http://hdl.handle.net/2117/635 Sat, 25 May 2013 04:29:15 GMT 2013-05-25T04:29:15Z webmaster.bupc@upc.edu Universitat Politècnica de Catalunya. Servei de Biblioteques i Documentació no http://hdl.handle.net/2117/19055 Title: Stochastic Phase-Field simulations of symmetric alloy solidification Authors: Benítez Iglesias, Raúl; Ramírez de la Piscina Millán, Laureano Abstract: We study initial transient stages in directional solidification by means of a non-variational phase field model with fluctuations. This model applies for the symmetric solidification of dilute binary solutions and does not invoke fluctuation-dissipation theorem to account for the fluctuation statistics. We devote our attention to the transient regime during which concentration gradients are building up and fluctuations act to destabilize the interface. To this end, we calculate both the temporally dependent growth rate of each mode and the power spectrum of the interface evolving under the effect of fluctuations. Quantitative agreement is found when comparing the phase-field simulations with theoretical predictions. Tue, 30 Apr 2013 11:08:59 GMT http://hdl.handle.net/2117/19055 2013-04-30T11:08:59Z Benítez Iglesias, Raúl; Ramírez de la Piscina Millán, Laureano no We study initial transient stages in directional solidification by means of a non-variational phase field model with fluctuations. This model applies for the symmetric solidification of dilute binary solutions and does not invoke fluctuation-dissipation theorem to account for the fluctuation statistics. We devote our attention to the transient regime during which concentration gradients are building up and fluctuations act to destabilize the interface. To this end, we calculate both the temporally dependent growth rate of each mode and the power spectrum of the interface evolving under the effect of fluctuations. Quantitative agreement is found when comparing the phase-field simulations with theoretical predictions. http://hdl.handle.net/2117/19053 Title: Phase-field models in interfacial pattern formation out of equilibrium Authors: González Cinca, Ricardo; Folch, R.; Benítez Iglesias, Raúl; Ramírez de la Piscina Millán, Laureano; Casademunt, Jaume; Hernández-Machado, A. Tue, 30 Apr 2013 10:58:31 GMT http://hdl.handle.net/2117/19053 2013-04-30T10:58:31Z González Cinca, Ricardo; Folch, R.; Benítez Iglesias, Raúl; Ramírez de la Piscina Millán, Laureano; Casademunt, Jaume; Hernández-Machado, A. no http://hdl.handle.net/2117/18961 Title: Dependency of calcium alternans on Ryanodine Receptor Refractoriness Authors: Álvarez Lacalle, Enrique; Rodríguez Cantalapiedra, Inma; Peñaranda Ayllón, Angelina; Cinca, J.; Hove-Madsen, Leif; Echebarría Domínguez, Blas Abstract: Background Rapid pacing rates induce alternations in the cytosolic calcium concentration caused by fluctuations in calcium rele However, the relationship between calcium alternans and refractoriness of the SR calcium release channel (RyR2) Methodology/Principal Findings To investigate how ryanodine receptor (RyR2) refractoriness modulates calcium handling on a beat-to-beat basis u used a mathematical rabbit cardiomyocyte model to study the beat-to-beat calcium response as a function of RyR were constructed depicting the beat-to-beat response. When alternans was observed, a novel numerical clamping was caused by oscillations in SR calcium loading or by RyR2 refractoriness. Using this protocol, we identified regio loading or RyR2 refractoriness underlie the induction of calcium alternans, and we found that at the onset of altern inactivation rates of the RyR2, calcium alternans was caused by alternation in SR calcium loading, while at low act level of available RyR2s. Conclusions/Significance We have mapped cardiomyocyte beat-to-beat responses as a function of RyR2 activation and inactivation, identify refractoriness underlie the induction of calcium alternans. A corollary of this work is that RyR2 refractoriness due t of calcium alternans even when alternation in SR calcium load is present. Tue, 23 Apr 2013 13:08:23 GMT http://hdl.handle.net/2117/18961 2013-04-23T13:08:23Z Álvarez Lacalle, Enrique; Rodríguez Cantalapiedra, Inma; Peñaranda Ayllón, Angelina; Cinca, J.; Hove-Madsen, Leif; Echebarría Domínguez, Blas no Background Rapid pacing rates induce alternations in the cytosolic calcium concentration caused by fluctuations in calcium rele However, the relationship between calcium alternans and refractoriness of the SR calcium release channel (RyR2) Methodology/Principal Findings To investigate how ryanodine receptor (RyR2) refractoriness modulates calcium handling on a beat-to-beat basis u used a mathematical rabbit cardiomyocyte model to study the beat-to-beat calcium response as a function of RyR were constructed depicting the beat-to-beat response. When alternans was observed, a novel numerical clamping was caused by oscillations in SR calcium loading or by RyR2 refractoriness. Using this protocol, we identified regio loading or RyR2 refractoriness underlie the induction of calcium alternans, and we found that at the onset of altern inactivation rates of the RyR2, calcium alternans was caused by alternation in SR calcium loading, while at low act level of available RyR2s. Conclusions/Significance We have mapped cardiomyocyte beat-to-beat responses as a function of RyR2 activation and inactivation, identify refractoriness underlie the induction of calcium alternans. A corollary of this work is that RyR2 refractoriness due t of calcium alternans even when alternation in SR calcium load is present. http://hdl.handle.net/2117/18794 Title: Experimental analysis of the bubble-slug transition in a flow generated by a T-junction in a minichannel with air/water and air/ethanol mixtures in conditions relevant to microgravity Authors: Arias Calderón, Santiago; González Cinca, Ricardo Abstract: We present an experimental study on the bubble–slug flow pattern transition which takes place in two-phase flows in microgravity related conditions. Two different sets of experiments were performed on ground with air/water and air/ethanol mixtures in a minichannel with an internal diameter of 1 mm. We address questions regarding the existence of a critical void fraction for the bubble–slug transition to occur. Experimental data are compared to the drift-flux and Suratman models predictions. We obtain results on the extension of applicability of these models for different boundary conditions. Mon, 15 Apr 2013 13:43:05 GMT http://hdl.handle.net/2117/18794 2013-04-15T13:43:05Z Arias Calderón, Santiago; González Cinca, Ricardo no We present an experimental study on the bubble–slug flow pattern transition which takes place in two-phase flows in microgravity related conditions. Two different sets of experiments were performed on ground with air/water and air/ethanol mixtures in a minichannel with an internal diameter of 1 mm. We address questions regarding the existence of a critical void fraction for the bubble–slug transition to occur. Experimental data are compared to the drift-flux and Suratman models predictions. We obtain results on the extension of applicability of these models for different boundary conditions. http://hdl.handle.net/2117/18789 Title: Study of a system to generate electricity on the Moon's surface during the lunar night Authors: Torroba, Oscar; Climent, Blai; González Cinca, Ricardo; Ramachandran, Narayanan; Griffin, Michael D. Abstract: One of the biggest challenges of the exploration of the Moon is the survival of the crew and the lunar assets during the lunar night. The environmental conditions on the lunar surface and its cycle, with long periods of darkness, make any long mission in need of specific amounts of heat and electricity to be successful. We have analyzed two different systems to produce heat and electricity on the Moon’s surface. The first system consists of the Thermal Wadis, sources of thermal power that can be used to supply heat to protect the exploration systems from the extreme cold during periods of darkness. Previous results showed that Wadis can supply enough heat to keep lunar devices such as rovers above their minimum operating temperature (approximately 243 K). The second system studied here is the Thermal Energy Storage (TES), which is able to run a heat engine during the lunar night to produce electricity. When the Sun is shining on the Moon’s surface, the system can run the engine directly using the solar power and simultaneously heat a thermal mass. This thermal mass is used as a high temperature source to run the heat engine during the night. We present analytical and numerical calculations for the determination of an appropriate thermal mass for the TES system. Mon, 15 Apr 2013 11:04:41 GMT http://hdl.handle.net/2117/18789 2013-04-15T11:04:41Z Torroba, Oscar; Climent, Blai; González Cinca, Ricardo; Ramachandran, Narayanan; Griffin, Michael D. no One of the biggest challenges of the exploration of the Moon is the survival of the crew and the lunar assets during the lunar night. The environmental conditions on the lunar surface and its cycle, with long periods of darkness, make any long mission in need of specific amounts of heat and electricity to be successful. We have analyzed two different systems to produce heat and electricity on the Moon’s surface. The first system consists of the Thermal Wadis, sources of thermal power that can be used to supply heat to protect the exploration systems from the extreme cold during periods of darkness. Previous results showed that Wadis can supply enough heat to keep lunar devices such as rovers above their minimum operating temperature (approximately 243 K). The second system studied here is the Thermal Energy Storage (TES), which is able to run a heat engine during the lunar night to produce electricity. When the Sun is shining on the Moon’s surface, the system can run the engine directly using the solar power and simultaneously heat a thermal mass. This thermal mass is used as a high temperature source to run the heat engine during the night. We present analytical and numerical calculations for the determination of an appropriate thermal mass for the TES system. http://hdl.handle.net/2117/18515 Title: Low density lipoproteins promote unstable calcium handling accompanied by reduced SERCA2 and connexin-40 expression in cardiomyocytes Authors: Barriga, Montserrat; Cal, Roi; Cabello, Nuria; Llach, Anna; Vallmitjana Lees, Alex; Benítez Iglesias, Raúl; Badimon, Lina; Cinca, J.; Llorente-Cortés, Vicenta; Hove-Madsen, Leif Abstract: The damaging effects of high plasma levels of cholesterol in the cardiovascular system are widely known, but little attention has been paid to direct effects on cardiomyocyte function. We therefore aimed at testing the hypothesis that Low Density Lipoprotein (LDL) cholesterol affects calcium dynamics and signal propagation in cultured atrial myocytes. For this purpose, mRNA and protein expression levels were determined by real time PCR and western blot analysis, respectively, and intracellular calcium was visualized in fluo-4 loaded atrial HL-1 myocyte cultures subjected to field stimulation. At low stimulation frequencies all cultures had uniform calcium transients at all tested LDL concentrations. However, 500 mg LDL/mL maximally reduced the calcium transient amplitude by 43% from 0.3060.04 to 0.1760.02 (p,0.05). Moreover, LDLcholesterol dose-dependently increased the fraction of alternating and irregular beat-to-beat responses observed when the stimulation interval was shortened. This effect was linked to a concurrent reduction in SERCA2, RyR2, IP3RI and IP3RII mRNA levels. SERCA2 protein levels were also reduced by 43% at 200 mg LDL/mL (p,0.05) and SR calcium loading was reduced by 3866% (p,0.001). By contrast, HDL-cholesterol had no significant effect on SERCA expression or SR calcium loading. LDLcholesterol also slowed the conduction velocity of the calcium signal from 3.2+0.2 mm/s without LDL to 1.760.1 mm/s with 500 mg LDL/mL (p,0.05). This coincided with a reduction in Cx40 expression (by 4463%; p,0.05 for mRNA and by 7962%; p,0.05 for Cx40 protein at 200 mg/ml LDL) whereas the Cx-43 expression did not significantly change. In conclusion, LDLcholesterol destabilizes calcium handling in cultured atrial myocytes subjected to rapid pacing by reducing SERCA2 and Cx40 expression and by slowing the conduction velocity of the calcium signal. Tue, 02 Apr 2013 09:45:58 GMT http://hdl.handle.net/2117/18515 2013-04-02T09:45:58Z Barriga, Montserrat; Cal, Roi; Cabello, Nuria; Llach, Anna; Vallmitjana Lees, Alex; Benítez Iglesias, Raúl; Badimon, Lina; Cinca, J.; Llorente-Cortés, Vicenta; Hove-Madsen, Leif no The damaging effects of high plasma levels of cholesterol in the cardiovascular system are widely known, but little attention has been paid to direct effects on cardiomyocyte function. We therefore aimed at testing the hypothesis that Low Density Lipoprotein (LDL) cholesterol affects calcium dynamics and signal propagation in cultured atrial myocytes. For this purpose, mRNA and protein expression levels were determined by real time PCR and western blot analysis, respectively, and intracellular calcium was visualized in fluo-4 loaded atrial HL-1 myocyte cultures subjected to field stimulation. At low stimulation frequencies all cultures had uniform calcium transients at all tested LDL concentrations. However, 500 mg LDL/mL maximally reduced the calcium transient amplitude by 43% from 0.3060.04 to 0.1760.02 (p,0.05). Moreover, LDLcholesterol dose-dependently increased the fraction of alternating and irregular beat-to-beat responses observed when the stimulation interval was shortened. This effect was linked to a concurrent reduction in SERCA2, RyR2, IP3RI and IP3RII mRNA levels. SERCA2 protein levels were also reduced by 43% at 200 mg LDL/mL (p,0.05) and SR calcium loading was reduced by 3866% (p,0.001). By contrast, HDL-cholesterol had no significant effect on SERCA expression or SR calcium loading. LDLcholesterol also slowed the conduction velocity of the calcium signal from 3.2+0.2 mm/s without LDL to 1.760.1 mm/s with 500 mg LDL/mL (p,0.05). This coincided with a reduction in Cx40 expression (by 4463%; p,0.05 for mRNA and by 7962%; p,0.05 for Cx40 protein at 200 mg/ml LDL) whereas the Cx-43 expression did not significantly change. In conclusion, LDLcholesterol destabilizes calcium handling in cultured atrial myocytes subjected to rapid pacing by reducing SERCA2 and Cx40 expression and by slowing the conduction velocity of the calcium signal. http://hdl.handle.net/2117/18395 Title: Bubble and slug flows characteristic lengths in a microchannel Authors: Arias Calderón, Santiago; González Cinca, Ricardo Tue, 19 Mar 2013 11:32:46 GMT http://hdl.handle.net/2117/18395 2013-03-19T11:32:46Z Arias Calderón, Santiago; González Cinca, Ricardo no http://hdl.handle.net/2117/17509 Title: Nonlinearities due to refractoriness in SR Ca release Authors: Peñaranda Ayllón, Angelina; Álvarez Lacalle, Enrique; Rodríguez Cantalapiedra, Inma; Hove-Madsen, Leif; Echebarría Domínguez, Blas Abstract: Calcium alternans is a pro-arrhytmic cardiac dysfunction related to beat-to-beat changes in the amplitude of intracellular calcium transient, that typically occurs at rapid pacing rates. Although oscillations in sarcoplasmic reticulum (SR) content have been related with calcium alternans, the experimental appearance of alternans without these oscillations suggests that another mechanism related with refractoriness of SR calcium release might be key, at least, under certain conditions. We investigate how RyR2 refractoriness modulates calcium handling on a beat-to-beat basis using a numerical rabbit cardiomyocyte model. We find that a slow recovery from inactivation of the RyR2 might be crucial. On one hand, a steep relation between sarcoplasmic reticulum (SR) load and calcium release makes regular calcium cycling unstable at high SR calcium load and/or fast pacing rates, in agreement with previous explanation when RyR2 inactivation is not important. On the other hand, we show that calcium release can also depend strongly on the number of RyR2 ready to open if an important number of RyR2s inactivate after the release. This gives rise to a steep nonlinear relation between the calcium release and the level of recovered RyR2, so that a small change in the later produces big changes in calcium release. A conclusion of this result is that RyR2 refractoriness can be the main nonlinearity behind alternans even when alternation in SR-Ca load is present Thu, 24 Jan 2013 13:11:32 GMT http://hdl.handle.net/2117/17509 2013-01-24T13:11:32Z Peñaranda Ayllón, Angelina; Álvarez Lacalle, Enrique; Rodríguez Cantalapiedra, Inma; Hove-Madsen, Leif; Echebarría Domínguez, Blas no Calcium alternans is a pro-arrhytmic cardiac dysfunction related to beat-to-beat changes in the amplitude of intracellular calcium transient, that typically occurs at rapid pacing rates. Although oscillations in sarcoplasmic reticulum (SR) content have been related with calcium alternans, the experimental appearance of alternans without these oscillations suggests that another mechanism related with refractoriness of SR calcium release might be key, at least, under certain conditions. We investigate how RyR2 refractoriness modulates calcium handling on a beat-to-beat basis using a numerical rabbit cardiomyocyte model. We find that a slow recovery from inactivation of the RyR2 might be crucial. On one hand, a steep relation between sarcoplasmic reticulum (SR) load and calcium release makes regular calcium cycling unstable at high SR calcium load and/or fast pacing rates, in agreement with previous explanation when RyR2 inactivation is not important. On the other hand, we show that calcium release can also depend strongly on the number of RyR2 ready to open if an important number of RyR2s inactivate after the release. This gives rise to a steep nonlinear relation between the calcium release and the level of recovered RyR2, so that a small change in the later produces big changes in calcium release. A conclusion of this result is that RyR2 refractoriness can be the main nonlinearity behind alternans even when alternation in SR-Ca load is present http://hdl.handle.net/2117/16978 Title: Percolation thresholds in chemical excitable media. Authors: Sendiña-Nadal, I.; Roncaglia, D.; Vives, D.; Pérez Muñuzuri, Vicente; Gómez Gesteira, Moncho; Pérez Villar, Vicente; Echave, J.; Casademunt, Jaume; Ramírez de la Piscina Millán, Laureano; Sagués Mestre, Francesc Abstract: The behavior of chemical waves advancing through a disordered excitable medium is investigated in terms of percolation theory and autowave properties in the framework of the light-sensitive Belousov-Zhabotinsky reaction. By controlling the number of sites with a given illumination, different percolation thresholds for propagation are observed, which depend on the relative wave transmittances of the two-state medium considered. Tue, 20 Nov 2012 13:41:34 GMT http://hdl.handle.net/2117/16978 2012-11-20T13:41:34Z Sendiña-Nadal, I.; Roncaglia, D.; Vives, D.; Pérez Muñuzuri, Vicente; Gómez Gesteira, Moncho; Pérez Villar, Vicente; Echave, J.; Casademunt, Jaume; Ramírez de la Piscina Millán, Laureano; Sagués Mestre, Francesc no Chemival waves, Disorder, Excitable media, Belousov-Zhabotinsky reaction, Percolation The behavior of chemical waves advancing through a disordered excitable medium is investigated in terms of percolation theory and autowave properties in the framework of the light-sensitive Belousov-Zhabotinsky reaction. By controlling the number of sites with a given illumination, different percolation thresholds for propagation are observed, which depend on the relative wave transmittances of the two-state medium considered. http://hdl.handle.net/2117/16977 Title: Wave propagation in a medium with disordered excitability Authors: Sendiña-Nadal, I.; Pérez Muñuzuri, Alberto; Vives, D.; Pérez Muñuzuri, Vicente; Casademunt, Jaume; Ramírez de la Piscina Millán, Laureano; Sancho, Jose Maria; Sagués Mestre, Francesc Abstract: The effect of quenched disorder on the propagation of autowaves in excitable media is studied both experimentally and numerically in relation to the light-sensitive Belousov-Zhabotinsky reaction. The spatial disorder is introduced through a random distribution with two different levels of transmittance. In one dimension the (time-averaged) wave speed is smaller than the corresponding to a homogeneous medium with the mean excitability. Contrarily, in two dimensions the velocity increases due to the roughening of the front. Results are interpreted using kinematic and scaling arguments. In particular, for d ­ 2 we verify a theoretical prediction of a power-law dependence for the relative change of the propagation speed on the disorder amplitude. Tue, 20 Nov 2012 13:06:17 GMT http://hdl.handle.net/2117/16977 2012-11-20T13:06:17Z Sendiña-Nadal, I.; Pérez Muñuzuri, Alberto; Vives, D.; Pérez Muñuzuri, Vicente; Casademunt, Jaume; Ramírez de la Piscina Millán, Laureano; Sancho, Jose Maria; Sagués Mestre, Francesc no Chemical waves, Disorder, Excitable media, Belousov-Zhabotinsky reaction The effect of quenched disorder on the propagation of autowaves in excitable media is studied both experimentally and numerically in relation to the light-sensitive Belousov-Zhabotinsky reaction. The spatial disorder is introduced through a random distribution with two different levels of transmittance. In one dimension the (time-averaged) wave speed is smaller than the corresponding to a homogeneous medium with the mean excitability. Contrarily, in two dimensions the velocity increases due to the roughening of the front. Results are interpreted using kinematic and scaling arguments. In particular, for d ­ 2 we verify a theoretical prediction of a power-law dependence for the relative change of the propagation speed on the disorder amplitude. http://hdl.handle.net/2117/16975 Title: Wave competition in excitable modulated media Authors: Sendiña-Nadal, I.; Gómez Gesteira, Moncho; Pérez Muñuzuri, Vicente; Pérez Villar, Vicente; Armero, J.; Ramírez de la Piscina Millán, Laureano; Casademunt, Jaume; Sancho, Jose Maria; Sagués Mestre, Francesc Abstract: The propagation of an initially planar front is studied within the framework of the photosensitive Belousov- Zhabotinsky reaction modulated by a smooth spatial variation of the local front velocity in the direction perpendicular to front propagation. Under this modulation, the wave front develops several fingers corresponding to the local maxima of the modulation function. After a transient, the wave front achieves a stationary shape that does not necessarily coincide with the one externally imposed by the modulation. Theoretical predictions for the selection criteria of fingers and steady-state velocity are experimentally validated. Tue, 20 Nov 2012 12:49:24 GMT http://hdl.handle.net/2117/16975 2012-11-20T12:49:24Z Sendiña-Nadal, I.; Gómez Gesteira, Moncho; Pérez Muñuzuri, Vicente; Pérez Villar, Vicente; Armero, J.; Ramírez de la Piscina Millán, Laureano; Casademunt, Jaume; Sancho, Jose Maria; Sagués Mestre, Francesc no Front propagation, Chemical waves, Excitable media, Belousov-Zhabotinsky reaction, Inhomogeneous media The propagation of an initially planar front is studied within the framework of the photosensitive Belousov- Zhabotinsky reaction modulated by a smooth spatial variation of the local front velocity in the direction perpendicular to front propagation. Under this modulation, the wave front develops several fingers corresponding to the local maxima of the modulation function. After a transient, the wave front achieves a stationary shape that does not necessarily coincide with the one externally imposed by the modulation. Theoretical predictions for the selection criteria of fingers and steady-state velocity are experimentally validated. http://hdl.handle.net/2117/16974 Title: Universality of the Gunn effect: self-sustained oscillations mediated by solitary waves Authors: Bonilla, Luis L.; Rodríguez Cantalapiedra, Inma Abstract: The Gunn effect consists of time-periodic oscillations of the current flowing through an external purely resistive circuit mediated by solitary wave dynamics of the electric field on an attached appropriate semiconductor. By means of an asymptotic analysis, it is argued that Gunn-like behavior occurs in specific classes of model equations. As an illustration, an example related to the constrained Cahn-Allen equation is analyzed. Tue, 20 Nov 2012 12:34:49 GMT http://hdl.handle.net/2117/16974 2012-11-20T12:34:49Z Bonilla, Luis L.; Rodríguez Cantalapiedra, Inma no Transport processes, Solitary waves The Gunn effect consists of time-periodic oscillations of the current flowing through an external purely resistive circuit mediated by solitary wave dynamics of the electric field on an attached appropriate semiconductor. By means of an asymptotic analysis, it is argued that Gunn-like behavior occurs in specific classes of model equations. As an illustration, an example related to the constrained Cahn-Allen equation is analyzed. http://hdl.handle.net/2117/16912 Title: Cementos químicos formulados con subproductos de óxido de magnesio Authors: Formosa Mitjans, Joan; Aranda, Miguel Ángel; Chimenos Ribera, Josep Ma.; Rosell Amigó, Juan Ramón; Fernández Renna, Ana Inés; Ginés, O. Abstract: Es posible obtener cementos de fosfato de magnesio y potasio (KMgPO4·6H2O; K-estruvita), mediante la reacción en medio acuoso del óxido de magnesio y el dihidrogenofosfato de potasio, siendo ésta una reacción exotérmica muy rápida que permite el fraguado del material en pocos minutos. Estos cementos, formulados a partir de óxidos de magnesio de elevada pureza y coste elevado, se encuentran descritos en la bibliografía para su utilización en el encapsulamiento de residuos especiales y como morteros de cemento para la reparación de hormigón. Sin embargo cabe la posibilidad de poder formular este mismo tipo de cementos con óxidos de magnesio de bajo contenido, cuyo precio es del orden de 10 a 15 veces más barato que el óxido de magnesio de elevada pureza. En el presente estudio se evalúa la utilización de óxidos de magnesio de bajo contenido (≈70% MgO), obtenidos en el proceso de calcinación de la magnesita natural, para la formulación de cementos de K-estruvita. En este estudio se pretende determinar la formulación óptima de estos cementos a partir de la evaluación tanto de las propiedades mecánicas como de los tiempos de fraguado de las diferentes composiciones Tue, 13 Nov 2012 13:02:11 GMT http://hdl.handle.net/2117/16912 2012-11-13T13:02:11Z Formosa Mitjans, Joan; Aranda, Miguel Ángel; Chimenos Ribera, Josep Ma.; Rosell Amigó, Juan Ramón; Fernández Renna, Ana Inés; Ginés, O. no cementos de fosfato, K-estruvita, cementos químicos, fosfato de magnesio y potasio Es posible obtener cementos de fosfato de magnesio y potasio (KMgPO4·6H2O; K-estruvita), mediante la reacción en medio acuoso del óxido de magnesio y el dihidrogenofosfato de potasio, siendo ésta una reacción exotérmica muy rápida que permite el fraguado del material en pocos minutos. Estos cementos, formulados a partir de óxidos de magnesio de elevada pureza y coste elevado, se encuentran descritos en la bibliografía para su utilización en el encapsulamiento de residuos especiales y como morteros de cemento para la reparación de hormigón. Sin embargo cabe la posibilidad de poder formular este mismo tipo de cementos con óxidos de magnesio de bajo contenido, cuyo precio es del orden de 10 a 15 veces más barato que el óxido de magnesio de elevada pureza. En el presente estudio se evalúa la utilización de óxidos de magnesio de bajo contenido (≈70% MgO), obtenidos en el proceso de calcinación de la magnesita natural, para la formulación de cementos de K-estruvita. En este estudio se pretende determinar la formulación óptima de estos cementos a partir de la evaluación tanto de las propiedades mecánicas como de los tiempos de fraguado de las diferentes composiciones http://hdl.handle.net/2117/16911 Title: Asymptotic analysis of the Gunn effect with realistic boundary conditions Authors: Bonilla, L. L.; Rodríguez Cantalapiedra, Inma; Gomila Lluch, Gabriel; Rubí Capaceti, José Miguel Abstract: A general asymptotic analysis of the Gunn effect in n-type GaAs under general boundary conditions for metal-semiconductor contacts is presented. Depending on the parameter values in the boundary condition of the injecting contact, different types of waves mediate the Gunn effect. The periodic current oscillation typical of the Gunn effect may be caused by moving charge-monopole accumulation or depletion layers, or by low- or high-field charge-dipole solitary waves. A new instability caused by multiple shedding of ~low-field! dipole waves is found. In all cases the shape of the current oscillation is described in detail: we show the direct relationship between its major features ~maxima, minima, plateaus, etc.! and several critical currents ~which depend on the values of the contact parameters!. Our results open the possibility of measuring contact parameters from the analysis of the shape of the current oscillation Tue, 13 Nov 2012 12:33:38 GMT http://hdl.handle.net/2117/16911 2012-11-13T12:33:38Z Bonilla, L. L.; Rodríguez Cantalapiedra, Inma; Gomila Lluch, Gabriel; Rubí Capaceti, José Miguel no Boundary A general asymptotic analysis of the Gunn effect in n-type GaAs under general boundary conditions for metal-semiconductor contacts is presented. Depending on the parameter values in the boundary condition of the injecting contact, different types of waves mediate the Gunn effect. The periodic current oscillation typical of the Gunn effect may be caused by moving charge-monopole accumulation or depletion layers, or by low- or high-field charge-dipole solitary waves. A new instability caused by multiple shedding of ~low-field! dipole waves is found. In all cases the shape of the current oscillation is described in detail: we show the direct relationship between its major features ~maxima, minima, plateaus, etc.! and several critical currents ~which depend on the values of the contact parameters!. Our results open the possibility of measuring contact parameters from the analysis of the shape of the current oscillation http://hdl.handle.net/2117/16910 Title: Stationary states and phase diagram for a model of the Gunn effect under realistic boundary conditions Authors: Gomila Lluch, Gabriel; Rubí Capaceti, José Miguel; Rodríguez Cantalapiedra, Inma; Bonilla, L. L. Abstract: A general formulation of boundary conditions for semiconductor-metal contacts follows from a phenomenological procedure sketched here. The resulting boundary conditions, which incorporate only physically well-defined parameters, are used to study the classical unipolar drift-diffusion model for the Gunn effect. The analysis of its stationary solutions reveals the presence of bistability and hysteresis for a certain range of contact parameters. Several types of Gunn effect are predicted to occur in the model, when no stable stationary solution exists, depending on the value of the parameters of the injecting contact appearing in the boundary condition. In this way, the critical role played by contacts in the Gunn effect is clearly stablished Tue, 13 Nov 2012 12:32:04 GMT http://hdl.handle.net/2117/16910 2012-11-13T12:32:04Z Gomila Lluch, Gabriel; Rubí Capaceti, José Miguel; Rodríguez Cantalapiedra, Inma; Bonilla, L. L. no Statistical mechanics, Soft condensed matter A general formulation of boundary conditions for semiconductor-metal contacts follows from a phenomenological procedure sketched here. The resulting boundary conditions, which incorporate only physically well-defined parameters, are used to study the classical unipolar drift-diffusion model for the Gunn effect. The analysis of its stationary solutions reveals the presence of bistability and hysteresis for a certain range of contact parameters. Several types of Gunn effect are predicted to occur in the model, when no stable stationary solution exists, depending on the value of the parameters of the injecting contact appearing in the boundary condition. In this way, the critical role played by contacts in the Gunn effect is clearly stablished