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Search Results: 1 - 10 of 12318 matches for " Lorena álvarez-Contreras "
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LiMnPO4: Review on Synthesis and Electrochemical Properties  [PDF]
Joel O. Herrera, Héctor Camacho-Montes, Luis E. Fuentes, Lorena álvarez-Contreras
Journal of Materials Science and Chemical Engineering (MSCE) , 2015, DOI: 10.4236/msce.2015.35007
Abstract: The olivine structured mixed lithium-transition metal phosphates LiMPO4 (M = Fe, Mn, Co) have attracted tremendous attention of many research teams worldwide as a promising cathode material for lithium batteries and for studying their magnetic and electrical properties. High energy density is required for mind- to large-scale batteries because the mounting spaces are quite small for vehicles and other energy storage applications. This constraint necessitates synthesis to yield high volumetric energy density and reliable battery performance. Development of eco-friendly, low cost and high energy density, LiMnPO4 cathode material became attractive due to its high operating voltage, which can be located inside of the electrochemical stability window of conventional electrolyte solutions and it offers more safety features due to the presence of a strong P-O covalent bond. This review is an overview of current research activities on LiMnPO4 cathodes and its carbon coating synthesized by various synthetic techniques.
Metal Nanoparticles Supported on Al-MCM-41 via In Situ Aqueous Synthesis
Ivonne Alonso-Lemus,Ysmael Verde-Gomez,Alfredo Aguilar-Elguézabal,Lorena álvarez-Contreras
Journal of Nanomaterials , 2010, DOI: 10.1155/2010/302898
Abstract: MCM-41 have been used to custom synthesize catalysts in because of the controllable properties, such as pore size, active phase incorporation, crystal size, and morphology, among others. In this paper, a simple and versatile method for the incorporation of platinum, ruthenium, and palladium onto Al-MCM-41 mesoporous silica by direct inclusion of various precursors was studied. M/Al-MCM-41 structure, textural properties, morphology, and elemental composition were analyzed. The results obtained indicate that the Al-MCM-41 mesoporous-ordered structure was not affected by metallic particle incorporation. High-surface areas were obtained (1131? /g). Metallic nanoparticles dispersion on Al-MCM-41 was homogeneous for all samples and its particles sizes were between 6?nm to 20?nm. Microscopy results show round shape particles in platinum and palladium samples; however, ruthenium catalysts exhibit a spherical and rod shapes. Electrochemical testing for Pt/Al-MCM-41 showed electrocatalytic activity for oxidation which indicates that these materials can be used as a catalyst in electrochemical devices. 1. Introduction Mesoporous materials have played an important role in catalytic applications [1–3]. Despite the availability of natural mesoporous materials, their use is limited in catalytic applications due to the presence of undesired phases. The chemical composition of natural mesoporous materials changes from one mineral deposit to another, even from one stratum to another in the same mine. Unfortunately, this makes it difficult to optimize their properties for tailor applications. Thus, custom synthesizing mesoporous materials by incorporating metallic particles is an important issue for the development of catalytic applications [4]. Kresge et al. discovered a series of mesoporous materials named as the M41S family [5]. The M41S family has three members: MCM-41 with a hexagonal array of uni-directional pores, MCM-48 with a three-dimensional cubic pore structure, and MCM-50 with an unstable lamellar structure. These materials have exceptionally high-surface areas ( 1000?m2/g) and narrow pore size distributions [6]. Among the three members of the M41S family, MCM-41 became the most popular mesoporous molecular sieve. MCM-41 has been used in many applications such as production of intermediates/fine chemicals, petrochemical reactions [7, 8], optical applications [9], development of sensors [10], and different industrial reactions (e.g., oxidation, condensation, and enzymatic reactions) [11–15]. MCM-41 is better option than microporous zeolites for some
SYNTHESIS AND CONFORMATIONAL ANALYSIS OF LEPTOCARPIN DERIVATIVES: INFLUENCE OF MODIFICATION OF THE OXIRANE RINGON LEPTOCARPIN'S CYTOTOXIC ACTIVITY
ROLANDO MARTíNEZ,VICTOR KESTERNICH,HéCTOR CARRACSO,CAROLINA áLVAREZ-CONTRERAS
Journal of the Chilean Chemical Society , 2006,
Abstract: The reaction in acidconditions of Leptocarpin 1, a compound with antitumor activity, formed two new isomeric products, 8b-angeloyl-1b,3b-dihydroxy-4,10-dimethyl,-D11(13) methylen-4Z,9Z-dieneheliangol-6,12-olide 2 and 8b-angeloyl-1b,3b-dihydroxy-4-methyl-D11(13),D11(14)-dimethylen-4Z-eneheliangol-6,12-olide 3, whose structures reported in this study were established by spectroscopy (1H-NMR, 13C-NMR, MS and IR) and confirmedthrough ROESY experiments and theoretical studies by molecular mechanics. The in vitro cytotoxicity of these isomeric compounds was less active than leptocarpin, showing the importance of the oxirane ring in the biological activity. Cytotoxic activity was measured in six cancer cell lines
Effect of Nitriding Current Density on the Surface Properties and Crystallite Size of Pulsed Plasma-Nitrided AISI 316L  [PDF]
J. C. Díaz-Guillén, E. E. Granda-Gutiérrez, G. Vargas-Gutiérrez, M. R. Díaz-Guillén, J. A. Aguilar-Martínez, L. álvarez-Contreras
Journal of Materials Science and Chemical Engineering (MSCE) , 2015, DOI: 10.4236/msce.2015.31007
Abstract:

In this work, plasma-nitrided AISI 316L stainless steel samples were performed by ion nitriding process under pulsed direct current (DC) discharge at different current densities (1 to 2.5 mA/ cm2). The effect of nitriding current density on the size of crystalline coherently diffracting domains (crystallite size) and strain grade was investigated using X-ray diffraction (XRD) coupled with Williamson-Hall method. Additionally, hardness and wear resistance of the nitriding layer were characterized using a Vickers indenter and pin-on-disk technique respectively. Results showed a decrease in crystallite size from 99 nm for untreated samples to 1.4 nm for samples nitrided at 2.5 mA/cm2 promoted both: an increase in hardness from 226 HV25g to 1245 HV25g, and a considerably decrease in volume loss by wear effect.

SYNTHESIS AND CONFORMATIONAL ANALYSIS OF LEPTOCARPIN DERIVATIVES: INFLUENCE OF MODIFICATION OF THE OXIRANE RINGON LEPTOCARPIN'S CYTOTOXIC ACTIVITY
MARTíNEZ,ROLANDO; KESTERNICH,VICTOR; CARRACSO,HéCTOR; áLVAREZ-CONTRERAS,CAROLINA; MONTENEGRO,CAROLINA; UGARTE,RICARDO; GUTIéRREZ,ELENA; MORENO,JOSé; GARCíA,CARLOS; WERNER,ENRIQUE; CáRCAMOS,JUAN;
Journal of the Chilean Chemical Society , 2006, DOI: 10.4067/S0717-97072006000400003
Abstract: the reaction in acidconditions of leptocarpin 1, a compound with antitumor activity, formed two new isomeric products, 8b-angeloyl-1b,3b-dihydroxy-4,10-dimethyl,-d11(13) methylen-4z,9z-dieneheliangol-6,12-olide 2 and 8b-angeloyl-1b,3b-dihydroxy-4-methyl-d11(13),d11(14)-dimethylen-4z-eneheliangol-6,12-olide 3, whose structures reported in this study were established by spectroscopy (1h-nmr, 13c-nmr, ms and ir) and confirmedthrough roesy experiments and theoretical studies by molecular mechanics. the in vitro cytotoxicity of these isomeric compounds was less active than leptocarpin, showing the importance of the oxirane ring in the biological activity. cytotoxic activity was measured in six cancer cell lines
Rese a de "La casa barroca de Pátzcuaro" de GABRIEL SILVA MANDUJANO,,
Héctor álvarez Contreras
Tzintzun. Revista de Estudios Históricos , 2005,
Abstract:
Conflictividad socioambiental en América Latina. El escenario post crisis de 2001 en Argentina
Lorena Bottaro,Marian Sola álvarez
Política y cultura , 2012,
Abstract: En este artículo presentamos una aproximación a los casos que inauguraron un nuevo ciclo de conflictos relacionados con la defensa de los "recursos naturales" y los territorios en Argentina. Para ello realizaremos, en la primera parte del texto, una breve descripción de los primeros momentos de los conflictos socioambientales que tuvieron lugar, por un lado, alrededor de la instalación de plantas de procesamiento de celulosa al margen del Río Uruguay -en la localidad de Gualeguaychú, litoral argentino-, y por otro, en torno a la llegada de un proyecto para la explotación minera a cielo abierto en la localidad de Esquel, patagonia argentina. En un segundo momento, presentaremos una caracterización de los movimientos sociales de la década de 1990 en Argentina con el objeto de identificar similitudes y divergencias entre estos actores y los movimientos socioambientales propios de principio de siglo.
CONFORMATIONAL PREFERENCE OF 4-ETHYL- 6-METHYL-1,3-DITHIANES
CONTRERAS,J. GUILLERMO; GERLI,LORENA A.;
Journal of the Chilean Chemical Society , 2007, DOI: 10.4067/S0717-97072007000300018
Abstract: interconversion reaction of i ? ii conformers of 4-ethyl-6-methyl-1,3-dithiane (metdit) has been studied by means of ab initio methods on the frame of mo theory. optimized geometries at hf/6-31g** level correlate well with that found for the parent 1,3-dithianefrom an x-ray diffraction study. both conformers possess similar bond distances, but differ in up 12° in some dihedrals. conformer i geometry presents the larger distortion from the regular 1,3-dithiane mainly due to difference between angles κ1 and κ2 (ca. 14°). in conformer ii this difference is just ca. 3°. the energetics and thermodynamics were obtained with basis sets that include diffuse, polarization functions and electronic correlations at the second-order perturbation meller plesset theory. gas phase thermodynamic predicts ii to be in a 98% concentration. low and medium high polarity solvents seem to exert no influence on the conformers concentrations. thus the i ? ii interconversion reaction is largely displaced to ii formation, both in the gas phase and solution. the calculated 1h-nmr data, i.e., chemical shifts and one-bond c-h spin-spin coupling constants are predicted on the basis of the consistent results obtained for the parent 1,3-dithiane. for both conformers reverse perlin effect takes place at c2 and c5. the c2 - hax and c2 - heq bond distances are similar ( ca. 1.092 ?), whereas the c5- heq are larger than the axial ones yielding smaller 1jc-h coupling constants
CONFORMATIONAL PREFERENCE IN 4,6-DIMETHYL-l,3-THIOXANE
CONTRERAS,J. GUILLERMO; GERLI,LORENA A.;
Journal of the Chilean Chemical Society , 2008, DOI: 10.4067/S0717-97072008000100014
Abstract: trans-4,6-dimethyl-l,3-thioxane shows two conformations as a result of the heterocyclic ring inversion. the i and ii conformers interconversion process has been studied by means of ab initio methods. the thermodynamics of this reaction calculated at the mp2/6-311+g**// hf/6-31g" and b3lyp/6-311+g"// b3lyp/6-31g" levels yield ag° values of ca. -1.5 kcal/mol in the gas phase, implying that ii is the most important species in the gas phase and that its relative concentration is ca. 93%. in solution of low and medium polarity solvents, pcm method predicts an 85% concentration for this species, consistent with an 83% estimated from 'h-nmr spectra. the experimental nmr properties ( 8 and 'jc_h ) have not been reported since i and ii separation could not be achieved. the results obtained for two long range "j2ax6e and 6j2e je coupling constants of ca. 0.5 hz are in good agreement with 0.41 hz, calculated in the present work. the calculated chemical shifts for the c2 and c5 protons show that 8heq < 8hax implying that some anomalous effects are operating, though the 8 seems to be moderately insensitive c-h bond distances changes. the spin-spin coupling constants are much more sensitive to changes occurring in the ring. thus, normal and reverse perlin effects for 'j are predicted for c2 -h and c5-h are operating, respectively. the cc5 - heq -> (7*c - s interaction would be the responsible for the very small elongation of the c5 - heq over the c5 - hax bond distances
Dise?o y puesta en marcha de un sistema de vigilancia epidemiológica en salud mental
Goldenberg,Judith Scharager; Contreras Escudero,Lorena;
Revista Panamericana de Salud Pública , 2002, DOI: 10.1590/S1020-49892002000200004
Abstract: objective. to design and test a surveillance system to detect and rank the most significant mental health problems in a community and to establish their association with the living conditions in that community. methods. the system was designed and tested at a primary health care center in the community of la florida, which is in the metropolitan region of santiago, the capital of chile. the health problems that were under surveillance, with monthly monitoring, were: addiction (to alcohol, tobacco, and benzodiazepines or other drugs), violence (domestic violence and sexual abuse, real or suspected), and disorders in children (attention deficit and behavioral disorders). by means of a computerized geographical information system (gis), those surveillance data were combined with other data. these other data included sociodemographic information on the persons who went to the health center for care as well as data on local risk factors and protective factors for health problems, some of which are indicators of the local residents' living conditions. results. the proposed epidemiological surveillance system, which takes into account local living conditions and environmental variables, is a good tool for health action. the proposed system also complements the incomplete perspective of monitoring systems, which do not include variables describing the local context and that focus only on health problems. the information synthesized in the gis maps makes it possible to simultaneously display different layers of information on factors related to the health problems studied, linked to a specific area and all its environmental variables, in an easy-to-read, self-explanatory format. conclusions. these results and the concurring opinions of the participating physicians show that the experimental system effectively met the basic requirements of an epidemiological surveillance system of this kind.
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