RENi5贮氢合金的组织与电化学性能

【作者】 田娜；

【导师】 井晓天；

【作者基本信息】 西安理工大学 ， 材料学， 2001， 硕士

【摘要】 本文在 LaNi₅贮氢合金的基础上，以混合富镧稀土Ml和混合富铈稀土 Mm相互取代制得Ml_αMm_1-αNi₅（α＝l，0．8，0．6，0．4，0．2，0）系列贮氢合金，采用XRD、金相检测、扫描电镜、能谱分析、电化学方法等对所得合金的相结构、晶体结构、组织与形貌及电化学性能、稀土成分对它们的影响等进行了测试分析。研究结果表明： 1.铸态Ml_αMm_1-αNi₅合金的主相为CaCu₅型结构（除极少量的稀土氧化物和镍）。晶格参数计算结果表明，随α值的减小，点阵常数a减小，而c变化不大，晶胞体积V减小，且与α呈线性关系：V=83．81474＋1．96128α。同时，MI_αMm_1-αNi₅合金中多面体间隙随之减小。合金熔炼过程中发生离异共晶，使微量镍在晶界和晶内偏析。合金成分偏聚主要以析出相的方式进行，晶界与晶内的析出相的合金成分与相应晶界合金成分一致。 2．La被Ce、Pr、Nd取代后，放氢平台压力升高，而且升高程度从大到小的次序为。Ce＞Nd＞Pr：La、Pr、Nd降低滞后，其次序为：Nd＞La＞Pr，Ce增大滞后，合金中铈的含量不宜过多；La被Ce、Pr、Nd取代后，合金热力学稳定性下降。且Pf＞Ce＞Nd。通过改变原子半径不同的金属的比例，使晶胞体积发生变化，可调节平衡氢压。 3．Ml_αMm_1-αNi₅（α=1，0.6，0）活化循环次数n_a的大小存在下列次序：Ml_0.6Mm_0.4Ni₅（6次）＜MlNi₅（8次）＜MmNi₅（10次）。富镧混合稀土系合金的放电容量高于富铈混合稀土系合金；MI_αMm（1-α）Ni₅（α=1，0.6，0）贮氢合金的最大放电容量C_50-max随α的增大先增后减，在a=0.6处取得最大值228mAh/g。在实验研究的30次充放电循环过程中，MI_αMm_1-αNi₅合金的容量衰退存在以下顺序：MmNi₅＜MI_0.6Mm_0.4Ni₅＜MlNi₅。放电电流增大时，由于极化增大，电池放电容量减小，工作电压降低。 4．放电过程中，负极的电化学极化随放电深度的增加而略有增加，放电初期极化主要由电化学极化控制，随着放电的进行，浓差极化逐渐增大并控制负极电位的衰减，而欧姆极化在放电过程中变化不明显。更多还原

【Abstract】 On the basis of LaNi₅ hydrogen storage alloy, Mi_α Mm_1-αNi₅ （α=1, 0.8, 0.6, 0.4,0.2, 0） alloys were Wared by means of substitution for La with Lthe-rich（Ml） and Cerium-rich（Mm）mishchmetal. The phase structure, crystal StructUe,microstrucfore, hydrogen storage property and the influence of rare earthcomposition on them were systematically investigated by means Qf X-ray diffiaction,light and SEM metallographic examination, energy spectrum analysis,electTochemical method and so on. The results Showed that:The main phases of Ml_αMm_1-αNi₅ alloys as cast were hp6-CaCu₅ tyPe structurewith occasional traces of rear earth oxide and Ni. The calculation of crystalparameter showed that, with the decrease of α, parameter ’a’ and cell volumedecreased, ’c’ had sligh variation, and thus polyhedron interstice in the alloysreduced. The unit cell volume （V） was linear function to the Ml content α in Ml_αMm（1-α）Ni₅ alloys, Which can be expressed as V= 83.8l474 + 1.96l28 α.Nonequilibrium eutectic led to small amount of Ni segregation at grain boundary andintracrystal during solidification process. The segregation of alloy elemellt formedprecipitate phase and the composition of them at grain boundary and intracryStal wasas same as corresponding grain boundarypartial substitutions for La with Ce, Pr and Nd brought about increase in plateaupressures of alloys. The order of increasing degree from high to low was Ce, Nd andPrin truns. La, Pr and Nd reduced hysteresis of alloys. Its order was that Nd>La>Pr,whereas Ce enhanced hysteresis. The thermodynamic stability was deteriorated withpartial substituions for La with Ce, Pr and Nd. Plateau pressure of alloys can beadjusted by means of change the ratio of different radium element, which led to thevariation of cell volume.The order of activation rate （n_a） of Ml_αMm_1-αNi₅（α=l, 0.6, 0） alloys can beexpressed as Ml_0.6Mm_0.4Ni₅ （6 cycles） < MlNi₅ （8 cycles）< MmNi₅（l0 cycles）. Thedischarge caPacity of Lanthanum-rich class hydrogen storage alloys was higher thanthat of Cerium-rich class alloys. The discharge capacity of Ml_αMm（1-α）Ni₅ alloysincreased first and then decreased. The max value （228mAh/g） was gained at α= 0.6.During the process of first 30 cycles’ charge-discharge, MlNi₅ alloys decayed raPidly,while MmNi₅, alloys decayed slowly. High rate discharge led to drop of dischargecaPacity and voltage due tO stongly polarization.The electrochemical polarization determined the decay of potential at beginningof discharge. Then, the diffession polarization ialluened the polarization processand at lat becarne controIling step. OhInic polariZation eallibits a slight change.更多还原