Determining factors of thermoelectric properties of semiconductor nanowires

It is widely accepted that low dimensionality of semiconductor heterostructures and nanostructures can significantly improve their thermoelectric efficiency. However, what is less well understood is the precise role of electronic and lattice transport coefficients in the improvement. We differentiate and analyze the electronic and lattice contributions to the enhancement by using a nearly parameter-free theory of the thermoelectric properties of semiconductor nanowires. By combining molecular dynamics, density functional theory, and Boltzmann transport theory methods, we provide a complete picture for the competing factors of thermoelectric figure of merit. As an example, we study the thermoelectric properties of ZnO and Si nanowires. We find that the figure of merit can be increased as much as 30 times in 8-Å-diameter ZnO nanowires and 20 times in 12-Å-diameter Si nanowires, compared with the bulk. Decoupling of thermoelectric contributions reveals that the reduction of lattice thermal conductivity is the predominant factor in the improvement of thermoelectric properties in nanowires. While the lattice contribution to the efficiency enhancement consistently becomes larger with decreasing size of nanowires, the electronic contribution is relatively small in ZnO and disadvantageous in Si.     

Manifold-based surfaces with boundaries

We present a manifold-based surface construction extending the C^∞ construction of Ying and Zorin (2004). Our surfaces allow for piecewise-smooth boundaries and have an improved derivative and visual behavior. Their degree of smoothness can be controlled by the user. Two-flexibility of our surface construction is confirmed numerically for a range of local mesh configurations.     

Contracting preference relations for database applications

The binary relation framework has been shown to be applicable to many real-life preference handling scenarios. Here we study preference contraction: the problem of discarding selected preferences. We argue that the property of minimality and the preservation of strict partial orders are crucial for contractions. Contractions can be further constrained by specifying which preferences should be protected. We consider preference relations that are finite or finitely representable using preference formulas. We present algorithms for computing minimal and preference-protecting minimal contractions for finite as well as finitely representable preference relations. We study relationships between preference change in the binary relation framework and belief change in the belief revision theory. We evaluate the proposed algorithms experimentally and present the results.     

Macroporous polymeric hydrogels formed from acrylate modified polyvinyl alcohol macromers

Macroporous polymeric hydrogels for the last several years have found broad application in areas connected with medicine, especially in such new disciplines as cell and tissue engineering. In present work a novel combine approach is proposed for preparation of polyvinyl alcohol macroporous hydrogels by cross-linking of polyvinyl alcohol acrylic derivatives in the presence of heterophase of frozen aqueous media. Hydrogels prepared using this method does not need additional structure fixing and are characterized by high thermal stability in swollen state sustaining even heating to more than 100 °С. The influence of different factors and reaction conditions on the cross-linked hydrogel formation process was studied. The high yield of products (80 ÷ 95%) was observed when reaction was conducted at temperature range −12 ÷ −18 °С, concentration of macromer 4–12 weight %, and amount of initiator 0.8 ÷ 1.6 mg/ml. Moreover, the equilibrium swelling of synthesized macroporous hydrogels was investigated and it was shown that synthesized cross-linked hydrogels are characterized by high water absorption which is weakly depended on solution pH and ionic force values.     

Measurement of caffeic and ferulic acid equivalents in plasma after coffee consumption: Small intestine and colon are key sites for coffee metabolism

Previous studies on coffee examined absorption of phenolic acids (PA) in the small intestine, but not the contribution of the colon to absorption. Nine healthy volunteers ingested instant soluble coffee (～335 mg total chlorogenic acids (CGAs)) in water. Blood samples were taken over 12 h, and at 24 h to assess return to baseline. Many previous studies, which used glucuronidase and sulfatase, measured only PA and did not rigorously assess CGAs. To improve this, plasma samples were analyzed after full hydrolysis by chlorogenate esterase, glucuronidase and sulfatase to release aglycone equivalents of PA followed by liquid–liquid extraction and ESI‐LC‐ESI‐MS/MS detection. Ferulic, caffeic and isoferulic acid equivalents appeared rapidly in plasma, peaking at 1–2 h. Dihydrocaffeic and dihydroferulic acids appeared in plasma 6–8 h after ingestion (T_max=8–12 h). Substantial variability in maximum plasma concentration and T_max was also observed between individuals. This study confirms that the small intestine is a significant site for absorption of PA, but shows for the first time that the colon/microflora play the major role in absorption and metabolism of CGAs and PA from coffee.     

Microwave synthesis of yttria stabilized zirconia

Yttria stabilised zirconia (YSZ) nanocrystals, with a mean size between 5 and 10 nm, were prepared by microwave flash synthesis. Flash synthesis was performed in alcoholic solutions of yttrium, zirconium chloride and sodium ethoxide (EtONa) using a microwave autoclave (RAMO system) specially designed by authors. Energy dispersive X-ray analysis (EDX), X-ray powder diffraction (XRD), BET adsorption technique, photon correlation spectroscopy (PCS) transmission and scanning electron microscopy (TEM and SEM) are used to characterized these nanoparticles. Compared with conventional synthesis, nanopowders can be produced in a short period (e.g. 10 s), both high purity and stoechiometric control are obtained. Nevertheless, this mean of production is more cheaper and much faster than the ones commonly used to produce yttria stabilized zirconia (YSZ) by conventional sol-gel techniques.     

Advantages of graded-channel SOI nMOSFETs for application as source-follower analog buffer

In this work the performance of graded-channel (GC) SOI MOSFETs operating as source-follower buffers is presented. The experimental analysis is performed by comparing the gain and linearity of buffers implemented with GC and standard SOI MOS devices considering the same mask dimensions. It is shown that by using GC devices, buffer gain very close to the theoretical limit can be achieved, with improved linearity, while for standard devices the gain departs from the theoretical value depending on the inversion level imposed by the bias current and input voltage.Two-dimensional numerical simulations were performed in order to confirm some hypotheses proposed to explain the gain behavior observed in the experimental data. By using numerical simulations the channel length has been varied, showing that the gain of buffers implemented with GC devices remains close to the theoretical limit even when short-channel devices are adopted. It has also been shown that the length of a source-follower buffer using GC devices can be reduced by a factor of 5, in comparison with a standard SOI MOSFET, without gain loss or linearity degradation.     

Self-Sustained Microwave Absorption Induced by Extremely High Radiation Intensities in Surface Electrons on Liquid Helium

A new nonlinear-optical absorption effect is observed in electrons bound to the liquid helium surface. We study absorption of mm-wave radiation due to resonant excitation of electron bound states. Below 1 K, almost all electrons occupy the ground state. Therefore, the system should be transparent for resonant radiation connecting any two excited states. On the contrary, we observe strong absorption peaks associated with transitions between the first excited and the higher excited states. We show that this anomaly results from the bistability of the electron system induced by extremely high radiation intensities and the long electron relaxation time.