Gas atomization processing of designed dual-phase intermetallic hydrogen storage alloys

doi:10.1016/S0921-5093(01)01605-7

Materials Science and Engineering: A

Volumes 329–331, June 2002, Pages 372-376

https://doi.org/10.1016/S0921-5093(01)01605-7 Get rights and content

Abstract

Two dual-phase nickel–metal hydride (Ni–MH) battery powders for battery applications were produced by gas atomization. They are characterized by a fine and uniform dispersion of rare-earth (RE)-containing second phase in the Zr-containing base alloy, achieved from gas atomizing molten mixtures of AB₂ and AB₅ alloys. The dual-phase alloy formulations were (AB₂)_100−x(AB₅)_x, where x is 1 and 5. The new dual-phase alloys were found to have high C-rate discharge capacities. Crushing and annealing the dual-phase powders improved the discharge capacity of as-gas atomized powder. C- and C/8-rate discharge capacities of 383 and 425 mAh g⁻¹ (or power density of 134 and 148 Wh kg⁻¹), respectively, were measured for the crushed and annealed dual-phase MmZr05 alloy powder. The possible commercial powder production of the Ni–MH battery powder using a gas atomization process was demonstrated. Four 500 lbs lots of a Zr-based AB₂ alloy were successfully gas atomized in a pilot gas atomizer unit. The discharge capacities of this powder after processing exhibited 349 and 414 mAh g⁻¹ at C- and C/8-rate discharges, respectively.

Introduction

AB₂ Laves phase hydride compounds have attracted much attention for negative electrode applications in nickel–metal hydride (Ni–MH) batteries. The Zr-based AB₂ alloys have the potential to offer higher discharge capacities and a longer cycle life than the rare-earth based AB₅ compositions [1]. The stable dense oxide protective layer in some Zr-based AB₂ materials lead to poor charge/discharge kinetics, leading to inferior initial activation compared to AB₅ alloys [2].

Previous studies have revealed that the hydrogenation kinetics of AB₂ alloy could be improved with a modification adding rare-earth elements in ingot cast materials [3], [4], [5], [6], blended materials [7], and mechanically ball-milled materials [8]. The observed enhancements were attributed to the appearance of La–Ni related phases [6]. Conventional ingot cast and ball-milling processing result in non-uniform second phase distribution in the matrix [9]. Recently, rapid solidification processes of tin modified LaNi₅ powders by gas atomization has been suggested as a mean to suppress the Sn segregation effect observed in conventional chill casting of LaNi_4.75Sn_0.25 [10], [11]. Gas atomization of the dual-phase alloys would not only produce uniformly distributed La–Ni rich phase by suppressing macro-segregation, but would also produce useful powder directly.

In this study, two designed dual-phase alloys were produced by inert gas atomization with the intention of combining the good kinetic properties of the AB₅ alloy with the high storage capacity of the AB₂ alloy. By using the inherent high solidification rate of the gas atomization process (10⁴ K s⁻¹ [12]), an uniform dispersion of AB₅ phase is precipitated in the AB₂ matrix when a pre-alloyed powders are produced. The dispersion of nickel-rich AB₅ precipitates is designed to form catalytic active sites, and should enhance rapid desorption of hydrogen across the electrode/electrolyte interface during high rate discharge.

Several 50 lbs experimental melts of Zr-based alloys with different degrees of rare earth additions were used to create dual-phase alloys. Electrochemical results of these powders were analyzed. The gas atomization process was scaled up to produce four 500 lbs melt batches of a Zr-based AB₂ alloy composition. The electrochemical capacities of the powder produced in the large scale batch are presented.

Section snippets

Experimental methods

The dual-phase MmZr01 and MmZr05 alloys are modified from two Zr-based AB₂ alloys designated Ov313 and Ov474, respectively. (These Zr-based alloys composition are based upon the concept of disordered, multi-element patented by Energy Conversion Devices, Inc.). Earlier generation of Zr-based AB₂ (Ov313) the material had rate limiting capability and slower activation [13]. An new AB₂ (Ov474) improved these properties. The composition of the AB₂ and dual-phase alloys are given in Table 1.

In the

Results and discussion

Electrochemical capacities for as-atomized Zr-based alloys are plotted in Fig. 2, Fig. 3. The highest C/8-rate discharge capacity found for gas atomized Ov313 was 389 mAh g⁻¹. The Ov313 ingot powder had C/8- and C- rate discharge capacity of 410 and 373 mAh g⁻¹, respectively. For Ov474, capacities obtained with finer powders (<75 μm) are higher than capacities obtained from coarser powders (<150 μm) as shown. This may be attributed to the larger surface area in finer powders for equal sample

Conclusions

This study showed that the C-rate discharge capacity of Zr-based AB₂ alloy can be significantly improved by small additions of rare-earth based AB₅ alloy in the melt chemistry. The dual-phase MmZr05 alloy powder made by gas atomization showed C/8- and C-rate discharge of 427 and 383 mAh g⁻¹, respectively. The fine distribution of the second phase can only be achieved by a rapid solidification.

This study has demonstrated that commercial quantities of Ni–MH powder, with high discharge capacities,

Acknowledgements

The authors would like to acknowledge the assistance of James Bauer, Charles Rader, Melissa Devinney, and Joanne Cheng for their help in the Ni–MH negative electrode capacity measurements and James McCalla's assistance for SEM imaging. This work is funded by US DOE, under NIST-ATP contract number 70NANB7H3019.

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For an ingot prepared by prevalent methods such as arc and induction melting, the segregation occurs during the alloy ingot solidification. The newer methods such as melt spinning and gas atomization with their high cooling rates and also mechanical alloying may produce ingots and powders with a better chemical homogeneity and smaller grain size than those synthesized by conventional casting methods, that leads to improve hydrogen storage characteristics, particularly the hydrogen absorption and desorption kinetics and cyclability [36,47,49–51]. The relatively novel vacuum plasma spray process; a rapid solidification and commonly used deposition technique in the industry, can be a promising method for mass production of the hydrogen storage alloys with suitable properties.
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^☆: 5^th International Conference on Structural and Functional Intermetallics, Vancouver, Canada July 16-20, 2000.

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Gas atomization processing of designed dual-phase intermetallic hydrogen storage alloys☆

Abstract

Introduction

Section snippets

Experimental methods

Results and discussion

Conclusions

Acknowledgements

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.