Roberto Rodríguez Gómez

NANOCRYSTALLIZATION STUDIES IN CO-RICH AMORPHOUS-ALLOYS

Abstract Nanocrystallization from Vitrovac® amorphous ribbon was studied by Mossbauer spectroscopy, transmission electron microscopy and differential scanning calorimetry. Results are interpreted in terms of a crystallization of Co, Co 2 B, and Co 3 B, leading to an increase in Fe content in the residual amorphous phase, which in turn results in an increase in hyperfine field and Curie temperature.

The modernisation of higher education in Mexico

Following the Second World War, higher education in Mexico began a series of fundamental changes that taken together, altered the profile of the traditional university. It developed into a system which in its academic, social and political functions became highly complex as well as diverse, given the variety of institutional options and professional development schemes.The transition took place both at the institutional level and was system wide. The former was accompanied by expansion in the organisation as a whole. It involved updating the curriculum and the overhaul of administrative, managerial and planning mechanisms. At the latter level, the most significant change involved reforming the major component parts; the territorial distribution of access to education, and carried across the various fields of knowledge and disciplines, bore down on the level and allocation of different funding sources. These developments are best analysed within the perspective context of academic diversification and differentiation.

Structural Model for Stress Evaluation of Prestressed Concrete Pipes of the Cutzamala System

The Cutzamala aqueduct is one of the main works constructed to provide water to more than 6 million residents of Mexico City. The buried aqueduct comprises prestressed concrete pipes of 2.50 m (99 in.) diameter. The main components of these pipes are a steel cylinder embedded in two concentric rings of concrete, prestressed steel wires wrapped on the exterior concrete liner and an exterior coating of mortar. Failure of two buried prestressed pipes in the aqueduct occurred last year. An afterward inspection showed that some more pipes were showing important levels of corrosion, particularly that related to the prestressed wires. These facts prompted the development of technical studies oriented to evaluate the possible causes of failure, particularly those associated with the loss of prestress due to corrosion of the prestressed wires. In this paper, the methodology used to predict failures of pipes when subjected to service loads under a progressive loss of prestress forces is described. The analysis used to evaluate the failure process of pipes when the prestressed wires lose their strength capabilities is presented, as well as 2D and 3D finite element models of the buried pipe that were used to simulate different load conditions and distributions of prestress wires. Finally, a criterion for evaluation of failures of prestressed concrete pipes is proposed.

Structural transitions in a TiNiFe shape memory alloy

Results for resistance versus temperature, differential scanning calorimetry (DSC), Moessbauer spectroscopy, and magnetic susceptibility measurements in a Ti[sub 50]Ni[sub 47]Fe[sub 3] sample are presented. The resistance versus temperature curve shows a sharp increase at 245K and then rises slowly until a maximum is attained at [approx]150K. The DSC curve also exhibits a peak at the former temperature. The room temperature Moessbauer spectrum consists of a single line. As the temperature is lowered to 270K, a small asymmetric quadrupole doublet appears. The isomer shift and quadrupole splitting values of this doublet increase with decreasing temperature until almost constant values are achieved at temperatures [approx]100K. The magnetic susceptibility curve shows a sharp decrease that begins at 255K and then an unexpected increase at 90K. The authors relate these observations with a charge density wave-driven premartensitic transition in this system, and with the final martensitic phase.

Study of the crystal structure, superconducting and magnetic properties of Ru1−xFexSr2GdCu2O8

Samples from the Ru1−xFexSr2GdCu2O8 system with x = 0, 0.025, 0.05, 0.075, 0.1 and 0.2, were prepared and their structural, superconducting and magnetic properties were studied. Rietveld refinement of the x-ray diffraction patterns shows that the Fe substitution occurs at both Ru and Cu sites. An increase of Fe concentration produces no significant changes in the Ru–O(3)–Ru bond angle, which is a measure of the rotation of the RuO6 octahedra around the c-axis, or in the Cu–O(1)–Ru bond angle , which is a measure of the canting of the RuO6 octahedra. On the other hand, the Cu–O(2)–Cu bond angle, which is a measure of the buckling of the CuO2 layer, has a slight tendency to decrease with increase of the Fe content. We found that both ferromagnetic and superconducting transition temperatures are reduced with increase of the Fe concentration. Analysis related to the decay of the superconducting and ferromagnetic states is presented.

Site occupancy and Tc degradation in iron substituted YBa2(Cu1−xFex)4O8+δ

Abstract A series of YBa2(Cu1−xFex)4O8+δ samples with x=0.00625, 0.0125, 0.01875, 0.025 and 0.05, were synthesized and characterized by resistance vs. temperature measurements, X-ray diffraction and by Mossbauer spectroscopy. In order to make the site assignment for the Fe atoms, a Rietveld refinement of the X-ray spectra was performed and a point charge calculation of the quadrupole splitting for all the possible oxygen environments around the two Cu sites was carried out, taking into account the two possible ionic states of the iron atoms that occupy those sites. We found that, for low level iron concentrations, the Fe atoms occupy only the Cu(1) sites of the structure in a fivefold pyramidal coordination, with the apical oxygen atom placed along the a-axis between the double chains of the unit cell. The presence of these extra oxygen atoms in the crystal structure could be related to the rapid Tc reduction as iron concentration increases.

Systematic Mössbauer study of YBa2Cu3−xFe x Oδ and PrBa2Cu3−xFe x Oδ as a function of temperature

We study the similitudes and differences between YBa2Cu3−xFexOδ and PrBa2Cu3−xFexOδ systems, using Mössbauer spectroscopy. Both systems, withz=0.04, were studied at several temperatures. The spectra of PrBa2Cu3−xFexOδ showed four asymmetric quadrupole doublets with apparently different line widths, which were fitted with four symmetric superimposed quadrupole doublets with the same line width, but assuming a small (∼4–6 kOe) residual magnetic field in the Fe sites, which are mainly the Cu(1) sites of the 1–2–3 structure. On the other hand, the PrBa2Cu3−xFexOδ spectra showed only three asymmetric quadrupole doublets which can be fitted with three superimposed symmetric doublets and a residual magnetic field. In this case, the Fe atoms also occupy the Cu(1) sites of the 1–2–3 structure. The temperature variations of the Mössbauer spectra and their parameters seem to indicate that, in the Pr-system, a structural change takes place between 12 K and 40 K.

Seismic hazard estimation: directly using observations versus applying seismic hazard model

Seismic hazard evaluation for a site can be carried out based on direct observations such as ground motion records at the site or based on an adopted seismic hazard model, including the distribution of earthquake intensity, the spatio-temporal occurrence model of earthquakes in a region, and the ground motion prediction equations. The former is adequate if sufficient records at the site are available, while the latter is capable to incorporate information from similar tectonic regions. This study compares the seismic hazard estimated using these two approaches for a recording site in Mexico City. The comparison is carried out in terms of uniform hazard spectra by considering interplate earthquakes alone, inslab earthquakes alone, and both interplate and inslab earthquakes. Up to date ground motion records obtained at the site are employed for the analysis. The results showed quantitatively the differences between the uniform hazard spectrum (UHS) obtained by the two approaches and the differences between the estimated UHS and the design spectrum, which is important for seismic risk assessment and design code making. The results are also used to identify the differences between the standard deviation of the residuals for GMPEs developed using records from a single station and that obtained considering a broad network of stations.

Use of Neural network to predict the peak ground accelerations and pseudo spectral accelerations for Mexican Inslab and Interplate Earthquakes

The use of Artificial Neural Networks (ANN) is explored to predict peak ground accelerations (PGA) and pseudospectral acceleration (SA) for Mexican inslab and interplate earthquakes. A total of 277 and 418 seismic records with two horizontal components for inslab and interplate earthquakes, respectively, are used to train the ANN models by using an ANN with a feed-forward architecture with a back-propagation learning algorithm. Both ANN with single and two hidden layers are considered. For comparison purposes, the PGA and SA values predicted by the trained ANN models are compared with those estimated with attenuation relations or ground motion prediction equations (GMPEs). The comparison indicates that the predicted PGA and SA values by the trained ANN models, in general, follow the trends predicted by the GMPEs. However, an extensive verification of the trained models must be conducted before they can be used for seismic hazard and risk analysis since, on occasion, the PGA and SA values predicted by the trained ANN models depart from the behaviour observed from the actual records.

Effect of Fe substitution in the structure and superconducting properties of the (Y0.8Pr0.2)Ba2Cu4−xFexO8 system

Abstract The effects of iron substitutions on the superconducting and structural properties of (Y 0.8 Pr 0.2 )Ba 2 Cu 4− x Fe x O 8 were studied. As iron concentration increases, the rate at which T c diminishes with x is the same as for the YBa 2 Cu 4− x Fe x O 8 system, but faster than in the YBa 2 Cu 3− x Fe x O 7− δ system. The Mossbauer spectra of the different samples, together with Rietveld refinements of their corresponding X-ray spectra, indicate that the iron atoms occupy preferably the Cu(1) sites of the double (CuO) 2 chains in fivefold coordination; that is, due to their tri-valency, the iron atoms attract extra oxygen atoms, named O(5), that place themselves along the a -axis in between two adjacent planes of the double (CuO) 2 chains. As x is increased, the Cu(1)–O(4) bond length decreases, whereas the Cu(2)–O(4) one increases. Also, a reduction of the occupation factors of the oxygen atoms in the CuO 2 planes is observed. Some of these changes may be due to the presence of O(5) atoms, which in turn affect the charge-transfer mechanism between the (CuO) 2 double chains and the CuO 2 planes where superconductivity takes place, and could be responsible of the rate at which T c decreases.