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Novedrate, Italy

This review is focused on the influence of interstitial hydrogen and alloy compositional changes on the internal friction (IF) spectrum and elastic Young's modulus (E) of NiTi based shape memory alloys. In the martensitically transforming binary alloys Ni 50+xTi 50-x (x≤1.3) vacuum annealed and furnace cooled (H-free), besides the well known IF peak associated with the martensitic transition two additional non-thermally activated peaks (P 150K and P 200K′) are present due to some sort of second-order phase transitions. In martensitically transforming Ni 50+xTi 50-x and Ti 50Ni 50-yCu y alloys doped with hydrogen two thermally activated peaks, P TWH and P H, appear which originate from stress-assisted motions of H-twin boundary complexes and isolated H-elastic dipoles (Snoek effect), respectively. In a H-free martensitically non-trasforming alloy (x=2), besides the non-thermally activated peak P 150K, a frequency dependent dip is observed in the E(T) curves at a temperature Tg. This dip is similar to that reported in the literature for two other non-transforming alloys (x=1.5 and x=2.5), which, however, were also found to exhibit a thermally activated IF peak just below Tg. Most likely, these two alloys were contaminated with hydrogen during the preliminary solubilization in argon atmosphere and subsequent water quenching treatments given to them. The Young's modulus dip and the lower temperature IF peak have been both attributed to a novel type of phase transition reported in the literature as "strain glass transition". The introduction of hydrogen into the non-transforming alloy with x=2 enhances the Young's modulus dip and gives rise to the H-Snoek peak P H just below T g, which clearly appears to be the counterpart of the peak observed in the alloys (x=1.5 and x=2.5) solubilized in argon atmosphere and water quenched. The conclusion was reached in the present work that this last peak is not related to the strain glass transition but is rather an H-Snoek relaxation. © 2011 Copyright 2011 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license.

Mazzolai G.,University Telematica mpus
International Journal of Hydrogen Energy | Year: 2013

Absorption and desorption of hydrogen have been investigated in Nb 95Mo5 and Nb80Mo20 alloys over wide temperature ranges. On continuous heating H desorption from Nb 95Mo5 was found to take place between 800 and 1000 K and from Nb80Mo20 between 900 and 1100 K. The observed increase in the desorption temperature with increasing Mo content has been attributed to a higher stability of the Mo and NbMo oxides with respect to those of Nb. The solid-gas reaction during absorption was first order and the rate limiting process consisted in the penetration of H atoms through surface oxides. At high temperatures and in the presence of H the oxides are expected to become permeable to H due to the reduction of higher valence to lower valence oxides. The values of the activation energy for H diffusion within the oxide films were 0.82 ± 0.04 eV for Nb95Mo5 and 1.1 ± 0.1 eV for Nb80Mo20. The thicknesses of the oxide films estimated from the absorption data were of the order of 1 μm. Copyright © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Mazzolai G.,University Telematica mpus
International Journal of Hydrogen Energy | Year: 2013

The diffusion of H and D has been investigated in the Nb 0.80Mo0.20 alloy by Gorsky and dipole reorientation/ reaction relaxations in the temperature ranges 130-370 K and 70-100 K, respectively. Between 130 and 370 K the chemical diffusion coefficient at infinite dilution of H and D obeys Arrhenius laws. Differently from the case of pure Nb no deviation from an exponential temperature dependence has been observed below 250 K. The activation energies in the alloy (0.113 ± 0.003 eV for H and 0.13 ± 0.02 eV for D) are slightly higher than in pure Nb (0.106 ± 0.006 eV for H and 0.127 ± 0.006 for D) for T > 250 K. The H diffusion data were fitted to a relationship provided by a trapping model and the binding energy of H to Mo was found to be as small as 0.009 eV. An internal friction (IF) peak occurring between about 70 and 100 K for frequencies in the range 0.50-6.62 kHz was proved to be due to stress-assisted changes in the directional short-range order of Mo-H(D) bonds. The Einstein diffusion coefficient of H(D) deduced from the IF peak does not match the higher temperature data due to differences in the tunneling regimes of free and trapped H atoms operating over the two temperature ranges. The mismatch is small for D for which the tunneling regime appears to be the same in the free and trapped states. © 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Mazzolai G.,University Telematica mpus
International Journal of Hydrogen Energy | Year: 2011

Hydrogen absorption by Sc has been investigated over wide ranges of temperature (790-1280 K) and pressure (10-150 mbar). The absorbed quantities of H were in agreement with those expected from p-x-T isotherms, available in the literature, only for temperatures higher than 1000 K, where the absorption curves could be fitted to a Johnson-Mehl-Avrami type of relationships. The gas-solid reaction was first-order and the reaction-rate-constant k exhibited an Arrhenius type of temperature dependence with an associated activation energy of 2.1 ± 0.1 eV (202 ± 10 kJ/mol·H2). The step controlling the absorption rate turned out not to be H diffusion in the bulk. Namely, the values of the apparent H diffusion coefficient deduced from absorption data were found to be some orders of magnitude smaller than expected from extrapolations of lower temperature anelastic and spin-lattice relaxation data. The absorption rate appears to be governed by H penetration through a sub-surface Sc layer containing a high concentration of interstitial oxygen, originated from the decomposition of surface oxides occurring between 900 K and 1000 K. © 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Caglieris F.,CNR Institute of Neuroscience | Melone L.,Polytechnic of Milan | Melone L.,University Telematica mpus | Canepa F.,CNR Institute of Neuroscience | And 11 more authors.
RSC Advances | Year: 2015

Nitroxides have great potential as contrast agents for Magnetic Resonance Imaging (MRI). Two β-cyclodextrin (βCD) derivatives bearing one or seven (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) units on the small rim of βCD (CD3 and CD6 respectively) were synthesized. Their effective magnetic moments were measured by DC-SQUID magnetometry obtaining the values μeffB ≈ 1.7 and μeffB ≈ 4.2 for CD3 and CD6 respectively. Interestingly, while isothermal magnetization data of CD3 were well described by a Brillouin function for a S = 1/2 single spin system, those associated with CD6 could not be explained in the framework of a non-interacting spins model. For this reason, four different configurations for the seven interacting nitroxides were considered and modeled. The numerical results evidenced that only the configurations with a privileged central spin could take into account the experimental observations, thus justifying the reduced effective magnetic moment of CD6. The water relaxivity (r1) in DMSO-d6-water (9:1 v:v) solutions was also measured for both the derivatives obtaining the values r1 = 0.323 mM-1 s-1 and r1 = 1.596 mM-1 s-1 for CD3 and CD6 respectively. © 2015 The Royal Society of Chemistry.

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