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Hydrogenation of amorphous and nanocrystalline Mg-based alloys
Journal of Alloys and Compounds 287 (1999) 243–250
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Hydrogenation of amorphous and nanocrystalline Mg-based alloys ¨ Tony Spassov* ,1 , Uwe Koster Department of Chemical Engineering, University of Dortmund, Emil-Figge-Strasse 66, D-44221 Dortmund, Germany Received 8 January 1999
Abstract Amorphous and nanocrystalline Mg-based alloys were produced by rapid quenching (melt-spinning) and their hydrogenation properties were studied. The thermal stability and crystallization behaviour of the as-quenched and hydrogenated alloys were investigated as well. It was found that the as-cast nanocrystalline / amorphous Mg 75 Ni 20 Mm 5 (Mm5Ce, La-rich mischmetal) alloy possesses the best hydriding properties (hydrogenation kinetics and hydrogen absorption capacity) with maximum hydrogen capacity of 4.0 wt.% H. The difference in the hydriding properties of the as-quenched nanocrystalline and completely crystallized (with grain size in the range of 100–150 nm) Mg 75 Ni 20 Mm 5 alloys was found to be insignificant. The amorphous and crystallized (after heat treatment) Mg 87 Ni 12 Y 1 alloys show slower hydriding kinetics and lower hydrogen absorption capacity compared to the other Mg-based alloys studied. The amorphous Mg 87 Ni 12 Y 1 exhibits faster initial hydrogenation kinetics than the partially and fully crystallized alloys with the same composition, due to faster hydrogen diffusion in the amorphous phase, but the hydrogen absorption capacity of all Mg 87 Ni 12 Y 1 alloys having different microstructure is practically the same. The crystallization of melt-spun Mg 75 Ni 20 Mm 5 and Mg 87 Ni 12 Y 1 alloys is a two-step process. The primary crystallization of a-Mg (for Mg 87 Ni 12 Y 1 ) takes place at about 1608C, followed by transformation of the residual amorphous matrix into a metastable phase, assigned as fcc Mg 6 Ni (isomorphic with fcc Mg 6 Pd (a o 52.0108 nm)). This intermediate metastable phase decomposes during further annealing (at about 3008C) into the equilibrium phases Mg 2 Ni and Mg. The product of the first crystallization reaction for the as-cast Mg 75 Ni 20 Mm 5 alloy is Mg 2 Ni, most probably realized by growth of the quenched-in Mg 2 Ni nanocrystals. The second reaction corresponds to transformation of the residual amorphous matrix into Mg 17 Mm 2 . 1999 Elsevier Science S.A. All rights reserved. Keywords: Magnesium alloys; Rapid quenching; Nanocrystalline; Amorphous alloy; Hydrogen storage; Crystallization
1. Introduction Mg-based hydrogen storage alloys continue to attract the interest of many investigators, in spite of the difficulties for their practical application due to their high hydrogen desorption temperature and relatively slow kinetics of absorption / desorption. Nanocrystalline alloys as well as alloys containing nanocrystalline and amorphous phases exhibit much faster kinetics of hydrogen absorption and desorption and lower temperature of hydriding / dehydriding compared to the conventional crystalline materials with the same composition. The large number of interfaces and grain boundaries available in the nanocrystalline materials promotes the absorption of hydrogen providing easy pathways for hydrogen diffusion. Nanocrystalline Mg 2 Ni-based alloys produced by ball *Corresponding author. 1 On leave from: Department of Chemistry, University of Sofia, 1126Sofia, Bulgaria.
milling [1–4] and by rapid quenching [5] are known to exhibit higher H-absorption capacity (about 3–3.5 wt.% H) and faster kinetics of hydriding / dehydriding than crystalline Mg 2 Ni. Amorphous (or partially amorphous) and nanocrystalline Mg-based alloys show, however, relatively low thermal stability (crystallization and coarsening temperatures in the range of 150–2508C). That is, the as-cast amorphous / nanocrystalline microstructures undergo some phase transformations and grain growth at temperatures below the H-desorption temperature and therefore are not suitable for repeating hydrogen absorption / desorption. On the other hand, by crystallization of amorphous or partially amorphous precursors nanocrystalline microstructures favourable for hydrogen storage can be produced. In this connection it is important to study the crystallization of the as-quenched alloys as well as the thermal stability of nanocrystalline alloys in order to obtain stable microstructures for H-storage. In our previous studies [5,6] the hydrogenation characteristics of nanocrystalline melt-spun Mg 63 (Ni,Y) 37 and
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00035-3
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Mg 82 (Ni,Y) 18 alloys were investigated. The observed very fast initial hydrogen absorption and high hydrogen capacity, especially for the Mg 63 (Ni,Y) 37 , were assumed to be due to the presence of a large amount of an amorphous (disordered) phase between the nanocrystals in the asquenched nanocrystalline material. During heating the ascast amorphous / nanocrystalline Mg 63 (Ni,Y) 37 alloy crystallizes completely (at about 1908C) by three dimensional growth of a constant number of nanocrystals, already existing in the as-quenched material. Hydrogenation of the as-cast alloy causes a change in the crystallization mechanism and leads to nanocrystalline microstructure after complete crystallization of the alloy. In the present study we focused our attention on the hydrogenation behaviour of two amorphous and nanocrystalline Mg-based alloys produced by melt-spinning. The thermal stability and crystallization of the as-cast alloys were also studied in order to obtain stable microstructures favourable for hydrogen storage. The hydrogenation characteristics (H-absorption capacity, hydrogen kinetics) of the as-quenched and heat treated alloys (partially and fully crystallized) were compared as well.
2. Experimental methods The pre-alloys were prepared by induction melting from high purity Mg, Ni, Y and Mm (61 at.% Ce, 38 at.% La and 1 at.% Sm) in a vacuum induction furnace under the protection of argon with a pressure of 500 mbar. From the master alloy ingots ribbons were produced by melt-spinning with different surface velocities of the quenching disc in a helium atmosphere of 400 mbar. The microstructure of the melt-spun materials as well as the crystalline phases in the heat treated and hydrogenated alloys were characterized by transmission electron microscopy (Philips CM200 operated at 200 kV) and by X-ray (Cu K a ) and electron diffraction. The chemical composition of the alloys as well as the surface condition of the ribbons before and after hydrogen charging were examined by SEM with an energy dispersive spectrometry. Thermal stability and crystallization of the as-quenched and hydrogen charged alloys were studied by means of DSC (TA-Instruments, DSC 910). Hydrogen charging was carried out electrolytically under galvanostatic conditions in 0.5 M KOH at 258C and a current density of 10 A / m 2 . The hydrogen content was measured by a microbalance with an accuracy of 1mg.
3. Results and discussion
3.1. Hydrogenation of the as-quenched alloys Figs. 1 and 2 present the X-ray diffraction patterns and TEM (micrographs and electron diffraction patterns) of the
Fig. 1. X-ray diffraction patterns (Cu K a ) of (a) as-quenched and (b) hydrogenated Mg 87 Ni 12 Y 1 and (c) as-quenched Mg 75 Ni 20 Mm 5 .
as-quenched Mg 87 Ni 12 Y 1 and Mg 75 Ni 20 Mm 5 (Mm5 mischmetal containing Ce, La and Sm) alloys. Both meltspun alloys are X-ray amorphous (Fig. 1 curves a and c). The electron diffraction of the as-quenched Mg 75 Ni 20 Mm 5 alloy reveals a nanocrystalline microstructure with some evidence for the existence of amorphous phase between the nanocrystals, whereas the Mg 87 Ni 12 Y 1 alloy is fully amorphous. From the electron diffraction pattern of the as-cast Mg 75 Ni 20 Mm 5 alloy the hexagonal Mg 2 Ni was detected, i.e. the as-quenched alloy contains Mg 2 Ni nanocrystals obviously embedded in an amorphous matrix. The rate of hydrogen absorption at room temperature for both as-cast alloys studied (Mg 75 Ni 20 Mm 5 and Mg 87 Ni 12 Y 1 ) is plotted in Fig. 3 together with the hydrogenation kinetic curve for Mg 63 Ni 30 Y 7 , obtained in our previous study at similar charging conditions [5]. All kinetic curves of hydriding show an initial fast hydrogen absorption stage after which the hydrogen content is saturated at longer hydrogenation times, reaching about 2.0 wt.% for the Mg 87 Ni 12 Y 1 and 4.0 wt.% for Mg 75 Ni 20 Mm 5 alloy. The course of the kinetic curves is similar to that observed by hydrogenation from the gas phase, although direct comparison between the kinetics of hydrogenation from hydrogen gas phase and by electrolytical H-charging is not correct, because of big differences in the hydrogen pressure realized by both charging techniques. Here it has to be mentioned, that whereas for the amorphous Mg 87 Ni 12 Y 1 alloy the hydrogen amount absorbed depends strongly on the charging current density i (in the range of i53–20 A / m 2 ) for the nanocrystalline / amorphous Mg 75 Ni 20 Mm 5 and Mg 63 Ni 30 Y 7 as-cast alloys this dependence is not so strongly pronounced. The hydrogenation kinetics and storage capacity of all as-cast amorphous and nanocrystalline / amorphous Mgbased alloys studied are superior to those of conventional polycrystalline materials with similar composition. The improved hydrogenation characteristics can be explained with the enhanced hydrogen diffusivity and solubility in amorphous and nanocrystalline microstructures, associated with the wide energy distribution of the available sites for hydrogen in the glassy structure as well as in the disordered (amorphous) nanograin boundary regions in the
¨ / Journal of Alloys and Compounds 287 (1999) 243 – 250 T. Spassov, U. Koster
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Fig. 2. TEM micrographs and electron diffraction of as-quenched (a) Mg 75 Ni 20 Mm 5 (bright field image) and (b) Mg 87 Ni 12 Y 1 (dark field image).
nanocrystalline microstructures. In the case of the nano/ amorphous Mg 75 Ni 20 Mm 5 and Mg 63 Ni 30 Y 7 composites the amorphous phase around the nanocrystals leads to an easier access of hydrogen to the nanograins, avoiding the long-range diffusion of hydrogen through an already formed hydride, which is often the slowest stage of absorption. Among the as-quenched Mg-based alloys studied, the nanocrystalline / amorphous Mg 75 Ni 20 Mm 5 shows the best hydrogenation properties: fastest hydriding kinetics (about 3 wt.% H in 10 min) and highest Habsorption capacity (about 4 wt.% H in 60 min). As-cast Mg 87 Ni 12 Y 1 possesses initial hydrogen charging kinetics similar to the Mg 63 Ni 30 Y 7 alloy and slower than that of
Mg 75 Ni 20 Mm 5 . The observed essential differences in the hydrogenation of the melt-spun amorphous and nanocrystalline Mg-based alloys studied most probably have to be associated with the composition of the alloys as well as with the differences in their microstructure (due to the different quenching rates). It is necessary to mention the fact that the alloy with the lowest H-absorption capacity is fully amorphous in the as-cast state, whereas the other two alloys contain nanocrystalline phase with a significant amount of amorphous phase between the nanocrystals. Unfortunately we were not able to produce (by melt spinning) fine nanocrystalline Mg 87 Ni 12 Y 1 , in order to compare the hydrogenation characteristics of alloys with different compositions, but in a similar microstructural state. Another possible reason for the lower H-capacity of the Mg 87 Ni 12 Y 1 could be the lower yttrium content (1 at.%) in this alloy as compared to the other two alloys. Yttrium and Mm (Ce,La-rich mischmetal) are known to work as catalysts for the hydrogenation of Mg. Dariet et al. [7] observed an improvement in the hydrogenation kinetics of Mg–La alloys due to the catalytic effect of lanthanum.
3.2. Thermal stability and crystallization of the meltspun Mg75 Ni20 Mm5 and Mg87 Ni12 Y1 alloys
Fig. 3. Rate of hydrogen absorption for as-quenched Mg 87 Ni 12 Y 1 , Mg 63 Ni 30 Y 7 and Mg 75 Ni 20 Mm 5 (m, as-cast; ., crystallized) alloys. (H-charging conditions: 0.5 M KOH, 10 A / m 2 , 258C.)
Thermal stability and crystallization of the as-quenched amorphous and nanocrystalline / amorphous Mg-alloys were studied in order to investigate the possibilities for producing stable nanocrystalline microstructures with improved hydrogenation properties by crystallization of glassy precursor alloys or nanocrystalline materials con-
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taining large amounts of amorphous phase. On heating of both melt-spun Mg-based alloys (Mg 87 Ni 12 Y 1 and Mg 75 Ni 20 Mm 5 ) two crystallization DSC peaks were observed. For the Mg 87 Ni 12 Y 1 alloy, close to the eutectic composition, the crystallization peaks are distinctly separated (about 408C between the peaks), while for the Mg 75 Ni 20 Mm 5 alloy the exothermic DSC peaks are slightly overlapped. The temperatures related to the maximum transformation rates of the amorphous into crystalline state, T max1 and T max2 , as well as the enthalpies of the first and second crystallization reaction DH1 and DH2 at constant heating rate of 5 K / min are presented in Table 1. Although the total enthalpy of transformation (DH5DH1 1 DH2 ) for the Mg 75 Ni 20 Mm 5 alloy (associated with crystallization of the amorphous phase existing between the nanocrystals in the as-cast material) is smaller than that for the Mg 87 Ni 12 Y 1 alloy (associated with crystallization of the fully amorphous alloy), it is still relatively large which indicates a significant volume fraction of the amorphous phase. Consequently, the as-quenched Mg 75 Ni 20 Mm 5 alloy can be regarded as a composite consisting of nanocrystalline Mg 2 Ni grains embedded in an amorphous phase. These results are in agreement with the TEM observations and electron diffraction (Fig. 2) of the as-cast material. For the Mg 75 Ni 20 Mm 5 alloy the first transformation step is primary crystallization of Mg 2 Ni (Fig. 4a, curve d), most probably realized by growth of the quenched-in nanocrystals. During the second crystallization reaction of the Mg 75 Ni 20 Mm 5 alloy mainly hexagonal Mg 17 Mm 2 is formed (Fig. 4a, curve e and Fig. 5). For comparison, the glass transition T g and crystallization T x temperatures of melt-spun Mg 75 Ni 20 Nd 5 alloy, determined by DSC (with 40 K / min) are 450 K and 456 K, respectively [8], as the crystallization is also a two-step process. The microstructure of the completely crystallized Mg 75 Ni 20 Mm 5 alloy is shown in the TEM micrograph in Fig. 5. The Mg 2 Ni crystals formed during the first crystallization stage show a rod-like shape. A similar morphology was observed during crystallization of melt-spun Mg 82 Ni 18 [9]. The average crystal size is about 100–150 nm. Annealing up to 4008C of the Mg 75 Ni 20 Mm 5 alloy does not lead to changes in the phase composition as well as in the microstructure of the fully crystallized alloy. The crystallization of the as-quenched amorphous Mg 87 Ni 12 Y 1 starts with primary crystallization of a-Mg as the first step (T max1 51588C at 5 K / min). The X-ray diffractogram taken from the ribbon after the first crystallization reaction reveals stronger Mg (002) and (004)
Fig. 4. (a) XRD patterns of the products of the different crystallization stages of Mg 87 Ni 12 Y 1 (curves a–c) and Mg 75 Ni 20 Mm 5 (curves d and e) alloys. The corresponding annealing temperatures are as follows: (a) 1808C, (b) 2208C, (c) 4008C, (d) 1808C and (e) 2508C. (b) XRD patterns of the intermediate and final crystallization products of Mg 87 Ni 12 Y 1 . The following annealing temperatures are applied: (a) 2508C, (b) 3258C, (c) 3758C.
peaks, thus indicating a preferential orientation of the Mg crystallites with the (002) axis perpendicular to the ribbon surface, (Fig. 4a curve a). The (002) XRD peak is superimposed on a broad peak around 2u 5408 associated
Table 1 Temperatures corresponding to the maximum of the DSC crystallization peaks (T max1 and T max2 ), enthalpies (DH1 and DH2 ) and activation energies (Q 1 and Q 2 ) of the first and second crystallization reactions for the amorphous Mg 87 Ni 12 Y 1 alloy Alloy
T max1 (8C)
DH1 (J / g)
T max2 (8C)
DH2 (J / g)
Q 1 (kJ / mol)
Q 2 (kJ / mol)
Mg 75 Ni 20 Mm 5 Mg 87 Ni 12 Y 1
174 158
35 54
208 199
46 32
205615 295622
177613 180615
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Fig. 5. TEM and electron diffraction pattern of crystallized (a) Mg 87 Ni 12 Y 1 (2708C) and (b) Mg 75 Ni 20 Mm 5 (3008C).
with the glassy matrix. This preferential orientation of Mg was not observed in the diffractograms from powder samples (Fig. 4b, curve a) as well as from ribbons mechanically polished before the X-ray experiment. These results indicate that the oriented Mg crystals are located only in a thin (several mm) layer at the surface of the ribbon. The existence of quenched-in nucleation centres or extremely fine Mg nanocrystals at the surface of the as-cast ribbon, which grow during the heat treatment, is probably the reason for the preferred orientation observed. Short-
time hydrogenation of the ribbons also leads to the disappearance of the intensive Mg (002) peaks, due to volume expansion of the ribbon during hydrogen absorption. The residual amorphous phase (after the first crystallization step) transforms mainly into an intermediate metastable phase, which is associated with the second exothermal DSC peak at 1998C. The structure of the intermediate phase was found to be identical with the fcc Mg 6 Pd phase (a 0 52.0108 nm), as the lattice constant determined is only very slightly decreased (a 0 52.009 nm)
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(Figs. 4 and 5), probably due to the small difference in the atomic sizes of Ni and Pd. This new intermediate metastable phase, isomorphic with the fcc Mg 6 Pd, is obviously fcc Mg 6 Ni. The experimental interlattice distances, d, Miller indices (hkl) and observed intensities (I) of the XRD peaks of the metastable fcc Mg 6 Ni phase are presented in Table 2. The existence of an intermediate metastable phase during crystallization of rapidly quenched Mg-rich Mg–Ni amorphous alloys was mentioned earlier by Sommer et al. [10]. Only recently Ong et al. [9] have also reported about the existence of an unknown phase during crystallization of rapidly solidified amorphous ˚ Other Mg 82 Ni 18 with an X-ray reflection at d52.37 A. data about composition and structure of this metastable phase are not available in the literature. Besides the two main crystalline phases (Mg1Mg 6 Ni) traces of Mg 2 Ni can be also detected after annealing in the temperature range 200–3008C. The thermal stability of the intermediate Mg 6 Ni phase is relatively high. Its transformation (decomposition) into the equilibrium hexagonal Mg 2 Ni and a-Mg was observed to take place in a wide temperature range (300–3508C), as the exothermic peak associated with this transformation is rather small. Fig. 4b shows X-ray diffraction patterns taken from the Mg 87 Ni 12 Y 1 alloy annealed for a short time at three different temperatures (250, 325 and 3758C) before, during and after the DSC exothermal effect. The broad DSC peak and extremely low enthalpy of this transformation imply very slow kinetics of this reaction. After heat treatment at temperatures higher than 350–3608C only the stable a-Mg and Mg 2 Ni phases can be detected (Fig. 4a, curve c and Fig. 4b, curve c). Yttrium, which is about 1 at.%, probably replaces some Ni in the lattice of Mg 2 Ni. The activation energies of the first and second crystallization reactions for both Mg 75 Ni 20 Mm 5 and Mg 87 Ni 12 Y 1 alloys were estimated according to the Kissinger method [11], and are presented in Table 1. Whereas for the Mg 75 Ni 20 Mm 5 alloy both activation energies 205615 and 177613 kJ / mol are practically equal (in the limits of the error), the Mg 87 Ni 12 Y 1 alloy exhibits a Table 2 Experimental interlattice distances d (in nm), Miller indices (hkl) and intensities I of the XRD peaks of the intermediate metastable phase, assigned as fcc Mg 6 Ni (a 0 52.009 nm) Line number
(hkl)
d (nm)
I /I0
1 2 3 4 5 6 7 8 9 10
(220) (222) (400) (422) (820), (644) (822), (660) (751), (555) (664) (844) (864)
0.7101 0.5800 0.5027 0.4102 0.2436 0.2367 0.2321 0.2141 0.2050 1.866
9 10 17 30 – 100 – 13 10 7
distinctly larger activation energy corresponding to the primary crystallization of a-Mg, 295622 kJ / mol, than to the second crystallization reaction (Mg 6 Ni-formation). This definitely higher value for the activation energy (295 kJ / mol) obviously has to be attributed to a process of nucleation and further diffusion controlled growth of a-Mg in the amorphous matrix. This result is in agreement with our electron diffraction study, that the as-cast Mg 87 Ni 12 Y 1 is fully amorphous, while the as-cast Mg 75 Ni 20 Mm 5 is nanocrystalline / amorphous. For Mg 90 Ni 10 amorphous alloy Sommer et al. [10] determined an activation energy of 231 kJ / mol for both crystallization steps (formation of a-Mg and Mg 2 Ni). Activation energies in the range of 163–240 kJ / mol for crystallization of Mg-based amorphous alloys were also reported [12]. After hydrogen charging of the as-cast Mg 87 Ni 12 Y 1 alloy, the XRD pattern (Fig. 1) contains only broad and diffuse maxima similar to the as-quenched H-free sample. Hydrogenation of the as-quenched amorphous Mg 87 Ni 12 Y 1 alloys leads only to a slight decrease of the thermal stability, but influences strongly the second crystallization reaction, which takes place at about 308C lower temperature as compared to the unhydrogenated alloy. The preliminary hydrogenated alloy crystallizes (during annealing in the temperature range 200–2508C) mainly into the stable Mg and Mg 2 Ni phases. The intermediate Mg 6 Ni phase is also present, but in a much smaller degree. The influence of hydrogen on the crystallization of Mg 87 Ni 12 Y 1 can be due to the stronger chemical interaction between the magnesium and hydrogen atoms in the amorphous phase, which would lead during annealing to formation of crystalline phases with lower Mg content. Hydrogen-enhanced host diffusion can be also a reason for the observed decrease in the alloy thermal stability. The microstructure of the fully crystallized hydrogen charged Mg 87 Ni 12 Y 1 alloy is similar to that of the completely crystallized hydrogen free alloy. Desorption of hydrogen was observed by DSC to proceed at about 2508C, as the endothermic peak is rather broad.
3.3. Hydriding properties of crystallized Mg75 Ni20 Mm5 and Mg87 Ni12 Y1 alloys As already mentioned one of the main reasons to study the thermal stability and crystallization of the melt-spun Mg-based alloys was to obtain stable microstructures (nano- or microcrystalline) with improved hydrogen storage properties. Therefore partially and fully crystalline microstructures were produced (by crystallization at different annealing conditions) and their hydrogenation properties were studied and compared with those of the asquenched amorphous and nano- / amorphous materials with the same composition. Fig. 6 shows the rate of hydrogen absorption for the Mg 87 Ni 12 Y 1 alloy at three different microstructural states: amorphous, partially crystalline (aMg nanocrystals embedded in amorphous matrix) and
¨ / Journal of Alloys and Compounds 287 (1999) 243 – 250 T. Spassov, U. Koster
Fig. 6. Kinetics of hydrogenation for amorphous, partially and fully crystallized Mg 87 Ni 12 Y 1 .
completely crystallized (a-Mg1Mg 6 Ni). As already shown the amorphous alloy exhibits very fast initial hydrogenation kinetics due to the rapid diffusion of hydrogen. The fully crystallized Mg 87 Ni 12 Y 1 alloy shows a lower initial hydrogenation rate as compared to the amorphous alloy, but at longer hydriding times the amount of hydrogen absorbed in the amorphous and crystalline (nanocrystalline) alloys is practically the same. This result also supports the suggestion that the amorphous (disordered) phase is mainly responsible for the fast initial hydriding kinetics. Although its initial hydrogenation kinetics is slower, the crystalline alloy attains the same hydrogen absorption capacity (about 2 wt.% H) as the amorphous alloy with the same composition, due to the fine two-phase microstructure (crystal size about 100 nm), favourable for hydrogen storage. It is interesting to note, that once the metastable Mg 6 Ni phase is formed, it does not decompose to the equilibrium phases during further hydrogen charging. The partially crystalline alloy shows hydrogenation kinetics only slightly better than those of the fully crystallized alloy. The hydriding kinetics of the completely crystallized Mg 75 Ni 20 Mm 5 alloy does not show essential differences compared to the as-cast nano/ amorphous alloy (Fig. 3). After about 30 min of Hcharging nearly the same hydrogen contents are attained. The crystalline (with average grain sizes of about 100– 150 nm) Mg 75 Ni 20 Mm 5 exhibits much better hydrogenation properties than the crystallized Mg 87 Ni 12 Y 1 (with grain sizes about 100 nm) as well as better than the nanocrystalline Mg 63 (Ni,Y) 37 [5]. Although systematic quantitative investigation of the effect of yttrium and Mm (Ce,La-rich mischmetal) was not carried out in the present study, it seems that the large difference in the hydrogenation properties of the crystallized Mg 75 Ni 20 Mm 5 and Mg 87 Ni 12 Y 1 alloys cannot be attributed only to the different Y or Mm content. The observed differences are most probably associated with the differences in the phase composition. Obviously the phase proportion of Mg 2 Ni to
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Mg 17 Mm 2 (Mg 17 Mm 2 undergoes a disproportionation to MmH x and Mg on hydrogenation / dehydrogenation [7]) for the Mg 75 Ni 20 Mm 5 alloy is close to the optimum with respect to the hydriding properties. Lower (in Mg 87 Ni 12 Y 1 ) as well as much higher (in Mg 63 Ni 30 Y 7 ) volume fractions of Mg 2 Ni than in the Mg 75 Ni 20 Mm 5 alloy make the hydrogenation kinetics worse. Liang et al. [13] showed the hydrogen storage properties of different Mg–Ni and Mg–LaNi 5 mechanically milled composites at room temperature and found a better hydrogen absorption kinetics of Mg–50 wt.% LaNi 5 (corresponding to alloy composition Mg 75 Ni 21 La 4 ), 2.5 wt.% hydrogen at RT in 500 s under 1.5 MPa hydrogen pressure, than those of Mg–30 wt.% LaNi 5 and Mg–30 wt.% Ni. The authors proposed that there should be an optimum proportion of Mg to Mg 2 Ni and lanthanum hydride. The optimum capacity for the Mg–50 wt.% LaNi 5 was found to be 4.1 wt.% at intermediate temperatures (523–573 K). An increase in the absorption–desorption characteristics (kinetics and capacity) of Mg–x wt.% LaNi 5 (x510, 20, 30) mechanically alloyed composites with the addition of LaNi 5 has been also recently reported by Terzieva et al. [14], as the best achievements belong to the composite with the maximum LaNi 5 content (corresponding to Mg 87 Ni 11 La 2 ). A recent study on the hydrogen storage properties of Mg 17 La 2 –x wt.% LaNi 5 (x50, 10, 20, 30, 40, 50, 60) mechanically milled composites show also the best overall hydriding kinetics for the composite with 40 wt.% LaNi 5 [15,16]. Studying the hydrogen absorption / desorption properties of different multi-component Mgbased alloys Au et al. [17] found the best kinetic properties for Mg 83.3 Ni 6.6 Cu 9.5 MI 0.6 (MI5La-rich mischmetal). The authors suggest that the interface between the Mg and Mg 2 (Cu,Ni) phases and the microcrack developed in the latter play the main role in improving the hydriding and dehydriding kinetics. Our results obtained for the meltspun Mg-based alloys show a very good agreement with all these findings. Another important factor, which has significant effect on the hydrogen storage properties is the microstructure of the alloy. The present study on the hydrogenation kinetics and capacity of nanocrystalline Mg-based alloys confirms that nanocrystalline (and microcrystalline) structures improve the absorption kinetics, especially at room temperature [5,13]. The differences in the hydrogenation characteristics between the as-cast and crystallized Mg-based alloys found in this work are not very large due to the fine microstructure (i.e. high amount of phase and grain boundaries) of the crystallized alloys, favourable for hydrogen absorption.
4. Conclusions The hydrogenation properties of rapidly quenched (meltspun) amorphous and nanocrystalline as well as of crys-
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tallized Mg-based alloys were studied and compared with the results of our previous works on similar alloys [5,6]. The as-cast nanocrystalline Mg 75 Ni 20 Mm 5 alloy (containing Mg 2 Ni nanocrystals embedded into amorphous matrix) shows the best hydrogenation properties (hydriding kinetics — 0.3 wt.% H / min and absorption capacity — 4 wt.% H) among the melt-spun Mg-based alloys studied. The fully amorphous Mg 87 Ni 12 Y 1 alloy exhibits lower hydrogen absorption capacity as compared with the other as-cast nanocrystalline / amorphous alloys, but it shows stronger charging current density dependence of the amount of hydrogen absorbed, which might be due to its amorphous microstructure. The thermal stability and crystallization of the as-cast alloys were also studied and the hydrogenation properties of the crystallized alloys were compared with those of the as-quenched materials. The crystallization of the Mg 75 Ni 20 Mm 5 alloy is a two-step process, starting with primary crystallization of Mg 2 Ni, probably realized by growth of quenched-in Mg 2 Ni nanocrystals (2–3 nm). The crystallization of the Mg 87 Ni 12 Y 1 alloy takes place through an intermediate metastable phase, isomorphic with the fcc Mg 6 Pd. The fully crystallized Mg 75 Ni 20 Mm 5 shows kinetics of hydrogen absorption and H-capacity comparable to those of the as-cast nanocrystalline / amorphous alloy. The partially (after the first crystallization reaction) and fully crystallized Mg 87 Ni 12 Y 1 alloys exhibit slower initial hydrogen absorption kinetics than the amorphous alloy, but the hydrogen amount absorbed at longer hydrogenation times is practically the same. The observed differences in the hydrogenation characteristics of the asquenched nanocrystalline / amorphous and the crystallized Mg-based alloys are relatively small due to the fine microstructure of the crystallized alloys, favourable for hydrogen absorption. The crystallized Mg 75 Ni 20 Mm 5 alloy exhibits much better hydrogen absorption characteristics than the crystallized Mg 87 Ni 12 Y 1 , which is most probably associated with the optimal phase proportion in the former alloy.
Acknowledgements One of the authors (T. Spassov) is very grateful to the Alexander von Humboldt Foundation (Germany) for financial support.
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Hydrogenation of amorphous and nanocrystalline Mg-based alloys ¨ Tony Spassov* ,1 , Uwe Koster Department of Chemical Engineering, University of Dortmund, Emil-Figge-Strasse 66, D-44221 Dortmund, Germany Received 8 January 1999
Abstract Amorphous and nanocrystalline Mg-based alloys were produced by rapid quenching (melt-spinning) and their hydrogenation properties were studied. The thermal stability and crystallization behaviour of the as-quenched and hydrogenated alloys were investigated as well. It was found that the as-cast nanocrystalline / amorphous Mg 75 Ni 20 Mm 5 (Mm5Ce, La-rich mischmetal) alloy possesses the best hydriding properties (hydrogenation kinetics and hydrogen absorption capacity) with maximum hydrogen capacity of 4.0 wt.% H. The difference in the hydriding properties of the as-quenched nanocrystalline and completely crystallized (with grain size in the range of 100–150 nm) Mg 75 Ni 20 Mm 5 alloys was found to be insignificant. The amorphous and crystallized (after heat treatment) Mg 87 Ni 12 Y 1 alloys show slower hydriding kinetics and lower hydrogen absorption capacity compared to the other Mg-based alloys studied. The amorphous Mg 87 Ni 12 Y 1 exhibits faster initial hydrogenation kinetics than the partially and fully crystallized alloys with the same composition, due to faster hydrogen diffusion in the amorphous phase, but the hydrogen absorption capacity of all Mg 87 Ni 12 Y 1 alloys having different microstructure is practically the same. The crystallization of melt-spun Mg 75 Ni 20 Mm 5 and Mg 87 Ni 12 Y 1 alloys is a two-step process. The primary crystallization of a-Mg (for Mg 87 Ni 12 Y 1 ) takes place at about 1608C, followed by transformation of the residual amorphous matrix into a metastable phase, assigned as fcc Mg 6 Ni (isomorphic with fcc Mg 6 Pd (a o 52.0108 nm)). This intermediate metastable phase decomposes during further annealing (at about 3008C) into the equilibrium phases Mg 2 Ni and Mg. The product of the first crystallization reaction for the as-cast Mg 75 Ni 20 Mm 5 alloy is Mg 2 Ni, most probably realized by growth of the quenched-in Mg 2 Ni nanocrystals. The second reaction corresponds to transformation of the residual amorphous matrix into Mg 17 Mm 2 . 1999 Elsevier Science S.A. All rights reserved. Keywords: Magnesium alloys; Rapid quenching; Nanocrystalline; Amorphous alloy; Hydrogen storage; Crystallization
1. Introduction Mg-based hydrogen storage alloys continue to attract the interest of many investigators, in spite of the difficulties for their practical application due to their high hydrogen desorption temperature and relatively slow kinetics of absorption / desorption. Nanocrystalline alloys as well as alloys containing nanocrystalline and amorphous phases exhibit much faster kinetics of hydrogen absorption and desorption and lower temperature of hydriding / dehydriding compared to the conventional crystalline materials with the same composition. The large number of interfaces and grain boundaries available in the nanocrystalline materials promotes the absorption of hydrogen providing easy pathways for hydrogen diffusion. Nanocrystalline Mg 2 Ni-based alloys produced by ball *Corresponding author. 1 On leave from: Department of Chemistry, University of Sofia, 1126Sofia, Bulgaria.
milling [1–4] and by rapid quenching [5] are known to exhibit higher H-absorption capacity (about 3–3.5 wt.% H) and faster kinetics of hydriding / dehydriding than crystalline Mg 2 Ni. Amorphous (or partially amorphous) and nanocrystalline Mg-based alloys show, however, relatively low thermal stability (crystallization and coarsening temperatures in the range of 150–2508C). That is, the as-cast amorphous / nanocrystalline microstructures undergo some phase transformations and grain growth at temperatures below the H-desorption temperature and therefore are not suitable for repeating hydrogen absorption / desorption. On the other hand, by crystallization of amorphous or partially amorphous precursors nanocrystalline microstructures favourable for hydrogen storage can be produced. In this connection it is important to study the crystallization of the as-quenched alloys as well as the thermal stability of nanocrystalline alloys in order to obtain stable microstructures for H-storage. In our previous studies [5,6] the hydrogenation characteristics of nanocrystalline melt-spun Mg 63 (Ni,Y) 37 and
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00035-3
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Mg 82 (Ni,Y) 18 alloys were investigated. The observed very fast initial hydrogen absorption and high hydrogen capacity, especially for the Mg 63 (Ni,Y) 37 , were assumed to be due to the presence of a large amount of an amorphous (disordered) phase between the nanocrystals in the asquenched nanocrystalline material. During heating the ascast amorphous / nanocrystalline Mg 63 (Ni,Y) 37 alloy crystallizes completely (at about 1908C) by three dimensional growth of a constant number of nanocrystals, already existing in the as-quenched material. Hydrogenation of the as-cast alloy causes a change in the crystallization mechanism and leads to nanocrystalline microstructure after complete crystallization of the alloy. In the present study we focused our attention on the hydrogenation behaviour of two amorphous and nanocrystalline Mg-based alloys produced by melt-spinning. The thermal stability and crystallization of the as-cast alloys were also studied in order to obtain stable microstructures favourable for hydrogen storage. The hydrogenation characteristics (H-absorption capacity, hydrogen kinetics) of the as-quenched and heat treated alloys (partially and fully crystallized) were compared as well.
2. Experimental methods The pre-alloys were prepared by induction melting from high purity Mg, Ni, Y and Mm (61 at.% Ce, 38 at.% La and 1 at.% Sm) in a vacuum induction furnace under the protection of argon with a pressure of 500 mbar. From the master alloy ingots ribbons were produced by melt-spinning with different surface velocities of the quenching disc in a helium atmosphere of 400 mbar. The microstructure of the melt-spun materials as well as the crystalline phases in the heat treated and hydrogenated alloys were characterized by transmission electron microscopy (Philips CM200 operated at 200 kV) and by X-ray (Cu K a ) and electron diffraction. The chemical composition of the alloys as well as the surface condition of the ribbons before and after hydrogen charging were examined by SEM with an energy dispersive spectrometry. Thermal stability and crystallization of the as-quenched and hydrogen charged alloys were studied by means of DSC (TA-Instruments, DSC 910). Hydrogen charging was carried out electrolytically under galvanostatic conditions in 0.5 M KOH at 258C and a current density of 10 A / m 2 . The hydrogen content was measured by a microbalance with an accuracy of 1mg.
3. Results and discussion
3.1. Hydrogenation of the as-quenched alloys Figs. 1 and 2 present the X-ray diffraction patterns and TEM (micrographs and electron diffraction patterns) of the
Fig. 1. X-ray diffraction patterns (Cu K a ) of (a) as-quenched and (b) hydrogenated Mg 87 Ni 12 Y 1 and (c) as-quenched Mg 75 Ni 20 Mm 5 .
as-quenched Mg 87 Ni 12 Y 1 and Mg 75 Ni 20 Mm 5 (Mm5 mischmetal containing Ce, La and Sm) alloys. Both meltspun alloys are X-ray amorphous (Fig. 1 curves a and c). The electron diffraction of the as-quenched Mg 75 Ni 20 Mm 5 alloy reveals a nanocrystalline microstructure with some evidence for the existence of amorphous phase between the nanocrystals, whereas the Mg 87 Ni 12 Y 1 alloy is fully amorphous. From the electron diffraction pattern of the as-cast Mg 75 Ni 20 Mm 5 alloy the hexagonal Mg 2 Ni was detected, i.e. the as-quenched alloy contains Mg 2 Ni nanocrystals obviously embedded in an amorphous matrix. The rate of hydrogen absorption at room temperature for both as-cast alloys studied (Mg 75 Ni 20 Mm 5 and Mg 87 Ni 12 Y 1 ) is plotted in Fig. 3 together with the hydrogenation kinetic curve for Mg 63 Ni 30 Y 7 , obtained in our previous study at similar charging conditions [5]. All kinetic curves of hydriding show an initial fast hydrogen absorption stage after which the hydrogen content is saturated at longer hydrogenation times, reaching about 2.0 wt.% for the Mg 87 Ni 12 Y 1 and 4.0 wt.% for Mg 75 Ni 20 Mm 5 alloy. The course of the kinetic curves is similar to that observed by hydrogenation from the gas phase, although direct comparison between the kinetics of hydrogenation from hydrogen gas phase and by electrolytical H-charging is not correct, because of big differences in the hydrogen pressure realized by both charging techniques. Here it has to be mentioned, that whereas for the amorphous Mg 87 Ni 12 Y 1 alloy the hydrogen amount absorbed depends strongly on the charging current density i (in the range of i53–20 A / m 2 ) for the nanocrystalline / amorphous Mg 75 Ni 20 Mm 5 and Mg 63 Ni 30 Y 7 as-cast alloys this dependence is not so strongly pronounced. The hydrogenation kinetics and storage capacity of all as-cast amorphous and nanocrystalline / amorphous Mgbased alloys studied are superior to those of conventional polycrystalline materials with similar composition. The improved hydrogenation characteristics can be explained with the enhanced hydrogen diffusivity and solubility in amorphous and nanocrystalline microstructures, associated with the wide energy distribution of the available sites for hydrogen in the glassy structure as well as in the disordered (amorphous) nanograin boundary regions in the
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Fig. 2. TEM micrographs and electron diffraction of as-quenched (a) Mg 75 Ni 20 Mm 5 (bright field image) and (b) Mg 87 Ni 12 Y 1 (dark field image).
nanocrystalline microstructures. In the case of the nano/ amorphous Mg 75 Ni 20 Mm 5 and Mg 63 Ni 30 Y 7 composites the amorphous phase around the nanocrystals leads to an easier access of hydrogen to the nanograins, avoiding the long-range diffusion of hydrogen through an already formed hydride, which is often the slowest stage of absorption. Among the as-quenched Mg-based alloys studied, the nanocrystalline / amorphous Mg 75 Ni 20 Mm 5 shows the best hydrogenation properties: fastest hydriding kinetics (about 3 wt.% H in 10 min) and highest Habsorption capacity (about 4 wt.% H in 60 min). As-cast Mg 87 Ni 12 Y 1 possesses initial hydrogen charging kinetics similar to the Mg 63 Ni 30 Y 7 alloy and slower than that of
Mg 75 Ni 20 Mm 5 . The observed essential differences in the hydrogenation of the melt-spun amorphous and nanocrystalline Mg-based alloys studied most probably have to be associated with the composition of the alloys as well as with the differences in their microstructure (due to the different quenching rates). It is necessary to mention the fact that the alloy with the lowest H-absorption capacity is fully amorphous in the as-cast state, whereas the other two alloys contain nanocrystalline phase with a significant amount of amorphous phase between the nanocrystals. Unfortunately we were not able to produce (by melt spinning) fine nanocrystalline Mg 87 Ni 12 Y 1 , in order to compare the hydrogenation characteristics of alloys with different compositions, but in a similar microstructural state. Another possible reason for the lower H-capacity of the Mg 87 Ni 12 Y 1 could be the lower yttrium content (1 at.%) in this alloy as compared to the other two alloys. Yttrium and Mm (Ce,La-rich mischmetal) are known to work as catalysts for the hydrogenation of Mg. Dariet et al. [7] observed an improvement in the hydrogenation kinetics of Mg–La alloys due to the catalytic effect of lanthanum.
3.2. Thermal stability and crystallization of the meltspun Mg75 Ni20 Mm5 and Mg87 Ni12 Y1 alloys
Fig. 3. Rate of hydrogen absorption for as-quenched Mg 87 Ni 12 Y 1 , Mg 63 Ni 30 Y 7 and Mg 75 Ni 20 Mm 5 (m, as-cast; ., crystallized) alloys. (H-charging conditions: 0.5 M KOH, 10 A / m 2 , 258C.)
Thermal stability and crystallization of the as-quenched amorphous and nanocrystalline / amorphous Mg-alloys were studied in order to investigate the possibilities for producing stable nanocrystalline microstructures with improved hydrogenation properties by crystallization of glassy precursor alloys or nanocrystalline materials con-
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taining large amounts of amorphous phase. On heating of both melt-spun Mg-based alloys (Mg 87 Ni 12 Y 1 and Mg 75 Ni 20 Mm 5 ) two crystallization DSC peaks were observed. For the Mg 87 Ni 12 Y 1 alloy, close to the eutectic composition, the crystallization peaks are distinctly separated (about 408C between the peaks), while for the Mg 75 Ni 20 Mm 5 alloy the exothermic DSC peaks are slightly overlapped. The temperatures related to the maximum transformation rates of the amorphous into crystalline state, T max1 and T max2 , as well as the enthalpies of the first and second crystallization reaction DH1 and DH2 at constant heating rate of 5 K / min are presented in Table 1. Although the total enthalpy of transformation (DH5DH1 1 DH2 ) for the Mg 75 Ni 20 Mm 5 alloy (associated with crystallization of the amorphous phase existing between the nanocrystals in the as-cast material) is smaller than that for the Mg 87 Ni 12 Y 1 alloy (associated with crystallization of the fully amorphous alloy), it is still relatively large which indicates a significant volume fraction of the amorphous phase. Consequently, the as-quenched Mg 75 Ni 20 Mm 5 alloy can be regarded as a composite consisting of nanocrystalline Mg 2 Ni grains embedded in an amorphous phase. These results are in agreement with the TEM observations and electron diffraction (Fig. 2) of the as-cast material. For the Mg 75 Ni 20 Mm 5 alloy the first transformation step is primary crystallization of Mg 2 Ni (Fig. 4a, curve d), most probably realized by growth of the quenched-in nanocrystals. During the second crystallization reaction of the Mg 75 Ni 20 Mm 5 alloy mainly hexagonal Mg 17 Mm 2 is formed (Fig. 4a, curve e and Fig. 5). For comparison, the glass transition T g and crystallization T x temperatures of melt-spun Mg 75 Ni 20 Nd 5 alloy, determined by DSC (with 40 K / min) are 450 K and 456 K, respectively [8], as the crystallization is also a two-step process. The microstructure of the completely crystallized Mg 75 Ni 20 Mm 5 alloy is shown in the TEM micrograph in Fig. 5. The Mg 2 Ni crystals formed during the first crystallization stage show a rod-like shape. A similar morphology was observed during crystallization of melt-spun Mg 82 Ni 18 [9]. The average crystal size is about 100–150 nm. Annealing up to 4008C of the Mg 75 Ni 20 Mm 5 alloy does not lead to changes in the phase composition as well as in the microstructure of the fully crystallized alloy. The crystallization of the as-quenched amorphous Mg 87 Ni 12 Y 1 starts with primary crystallization of a-Mg as the first step (T max1 51588C at 5 K / min). The X-ray diffractogram taken from the ribbon after the first crystallization reaction reveals stronger Mg (002) and (004)
Fig. 4. (a) XRD patterns of the products of the different crystallization stages of Mg 87 Ni 12 Y 1 (curves a–c) and Mg 75 Ni 20 Mm 5 (curves d and e) alloys. The corresponding annealing temperatures are as follows: (a) 1808C, (b) 2208C, (c) 4008C, (d) 1808C and (e) 2508C. (b) XRD patterns of the intermediate and final crystallization products of Mg 87 Ni 12 Y 1 . The following annealing temperatures are applied: (a) 2508C, (b) 3258C, (c) 3758C.
peaks, thus indicating a preferential orientation of the Mg crystallites with the (002) axis perpendicular to the ribbon surface, (Fig. 4a curve a). The (002) XRD peak is superimposed on a broad peak around 2u 5408 associated
Table 1 Temperatures corresponding to the maximum of the DSC crystallization peaks (T max1 and T max2 ), enthalpies (DH1 and DH2 ) and activation energies (Q 1 and Q 2 ) of the first and second crystallization reactions for the amorphous Mg 87 Ni 12 Y 1 alloy Alloy
T max1 (8C)
DH1 (J / g)
T max2 (8C)
DH2 (J / g)
Q 1 (kJ / mol)
Q 2 (kJ / mol)
Mg 75 Ni 20 Mm 5 Mg 87 Ni 12 Y 1
174 158
35 54
208 199
46 32
205615 295622
177613 180615
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Fig. 5. TEM and electron diffraction pattern of crystallized (a) Mg 87 Ni 12 Y 1 (2708C) and (b) Mg 75 Ni 20 Mm 5 (3008C).
with the glassy matrix. This preferential orientation of Mg was not observed in the diffractograms from powder samples (Fig. 4b, curve a) as well as from ribbons mechanically polished before the X-ray experiment. These results indicate that the oriented Mg crystals are located only in a thin (several mm) layer at the surface of the ribbon. The existence of quenched-in nucleation centres or extremely fine Mg nanocrystals at the surface of the as-cast ribbon, which grow during the heat treatment, is probably the reason for the preferred orientation observed. Short-
time hydrogenation of the ribbons also leads to the disappearance of the intensive Mg (002) peaks, due to volume expansion of the ribbon during hydrogen absorption. The residual amorphous phase (after the first crystallization step) transforms mainly into an intermediate metastable phase, which is associated with the second exothermal DSC peak at 1998C. The structure of the intermediate phase was found to be identical with the fcc Mg 6 Pd phase (a 0 52.0108 nm), as the lattice constant determined is only very slightly decreased (a 0 52.009 nm)
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(Figs. 4 and 5), probably due to the small difference in the atomic sizes of Ni and Pd. This new intermediate metastable phase, isomorphic with the fcc Mg 6 Pd, is obviously fcc Mg 6 Ni. The experimental interlattice distances, d, Miller indices (hkl) and observed intensities (I) of the XRD peaks of the metastable fcc Mg 6 Ni phase are presented in Table 2. The existence of an intermediate metastable phase during crystallization of rapidly quenched Mg-rich Mg–Ni amorphous alloys was mentioned earlier by Sommer et al. [10]. Only recently Ong et al. [9] have also reported about the existence of an unknown phase during crystallization of rapidly solidified amorphous ˚ Other Mg 82 Ni 18 with an X-ray reflection at d52.37 A. data about composition and structure of this metastable phase are not available in the literature. Besides the two main crystalline phases (Mg1Mg 6 Ni) traces of Mg 2 Ni can be also detected after annealing in the temperature range 200–3008C. The thermal stability of the intermediate Mg 6 Ni phase is relatively high. Its transformation (decomposition) into the equilibrium hexagonal Mg 2 Ni and a-Mg was observed to take place in a wide temperature range (300–3508C), as the exothermic peak associated with this transformation is rather small. Fig. 4b shows X-ray diffraction patterns taken from the Mg 87 Ni 12 Y 1 alloy annealed for a short time at three different temperatures (250, 325 and 3758C) before, during and after the DSC exothermal effect. The broad DSC peak and extremely low enthalpy of this transformation imply very slow kinetics of this reaction. After heat treatment at temperatures higher than 350–3608C only the stable a-Mg and Mg 2 Ni phases can be detected (Fig. 4a, curve c and Fig. 4b, curve c). Yttrium, which is about 1 at.%, probably replaces some Ni in the lattice of Mg 2 Ni. The activation energies of the first and second crystallization reactions for both Mg 75 Ni 20 Mm 5 and Mg 87 Ni 12 Y 1 alloys were estimated according to the Kissinger method [11], and are presented in Table 1. Whereas for the Mg 75 Ni 20 Mm 5 alloy both activation energies 205615 and 177613 kJ / mol are practically equal (in the limits of the error), the Mg 87 Ni 12 Y 1 alloy exhibits a Table 2 Experimental interlattice distances d (in nm), Miller indices (hkl) and intensities I of the XRD peaks of the intermediate metastable phase, assigned as fcc Mg 6 Ni (a 0 52.009 nm) Line number
(hkl)
d (nm)
I /I0
1 2 3 4 5 6 7 8 9 10
(220) (222) (400) (422) (820), (644) (822), (660) (751), (555) (664) (844) (864)
0.7101 0.5800 0.5027 0.4102 0.2436 0.2367 0.2321 0.2141 0.2050 1.866
9 10 17 30 – 100 – 13 10 7
distinctly larger activation energy corresponding to the primary crystallization of a-Mg, 295622 kJ / mol, than to the second crystallization reaction (Mg 6 Ni-formation). This definitely higher value for the activation energy (295 kJ / mol) obviously has to be attributed to a process of nucleation and further diffusion controlled growth of a-Mg in the amorphous matrix. This result is in agreement with our electron diffraction study, that the as-cast Mg 87 Ni 12 Y 1 is fully amorphous, while the as-cast Mg 75 Ni 20 Mm 5 is nanocrystalline / amorphous. For Mg 90 Ni 10 amorphous alloy Sommer et al. [10] determined an activation energy of 231 kJ / mol for both crystallization steps (formation of a-Mg and Mg 2 Ni). Activation energies in the range of 163–240 kJ / mol for crystallization of Mg-based amorphous alloys were also reported [12]. After hydrogen charging of the as-cast Mg 87 Ni 12 Y 1 alloy, the XRD pattern (Fig. 1) contains only broad and diffuse maxima similar to the as-quenched H-free sample. Hydrogenation of the as-quenched amorphous Mg 87 Ni 12 Y 1 alloys leads only to a slight decrease of the thermal stability, but influences strongly the second crystallization reaction, which takes place at about 308C lower temperature as compared to the unhydrogenated alloy. The preliminary hydrogenated alloy crystallizes (during annealing in the temperature range 200–2508C) mainly into the stable Mg and Mg 2 Ni phases. The intermediate Mg 6 Ni phase is also present, but in a much smaller degree. The influence of hydrogen on the crystallization of Mg 87 Ni 12 Y 1 can be due to the stronger chemical interaction between the magnesium and hydrogen atoms in the amorphous phase, which would lead during annealing to formation of crystalline phases with lower Mg content. Hydrogen-enhanced host diffusion can be also a reason for the observed decrease in the alloy thermal stability. The microstructure of the fully crystallized hydrogen charged Mg 87 Ni 12 Y 1 alloy is similar to that of the completely crystallized hydrogen free alloy. Desorption of hydrogen was observed by DSC to proceed at about 2508C, as the endothermic peak is rather broad.
3.3. Hydriding properties of crystallized Mg75 Ni20 Mm5 and Mg87 Ni12 Y1 alloys As already mentioned one of the main reasons to study the thermal stability and crystallization of the melt-spun Mg-based alloys was to obtain stable microstructures (nano- or microcrystalline) with improved hydrogen storage properties. Therefore partially and fully crystalline microstructures were produced (by crystallization at different annealing conditions) and their hydrogenation properties were studied and compared with those of the asquenched amorphous and nano- / amorphous materials with the same composition. Fig. 6 shows the rate of hydrogen absorption for the Mg 87 Ni 12 Y 1 alloy at three different microstructural states: amorphous, partially crystalline (aMg nanocrystals embedded in amorphous matrix) and
¨ / Journal of Alloys and Compounds 287 (1999) 243 – 250 T. Spassov, U. Koster
Fig. 6. Kinetics of hydrogenation for amorphous, partially and fully crystallized Mg 87 Ni 12 Y 1 .
completely crystallized (a-Mg1Mg 6 Ni). As already shown the amorphous alloy exhibits very fast initial hydrogenation kinetics due to the rapid diffusion of hydrogen. The fully crystallized Mg 87 Ni 12 Y 1 alloy shows a lower initial hydrogenation rate as compared to the amorphous alloy, but at longer hydriding times the amount of hydrogen absorbed in the amorphous and crystalline (nanocrystalline) alloys is practically the same. This result also supports the suggestion that the amorphous (disordered) phase is mainly responsible for the fast initial hydriding kinetics. Although its initial hydrogenation kinetics is slower, the crystalline alloy attains the same hydrogen absorption capacity (about 2 wt.% H) as the amorphous alloy with the same composition, due to the fine two-phase microstructure (crystal size about 100 nm), favourable for hydrogen storage. It is interesting to note, that once the metastable Mg 6 Ni phase is formed, it does not decompose to the equilibrium phases during further hydrogen charging. The partially crystalline alloy shows hydrogenation kinetics only slightly better than those of the fully crystallized alloy. The hydriding kinetics of the completely crystallized Mg 75 Ni 20 Mm 5 alloy does not show essential differences compared to the as-cast nano/ amorphous alloy (Fig. 3). After about 30 min of Hcharging nearly the same hydrogen contents are attained. The crystalline (with average grain sizes of about 100– 150 nm) Mg 75 Ni 20 Mm 5 exhibits much better hydrogenation properties than the crystallized Mg 87 Ni 12 Y 1 (with grain sizes about 100 nm) as well as better than the nanocrystalline Mg 63 (Ni,Y) 37 [5]. Although systematic quantitative investigation of the effect of yttrium and Mm (Ce,La-rich mischmetal) was not carried out in the present study, it seems that the large difference in the hydrogenation properties of the crystallized Mg 75 Ni 20 Mm 5 and Mg 87 Ni 12 Y 1 alloys cannot be attributed only to the different Y or Mm content. The observed differences are most probably associated with the differences in the phase composition. Obviously the phase proportion of Mg 2 Ni to
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Mg 17 Mm 2 (Mg 17 Mm 2 undergoes a disproportionation to MmH x and Mg on hydrogenation / dehydrogenation [7]) for the Mg 75 Ni 20 Mm 5 alloy is close to the optimum with respect to the hydriding properties. Lower (in Mg 87 Ni 12 Y 1 ) as well as much higher (in Mg 63 Ni 30 Y 7 ) volume fractions of Mg 2 Ni than in the Mg 75 Ni 20 Mm 5 alloy make the hydrogenation kinetics worse. Liang et al. [13] showed the hydrogen storage properties of different Mg–Ni and Mg–LaNi 5 mechanically milled composites at room temperature and found a better hydrogen absorption kinetics of Mg–50 wt.% LaNi 5 (corresponding to alloy composition Mg 75 Ni 21 La 4 ), 2.5 wt.% hydrogen at RT in 500 s under 1.5 MPa hydrogen pressure, than those of Mg–30 wt.% LaNi 5 and Mg–30 wt.% Ni. The authors proposed that there should be an optimum proportion of Mg to Mg 2 Ni and lanthanum hydride. The optimum capacity for the Mg–50 wt.% LaNi 5 was found to be 4.1 wt.% at intermediate temperatures (523–573 K). An increase in the absorption–desorption characteristics (kinetics and capacity) of Mg–x wt.% LaNi 5 (x510, 20, 30) mechanically alloyed composites with the addition of LaNi 5 has been also recently reported by Terzieva et al. [14], as the best achievements belong to the composite with the maximum LaNi 5 content (corresponding to Mg 87 Ni 11 La 2 ). A recent study on the hydrogen storage properties of Mg 17 La 2 –x wt.% LaNi 5 (x50, 10, 20, 30, 40, 50, 60) mechanically milled composites show also the best overall hydriding kinetics for the composite with 40 wt.% LaNi 5 [15,16]. Studying the hydrogen absorption / desorption properties of different multi-component Mgbased alloys Au et al. [17] found the best kinetic properties for Mg 83.3 Ni 6.6 Cu 9.5 MI 0.6 (MI5La-rich mischmetal). The authors suggest that the interface between the Mg and Mg 2 (Cu,Ni) phases and the microcrack developed in the latter play the main role in improving the hydriding and dehydriding kinetics. Our results obtained for the meltspun Mg-based alloys show a very good agreement with all these findings. Another important factor, which has significant effect on the hydrogen storage properties is the microstructure of the alloy. The present study on the hydrogenation kinetics and capacity of nanocrystalline Mg-based alloys confirms that nanocrystalline (and microcrystalline) structures improve the absorption kinetics, especially at room temperature [5,13]. The differences in the hydrogenation characteristics between the as-cast and crystallized Mg-based alloys found in this work are not very large due to the fine microstructure (i.e. high amount of phase and grain boundaries) of the crystallized alloys, favourable for hydrogen absorption.
4. Conclusions The hydrogenation properties of rapidly quenched (meltspun) amorphous and nanocrystalline as well as of crys-
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tallized Mg-based alloys were studied and compared with the results of our previous works on similar alloys [5,6]. The as-cast nanocrystalline Mg 75 Ni 20 Mm 5 alloy (containing Mg 2 Ni nanocrystals embedded into amorphous matrix) shows the best hydrogenation properties (hydriding kinetics — 0.3 wt.% H / min and absorption capacity — 4 wt.% H) among the melt-spun Mg-based alloys studied. The fully amorphous Mg 87 Ni 12 Y 1 alloy exhibits lower hydrogen absorption capacity as compared with the other as-cast nanocrystalline / amorphous alloys, but it shows stronger charging current density dependence of the amount of hydrogen absorbed, which might be due to its amorphous microstructure. The thermal stability and crystallization of the as-cast alloys were also studied and the hydrogenation properties of the crystallized alloys were compared with those of the as-quenched materials. The crystallization of the Mg 75 Ni 20 Mm 5 alloy is a two-step process, starting with primary crystallization of Mg 2 Ni, probably realized by growth of quenched-in Mg 2 Ni nanocrystals (2–3 nm). The crystallization of the Mg 87 Ni 12 Y 1 alloy takes place through an intermediate metastable phase, isomorphic with the fcc Mg 6 Pd. The fully crystallized Mg 75 Ni 20 Mm 5 shows kinetics of hydrogen absorption and H-capacity comparable to those of the as-cast nanocrystalline / amorphous alloy. The partially (after the first crystallization reaction) and fully crystallized Mg 87 Ni 12 Y 1 alloys exhibit slower initial hydrogen absorption kinetics than the amorphous alloy, but the hydrogen amount absorbed at longer hydrogenation times is practically the same. The observed differences in the hydrogenation characteristics of the asquenched nanocrystalline / amorphous and the crystallized Mg-based alloys are relatively small due to the fine microstructure of the crystallized alloys, favourable for hydrogen absorption. The crystallized Mg 75 Ni 20 Mm 5 alloy exhibits much better hydrogen absorption characteristics than the crystallized Mg 87 Ni 12 Y 1 , which is most probably associated with the optimal phase proportion in the former alloy.
Acknowledgements One of the authors (T. Spassov) is very grateful to the Alexander von Humboldt Foundation (Germany) for financial support.
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