Showing papers by "Instituto Politécnico Nacional published in 1991"
TL;DR: It has been known for many years that it is possible to produce mixtures of methanol and higher aliphatic alcohols from synthesis gas by alkali promotion of the synthesis catalysts and by appropriate modification of the reaction conditions as mentioned in this paper.
Abstract: It has been known for many years that it is possible to produce mixtures of methanol and higher alcohols from synthesis gas by alkali promotion of the methanol synthesis catalysts and by appropriate modification of the reaction conditions [l]. From 1927 to 1945 plants were in operation in the United States and Germany, that were dismantled with the coming of different feedstocks and the necessity of obtaining pure alcohols for chemical use. In the last decade the chemical and petroleum industry has shown a renewed and growing interest in the use of mixtures of methanol and higher aliphatic alcohols. The original goal in the late '70s was mainly to reduce oil dependence by producing synthetic components for gasoline blends. More recently the trend to lead phase-down due to environmental protection has focused the attention on the product performances of such alcohol mixtures as high octane blending stock for gasoline. It has been shown that the addition of higher alcohols to methanol increases the...
231 citations
TL;DR: In this article, the electron avalanche ionization in water was considered experimentally and theoretically, and the effect appeared to be due to electron avalanches ionization due to the dependence of the breakdown probability on the laser field.
Abstract: Dielectric breakdown induced in water by Nd: YAG laser pulses is considered experimentally and theoretically. The effect appears to be due to electron avalanche ionization. The aspects of this process considered here are the following: (i) The dependence of the breakdown probability on the laser field. At high fields, electron interaction with molecular (Raman) vibrations or with collective molecular motions occurs. (ii) Bragg scattering, which contributes to keeping the electron motion in phase with the optical field. (iii) The role of the electron mobility, which contributes to stabilizing the process. (iv) The generation of the electrons that start the avalanche in relation to different laser-pulse durations and irradiances.
209 citations
TL;DR: In this paper, specific similarities and differences of UASB, filter and expanded/fluidized bed reactors with respect to start-up, operation, parameter monitoring and process control are discussed.
146 citations
TL;DR: High output power and the highest slope efficiency reported to date for a bulk Er:glass laser are reported for the first time to date.
Abstract: Continuous-wave laser operation of bulk Er:Yb:phosphate glass pumped at 980 nm by an InGaAs index-guided diode laser has been achieved for the first time to our knowledge, with 2 mW of output power obtained at 1.54 μm. To optimize pumping conditions and to investigate the effects of pump wavelength, the Er3+:Yb3+ glass disk has also been pumped by a Ti:Al2O3 laser at 960 and 980 nm, and we obtained high output power (70 mW) and the highest slope efficiency (21% for 980-nm pumping) reported to date for a bulk Er:glass laser.
117 citations
TL;DR: Spectral analysis of cardiovascular variabilities suggests that in man and in conscious dogs psychological challenges induce a profound re-arrangement of neural control of the circulation, which appears to be characterised by sympathetic predominance and which can be monitored by this technique.
Abstract: In this study, we used spectral analysis of short-term R-R and systolic arterial pressure (SAP) variabilities to estimate the changes in neural control of the circulation produced by psychological stress. The 0.1 Hz low-frequency (LF) component of R-R and SAP variabilities provided a quantitative index of the sympathetic activity controlling heart rate and vasomotion. Conversely the high-frequency (HF) respiratory component of R-R variability provided an index of vagal tone. In conscious dogs we used the seemingly stressful situation of being accompanied for the first time to the experimental laboratory as a stimulus. In human subjects we used mental arithmetic. In both cases LF of R-R and SAP variabilities increased significantly suggesting enhanced sympathetic activity both to the SA node and the vasculature. In man, the index α, a measure of the overall gain of baroreceptor mechanisms, was found to be reduced during mental arithmetic. Spectral analysis of cardiovascular variabilities thus suggests that in man and in conscious dogs psychological challenges induce a profound re-arrangement of neural control of the circulation, which appears to be characterised by sympathetic predominance and which can be monitored by this technique.
116 citations
TL;DR: In this paper, the analysis of the dynamic response of a homogeneous linear-elastic solid to external actions is formulated in the time domain by means of suitable integral representations of boundary displacements and of boundary tractions using time-dependent fundamental solutions.
80 citations
TL;DR: In this paper, the authors studied the dynamical effects induced by a squeezed vacuum field interacting with the atoms of a standard single mode optical bistable system and showed that by varying the relative phase between the squeezed vacuum and the coherent input light one can force the system to switch from a low to a high transmission state and vice versa.
57 citations
TL;DR: In this paper, the authors derived an explicit criterion indicating the critical value of the plastic modulus at the onset of a stationary discontinuity (one wave-speed is zero) in both cases where the fluid and solid constituents are compressible and where they are not.
Abstract: coupled constitutive equations for a saturated porous medium are developed in the frame of the theories of mixtures : the fluid constituent is elastic and the solid skeleton behaves as a rate-independent elastic-plastic solid. Then the existence of real acceleration wave-speeds is considered : actually the analysis centers on the modes in which these wave-speeds cease to be real. An explicit criterion indicating the critical value of the plastic modulus at the onset of a stationary discontinuity (one wave-speed is zero) is derived in both cases where the fluid and solid constituents are compressible and where they are not. Furthermore, it is shown that in some circumstances, some wave-speeds cease to be real in the very early stages of the inelastic deformation process due to the incipience of a flutter instability (two wave-speeds are complex conjugate). When it is not excluded, this mode of loss of hyperbolicity of the dynamic equilibrium equations usually precedes the onset of a stationary discontinuity and may occur right at the inception of plastic loading, that is for an infinitely large plastic modulus. Flutter instability is excluded when the plastic behavior of the solid skeleton is associative and its existence depends strongly on the relative positions of the shear and longitudinal elastic wave-speeds. It is not likely to occur if the shear wave-speed is the smallest elastic wave-speed and then a stationary discontinuity prevails.
55 citations
TL;DR: In this article, the problems related to resonators suitable for generation of diffraction-limited beams of high power or energy, and a few of the most significant recent solutions, are reviewed.
Abstract: The problems related to resonators suitable for generation of diffraction-limited beams of high power or energy, and a few of the most significant recent solutions, are reviewed. In particular, this paper is addressed to two promising resonator configurations developed mainly for Nd:YAG (yttrium aluminium garnet) lasers: dynamically stable resonators of minimum misalignment sensitivity for lasers with a strong thermal lensing in the active rod and unstable resonators with variable reflectivity output mirrors of super-Gaussian profile. For both cases experimental data and simple design guidelines are discussed.
47 citations
TL;DR: This study found that betaine deficiency in diverse maize germplasm is conditioned by the same genetic locus, and locates the lesion in betaine-deficient plants at the choline --> betaine aldehyde step of betaine synthesis.
Abstract: Maize (Zea mays L.) is a betaine-accumulating species, but certain maize genotypes lack betaine almost completely; a single recessive gene has been implicated as the cause of this deficiency (D Rhodes, PJ Rich [1988] Plant Physiol 88: 102-108). This study was undertaken to determine whether betaine deficiency in diverse maize germplasm is conditioned by the same genetic locus, and to define the biochemical lesion(s) involved. Complementation tests indicated that all 13 deficient genotypes tested shared a common locus. One maize population (P77) was found to be segregating for betaine deficiency, and true breeding individuals were used to produce related lines with and without betaine. Leaf tissue of both betaine-positive and betaine-deficient lines readily converted supplied betaine aldehyde to betaine, but only the betaine-containing line was able to oxidize supplied choline to betaine. This locates the lesion in betaine-deficient plants at the choline → betaine aldehyde step of betaine synthesis. Consistent with this location, betaine-deficient plants were shown to have no detectable endogenous pool of betaine aldehyde.
44 citations
TL;DR: In this article, the class of elastic-plastic constitutive laws assumed in this paper can be described as follows: y ⩾ 1 yield criteria (or "modes") which define the current yield surface; each yield function φα consists of two addends: an effective stress Φα and a yield limit Yα (α = 1,…, y), which are generally nonlinear functional of the plastic strain history, so that kinematic hardening is accounted for.
TL;DR: In this article, the problem of finding the optimal orientation of orthotropic properties for an elastic body, subjected to a plane state of stress, in order to maximize the stiffness of the body itself is addressed.
Abstract: The paper deals with the problem of finding the optimal orientation of orthotropic properties for an elastic body, subjected to a plane state of stress, in order to maximize the stiffness of the body itself
01 Jan 1991
TL;DR: The fast development of modern material science is based on the existence of many physical techniques which are used for the characterisation of the molecular and/or bulk properties of the materials.
Abstract: The fast development of modern material science is based on the existence of many physical techniques which are used for the characterisation of the molecular and/or bulk properties of the materials. The data so collected form the basis of any attempt to understand on a theoretical basis the physics of the phenomena which originate the properties of the material.
TL;DR: The results suggest that the measure of ATPase activities in erythrocytes membranes could be a simple, safe and useful early marker of liver damage and also valuable to test the effectiveness of a given drug therapy.
TL;DR: The results suggest that L-DOPA is able to activate D1 receptors located on the terminals of striatal projections via the dopamine formed by a decarboxylation mediated by an NSD-1015-resistant enzyme.
TL;DR: In this paper, the authors applied functional integral techniques to the problem of level densities for interacting Fermi systems and derived a mean-field approximation of the partition function with a high level of accuracy.
TL;DR: In this article, the synthesis, conformational and spectroscopic studies of N-borane adducts of 14 nitrogen containing six-membered ring heterocycles are reported.
TL;DR: There was a close correlation between glucose-stimulated motility and enhancement of ATP (P less than 0.05) by glucose even under incubation conditions in which glucose caused the Crabtree effect (decrease in respiration rate).
Abstract: Washed guinea-pig spermatozoa from the vas deferens re-acquired progressive motility within 1-2 min of incubation in minimal culture medium containing pyruvate and lactate. When glucose was added, either at the beginning or after 15 min of incubation, the cells showed stimulated motility (increased straight-line velocity, linearity and beat-cross frequency, P less than 0.01). Re-acquisition of progressive motility was preceded by a significant (P less than 0.005) transient increase in sperm concentration of cyclic adenosine 5'-phosphate (cAMP) with or without glucose in the medium. Papaverine caused another large significant (P less than 0.001) increase in cAMP concentration; and 5.56mM glucose with papaverine caused a further stimulation in cAMP beyond that with papaverine alone (P less than 0.005). Although 0.05 or 5.56mM glucose plus alpha-chlorohydrin stimulated sperm motility, they did not further stimulate the increase in cAMP after 30 s of incubation. Thus, there was no apparent correlation between the glucose-stimulating effect on sperm motility and the enhancement of cAMP at 30 s. However, there was a close correlation between glucose-stimulated motility and enhancement of ATP (P less than 0.05) by glucose even under incubation conditions in which glucose caused the Crabtree effect (decrease in respiration rate).
TL;DR: In this paper, a systematic investigation on the elemental composition, structural, optical and electrical properties of undoped and doped microcrystalline silicon carbide films deposited by plasma enhanced chemical vapor deposition was conducted.
Abstract: Experimental results on a systematic investigation on the elemental composition, structural, optical and electrical properties of undoped and doped microcrystalline silicon carbide films deposited by Plasma Enhanced Chemical Vapor Deposition. The doped samples show high values of dark conductivity accompanied by good optical properties so to satisfy the requirements for heterojunction window material.
TL;DR: In this article, the authors describe a bimetallic complex from which single crystals were obtained after recrystallization from n-heptane solution using X-ray diffraction.
TL;DR: In this paper, the shakedown and nonshakedown theory in elastic-plastic dynamics is dealt with on the following basis: for the material behaviour a quite general class of internal variables constitutive model is adopted allowing for general nonlinear hardening law; discrete structural models described in terms of generalized stresses and strains are referred to.
TL;DR: In this article, the authors focus on the orientación cultural de los ninos mazahuas de Mexico hacia the observación como estrategia preferida in la organización del aprendizaje, and muestra como a partir de esta orientacion se desarrollan arreglos interaccionales for realizar actividades de aprendía dentro del salon de clase.
Abstract: ResumenEn el salon de clase los alumnos manejan un conocimiento cultural que han adquirido por ser miembros de un grupo sociocultural particular. En parte este conocimiento se trata de formas sociales particulares de organizar el aprendizaje que se pueden identificar en los comportamientos y actitudes de los ninos frente el quehacer escolar. Este articulo indaga acerca de la naturaleza de este tipo de conocimiento y su alcance dentro del salon de clase. El analisis se centra en la orientacion cultural de los ninos mazahuas de Mexico hacia la observacion como estrategia preferida en la organizacion del aprendizaje, y muestra como a partir de esta orientacion se desarrollan arreglos interaccionales para realizar actividades de aprendizaje dentro del salon de clase. Las descripciones y los analisis se basan en los datos provistos por las observaciones intensivas y entrevistas abiertas realizadas en una escuela rural de Mexico. Se enfoca la practica escolar en un aula a la que asisten alumnos de quinto y sext...
TL;DR: Amealco Caldera, located in the central part of the Mexican Volcanic Belt (MVB), about 130 km NW of Mexico City, is characterized by a quasi-circular, ∼11-km-diameter structure whose southern portion is partly truncated by a regional fault.
TL;DR: In this paper, general analytic expressions concerning optical stability, mode spot size in the gain and saturable absorber jets, and misalignment sensitivity of femtosecond dye laser resonators are derived by using the one-way ABCD ray matrix technique.
TL;DR: In this article, the homolytic methylation of naphthoquinone to obtain menadione has been investigated by three sources of methyl radical: t-BuOOH DMSO and H 2 O 2 acetone and H2 O 2.