1H NMR study of hydrogen self-diffusion in ternary Ti–V–Cr alloys

Journal of Alloys and Compounds 614 (2014) 364–367

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1

H NMR study of hydrogen self-diffusion in ternary Ti–V–Cr alloys

A.V. Vyvodtceva a, M.G. Shelyapina a,⇑, A.F. Privalov b, Yu.S. Chernyshev a, D. Fruchart c a

Faculty of Physics, Saint-Petersburg State University, 1 Ulyanovskaya St., Peterhof, Saint Petersburg 198504, Russia Institute für Ferstkoerperphysik, TU Darmstadt, 6 Hochschulstarsse, Darmstadt 64289, Germany c MCMF Institut Néel, CNRS, BP 166, 38042 Grenoble Cedex 9, France b

a r t i c l e

i n f o

Article history: Received 26 May 2014 Received in revised form 3 June 2014 Accepted 4 June 2014 Available online 14 June 2014 Keywords: Metal hydrides Diffusion Nuclear resonances

a b s t r a c t Here we report on the results of proton NMR study of hydrogen self-diffusion in hydrides of Ti–V–Cr alloys of different composition, pure and with 4 wt.% of Zr7Ni10 additives, namely, TiV0.8Cr1.2H5.29, Ti0.5V1.9Cr0.6H5.03 and Ti0.33V1.27Cr1.4H1.13. The measurements have been made using the static field gradient nuclear magnetic resonance technique. The hydrogen self-diffusion coefficient at room temperature lies within the range of 1.4–3.7  1011 m2/s. The activation energy Ea strongly depends on the both composition and structure type of the hydride. The samples with bcc structure (and with the lowest hydrogen concentration) exhibit the highest Ea value of 0.2 eV. For the samples with fcc structure the Ea value is lower and decreases with increasing the vanadium fraction. The influence of Zr7Ni10 additives on the measured parameters is also discussed. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction During the last decades Ti–V–Cr alloys with bcc structure have been extensively studied as promising materials for hydrogen storage [1]. These systems exhibit sufficiently high hydrogen sorption capacity with maximum hydrogen uptake up to 3.8 wt.% for the most appropriate compositions [2,3]. However, after multiple cycles their reversible hydrogen storage capacity does not exceed 2.3 wt.%. [4]. Nevertheless, these alloys with bcc structure display rather high hydrogen sorption kinetics and can be used as additives to MgH2 promoting formation of the bcc structure of magnesium [5] and by mean of this accelerating the hydrogen sorption kinetics of magnesium. Recently it has been shown that both the hydrogen sorption rate and the reversible storage capacity of Ti–V–Cr alloys can be further improved by alloying with small amount of Zr7Ni10 (of about 4 wt.%) [4]. Such an improvement can be explained by the fact that addition of Zr7Ni10 leads to microstructuring of the sample. SEM patterns reveal that the resulting composite materials are not homogenous: there are a ‘‘main matrix’’ phase, which mostly consists of Ti, V and Cr, and an intergranular phase, which includes a significant fraction of Zr and Ni. Nevertheless, it remains unclear how such a microstructure affects the hydrogen mobility in these materials.

⇑ Corresponding author. Tel.: +7 812 428 44 69; fax: +7 812 428 72 40. E-mail address: [email protected] (M.G. Shelyapina). http://dx.doi.org/10.1016/j.jallcom.2014.06.023 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

Earlier we reported on the results of 1H NMR (nuclear magnetic resonance) study of hydrogen mobility in ternary Ti–V–Cr alloys of various compositions [6]. It was found that in Ti–V–Cr hydrides the 1 H relaxation strongly depends on the composition of the hydride, especially on the vanadium fraction. In vanadium rich hydrides the hydrogen motion starts at lower temperatures. For the TiV0.8Cr1.2 compound the Zr7Ni10 or Hf7Ni10 additives enhance the hydrogen mobility but simultaneously increase of the activation energy. However, the latter may be caused by the loss of vanadium by melting the TiV0.8Cr1.2 sample with additives [6]. Commonly the proton relaxation in metallic hydrides is analyzed using the isotropic model firstly proposed by Bloembergen, Purcell and Pound (BPP) [7] and initially developed to describe rotational motion of atoms (or groups of atoms) in liquids. In metal hydrogen systems translational motion of hydrogen atoms can be considered as a continuous change of their orientations. Metal hydrogen systems are usually inhomogeneous, leading to a temperature dependence of the spin–lattice relaxation time broader than the BPP model predicts [8,9]. An improved data analysis uses a distribution of hydrogen motional parameters [10,11]. Nevertheless, there are some features of the temperature dependencies of the proton relaxation in metallic hydrides that cannot be explained within the framework of the BPP model even taking into account distribution of activation energies and correlation times, namely: the disagreement in the spin–lattice relaxation time values at high temperature measured at different frequencies [8,12]; extremely low values of the second moment of a spectral line; the disagreement between spin–spin (T2) and spin–lattice (T1) relaxation times

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at high temperatures [13,14]. As it was shown in Refs. [6,15,16] it is possible to avoid the first two drawbacks by introducing a so-called exchange model. However, all these models contain a number of fitting parameters. To justify the model it would be expedient to determine some parameters from other experiments; for example, the activation energy can be obtained directly from 1H NMR diffusion measurements. Moreover, on a microscopic level the Field-Gradient Spin Echo NMR provide a direct access to the hydrogen diffusion coefficient. In this contribution we report on the results of the NMR study of hydrogen self-diffusion in hydrides of Ti–V–Cr alloys. The following hydrides have been studied: TiV0.8Cr1.2H5.29, Ti0.5V1.9Cr0.6H5.03 and Ti0.33V1.27Cr1.4H1.13. To reveal how the Zr7Ni10 additives affect the hydrogen diffusion we have also investigated the hydrides of the following alloys: Ti0.5V1.9Cr0.6 + 4 wt.% Zr7Ni10 and Ti0.33V1.27Cr1.4 + 4 wt.% Zr7Ni10.

Fig. 2. Stimulated echo decays in the static field gradient for Ti0.5V1.9Cr0.6H5.03 at 313 K and tm = 1 and 5 ms.

2. Sample preparation and experimental method

      2s tm exp  ; Sðtm ; sÞ ¼ exp Q 2 Dtm exp  T2 T1

ð1Þ

where Q = csg is the generalized scattering vector, c is the gyromagnetic ratio, g is the magnetic field gradient, s and tm are the times between the pulses (tm  s). To eliminate the contribution of T2 into the S(tm, s) dependence the Hahn echo pulse sequence was used. Both used pulse sequences are shown in Fig. 1.

TiV0.8Cr1.2H5.29 Ti0.5V1.9Cr0.6H5.03

4

-11

2

(m /s)

6

D 10

The TiV0.8Cr1.2, Ti0.5V1.9Cr0.6 and Ti0.33V1.27Cr1.4 samples have been prepared by induction melting of the pure elements (Treibacher Industries AG) in argon atmosphere. The melting has been repeated three times. The obtained ingots were hydrogenated in an autoclave at a pressure of 20 bar at 200 °C. The estimation of the quantities of absorbed hydrogen was made by the weighing the samples before and after their hydrogenation that results in the following formulas: TiV0.8Cr1.2H5.29, Ti0.5V1.9Cr0.6H5.03 and Ti0.33V1.27Cr1.4H1.13. The studied Ti–V–Cr alloys crystallize into the body centered cubic (bcc) structure [6]. Upon hydrogenation, for the TiV0.8Cr1.2 and Ti0.5V1.9Cr0.6 alloys a phase transition to the face centered cubic (fcc) structure occurs, which is confirmed by both, ab initio calculations and neutron diffraction measurements [3,17]. The structure type of the Ti0.33V1.27Cr1.4H1.13 hydride remains bcc due to the low hydrogen content [6]. To prepare the samples with additives the initial alloys were melted with 4 wt.% of Zr7Ni10 and hydrogenated using the technique described above. The structural studies of these composite alloys were reported in Refs. [4,6]. For all studied samples the grain size after hydrogenation was less than 100 lm; this is comparable to the radiofrequency skin depth in such materials at applied frequencies. The hydrogen diffusion measurements were performed using NMR in a static field gradient (SFG NMR) [18,19]. This method allows us to measure diffusion coefficients (D) in systems with short T2 values, which are typical for solids. To determine diffusion coefficients the stimulated echo pulse sequence was used. The observed echo amplitude can be described by the following exponential function:

2

0 10

-3

10

-2

10

-1

tm (s) Fig. 3. Hydrogen diffusion coefficient as a function of mixing time in Ti0.5V1.9Cr0.6H5.03 and TiV0.8Cr1.2H5.29.

The experiment was carried out using the home-made1 SFG NMR relaxometer at 100 MHz in a static field gradient of 51.3 ± 0.3 T/m within the temperature range from 294 K to 433 K using a high temperature probe [20]. The temperature of the samples was controlled with the accuracy of 0.5 K.

3. Results and discussion To determine the hydrogen diffusion coefficient we have recorded the stimulated echo decay. The results for the Ti0.5V1.9Cr0.6H5.03 hydride at 313 K are shown in Fig. 2 for two different mixing time values. It is to be noted that for an accurate determination of the diffusion coefficient the tm value must be chosen in a correct way. In Fig. 3 we plot the dependence of the hydrogen diffusion coefficient versus the mixing time in TiV0.8Cr1.2H5.29 and Ti0.5V1.9Cr0.6H5.03 measured at room temperature. As it is seen, for TiV0.8Cr1.2H5.29 the D value does not depend on tm, whereas for Ti0.5V1.9Cr0.6H5.03 starting from tm  10 ms it slightly decreases with tm. The similar dependences (slight decrease at high tm values) were obtained for other hydrides. Such dependencies indicate that the hydrogen diffusion in these systems can be restricted (for example, by the crystallite size). However, as the inaccuracy of the D measurement is rather high it is difficult to make an unambiguous conclusion. Therefore, to avoid this possible effect we used short tm (1 and 5 ms). Fig. 1. Hahn echo and stimulated echo pulse sequences.

1

TU Darmstadt.

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10

-9

Ti0.33V1.27Cr1.4H1.13 Ti0.5V1.9Cr0.6H5.03

10

-10

10

-11

2

D (m /s)

TiV0.8Cr1.2H5.29

2.0

2.4

2.8

3.2

3.6

1/T (1000/K) Fig. 4. Hydrogen diffusivity in Ti0.5V1.9Cr0.6H5.03, TiV0.8Cr1.2H5.29 in Ti0.33V1.27Cr1.4H1.13. The solid lines represent fits by the Arrhenius law (Eq. (2)).

10

-9

Ti0.33V1.27Cr1.4H1.13+ 4 wt.% Zr7Ni10

10

-10

10

-11

2

D (m /s)

Ti0.5V1.9Cr0.6H5.03 + 4 wt.% Zr7Ni10

2.0

2.4

2.8

3.2

3.6

1/T (1000/K) Fig. 5. Hydrogen diffusivity in Ti0.33V1.27Cr1.4H1.13 + 4 wt.%Zr7Ni10 and Ti0.5V1.9Cr0.6H5.03 + 4 wt.%Zr7Ni10. The solid lines represent fits by the Arrhenius law (Eq. (2)).

In metal hydrogen system hydrogen diffusion processes usually can be described within the activation model. In this case the temperature dependence of the diffusion coefficient obeys the Arrhenius law:

  Ea D ¼ D0 exp  ; kT

ð2Þ

where D0 is the pre-exponential factor, Ea is the activation energy, k is the Boltzmann constant. The experimental temperature dependencies of the hydrogen diffusion coefficients for all investigated samples are shown in Figs. 4 and 5. As it is seen within the studied temperature range all the dependencies can be described good enough by the

activation model, though the D(T) dependence for TiV0.8Cr1.2H5.29 exhibits considerably more scattering in comparison with other samples. The diffusion coefficients, the activation energies and the pre-exponential factors obtained from the Arrhenius fits are listed in Table 1. By analyzing the data one can conclude that in hydrides of Ti–V–Cr alloys the hydrogen diffusion coefficient value at T = 294 K (D294K) increases with increasing vanadium concentration. The highest D294K was determined for Ti0.5V1.9Cr0.6H5.03 (D294K = 3.74  1011 m2/s). Such low values of the hydrogen diffusion coefficient are quite typical for transition metal hydrides with a high hydrogen concentration [21] (the formulas TiV0.8Cr1.2 H5.29, Ti0.5V1.9Cr0.6H5.03 and Ti0.33V1.27Cr1.4H1.13 correspond to MH1.76, MH1.68 and MH0.38, respectively). For example, in transition metal hydrides with low hydrogen concentration the D294K value is about 1–5  109 m2/s [22] and rapidly decreases with hydrogen concentration increasing [23]. According to our previous 1H NMR studies [16] hydrogen occupies mainly tetrahedral interstitial sites. It means that in hydrides with fcc structure almost all tetrahedral sites are occupied. For the samples without additives the lowest activation energy Ea = 0.12 eV was obtained for the vanadium rich composition Ti0.5V1.9Cr0.6H5.03. The Ti0.33V1.27Cr1.4H1.13 hydride with bcc structure exhibits the highest activation energy (Ea = 0.23 eV). In Table 1 we have also listed the activation energy values obtained from the proton relaxation study [6]. Both the relaxation and diffusion measurements exhibit rather similar activation energies for the TiV0.8Cr1.2H5.29 and Ti0.5V1.9Cr0.6H5.03 hydrides. However, there is a noticeable disagreement for Ti0.33V1.27Cr1.4H1.13. Such a discrepancy appears rather often in metal hydrides. For example, in b-LaNi5H6.5 diffusion measurement leads to Ea = 0.41 eV, however, from spin–lattice relaxation data this value is two times lower: Ea = 0.2 eV [24,25]. It can be explained by following. On the one hand, different methods ‘‘see’’ different processes. Diffusiometry is sensitive on the long-range scale and sees mass transport, whereas relaxation measurements are sensitive on the atomic scale, where forward and backward atomic jumps in a potential landscape may take place without effective mass transport. On the other hand, in diffusion measurements the activation energy is determined directly from the slope of the D(T) dependence, whereas the Ea value determined from T1(1/T) is affected by the proper model choice. Indeed, the activation energy of the hydrogen motion in hydrides of Ti–V–Cr alloys were calculated in Ref. [6] using a number of approximations. Particularly, the ratio of the hydrogen concentration in the two states with different mobility (pa/pb), was supposed to be independent of temperature and equal to its value at room temperature. However, the correction that takes into account the temperature dependence of the pa/pb ratio leads to further decreasing of the Ea values determined from the proton relaxation experiment [25]. Fig. 5 shows the temperature dependences of the hydrogen selfdiffusion coefficient in hydrides of Ti–V–Cr alloys with Zr7Ni10. The

Table 1 Hydrogen self-diffusion parameters for the studied hydrides. The diffusivities at 294 K, D294K were calculated using Eq. (2). The activation energy values obtained from relaxation measurements (Refs. [6] and [26]) are given for comparison. Hydrides

TiV0.8Cr1.2H5.29 TiV0.8Cr1.2 H5.29 + 4 wt.% Zr7Ni10 Ti0.5V1.9Cr0.6 H5.03 Ti0.5V1.9Cr0.6 H5.03 + 4 wt.% Zr7Ni10 Ti0.33V1.27Cr1.4H1.13 Ti0.33V1.27Cr1.4 H1.13 + 4 wt.% Zr7Ni10

D0  108 (m2/s)

0.53 ± 0.05 – 0.43 ± 0.05 0.65 ± 0.07 6.98 ± 0.91 7.65 ± 0.99

D294K  1011 (m2/s)

1.43 ± 0.13 – 3.74 ± 0.41 3.87 ± 0.43 2.62 ± 0.34 0.91 ± 0.12

Ea (eV) Diffusion measurements (this work)

Relaxation measurements Ref. [6]

Ref. [26]

0.15 ± 0.01 – 0.12 ± 0.01 0.13 ± 0.01 0.20 ± 0.01 0.23 ± 0.01

0.13 0.14 0.11 – 0.11 –

0.11 – 0.09 0.09 – –

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D(T) dependence can be also tolerably fitted by the Arrhenius law. The fitting parameters are also listed in Table 1. It is clearly seen that the influence of the additives on the Ti–V–Cr hydrides with fcc and bcc structure is rather different. In Ti0.5V1.9Cr0.6H5.03 addition of Zr7Ni10 almost does not change the hydrogen diffusion parameters, whereas in Ti0.33V1.27Cr1.4H1.13 it affects both the activation energy and the pre-exponential factor, that leads to three times lower hydrogen self-diffusion coefficient at room temperature, as compared to the hydride without additives. A speculative explanation of such results can be following. According to Ref. [4] alloying the Ti–V–Cr samples with Zr7Ni10 leads to a partial substitution of the Ti, V, Cr atoms by Ni and Zr, especially near the intergranular phases, that causes changes of the activation energy. The deviation of the result composition from the initial one after the melting with Zr7Ni10 may be different for Ti0.5V1.9Cr0.6 and Ti0.33V1.27Cr1.4. However, to prove this reasoning an additional microstructural analysis of the samples is required.

[4]

[5] [6]

[7] [8]

[9]

[10]

[11]

4. Conclusion SFG NMR was applied to study the hydrogen self-diffusion in hydrides of disordered Ti–V–Cr alloys. To describe the temperature dependences of the diffusion coefficient the activation model was used. All studied hydrides exhibit slow hydrogen diffusion that is usual for hydrides with high hydrogen concentration. It was found that the activation energy increases in the series Ti0.5V1.9Cr0.6H5.03 < TiV0.8Cr1.2H5.29 < Ti0.33V1.27Cr1.4H1.13. The additives of Zr7Ni10 have no significant effect on the hydrogen self-diffusion parameters in Ti0.5V1.9Cr0.6H5.03, but slow down the hydrogen diffusion in Ti0.33V1.27Cr1.4H1.13. The obtained results are partially in agreement with recent research of proton relaxation studies [6]. However, diffusion measurements results in higher activation energy values for hydrogen motion, especially in hydrides with bcc structure.

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Acknowledgments [20]

This work was supported by Saint-Petersburg State University (Pulse NMR spectroscopy of anisotropic and nanostructured materials; No. 11.0.63.2010). The authors also thank the GermanRussian Interdisciplinary Science Center (G-RISC), which sponsored the stay of Anna Vyvodtceva at TU Darmstadt. References [1] E. Akiba, H. Iba, Hydrogen absorption by Laves phase related BCC solid solution, Intermetallics 6 (1998) 461–470. [2] A. Kamegawa, T. Tamura, H. Takamura, M. Okada, Protium absorption– desorption properties of Ti–Cr–Mo bcc solid solution alloys, J. Alloys Comp. 356–357 (2003) 447–451. [3] S. Miraglia, D. Fruchart, N. Skryabina, M. Shelyapina, B. Ouladiaf, E.K. Hlil, P. de Rango, J. Charbonnier, Hydrogen-induced structural transformation in

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