Showing papers by "University of Seville published in 2008"

The age of multilevel converters arrives

Abstract: This work is devoted to review and analyze the most relevant characteristics of multilevel converters, to motivate possible solutions, and to show that we are in a decisive instant in which energy companies have to bet on these converters as a good solution compared with classic two-level converters. This article presents a brief overview of the actual applications of multilevel converters and provides an introduction of the modeling techniques and the most common modulation strategies. It also addresses the operational and technological issues.

Entrepreneurial Orientation and Growth of SMEs: A Causal Model

Abstract: The literature existing on entrepreneurship implicitly assumes that entrepreneurial orientation (EO) and growth orientation are positively related with each other. However, few studies, whether theoretical or empirical, analyze such relation in an explicit manner. Instead, most previous works have focused on the EO-performance relation, even though growth and profitability do not always correlate positively. This work has been carried out on a sample of 434 SMEs, and contributes two novelties with regard to previous research: (1) the analysis focuses on the EO-growth relation; and (2) it uses a flexible method (Partial Least Squares) which allows the study of several simultaneous relationships. The results reveal the complexity of the relationships between EO, strategy, environment, resources and growth.

Skill and value perceptions: how do they affect entrepreneurial intentions?

Abstract: This paper starts from Ajzen’s theory of planned behavior to test the role of different perceptions on the individual’s intention to become an entrepreneur. Support has most often been found for this theory in the field of entrepreneurship. However, little is yet known about the way in which perceptions are formed. It may be argued that social values regarding entrepreneurship, and also personal skill perceptions, would both affect entrepreneurial intentions. Our objective, therefore, is testing the existence and reach of both effects. Empirical analysis has been carried out on a sample of 249 university students. Structural equations models have been used to test our hypotheses. Results generally confirm them, since values and skills do play a significant role in explaining intention. However, the role of perceived skills seems to be more relevant. Implications may be derived in several areas, and especially regarding entrepreneurship education.

Electric Energy Systems : Analysis and Operation

Abstract: Electric Energy Systems-An Overview I.J. Perez Arriaga, H. Rudnick, and M. Rivier Steady-State Single-Phase Models of Power System Components E. Handschin, A. F. Otero, and J. Cidras Load Flow A. Gomez-Exposito and F. L. Alvarado State Estimation A. Gomez-Exposito and A. Abur Economics of Electricity Generation F. D. Galiana and A. J. Conejo Optimal and Secure Operation of Transmission Systems J. L. Martinez Ramos and V. H. Quintana Three-Phase Linear and Nonlinear Models of Power System Components E. Acha and J. Usaola Fault Analysis and Protection Systems J. Cidras, J. F. Minambres, and F. L. Alvarado Frequency and Voltage Control G. Andersson, C. Alvarez Bel, and C. Canizares Angle, Voltage, and Frequency Stability C. Canizares, L. Rouco, and G. Andersson Three-Phase Power Flow and Harmonic Analysis W. Xu and J. G. Mayordomo Electromagnetic Transients Analysis J. A. Martinez-Velasco and J. Marti Appendix A: Solution of Linear Equation Systems F. L. Alvarado and Antonio Gomez-Exposito Appendix B:Mathematical Programming A. J. Conejo Appendix C:Dynamic Models of Electric Machines L. Rouco Index

Experimentally testable state-independent quantum contextuality.

Abstract: We show that there are Bell-type inequalities for noncontextual theories that are violated by any quantum state. One of these inequalities between the correlations of compatible measurements is particularly suitable for testing this state-independent violation in an experiment.

Inflammatory Response in the Hippocampus of PS1M146L/APP751SL Mouse Model of Alzheimer's Disease: Age-Dependent Switch in the Microglial Phenotype from Alternative to Classic

Abstract: Although the microglial activation is concomitant to the Alzheimer's disease, its precise role (neuroprotection vs neurodegeneration) has not yet been resolved. Here, we show the existence of an age-dependent phenotypic change of microglial activation in the hippocampus of PS1xAPP model, from an alternative activation state with Abeta phagocytic capabilities (at 6 months) to a classic cytotoxic phenotype (expressing TNF-alpha and related factors) at 18 months of age. This switch was coincident with high levels of soluble Abeta oligomers and a significant pyramidal neurodegeneration. In vitro assays, using astromicroglial cultures, demonstrated that oligomeric Abeta42 and soluble extracts from 18-month-old PS1xAPP hippocampus produced a potent TNF-alpha induction whereas monomeric Abeta42 and soluble extract from 6- or 18-month-old control and 6-month-old PS1xAPP hippocampi produced no stimulation. This stimulatory effect was avoided by immunodepletion using 6E10 or A11. In conclusion, our results show evidence of a switch in the activated microglia phenotype from alternative, at the beginning of Abeta pathology, to a classical at advanced stage of the disease in this model. This change was induced, at least in part, by the age-dependent accumulation of extracellular soluble Abeta oligomers. Finally, these cytotoxic activated microglial cells could participate in the neuronal lost observed in AD.

Anticancer and carcinogenic properties of curcumin: considerations for its clinical development as a cancer chemopreventive and chemotherapeutic agent.

Abstract: A growing body of research suggests that curcumin, the major active constituent of the dietary spice turmeric, has potential for the prevention and therapy of cancer. Preclinical data have shown that curcumin can both inhibit the formation of tumors in animal models of carcinogenesis and act on a variety of molecular targets involved in cancer development. In vitro studies have demonstrated that curcumin is an efficient inducer of apoptosis and some degree of selectivity for cancer cells has been observed. Clinical trials have revealed that curcumin is well tolerated and may produce antitumor effects in people with precancerous lesions or who are at a high risk for developing cancer. This seems to indicate that curcumin is a pharmacologically safe agent that may be used in cancer chemoprevention and therapy. Both in vitro and in vivo studies have shown, however, that curcumin may produce toxic and carcinogenic effects under specific conditions. Curcumin may also alter the effectiveness of radiotherapy and chemotherapy. This review article analyzes the in vitro and in vivo cancer-related activities of curcumin and discusses that they are linked to its known antioxidant and pro-oxidant properties. Several considerations that may help develop curcumin as an anticancer agent are also discussed.

A universal curve for the magnetocaloric effect: an analysis based on scaling relations

Abstract: The universal character of the recent experimentally found master curve for the magnetic entropy change, ΔSM, in studies of the magnetocaloric response of materials is analytically justified by using scaling arguments. The validity of the obtained scaling relations is checked against experimental data as well as the mean field and Heisenberg models. The curves are unique for each universality class. It is shown that the universal curve can be practically constructed in two different ways, reducing the number of required parameters with respect to the previous phenomenological derivation. This opens the possibility of an inexpensive screening of the performance of magnetocaloric materials, as it allows extrapolations to magnetic fields or temperatures not available in some laboratories.

Antioxidant activity of phenolic compounds: from in vitro results to in vivo evidence.

Abstract: Over last decade an increasing interest for antioxidants in foods has arisen. The healthy properties of antioxidants related to the prevention of degenerative diseases are the main cause of this boom. An antioxidant prevents the oxidation process, the initial step of development of degenerative diseases, cancer and many others. Literature encompasses analytical methodology development to assess antioxidant properties of foods and beverages. The screening of antioxidant activity of foodstuffs is the subject of a large number of articles. Special interest has been addressed to wine, tea and chocolate. However, the crucial key in the prevention of disease is the action these antioxidants exert after their consumption. Studies involving human subjects are scarce due to the requirements of availability of volunteers and conditions to test are limited. This review summarizes data related to in vitro antioxidant activity of foods, emphasizing the main role of phenolic compounds. A critical comparison is realized between the biological significance of these values and the biological significance of in vivo measurements. In addition, the Plasma Antioxidant Capacity is evaluated and selected as biomarker for in vivo antioxidant status of human organism. In a second part, data collected from different intervention studies performed up to date are compiled and discussed. This review summarized data related to in vitro antioxidant activity of foods, emphasizing the main role of phenolic compounds. A critical comparison is realized between the biological significance of these values and the biological significance of in vivo measurements. In addition, the Plasma Antioxidant Capacity is evaluated and selected as biomarker for in vivo antioxidant status of human organism. In a second part, data collected from different intervention studies performed up to date are compiled and discussed. The original contribution of this work is to compile data of Plasma Antioxidant Capacity after dietetic intervention studies taking into account the portion of food ingested. In addition, we calculated the antioxidant compounds content (phenolic content, ascorbic acid, vitamin E and carotenoids) contained in each food ingested to evaluate better their impact in Plasma Antioxidant Capacity. Intervention studies are grouped by the length of intervention and type of food ingested. Results reported in literature reveal that the increment in Plasma Antioxidant Capacity largely depends on analytical method used.

Market Orientation and Internationalization in Small and Medium‐Sized Enterprises

Abstract: This article builds on studies from the literature on market orientation (MO) and internationalization to develop a model and a set of hypotheses regarding the relationships among MO, knowledge acquisition (KA), and market commitment (MC), and the direct and indirect effects of these variables on the performance of small and medium-sized enterprises (SMEs) in foreign markets. The model and its hypotheses are tested by means of an empirical study of a multi-industry sample of Spanish SMEs operating in foreign markets. The results, obtained by structural equation modeling, indicate that a direct positive relationship exists between MO and a strategy of internationalization, and that the effect of MO on performance in foreign markets is moderated by KA and MC.

Lyapunov-Based Model Predictive Control of Nonlinear Systems Subject to Data Losses

Abstract: In this work, we focus on model predictive control of nonlinear systems subject to data losses. The motivation for considering this problem is provided by wireless networked control systems and control of nonlinear systems under asynchronous measurement sampling. In order to regulate the state of the system towards an equilibrium point while minimizing a given performance index, we propose a Lyapunov-based model predictive controller which is designed taking data losses explicitly into account, both in the optimization problem formulation and in the controller implementation. The proposed controller allows for an explicit characterization of the stability region and guarantees that this region is an invariant set for the closed-loop system under data losses, if the maximum time in which the loop is open is shorter than a given constant that depends on the parameters of the system and the Lyapunov-based controller that is used to formulate the optimization problem. The theoretical results are demonstrated through a chemical process example.

The Warburg Effect: Why and How Do Cancer Cells Activate Glycolysis in the Presence of Oxygen?

Abstract: Cells can obtain energy through the oxygen-dependent pathway of oxidative phosphorylation (OXPHOS) and through the oxygen-independent pathway of glycolysis. Since OXPHOS is more efficient in generating ATP than glycolysis, it is recognized that the presence of oxygen results in the activation of OXPHOS and the inhibition of glycolysis (Pasteur effect). However, it has been known for many years that cancer cells and non-malignant proliferating cells can activate glycolysis in the presence of adequate oxygen levels (aerobic glycolysis or Warburg effect). Accumulating evidence suggests that the persistent activation of aerobic glycolysis in tumor cells plays a crucial role in cancer development; the inhibition of the increased glycolytic capacity of malignant cells may therefore represent a key anticancer strategy. Although some important knowledge has been gained in the last few years on this growing field of research, the basis of the Warburg effect still remains poorly understood. This communication analyzes why cancer cells switch from OXPHOS to glycolysis in the presence of adequate oxygen levels, and how these cells manage to avoid the inhibition of glycolysis induced by oxygen. Several strategies and drugs that may interfere with the glycolytic metabolism of cancer cells are also shown. This information may help develop anticancer approaches that may have clinical relevance.

Automatic Online Diagnosis Algorithm for Broken-Bar Detection on Induction Motors Based on Discrete Wavelet Transform for FPGA Implementation

Abstract: Overall system performance on a production line is one of the major concerns in modern industry where induction motors are present and their condition monitoring is mandatory. Periodic offline monitoring of the motor condition is usually performed in the industry, consuming production time and increasing cost. Broken rotor bars are among the most common failures in induction motors. Reported research projects give a broken-rotor-bar-detection methodology based on personal-computer implementation that is performed offline and requires an expert technician interpretation which is not a cost-effective solution. The novelty of this paper is the development of an automatic online diagnosis algorithm for broken-rotor-bar detection, optimized for single low-cost field-programmable gate-array (FPGA) implementation, which guarantees the development of economical self-operated equipment. The proposed algorithm requires less computation load than the previously reported algorithms, and it is mainly based on the discrete-wavelet-transform application to the start-up current transient; a further single mean-square computation determines a weighting function that, according to its value, clearly points the motor condition as either healthy or damaged. In order to validate the proposed algorithm, several tests were performed, and an FPGA implementation was developed to show the algorithm feasibility for automatic online diagnosis.

Effect of Azospirillum brasilense coinoculated with Rhizobium on Phaseolus vulgaris flavonoids and Nod factor production under salt stress

Abstract: The effects of salt upon Azospirillum brasilense strain Cd on plant growth, nodulation, flavonoid and lipochitooligosaccharide (LCOs-Nod factor) production, were sequentially followed after 4, 7 and 14 days during a Rhizobium-Phaseolus vulgaris cv. Negro Jamapa interaction, in a hydroponics growth system. Azospirillum brasilense promoted root branching in bean seedling roots and increased secretion of nod-gene-inducing flavonoid species, as detected by high-performance liquid chromatography (HPLC). The results also support that A. brasilense allows a longer, more persistent exudation of flavonoids by bean roots. A general positive effect of Azospirillum-Rhizobium coinoculation on the expression of nod-genes by Rhizobium tropici CIAT899 and Rhizobium etli ISP42, and on nodulation factor patterns, was observed in the presence of root exudates. The negative effects obtained under salt stress on nod-gene expression and on Nod factors' appearance were relieved in coinoculated plants.

Faecal carriage of extended-spectrum β-lactamase-producing Escherichia coli: prevalence, risk factors and molecular epidemiology

Abstract: Results: The prevalence of faecal carriage was 67.9% in patients with UTI, 27.4% in household members, 15.6% in non-household relatives and 7.4% in unrelated patients. Being a relative of a patient with UTI was independently associated with an increased risk of being a carrier. Among the relatives, multivariate analysis showed that those eating their main meal outside their own home >15 days during the previous month were less likely to be faecal carriers (OR 5 0.2; 95% CI: 0.06–0.6; P 5 0.007). The faecal isolates of patients with UTI were CTX-M-producers in 66.6% and SHV-producers in 33.3% of the cases, while the percentages for other population groups were 40% to 55.5% and 50% to 75%, respectively. Of the 19 families with >1 carrier member, 8 families had 2 members who shared clonally related isolates, 8 families had 2 members carrying different clones producing the same enzymes and there were 3 families where all members had different enzyme-producing clones. Conclusions: Our results suggest that both acquisition from a common source and person-to-person transmission might contribute to ESBL dissemination.

Catalytic enantioselective hydrophosphonylation of aldehydes and imines

Abstract: In this review, recent advances in catalytic asymmetric hydrophosphonylation of aldehydes and imines are discussed. We also summarize several proposed mechanisms for the different possibilities of asymmetric induction and the application of this catalytic methodology to the enantioselective synthesis of α-amino and α-hydroxy phosphonates. A variety of metal-based chiral catalysts as well as several organic catalysts have been exploited as suitable systems for the preparation of enantiomerically pure phosphonates. The recent evolution and future trends of those and other catalytic systems are described.

qnr Gene nomenclature.

Abstract: Since the plasmid-borne quinolone resistance gene qnr was reported in 1998 (8), many additional qnr alleles have been discovered on plasmids or the bacterial chromosome (reviewed in references 9 and 13). The plasmid-borne qnr genes currently comprise three families, qnrA, qnrB, and qnrS, differing from each other 40% or more in nucleotide sequence. Within each family, minor (≤10%) variation in sequence has defined a growing number of alleles. For the qnrA and qnrS families, the number of variants has been manageable, with general agreement on allele designations, but lately, the number of qnrB sequences submitted to GenBank has exploded, with the same qnrB allele number claimed for dissimilar sequences by different investigators and the same entry given new allele numbers from week to week.

Extraordinary Transmission Through Arrays of Electrically Small Holes From a Circuit Theory Perspective

Abstract: Extraordinary optical transmission of light or electromagnetic waves through metal plates periodically perforated with subwavelength holes has been exhaustively analyzed in the last ten years. The study of this phenomenon has attracted the attention of many scientists working in the fields of optics and condensed matter physics. This confluence of scientists has given rise to different theories, some of them controversial. The first theoretical explanation was based on the excitation of surface plasmons along the metal-air interfaces. However, since periodically perforated dielectric (and perfect conductor) slabs also exhibit extraordinary transmission, diffraction by a periodic array of scatterers was later considered as the underlying physical phenomenon. From a microwave engineering point of view, periodic structures exhibiting extraordinary optical transmission are very closely related to frequency-selective surfaces. In this paper, we use simple concepts from the theory of frequency-selective surfaces, waveguides, and transmission lines to explain extraordinary transmission for both thin and thick periodically perforated perfect conductor screens. It will be shown that a simple transmission-line equivalent circuit satisfactorily accounts for extraordinary transmission, explaining all of the details of the observed transmission spectra, and easily gives predictions on many features of the phenomenon. Although the equivalent circuit is developed for perfect conductor screens, its extension to dielectric perforated slabs and/or penetrable conductors at optical frequencies is almost straightforward. Our circuit model also predicts extraordinary transmission in nonperiodic systems for which this phenomenon has not yet been reported.

Absolute instability of a liquid jet in a coflowing stream.

Abstract: Cylindrical liquid jets are inherently unstable and eventually break into drops due to the Rayleigh-Plateau instability, characterized by the growth of disturbances that are either convective or absolute in nature. Convective instabilities grow in amplitude as they are swept along by the flow, while absolute instabilities are disturbances that grow at a fixed spatial location. Liquid jets are nearly always convectively unstable. Here we show that two-phase jets can breakup due to an absolute instability that depends on the capillary number of the outer liquid, provided the Weber number of the inner liquid is $gO(1)$. We verify our experimental observations with a linear stability analysis.

Abnormal sympathoadrenal development and systemic hypotension in PHD3-/- mice.

Abstract: Cell culture studies have implicated the oxygen-sensitive hypoxia-inducible factor (HIF) prolyl hydroxylase PHD3 in the regulation of neuronal apoptosis. To better understand this function in vivo, we have created PHD3−/− mice and analyzed the neuronal phenotype. Reduced apoptosis in superior cervical ganglion (SCG) neurons cultured from PHD3−/− mice is associated with an increase in the number of cells in the SCG, as well as in the adrenal medulla and carotid body. Genetic analysis by intercrossing PHD3−/− mice with HIF-1a+/− and HIF-2a+/− mice demonstrated an interaction with HIF-2α but not HIF-1α, supporting the nonredundant involvement of a PHD3-HIF-2α pathway in the regulation of sympathoadrenal development. Despite the increased number of cells, the sympathoadrenal system appeared hypofunctional in PHD3−/− mice, with reduced target tissue innervation, adrenal medullary secretory capacity, sympathoadrenal responses, and systemic blood pressure. These observations suggest that the role of PHD3 in sympathoadrenal development extends beyond simple control of cell survival and organ mass, with functional PHD3 being required for proper anatomical and physiological integrity of the system. Perturbation of this interface between developmental and adaptive signaling by hypoxic, metabolic, or other stresses could have important effects on key sympathoadrenal functions, such as blood pressure regulation.

A Model-Based Direct Power Control for Three-Phase Power Converters

Abstract: The direct power control (DPC) technique has been widely used as a control strategy for three-phase power rectifiers due to its simplicity and good performance. DPC uses the instantaneous active and reactive power to control the power converter. The controller design has been proposed as a direct control with a lookup table and, in recent works, as an indirect control with an inner control loop with proportional-plus-integral controllers for the instantaneous active and reactive power errors. In this paper, a model-based DPC for three-phase power converters is designed, obtaining expressions for the input control signal, which allow the design of an adaptive control law that minimizes the errors introduced by parameter uncertainties as the smoothing inductor value or the grid frequency. A controller design process, a stability study of the system, and experimental results for a synchronous three-phase power rectifier prototype are presented to validate the proposed controller.

A technological acceptance of e-learning tools used in practical and laboratory teaching, according to the European higher education area

Abstract: The application of scientific tools to analyse the use of Internet-based e-learning tools in academic settings is in general an ignored area. E-learning tools are actually an emergent topic as a result of the new ideas introduced by the European Higher Education Area. Lifelong learning, or the promotion of student initiative, is the new paradigm of a learner-centred education. In this context, e-learning tools can represent an effective way of supporting this new trend in education. Assuming the premise that successful use of these web-based tools depends primarily on a user's behaviour, the objective of this research is to examine the technology acceptance model (TAM) of web-based e-learning tools used in practical and laboratory teaching. The research hypotheses derived from this model have empirically been validated using the responses to a survey on e-learning usage among 220 users. These responses have been examined through partial least square. The obtained results strongly support the extended TAM in predicting a student's intention to use e-learning and define a set of external variables with a significant influence in the original TAM variables. Surprisingly, perceived ease of use did not posit a significant impact on student attitude or intention towards e-learning tool usage. Therefore, early evaluation of e-learning material is considered essential to providing a framework for further improvements of the tool.