【作者】 潘崇超；

【导师】 于荣；

【作者基本信息】 清华大学 ， 材料科学与工程， 2009， 博士

【摘要】 本文在全面综述La-Mg-Ni储氢合金的国内外研究基础上,确定Nd-Mg-Ni-Co系储氢合金作为研究对象。采用了XRD、TEM、SEM、PCT、EIS、VSM、MSTAT等材料的结构表征和性能测试方法,分别研究了非化学计量、磁场热处理、退火热处理和快速凝固对合金结构和性能的影响,发现了通过磁场热处理大幅改善储氢合金电极倍率性能的新方法,并探索相关机理。通过改变化学计量比优化确定Nd0.75Mg0.25(Ni0.8Co0.2)3.8合金具有较好的综合电化学性能。采用透射电子显微镜确定合金中的Ce2Ni7型超结构,根据Van’t Hoff方程拟合得到了Nd0.75Mg0.25(Ni0.8 Co0.2)3.8合金吸放氢过程的热力学参数。系统研究了温度(100℃~ 650℃)和磁场强度(2 T~10 T)对磁场热处理后Nd0.75Mg0.25(Ni0.8Co0.2)3.8储氢合金组织结构、吸放氢动力学性能和磁性能的影响。结果表明,磁场热处理可以大幅改善合金电极的倍率性能和气态吸氢速率。发现在场强为10 T和650℃下磁场热处理3 h,合金的动力学性能最好。对铸态、退火热处理和磁场热处理三种状态储氢合金的进行对比分析,XRD图谱全谱拟合和TEM组织观察发现磁场热处理没有改变合金中的相组成,但使得Ce2Ni7型(Nd,Mg)2(Ni,Co)7相单胞沿c轴方向被拉长,a轴不变,从而导致四面体间隙尺寸增大。电化学阻抗谱和线性极化曲线实验结果表明磁场热处理后合金的反应阻抗减小,而极化电流增大,从而导致氢在合金中的扩散所需克服的阻力减小,动力学性能大幅增加。c轴被拉长是由于磁场热处理作用导致合金中的(Nd,Mg)2(Ni,Co)7相发生单轴取向,通过第一性原理计算也表明磁场热处理使得(Nd,Mg)2(Ni,Co)7相磁矩增加,合金磁化强度也增强。退火热处理可以改善Nd0.75Mg0.25(Ni0.8Co0.2)3.5储氢合金电极的循环稳定性。研究了电化学循环过程中的晶体结构稳定性、颗粒粉化情况和表面腐蚀行为,系统分析了循环衰退机理。退火热处理降低合金在氢化过程中的体积膨胀、减小合金颗粒粉化和晶格内应力,同时增加合金成份的均匀性,提高合金的晶体结构稳定性,减缓了合金放电容量的衰减。快淬可以改善Nd0.75Mg0.25(Ni0.8Co0.2)3.8储氢合金电极的循环稳定性。由于淬速越高,晶粒尺寸变小或出现部分非晶,成分均匀性得到提高,晶格内应力增加。在带速为20 m/s时合金电极具有较好的综合电化学性能。更多还原

【Abstract】 In this thesis, based on a comprehensive review of the research and development of the La-Mg-Ni hydrogen storage alloys, the Nd-Mg-Ni-Co system hydrogen storage alloys have been selected and studied in order to optimize the overall electrochemical properties and to explore new method and mechanism to modify the rate properties. The effect of non-stoichiometry, magnetic annealing, annealing treatment and rapid solidification on the structural, electrochemical and magnetic properties of Nd-Mg-Ni-Co system alloys have been investigated by means of XRD, TEM, SEM, PCT, EIS and VSM analyses.The Nd0.75Mg0.25(Ni0.8Co0.2)3.8 hydrogen storage alloy has better overall electrochemical properties by non-stoichiometric optimization. The Ce2Ni7 type superstructure of alloys was characterized by TEM. The thermal dynamic parameters of hydrogen absorption and desorption have been obtained by modeling according to Van’t Hoff equations for the Nd0.75Mg0.25(Ni0.8Co0.2)3.8 hydrogen storage alloys.The effect of temperature and magnetic field strength on the structural, electrochemical and magnetic properties have been studied systemically for Nd0.75Mg0.25(Ni0.8Co0.2)3.8 hydrogen storage alloys. The results show that magnetic annealing can improve the rate properties and hydrogen absorption rate largely. It is better to do magetic annealing for 3 hours at 10 T and 650 oC. The as-cast, magnetic annealing and annealing Nd0.75Mg0.25(Ni0.8Co0.2)3.8 hydrogen storage alloys have been studied for comparison. Rietveld analyses results show that magnetic annealing has little effect on the phase composition for the Nd0.75Mg0.25 (Ni0.8Co0.2)3.8 hydrogen storage alloys, the lattice parameter c of Ce2Ni7 type (Nd,Mg)2(Ni,Co)7 phase increases, but the lattice parameter a has little change, the size of tetrahedral interstitial increase accordingly, the results are identical with TEM characterization. The decrease of reaction resistance and the increases of polar current for the alloys have been obtained by the test of EIS and polar curves respectively. So the resistance of hydrogen diffusion in the alloys decreases. The dynamic property increases largely. The c-axis was elongated by uniaxial anisotropy along the c-axis in magnetic annealing. In addition, the results which magnetic moments is increased for (Nd,Mg)2(Ni,Co)7 phase by magnetic annealing are confirmed by the first principle calculation.The cycle stability has been improved by annealing for Nd0.75Mg0.25(Ni0.8 Co0.2)3.5 hydrogen storage alloys. The three elements which include stability of crystal structure, grain pulverization, and surface corrosion have been analyzed systemically during the electrochemical cycle. After annealing treatment, the rate of volume expansion during of hydrogenation, the grain pulverization and the inter stress of grain decreases, the element composition is more uniformly, the structural stability is improved, the corrosion of alloy element is restrained, so the cycle stability properties is improved.The cycle stability of Nd0.75Mg0.25(Ni0.8 Co0.2)3.8 hydrogen storage alloy electrode has been modified by rapid solidification. The speed of melt-spun is higher, the grain size is smaller or some amorphous grain is produced, the element composition is more uniformly, the inter grain stress increases. It exhibits good overall electrochemical properties when the speed of melt-spun is 20 m/s.更多还原