It was recently suggested theoretically that atomically thin films of Bi2Te3 topological insulators have strongly enhanced thermoelectric figure of merit. We used the “graphene-like” exfoliation process to obtain Bi...
详细信息
It was recently suggested theoretically that atomically thin films of Bi2Te3 topological insulators have strongly enhanced thermoelectric figure of merit. We used the “graphene-like” exfoliation process to obtain Bi2Te3 thin films. The films were stacked and subjected to thermal treatment to fabricate pseudo-superlattices of single crystal Bi2Te3 films. Thermal conductivity of these structures was measured by the “hot disk” and “laser flash” techniques. The room temperature in-plane and cross-plane thermal conductivity of the stacks decreased by a factor of ~2.4 and 3.5 respectively as compared to that of bulk. The strong decrease of thermal conductivity with preserved electrical properties translates to ~140–250% increase in the thermoelectric figure if merit. It is expected that the film thinning to few-quintuples, and tuning of the Fermi level can lead to the topological insulator surface transport regime with the theoretically predicted extraordinary thermoelectric efficiency.
In this paper we are concerned with the morphology of the polymers adsorbedon surfaces, in particular di-block copolymers. Our work is motivated by the experimental findings of Fladziioannou et al. [1] on the steric f...
In this paper we are concerned with the morphology of the polymers adsorbedon surfaces, in particular di-block copolymers. Our work is motivated by the experimental findings of Fladziioannou et al. [1] on the steric forces between two adsorbed layers of di-block poly(vinyl-2-pyridine)\ polystyrene (PV2P\ PS) copolymer on mica surfaces. The PV2P block binds strongly on the mica surfaces and the PS block extends into thesolvent toluene (good solvent for PS). Hadziiouannou et al. found that the repulsive forces between the two surfaces start at a distance 1) larger than 10 times the radius of gyration RG of a free P’ in toluene. Furthermore, the starting distance D increases with increasing degree of polymerization N of PS in a fashion I) ~ Na with a close to I. We,tudy the adsorption of di-block copolymer with Monte Carlo simulations. The Monte Carlo simulations are especially powerful in dealing with kinetics which is important in systems where hysteresis is observed II1 and cannot be appropriately taken into account by analytical (or numerical) calculations based onequilibrium assumptions.
We report in situ transmission electron microscopy observations of the dependence of the degree of orientations of oxide nuclei on oxygen pressure during the initial-stage oxidation of Cu(100) surfaces, which reveal a...
详细信息
We report in situ transmission electron microscopy observations of the dependence of the degree of orientations of oxide nuclei on oxygen pressure during the initial-stage oxidation of Cu(100) surfaces, which reveal a transition from nucleating epitaxial Cu2O islands to randomly oriented oxide islands with increasing the oxygen pressure from 5 × 10−5 to 150 Torr. We show by a kinetic model that this transition of oxide nucleation orientation is driven by the influence of oxygen pressure on the oxide nucleation barrier and atom collision rate.
We have used transmitted polarized light microscopy to examine the fluid contents of silk glands taken from Bombyx mori silkworms and Nephila clavipes orb-weaving spiders. In the absence of shear, the secretions are o...
We have used transmitted polarized light microscopy to examine the fluid contents of silk glands taken from Bombyx mori silkworms and Nephila clavipes orb-weaving spiders. In the absence of shear, the secretions are optically isotropic. As the concentration is allowed to increase by evaporation, microstructures typical of the nematic liquid crystalline state are observed. Thus it appears that naturally spun silk becomes liquid crystalline en route to solidifying into fiber - which is advantageous to introducing and retaining global molecular alignment. This will facilitate the formation of strong, stiff fibers without the need for a significant post-spinning draw. We have also found that natural silk does not exhibit the defects in molecular alignment that are typical of synthetic polymer fibers spun from liquid crystalline solutions or melts.
This paper presents a deterministic and adaptive spike model derived from radial basis functionsand a leaky integrate-and-fire sampler developed for training spiking neural networks without directweight manipulation. ...
This paper presents a deterministic and adaptive spike model derived from radial basis functions
and a leaky integrate-and-fire sampler developed for training spiking neural networks without direct
weight manipulation. Several algorithms have been proposed for training spiking neural networks
through biologically-plausible learning mechanisms, such as spike-timing-dependent synaptic plasticity
and Hebbian plasticity. These algorithms typically rely on the ability to update the synaptic strengths,
or weights, directly, through a weight update rule in which the weight increment can be decided
and implemented based on the training equations. However, in several potential applications of
adaptive spiking neural networks, including neuroprosthetic devices and CMOS/memristor nanoscale
neuromorphic chips, the weights cannot be manipulated directly and, instead, tend to change over time
by virtue of the pre- and postsynaptic neural activity. This paper presents an indirect learning method
that induces changes in the synaptic weights by modulating spike-timing-dependent plasticity by means
of controlled input spike trains. In place of the weights, the algorithm manipulates the input spike trains
used to stimulate the input neurons by determining a sequence of spike timings that minimize a desired
objective function and, indirectly, induce the desired synaptic plasticity in the network.
It was recently discovered that in addition to its exotic electronic properties graphene reveals unusually high intrinsic thermal *** physics of phonons–the main heat carriers in graphene–was shown to be substantial...
详细信息
It was recently discovered that in addition to its exotic electronic properties graphene reveals unusually high intrinsic thermal *** physics of phonons–the main heat carriers in graphene–was shown to be substantially different in two-dimensional(2D)crystals,such as graphene,than in three-dimensional(3D)***,we report
We show evidence of electrical and thermal conductivity percolation in polymer based carbon nanotube (CNT) composites, which follow power law variations with respect to the CNT concentrations in the matrix. The experi...
详细信息
We show evidence of electrical and thermal conductivity percolation in polymer based carbon nanotube (CNT) composites, which follow power law variations with respect to the CNT concentrations in the matrix. The experimentally obtained percolation thresholds, i.e., ~ 0.074 vol % for single walled CNTs and ~ 2.0 vol % for multi-walled CNTs, were found to be aspect ratio dependent and in accordance with those determined theoretically from excluded volume percolation theory. A much greater enhancement, over 10 orders of magnitude, was obtained in the electrical conductivity at the percolation threshold, while a smaller increase of ~ 100 % was obtained in the thermal conductivity values. Such a difference is qualitatively explained on the basis of the respective conductivity contrast between the CNT filler and the polymer matrix.
Sapphire wafers are widely used as the substrates for fabricating GaN light-emitting diodes (LEDs). The quality of LEDs depends directly on the quality of the sapphire wafers. A series of machining processes is requir...
详细信息
Sapphire wafers are widely used as the substrates for fabricating GaN light-emitting diodes (LEDs). The quality of LEDs depends directly on the quality of the sapphire wafers. A series of machining processes is required to turn sapphire barstock into wafers with the desired geometry and surface quality. This paper reviews the literature on various machining processes for sapphire wafers: slicing (including outer diameter cut-off grinding, inner diameter cut-off grinding, loose abrasive multi-wire sawing, fixed abrasive slicing, and fixed abrasive multi-wire sawing), flattening (including lapping and grinding), and surface finishing (including mechanical polishing and chemical mechanical polishing). New machining technologies are also introduced.
暂无评论