Dielectric Relaxations and Conductivity of Crosslinked PVA/SSA/GO Composite Membranes for Fuel Cells

[EN] Composite membranes obtained from Poly(vinyl Alcohol) (PVA) with sulfosuccinic acid (SSA) as crosslinking agent, and two different proportions of graphene oxide (GO), were prepared to be used in Proton Exchange Membrane Fuel Cells (PEMFCs). The superficial micrographs from transmission electron microscopy (TEM) confirmed a good dispersion of GO. Fourier Transform Infrared spectroscopy (FTIR) was used to evaluate the final chemical structure. Differential Scanning Calorimetry (DSC) showed that glass transition and crystalline phase were not present in the cross-linked PVA/SSA/GO composites membranes. Thermogravimetric analysis (TGA) demonstrated that the addition of GO reduced the moisture content and increased the thermal stability of the membranes. The electrical properties of PVA/SSA and PVA/SSA/GO composite membranes and the effect of GO concentration were evaluated by means of dielectric spectra in a broad range of temperatures and frequencies. The dielectric permittivity of these membranes was significantly promoted at low filler concentration due to an interfacial polarization effect. From the analysis of the dielectric relaxation spectrum, it can be deduced that the origin of the associated molecular movements is intramolecular and occurs in the working range of the PMEFC. In addition, the direct current conductivity, the protonic conductivity, and the polarization currents were correlated to the power produced in a hydrogen monocell. It was observed that low and no high GO concentrations of filler in PVA/SSA composite membranes enhanced their performance. The systematic characterization procedure based on the study of dielectric spectra and conductivity allowed to establish a potential approach to control the addition of GO in the design of other composite membranes for PEMFC with improved properties.The authors would like to thank Dr. Roberto Teruel Juanes and Dr. Victor Saenz de Juano for their advice in the treatment of results. The authors are also thankful to Generalitat Valenciana and the European Social Fund for the Santiago Grisolia scholarship, GRISOLIA/2013/031, and the Spanish Ministry of Science and Innovation for the concession of Research Project ENE2014-53734-C2-1-R.González-Guisasola, C.; Ribes-Greus, A. (2018). Dielectric Relaxations and Conductivity of Crosslinked PVA/SSA/GO Composite Membranes for Fuel Cells. Polymer Testing. 67:55-67. https://doi.org/10.1016/j.polymertesting.2018.01.024S55676

Concepciones de futuros profesores de ciencias sobre un concepto "olvidado" en la enseñanza de la electricidad : la fuerza electromotriz

El trabajo que aquí se presenta recoge los resultados de una investigación en torno al aprendizaje del concepto de fuerza electromotriz a través de las siguientes etapas: a) conocer las principales dificultades que se produjeron en el desarrollo del concepto a lo largo de la historia; b) establecer las principales características de un aprendizaje significativo del concepto de fuerza electromotriz; c) averiguar en qué medida se presentan dificultades de aprendizaje de este concepto en estudiantes de último curso de carrera y recién licenciados en ciencias. Los resultados obtenidos parecen confirmar que los estudiantes, al acabar sus estudios de licenciatura no han logrado un claro significado de la magnitud fuerza electromotriz y, en consecuencia, presentan confusiones entre ésta y la diferencia de potencial en circuitos eléctricos y en fenómenos sencillos de inducción electromagnética. En la situación particular de un fenómeno sencillo de inducción electromagnética, casi la totalidad de los estudiantes presentan serias deficiencias en los significados de la fuerza electromotriz y diferencia de potencial.This paper offers the result of our research into the learning of the concept of EMF at an introductory level according to the following stages: a) To know the main difficulties when defining the concept throughout history. b) To establish the main characteristics of the meaningful learning of the concept of EMF. c) To analyse to what extent students have a meaninful learning of EMF when they finish their degrees in science. The results seem to confirm that university students when finishing their science degrees have not got a clear idea of the notion of EMF. Therefore they often mistake it for potential difference. In the specific test of an easy experience of EMF induction, nearly all the students had serious problems in differentiating between EMF and potential difference

Probing university students’ understanding of electromotive force in electricity

The goal of this study is to identify students’ difficulties with learning the concepts of electromotive force (emf) and potential difference in the context of transitory currents and resistive direct-current circuits. To investigate these difficulties, we developed a questionnaire based on an analysis of the theoretical and epistemological framework of physics, which was then administered to first-year engineering and physics students at universities in Spain, Colombia, and Belgium. The results of the study show that student difficulties seem to be strongly linked to the absence of an analysis of the energy balance within the circuit and that most university students do not clearly understand the usefulness of and the difference between the concepts of potential difference and emf

Influence of the Molecular Weight on PVA/GO Composite Membranes for Fuel Cell Applications

Composite polymer electrolyte membranes were prepared with poly (vinyl alcohol) (PVA). Two different molecular weight (Mw), 67·103 and 130·103 g·mol−1 were selected, cross-linked with sulfosuccinic acid (SSA) and doped graphene oxide (GO). The effects on the membranes obtained from these polymers were characterized in order to evaluate the fuel cell performance. Electron microscopy showed a proper nanoparticle distribution in the polymer matrix. The chemical structure was evaluated by Fourier transform infrared spectroscopy. The absence of a crystalline structure and the enhancement on the thermal stability with the addition of 1% of GO was demonstrated by thermal characterization. Total transference number and protonic conductivity were correlated to the performance of a hydrogen fuel cell. Overall, a power increase in the composite membranes with lower molecular weight was observed. Shorter polymer chains may improve protonic conductivity and consequently the fuel cell performance

Student understanding of the Special Theory of Relativity and design for a guided visit to a science museum

The present paper describes the design of teaching materials that are used as learning tools in school visits to a science museum. An exhibition on 'A century of the Special Theory of Relativity', in the Kutxaespacio Science Museum, in San Sebastian, Spain, was used to design a visit for first- year engineering students at the university and assess the learning that was achieved. The first part of the paper presents the teaching sequence that was designed to build a bridge between formal teaching and the exhibition visit. The second part analyses the potential of the exhibition and the aforementioned teaching sequence to influence the students' knowledge of three aspects of the Special Theory of Relativity. The results obtained show that the design of the visit, with both pre- visit and follow-up activities, was effective as a means of increasing students' understanding and stimulating their ability to argue scientifically

Development of a kinetic model for elemental sulfur and sulfate formation from the autotrophic sulfide oxidation using respirometric techniques

©IWA Publishing 2009. The definitive peer-reviewed and edited version of this article is published in Water science and technology, vol. 59, núm. 7, p. 1323-1329, 2009. DOI: 10.2166/wst.2009.110 and is available at www.iwapublishing.com.A kinetic model for the elemental sulfur and sulfate production from the autotrophic sulfide oxidation has been proposed. It is based on two kinetic equations able to describe the simultaneous microbial consumption of oxygen and sulfide (OUR and SUR) as a function of a particular sulfide-oxidizing microorganism or its physiological state, these can be characterized by the assessment of their kinetic constants. The respirometric technique allowed to estimate the dynamic experimental OUR and SUR profiles, which were used to calibrate the kinetic model. The ratio OUR/SUR was proposed to predict the sulfide oxidation extent and then the fate of sulfide to elemental sulfur and sulfate.Peer ReviewedPostprint (author's final draft

P28 85. Análisis de la supervivencia a corto y medio plazo de la cirugía de aorta ascendente

ObjetivosEvaluar la mortalidad hospitalaria y la supervivencia a medio plazo de los pacientes sometidos a una cirugía de aorta ascendente y determinar cuáles son sus factores de riesgo.Material y métodosSe analizó de forma retrospectiva toda la población de pacientes sometidos a un procedimiento quirúrgico de aorta ascendente (sustitución con o sin cirugía valvular o de raíz asociada) entre enero de 2003 y diciembre de 2008. Se evaluó la mortalidad hospitalaria y la supervivencia absoluta en el seguimiento, y mediante análisis uni y multivariante se determinaron las correspondientes variables predictoras.ResultadosTrescientos sesenta y cinco pacientes fueron sometidos de forma consecutiva a algún procedimiento quirúrgico de la aorta ascendente. Edad media: 63,1 años (desviación estándar [DE] 13); n = 99 (27,1%) fueron mujeres; n = 58 (15,9%) habían sido sometidos a una cirugía previa. En 32 pacientes se sustituyó total o parcialmente el arco aórtico. EuroSCORE mediano de 11,72% (interquartile range [IQR]: 6,78-20,43). La mortalidad hospitalaria fue 7,7% (n = 28). Los predictores de mortalidad en análisis multivariante fueron: cirugía cardíaca previa (odds ratio [OR]: 3,18; p = 0,045), tabaquismo activo (OR: 5,7; p = 0,021), insuficiencia renal (OR: 3,03; p = 0,043), estado crítico preoperatorio (OR: 5,63; p = 0,005), edad (OR: 1,06; p = 0,018) y cirugía coronaria asociada (OR: 3,17; p = 0,35). La supervivencia a 1, 3 y 5 años fue 96, 94 y 92%. Sólo edad (hazard ratio [HR]: 1,08; p = 0,022) e hipertensión pulmonar grave (HR: 3,5; p = 0,026) fueron predictores independientes de mortalidad en el seguimiento.ConclusionesEn nuestra experiencia, la sustitución de la aorta ascendente demuestra una baja mortalidad hospitalaria y una adecuada supervivencia a medio plazo

Magnetic-Responsive Release Controlled by Hot Spot Effect

Magnetically triggered drug delivery nanodevices have attracted great attention in nanomedicine, as they can feature as smart carriers releasing their payload at clinician's will. The key principle of these devices is based on the properties of magnetic cores to generate thermal energy in the presence of an alternating magnetic field. Then, the temperature increase triggers the drug release. Despite this potential, the rapid heat dissipation in living tissues is a serious hindrance for their clinical application. It is hypothesized that magnetic cores could act as hot spots, this is, produce enough heat to trigger the release without the necessity to increase the global temperature. Herein, a nanocarrier has been designed to respond when the temperature reaches 43 degrees C. This material has been able to release its payload under an alternating magnetic field without the need of increasing the global temperature of the environment, proving the efficacy of the hot spot mechanism in magnetic-responsive drug delivery devices