Growth optimization and doping with Si and Be of high quality GaN on Si(111) by molecular beam epitaxy

GaN layers have been grown by plasma-assisted molecular beam epitaxy on AlN-buffered Si(111) substrates. An initial Al coverage of the Si substrate of aproximately 3 nm lead to the best AlN layers in terms of x-ray diffraction data, with values of full-width at half-maximum down to 10 arcmin. A (2×2) surface reconstruction of the AlN layer can be observed when growing under stoichiometry conditions and for substrate temperatures up to 850°C. Atomic force microscopy reveals that an optimal roughness of 4.6 nm is obtained for AlN layers grown at 850°C. Optimization in the subsequent growth of the GaN determined that a reduced growth rate at the beginning of the growth favors the coalescence of the grains on the surface and improves the optical quality of the film. Following this procedure, an optimum x-ray full-width at half-maximum value of 8.5 arcmin for the GaN layer was obtained. Si-doped GaN layers were grown with doping concentrations up to 1.7×10¹⁹ cm⁻³ and mobilities approximately 100 cm²/V s. Secondary ion mass spectroscopy measurements of Be-doped GaN films indicate that Be is incorporated in the film covering more than two orders of magnitude by increasing the Be-cell temperature. Optical activation energy of Be acceptors between 90 and 100 meV was derived from photoluminescence experiments.     

Juridical and sociocultural problems on the definition of a law concerning property, usage and access to genetic resources in Colombia

The property, usage, and access to genetic resources, is today one of the primary topics in international business, as a result of the strategic importance of the resources for the biotechnology industry. Internationally, the sovereignty that each country has over its natural patrimony is recognized. However, the new laws of international marketing have obligated countries in the process of development, such as Colombia, to adopt and copy a concept of intellectual property on living resources that does not have anything to do with the country's sociocultural identity, and sometimes even does not take into account its material enjoyment. The new juridical movement that treats genetic resources as private property produces a cultural conflict between indigenous populations, Afro-Americans and peasants, because for them the genetic resources are an element of community life. In these communities, knowledge is freely transmitted; it is an understanding that they have to conserve their agricultural customs and the relationship that they have with the environment. They do not recognize the term "property' according to patenting laws. These elements have to be considered, respected, and guaranteed in the laws that recognize the genetic resources in the country. On the other hand, not even countries that are pioneers in biotechnological development can adopt a concept about patents that is in agreement with the particularities that the living materials possess. This is obviously the reason for the numerous discussions on the legal interpretation, as well as complicated debates in court. Confronting that situation, there are countries rich in biodiversity, such as Colombia, but which do not have a proper concept and are not economically strong in the international context. These countries have to copy inadequate protection policies that do not take into account all their rights. This paper describes some of the technical, juridical, and sociocultural difficulties which Colombia has to confront, in order to set a guideline on patenting living organisms, and on the access and usage of the genetic resources.     

A method for thermal characterization and modeling of large gantry-type machine tools

In this paper, a method specially designed for the assessment of repeatability and accuracy of large machine tools is proposed, along with results for a large gantry-type milling machine, recorded in a medium-term period. For this purpose, temperatures were recorded and a metrological frame was used along with inductive sensors in the tool tip, performing repetitive measurements. As initial results, origin, weight and influence of every heat source on this kind of machines were found. Afterwards, high precision measurements of thermal deformations were obtained. Mechanism of errors was found, discerning between main thermal error in the vertical Z-axis and secondary error in longitudinal X-axis towards out of the plane of the gantry bridge. Finally, a finite element model was developed which showed main thermal behavior identified experimentally. This will permit to make simulations of the thermal response of the machine and to choose machining strategies for future parts. Proposed methodology was therefore proved satisfactory for thermal characterization of this kind of big machine tools. This knowledge will make possible to improve thermal design of machines and to develop error compensation procedures. This method can also be applied on workshop conditions for recalibration purposes.     

Fast AlGaN metal-semiconductor-metal photodetectors grown onSi(111)

Metal-semiconductor-metal photodetectors have been fabricated on AlGaN grown on Si(111) by molecular beam epitaxy for solar-blind applications. A cutoff wavelength of 290 nm and an ultraviolet/visible contrast of more than three orders of magnitude are achieved. Time response measurements show fast exponential decays. With a minimum decay time of 150 ns. The photodetectors present responsivities that increase with bias, reaching 15 mA/W at 4 V bias     

Modeling of a two-step solar hydrogen production plant

The promising advances in research in two-step solar hydrogen production from water have increased interest in producing hydrogen with this technology. In this framework, the Hydrosol II Project pilot plant for producing continuous solar hydrogen from water using a ferrite-based redox technology was erected at the CIEMAT-Plataforma Solar de Almería. Two reactors allow the oxidation and reduction steps to be performed in parallel, which, sequentially switched, make hydrogen production quasi-continuous.     

Stability charts with large curve-flute end-mills for thin-walled workpieces

The development of advanced performance materials by foundrymen allows new designs of components with similar mechanical resistance but lower thicknesses. This accentuates the well-known problems in machining cutting systems (static form errors, forced vibrations, and chatter), which appear increasingly early. These problems are particularly severe in the case of rotating parts in the field of aeronautics. Currently, the machining time for impellers and blisks can be improved so that new cutting tools with innovative features are continually developed. Barrel type or curved end mills will have a significant impact on the manufacturing of aircraft components during incoming years. In this paper, a time domain method is proposed for the first time to predict stability in milling operations when using these tools. The dynamic cutting forces are first obtained and then used to plot stability contour maps. Key aspects, such as tool orientation angles, several dynamic modes, and runout were also considered. Finally, chatter tests were performed for a low-stiffness mechanical system to validate the model. The presented maps indicate chatter severity and may be used for chatter prediction and productivity improvement.     

Optimization of the Biocatalysis for D-DIBOA Synthesis Using a Quick and Sensitive New Spectrophotometric Quantification Method

D-DIBOA (4-hydroxy-(2H)-1,4-benzoxazin-3-(4H)-one) is an allelopathic-derived compound with interesting herbicidal, fungicidal, and insecticide properties whose production has been successfully achieved by biocatalysis using a genetically engineered Escherichia coli strain. However, improvement and scaling-up of this process are hampered by the current methodology for D-DIBOA quantification, which is based on high-performance liquid chromatographic (HPLC), a time-consuming technique that requires expensive equipment and the use of environmentally unsafe solvents. In this work, we established and validated a rapid, simple, and sensitive spectrophotometric method for the quantification of the D-DIBOA produced by whole-cell biocatalysis, with limits of detection and quantification of 0.0165 and 0.0501 µmol·mL⁻¹ respectively. This analysis takes place in only a few seconds and can be carried out using 100 µL of the sample in a microtiter plate reader. We performed several whole-cell biocatalysis strategies to optimize the process by monitoring D-DIBOA production every hour to keep control of both precursor and D-DIBOA concentrations in the bioreactor. These experiments allowed increasing the D-DIBOA production from the previously reported 5.01 mM up to 7.17 mM (43% increase). This methodology will facilitate processes such as the optimization of the biocatalyst, the scaling up, and the downstream purification.     

Analytical methods for the determination of acrylamide in food products: a review

In early 2002, the Swedish National Food Administration reported high acrylamide levels in heat-treated carbohydrate-rich foods. Consequently, intensive activity began examining the many different types of food, and thousands of analyses have been undertaken world wide. Measurement data have been published in many different types of media. Within this flood of publications, there are only a limited number of articles concerned with the technical aspects of the measurements. This review focuses on the state-of-the-art in the analysis of acrylamide in foodstuffs. It covers information on methods from peer-reviewed articles and other sources (e.g. a survey carried out among official and private laboratories of the Member States of the European Union). Alternative methods are presented and discussed alongside the more common measurement techniques for acrylamide in foodstuffs. Special attention is given to sample preparation. The greatest differences between the analytical methods was for acrylamide extraction and clean-up. The influence of different extraction techniques or extraction solvents/solvent mixtures on the measurement results has not yet been fully investigated. There is also a lack of understanding about the sample clean-up. Since both might have a large impact on the results of the analysis, this review should also be considered as a basis for further investigations.     

Germ cell control of testin production is inverse to that of other Sertoli cell products

Recent studies have shown that germ cells can regulate testins, two newly identified Sertoli cell proteins that are associated with junctional complexes. To investigate this possibility, several parameters of Sertoli cell function were investigated over 2-120 days post exposure of the rat testes to x-rays (3 Grays). The irradiation-induced loss of spermatogonia resulted in a maturation-depletion process progressively affecting all germ cell classes. Testis weight began to decrease when the most numerous germ cell type (spermatids) began to decline. A complete or near complete recovery of spermatogenesis and of the testis weight had occurred by day 120 post irradiation. There was no significant change in FSH, epididymal androgen-binding protein, and tubule fluid levels during the first weeks after irradiation, when the seminiferious epithelium was depleted of spermatogonia and germ cells up to early spermatids. In contrast, when the number of the more mature forms of spermatids declined (between day 21 and 54), FSH rose and androgen-binding protein as well as fluid production declined. The subsequent recovery of these parameters was also highly correlated with the number of late spermatids. By contrast, testicular testin contents reacted to the depletion of germ cells with a biphasic increase; a doubling occurred when spermatogonia, spermatocytes, and early spermatids were absent (days 4-28), and a 7-fold rise occurred by day 37 when the number of late spermatids had decreased by 50%. By day 54, when the sperm counts had reached a nadir, testin contents had returned to levels corresponding to about four times the control levels; they progressively recovered thereafter. These observations support the postulate that germ cells negatively regulate testins. This possibility was investigated with in vitro experiments showing that addition of germ cell-conditioned medium to Sertoli cell monolayers inhibited testin secretion in a dose-dependent manner. In conclusion this study; 1) highlights the complex interplay between the various germ cell classes in the control of the Sertoli cell function in the adult testis; 2) establishes that germ cell effects may be opposite on different Sertoli cell products; 3) demonstrates that several classes of germ cells negatively control testicular testin contents; and 4) emphasizes the particular role of late spermatids in Sertoli cell regulation.     

Blood fractionation based on the extraction of the buffy-coat layer. Analysis of our results

T-cell growth is controlled to a large degree by extracellular signals that bind to specific receptors on the surface of cells. A number of these receptors have intrinsic protein tyrosine kinase (PTK) activity. Their action on second messenger generation, and thus on cell proliferation, has been indirectly demonstrated by the decrease in [3H]-thymidine (TdR) uptake that follows co-stimulation of T-cells with mitogens and PTK inhibitors such as genistein (GEN). In this paper we report that the [3H]-TdR uptake assay is not a valid and reliable tool for investigating the proliferative activity of certain T-cell lines. In fact, a concomitant assessment of both [3H]-TdR uptake and cell cycle progression demonstrated that GEN is able to block G2/M progression of Jurkat T-lymphocytes even at doses (5 micrograms/ml) that do not influence [3H]-TdR uptake. Pretreatment with sodium o-vanadate (100 nM) could not reverse the GEN-related cell cycle perturbation, but was able to restore optimal [3H]-TdR uptake. Finally, GEN treatment was able to induce concentration-dependent apoptotic cell death of Jurkat T-cells. The control of cell activation, proliferation and programmed cell death is undoubtedly influenced by receptor-associated PTKs. The final effect on cell survival is almost entirely dependent on the activation state of the cell. The [3H]-TdR uptake assay seems to be inadequate for a correct interpretation of the expected results.