Monte Carlo simulations of densely-packed athermal polymers in the bulk and under confinement

We review the main results from extensive Monte Carlo (MC) simulations on athermal polymer packings in the bulk and under confinement. By employing the simplest possible model of excluded volume, macromolecules are represented as freely-jointed chains of hard spheres of uniform size. Simulations are carried out in a wide concentration range: from very dilute up to very high volume fractions, reaching the maximally random jammed (MRJ) state. We study how factors like chain length, volume fraction and flexibility of bond lengths affect the structure, shape and size of polymers, their packing efficiency and their phase behaviour (disorder–order transition). In addition, we observe how these properties are affected by confinement realized by flat, impenetrable walls in one dimension. Finally, by mapping the parent polymer chains to primitive paths through direct geometrical algorithms, we analyse the characteristics of the entanglement network as a function of packing density.

Optimizing phosphorus diffusion for photovoltaic applications: peak doping, inactive phosphorus,gettering, and contact formation

The phosphosilicate glass (PSG), fabricated by tube furnace diffusion using a POCl3 source, is widely used as a dopant source in the manufacturing of crystalline silicon solar cells. Although it has been a widely addressed research topic for a long time, there is still lack of a comprehensive understanding of aspects such as the growth, the chemical composition, possible phosphorus depletion, the resulting in-diffused phosphorus profiles, the gettering behavior in silicon, and finally the metal-contact formation. This paper addresses these different aspects simultaneously to further optimize process conditions for photovoltaic applications. To do so, a wide range of experimental data is used and combined with device and process simulations, leading to a more comprehensive interpretation. The results show that slight changes in the PSG process conditions can produce high-quality emitters. It is predicted that PSG processes at 860 °C for 60 min in combination with an etch-back and laser doping from PSG layer results in high-quality emitters with a peak dopant density Npeak = 8.0 × 1018 cm−3 and a junction depth dj = 0.4 μm, resulting in a sheet resistivityρsh = 380 Ω/sq and a saturation current-density J0 below 10 fA/cm2. With these properties, the POCl3 process can compete with ion implantation or doped oxide approaches.

Joule heating in ferromagnetic nanostripes with a notch

The temperature in a ferromagnetic nanostripe with a notch subject to Joule heating has been studied in detail. We first performed an experimental real-time calibration of the temperature versus time as a 100 ns current pulse was injected into a Permalloy nanostripe. This calibration was repeated for different pulse amplitudes and stripe dimensions and the set of experimental curves were fitted with a computer simulation using the Fourier thermal conduction equation. The best fit of these experimental curves was obtained by including the temperature-dependent behavior of the electrical resistivity of the Permalloy and of the thermal conductivity of thesubstrate(SiO2). Notably, a nonzero interface thermal resistance between the metallic nanostripe and thesubstrate was also necessary to fit the experimental curves. We found this parameter pivotal to understand ourresults and the results from previous works. The higher current density in the notch, together with the interface thermal resistance, allows a considerable increase of the temperature in the notch, creating a large horizontal thermal gradient. This gradient, together with the high temperature in the notch and the larger current density close to the edges of the notch, can be very influential in experiments studying the current assisted domain wall motion.

Analysis of the behavior of multijunction solar cells under high irradiance Gaussian light profiles showing chromatic aberration with emphasis on tunnel junction performance

In this work, we explain the behavior of multijunction solar cells under non-uniform (spatially and in spectral content) light profiles in general and in particular when Gaussian light profiles cause a photo-generated current density, which exceeds locally the peak current density of the tunnel junction. We have analyzed the implications on the tunnel junction's limitation, that is, in the loss of efficiency due to the appearance of a dip in the I–V curve. For that, we have carried out simulations with our three-dimensional distributed model for multijunction solar cells, which contemplates a full description of the tunnel junction and also takes into account the lateral resistances in the tunnel junction. The main findings are that the current density photo-generated spreads out through the lateral resistances of the device, mainly through the tunnel junction layers and the back contact. Therefore, under non-uniform light profiles these resistances are determinant not only to avoid the tunnel junction's limitation but also for mitigating losses in the fill factor. Therefore, taking into account these lateral resistances could be the key for jointly optimizing the concentrator photovoltaic system (concentrator optics, front grid layout and semiconductor structure)

Cyclic AMP regulates potassium channel expression in C6 glioma by destabilizing Kv1.1 mRNA

The tissue distributions and physiological properties of a variety of cloned voltage-gated potassium channel genes have been characterized extensively, yet relatively little is known about the mechanisms controlling expression of these genes. Here, we report studies on the regulation of Kv1.1 expressed endogenously in the C6 glioma cell line. We demonstrate that elevation of intracellular cAMP leads to the accelerated degradation of Kv1.1 RNA. The cAMP-induced decrease in Kv1.1 RNA is followed by a decrease in Kv1.1 protein and a decrease in the whole cell sustained K+ current amplitude. Dendrotoxin-I, a relatively specific blocker of Kv1.1, blocks 96% of the sustained K+ current in glioma cells, causing a shift in the resting membrane potential from −40 mV to −7 mV. These data suggest that expression of Kv1.1 contributes to setting the resting membrane potential in undifferentiated glioma cells. We therefore suggest that receptor-mediated elevation of cAMP reduces outward K+ current density by acting at the translational level to destabilize Kv1.1 RNA, an additional mechanism for regulating potassium channel gene expression.

Antisense oligonucleotides against α1E reduce R-type calcium currents in cerebellar granule cells

Many neurons of the central nervous system display multiple high voltage-activated Ca2+ currents, pharmacologically classified as L-, N-, P-, Q-, and R-type. Of these current types, the R-type is the least understood. The leading candidate for the molecular correlate of R-type currents in cerebellar granule cells is the α1E subunit, which yields Ca2+ currents very similar to the R-type when expressed in heterologous systems. As a complementary approach, we tested whether antisense oligonucleotides against α1E could decrease the expression of R-type current in rat cerebellar granule neurons in culture. Cells were supplemented with either antisense or sense oligonucleotides and whole-cell patch clamp recordings were obtained after 6–8 days in vitro. Incubation with α1E antisense oligonucleotide caused a 52.5% decrease in the peak R-type current density, from −10 ± 0.6 picoamperes/picofarad (pA/pF) (n = 6) in the untreated controls to −4.8 ± 0.8 pA/pF (n = 11) (P < 0.01). In contrast, no significant changes in the current expression were seen in sense oligonucleotide-treated cells (−11.3 ± 3.2 pA/pF). The specificity of the α1E antisense oligonucleotides was supported by the lack of change in estimates of the P/Q current amplitude. Furthermore, antisense and sense oligonucleotides against α1A did not affect R-type current expression (−11.5 ± 1.7 and −11.7 ± 1.7 pA/pF, respectively), whereas the α1A antisense oligonucleotide significantly reduced whole cell currents under conditions in which P/Q current is dominant. Our results support the hypothesis that members of the E class of α1 subunits support the high voltage-activated R-type current in cerebellar granule cells.

Tropomyosin directly modulates actomyosin mechanical performance at the level of a single actin filament

Muscle contraction is the result of myosin cross-bridges (XBs) cyclically interacting with the actin-containing thin filament. This interaction is modulated by the thin filament regulatory proteins, troponin and tropomyosin (Tm). With the use of an in vitro motility assay, the role of Tm in myosin’s ability to generate force and motion was assessed. At saturating myosin surface densities, Tm had no effect on thin filament velocity. However, below 50% myosin saturation, a significant reduction in actin–Tm filament velocity was observed, with complete inhibition of movement occurring at 12.5% of saturating surface densities. Under similar conditions, actin filaments alone demonstrated no reduction in velocity. The effect of Tm on force generation was assessed at the level of a single thin filament. In the absence of Tm, isometric force was a linear function of the density of myosin on the motility surface. At 50% myosin surface saturation, the presence of Tm resulted in a 2-fold enhancement of force relative to actin alone. However, no further potentiation of force was observed with Tm at saturating myosin surface densities. These results indicate that, in the presence of Tm, the strong binding of myosin cooperatively activates the thin filament. The inhibition of velocity at low myosin densities and the potentiation of force at higher myosin densities suggest that Tm can directly modulate the kinetics of a single myosin XB and the recruitment of a population of XBs, respectively. At saturating myosin conditions, Tm does not appear to affect the recruitment or the kinetics of myosin XBs.

An initiation element in the yeast CUP1 promoter is recognized by RNA polymerase II in the absence of TATA box-binding protein if the DNA is negatively supercoiled

Purified RNA polymerase II initiated transcription from the yeast CUP1 promoter fused to a C-less cassette if the DNA was negatively supercoiled. Relaxed plasmid was not transcribed. Transcription did not require addition of any other transcription factors. TATA box-binding protein (TBP) was not detectable in the polymerase preparation and the TATA box was not required. Deletion analysis of the CUP1 promoter revealed that a 25-bp element containing the initiation region was sufficient for recognition by polymerase. Two transcription start sites were mapped, one of which is identical to one of the two major start sites observed in vivo. Our observations can be accounted for by using a theoretical analysis of the probability of DNA melting within the plasmid as a function of superhelix density: the CUP1 initiation element is intrinsically unstable to superhelical stress, permitting entry of the polymerase, which then scans the DNA to locate the start site. In support of this analysis, the CUP1 promoter was sensitive to mung bean nuclease. These observations and a previous theoretical analysis of yeast genes support the idea that promoters are stress points within the DNA superhelix. The role of transcription factors might be to mark the promoter and to regulate specific melting of promoter DNA.

Estudo da degradação do dietil ftalato por processo eletroquímico com ânodo dimensionalmente estável em sistemas aquosos

Os ésteres de ftalato (PAEs) são compostos produzidos em grandes quantidades, amplamente utilizados industrialmente como agentes plastificantes. Seus resíduos são lixiviados pela água tornando-se poluentes orgânicos persistentes (POPs) no meio ambiente aquoso, além de apresentar características de interferência endócrina. O dietil ftalato (DEP) é frequentemente encontrado nas amostras ambientais, pois possui elevada solubilidade na água e pode ser gerado durante a degradação de outros PAEs. Assim, este trabalho teve como objetivo a degradação do dietil ftalato em meio aquoso por método eletroquímico utilizando um ânodo dimensionalmente estável (ADE) comercial representado como Ti/Ru0,3Ti0,7O2 em uma célula do tipo filtro-prensa. As eletrólises foram de 120 minutos contendo uma concentração inicial de 100,3 mg L-1 de DEP, pH inicial igual a 3, a temperatura em 25 °C e vazão em 250 mL min-1. Os experimentos foram feitos utilizando planejamento fatorial do tipo 32 com duas réplicas no ponto central, apresentando como variáveis independentes a densidade de corrente (10, 25 e 40 mA cm-2) e o logaritmo em base 10 da forca iônica do eletrólito suporte, NaCl e Na2SO4 (µ = 0,05, 0,15 e 0,5 mol L-1), com o intuito de estudar o efeito da densidade de corrente, concentração e natureza do eletrólito para determinar a melhor condição de degradação do dietil ftalato. O monitoramento da concentração do DEP foi feito com cromatografia líquida de alta eficiência (CLAE) e a mineralização foi acompanhada pelas análises de carbono orgânico total (COT). Foram obtidas maiores porcentagens de remoção e mineralização com uso das maiores densidades de corrente e na presença de altas concentrações de NaCl em comparação com Na2SO4. Dessa maneira, se obteve remoção de 63,2 % e mineralização de 63,9 % em solução 0,5 mol L-1 NaCl e densidade de corrente de 40 mA cm-2, enquanto que para Na2SO4 (µ = 0,5 mol L-1) e 40 mA cm-2 foi removido 51,3 % e mineralizado 53,0 % de DEP. O mecanismo de degradação de DEP foi determinado em meio de NaCl e Na2SO4, através de CLAE-MS nas condições citadas anteriormente, identificando-se os íons moleculares de m/z 149 e 177 em ambos eletrólitos, correspondentes ao anidrido ftálico protonado e ao aduto do anidrido ftálico com C(2)H(5)(+) respectivamente, íons característicos da fragmentação do DEP, além do íon m/z 239 em Na2SO4 correspondente ao dietil 3-hidroxiftalato. A degradação do DEP acontece através da cadeia alifática.

Chronopotentiometric study of a Nafion membrane in presence of glucose

Chronopotentiometric and swelling experiments have been conducted to characterize the behavior of a Nafion membrane in NaCl and KCl aqueous solutions without and with glucose. A mixture solution with similar composition to the cerebrospinal fluid and blood plasma has also been studied. From the chronotentiograms, current-voltage curves have been obtained, and the values of the limiting current density, diffusion boundary layer thickness, difference between counter-ion transport number in membrane and free solution, and transition times have been determined for the investigated membrane systems. The obtained results indicate that the presence of glucose affects the ion transport through the membrane depending on the electrolyte and glucose concentrations. At low electrolyte concentration, experimental transition times are found to be smaller in presence of glucose, which has been related to an effective membrane area reduction in presence of glucose. The membrane system corresponding to the mixture solution shows a behavior similar to the single high concentration NaCl membrane system, indicating that the observed behavior is mainly associated to the Na^+ ions transport in higher proportion. In this case, the glucose presence does not affect significantly the investigated properties of the membrane, which is interesting for its utilization in a glucose fuel cell.

Carbon–carbon asymmetric aqueous capacitor by pseudocapacitive positive and stable negative electrodes

An asymmetric aqueous capacitor was constructed by employing zeolite-templated carbon (ZTC) as a pseudocapacitive positive electrode and KOH-activated carbon as a stable negative electrode. The asymmetric capacitor can be operated with the working voltage of 1.4 V, and exhibits an energy density that is comparable to those of conventional capacitors utilizing organic electrolytes, thanks to the large pseudocapacitance of ZTC. Despite relatively thick electrode (0.2 mm) configuration, the asymmetric capacitor could be well operated under a current density of 500 mA g −1.

Electrocatalytic hydrogenation of acetophenone using a Polymer Electrolyte Membrane Electrochemical Reactor

The use of a solid polymeric electrolyte, spe, is not commonly found in organic electrosynthesis despite its inherent advantages such as the possible elimination of the electrolyte entailing simpler purification processes, a smaller sized reactor and lower energetic costs. In order to test if it were possible to use a spe in industrial organic electrosynthesis, we studied the synthesis of 1-phenylethanol through the electrochemical hydrogenation of acetophenone using Pd/C 30 wt% with different loadings as cathode and a hydrogen gas diffusion anode. A Polymer Electrolyte Membrane Electrochemical Reactor, PEMER, with a fuel cell structure was chosen to carry out electrochemical reduction with a view to simplifying an industrial scale-up of the electrochemical process. We studied the influence of current density and cathode catalyst loading on this electroorganic synthesis. Selectivity for 1-phenylethanol was around 90% with only ethylbenzene and hydrogen detected as by-products.

Oxygen evolution at ultrathin nanostructured Ni(OH)2 layers deposited on conducting glass

Ultrathin and transparent nanostructured Ni(OH)2 films were deposited on conducting glass (F:SnO2) by a urea-based chemical bath deposition method. By controlling the deposition time, the amount of deposited Ni(OH)2 was varied over 7 orders of magnitude. The turnover number for O2 generation, defined as the number of O2 molecules generated per catalytic site (Ni atom) and per second, increases drastically as the electrocatalyst amount decreases. The electrocatalytic activity of the studied samples (measured as the current density at a certain potential) increases with the amount of deposited Ni(OH)2 until a saturation value is already obtained for a thin film of around 1 nm in thickness, composed of Ni(OH)2 nanoplatelets lying flat on the conductive support. The deposition of additional amounts of catalyst generates a porous honeycomb structure that does not improve (only maintains) the electrocatalytic activity. The optimized ultrathin electrodes show a remarkable stability, which indicates that the preparation of highly transparent electrodes, efficient for oxygen evolution, with a minimum amount of nickel is possible.

Sonochemical synthesis of graphene oxide supported Pt–Pd alloy nanocrystals as efficient electrocatalysts for methanol oxidation

Pt–Pd bimetallic nanoparticles supported on graphene oxide (GO) nanosheets were prepared by a sonochemical reduction method in the presence of polyethylene glycol as a stabilizing agent. The synthetic method allowed for a fine tuning of the particle composition without significant changes in their size and degree of aggregation. Detailed characterization of GO-supported Pt–Pd catalysts was carried out by transmission electron microscopy (TEM), AFM, XPS, and electrochemical techniques. Uniform deposition of Pt–Pd nanoparticles with an average diameter of 3 nm was achieved on graphene nanosheets using a novel dual-frequency sonication approach. GO-supported bimetallic catalyst showed significant electrocatalytic activity for methanol oxidation. The influence of different molar compositions of Pt and Pd (1:1, 2:1, and 3:1) on the methanol oxidation efficiency was also evaluated. Among the different Pt/Pd ratios, the 1:1 ratio material showed the lowest onset potential and generated the highest peak current density. The effect of catalyst loading on carbon paper (working electrode) was also studied. Increasing the catalyst loading beyond a certain amount lowered the catalytic activity due to the aggregation of metal particle-loaded GO nanosheets.

Electrochemical behaviour of activated carbons obtained via hydrothermal carbonization

Activated carbons were prepared by chemical activation of hydrochars, obtained by hydrothermal carbonisation (HTC) using low cost and abundant precursors such as rye straw and cellulose, with KOH. Hydrochars derived from rye straw were chemically activated using different KOH/precursor ratios, in order to assess the effect of this parameter on their electrochemical behaviour. In the case of cellulose, the influence of the hydrothermal carbonisation temperature was studied by fixing the activating agent/cellulose ratio. Furthermore, N-doped activated carbons were synthesised by KOH activation of hydrochars prepared by HTC from a mixture of glucose with melamine or glucosamine. In this way, N-doped activated carbons were prepared in order to evaluate the influence of nitrogen groups on their electrochemical behaviour in acidic medium. The results showed that parameters such as chemical activation or carbonisation temperature clearly affect the capacitance, since these parameters play a key role in the textural properties of activated carbons. Finally, symmetric capacitors based on activated carbon and N-doped activated carbon were tested at 1.3 V in a two-electrode cell configuration and the results revealed that N-groups improved the capacitance at high current density.