TWO-PION EXCHANGE NUCLEAR POTENTIAL - CHIRAL CANCELLATIONS

We show that chiral symmetry is responsible for large cancellations in the two-pion exchange nucleon-nucleon interaction, which are similar to those occuring in free pion-nucleon scattering.Comment: REVTEX style, 5 pages, 3 PostScrip figures compressed, tarred and uuencode

NN Scattering: Chiral Predictions for Asymptotic Observables

We assume that the nuclear potential for distances larger than 2.5 fm is given just by the exchanges of one and two pions and, for the latter, we adopt a model based on chiral symmetry and subthreshold pion-nucleon amplitudes, which contains no free parameters. The predictions produced by this model for nucleon-nucleon observables are calculated and shown to agree well with both experiment and those due to phenomenological potentials.Comment: 16 pages, 12 PS figures included, to appear in Physical Review

Markov Chain Beam Randomization: a study of the impact of PLANCK beam measurement errors on cosmological parameter estimation

We introduce a new method to propagate uncertainties in the beam shapes used to measure the cosmic microwave background to cosmological parameters determined from those measurements. The method, which we call Markov Chain Beam Randomization, MCBR, randomly samples from a set of templates or functions that describe the beam uncertainties. The method is much faster than direct numerical integration over systematic `nuisance' parameters, and is not restricted to simple, idealized cases as is analytic marginalization. It does not assume the data are normally distributed, and does not require Gaussian priors on the specific systematic uncertainties. We show that MCBR properly accounts for and provides the marginalized errors of the parameters. The method can be generalized and used to propagate any systematic uncertainties for which a set of templates is available. We apply the method to the Planck satellite, and consider future experiments. Beam measurement errors should have a small effect on cosmological parameters as long as the beam fitting is performed after removal of 1/f noise.Comment: 17 pages, 23 figures, revised version with improved explanation of the MCBR and overall wording. Accepted for publication in Astronomy and Astrophysics (to appear in the Planck pre-launch special issue

Design of a biosynthetic pathway for curcumin production in Escherichia coli

Curcumin is the yellow pigment from turmeric, a well known culinary spice produced from the herb Curcuma longa. Research over the last years has shown that curcumin presents a wide range of pharmacological effects, including anti-inflammatory, anti-oxidant and anticarcinogenic activity. Given its potential application in cancer treatment, there is an interest for industrial production of this natural compound. This work consists on a synthetic biology approach for the design of a heterologous pathway for curcumin synthesis in Escherichia coli, a widely used microbe in industrial biotechnology. Using pathway databases and literature research we have selected the best gene candidates for heterologous expression of a curcumin synthesis pathway in E. coli. The DNA sequences for these genes were retrieved from public databases and can be readily synthesized for insertion into the host using molecular biology techniques. The inclusion of this pathway in a recent genome-scale reconstruction of the metabolism of E. coli has enabled the in silico analysis of the production capabilities for this host. We have analysed the theoretical production yields and biomass growth under different experimental conditions. Using this model we have also searched for potential gene knockouts that partially redirect the metabolic flux to the heterologous pathway without compromising cellular growth. In overall, the methods used in this work allow the selection of the most suitable combination of experimental conditions and genetic manipulations for the design of an efficient biosynthetic pathway for curcumin production in E.coli

Influência da incorporação de partículas de SiC na microestrutura e no comportamento tribológico de compósitos Al/SiCp com gradiente funcional

Devido à boa relação entre densidade e resistência mecânica apresentada pelos compósitos de matriz de alumínio, estes materiais têm-se destacado em algumas aplicações industriais. Com efeito, a adição de reforços cerâmicos às ligas de alumínio, nomeadamente partículas de SiC, resulta na melhoria das suas propriedades mecânicas com particular impacto no aumento da resistência ao desgaste, ampliando o seu campo de aplicação como tribo-materiais. O desenvolvimento complementar de gradientes funcionais de propriedades neste tipo de compósitos permite a melhoria de uma grande variedade de propriedades, possibilitando a utilização destes tribo-materiais na indústria automóvel e aeronáutica. Exemplos de aplicações são cilindros, válvulas e seguidores de cames em motores de combustão interna, onde estes materiais operam como parte integrante de diversos sistemas tribológicos. Este estudo teve como objectivo verificar a influência da incorporação de partículas de SiC na microestrutura da liga Al-10Si-4.5Cu-2Mg processada por fundição centrífuga, tendo como referência a matriz não reforçada mas também processada por fundição centrífuga e caracterizada por um gradiente na microestrutura. A fracção de volume de partículas de SiC adicionadas foi de 10%. O comportamento tribológico dos dois materiais foi avaliado a fim de compreender o modo como as alterações na microestrutura promovidas pela adição de partículas de SiC influencia a resposta tribológica. Os testes tribológicos foram realizados na ausência de lubrificação e na geometria esfera-disco. A carga normal aplicada e a velocidade de deslizamento foram mantidas constantes com os valores de 3 N e 0.5 ms-1, respectivamente. Como contra-corpo foi usado aço de rolamento AISI 52100. A microestrutura e os principais mecanismos de desgaste foram caracterizados por recurso a SEM/EDS. Os resultados demonstraram a existência de fadiga extrema e a predominância de fenómenos de abrasão como mecanismos de desgaste. A resposta tribológica dos materiais caracterizados indiciou poder ser fortemente influenciada pela quantidade de partículas de reforço presente na matriz de alumínio.Fundação para a Ciência e Tecnologia (FCT) - Programa POCTI (Projecto POCTI/CTM/46086/2002)

Effect of silicon carbide particles on the microstructure and tribological behaviour of functional graded Al/SiCp composites

Aluminium matrix composites have been wide used essentially due to the good relation between weight and mechanical resistance. Also, the additions of ceramic reinforcement, mainly SiC particles, improve some properties of these composites, particularly wear resistance. Additionally, when functional gradients of properties are promoted in this type of composites, a wide range of properties are improved. This type of materials could be used as tribomaterials of high potential for automotive and aeronautic industries, particularly in applications such as cylinder liners, valves and came followers. In those situations a tribological system is created. In this work, the influence of the incorporation of SiCp on the microstructure of the Al matrix (Al-10Si-4,5Cu-2Mg) processed by centrifugal casting was evaluated by comparing the unreinforced alloy, in which a graded microstructure is present, with that obtained in the functionally graded Al-alloy composite reinforced with SiCp. The volume fraction of SiC particles in the precursor composite was 10%. Additionally, the tribological behaviour of the two materials was evaluated and compared in order to estimate how the changes in microstructure promoted by SiCp addition would influence the tribological response. Unlubricated tribological tests were performed in a pin-on-disc tribometer where the normal load, frequency and linear velocity were kept constant (3 N, 1 Hz, 0.5 m/s respectively). As conterbody an AISI 52100 steel was used. The microstructures and the wear mechanisms were identified by SEM/EDS analysis. Worn surfaces were also characterised by AFM. Results show that some extreme fatigue and abrasion phenomena dominate the wear mechanisms presented in this type of materials. However, the tribological response appears to be strongly influenced by the volume fraction of reinforcing particles. Also, the role of the matrix microstructure on the tribological behaviour, essentially in what concerns to the particles/matrix interfacial region is discussed.Fundação para a Ciência e aTecnnologia (FC