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.

Phase characterization in Ni alloy-hard carbide composites for fused coatings

Abstract: Ni-based alloys containing low amounts of boron and silicon, reinforced with hard carbides, are extensively used for wear-resistant coatings. A powdered alloy of NiBSi and two of NiCrBSi (with 10.5 and 15 wt.% Cr, respectively) were mixed with VC and WC in proportions of 15 and 50 vol.% and then processed by either thermal spraying or liquid phase sintering. Phase identification was performed by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), and energy and wavelength dispersion spectroscopy (EDS and WDS). Materials free of Cr and those containing 10.5% Cr were characterized by the presence of a boron-rich net. At 15% Cr, the distribution of the boron-rich phase was sensibly modified. Further, Cr addition produced finely dispersed carbides with variable contents of V or W. Vanadium diffusion in the matrix, as well as metal interchange between vanadium and chromium carbides is, apparently, enhanced by increasing Cr content. Mass transfer processes in carbide-matrix interface seems to be slower when WC is used as reinforcement.

Crystallization in ABC Triblock Copolymers with Two Different Crystalline End Blocks: Influence of Confinement on Self-Nucleation Behavior

Abstract: The influence of different confinements active during crystallization within polybutadiene-block-polyisoprene-block-poly(ethylene oxide) (PB-b-PI-b-PEO) and the corresponding hydrogenated polyethylene-block-poly(ethylene-alt-propylene)-block-poly(ethylene oxide) (PE-b-PEP-b-PEO) triblock copolymers on the self-nucleation behavior of the crystallizable PEO and PE blocks is investigated by means of differential scanning calorimetry (DSC). In triblock copolymers with PEO contents ≤ 20 wt.-% crystallizationof PEO is confined within small isolated microdomains (spheres or cylinders), and PEO crystallization takes place exclusively at high supercoolings. Self-nucleation experiments reveal an anomalous behavior in comparison to the classical self-nucleation behavior found in semicrystalline homopolymers. In these systems, domain II (exclusive self-nucleation domain) vanishes, and self-nucleation can only take place at lower temperatures in domain III SA , when annealing is already active. The self-nucleation behavior of the PE blocks is significantly different compared with that of the PEO blocks. Regardless of the low PE content (10-25 wt.-%) on the investigated PE-b-PEP-b-PEO triblock copolymers a classical self-nucleation behavior is observed, i.e., all three self-nucleation domains, usually present in crystallizable homopolymers, can be located. This is a direct result of the small segmental interaction parameter of the PEP and PE segments in the mel. As a consequence, crystallization of PE occurs without confinement from a homogeneous mixture of PE and PEP segments. Self-nucleation regimes of a block copolymer showing confined crystallization by means of DSC.

Molecular Simulation of Joule–Thomson Inversion Curves

Abstract: A method to determine Joule–Thomson inversion curves, using isobaric-isothermal Monte Carlo molecular simulations, is presented. The usual experimental practice to obtain the locus of points in which the isenthalpic derivative of temperature with respect to pressure vanishes is to process volumetric data by means of thermodynamic relations. This experimental procedure requires the very precise measurement of volumetric properties at conditions up to five times the fluid's critical temperature and twelve times its critical pressure. These harsh experimental conditions have hindered the publication of data for even simple fluids and mixtures. By using molecular simulation, these problems may be circumvented, since the computational effort is roughly independent of the actual value of the pressure or the temperature. In general, Joule–Thomson inversion curves obtained by molecular simulation may be used either as an unambiguous test for equations of state in the supercritical and high-pressure regions or for the prediction of real fluid behavior, should the potential be well known. Both applications are exemplified for a Lennard-Jones fluid for which the complete inversion curve is obtained.