Jorrrncrl ofthe Le.wCbmmon Me& 169 ( 199 1 ) V- 17 9

Magnesium-nickel alloy hydride compacts prepared by cylindrical explosion shock compression

Kei Nomura, Shuuzou Fujiwara, Hiroshi Hayakawa, Etsuo Akiba, Yoshihiko Ishido and Shuitiro Ono Nutionul Chemicnl Luboratonj for Industy. I-I Higushi. Tsukuba, Ibarmki 305 (Japan)

(Received April 2, 1990; in final form September 18, 199Oj

Abstract

High density sintered solid forms of Mg,Ni hydride and Mg-Swt.%Ni alloy hydride were prepared

using the cylindrical explosion shock compression technique. During such treatment, both hydrides

lost about 10% of their hydrogen. The colour of the Mg,NiH, product was black and uniform, and it

was mainly composed of Mg,NiH, and MgZNi, with some MgNi, and MgH, phases. This material was

hard and tough, and rather stable in air. On the contrary, the Mg-Swt.%NiH, product was greyish in

colour. including white stripes and cracks. The sintering process did not proceed in the white part.

This material was composed of MgH,, magnesium metal and some Mg:Ni phase. The Vickers hard-

ness was 232 and I85 respectively. The porosity was 5.29% and 4.739/o, and the hydrogen content was

3.33 wt.% (theoretically 3.73 wt.%) and 6.97 wt.%, (7.88 wt.%). The former exhibited metallic conduc-

tivity, but the latter was a electrical insulator.

1. Introduction

The cylindrical explosion shock compression (CESC) and the shock wave loading techniques were developed about two decades ago. Since then, using this powerful and effective method, many studies have been performed mainly on ceramic powders such as oxides, nitrides and carbides [ 1, 21.

Metal hydrides (MHs) are ordinarily bulky fine powders with particles of a few microns in size and obtaining single crystals or sintered materials of MH has been thought to be difficult. Most of the studies relating to measuring the physical properties of MHs have been performed using a pellet compacted by using a diamond anvil and a mould.

Thin films such as LaNi,H, and PdH., have also been used successfully to investigate the mechanical properties, hydrogen diffusivity and solubility in a metal

[3,41. The object of the present study is to explore the possibility of preparing high

density MH sintered materials using the CESC technique. We selected Mg,Ni hydride and Mg-Swt.%Ni hydride as the raw powder

materials, because magnesium nickel alloy hydride is reported to be a semi- conductor [5-S], and the equilibrium hydrogen pressures of these materials are as low as 1 Pa at room temperature [9] and they are rather stable in air. Some physical properties relating to these hydrides have been measured.

0022-SO88/91/$3.S0 0 Elsevier Sequoia/Printed in The Netherlands

2. Experimental details

The metal ingots, used in these experiments were from the same batches employed in the work described in the previous papers [9-12). They were hydrided five times at 623 K and the hydrided powders were kept in an argon dry-box.

Figure 1 shows the CESC treatment process schematically. The stainless steel (~~304) container used was 27 mm in outer diameter 21 mm in inner diameter and 80 mm in height. Both samples were moulded at 0.3 GPa in air. The gap between the sample powder and the lid was a few millimetres. Next, the sample part of the container was immersed in cooling water and the lid was arc welded. Then, the container was evacuated through a copper tube, and the tube was sealed under vacuum.

The container was placed vertically in a plastic vessel and surrounded by black gunpowder. A wooden plate was placed on a water bath, and the plastic vessel was placed on it. All of these were covered with sand, and the percussion cap was fired. Ring-shaped shock waves ran along the container axis, compressing the container in the radial direction. The pressure was estimated to be around 5 GPa, and lasted for a few microseconds. The highest temperature in the con- tainer, caused by the adiabatic compression, was calculated to be about 800 K. The container fell into the water and was quenched.

(A)

n

(D)

lz!lHD . . :

:, .’

: ‘. :

: .,

(6) (C)

Fig. 1. Schematic drawings of the CESC process: (A) pressing the raw powder material in the con-

tainer; (B) after the arc welding; (C) evacuated and sealed; (D) placed in a plastic vessel; (E) after

CESC treatment.

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3. Results and discussion

After the treatment, it was found that the containers were reduced to 24.5 mm in diameter at the middle, but the length did not change at all. A carborundum blade and kerosene oil were used to cut the containers into slices 25 mm in thick- ness, and then along the axial direction.

In the case of Mg,NiH,, the colour of the raw powder was light brown. After the treatment, it changed to perfectly black. It had the appearance of a diatoma- ceous earth or a low grade coal. A 5 mm X 5 mm X 25 mm and two 10 mm X 10 mm x 10 mm tips were cut out. The hardness and the toughness of these tips were sufficient to resist breakage; when the former was dropped to the plastic floor from a height of 1 m, it did not break, and window glass was scratched readily by its corner edge.

In the case of Mg-Swt.%NiH.,, the raw powder was grey in colour. After the treatment, ten or more white stripes (1 mm wide, 10 mm long) were observed diagonally to the axial direction on the greyish cut section like the pattern of a zebra and, in addition, many small white spots were distributed in the greyish part (see Fig. 4). Many cracks (a few millimetres long) were also observed across the stripes. During the cutting treatment, the sample was broken into several segments and we could not obtain a rectangular tip for a precise determination of the physical properties.

Figure 2 shows a metallurgical microscopic observation of the cut surface of the Mg,NiH, section. The grain size of the raw material was a few microns but, after the treatment, the particles had grown to 100-200 pm in diameter. All of these particles were surrounded by narrow black boundaries, which are also occa- sionally observed in sintered ceramics. Many small hollows were also seen scattered on the section.

Figure 3 shows the scanning electron microscopy (SEM) observation. The characteristics of a hollow are seen precisely, and it is observable that the sintering occurred well into this section.

Fig. 2. Metallurgical microscopic observation of the section of a CESC Mg,NiH, material. The scale bar represents I24 ,um.

Fig. 3. SEM observation of the CESC Mg?NiH, section. The scale bar represents 20 pm.

Fig. 4. Metallurgical microscopic observation of the CESC Mg-Swt.%NiH, section. Raw powder still remains in the white stripes. The scale bar represents 5 mm.

Fig. 5. SEM observation of the white stripes shown in Fig. 4. The scale bar represents 20 pm.

Mg

Ni

Fig. 6. Electron microprobe analysis (EPMA) of an Mg-Swt.%NiH, section. The elemental nickel concentration is very high in this particle. The particle is composed of Mg,Ni phase and the greyish outside region is MgH,. The scale bar represents 20 pm.

Figure 4 shows the metallurgical microscopic observation of collapsed Mg-Swt.%NiH, segments. First, we checked the difference in the formation between the white stripes and the uniform greyish part. As this material had no electrical conductivity, vapour deposition of gold was needed for SEM. The white part is shown in Fig. 5, where it is clear that the sintering had not progressed at all. The greyish part is shown in Fig. 6; there the sintering was well established. The EPMA analysis was performed across the small particle at the centre. The concen- tration of nickel was very high in this particle and very low in the outer greyish part. We estimate that this particle is composed of the Mg,Ni phase.

The hydrogen content in the samples before and after the treatment was analysed. The results are shown in Table 1. After the treatment, it was seen to

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TABLE 1

Hydrogen content (wt.“/o)

MgZNiH, Mg-Swt~NiH,

Theoretical 3.73 7.8X Raw powder 3.6 I 7.70 CESC tip 3.33 6.07

20 30 40 50

28

Fig. 7. X-ray diffraction profiles before and after the CESC for an Mg?NiH, section: peaks A, Mg?NiH,; peaks B, Mg>Ni; peaks C, MgNi,; peaks D, MgH2; peak E, Mg.

decrease by about 10% in both cases. This decrease is thought to have happened during the welding process of the lid, but we have, at present, no idea of how to prevent this decomposition of the hydride.

Figure 7 shows the powder X-ray diffraction patterns of the raw material and the CESC Mg,NiH,, section. All the peaks which appeared on the raw powder material could be indexed, and were from Mg,NiH, (low temperature form), Mg,Ni, MgNi, and some MgHz phases 19-l 11, It is clear that the peak position before and after the CESC treatment did not change, but the peak shapes were severely broadened and diversed significantly from the normal Cauchy or gaussian profiles. This diffraction pattern of the CESC hydride resembled closely that of shocked single-crystal MgO reported by Klein and Rudman [ 131.

Once, after thousands of hydriding and dehydriding cycles, we have observed that the peaks in the di~raction pattern of Mg~NiH~ tended to become broad, because of the increasing lattice strain and the decreasing crystallite size, but all of

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these peaks still retained a Cauchy or gaussian profile [ 141. It is clear that the crystal lattice was deformed severely by the huge power of the explosion, We tried to estimate the lattice strain stored in the lattice, but in this case it was difficult to separate each peak and to fit the profile function to it.

Relating to the Mg-Swt.%NiH, ECSC specimen, all the peaks could also be indexed, and the section consisted of MgH,, magnesium metal and some Mg,Ni phases (Fig. 8). After the CESC treatment, in comparison with the raw powder material, the peak heights of the magnesium metal were increased and those of MgH, decreased, and all the peaks were not so severely distorted.

Using a micrometer and chemical balance, the densities of the tips were measured. The results are shown in Table 2. The theoretical densities were calculated from the lattice constants and the hydrogen contents.

D

D

I 20 30 40 50

20

Fig. 8. X-ray diffraction profiles before and after the CESC for an Mg-Swt.%NiH, section.

TABLE 2

Density (g cm-‘)

Theoretical Moulding CESC tip Porosity (%)

Mg,NiH, Mg-Swt.%NiH,

2.124 1.564 2.00 1.01 2.58 1.49 5.29 4.73

Remarks

100% hydrided 0.3 GPa 5 GPa

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TABLE 3

Vickers hardness

Direction Mg$‘iH, Mg-Swt.%NiH,

Axial Diametral Average

194-239 158-226 193-270 158-187 232k40 185&41

Table 3 shows the Vickers hardness of both sections. Measurements were repeated six times, and the average values were 232 for Mg,NiH, and 185 for the greyish part of Mg-Swt.%NiH, respectively.

The electrical conductivity of the Mg,NiH, specimen was measured. Indium metal and the dc. two-terminal method was employed. First, the sample was cooled rapidly to 2 K, and then, during a period of 12 h, the conductivity was measured while the temperature was increased to 290 K at a constant rate.

The values obtained were 0.15 (52 cm)-’ and 0.12 (& cm)-’ at 2 K and 290 K respectively. Figure 9 shows the conductivity vs. 1/7X The activation energy was calculated to be almost equal to zero.

Gupta et al. I.51 estimated that Mg,NiH, was a semiconductor with a band gap of 1.36 eV. Gavra et al. [6] reported that, at x = 4.0, an Mg,NiH, powder compact was an insulator but, at around x = 3.96, it became a conductor. Noreus et al. [7] reported that the activation energy of the Mg*NiH, powder compact was - 0.05 eV, and the electrical conductivity was 1.5 X 10 - * (52 cm)- ’ at 350 K. Lupu et al.

[8] reported that they made an Mg,NiH, powder compact and that it was a semi- conductor with a band gap of 1.68 eV. From Lupu et al.3 value, the intrinsic specific electrical conductivity can be estimated to be around 10eh (Q cm)-’ at 500 K.

Our CESC specimen consists of about 85% of the Mg,NiH, phase and 15% of the metal phase. Accordingly, it should a quasimetallic conductor at 290 K and the resulting flat line of our experimental result is reasonable and can be explained

T IKI 10 5

0 100 200

1000/T(K)

Fig. 9. Electrical conductivity vs. 1 /T for the CESC Mg,NiH, material.

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by the charge carrier concentration being independent of its temperature, as was reported by Debye for a highly impure germanium sample [ 151.

Using a 5.0 mm X 3.8 mm X 0.6 mm Mg,NiH,tip, the Hall effect was checked at room temperature. The e.m.f. was directly proportional to the current supplied, and the magnetic field (maximum * 10 kG) had no effect on it. We can conclude again that this specimen has metallic conductivity and the e.m.f. generated was caused by the non-uniformity of the composition.

In order to check the stability of Mg,NiH, tips, one of the two tips (10 mm X 10 mm X 10 mm) was left at room temperature in air and the other was soaked in silicon oil. After half a month, small cracks began to appear from the one corner of the tip which had been in air. Then, in a month, the cracks grew to the middle of the tip and at the same time, from another edge, some cracks appeared and the tip began to crumble little by little. In addition, a swelling phenomenon also occurred and after half a year the tip collapsed into a small heap of powder.

The other tip was taken out of the oil after one year. No change in the outer size was found, and then we wiped off the oil with tissue paper and left it in air. Since then four years have passed and it has expanded a little, but it has still kept its hardness and the original cubic shape.

4. Conclusion

Fine powders of Mg,NiH, and Mg-Swt.%NiH, were sintered by using the CESC treatment.

Newly developed solid Mg,NiH, material was hard and tough. The density attained nearly 95% of its theoretical value and was confirmed that it was rather homogeneous and was a metallic conductor. However, in the case of Mg-Swt.%NiH,, much unreacted powder was left, forming white stripes and spots.

As a whole, we may conclude that CESC is one of the best methods of obtain- ing a sintered MH material. The only difficulty encountered is that a portion of the MH is inevitably decomposed during the welding process, resulting in the presence of metallic phases. Methods to achieve a stoichiometric hydride sintered material are under consideration.

Acknowledgments

The authors acknowledge the technical assistance of Mr. S. Higano (Mitsubishi Steel Manufacturing Co. Ltd.) and Dr K. Kawabata (Idemitsu Kosan Co. Ltd.).

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