Simón Bolívar University

About: Simón Bolívar University is a education organization based out in Caracas, Venezuela. It is known for research contribution in the topics: Population & Crystallization. The organization has 5912 authors who have published 8294 publications receiving 126152 citations.

Rigorous analytic solution for the drain current of undoped symmetric dual-gate MOSFETs

Abstract: We describe a new drain current model for nanoscale undoped-body symmetric dual-gate MOSFETs based on a fully consistent physical description. The model consists on a single analytic equation that includes both drift and diffusion contributions. It is built on the basis of the potentials at the surface and at the center of the silicon film evaluated at the source and drain ends. The derivation is completely rigorous and is based on a procedure previously enunciated for long-channel bulk SOI MOSFETs. The expression is a continuous description valid for all bias conditions, from subthreshold to strong inversion and from linear to saturation operation. The validity of the model has been ascertained by extensive comparison to exact numerical simulations. The results attest to the excellent accuracy of this formulation.

Estimation of the nucleation and crystal growth contributions to the overall crystallization energy barrier

Abstract: Classical kinetic theories of polymer crystallization were applied to iso- thermal crystallization kinetics data obtained by polarized optical microscopy (PLOM) and differential scanning calorimetry (DSC). The fitted parameters that were proportional to the energy barriers obtained allow us to quantitatively estimate the nucleation and crystal growth contributions to the overall energy barrier associ- ated to the crystallization process. It was shown that the spherulitic growth rate energy barrier found by fitting PLOM data is almost identical to that obtained by fit- ting the isothermal DSC crystallization data of previously self-nucleated samples. Therefore, we demonstrated that by self-nucleating the material at the ideal self- nucleation (SN) temperature, the primary nucleation step can be entirely completed and the data obtained after subsequent isothermal crystallization by DSC contains only contributions from crystal growth or secondary nucleation. In this way, by employing SN followed by isothermal crystallization, we propose a simple method to obtain separate contributions of energy barriers for primary nucleation and for crys- tal growth, even in the case of polymers where PLOM data are very difficult to obtain (because they exhibit very small spherulites). Comparing the results obtained with poly(p-dioxanone), poly(e-caprolactone), and a high 1,4 model hydrogenated polybuta- diene, we have interpreted the differences in primary nucleation energy barriers as

Relación del burnout y el rendimiento académico con la satisfacción frente a los estudios en estudiantes universitarios

Abstract: Los estudiantes se enfrentan cada vez mas a retos y exigencias del medio que demandan una gran cantidad de recursos fisicos y psicologicos de diversa indole. Muchos de ellos dejan sus estudios o disminuyen de manera considerable su rendimiento academico sin encontrar una explicacion. Dos variables que han sido poco estudiadas en nuestro contexto y que pueden dar claridad a buena parte de esta situacion son el burnout y la satisfaccion. En esta investigacion se establecio la relacion del burnout, el rendimiento academico, con la satisfaccion frente a los estudios en una muestra de 202 universitarios de la jornada nocturna de una universidad privada de Barranquilla, quienes respondieron el MBI-SS (Schaufeli, Martinez, Marques Pinto, Salanova, & Bakker, 2002). Los resultados indican correlaciones negativas entre agotamiento, cinismo, autoeficacia y rendimiento academico con la satisfaccion frente al estudio, mientras que el vigor, dedicacion y absorcion se correlacionaron positivamente con esta. Por otro lado, el buen funcionamiento academico se correlaciona positivamente con las dimensiones del engagement y negativamente con el cinismo; de manera particular, ninguna de las variables se correlaciona con el agotamiento, a pesar de evidenciar cinismo y baja autoeficacia.

Melt Structure and its Transformation by Sequential Crystallization of the Two Blocks within Poly(L‐lactide)‐block‐Poly(ε‐caprolactone) Double Crystalline Diblock Copolymers

Abstract: Sequential crystallization of poly(L-lactide) (PLLA) followed by poly(epsilon-caprolactone) (PCL) in double crystalline PLLA-b-PCL diblock copolymers is studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS). Three samples with different compositions are studied. The sample with the shortest PLLA block (32 wt.-% PLLA) crystallizes from a homogeneous melt, the other two (with 44 and 60% PLLA) from microphase separated structures. The microphase structure of the melt is changed as PLLA crystallizes at 122 degrees C (a temperature at which the PCL block is molten) forming spherulites regardless of composition, even with 32% PLLA. SAXS indicates that a lamellar structure with a different periodicity than that obtained in the melt forms (for melt segregated samples). Where PCL is the majority block, PCL crystallization at 42 degrees C following PLLA crystallization leads to rearrangement of the lamellar structure, as observed by SAXS, possibly due to local melting at the interphases between domains. POM results showed that PCL crystallizes within previously formed PLLA spherulites. WAXS data indicate that the PLLA unit cell is modified by crystallization of PCL, at least for the two majority PCL samples. The PCL minority sample did not crystallize at 42 degrees C (well below the PCL homopolymer crystallization temperature), pointing to the influence of pre-crystallization of PLLA on PCL crystallization, although it did crystallize at lower temperature. Crystallization kinetics were examined by DSC and WAXS, with good agreement in general. The crystallization rate of PLLA decreased with increase in PCL content in the copolymers. The crystallization rate of PCL decreased with increasing PLLA content. The Avrami exponents were in general depressed for both components in the block copolymers compared to the parent homopolymers. Polarized optical micrographs during isothermal crystalli zation of (a) homo-PLLA, (b) homo-PCL, (c) and (d) block copolymer after 30 min at 122 degrees C and after 15 min at 42 degrees C.