Influence Of Strip Casting And Heat Treatment On The Electrochemical Properties Of Nd-Free Lpc-Type Hydrogen Storage Alloys

At present, Nd and Ce are required mostly by the utilization and development of NdFeB and automobile catalyst, the mountains of Nd-free LPC mischmetal, in which LPC represents the abbreviation of La -Pr-Ce mischmetal after the extraction of most Ce and Nd, hindered the healthy development of rare earth industries, so the feasibility of utilizing LPC mischmetal to produce hydrogen storage alloys was demonstrated.As we know, the thickness of hydrogen storage alloy ingots in practical production was popularly within 10-30mm and in research is usually less than 1mm, such as rapid quench and gas atomization. In this study, strip casting was introduced and the influence of the thickness of ingots within 3-10mm on the electrochemical properties of high-Co ( LPCNi3.55Co0.75Mn0.4Al0.3 ) and low-Co (LPCNi3.55Co0.4Mn0.4Al0.3Fe0.15Cu0.1Si0.1) LPC-type hydrogen storage alloys was investigated using open and H-type three-electrode system, and the Sichuan high-Co Mm-type hydrogen storage alloy (MmNi3.55Co0.75Mn0.4Al0.3) was used as comparison.It was found that the comprehensive electrochemical properties of high-Co hydrogen storage alloys were improved with the increase of the thickness of ingots. It was probably attributed to three factors: the expansion of unit cell volume, the decrease of the crystal defects and the diminishing of the internal stress. Theelectrochemical properties of low-Co LPC-type hydrogen storage alloys were insensitive to the thickness of ingots, which were maybe due to the substitution of Cu, Fe and Si for Co element.As is known to all, heat treatment, which was usually at high temperature(>1000 ℃), is an effective method to improve the comprehensive electrochemical properties of hydrogen storage alloys. In this work, the effect of heat treatment procedure, namely at middle temperature (<900℃) and different heat holding time, on the electrochemical properties of high-Co and low-Co LPC-type hydrogen storage alloys was studied. It was found that after heat treatment the maximum discharge capacities, especially those at high rate, of this two alloys were improved, and so the activation property, the cyclic life and high-rate discharge ratio (HRDR). The alloys with different thickness after heat-treated at middle temperature for 4 hours could show better comprehensive electrochemical properties. The thinner the thickness of ingots, the better the effect of heat treatment. The comprehension electrochemical properties of the high-Co LPC-type hydrogen storage alloys with different thickness have been improved to close to each other after heat treatment. The improved properties of low-Co alloys with different thickness were still approximate and insensitive to the thickness of ingots. The improvement of the electrochemical properties of hydrogen storage alloys by heat treatment was mainly attributed to the diminishing of internal stress and defects introduced by casting.It was also found that the excessive prolongation of annealing time, for example 10 hours, would deteriorate the comprehensive electrochemical properties of alloys, so it was extremely important to select an optimal heat treatment procedure to obtain alloys possessing excellent comprehensive electrochemical properties.Moreover, the properties of annealed low-Co LPC-type alloys exceeded those of high-Co LPC-type hydrogen storage alloys except for the discharge capacity. It was worth paying attention to reduce production costs of hydrogen storage alloys by lowering the Co content.The electrochemical properties of annealed high-Co LPC-type alloys with different thickness also exceeded those of Sichuan high-Co Mm-type alloys, whichillustrated the fact that the Nd-free LPC mischmetal could be used to produce hydrogen storage alloys and would possess well performance.