H Diffusion in the H-Storage TiVMnCr Alloys as Deduced from Absorption and Mechanical Spectroscopy Experiments
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H Diffusion in the H-Storage TiVMnCr Alloys as Deduced from Absorption and Mechanical Spectroscopy Experiments Giovanni Mazzolai1, Andrea Biscarini1, Bruno Coluzzi1, Fabio M. Mazzolai1, and Ausonio Tuissi2 1 University of Perugia, Department of Physics, Via A. Pascoli 5, 06100 Perugia, Italy 2 Istituto per l'Energetica e le Interfasi, CNR-IENI, Lecco, Italy
ABSTRACT The low H content portion of the PCT absorption curves of the 40Ti-xV-(50-x)Mn-10Cr alloys exhibits a plateau, whose pressure has been determined together with the solubility limit of H in the α−phase (solid solution). The chemical (Dc) and Einstein (DE) diffusion coefficients of H have been deduced from H absorption kinetics (273 K-1100 K) and from mechanical spectroscopy (150 K-200 K), which has revealed a Snoek-type of anelastic relaxation. The combined Arrhenius plot of DE and Dc gives the following parameters for H diffusion: W=0.318±0.005 eV, D0=(4±1)×10-7 m2/s (36 V alloy) and W=0.352±0.005 eV, D0=(5±1)10-7 m2/s (32 V alloy).
INTRODUCTION The TiVCrMn bcc alloys, particularly those of the series 40Ti-xV-(50-x)Mn-10Cr, have been indicated as potential H storage materials [1-4]. As a matter of fact, the mass capacity of these alloys may reach values as high as 3.98 % [1]. Actually, the amount of H that can be reversibly absorbed and desorbed in practice is not higher than about 2.5 wt %; this is due to the exceedingly small equilibrium gas pressures exhibited by these alloys for H contents lower than 1-1.5 wt %. In this concentration range the PCT isotherms have not been traced so far, thus, the solubility limit of H in the solid solution as well as the equilibrium pressures are not known as yet. It appears that also the mobility and localization of H within the crystal lattice have not been investigated. Thus, a research program has been undertaken with the aim of studying the absorption kinetics, the PCT relationships at low H contents and the H mobility as a function of temperature. Results on the first issue have already been reported [5], those on PCT relationships and on diffusion are presented here.
EXPERIMENT The Ti-xV-(50-x)Mn-10Cr alloys (x=32, 36) were prepared by arc melting at IENI-CNR. The compositions were chosen in such a way to have a single bcc phase within the material [1]. The buttons were re-melted several times to improve the alloy homogeneity. Rectangular bars of 0.25, 0.50, 1.00, 1.50 mm thickness were cut from the ingot by spark erosion. The bars were then chemically etched by using a water solution of hydrofluoric and nitric acid (50, 10 and 40 vol. %, respectively). One at a time the samples were exposed to an H2 gas atmosphere in the reaction
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nH
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 10 10 32 V alloy 36 V alloy T=273 K
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5
Peq (x10 Pa)
0,1
1 0,1
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0,01
1E-3
1E-3
1E-4
1E-4
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1E-5 1E-6
1E-6 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2
H concentration (Weight %) Figure 1. PCT isotherms for the two alloys at 273 K.
Heat Flow (arbitrary units)
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