Evaluation of decay photocurrent measurements in dye‐sensitized solar cells: Application to laser beam‐induced current technique

The use of the laser beam‐induced current (LBIC) technique in photovoltaic devices is widespread, but its use in photoelectrochemical cells, such as dye‐sensitized solar cells (DSSCs), is limited due to the configuration of these devices. The main reason is the very slow response time of DSSCs in the decay process, and therefore LBIC scans take too long to perform. We have designed a procedure published in the literature to correct the photocurrent values obtained by the LBIC technique, based on an algorithm that uses a decreasing mono‐exponential model. This work presents a study of the decay measurements in DSSCs using several functions, in order to improve the algorithm designed. It concludes that functions such as a decreasing bi‐exponential or Becquerel function generate better fits to experimental data. Copyright © 2010 John Wiley & Sons, Ltd.     

Enhanced photocatalytic hydrogen production by improving the Pt dispersion over mesostructured TiO2

Among other potential applications, mesoporous titania with high surface area and crystalline framework is attractive in photocatalytic hydrogen production. The mesoporous structure with pore walls formed by nanocrystals of anatase would provide a shorter distance of the electron–hole pairs to reach the photocatalyst surface and a higher surface area to deposit modifiers of its photocatalytic activity. In this work, we have successfully applied a hard-templating pathway to obtain ordered mesoporous titania (m-TiO₂) with high surface area and anatase as main crystalline phase. Subsequently, various amounts of metallic Pt have been deposited using different impregnation methods. All reactions performed exhibit, at short times, a rapid increase in the hydrogen production rate until a point in which a nearly constant value is achieved. The material prepared by the “citrate method”, based on reduction and encapsulation with sodium citrate of Pt nanoparticles before the photocatalytic reaction, leads to the highest hydrogen production rates with the shortest time to reach the change on the trend of the activity curve. The reason of this result is that citrate method provides very good dispersion and, specially, because the Pt nanoparticles are deposited and reduced preferentially within the pores of m-TiO₂, leading to stronger interactions than the other two explored dispersion routes (wetness impregnation and photodeposition). Thus, despite introducing less than half of the theoretical amount of Pt, citrate method produces close to twice the amount of hydrogen obtained by the other dispersion routes. This production capacity is even higher when the amount of Pt loaded is increased, with the optimal concentration being determined as 2% (w/w).     

Electroplated Au-GaAs1?xPx and Au-Ga1?xAlxAs Schottky barriers

A novel technique based on pulse electroplating and surface treatment with very slow etching rate solutions has been applied to fabricate Au Schottky barriers on GaAs1?xPx and Ga1?xAlxAs. Device characteristics are compared to the behaviour of vacuum-deposited barriers. The present technique allows a simple and fast procedure to obtain Schottky barriers for material evaluation and device applications.     

Improvements in the estimation of daily minimum air temperature in peninsular Spain using MODIS land surface temperature

Air temperature (T_a) is a key variable in many environmental risk models and plays a very important role in climate change research. In previous studies we developed models for estimating the daily maximum (T_max), mean (T_mean), and minimum air temperature (T_min) in peninsular Spain over cloud-free land areas using Moderate Resolution Imaging Spectroradiometer (MODIS) data. Those models were obtained empirically through linear regressions between daily T_a and daytime Terra-MODIS land surface temperature (LST), and then optimized by including spatio-temporal variables. The best T_mean and T_max models were satisfactory (coefficient of determination (R²) of 0.91–0.93; and residual standard error (RSE) of 1.88–2.25 K), but not the T_min models (R² = 0.80–0.81 and RSE = 2.83–3.00 K). In this article T_min models are improved using night-time Aqua LST instead of daytime Terra LST, and then refined including total precipitable water (W) retrieved from daytime Terra-MODIS data and the spatio-temporal variables curvature (c), longitude (λ), Julian day of the year (JD) and elevation (h). The best T_min models are based on the National Aeronautics and Space Administration (NASA) standard product MYD11 LST; and on the direct broadcast version of this product, the International MODIS/AIRS Processing Package (IMAPP) LST product. Models based on Sobrino’s LST1 algorithm were also tested, with worse results. The improved T_min models yield R² = 0.91–0.92 and RSE = 1.75 K and model validations obtain similar R² and RSE values, root mean square error of the differences (RMSD) of 1.87–1.88 K and bias = 0.11 K. The main advantage of the T_min models based on the IMAPP LST product is that they can be generated in nearly real-time using the MODIS direct broadcast system at the University of Oviedo.     

Differences between the isostructural IRMOF-1 and MOCP-L porous adsorbents

Several discrepancies have emerged about the preparation methods of MOF-5 material, one of the adsorbents more studied for hydrogen storage, because different synthesis procedures give rise to apparently isostructural materials but actually having very different textural properties and also showing different hydrogen adsorption capacities. In this work, MOF-5 material has been successfully synthesized according to the two most extended methods, and the respective resulting materials, IRMOF-1 and MOCP-L, have been widely characterized. Powder X-ray diffraction confirmed that MOF-5 is the main crystallized phase obtained in both cases, and that the MOCP-L product contains some ZnO impurities, mainly in the form of segregated crystals that appear preferentially outside the micropores. The presence of ZnO in the MOCP-L material has been also confirmed by other characterization techniques. On the other hand, interpenetrated networks in the MOCP-L material are not significant, appearing in any case at a similar extent than in IRMOF-1; consequently it cannot be considered for explaining the differences observed in the adsorption capacity of both materials. These differences cannot be completely explained simply by the segregated ZnO impurities, so the presence of some ZnO nanocrystals partially blocking the micropores of MOCP-L must be admitted.     

Analysis and experimental results of a single-stagehigh-power-factor electronic ballast based on flyback converter

A new single-stage high-power-factor electronic ballast based on a flyback converter is presented in this paper. The ballast is able to supply a fluorescent lamp assuring a high-input power factor for the utility line. Other features are lamp power regulation against line voltage variations and low lamp current crest factor, both assuring long lamp life. The ballast is analyzed at steady-state operation, and design equations and characteristics are obtained. Also, a procedure for the ballast design is presented. Finally, simulation and experimental results from a laboratory prototype are shown     

Dynamic-mechanical relaxations in high and low density polyethylenes. Effects of irradiation and annealing

A study of the dynamic-mechanical relaxation spectrum in a series of commercial high and low density polyethylenes (Dow Chemical), irradiated as well as unirradiated, and subjected to different annealing process, has been performed. The effect of 20-Mrad dose of irradiation on the chemical structure has been analyzed and an increase in the number of aldehyde, ketone, and transvinylene groups and a decrease in the number of vinyl and vinylidene groups has been observed. The dynamic-mechanical spectrum of irradiated and unirradiated high and low density polyethylenes contains the γ_II-, γ_I-, β-, α_I-, α_II-, and α_III- relaxations, in order of increasing temperature. It has been observed that γ-irradiation followed by annealing modified the intensity and the position of relaxations in these polyethylenes.     

On polyethylene chain generation through chemical crosslinking of isotactic polypropylene

Crosslinked isotactic polypropylene (iPP) was prepared by reactive blending using dicumyl peroxide and active sulfur. The modified material was characterized by means of several techniques: dynamic mechanical thermal analysis, differential scanning calorimetry, Fourier transform infrared spectroscopy, and nuclear magnetic resonance (¹³C solid state NMR). Analysis of results using the above techniques confirms that, during the crosslinking process, new ethylenic chains are generated at expenses of the initially present iPP. Furthermore, a balance between the amount of missing iPP and the proportion of newly created polyethylene (PE) is observed. The newly formed PE is semicrystalline, having a degree of crystallinity of about 30%. The amorphous component of the generated PE is thought to contribute to the high impact strength of the crosslinked samples when compared to the unmodified iPP. In light of the results obtained by the different techniques, an attempt to describe the emerging structure of the new crosslinked iPP material is proposed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010