solution,such as NaCl solution,for the purpose of the regeneration of GS-Hep for further LDL adsorption.
Previously crack propagation and joint failure in thermal cycling tests were correlated with recrystallization of Sn grains in SnAgCu (SAC) ball grid array (BGA) solder joints. Generally recrystallization of the Sn gr...
详细信息
ISBN:
(纸本)9781479902330
Previously crack propagation and joint failure in thermal cycling tests were correlated with recrystallization of Sn grains in SnAgCu (SAC) ball grid array (BGA) solder joints. Generally recrystallization of the Sn grains was observed to occur in the high strain region before solder joint failure. In an effort to better understand this failure mechanism in SnAgCu solder joints subjected to mild thermal cycling profiles, and in smaller solder joints that have interlaced Sn grain morphologies, both conventional （-40/125°C, 0/100°C） and mild (20/80°C) accelerated thermal cycling (ATC) tests were performed on various SAC solder joints. Correlations between microstructure and failure mechanism for solder joints on various BGA packages, chip scale packages (CSP), and quad-flat no-lead (QFN) packages were examined. The microstructure of samples was carefully analyzed;selected samples were removed from the chamber after different numbers of cycles in order to investigate the evolution of the SAC solder joint microstructure. Both recrystallization and intergranular crack growth were observed in these SAC solder joints after thermal cycling. Distinct coarsening of precipitates was observed in the recrystallized areas adjacent to cracks, consistent with strain enhanced coarsening. The 20/80°C reliability test results suggested that the failure mechanism of SAC assemblies is similar to that of conventional ATC profiles (0/100°C, -40/125 C) commonly performed in industry. After the same percentage of projected characteristic life, crack lengths were observed to be much smaller for interlaced twinning structures than for larger beach ball structures. This correlation of longer SAC solder joint lifetimes with interlaced Sn grain morphologies suggests that optimized control of Sn grain morphology in SAC solder joints may significantly enhance Pb free solder joint lifetime.
The photoelectrochemical responses of two archetypal metal–organic frameworks (MOFs), MOF-5 and MOF-177, have been assessed. Films of MOF-5 and MOF-177 were grown on carboxylic-acid-terminated conductive fluorine-dop...
详细信息
The photoelectrochemical responses of two archetypal metal–organic frameworks (MOFs), MOF-5 and MOF-177, have been assessed. Films of MOF-5 and MOF-177 were grown on carboxylic-acid-terminated conductive fluorine-doped tin oxide substrates. Separate analyses by powder X-ray diffraction, Raman spectroscopy, and fluorescence spectroscopy collectively indicated these films prepared via a solvothermal method in diethylformamide were free of residual impurities such as ZnO clusters and residual organics. Exposure of these films to white light illumination while immersed in acetonitrile electrolytes elicited measurable photocurrents. Wavelength-dependent analysis of the photoresponses showed that the measured photocurrents were induced by ultraviolet light and that the spectral response profiles followed closely the light absorption profiles of each respective material. Attenuation of the induced photocurrents was noted after prolonged ultraviolet light illumination and/or exposure of the films to H2O(l), indicating that the observed photoresponse properties are directly related to the structural integrity of these MOFs. The cumulative data illustrate that such MOFs have innately light-sensitive properties that are atypical in high surface area materials.
We investigated ultra-efficient nano-photonic modulators based on silicon photonic crystal slot waveguides infiltrated with electro-optic polymers. The integration of Si3N4 guided mode resonance grating with plasmonic...
详细信息
Cobalt-based alloys are widely used as hardfacing materials when wear resistance is required at room temperature or high temperature applications. However, their performance is a consequence of their microstructures t...
详细信息
Cobalt-based alloys are widely used as hardfacing materials when wear resistance is required at room temperature or high temperature applications. However, their performance is a consequence of their microstructures that depends on the processing conditions. This work focused on the influence of solidification rate on the structure development by processing the alloys with and without the interference of the substrate. The coatings were characterized by scanning electron microscopy, energy dispersive spectrometer, optical microscopy and instrument indentation tests. Results showed that despite the same phases developed in tested conditions, differences in the solidification microstructure and the influence of Fe diffusing from the substrate accounted for the measured variation in hardness. Higher hardness values were obtained for the samples processed free-standing (mini billets) with respect to the coatings and they were independent of the processing parameters, indicating that the substrate compromise the properties of hardness, as expected.
暂无评论