Directional lithium hydride Compton profiles of 0.1% statistical accuracy

Peer reviewe

Intermediate range O-O correlations in supercooled water down to 235 K

Wide angle x-ray scattering of supercooled water down to 234.8 K was studied using high energy x rays at the European Synchrotron Radiation Facility. The oxygen-oxygen pair distribution function (PDF) was calculated from the scattering pattern out to the 5th peak at an intermolecular distance, r approximate to 11 angstrom. We observe that the 4th peak and the 5th peak in the PDF increase in height upon supercooling. We also observe that the 4th peak position (r(4)) shifts to shorter distances upon supercooling consistent with previous studies, but we see a more rapid change at the lowest temperature. The running oxygen-oxygen coordination number is calculated for 5 different temperatures, and an isosbestic point at r(iso) = 3.31 +/- 0.05 angstrom was found corresponding to a coordination number of 4.39 +/- 0.15. The comparison of the PDF of the coldest water with that of amorphous ice shows distinct differences. We propose that there are 5-member pentamer rings in low density liquid-like structures giving rise to the sharp correlations at r approximate to 9 angstrom and r approximate to 11 angstrom.Peer reviewe

Projetos de Aprendizagem Mediados por Ambientes Virtuais no Ensino de EngenhariaEl trica

As experiências relacionadas à metodologia de projetos de aprendizagem têm sua origem nas formulações de John Dewey no início do século XX. A concepção de projeto de aprendizagem defendida nesta pesquisa alia às formulações de Dewey as propostas da professora Lea Fagundes. Esta metodologia tem o aluno como responsável pela sua aprendizagem e reflete os conceitos construtivistas de Piaget. A integração e a construção de conhecimento via projetos é facilitada com o uso dos ambientes virtuais para apoio à aprendizagem que são softwares que permitem interações síncronas ou assíncronas além de possibilitar o registro de todo o caminho percorrido pelo estudante e de todas as atividades de uma disciplina. A partir de dados coletados, no período de 2003 a 2007, em atividades escolares de disciplinas do terceiro período letivo do curso de Engenharia Elétrica da Universidade Federal do Espírito Santo pretende-se analisar (a) em que medida os ambientes virtuais de aprendizagem e a apropriação das tecnologias de informação e comunicação acrescentam na formação do estudante de engenharia, em relação às práticas pedagógicas presenciais e (b) em que medida os projetos de aprendizagem contribuem para o enfrentamento dos problemas que temos, hoje, no ensino de engenharia elétric

Hole depletion and localization due to disorder in insulating PrBa2Cu3O7-d: a Compton scattering study

The (mostly) insulating behaviour of PrBa2Cu3O7-d is still unexplained and even more interesting since the occasional appearance of superconductivity in this material. Since YBa2Cu3O7-d is nominally iso-structural and always superconducting, we have measured the electron momentum density in these materials. We find that they differ in a striking way, the wavefunction coherence length in PrBa2Cu3O7-d being strongly suppressed. We conclude that Pr on Ba-site substitution disorder is responsible for the metal-insulator transition. Preliminary efforts at growth with a method to prevent disorder yield 90K superconducting PrBa2Cu3O7-d crystallites.Comment: 4 pages, 3 figures, revised version submitted to PR

Sur la précision des estimations de température à partir de mesures in operando de diffraction des rayons X lors de la fabrication additive

Lattice strains obtained from in operando synchrotron X-ray diffraction measurements during metal additive manufacturing are being increasingly used to deduce temperature and cooling rates occurring during the process. The underlying assumption behind this deduction is that lattice strain evolution occurs solely due to temperature changes caused by the passage of the heat-source. However, this assumption crucially neglects the role of mechanical constraints on the sample and the evolution of internal stresses generated due to microstructure evolution. The ensuing elastic strains can have a significant contribution to lattice strains and failure to decouple them from the contribution of thermal strains may induce significant errors in temperature and cooling rate estimates. In this study, we quantify these errors using a combined experimental and numerical approach. First, lattice strains are obtained from in operando synchrotron X-ray diffraction measurements during additive manufacturing of a 316L stainless steel thin-wall. Then, the contributions of elastic and thermal strains to the lattice strain are deconvoluted using experimentally validated fast large-scale thermomechanical simulations of the entire AM process. Results reveal that even in the simple case of AM of a single-phase material such as 316L and a thin-wall geometry, significant errors in both temperature amplitudes (up to 30\%) and cooling rates (up to 27\%) occur when the contribution of elastic strains is neglected. Furthermore, mechanical and thermal effects cannot be trivially decoupled from X-ray diffraction data; fast large-scale thermomechanical simulations become necessary to perform this decoupling