La Enseñanza y el aprendizaje de la química en el contexto de un currículo articulado desde concepciones científicas de interacción

En el presente trabajo se muestra la construcción, desarrollo y aplicación de unidades didácticas innovadoras abordando conceptos estructurantes como naturaleza corpuscular de la materia, cambio químico y cuantificación de relaciones, orientadas por un eje didáctico de resolución de problemas teniendo en cuenta los estándares curriculares nacionales y concepciones científicas de interacción. Las unidades didácticas derivadas de la conjugación de estas perspectivas científicas y educativas fueron aplicadas con estudiantes de educación media de varios Colegios de la ciudad de Bogotá

Abnormal white matter tracts resembling pencil fibers involving prefrontal cortex (Brodmann area 47) in autism: a case report.

BackgroundAutism is not correlated with any neuropathological hallmark as the brain of autistic individuals lack defined lesions. However, previous investigations have reported cortical heterotopias and local distortion of the cytoarchitecture of the neocortex in some cases of autism.Case presentationOur patient was a 40-year-old white woman diagnosed at an early age with autism and mental retardation. Pencil fibers were present within the prefrontal cortex (Brodmann area 47) and its composition resembled that of the underlying white matter region. Pencil fibers encompassed most of the extent of the cortical grey matter and were populated by oligodendrocytes, astrocytes, and microglial cells, but not by neurons.ConclusionsHere we report a new cytoarchitectural abnormality that has not been previously described in autism. Future pathological examinations should keep in mind the potential presence of pencil fibers within the prefrontal cortex of cases with autism

Corporate information transparency on the Internet by listed companies in Spain (IBEX35) and Mexico (IPYC)

Financial and non-financial disclosure play a central role in the functioning of capital markets. In this context, the Internet has been adopted as an effective mechanism for large companies to disseminate corporate information. The institutional theory approach has been applied to identify both formal (fundamentally legal and economic) and informal factors that significantly influence listed companies’ level of corporate transparency on the Internet. Our work aims to build on existing study by focusing on two main objectives. Firstly, to make a comparative study of the corporate transparency of listed companies from Mexico and Spain by creating an index of corporate transparency on the Internet (e-CTI). And secondly, to identify the factors that affect this index using multiple regression analysis. Our study population is comprised of 70 companies, of which 35 belong to the Mexican Price and Quotations Index (IPyC) and 35 to the Spanish IBEX 35 index. The descriptive analysis reveals significant differences in the level of information disclosure between the two countries. The companies listed in Mexico obtain an e-CTI of 59%, while the Spanish ones register 80%, i.e. more importance is assigned to the disclosure of corporate governance data in Spain than in Mexico. Furthermore, this analysis shows that the factors most telling with regards to corporate transparency are the strength and application of law, GDP per capita, inflation and firm-level variables such as ownership concentration and Chairman of the Board-Chief Executive Officer (COB-CEO) duality. However, other variables such as board size and composition, profitability, leverage and firm size are not significant for the purposes of this analysis. Our work is of great relevance today, since most studies have focused on developed countries, mainly in the U.S. and Europe, with few comparisons being made between developed and developing countries, such as Spain and Mexico

Corrosão de interfaces ti/al2o3 em solucões fisiologicas simuladas

Nos últimos anos o estudo das interfaces metal/cerâmico (M/C) tem despertado grande interesse devido às diversas utilidades que estas podem oferecer em diferentes campos de aplicação prática. Um destes campos é o relativo à área das aplicações biomédicas. De entre os vários processos existentes para a produção de uniões M/C, as técnicas de união por difusão e de brasagem activa são algumas das mais utilizadas. Em qualquer uma destas técnicas, a alta temperatura envolvida no processo é um parâmetro que favorece o desenvolvimento de reacções químicas complexas que dão origem a interfaces multi-camadas cujas propriedades físico-químicas são complexas. Esta complexidade da interface, unida ao facto, já conhecido, de que as propriedades mecânicas, físicas e químicas, incluindo a resistência à corrosão, dependem da composição química e da microestrutura da região interfacial, fazem com que seja necessário um melhor conhecimento da natureza e das características físico-químicas desta região, de forma a poder controlar mais estritamente as propriedades da interface. O principal objectivo deste trabalho é o estudo do comportamento à corrosão de interfaces Ti/Al2O3 produzidas pela técnica de união por brasagem activa. Para efectuar este estudo para além da interface, foram produzidas ligas com composições químicas similares a cada camada interfacial, as quais foram testadas através de ensaios electroquímicos de corrente directa, nomeadamente, polarização potenciodinâmica, resistência à polarização e corrosão galvânica. Neste trabalho, será descrito o comportamento à corrosão da interface, assim como o efeito que a interacção entre as diferentes camadas presentes na interface produz sobre o comportamento à degradação geral da interface Ti/Al2O3.Fundação para a Ciência e a Tecnologia (FCT), Portugal (Projectos POCTI/CTM/33384/2000 e SFRH/BPD/5518/2001)

Corrosion behaviour of Ti/Al2O3 interfaces produced by an active metal brazing methodology

Metal/ceramic joints are used in a broad range of applications in biomedicine, such as the encapsulation of implantable telemetric devices, the fabrication of crowns and bridges for dental restoration, or the production of drug delivery systems, biomedical sensors and electrodes. Apart of other characteristics, the corrosion resistance of metal/ceramic interfaces is of prime importance when biomedical applications are considered. Most of metal/ceramic joints are produced by the active metal brazing technique or by diffusion bonding. Both techniques originates a multi-layered interface which should be able of accommodating the abrupt electronic, crystallographic, chemical, mechanical and thermomechanical discontinuity that characterize these metal/ceramic systems. However, galvanic interactions between those chemically distinct layers are likely to occur, affecting the degradation behaviour of the interface. In this work the corrosion behaviour of Ti-Al2O3 interfaces produced by an active metal brazing methodology was studied. SEM analyses evidenced an interface mainly constituted by four different layers. A first layer rich on titanium and copper, located near to the pure titanium, another layer also rich on Ti and Cu, but with a higher Ag content, an intermediate layer rich in silver which contains some little precipitates of Ti-Cu and finally, a reaction layer with a composition profile containing Ti, Cu, Ag, and Al, located near the alumina part. Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) measurements, carried out in a simulated physiological solution at ambient temperature, revealed a strong influence of the rich silver layer on the passivating behaviour of the interface. On the other hand, the reaction layer appears to be the main responsible for the degradation process of the interface. This degradation is accompanied by a relatively high release of copper. Through EIS data simulation it was possible to obtain an electrochemical equivalent circuit that describes the corrosion process and allowed an estimation of the polarisation resistance of the constitute layers of the interface. The electrochemical interaction between the different constitutive layers was evaluated, and was correlated with the overall degradation behaviour of the Ti-Al2O3 interface.Fundação para a Ciência e a Tecnologia (FCT)

Use of Foundry Sands in the Production of Ceramic and Geopolymers for Sustainable Construction Materials

The aim of this research was to evaluate the possibility of reusing waste foundry sands derived from the production of cast iron as a secondary raw material for the production of building materials obtained both by high-temperature (ceramic tiles and bricks) and room-temperature (binders such as geopolymers) consolidation. This approach can reduce the current demand for quarry sand and/or aluminosilicate precursors from the construction materials industries. Samples for porcelain stoneware and bricks were produced, replacing the standard sand contained in the mixtures with waste foundry sand in percentages of 10%, 50%, and 100% by weight. For geopolymers, the sand was used as a substitution for metakaolin (30, 50, 70 wt%) as an aluminosilicate precursor rather than as an aggregate to obtain geopolymer pastes. Ceramic samples obtained using waste foundry sand were characterized by tests for linear shrinkage, water absorption, and colorimetry. Geopolymers formulations, produced with a Si/Al ratio of 1.8 and Na/Al = 1, were characterized to evaluate their chemical stability through measurements of pH and ionic conductivity, integrity in water, compressive strength, and microstructural analysis. The results show that the addition of foundry sand up to 50% did not significantly affect the chemical-physical properties of the ceramic materials. However, for geopolymers, acceptable levels of chemical stability and mechanical strength were only achieved when using samples made with 30% foundry sand as a replacement for metakaolin

Performance and abilities of family-member ceos in a context of formal institutional weakness

Antecedentes: desde la perspectiva de la teoría institucional, este artículo estudia la relación entre las habilidades de los directores generales pertenecientes a la familia y el desempeño financiero de las empresas familiares listadas en un entorno caracterizado por la debilidad de las instituciones formales. Metodología: la muestra se compone de empresas familiares no financieras listadas en la Bolsa Mexicana de Valores durante el periodo 2001-2014. El análisis econométrico utiliza modelos lineales para datos de panel estimados por mínimos cuadrados ordinarios (mco). Para tomar en cuenta la endogeneidad, las regresiones se corren con el método generalizado de momentos (mgm). Conclusiones: en un contexto de debilidad institucional, invertir en educación superior en el área de negocios —de alta calidad y proporcionada en el propio país— es una buena estrategia a largo plazo para empresas familiares que quieran promover a un director general de la familia.Background: From the perspective of institutional theory, this paper studies the relation between the abilities of family-member ceos and the financial performance of listed family companies, in a setting of formal institutional weakness. Methodology: The study sample is composed of non-financial family firms listed in the Mexican Stock Exchange during the 2001-2014 period. Econometric analysis is attained through linear models for panel data, estimated by ordinary least squares (ols). To take into account endogeneity concerns, regressions are run through the generalized method of moments (gmm). Conclusions: In a context of formal institutional weakness, investment in businessrelated higher education —of high quality, and in one’s own country— is a good long-term strategy for family businesses that wish to promote a family-member ce