| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 91 === Hydrogenation and electrochemical properties of MmNi5-based hydrogen storage alloys prepared by gas atomization (GA) were investigated. The results are compared with those of LaNi5-based alloys prepared by arc melting. The main differences between gas atomized and arc melted powders are particle shape and surface condition. The compositions of three gas atomized alloys were MmNi3.6Co0.84Mn0.28Al0.28 (GA-A), MmNi3.6Co0.70Mn0.42Al0.28 (GA-B), and MmNi3.6Co0.56Mn0.56Al0.28 (GA-C). Similar compositions but with the substitution of Mm by La was made by arc melting, i.e., LaNi3.6Co0.84Mn0.28Al0.28 (AM-A), LaNi3.6Co0.70Mn0.42Al0.28 (AM-B), and LaNi3.6Co0.56Mn0.56Al0.28 (AM-C). In addition, some alloys were also prepared with partial substitution of La by Ta, V, or Zr by arc melting.
This study includes three major parts. First is the hydrogenation and electrochemical properties of MmNi5-based hydrogen storage alloys prepared by gas atomization. It was observed that there is a trade-off in the contents of Co and Mn for the electrochemical performance. A higher content of Co and lower content of Mn gives lower electrochemical capacity but longer cycle life. A lower content of Co and Mn results in higher discharge capacity and decay rate. The particle size seems to be a very important factor influencing the electrochemical activation kinetics.
The second part focuses on the effect of particle size on hydrogenation properties of gas-atomized MmNi3.6Co0.56Mn0.56Al0.28. Four ranges of particles were selected for test, i.e., 75-38m, < 38m, 38-10m, and < 10m. It was observed that the hydrogenation kinetics, P-C-T curves and cyclic charge/discharge curves were affected by the particle size. For particles smaller than 10m, no cracks were observed. They could hardly react with hydrogen gas.
Finally, the compositional effect on the hydrogenation properties of gas-atomized and arc-melted alloy was investigated. The variation of P-C-T curves for partial substitutional of La with Zr, V, or Ta can be explained based on the synergistic effects of atomic size and hydride formation enthalpies of the substitutional elements. Substitution of La by 0.1 Zr, or 0.1 V is effective in improving the electrochemical activation rate and increasing the discharge capacity. The activation rate can also be improved by replacing La with 0.2 Zr or 0.1 Ta, but the discharge capacity is decreased slightly.
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