Showing papers by "Simón Bolívar University published in 1997"

Successive self-nucleation/annealing (SSA): A novel technique to study molecular segregation during crystallization

Abstract: A new procedure to fractionate ethylene/α-olefin copolymers using DSC is presented. This procedure allows melt/melt and melt/solid segregation to occur during thermal cycles that promote self-nucleation, crystallization and annealing processes (Successive Self-Nucleation/ Annealing, SSA). The SSA has been compared with the Step-Crystallization (SC) method proposed earlier in the literature to qualitatively characterize chain branching distribution in a faster and easier way than Temperature Rising Elution Fractionation (TREF). In general, SSA produces better fractionation than SC and the DSC derived chain branching distribution by SSA can be qualitatively comparable to that obtained by TREF. The SSA technique could have important applications for the characterization of polymers that crystallize over a broad temperature range.

A robust and fast action selection mechanism for planning

Abstract: The ability to plan and react in dynamic environments is central to intelligent behavior yet few algorithms have managed to combine fast planning with a robust execution. In this paper we develop one such algorithm by looking at planning as real time search. For that we develop a variation of Korf's Learning Real Time A* algorithm together with a suitable heuristic function. The resulting algorithm interleaves lookahead with execution and never builds a plan. It is an action selection mechanism that decides at each time point what to do next. Yet it solves hard planning problems faster than any domain independent planning algorithm known to us, including the powerful SAT planner recently introduced by Kautz and Selman. It also works in the presence of perturbations and noise, and can be given a fixed time window to operate. We illustrate each of these features by running the algorithm on a number of benchmark problems.

Comparative pollination biology of Venezuelan columnar cacti and the role of nectar-feeding bats in their sexual reproduction.

Abstract: The floral biology, reproductive system, and visitation behavior of pollinators of four species of columnar cacti, Stenocereus griseus, Pilosocereus moritzianus, Subpilocereus repandus, and Subpilocereus horrispinus,were studied in two arid zones in the north of Venezuela. Our results support the hypothesis that Venezuelan species of columnar cacti have evolved toward specialization on bat pollination. Additional information on the floral biology of a fifth species, Pilosocereus lanuginosus, was also included. All species showed the typical traits that characterize the pollination syndrome of chiropterophily. All species but Pilosocereus moritzianus were obligate outcrossers. Nectar and pollen were restricted to nocturnal floral visitors. Two species of nectar-feeding bats, Leptonycteris curasoae Miller and Glossophaga longirostris Miller, were responsible for practically all the fruit set in these cacti. Frequency of bat visitation per flower per night was highly variable within and between species of cactus, with average frequencies varying between 27 and 78 visits · flower 21 · night21 .I n general terms, the pattern of floral visitation through the night was significantly correlated with the pattern of nectar production and nectar sugar concentration for all species of cactus. Under natural pollination, fruit:flower ratios varied from 0.46 in Subpilocereus repandusto 0.76 in Stenocereus griseus. The efficiency of bat pollination in terms of seed:ovule ratio was high in all species, varying between 0.70 and 0.94.

A Markov Chain Approach to Baseball

Abstract: Most earlier mathematical studies of baseball required particular models for advancing runners based on a small set of offensive possibilities. Other efforts considered only teams with players of identical ability. We introduce a Markov chain method that considers teams made up of players with different abilities and which is not restricted to a given model for runner advancement. Our method is limited only by the available data and can use any reasonable deterministic model for runner advancement when sufficiently detailed data are not available. Furthermore, our approach may be adapted to include the effects of pitching and defensive ability in a straightforward way. We apply our method to find optimal batting orders, run distributions per half inning and per game, and the expected number of games a team should win. We also describe the application of our method to test whether a particular trade would benefit a team.

Moisture effect on the low‐ and high‐temperature dielectric relaxations in nylon‐6

Abstract: The effect of water sorption on the dielectric relaxation processes of nylon-6 samples with water concentrations ranging from the dry to the water-saturated polymer has been studied by thermally stimulated depolarization currents experiments in a broad temperature range, from 77 to 365 K. The strengths of the low-temperature modes, y- and β-peaks, are affected in opposite ways by the water concentration, h, as the first one shows a decrease in intensity and the second one grows as h increases. The precise determination of the relaxation parameters is made by the decomposition in elementary Debye processes and best fitting to the experimental profile of the complex peak. For h < 3%, the reorienting energies are almost independent of the water content, and the most significant intensity variations occur. The firmly bound water is held responsible for these effects. As for the higher temperature zone besides the α-peak, which is the dielectric manifestation of the glass transition, intermediate temperatures modes are observed at high h values and are originated by the loosely bound water, while the highest temperature peak is attributed to a Maxwell-Wagner interfacial polarization. The characteristic parameters of the α-mode are determined and related to the plasticization effect of water.

Embolism vulnerability in drought-deciduous and evergreen species of a tropical dry forest

Abstract: Plant water status plays an important role in determining phenological patterns of leaf expansion and abscission of drought-deciduous (DC) and evergreen (EG) species co-occurring in tropical dry forests. If vulnerability to embolism is related to the phenological pattern, we should find that drought-deciduous species are intrinsically more susceptible than evergreen species to drought-induced embolisms. The object of this study was to assess the embolism vulnerability curves and the percentage loss of hydraulic conductivity (PLC) due to embolisms in four DC and two EG species from a tropical dry forest in Venezuela. In DC, PLC was between 8 and 19% during the wet season but a significant increase to between 38 and 79% occurred during the dry season. Increment of embolism in EG was from 30 to 35% during the wet season to between 40 and 48% during the dry season. Regarding embolism vulnerability curves, a clear-cut difference between DC and EG species was not observed. Both plant types underwent 50 PLC at a cavitation tension between 1.65 and 3.82 MPa. In conclusion, the degree of embolism in DC and EG was not determined by differences in vulnerability curves. Moreover, no interrelation was found between xylem efficiency and tension at 50 PLC embolisms. Differences in root depth and water availability between DC and EG seemed to be the underlying cause of embolism in the field. Nevertheless, the occurrence of a lower susceptibility to drought-induced embolisms in the dominant tree B. cumanensis implied that risk from xylem dysfunction may have some ecological significance.

Analysis of abnormal signals within the QRS complex of the high-resolution electrocardiogram

Abstract: Presents a new, quantitative approach to measuring abnormal intra-QRS signals, using the high-resolution electrocardiogram (HRECG). These signals are conventionally known as QRS "notches and slurs." They are measured qualitatively and form the basis for the ECG identification of myocardial infarction. The HRECG is used for detection of ventricular late potentials (LP), which are linked with the presence of a reentry substrate for ventricular tachycardia (VT) after a myocardial infarction. LP's are defined as signals from areas of delayed conduction which outlast the normal QRS period. The authors' objective is to quantify very low-level abnormal signals that may not outlast the normal QRS period. In this work, abnormal intra-QRS potentials (AIQP) were characterized by removing the predictable, smooth part of the QRS from the original waveform. This was represented as the impulse response of an ARX parametric model, with model order selected empirically from a training data set. AIQP were estimated using the residual of the modeling procedure. Critical AIQP parameters to separate VT and non-VT subjects were obtained using discriminant functions. Results suggest that AIQP indexes are a new predictive index of the HRECG for VT. The concept of abnormal intra-QRS potentials permits the characterization of pathophysiological signals contained wholly within the normal QRS period, but related to arrhythmogenesis. The new method may have other applications, such as detection of myocardial ischemia and improved ECG identification of the site of myocardial infarction, particularly in the absence of Q waves.

Monte Carlo simulation of a mixed spin 2 and spin Ising ferrimagnetic system

Abstract: The critical behaviour of a mixed ferrimagnetic Ising system on a square lattice in which the two interpenetrating square sublattices have spins and has been studied. We carried out exact ground state calculations and performed Monte Carlo simulations to obtain the finite-temperature phase diagram of the model. We found that the system that includes only a nearest-neighbour interaction and the crystal field does not have a compensation point. Also, our study seems to indicate that, contrary to effective-field predictions, there is no tricritical point at nonzero temperature for this model; however a more elaborate analysis will be needed to draw a definite conclusion on this point.

Rheological and calorimetric evidences of the fractionated crystallization of iPP dispersed in ethylene/α‐olefin copolymers

Abstract: In this work, the melt crystallization of immiscible blends of isotactic poly- propylene (iPP) and branched polyethylenes (PE) was followed by oscillatory shear measurements during controlled cooling. All the blends contained 20% iPP finely dis- persed in several ethylene/a-olefin copolymer matrices (with and without a nucleating agent) with densities ranging from 0.88 to 0.92 g/cm 3 (linear low, very low, and ultra low density polyethylenes: LLDPE, VLDPE, and ULDPE). The rheological results were compared with parallel differential scanning calorimetry (DSC) measurements at the same cooling rate. During preliminary evaluations of the neat resins, no effect was found of the variation of the frequency of oscillation or the applied shear strain on their crystallization (at least in the range explored in this work). In the case of the blends, when the iPP crystallized in a fractionated fashion, only one sudden increase in the storage modulus (G*) was observed during cooling due to the partial coincident crystal- lization of both iPP and the PE matrix. In the presence of a nucleating agent, an almost complete separation between the crystallization of both components in the blend was achieved and two increases in G* were clearly observed upon cooling. A close match between the dynamic crystallization kinetics obtained by DSC and torsion rheometry was demonstrated by a direct comparison between calorimetrically measured solid conversion and G* during cooling from the melt. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2481-2493, 1997

Electroless nickel-boron coatings on metal carbides

Abstract: Electroless deposition is a very simple process which can be used to obtain amorphous metallic coatings of uniform thickness on metallic or non-metallic substrates. In this work, nickel-boron coatings were deposited by electroless plating on WC and VC powders and sintered substrates. Two electrolyte solutions for the deposition of Ni-B coatings were developed: an alkaline and an acidic bath, both based on nickel chloride. Coating characterization was carried out by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Auger electron spectroscopy (AES) and X-ray diffraction (XRD). Both electrolytes produced continuous firms of uniform thickness on sintered carbide substrates; the acidic bath showed a higher deposition rate. Furthermore on the carbide powders it produced crack-free Ni-B continuous coatings. Ni was detected by EDS and the coating purity was confirmed by AES. XRD proved that amorphous coatings were obtained in all cases.

Enhanced oily sludge biodegradation by a tensio-active agent isolated from Pseudomonas aeruginosa USB-CS1

Abstract: The biodegradation of an oily sludge is facilitated by a microbial tensio-active agent isolated from Pseudomonas aeruginosa USB-CS1. The optimal oil-in-water dispersion conditions are as follows: pH 6.5, temperature 30 °C, agitation 150 rev/min. The total hydrocarbon content shows that the biodegradation of the oily substrate mediated by the biosurfactant or by the biosurfactant–P. aeruginosa USB-CS1 complex is significantly higher after 30 days of incubation than that in other experimental conditions, by a mean of 70%. Substrate fractionation by column chromatography reveals that, if biosurfactant is present, saturated and aromatic compounds are more susceptible to microbial degradation than they are in other biodegradation systems by an average of 55% and 40% respectively. These results suggest that the stimulatory effects of the biosurfactant on the biodegradation of the oily substrate are limited over time by the loss of surface activity of the biosurfactant after 30 days of incubation.

Liquid-liquid phase separation in blends of a linear low-density polyethylene with a low-density polyethylene

Abstract: A linear low-density butene copolymer, of overall branch content 3 mol %, has been blended with a low-density polyethylene. The low-density polyethylene has an overall branch content of 5 mol %, including both long and short branches. The two materials were blended in a wide range of compositions and the phase behavior investigated using indirect experimental methods, the examination of quenched blends by differential scanning calorimetry, and transmission electron microscopy. After quenching from temperatures up to 170°C, blends, of almost all compositions, show two crystal populations, separated on a micron scale. It is argued that this implies that the blends were phase separated in the melt before quenching. This behavior shows good agreement with predictions based on previous extensive studies of binary and ternary blends of linear with lightly branched polyethylenes. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 1921–1931, 1997

Phase relations, microstructure and mechanical properties of VC substituted WC-10Co cemented carbide alloys

Abstract: As an effort to investigate a possible extent of substitution of W, the metal from strategic mineral, by a less strategic V, phase relations, microstructure, and hardness and fracture strength, of powder metallurgically processed 90 mass% [(1 − y )WC− y VC]−10 mass% Co alloys are studied. Throughout the composition eight phases, α-Co, β-Co, WC, V 4 C 3 , V 8 C 7 , η 3 (Co 3 W 3 C ), η 6 (Co 6 W 6 C) and graphite C(g), were encountered and a distinct microstructural difference existed across midcomposition. On the basis of combined mechanical properties of hardness and fracture strength the alloys between y = 0.2 and 0.4, which correspond to 18–36 mass % VC, were most promising. Contrary to the antagonistic relations between hardness and toughness commonly encountered in cemented carbides and hard materials, the hardness and fracture strength of the present WC-VC-10Co alloys were nearly parallel.

Genome composition in Venezuelan spiny-rats of the genus Proechimys (Rodentia, Echimyidae). I. Genome size, C-heterochromatin and repetitive DNAs in situ hybridization patterns

Abstract: The genome sizes of the Venezuelan spiny-rats Proechimys guairae guairae (2n = 48) and P. trinitatis (2n = 62) were evaluated and proved to be 12.5 ± 0.5pg and 12

Zirconia coatings by spray pyrolysis

Abstract: Zirconia coatings were obtained with the spray pyrolysis process, using a precursor solution of zirconium n-propoxide in 1-propanol. An investigation was undertaken into the influence of carrier gas (air, He, N2 and Ar), temperature (450 and 600 °C), time (1 and 2 h) and substrate composition (borosilicate glass, alumina, stainless steel and carbon steel) on coating characteristics. The most important deposition parameters were found to be the atomic or molecular weight of the carrier gas and the pyrolysis temperature. Cubic ZrO2 thin films, with dense and homogeneous nanostructrure were obtained using He and N2 as carrier gas, at 600 °C, on ceramic substrates.

A new approach to extract the threshold voltage of MOSFETs

Abstract: A new method is presented to extract the threshold voltage of MOSFETs. It is developed based on an integral function which is insensitive to the drain and source series resistances of the MOSFETs. The method is tested in the environments of circuit simulator (SPICE), device simulation (MEDICI), and measurements.

Magnetic Monopoles over Topologically Nontrivial Riemann Surfaces

Abstract: An explicit canonical construction of monopole connections on nontrivial U(1) bundles over Riemann surfaces of any genus is given. The class of monopole solutions depends on the conformal class of the given Riemann surface and a set of integer weights. The reduction of Seiberg--Witten 4-monopole equations to Riemann surfaces is performed. It is then shown that the monopole connections constructed are solutions to these equations.