No abstract

LaNi 5 and related substituted alloys belong to the most interesting electrode materials for rechargeable Ni/MH batteries, because large quantities of hydrogen can be rapidly and reversibly stored in these compounds. However, the storage capacity decreases during repeated charge/discharge cycles. This degradation process is closely related to corrosion of the alloy in the aqueous KOH electrolyte and is enhanced by decrepitation (fissuring of the powder particles). The latter phenomenon is the consequence of volume changes resulting from the hydrogen solid solution/hydride phase transformation upon cycling. 1 The exact mechanisms governing the initiation and propagation of alloy corrosion depend on the material's chemical composition and microstructure, on the nature of the electrolyte, and the cycling conditions and are controlled through a complex interplay between various chemical reaction and matter/charge-transport steps at an atomic scale. Accurate prediction of the lifetime behavior of Ni/MH batteries is hindered by a lack of knowledge about corrosion scale structure and chemistry, as well as about its time evolution during corrosion. Predictive models for the specific corrosion mechanisms and their interplay, together with a fundamental understanding of the processes, can be obtained only by systematic experimental analysis of the corrosion products. In the present work, a substituted AB 5 alloy with minimal decrepitation upon cycling was chosen to study corrosion mechanisms and kinetics. The alloy was cycled in a model cell for different time and temperature conditions. Corrosion products were identified by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In the past, several similar studies were made by surface analysis techniques with excellent depth resolution but poor lateral resolution. In this study, TEM offers the advantage to provide both high lateral and spatial resolution and yields insight about the corrosion scale features at the atomic level. Corrosion models are developed from the corrosion scale microstructure and time evolution.Experimental Description of alloy, electrodes, and cells.-The material studied in the present work was a mechanically ground industrial alloy annealed for 6 h at 1100ЊC with an average particle diameter of 40 m.Its crystal structure was hexagonal (CaCu 5 type) with lattice parameters a ϭ 0.4988 nm and c ϭ 0.4055 nm. The alloy was composed of rare earth metals (La, Ce, Pr, and Nd), nickel, cobalt, manganese, and aluminum (exact chemical composition given in Table I). The alloy was almost homogeneous, only a few rare earth precipitates being present (bright contrast in the SEM image of Fig. 1). Lanthanum was highly concentrated in these precipitates, probably due its larger atomic radius compared to the other Mm elements in presence.For forming the negative MH electrodes, a slurry of metal powder, carbon, and binder was pasted into a foam nickel substrate, dried, and compressed (details are given in Ref. 2). MH n...

No abstract

No abstract

No abstract

Parabolic rate constants describing the kinetics of the exclusively diffusion controlled growth of magnetite scales during simple reactions in the iron‐oxygen system are derived on basis of preceding studies on the defect structure and on the transport properties of this oxide. Furthermore, experimental results for the formation of magnetite by solid state reaction between the iron oxides wüstite and hematite are reported and compared with calculated data.

No abstract

▪ Abstract  Chemical reactivity at heterophase interfaces is reviewed with a special focus on metal-oxide and oxide-oxide interfaces. Equilibrium chemistry of interfaces is discussed in terms of processes at the macroscopic and atomic level. The dependency on thermodynamic and crystallographic variables is described and illustrated by experimental results. Any reaction-related motion of an interface is associated with numerous transport and reaction steps that include not only atom transfer across the interface, chemical reactions between species and defects and phase transitions, but also steps accommodating lattice mismatch and volume changes. Faceting and morphology evolution of interfaces are included in the considerations.