4,157 research outputs found
Modification of chiral dimethyl tartrate through transesterification : immobilisation on POSS and enantioselectivity reversion in Sharpless asymmetric epoxidation
Modification of dimethyl tartrate has been investigated through transesterification with aminoalcohols to provide reactive functionalities for the covalent bonding of chiral tartrate to polyhedral oligomeric silsesquioxanes. The transesterification of dimethyl tartrate has been widely studied by means of using different catalytic systems and reaction conditions. Through the proper selection of both, the catalytic system and the reaction conditions, it is possible to achieve the mono- or the bis-substituted tartrate derivative as sole products. All the intermediate chiral tartrate-derived ligands were successfully used in the homogeneous enantioselective epoxidation of allylic alcohols providing moderate enantiomeric excess over the products. Attached amine groups have been used to support the modified tartrate ligands onto a haloaryl-functionalized silsesquioxane moiety. This final chiral tartrate ligand displays enantioselectivity reversion in the asymmetric epoxidation of allylic alcohols with regards to the starting dimethyl tartrate ligand, having both molecules them the same chiral sign. However, the POSS-containing ligand can be easily recovered in almost quantitative yield and reused in asymmetric epoxidation reactions. In addition, recovered silsesquioxane-pendant ligand, though displaying decreasing catalytic activity in recycling epoxidation tests, showed very stable enantioselective behavior
Analysis of double-parallel amplified recirculating optical-delay lines
A novel method of analysis of double-parallel amplified recirculating optical-delay lines (DPAROD) is presented. The location of the maxima and the minima of the transfer function for this configuration is calculated and experimentally demonstrated. The influence of different parameters, such as the coupling coefficients, gains, lengths of the fiber loops and fractional losses of the directional couplers, on the shape of the transfer function are analyzed. Different measurements have been taken to verify this model. The potential application of these interconnected delay loops as filters is a reason for developing this method.Publicad
Low temperature dielectric relaxation in ordinary perovskite ferroelectrics: enlightenment from high-energy x-ray diffraction
Ordinary ferroelectrics exhibit a second order phase transition that is characterized by a sharp peak in the dielectric permittivity at a frequency-independent temperature. Furthermore, these materials show a low temperature dielectric relaxation that appears to be a common behavior of perovskite systems. Tetragonal lead zirconate titanate is used here as a model system in order to explore the origin of such an anomaly, since there is no consensus about the physical phenomenon involved in it. Crystallographic and domain structure studies are performed from temperature dependent synchrotron x-ray diffraction measurement. Results indicate that the dielectric relaxation cannot be associated with crystallographic or domain configuration changes. The relaxation process is then parameterized by using the VogelâFulcherâTammann phenomenological equation. Results allow us to hypothesize that the observed phenomenon is due to changes in the dynamic behavior of the ferroelectric domains related to the fluctuation of the local polarization.Postprint (author's final draft
Amplified Fiber-Optic Recirculating Delay Lines
Experimental and theoretical results on single- and double-amplified recirculating delay lines are presented. One of our aims is to emphasize their application as filters, showing a wide flexibility of design. Analysis of their performance in the spectral and time domains have been carried out. A novel method of understanding the behavior of double structures has been developed and successfully tested with experimental results employing Er-doped fiber amplifiers as delay lines.Publicad
Preisach modelling of nonlinear response in electrically biased lead zirconate titanate-based piezoceramics
The alteration of the high-field electrical permittivity (nonlinear response) of PZT-based ceramics when an electrical bias field is applied is reported in this work. Large differences are observed between soft and hard PZT behaviours. While in soft PZT a bias field does not modify the nonlinear behaviour, a notable dependence is verified in hard PZT. The Preisach model is satisfactorily used to describe experimental results. A distribution function containing the first terms of the Maclaurin development series of a function composed by two Gaussian-like functions of different amplitudes is proposed. The model gives a satisfactory explanation for the fact that the permittivity depends not only on the amplitude of the applied electric field, but also on the bias field, both for soft and hard ceramics and for poled or unpoled samples.Postprint (published version
Evaluation of a Bayesian Algorithm to Detect Burned Areas in the Canary Islandsâ Dry Woodlands and Forests Ecoregion Using MODIS Data
Burned Area (BA) is deemed as a primary variable to understand the Earthâs climate system. Satellite remote sensing data have allowed for the development of various burned area detection algorithms that have been globally applied to and assessed in diverse ecosystems, ranging from tropical to boreal. In this paper, we present a Bayesian algorithm (BY-MODIS) that detects burned areas in a time series of Moderate Resolution Imaging Spectroradiometer (MODIS) images from 2002 to 2012 of the Canary Islandsâ dry woodlands and forests ecoregion (Spain). Based on daily image products MODIS, MOD09GQ (250 m), and MOD11A1 (1 km), the surface spectral reflectance and the land surface temperature, respectively, 10 day composites were built using the maximum temperature criterion. Variables used in BY-MODIS were the Global Environment Monitoring Index (GEMI) and Burn Boreal Forest Index (BBFI), alongside the NIR spectral band, all of which refer to the previous year and the year the fire took place in. Reference polygons for the 14 fires exceeding 100 hectares and identified within the period under analysis were developed using both post-fire LANDSAT images and official information from the forest fires national database by the Ministry of Agriculture and Fisheries, Food and Environment of Spain (MAPAMA). The results obtained by BY-MODIS can be compared to those by official burned area products, MCD45A1 and MCD64A1. Despite that the best overall results correspond to MCD64A1, BY-MODIS proved to be an alternative for burned area mapping in the Canary Islands, a region with a great topographic complexity and diverse types of ecosystems. The total burned area detected by the BY-MODIS classifier was 64.9% of the MAPAMA reference data, and 78.6% according to data obtained from the LANDSAT images, with the lowest average commission error (11%) out of the three products and a correlation (R2) of 0.82. The Bayesian algorithmâoriginally developed to detect burned areas in North American boreal forests using AVHRR archival data Long-Term Data Recordâcan be successfully applied to a lower latitude forest ecosystem totally different from the boreal ecosystem and using daily time series of satellite images from MODIS with a 250 m spatial resolution, as long as a set of training areas adequately characterising the dynamics of the forest canopy affected by the fire is defined
Insight into the dynamics of low temperature dielectric relaxation of ordinary perovskite ferroelectrics
The temperature dependence of the dielectric response of ordinary ferroelectric materials exhibits a frequency-independent anomalous peak as a manifestation of the ferroelectric to paraelectric phase transition. A second anomaly in the permittivity has been reported in different ferroelectric perovskite-type systems at low temperatures, often at cryogenic temperatures. This anomaly manifests as a frequency-dependent local maximum, which exhibits similar characteristics to that observed in relaxor ferroelectrics around their phase transition. The origin of this unexpected behavior is still controversial. In order to clarify this phenomenon, a model-free route solution is developed in this work. Our findings reveal the same critical linear pattern/glass-like freezing behavior previously observed for glass-forming systems. Contrary to current thought, our results suggest that a critical-like dynamic parameterization could provide a more appropriate solution than the conventional VogelâFulcherâTammann equation. The implemented methodology may open a new pathway for analyzing relaxation phenomena in other functional materials like relaxor ferroics.Postprint (published version
Multicultural education: some reflections on the spanish case
In this paper we succinctly describe sorne aspects related to multicultural education in Spain. First we must recognise that there exists a somewhat extensive and significant work on Gypsies (the most important ethnic group in Spain) that we do not include in this paper, because it would require a separate article. It is much more interesting, we hope, to present the state of art of discourses and practices related to multicultural education in our country. In fact, we argue Spanish authorities have never considered the education of Gypsy children as a matter of multicultural education. The very phrases 'multicultural education' and 'intercultural education' have only recently been introduced in Spain. In the first section of this paper we describe the reasons put forth by sorne authors
to support intercultural education. Their rationale for more multicultural education is compared to our own reasons. Sections number two to five briefly review sorne congresses on multicultural education held in our country (II), current research (III), legislation (IV), and programs in action (V). The common feature of these sections is the emphasis on showing how scarce and recent the whole topic of multiculturalism is in Spain. However, many Spanish investigators and educators are beginning to work in thĂs fĂeld, and government Ănvolvement is rapidly growing
Efecto de dos especies de plantas invasoras fijadoras de nitrĂłgeno sobre las propiedades quĂmicas del suelo en el centro-sur de Chile
Dos especies invasoras leñosas, Acacia dealbata y Teline monspessulana (Fabaceae), ampliamente distribuidas en el centrosurde Chile, poseen la capacidad de fijar biolĂłgicamente el nitrĂłgeno atmosfĂ©rico y contribuir a la fertilidad del suelo. Sepresentan evidencias de potenciales cambios ocasionados por estas especies en las propiedades quĂmicas del suelo en ĂĄreasaltamente degradadas de la RegiĂłn del BĂo-BĂo, Chile.Dos especies invasoras leñosas, Acacia dealbata y Teline monspessulana (Fabaceae), ampliamente distribuidas en el centrosurde Chile, poseen la capacidad de fijar biolĂłgicamente el nitrĂłgeno atmosfĂ©rico y contribuir a la fertilidad del suelo. Sepresentan evidencias de potenciales cambios ocasionados por estas especies en las propiedades quĂmicas del suelo en ĂĄreasaltamente degradadas de la RegiĂłn del BĂo-BĂo, Chile
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