551 research outputs found
Análisis de las actividades sobre la historia de la química en los libros de física y química del segundo ciclo de la ESO
¿Son las nuevas empresas internacionales más eficientes que las domésticas en un periodo de crisis?
Esta investigación tiene como objetivo determinar si las nuevas empresas internacionales, establecidas en un período de crisis, son más eficientes que las nuevas empresas domésticas creadas en ese mismo período. Con esta finalidad, se aplica el Análisis Envolvente de Datos (DEA) y regresiones por cuantiles para más de 12.000 nuevas empresas españolas de diferentes industrias, creadas entre 2008 y 2010. Los resultados más relevantes sugieren que las empresas de temprana internacionalización tienen mejores niveles de eficiencia en comparación con las nuevas empresas domésticas en este período
Constant rate thermal analysis for thermal stability studies of polymers
This paper explores the relationship between the shapes of temperature-time curves obtained from experimental data recorded by means of constant rate thermal analysis (CRTA) and the kinetic model followed by the thermal degradation reaction. A detailed shape analysis of CRTA curves has been performed as a function of the most common kinetic models. The analysis has been validated with simulated data, and with experimental data recorded from the thermal degradation of polytetrafluoroethylene (PTFE), poly(1,4-butylene terephthalate) (PBT), polyethylene (PE) and poly(vinyl chloride) (PVC). The resulting temperature-time profiles indicate that the studied polymers decompose through phase boundary, random scission, diffusion and nucleation mechanisms respectively. The results here presented demonstrate that the strong dependence of the temperature-time profile on the reaction mechanism would allow the real kinetic model obeyed by a reaction to be discerned from a single CRTA curve.Junta de Andalucía TEP-03002Ministerio de Ciencia e Innovación MAT 2008-06619/MA
Assesing advertising efficiency : does the internet play a role?
This research focuses on a major concern for marketers addressing the claims of inefficiency of the spending on advertising. We examine whether the Internet can help increase overall advertising efficiency. Using a sample from the Spanish automobile industry, we combine a nonparametric method - Data Envelopment Analysis - with recent important insights from statistics and econometrics studies, and we find that online advertising improves the efficiency levels and this effect is more pronounced in the long-term temporal framework
Photoionization-ion mobility spectrometer for non-targeted screening analysis or for targeted analysis coupling a Tenax TA column
Photoionization-ion mobility spectrometer (UV-IMS) can be used as a screening
instrument to distinguish gasoline vapors from other similar samples, obtaining characteristic
fingerprints in a quick and simple analysis. A non-targeted approach can be used to resolve many
problems, moreover, this device can also be used for targeted analysis. In this case a Tenax TA
trap was connected to the UV-IMS as a pre-separation step in order to identify compounds
present in the gasoline gaseous sample, such as benzene and toluene. Firstly, target analytes were
trapped in a Tenax TA column at ambient temperature and then they were desorbed thermally in
an oven and connected to the UV-IMS for their analysis. Instrumental parameters such as
temperature ramp, sample gas flow rate, drift gas flow rate, adsorption time and adsorption flow rate were optimized to obtain high sensitivity, selectivity and precision to determine benzene and
toluene in vapour gaseous samples. The UV-IMS was calibrated with standard gases generated in
an exponential dilution flask, obtaining a limit of quantification of 1.8 and 1.9 mg m-3 for
benzene and toluene, respectively. The method developed was successfully applied to identify
these analytes in air samples close to a gasoline spillag
Dependence of the preexponential factor on temperature
The dependence of the preexponential factor on the temperature has been examined and the errors involved in the activation energy calculated from isothermal and non-isothermal methods without considering such dependence have been estimated. It has been shown that the error in the determination of the activation energy calculated ignoring the dependence of Aon Tcan be rather large and it is dependent on x=E/RT, but independent of the experimental method used. It has been also shown that the error introduced by omitting the dependence of the preexponential factor on the temperature is considerably larger than the error due to the Arrhenius integral approach used for carrying out the kinetic analysis of TG data.Peer reviewe
Quality production and sustainable development: the case of Merina de Grazalema Sheep cheese
This paper focus on the innovations made by local actors in the production and marketing of Merina de Grazalema Sheep Cheese, studying how these changes may improve or not the sustainability of Sierra de Grazalema as
an agro-ecosystem. These innovations answer to production system
and organization patterns within the supply chain, they seek to confront
a new agro-food context. This new context has been unfolded
by changes in three main elements of the equation:
(1) new EU policies,
(2) Globalization ofthe markets, and new consumption patterns. Merina de Gra lema. We found that the low scale and the high artisan level of production have been the key to the construction of positive answer
to this new context, and that the materialization of these answers is highly diversified
Limitations of model-fitting methods for kinetic analysis: Polystyrene thermal degradation
In this paper, some clarifications regarding the use of model-fitting methods of kinetic analysis are provided in response to the lack of plot linearity and dispersion in the activation energy values for the thermal degradation of polystyrene found in the literature and some results proposing an nth order model as the most suitable one. In the present work, two model-fitting methods based on the differential and integral forms of the general kinetic equation are evaluated using both simulated and experimental data, showing that the differential method is recommended due to its higher discrimination power. Moreover, the intrinsic limitations of model-fitting methods are highlighted: the use of a limited set of kinetic models to fit experimental data and the ideal nature of such models. Finally, it is concluded that a chain scission model is more appropriate than first order. © 2013 Elsevier B.V.Peer Reviewe
Critical study of the isoconversional methods of kinetic analysis
A critical study of the use of isoconversional methods for the kinetic analysis of non-isothermal data corresponding to processes with either a real or an apparent variation of the activation energy, E, with the reacted fraction, α, has been carried out using for the first time simulated curves. It has been shown that the activation energies obtained from model-free methods are independent of the heating rate. However, the activation energy shows a very strong dependence of the range of heating rates used for simulating the curves if the apparent change of E with α is caused by overlapping processes with different individual activation energies. This criterion perhaps could be used for determining if a real dependence between E and α is really occurringEspaña Ministerio de Educación y Ciencia No. MAT2004-0264
Kissinger kinetic analysis of data obtained under different heating schedules
The dynamic heating rate method developed by TA Instruments (
Hi-Res
™
) is a kind of sample
controlled thermal analysis in which a linear relati
onship between the logarith
m of the heating rate
and the rate of weight change is imposed. It is s
hown in this paper that the reacted fraction at the
maximum reaction rate strongly depends
on the parameters selected for the
Hi-Res
heating
algorithm, what invalidates the use of
the Kissinger method for analysing
Hi-Res
data unless that
the reaction fits a first order kinetic law. Only in this latter case, it has been demonstrated that it is
not required that a constant value of the reacted fr
action at the maximum reaction rate is fulfilled for
determining the activation energy from the Kissi
nger method. In such a case the Kissinger plot
gives the real activation energy, independently of
both the heating schedule used and the value of
the reacted fraction,
α
m
, at the maximum
- …