Showing papers by "University of Los Andes published in 1990"

Sound Velocities and Elastic Moduli in CuInTe2 and CuInSe2

Abstract: A dynamic pulse-echo overlap method is used to measure the longitudinal and transverse sound velocities of polycrystalline samples of CuInTe2 and CuInSe2 between 4.2 and 300 K. From these measurements the temperature dependence of the elastic moduli is obtained. From the low temperature value of the average sound velocity, the Debye temperature is calculated to be 197.5 and 243.7 K for CuInTe2 and CuInSe2, respectively. Using the Szigeti relationships the ionicities are estimated to be 0.39 and 0.34 for CuInSe2 and CuInTe2, respectively. Une methode dynamique de recouvrement de poulsation-echo a ete utilisee pour mesurer la vitesse du son longitudinale et transverse dans des echantillons polycristallins de CuInTe2 et CuInSe2 entre 4,2 et 300 K. Nous avons obtenu la dependence vis a vis de la temperature des modules elastiques. D'autre part a partir des valeurs moyennes de la vitesse du son a basses temperatures nous avons calcule la temperature de Debye et on a trouve 197,5 et 243,7 K pour le CuInTe2 et CuInSe2 respectivement. Au moyen des relations de Szigeti les ionicites ont ete estimees en 0,39 et 0,34 pour le CuInTe2 et CuInSe2, respectivement.

Blood serum selenium in the province of Mérida, Venezuela, related to sex, cancer incidence and soil selenium content.

Abstract: Blood serum selenium levels were measured in healthy subjects of six districts located throughout the province of Merida, Venezuela. There were either high (87-115 micrograms/L) or low (58-72 micrograms/L) serum selenium-containing districts. Serum selenium in males from three districts were statistically significantly lower (p less than 0.001) than in females. There was a general tendency for serum selenium to increase with age. On the average, a reduction in cancer was observed in the districts with high serum selenium content. Further, serum selenium was lower in male patients from the Merida district with cancer disease. Association between selenium in soil and in the blood serum of healthy subjects has been observed.

Systemic sulpiride increases dopamine metabolites in the lateral hypothalamus.

Abstract: In rats with microdialysis probes in the perifornical lateral hypothalamus (PFH) a single injection of the D2 receptor blocker 1-sulpiride (20 mg/kg IP) significantly increased extracellular dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), but not 5-hydroxyindoleacetic acid (5-HIAA). This suggests that sulpiride crosses the blood-brain barrier and blocks D2 dopamine receptors in the PFH leading to increased dopamine turnover reflected in increased extracellular DOPAC and HVA. We conclude that D2 blockade in the hypothalamus could play a role in the hyperphagia and body weight gain observed in female rats under chronic administration of the antipsychotic drug, sulpiride.

Design of P-I controllers for DC-to-DC power supplies via extended linearization

Abstract: In this article, the method of ‘extended linearization’ is used for designing stabilizing non-linear proportional-integral (P-I) controllers which regulate to a constant value either the average output voltage or the average input current of PWM controlled DC-to-DC power converters such as the boost, the buck-boost and the Cuk converters, The design is carried out on the basis of the Ziegler-Nichols P-I controller design method applied to a family of linearized transfer function models of converters parametrized by constant operating equilibrium points of the average PWM controlled circuit.

Chronic lithium administration enhances serotonin release in the lateral hypothalamus but not in the hippocampus in rats. A microdialysis study.

Abstract: Chronic administration of lithium displays therapeutic and prophylactic effects in bipolar affective disorders, but its mechanism of action remains unknown. Several studies in animals and humans strongly suggest that central serotonergic neurons might be involved in lithium effects. In the experiments reported here microdialysis with removable probes and high pressure liquid chromatography and electrochemical detection were used to assess the amphetamine-induced release of serotonin (5-HT) and the 5-hydroxy-indoleacetic acid (5-HIAA) levels in the perifornical hypothalamus (PFH) and hippocampus (HP) of freely moving rats before and after chronic lithium chloride administration (2 meq/kg, as intragastric daily injections for 14 days). The serum lithium levels were 0.66 ± 0.08 meq/1. After lithium treatment, the amphetamine-induced 5-HT release was significantly enhanced in the PFH but not so in the HP. Basal levels of 5-HIAA in the control group decreased but remained unchanged in the lithium group in the PFH. No change of basal levels of 5-HIAA was observed in the HP. The effect of lithium on the PFH could be related to the improvement of the autonomic and cyclic symptoms of patients with manic depressive disorders undergoing lithium therapy.

Structure at infinity, zero dynamics and normal forms of systems undergoing sliding motions

Abstract: In this article we examine the structure at infinity of non-linear closed-loop systems locally undergoing sliding regimes about a smooth surface defined in state space. By using a locally diffeomorphic state coordinate transformation, associated with the relative degree of the system, one obtains a normal form exhibiting the basic internal dynamic structure of the controlled system. It is found that the local existence of sliding motions demands a considerably simple local structure at infinity of the original non-linear system. The ideal sliding dynamics in local sliding surface coordinates is shown to coincide precisely with the zero dynamics. The stability properties of this internal behaviour model are studied. Several illustrative examples are presented.

How does a mistletoe affect the water, nitrogen and carbon balance of two mangrove ecosystem species?

Abstract: . The effect of a mistletoe, Phthirusa maritima, on the water, nitrogen and carbon balance of two mangrove host species, Conocarpus erectus and Coccoloba uvifera, was studied. Several daily cycles of water potential and its components (pressure-volume curves); leaf nitrogen content (Kjeldahl method); leaf conductance, transpiration rates and carbon assimilation (portable gas exchange system) were measured on mistletoe, infested and uninfested plants in the Caribbean coast of Venezuela. The mistletoe on both host species showed higher transpiration rates and lower CO2 assimilation rates, and therefore lower water use efficiencies. With respect to infested and uninfested plants, C. erectus did not show large differences in the parameters measured with the exception of assimilation rates which were significantly lower in the infested plants. On the other hand, C. uvifera did show differences in all parameters and, therefore, was affected to a greater degree by the mistletoe. The behaviour of mistletoeinfested and uninfested plants, with respect to habitats with different degrees of water stress and with respect to the salinity gradient in which these mangroves grow, is discussed.

New Coumarins from Pilocarpus goudotianus.

Abstract: The leaves of Pilocarpus goudotianus afforded in addition to eight known coumarins three new ones, donatin, alloxanthoxyletol, and (+)-elisin. The structures were elucidated on the basis of spectroscopic evidence and chemical derivatives.

Systems thinking and critique. I. What is critique

Abstract: This paper is the first in a trilogy which intends to discuss the notion of critique within two different contexts: Modern Western Philosophy and Contemporary Systems Thinking. This first paper presents a phenomenological inquiry into the concept of “critique.” The result of the inquiry is a model of the possibilities ofcritique which is used to interpret four “moments” of critique in Modern Western Philosophy.

Tissue water relations in elfin cloud forest tree species of Serrania de Macuira, Guajira, Colombia.

Abstract: Diurnal courses of stomatal conductance, leaf water potential, and the components of tissue water potential were measured in six canopy species in an elfin cloud forest. High values of stomatal conductance were measured on cloudy days and during early morning and late afternoon of sunny days. Decreases in stomatal conductance with increases in vapour pressure deficit may have been a response to avoid further water deficits and suggested a stomatal response to changes in relative humidity. Daily transpiration varied between 470 and 1014 g m-2 day-1 during cloudy days and between 532 and 944 g m-2 day-1 during clear days. Stomatal conductance may have also responded to changes in leaf water potential, which was minimum at noon. The minimum tissue water potential measured in the field was -1.8 MPa in Myrcianthes fragrans, and the minimum turgor pressure was 0.49 MPa also in M. fragrans. There was a correlation between the osmotic potential and the minimum tissue water potential, suggesting that osmotic potential plays a major role in the maintenance of turgor in these species, in spite of the great variability in the elastic properties of leaf tissues. Turgor pressure decreased during the day following the course of water potential but never approached the turgor loss point, as it has been measured in some lowland rain forest trees. This is a strong indication that elfin cloud forest trees do not suffer severe water deficits, and that small tree stature is not directly related to water shortage.

Proposal for experiments to detect the missing torque in special relativity

Abstract: The torque experienced by a current loop, of magnetic momentm, and moving with velocityv in an electric fieldE, cannot be explained in terms of the interaction between the loop current andE, but only by taking into account the internal stress in the loop. This subtle effect can be tested by measuring the frequency changes in the Zeeman splitting of moving atoms, the change of helicity of elementary particles, and the current induced in a moving coil. These tests may have relevance in the context of modern ether theories and of the Aharonov-Casher effect.

Foaming properties of surfactant-oil-vater systems in the neighbourhood of optimum formulation

Abstract: The foaming properties of. surfactant-oil-water systems are found to be closely related to the phase behavior at equilibrium. In Winsor I systems both the surfactant-oil-water emulsion and its aqueous phase produce stable foams. The foaminess and the foam stability follow the same pattern of variation than the emulsion stability. In Winsor III (three phase microemulsion-oil-water) and Winsor II systems no foam can be formed. The changes are the same no matter the formulation variable used to scan the physico-chemical formulation or the method used to measure the foam stability.

Speciation of tetraalkyllead compounds by flow injection — atomic absorption spectrophotometry

Abstract: The on-line demetallation of alkyllead compounds, with iodine, and further emulsification, provides a fast determination of total lead content in liposoluble matrices, such as gasolines. It can be carried out in a double channel manifold, using a magnetically well stirred dilution chamber for the emulsification of the samples, previous to their analysis by flame atomic absorption spectrophotometry. On the other hand, the different behaviour of tetraethyllead (TEL) and tetramethyllead (TML) can be employed for speciation of both compounds in the sample. The effect of the flow injection parameters on the sensitivity and accuracy has been studied and the figures of merit of the proposed methods evaluated.

Electronic Structure of (001) Semiconducting MTe Surfaces (M = Zn, Cd, Hg)

Abstract: By using the tight-binding scheme with ten orbitals per atom (SP3S* and two spin orientations) the electronic projected bulk band structure, surface band structure (both anion-and cation-terminated surfaces), and the wave-vector-resolved density of states for these II-VI semiconductors are calculated. A quickly convergent iterative technique is employed for calculating the Green function of the actual semi-infinite crystal. This calculational method allows to precise very accurately, the energy of localized surface states. All the materials considered in this work present surface states with energies lying in the fundamental gap as well as surface states in the lenses opened in the projected bulk valence-band continum. The atomic weight of each surface state is determined. The states in the fundamental gap turn out to be ‘bridge-bond’-type states for anion-terminated surfaces and ‘dangling-bond’-type states for cation-terminated surfaces. Dans l'approximation de liassons fortes avec dix orbitales per atome (SP3S* et deux orientations de spin) nous avons calcule la structures des bandes electroniques projectees, la structure des bandes de surface (surfaces terminees en anion et cation) et la densite d'etats resolue en vecteur d'ondes pour ces Semiconducteurs II-VI. Nous avons utilise un algorithme iteratif qui converge tres rapidement pour calculer la Fonctions de Green du vrai cristal semi-infinite. Cette methode de calcul nous a permis de trouver, avec une tres grande precision, les bordes des bands projectes ainsi comme les energies des etats de surface. Pour tous les composes ici etudies on a trouve des etats de surfaces avec des energies dans le gap fondamental ainsi comme des etats de surfaces dans les vides ouverts dans les bandes de valence projectes. Le poid de chaque orbitale atomique sur les etats de surfaces a ete determine. Les etats dans le gap fondamental sont du type “bridge-bond” pour les surfaces terminees en anion et du type “dangling-bond” pour les surfaces terminees en cation.

Temperature Variation of Optical Energy Gap and Deformation Potentials in AgInTe2

Abstract: Values of the optical energy gap E0 of AgInTe2 as a function of temperature T in the range 10 to 300 K are obtained by optical absorption measurements. The variation of E0 with T is fitted well by a Manoogian-Leclerc equation of the form E0(0) − E0(T) = UT + Vφ (coth φ/2T − 1). Values of (dE0/dT)1, the electron–phonon interaction contribution to the variation of the energy gap with temperature, and (dE0/dT)2, the lattice dilation contribution, are obtained from the Vφ and U terms, respectively. From these values of (dE0/dT)1 and (dE0/dT)2, values are determined for the acoustic deformation potentials of the conduction band Ce and the valence band Ch. Mit Hilfe optischer Absorptionsmessungen wird die optische Energielucke E0 als Funktion der Temperatur T im Bereich von 10 bis 300 K ermittelt. Die Variation von E0 mit T wird gut durch eine Manoogian-Leclerc-Gleichung der Form E0(0) − E0(T) = UT + Vφ (coth φ/2T − 1) approximiert Die Beitrage der Elektron–Phonon-Wechselwirkung, (dE0/dT)2, und der Gitterdilatation, (dE0/dT)1, und werden aus den Vφ- bzw. U-Termen bestimmt. Aus diesen Werten von (dE0/dT)1 und (dE0/dT)2 werden die akustischen Deformationspotentiale des Leitungsbandes, Ce, und des Valenzbandes, Ch, erhalten.