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Ramirez-Moreno M.A.,Escuela Superior de Fisica y Matematicas IPN | Angulo-Brown F.,Escuela Superior de Fisica y Matematicas IPN
Journal of Physics: Conference Series | Year: 2017

This paper presents a simple method based on Finite-Time Thermodynamics (FTT) to determine the thermal conductivity of atmospheric air. The method considers an atmospheric heat engine in which the air moves convectively on the border day-night driven by the Sun's energy. The numerical value obtained for the thermal conductivity reasonably accords with the reported value by Van Ness by the number of Rayleigh. © Published under licence by IOP Publishing Ltd.

Yee-Madeira H.,Escuela Superior de Fisica y Matematicas IPN
Journal of Agricultural and Food Chemistry | Year: 2011

Banana starch was chemically modified using single (esterification or cross-linking) and dual modification (esterification-cross-linking and cross-linking-esterification), with the objective to increase the slowly digestible starch (SDS) and resistant starch (RS) concentrations. Physicochemical properties and in vitro digestibility were analyzed. The degree of substitution of the esterified samples ranged from 0.006 to 0.020. The X-ray diffraction pattern of the modified samples did not show change; however, an increase in crystallinity level was determined (from 23.79 to 32.76%). The ungelatinized samples had low rapidly digestible starch (RDS) (4.23-9.19%), whereas the modified starches showed an increase in SDS (from 10.79 to 16.79%) and had high RS content (74.07-85.07%). In the cooked samples, the esterified starch increased the SDS content (21.32%), followed by cross-linked starch (15.13%). Dual modified starch (cross-linked-esterified) had the lowest SDS content, but the highest RS amount. The esterified and cross-linked-esterified samples had higher peak viscosity than cross-linked and esterified-cross-linked. This characteristic is due to the fact that in dual modification, the groups introduced in the first modification are replaced by the functional group of the second modification. Temperature and enthalpy of gelatinization decreased in modified starches (from 75.37 to 74.02 °C and from 10.42 to 8.68 J/g, respectively), compared with their unmodified starch (76.15 °C and 11.05 J/g). Cross-linked-esterified starch showed the lowest enthalpy of gelatinization (8.68 J/g). Retrogradation temperature decreased in modified starches compared with unmodified (59.04-57.47 °C), but no significant differences were found among the modified samples. © 2011 American Chemical Society.

Yee-Madeira H.,Escuela Superior de Fisica y Matematicas IPN
Plant Foods for Human Nutrition | Year: 2010

Fresh and stored maize (white and blue) tortillas were evaluated for physicochemical, rheological and structural characteristics assessed by calorimetry, x-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, dynamic viscoelastic tests, and high-performance sizeexclusion chromatography. Two endotherms were found in studies of fresh and stored tortillas. The low temperature endotherm (50-56 °C) was due to reorganized (retrograded) amylopectin, while the high temperature endotherm (105-123 °C) was attributed to retrograded amylose. The enthalpy value for the lower temperature transition was minor than that of the high temperature transition. Fresh tortillas showed an amorphous starch arrangement by x-ray diffraction study. Stored samples showed the presence of peaks at 2θ = 17° and 23°, indicating re-crystallization of starch components. FTIR results confirmed the development of higher levels of starch crystals during storage. Differences in the viscoelastic parameters were also observed between fresh and stored samples. At the longest storage times, white tortillas were more rigid than blue tortillas. Molar mass values for starch increased for both white and blue tortillas as storage time progressed, though relatively higher values were obtained for white tortillas. More starch reorganization occurred in white tortillas, in accordance to calorimetric, x-ray diffraction, FTIR and rheological results. These results corroborate that changes occurring in tortillas during storage are related to reorganization of starch components, and the maize variety more than the color plays an important role. © 2010 Springer Science+Business Media, LLC.

Rodriguez-Marin M.L.,National Polytechnic Institute of Mexico | Bello-Perez L.A.,National Polytechnic Institute of Mexico | Yee-Madeira H.,Escuela Superior de Fisica y Matematicas IPN | Zhong Q.,University of Tennessee at Knoxville | Gonzalez-Soto R.A.,National Polytechnic Institute of Mexico
Materials Science and Engineering C | Year: 2013

Rice and banana flours are inexpensive starchy materials that can form films with more improved properties than those made with their starch because flour and starch present different hydrophobicity. Montmorillonite (MMT) can be used to further improve the properties of starch-based films, which has not received much research attention for starchy flours. The aim of this work was to evaluate the mechanical and barrier properties of nanocomposite films of banana and rice flours as matrix material with addition of MMT as a nanofiller. MMT was modified using citric acid to produce intercalated structures, as verified by the X-ray diffraction pattern. The intercalated MMT was blended with flour slurries, and films were prepared by casting. Nanocomposite films of banana and rice flours presented an increase in the tensile at break and elongation percentage, respectively, more than their respective control films without MMT. This study showed that banana and rice flours could be alternative raw materials to use in making nanocomposite films. © 2013 Elsevier B.V.

Vigil-Galan O.,Escuela Superior de Fisica y Matematicas I.P.N. | Courel M.,Escuela Superior de Fisica y Matematicas I.P.N. | Cruz-Gandarilla F.,Escuela Superior de Fisica y Matematicas I.P.N. | Seuret-Jimenez D.,Autonomous University of the State of Morelos
Journal of Materials Science: Materials in Electronics | Year: 2016

In this work, the results about the properties of CdTe:Bi thin films grown by the Closed Space Vapor Transport (CSVT) method are presented. Two procedures were developed for the CdTe:Bi films deposition: (a) using powders with different Bi concentrations at a constant pressure in the CSVT chamber and (b) varying the total final Ar pressure in the CSVT chamber. The CdTe powders used in our experimental conditions were obtained by using CdTe:Bi crystals grown by the vertical Bridgman method, varying the nominal Bi-dopant concentration in the 1.0 × 1017 to 4 × 1019 at/cm3 range. Finally, the possible influence of both parameters on the existence of an intermediate band in CdTe:Bi thin films and CdS/CdTe solar cell characteristics is analyzed. © 2016 Springer Science+Business Media New York

Hernandez M.P.,University of Habana | Farias M.H.,National Autonomous University of Mexico | Castillon F.F.,National Autonomous University of Mexico | Diaz J.A.,National Autonomous University of Mexico | And 4 more authors.
Applied Surface Science | Year: 2011

Polycrystalline thin films of Ge-C were grown on Si (1 1 1) substrates by means of reactive pulsed laser deposition with methane pressure of 100 mTorr. Effect substrate temperature, Ts, on C incorporation to substitutional sites (x) in Ge1-xCx was investigated systematically by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyzes. The substrate temperatures were ranging from 250 to 400 °C. The substitutional C composition x in the films by XRD were estimated using the Vegard's linear law. The maximum value of x calculated by XRD was 0.032 for Ts of 350 °C. The position of the C 1s peak at 283.4 eV in the XPS spectrum confirmed the germanium-carbon alloys. XRD measurements indicated that x increased with Ts from 250 °C to 350 °C. At T s = 400 °C, the estimation of x was lowered. However, the C content calculated by XPS analyzes increased with Ts being more these values than substitutional C composition x. XPS and XRD analyzes demonstrate that the remaining C atoms are incorporated to interstitial sites. The use of the Ts plays important roles in the incorporation of substitutional C and in restraining C-cluster formation in the reactive pulsed laser deposition growth of Ge-C/Si. © 2010 Elsevier B.V. All rights reserved.

Castillo-Alvarado F.L.,Escuela Superior de Fisica y Matematicas IPN | Inoue-Chavez J.A.,Escuela Superior de Fisica y Matematicas IPN | Vigil-Galan O.,Escuela Superior de Fisica y Matematicas IPN | Sanchez-Meza E.,Escuela Superior de Fisica y Matematicas IPN | And 2 more authors.
Thin Solid Films | Year: 2010

Polycrystalline thin film CdS/CdTe heterojunction solar cells are important candidates for large scale photovoltaic applications. In this work we use a C-V (capacitance vs. voltage) theoretical method for the determination of the interface charge density σ and band discontinuity ΔEv of the CdS/CdTe heterojunction. The methodology is based on three cardinal equations: i) line up of the bands relative to the common Fermi level (at equilibrium) or the quasi-Fermi level (when voltage is applied), ii) charge neutrality and iii) the total capacitance of the heterostructure. We used CdS/CdTe solar cells, grown in our laboratory by the chemical bath deposition (for CdS film) and the close space vapor transport (for CdTe film) techniques. The interface parameters σ, and ΔEv are determined from C-V fitting between the calculated and the measured curve. The methodology presented in this study is general and can be applied to semiconductor-semiconductor and semimetal-semiconductor heterojunctions. © 2009 Elsevier B.V. All rights reserved.

Valencia G.,Escuela Superior de Fisica y Matematicas IPN | Arias L.A.,Escuela Superior de Fisica y Matematicas IPN
Journal of Physics: Conference Series | Year: 2015

In this work we show a RLC-circuit as energy converter within the context of first-order irreversible thermodynamics (FOIT). For our analysis, we propose an isothermic model with transient elements and passive elements. With the help of the dynamic equations, the Kirchhoff equations, we found the generalized fluxes and forces of the circuit, the equation system shows symmetry of the cross terms, this property is characteristic of the steady state linear systems, but in this case phenomenological coefficients are function of time. Then, we can use these relations, similar to the linear Onsager relations, to construct the characteristic functions of the RLC energy converter: the power output, efficiency, dissipation and ecological function, and study its energetic performance. The study of performance of the converter is based on two parameters, the coupling parameter and the "forces ratio" parameter, in this case as functions of time. We find that the behavior of the non-steady state converter is similar to the behavior of steady state energy converter. We will explain the linear and symmetric behavior of the converter in the frequencies space rather than in the time space. Finally, we establish optimal operation regimes of economic degree of coupling for this energy converter. © Published under licence by IOP Publishing Ltd.

Gonzalez-Cisneros A.,Escuela Superior de Fisica y Matematicas IPN | Castillo-Alvarado F.L.,Escuela Superior de Fisica y Matematicas IPN | Ortiz-Lopez J.,Escuela Superior de Fisica y Matematicas IPN | Contreras-Puente G.,Escuela Superior de Fisica y Matematicas IPN
International Journal of Photoenergy | Year: 2013

In CdS/CdTe solar cells, chemical interdiffusion at the interface gives rise to the formation of an interlayer of the ternary compound CdSxCdTe1-x. In this work, we evaluate the effects of this interlayer in CdS/CdTe photovoltaic cells in order to improve theoretical results describing experimental C-V (capacitance versus voltage) characteristics. We extended our previous theoretical methodology developed on the basis of three cardinal equations (Castillo-Alvarado et al., 2010). The present results provide a better fit to experimental data obtained from CdS/CdTe solar cells grown in our laboratory by the chemical bath deposition (for CdS film) and the close-spaced vapor transport (for CdTe film) techniques. © 2013 A. Gonzalez-Cisneros et al.

Gonzalez-Ayala J.,Escuela Superior de Fisica y Matematicas IPN | Angulo-Brown F.,Escuela Superior de Fisica y Matematicas IPN
Proceedings of the 25th International Conference on Efficiency, Cost, Optimization and Simulation of Energy Conversion Systems and Processes, ECOS 2012 | Year: 2012

It is common in many thermodynamics textbooks to illustrate the Carnot's theorem through the usage of diverse equations of state for gases, paramagnets, and other simple thermodynamic systems. As it is wellknown, the universality of the Carnot efficiency is easily demonstrated in a temperature-entropy diagram. In fact, any thermodynamic cycle depicted in a (W, Z) diagram, being the inexact differential of work, can be mapped into a (T, S) diagram. In this work we show that this property is consistent with a preserving area map with Jacobian[(W, Z), (T, S)] =-1, and that any equation of state satisfying this Jacobian transformation fulfills the Carnot's theorem.

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