稀土系过化学计量比储氢电极合金的相结构及电化学性能研究

【作者】 李芳红；

【导师】 康龙； 罗永春；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2003， 硕士

【摘要】 本文以低钴非化学计量比稀土系储氢合金MmNi_y-1.4-xAl_0.5Mn_0.5Cu_0.4Co_x（x=0～0.2；y=5.2～5.5）为研究对象，采用XRD、SEM/EDS以及电化学测试等手段，比较系统地研究了储氢合金的化学计量比以及退火处理和快速凝固等制备技术对上述储氢合金组织结构和电化学性能的影响，力求通过对储氢合金化学计量比和制备工艺的研究，进一步改善合金的相结构，提高储氢合金的电化学性能。 对常规熔铸及其退火处理合金的研究结果表明：过化学计量比的常规熔铸合金为粗大的树枝晶组织，合金中除LaNi₅主相外，还存在富含Mn、Al的第二相，第二相零散地分布在枝间区域，且其数量随化学计量比的增大而增多；常规熔铸合金经1000℃×10h退火处理后，少量第二相溶解，晶粒尺寸并没有发生明显变化。电化学测试结果表明，常规熔铸合金具有很好的活化性能，一般只需3-4次循环即可完全活化，合金的最大放电容量随化学计量比的增大而降低，循环稳定性随Co含量的增加和化学计量比的增大而提高；退火处理后，活化次数增加，最大放电容量和循环稳定性无明显变化，常规熔铸合金在退火处理前后都具有很好的高倍率放电性能；在所研究的不同计量比的合金中，以钴含量x=0.2，化学计量比y=5.3，经1000℃×10h热处理的合金的综合电化学性能较好，其活化次数为4次，最大放电容量C_max=281.58mAh/g，高倍率放电性能HRD₆₀₀=87.5％，经250mA/g、100次循环后的容量保持率为S₁₀₀=88.15％。 对快速凝固及退火处理合金的研究结果表明：快速凝固可以完全抑制过化学计量比合金中第二相的析出，合金均为CaCu₅型单相结构，具有胞状或长条状组织，晶粒细小，成分分布均匀，无明显偏析；快速凝固合金经低温退火处理（400℃×1h和600℃×1h）后仍保持单相结构，但高温退后处理（1000℃×10h）后又有少量第二相析出。电化学测试结果表明，快速凝固合金活化速度较慢，活化次数随冷却速度的增大而增大，一般需要6-9次或更多次才能完全活化，快速凝固合金的最大放电容量随着冷却速度的增大而明显下降，与相同成分的常规熔铸合金相比，最大放电容量有所降低，但循环稳定性大幅度的提高，高倍率放电性能明显降低，且计量比越大，高倍率放电性能越好；快速凝固合金经退火处理后活化性能明显改善，一般需要3-5次循环即可完全活化，低温退火使合金的放电容量增大，循环稳定性进一步提高，而高温退火则导致快速凝固合金的放电容量稍有下降，循环稳定性降低，高倍率放电性能随退火温度的升高有较大幅度的提高。对不同计量比和冷却速度的快速凝固合金的研究结果表明，冷却速度为20m/s，钴含量x=0.1，化学计量比y=5.2，经400℃×1h退火处理的合金具有较好的综合电化学性能，其活化次数为5次，最大放电容量C_max=280.38mAh/g，高倍率放电性能HRD₆₀₀=90.76％，经250mA/g、100次循环后的容量保持率为S₁₀₀=93.5％。 由此可见，将非化学计量比的储氢合金进行快速凝固加低温退火处理可有效改善其循环稳定性，从而达到降低钴含量的目的。更多还原

【Abstract】 In this paper, the MmNiyy-1.4-xAl0.5Mn0.5Cu0.4Cox(x=0~0.2; y=5.2~5.5) Co-low nonstoichiometric rare earth hydrogen storage alloys were studied. By means of XRD analysis, SEM/EDS investigation and electrochemical measurements, the effects of chemical composition, the composition and the preparation process such as annealing and rapidly quenching on the phase structures and electrochemical properties were studied systemically.For the as-cast and annealed alloys, the results show that the as-cast alloys have a typical coarse dendritic structure, containing the second phase besides the LaNi5 main phase. The second phase distributes along the grain boundary of the main phase and its amount increases with y increases. The as-cast+annealing alloy has nearly the same phase structure as that of the as-cast alloys. The electrochemical performance measurement shows that the as-cast alloys have fewer activation number, only 3 to 4 times is needed to reach the maximum discharge capacity, and the discharge capacity of the alloys decreases with the decrease of the y value. The cycle stability is improved with the increase of y and x value. The alloys annealed at 1000℃ for 10h have higher activation number, while the maximum discharge capacity and the cycle stability have no obvious change. Both the as-cast and annealed alloys have good high-rate discharge ability. Among all the alloy samples prepared, the one with x=0.2,y=5.3, and annealed at 1000℃ for 10h has the best general performance: its maximum discharge capacity Cmax=281.58mAh/g, high-rate discharge ability HRD600=87.5%, and capacity retention S100=88.15% after 100 cycles at 250 mA/g.The results of XRD analysis, SEM/EDS and matalograph observation revealed that melt-spun alloys with different cooling rates (10, 15, 20m/s) consist of single LaNi; phase with CaCus type crystal structure with fine and homogenous cellular and strip structure. The single phase structure is due to the rapid solidification process which can effectively restrain the occurrence of the second phase. Annealed at lower temperature, the alloy remains the single phase, and at higher temperature, it changed to two phases, ie, a second phase appears. The electrochemical performance measurement shows that the melt-spun alloys have the more activation number (usually 6 to 9 times), and the discharge capacity of the melt-spun alloys decreases with increasing the cooling rate. Compared to the as-cast alloys, the mult-spun alloys has lower discharge capacity, higher cycle stability, greatly high-rate discharge ability. After annealing, the activation property is largely improved (only needs 3 to 5 times). Alloys annealed at lower temperature have higher discharge capacity, higher capacity retention. While the ones annealed at higher temperature have relatively lower discharge capacity and capacity retention. The high-rate discharge ability has largely improved with increasing annealed temperature. Both the melt-spun andmelt-spun+annealed alloys have good high-rate discharge ability. Among all the melt-spun and melt-spun+annealed alloy samples prepared, the one with x=0.1,y=5.2, and annealed at 400℃ for qh has the best general performance: its activation number 5 times, the maximum discharge capacity Cmax=280.38mAh/g, high-rate discharge ability HRD600=90.76%, and capacity retention S100=93.5% after 100 cycles at 250 mA/g.It is seen therefore that nonstochiometric hydrogen storage alloys after melt-spun and low temperature annealing will have an improved cycle stability with decreased usage of cobalt metal.更多还原