Tribological behaviour of polyalphaolefins: wear and rolling contact fatigue tests

Polyalphaolefin fluids are gaining rapid acceptance as high-performance lubricants and functional fluids because they have certain inherent, and highly desirable, characteristics relative to mineral oils. One of these characteristics is their low toxicity. It combined with excellent viscometrics and lubricity, have made low-viscosity PAO fluids an important component in lubricant formulations. Typical data found in product specifications for lubricants are the kinematic viscosity and the viscosity index. These values do not give enough information to choose the optimum lubricant for a lubricated contact. In mechanical systems take place rolling, sliding and rolling/sliding contacts, therefore lubricants have to work the best possible in these operation conditions. In this study are experimentally determined the L50, L10 and Weibull´s slope () of polyalphaolefins with two different viscosities. This test was made on a four-ball machine (Stanhope Seta). Wear test also was made on a four-ball tester (Roxana) in order to measure the wear scar diameter (WSD), and the flash temperature parameter (FTP). Lubricants were identified through infrared spectroscopy, and ball´s pittings were observed with SEM

EuCo2P2: A Model Molecular-Field Helical Heisenberg Antiferromagnet

The metallic compound EuCo2P2 with the body-centered tetragonal ThCr2Si2 structure containing Eu spins 7/2 was previously shown from single-crystal neutron diffraction measurements to exhibit a helical antiferromagnetic (AFM) structure below TN = 66.5 K with the helix axis along the c axis and with the ordered moments aligned within the ab-plane. Here we report crystallography, electrical resistivity, heat capacity, magnetization and magnetic susceptibility measurements on single crystals of this compound. We demonstrate that EuCo2P2 is a model molecular-field helical Heisenberg antiferromagnet from comparisons of the anisotropic magnetic susceptibility chi, high-field magnetization and magnetic heat capacity of EuCo2P2 single crystals at temperature T < TN with the predictions of our recent formulation of molecular field theory. Values of the Heisenberg exchange interactions between the Eu spins are derived from the data. The low-T magnetic heat capacity ~ T^3 arising from spin-wave excitations with no anisotropy gap is calculated and found to be comparable to the lattice heat capacity. The density of states at the Fermi energy of EuCo2P2 and the related compound BaCo2P2 are found from the heat capacity data to be large, 10 and 16 states/eV per formula unit for EuCo2P2 and BaCo2P2, respectively. These values are enhanced by a factor of ~2.5 above those found from DFT electronic structure calculations for the two compounds. The calculations also find ferromagnetic Eu-Eu exchange interactions within the ab-plane and AFM interactions between nearest- and next-nearest planes, in agreement with the MFT analysis of chi{ab}(T < TN).Comment: 20 pages, 17 figures, 3 tables, 46 references. This is an extended replacement of arXiv:1512.02958 with an additional coautho

Non-affine geometrization can lead to nonphysical instabilities

Geometrization of dynamics consists of representing trajectories by geodesics on a configuration space with a suitably defined metric. Previously, efforts were made to show that the analysis of dynamical stability can also be carried out within geometrical frameworks, by measuring the broadening rate of a bundle of geodesics. Two known formalisms are via Jacobi and Eisenhart metrics. We find that this geometrical analysis measures the actual stability when the length of any geodesic is proportional to the corresponding time interval. We prove that the Jacobi metric is not always an appropriate parametrization by showing that it predicts chaotic behavior for a system of harmonic oscillators. Furthermore, we show, by explicit calculation, that the correspondence between dynamical- and geometrical-spread is ill-defined for the Jacobi metric. We find that the Eisenhart dynamics corresponds to the actual tangent dynamics and is therefore an appropriate geometrization scheme.Comment: Featured on the Cover of the Journal. 9 pages, 6 figures: http://iopscience.iop.org/1751-8121/48/7/07510

Metallic spin-glasses beyond mean-field: An approach to the impurity-concentration dependence of the freezing temperature

A relation between the freezing temperature ($T^{}_{\rm g}$) and the exchange couplings ($J^{}_{ij}$) in metallic spin-glasses is derived, taking the spin-correlations ($G^{}_{ij}$) into account. This approach does not involve a disorder-average. The expansion of the correlations to first order in $J^{}_{ij}/T^{}_{\rm g}$ leads to the molecular-field result from Thouless-Anderson-Palmer. Employing the current theory of the spin-interaction in disordered metals, an equation for $T^{}_{\rm g}$ as a function of the concentration of impurities is obtained, which reproduces the available data from {\sl Au}Fe, {\sl Ag}Mn, and {\sl Cu}Mn alloys well.Comment: 4 figures. This is a strongly revised version, where several aspects have been improved, and the equation for the freezing temperature has been refined. It is equivalent to the published version in J. Phys.: Condens. Matter 25 (2013) 13600

Food consumption analysis in Spanish elderly based upon the Mini Nutritional Assessment Test

Background/Aims: Aged people are an increasing population group worldwide, and nutritional impairments may contribute to additional health problems. The characterization of food consumption of elderly people is a good approach to implementing adequate nutritional policies in order to improve their nutritional status. The aim of this study was to describe and analyze specific aspects of food intake based upon the Mini Nutritional Assessment (MNA) test in a representative country sample. Methods: The survey consisted of a cross-sectional study assessing the nutritional status of 22,007 Spanish people aged 65 years or older using the MNA test. These data, including dietary information, were obtained by health professionals specifically trained to carry out the questionnaire. A multiple regression analysis was used to evaluate the contributing impact of the diet-related MNA questions on the total MNA score. Results: Differences in the food choices between men and women were found, but not in the mode of feeding. Obese and non-obese individuals, categorized by a body mass index (BMI) more or less than 30, showed similar intake patterns of protein-rich foods, but differences in fruits, vegetables and fluids. On the other hand, undernourished and non-undernourished subjects, according to the MNA criteria, revealed different food consumption. Loss of appetite and mode of feeding were the items with the highest influence the MNA total score. Conclusions: While age, gender and BMI considered together are responsible for 11.3% of the total MNA score, dietary-related items can predict the 62.4% of the total MNA classification in the overall elderly Spanish population

Verification of the Parallel Pin-Wise Core Simulator pCTF/PARCSv3.2 in Operational Control Rod Drop Transient Scenarios

This is an Accepted Manuscript of an article published by Taylor & Francis in Nuclear Science and Engineering on 2017, available online: https://www.tandfonline.com/doi/full/10.1080/00295639.2017.1320892[EN] Thanks to advances in computer technology, it is feasible to obtain detailed reactor core descriptions for safety analysis of the light water reactor (LWR), in order to represent realistically the fuel elements design, as is the case for three-dimensional coupled simulations for local neutron kinetics and thermal hydraulics. This scenario requires an efficient thermal-hydraulic code that can produce a response in a reasonable time for large-scale, detailed models. In two-fluid codes, such as the thermal-hydraulic subchannel code COBRA-TF, the time restriction is even more important, since the set of equations to be solved is more complex. We have developed a message passing interface parallel version of COBRA-TF, called pCTF. The parallel code is based on a cell-oriented domain decomposition approach, and performs well in models that consist of many cells. The Jacobian matrix is computed in parallel, with each processor in charge of calculating the coefficients related to a subset of the cells. Furthermore, the resulting system of linear equations is also solved in parallel, by exploiting solvers and preconditioners from PETSc. The goal of this study is to demonstrate the capability of the recently developed pCTF/PARCS coupled code to simulate large cores with a pin-by-pin level of detail in an acceptable computational time, using for this purpose two control rod drop operational transients that took place in the core of a three-loop pressurized water reactor. As a result, the main safety parameters of the core hot channel have been calculated by the coupled code in a pin level of detail, obtaining best estimate results for this transient.This work has been partially supported by the Universitat Politecnica de Valencia under Projects COBRA_PAR (PAID-05-11-2810) and OpenNUC (PAID-05-12), and by the Spanish Ministerio de Economia y Competitividad under Projects SLEPc-HS (TIN2016-75985-P) and NUC-MULTPHYS (ENE2012-34585).Ramos Peinado, E.; Roman Moltó, JE.; Abarca Giménez, A.; Miró Herrero, R.; Bermejo, JA.; Ortego, A.; Posada-Barral, JM. (2017). Verification of the Parallel Pin-Wise Core Simulator pCTF/PARCSv3.2 in Operational Control Rod Drop Transient Scenarios. Nuclear Science and Engineering. 187(3):254-267. https://doi.org/10.1080/00295639.2017.1320892S2542671873Cuervo, D., Avramova, M., Ivanov, K., & Miró, R. (2006). Evaluation and enhancement of COBRA-TF efficiency for LWR calculations. Annals of Nuclear Energy, 33(9), 837-847. doi:10.1016/j.anucene.2006.03.011Ramos, E., Roman, J. E., Abarca, A., Miró, R., & Bermejo, J. A. (2016). Control rod drop transient analysis with the coupled parallel code pCTF-PARCSv2.7. Annals of Nuclear Energy, 87, 308-317. doi:10.1016/j.anucene.2015.09.016T. DOWNAR et al. “PARCS v2.7 U.S. NRC Core Neutronics Simulator: User Manual” (2006).T. DOWNAR et al. “PARCS v2.7 U.S. NRC Core Neutronics Simulator: Theory Manual” (2006)

Solidification of small para-H2 clusters at zero temperature

We have determined the ground-state energies of para-H$_2$ clusters at zero temperature using the diffusion Monte Carlo method. The liquid or solid character of each cluster is investigated by restricting the phase through the use of proper importance sampling. Our results show inhomogeneous crystallization of clusters, with alternating behavior between liquid and solid phases up to N=55. From there on, all clusters are solid. The ground-state energies in the range N=13--75 are established and the stable phase of each cluster is determined. In spite of the small differences observed between the energy of liquid and solid clusters, the corresponding density profiles are significantly different, feature that can help to solve ambiguities in the determination of the specific phase of H$_2$ clusters.Comment: 17 pages, accepted for publication in J. Phys. Chem.