HYDROGEN DIFFUSION IN HCP Zr

We study the behavior of hydrogen (H) in the α and β phases of Zr and in the β phase of Nb, using the Siesta and Vasp first principles codes based on the density functional theory (DFT). We calculate the hydrogen solution energy, the binding energy of the H-H and H-Nb complexes, as well as, the mixing energy of the Zr-Nb system. Furthermore, we have proposed a simple model for the study of the hydrogen diffusion (DH) mediated by interstitial mechanism in hcp Zr. We find that interstitial hydrogen diffuses isotropically according toDH=1.1×10−7exp(−42 KJ.mol−1/RT)(m2/s). Our results are in good agreement with experimental data and other Kinetic Monte Carlo (KMC) calculations. Finally, the formation energies of the three most stable hydrides in the hcp Zr phase were also calculated

Thermodynamics of Two - Band Superconductors: The Case of MgB2_{2}

Thermodynamic properties of the multiband superconductor MgB$_{2}$ have often been described using a simple sum of the standard BCS expressions corresponding to $\sigma$- and $\pi$-bands. Although, it is \textit{a priori} not clear if this approach is working always adequately, in particular in cases of strong interband scattering. Here we compare the often used approach of a sum of two independent bands using BCS-like $\alpha$-model expressions for the specific heat, entropy and free energy to the solution of the full Eliashberg equations. The superconducting energy gaps, the free energy, the entropy and the heat capacity for varying interband scattering rates are calculated within the framework of two-band Eliashberg theory. We obtain good agreement between the phenomenological two-band $\alpha$-model with the Eliashberg results, which delivers for the first time the theoretical verification to use the $\alpha$-model as a useful tool for a reliable analysis of heat capacity data. For the thermodynamic potential and the entropy we demonstrate that only the sum over the contributions of the two bands has physical meaning.Comment: 27 pages, 10 figures, 1 table, submitted to Phys. Rev.

Interaction of hydrogen with the microstructure of low-carbon steel

The interaction of hydrogen (H) with the microstructure of SAE 1008 type low-carbon steel is studied in this work. The objective is to determine the binding energy of H with the cementite (Fe3C) for three different conditions of the carbide. Electrochemical permeation tests are combined with the numerical resolution of Fick´s equations assuming that hydrogen is delayed during de transport by reversible trapping sites. Techniques of H vacuum desorption are also employed to describe irreversible trapping. From permeation tests, we obtain the density of reversible trapping sites and the probabilities of H capture and release from which the binding energy is evaluated. From effusion test, weak and strong trapping sites with energies in the range of 10-50 kJ/mol H were observed.Fil: Ramunni, Viviana Patricia. Comisión Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: De Paiva Coelho, T.. No especifíca;Fil: de Miranda, P. E. V.. Universidade Federal do Rio de Janeiro; Brasi

THEORETICAL AND NUMERICAL STUDY OF HYDROGEN DIFFUSION IN Zr-NbALLOYS

Within the framework of the delayed hydride cracking phenomena reported for the pressure tubes of CANDU-typenuclear reactors, we study the influence of the Zr-β(∼20 %Nb -∼80 %Zr) continuous phase on the hydrogen diffusion coefficient,DH, through the Zr-α/ Zr-βbiphasic alloy. We propose an improved phenomenological model forDHwithrespect to those found in the literature. Furthermore, we study the influence of the Nb content onDHfor the cubic phase Zr-β, employing the transition state theory furnished with ab-initio parameters provided by the SIESTA code. In particular, 9 ordered alloys are considered with different Nb content and effective activation energies are computed by Arrhenius fits for each alloy. We find that activation energies vary in a non-monotonic way as Nb content increases, reaching a maximum value at about Zr-50 %Nb. Finally, we observe that the predicted and measured larger diffusivity along the tube axis vs. the radial direction, is consistent with the material texture. Moreover, we conclude that the loss of continuity of the Zr-βsheets present in the tube microstructure, is consistent with the decrease ofDHin time at a given temperature

A simple method for correcting overestimated glomerular filtration rate in obese subject evaluated by the Cockcroft and Gault formula: a comparison with 51Cr EDTA clearance

AIM: The Cockcroft and Gault formula is a quick and reliable method for calculating creatinine clearance without a 24-hour urine collection (CG-cl). In obese subjects an excess of fat mass provokes a reduction in daily creatinine urine excretion per body kilo weight and is responsible for overestimated renal function when calculated by CG-cl. The aim of this study was to devise a simple correction method which could also make use of CG-cl in obese subjects. PATIENTS AND METHODS: In 52 subjects with a body mass index (BMI) > 25, renal function was assessed by simultaneously determining creatinine clearance using 24-hour urine collection (Cr-cl) and the CG-cl. The percentage difference between the 2 clearances (delta %) was correlated with BMI for each patient using simple linear regression analysis. The estimated regression model (delta% = 1.217 BMI-- 24.81) provided the following CG-cl correction formula for obese subjects: Corrected CG-cl = CG-cl (1.25 - 0.012 BMI). Its validity was evaluated in another group of 20 subjects with BMI > 25 by comparing the results obtained with Corrected CG-cl to those obtained by CG-cl and MDRD formula (MDRD-cl) using the clearance of 51Cr-EDTA (5 Cr-EDTA-cl) as the GFR measurement gold standard. RESULTS AND CONCLUSION: Linear regression analysis of CG-cl, MDRD-cl and Corrected CG-cl compared to 5tCr-EDTA-cl (considered as the independent variable) resulted in the following determination coefficients (R2): 0.687; 0.818; 0.947, respectively. In conclusion, this formula can be considered a quick and reliable method for CG-cl correction in obese subjects