Enhancing the Operating Life and Performance of Lead-Acid Batteries via Grain-Boundary Engineering
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RS BULLETIN/NOVEMBER 1999
in dimensional changes in the electrodes over time (i.e., grid "growth"); this causes adjacentplates to short, leading to reduced battery capacity.4 Moreover, in order to maintain sufficient operating- and cyclelife performance, considerable thickness allowances are required (0.5-3 m m ) , which commensurately increase the size and weight of the battery. While new alloy developments have effected signifi cant improvements in positive-electrode Performance, intergranular-degradation susceptibility continues to restrict the ca-
pacity, increase the size and weight, and accordingly diminish the overall Perfor mance of lead-acid batteries. Iniergranular-degradation processes occur exclusively at grain boundaries and lead to component failure by propagating through the intercrystalline network. Ac cordingly, these processes are dependent upon specific grain-boundary structure, grain-boundary chemistry (i.e., sohlte segregation and precipitation), and grain size and shape (i.e., Connectivity). Previous studies (see Reference 5) have shown that grain boundaries crystallographically described by low-2 coincidence Site lattice (CSL)6 relationships (2 < 29) possess more ordered structures, are less prone to solute interaction, and can often selectively display a high resistance (and sometimes immunity) to corrosion, intergranular sliding, cavitation, and fracture. Recent advances in automated crystallographic orientation-determination techniques (e.g., Reference 7) have now made it possible to readily evaluate g r a i n boundary-character distributions in con ventional polycrystalline materials and allow for the optimization of materialsprocessing t e c h n i q u e s (e.g., casting, working, annealing, etc.) in Order to en hance the overall population of low-2 grain boundaries and yield microstructures that are resistant to intergranular degradation. Moreover, recent advances in the cost-effective synthesis of nanostructured materials (with grain sizes in the 3-100-nm ränge) have significantly expanded the allowable ränge of grain-size control and, consequently, the scope of opportunity for Controlling both the intercrystalline Connectivity and grain-boundary chemistry (i.e., dilution of harmful solutes) in polycrystalline materials. In this article, we present geometric modeis for grain-boundary structure and grain-size effects on intergranulardegradation susceptibility in materials, and we provide specific examples of the practical application of this "grainboundary-engineered" (GBE) approach to enhancing the performance and oper ating life of lead-acid batteries.
Grain-Boundary Structure Effects on Intergranular Cracking and Corrosion Intergranular Cracking Figure 1. Cross-sectional optical micrograph of a lead-acid positive battery grid (Pb-1wt%Sb) segment following approximately four years of Service.
In quantifying the effect of grain size and special grain-boundary (i.e., £ ^ 29) frequency on bulk intergranular-cracking susceptibility, a geometric model has been formulated 8 which con
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