Hydrides of metallic glass alloys

Journal of the Less-Common Metals, 74 (1980) 279 - 285 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

279

HYDRIDES OF METALLIC GLASS ALLOYS*

A. J. MAELAND, L. E. TANNER** Corporate

Research

Center, Allied

and G. G. LIBOWITZ Chemical Corp., Morristown,

N. J. 07960

(U.S.A.)

Summary The hydrogen absorption properties of several metallic glass alloys of the general formulas Tii_,Cu, and Zri _Cu, (3~= 0.3 - 0.7) were investigated and were compared with the absorption properties of the corresponding crystalline intermetallic compounds. Under similar conditions of temperature and pressure the metallic glass alloys had larger absorption capacities than their crystalline counterparts. “Amorphous” TiCu, for example, absorbed 35% more hydrogen than crystalline TiCu (TiCuH1.s5 compared with TiCuH). These results indicate the relative importance of electronic structure and crystal structure in hydrogen absorption; it appears that the maximum hydrogen absorption capacity is determined by the electronic structure but that the crystal structure, i.e. the type and size of interstitial sites in the lattice, may not always permit maximum absorption to take place readily.

1. Introduction A preliminary report on the absorption of hydrogen in the metallic glasses Ti,,ssCu,s, and TiCu was presented at the International Symposium on Hydrides for Energy Storage [l] . We have continued the studies of hydrogen absorption in metallic glasses in the Ti-Cu system and have also begun a study of the Zr-Cu system. Glasses form readily in both these systems over wide ranges of compositions (30 - 70 at.% Cu) when cooling rates in excess of lo6 “C s-l are used. The equilibrium phase diagrams show that several intermetallic compounds fall in this glass-forming composition range. This property makes these systems of particular interest as the hydrogen absorption in metallic glasses can be compared with the hydrogen absorption in the corresponding intermetallic compounds. Such a comparison may be useful in

*Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides, Colorado Springs, Colorado, U.S.A., April 7 - 11, 1980. **Present address: Manlabs Inc., Cambridge, Mass. 02139, U.S.A.

280

assessing the relative importance of electronic structure and crystal structure in hydrogen absorption. 2. Experimental The metallic glass samples were vacuum-cast ribbons [ 21 approximately 2.1 mm wide and 0.05 mm thick. The ribbons were first surface cleaned by abrasion with emery paper, were cut into short strips (less than 10 mm long) and were then ultrasonically cleaned in acetone followed by ether, before drying and weighing. A 1 - 3 g sample was placed in a vacuum system which was subsequently evacuated to 1.3 X 10m4 Pa. Pure hydrogen, generated by the decomposition of TiHs, was introduced into the samples at room temperature to avoid crystallization. The amount of hydrogen absorbed was calculated from the weight of the sample, the known volume of the system and the change in hydrogen pressure using the ideal gas law. The compositions were also checked by thermal analysis and the two methods agreed within experimental error (H/Metal values agreed to within kO.05). The intermetallic compounds were prepared by arc melting the appropriate amounts of the constituent metals under an argon atmosphere. The purities of the starting materials were titanium 99.99%, copper 99.9% and zirconium 99.5%. The resulting buttons were remelted at least four times to ensure homogeneity. Lattice parameters, determined from X-ray powder patterns, were in good agreement with published values [3 - 51. Prior to exposure to hydrogen, the samples were broken up into small chunks and degassed by heating to 500 - 600 ‘?I!in vacuum (1.3 X 10e4 Pa). After cooling to room temperature, hydrogen was admitted at approximately atmospheric pressure. Absorption started slowly at room temperature but proceeded rapidly when the samples were heated to 150 - 200 “C. When absorption was complete, the samples were slowly cooled to room temperature and were removed for X-ray examination. The compositions were calculated as before from the initial and final pressures, the known volume of the system and the weight of the samples. All the samples were handled and kept in an argon dry box. 3. Results and discussion The hydrogen absorption capacities at room temperature and atmospheric pressure of a number of metallic glass compositions are summarized in Table 1; the corresponding intermetallic compounds, when they exist, and their hydrogen absorption capacities under the same conditions are also listed. The hydrogen-charged metallic glasses showed small shifts in the broad maximum of their X-ray patterns to lower angles, indicating that hydrogen absorption had caused some volume expansion of the glass (see Table 2). By comparison, the increase in volume of the crystalline hydride phases (as determined from X-ray data) was generally 8 - 10% [ 1,8,9] .

281 TABLE 1 Hydrogen absorption capacity at room temperature and atmospheric pressure of hydrogen Glcss composition

H/Metal

Corresponding intermetallic compound

Reference

H/Metal

Ti0.S5CU0.65

0.37

Non-existent

6

-

Tio.so~o.4o

TiCu

0.68 0.96

TiCu Nonexistent

6 6

0.47 -

Ti0.65Cu0.35

1.15

T&&u

6

0.92

zr0.~Ocu0.60

0.64

Zr7%0

6, 7

n.a.a

%.a_, not available. TABLE 2 X-ray diffraction data on thermally treated metallic glasses TiCu and TiCuH1.33 Sample

Heat treatment

Time (h)

X-ray pattern

CuTi

-

Broad amorphous peak, 28 = 41.3”

CuTi

None (as cast) 245 “C

No change

CuTiH,_,a

None (as prepared)

24 -

~TiHl.33

90 “C

18

No change

CuTiH,.aa

Reheated to 170 “C

16

CuTiHl.33

134 “C

63

CuTiH1.33

245 “C

18

Medium peak at 28 = 35.0” (TiH,); medium broad peak at 28 = 40.8” (TiHg and metallic glass hydride); weak sharp peak at 28 = 43.2” (copper) Broad amorphous peak at 28 = 40.5” ; very weak peak at 28 = 35.1” (TiH,); very weak peak at 28 = 43.2” (copper) and very weak peak at 28 = 50.4” (copper) Strong peak at 28 = 35.1” (TiHs); medium peak at 28 = 40.8” (TiHz); very strong peak at 28 = 43.2” (copper); very weak peak at 28 = 50.4” (copper)

3.1.

Broad amorphous peak, 20 = 40.2”

Thermal analysis data The hydrogen absorption capacities of the Ti-Cu glasses listed in Table 1 are higher than those obtained previously [ 11. In our earlier work hydrogen had been introduced electrolytically. Non-homogeneous distribution resulted when pieces of ribbon broke off during electrolysis before having absorbed the maximum amount of hydrogen. The end result was that the samples contained both hydrided metallic glass and unreacted or partially hydrided glass. Evidence for this is clearly seen in the differential thermal

T&&h,,

g6./ , , g, :53/o , I

0

I

I

I

200

400

600

TEMPERATURE°C

600

25 100 200 300 400 500 600 700 TEMPERATURE'C

Fig. 1. The differential thermal analysis curve for an uncharged (no hydrogen) metallic glass sample. The 24.3 mg eample was initially heated to approximately at a rate. of 25 “C mine1 in purified helium in an alumina crucible.

TiCu 1060 “C

Fig. 2. Differential thermal analysis (. . .), differential thermogravimetric analysis (- - -) and thermogravimetric analysis (-) data for the metallic glass hydride TiCuH,.,,. The 24 mg sample was heated at a rate of 26 “C min-’ in purified helium in an alumina crucible.

analysis (DTA) data shown in Figs. 1 -3. The two exothermic peaks at 430 and 470 “C in the DTA curve for uncharged TiCu (Fig. 1) are due to the crystallization of the glass. When hydrogen is introduced from the gas phase and sufficient time is allowed for equilibration, the entire sample reacts and a homogeneously hydrided sample results. Figure 2 shows the thermal analysis of such a sample. The peaks of 430 and 470 “C are no longer present. The two exothermic peaks at 160 and 235 “C in the DTA curve are associated with the decomposition of the metallic glass hydride and the formation of crystalline titanium hydride; the strongly endothermic reaction at 535 “C, accompanied by the maximum weight loss (see the differential thermogravimetric analysis curve), is due to the reaction of TiHa-x with copper to form TiCu and hydrogen. If we look at the thermal analysis curve for the electrolyticallyprepared metallic glass that has been hydrogen charged to a composition of TiCuH1.iz (Fig. 3) we can see the peaks at 180 and 235 “C and the endotherm at 580 “C which are representative of the metallic glass hydride, but we can also see the crystallization peaks at 435 and 475 “C indicating the presence of uncharged metallic glass. The interpretation of the thermal analysis curves in Fig. 2 in terms of the decomposition of the glass and the formation of titanium hydride is confirmed by X-ray diffraction studies. Samples were sealed in evacuated glass ampules and were heated to a constant temperature for various time periods. The results of the X-ray diffraction measurements are summarized in Table 2. No change in the X-ray pattern was observed when the uncharged metallic glass TiCu was heated to 245 “C for 24 h. The hydrided metallic glass TiCuH1.33, however, when heated to this temperature decomposed complete-

283

25 100 200 300 400 500 600 700750 TEMPERATURE "c

T~MFERATU~E "C

Fig. 3. Differential thermal analysis (, . .), differential thermogravimetric analysis (- - -) and thermogravimetric analysis (-) curves for a metallic glass of nominal composition TiCuHl 12. The 14.8 mg sample was heated at a rate of 26 “C mine1 in purified helium in an alukina crucible. analysis (- - -) Fig. 4. Differential thermal analysis (. , .), differential thermogravimetric and thermogravimetric analysis (-) data for a mixture of TiH, 98 and copper metal. The 16.6 mg sample was heated at a rate of 25 “C min-’ in purihed helium in an alumina crucible.

ly into TiH,_, and metallic copper. Incomplete decomposition of the hydride was‘observed in the X-ray patterns at 134 “C whereas no change was apparent after heating at 90 “C for 18 h; reheating this sample to 170 “C for 16 h resulted in incomplete decomposition of the metallic glass hydride. Recent nuclear magnetic resonance work correlates well with these observations; an anomaly in the proton relaxation times of amorphous TiCuH, was observed near 147 - 157 “C where T,, (the spin-spin relaxation time determined by the Carr-Purcell-Meiboom-Gill pulse sequence technique [lo] ) decreased irreversibly by a factor of about 10 [ 111. The transformation was incomplete as indicated by the coexistence of both relatively slow diffusing hydrogen species (TiH,_,) and rapidly moving protons (metallic glass hydride) [ 1 l] . The broad DTA peak at 160 “C thus appears to represent a metastable stage of the decomposition whereas the peak at 235 “C is assoeiated with complete decomposition of the metallic glass hydride. The fact that the strongly endothermic reaction at 535 “C is due to the reaction of crystalline TiH2_% with copper is confirmed by the thermal analysis data shown in Fig. 4, which were obtained under identical conditions of heating rate and helium flow using an equimolar mechanical mixture of TiH1.9s and copper powders. The thermal effects in this temperature region are essentially the same, suggesting similar reactions in the two samples (Figs. 2 and 4). It should not be concluded that the low temperature decomposition of an alloy hydride is characteristic of a metallic glass hydride. Similar low temperature decomposition reactions occur in many hydrides of in~rme~lic compounds [ 121, including crystalline TiCuII which decomposes to TiW, and copper when heated to about 200 “C [ 11.

284

3.2. Hydrogen absorption capacities It is evident from Table 1 that, under similar conditions of temperature and pressure, the metallic Ti-Cu glass alloys had larger absorption capacities than their crystalline counterparts. “Amorphous” TiCu, for example, absorbed 35% more hydrogen than crystalline TiCu (TiCuHr.ss compared with TiCuH). We believe this to be significant in demonstrating the relative importance of electronic structure and crystal structure in hydrogen absorption [ 11. The maximum hydrogen absorption capacity is determined by the electronic structure which, to a first approximation, is quite similar in the metallic glass and the corresponding intermetallic compound [ 131. However, maximum hydrogen absorption in the crystalline counterpart may not always take place readily because of the crystal structure, i.e. the type and size of the interstitial sites in the lattice. In crystalline TiCuDc.a,,, for example, the deuterium atoms are located at the centers of distorted tetrahedra of titanium atoms [ 14) ; thus the crystallographically limiting composition is TiCuD. If further deuterium (hydrogen) absorption were to take place, higher energy sites would have to be occupied. At the present time there are not sufficient data available to make a similar comparison for the Zr-Cu alloys. The local environment of the hydrogen atoms in the corresponding metallic glass structure has recently been explored using inelastic neutronscattering techniques [ 151. The metallic glass hydride shows a very broad band of hydrogen vibrations (approximately 75 meV full width at halfmaximum) peaked at an energy of about 145 meV which is roughly the same energy as in the crystalline counterpart. The width, however, is much larger, about 4 times that observed in the polycrystalline material. This would indicate that, although on average hydrogen atoms are in tetrahedral-type sites, there is a wide distribution of local environments for the hydrogen atoms in the metallic glass. Such fluctuations in local symmetry may increase the number of sites available for hydrogen occupation (with respect to the crystalline structure) by lowering the energy. Thus, a higher hydrogen absorption capacity may be possible in certain metallic glass structures compared with their crystalline counterparts. It is of interest to compare the situation in TiCu hydride with that in palladium hydride, which has the NaCl structure where hydrogen atoms occupy octahedral sites in an f.c.c. palladium sublattice. Although the stoichiometric formula should be PdH, it is extremely difficult to fill the octahedral sites much beyond the composition PdHoes.This is because there are only 0.6 empty electronic states below the Fermi level in palladium hydride (0.36 holes in the d band plus 0.24 additional s-like states due to the presence of hydrogen in the lattice [ 161) which are available for filling by electrons from the hydrogen atoms. To add additional hydrogen requires putting electrons into higher energy levels. Therefore, in contrast with TiCuH, the maximum hydrogen content in palladium hydride is limited by electronic structure rather than crystal structure.

285

Acknowledgments We wish to thank Dr. E. Turi and Mr. W. M. Wenner for obtaining and discussing with us the thermal analysis data.

References 1 A. J. Maeland, in A. F. Andresen and A. J. Maeland (eds.), Hydrides for Energy Storage, Proc. Int. Symp., Geilo, 1977, Pergamon, Oxford, 1978, pp. 447 - 462. 2 J. J. Gilman, Phys. Today, 28 (1975) 46. 3 M. H. Mueller and H. W. Knott, Trans. Metall. Sot. AZME, 227 (1963) 674. 4 N. Karlson, J. Inst. Met., 79 (1951) 391. 5 J. M. Vitek, Z. Metallkd., 67 (1976) 559. 6 M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958. 7 L. Bsenko, J. Less-Common Met., 40 (1975) 365. 8 A. J. Maeland, Preparation and properties of TiCuH, Adu. Chem. Ser., 167 (1978) 302 - 311. 9 A. J. Maeland and G. G. Libowitz, Hydrogen absorption in some AzB inter-metallic compounds with the MoSiz-type structure (Cllb). In Proc. Int. Symp. on the Properties and Applications of Metal Hydrides, Colorado Springs, Colorado, April 7 - 11, 1980; J. Less-Common Met., 74 (1980) 295. 10 H. Y. Carr and E. M. Purcell, Phys. Rev., 94 (1954) 630. S. Meiboom and D. Gill, Rev. Sci. Instrum., 29 (1958) 688. 11 R. C. Bowman, Jr., and A. J. Maeland, paper presented at American Physical Society Meeting, Chicago, Illinois, March 19 - 23, 1979, to be published. 12 G. G. Libowitz and A. J. Maeland, paper presented at American Chemical Society Meeting, Washington, D. C., Septem her, 1979. 13 A. Amamou, Solid State Commun., 33 (1980) 1029. D. S. Boudreaux, Allied Chemical Corp., personal communication, 1980. 14 A. Santoro, A. J. Maeland and J. J. Rush, Acta Crystallogr., Sect. B, 34 (1978) 3059. 15 J. J. Rush, J. M. Rowe and A. J. Maeland, paper presented at American Physical Society Meeting, New York, March 1980; J. Phys. C, in the press. 16 A. C. Switendick, Ber. Bunsenges. Phys. Chem., 76 (1972) 535.