摘要
本文在 LaNi_5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-α)Ni_5(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-α)Ni_5合金的主相为CaCu_5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-α)Ni_5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-α)Ni_5(α=1,0.6,0)活化循环次数n_a的大小存在下列次序:
Ml_(0.6)Mm_(0.4)Ni_5(6次)<MlNi_5(8次)<MmNi_5(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)Ni_5(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-α)Ni_5合金的容量衰退存在以下
顺序:MmNi_5<MI_(0.6)Mm_(0.4)Ni_5<MlNi_5。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。
On the basis of LaNi_5 hydrogen storage alloy, Mi_α Mm_(1-α)Ni_5 (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 earth
composition 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-α)Ni_5 alloys as cast were hp6-CaCu_5 tyPe structure
with occasional traces of rear earth oxide and Ni. The calculation of crystal
parameter showed that, with the decrease of α, parameter 'a' and cell volume
decreased, 'c' had sligh variation, and thus polyhedron interstice in the alloys
reduced. The unit cell volume (V) was linear function to the Ml content α in Ml_α
Mm(1-α)Ni_5 alloys, Which can be expressed as V= 83.8l474 + 1.96l28 α.
Nonequilibrium eutectic led to small amount of Ni segregation at grain boundary and
intracrystal during solidification process. The segregation of alloy elemellt formed
precipitate phase and the composition of them at grain boundary and intracryStal was
as same as corresponding grain boundary
partial substitutions for La with Ce, Pr and Nd brought about increase in plateau
pressures of alloys. The order of increasing degree from high to low was Ce, Nd and
Prin 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 with
partial substituions for La with Ce, Pr and Nd. Plateau pressure of alloys can be
adjusted by means of change the ratio of different radium element, which led to the
variation of cell volume.
The order of activation rate (n_a) of Ml_αMm_(1-α)Ni_5(α=l, 0.6, 0) alloys can be
expressed as Ml_(0.6)Mm_(0.4)Ni_5 (6 cycles) < MlNi_5 (8 cycles)< MmNi_5(l0 cycles). The
discharge caPacity of Lanthanum-rich class hydrogen storage alloys was higher than
that of Cerium-rich class alloys. The discharge capacity of Ml_αMm(1-α)Ni_5 alloys
increased first and then decreased. The max value (228mAh/g) was gained at α= 0.6.
During the process of first 30 cycles' charge-discharge, MlNi_5 alloys decayed raPidly,
while MmNi_5, alloys decayed slowly. High rate discharge led to drop of discharge
caPacity and voltage due tO stongly polarization.
The electrochemical polarization determined the decay of potential at beginning
of discharge. Then, the diffession polarization ialluened the polarization process
and at lat becarne controIling step. OhInic polariZation eallibits a slight change.
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