Stoichiometric nonuniformity

Compositional homogeneity may strongly impact the observed properties of bulk material. Often the fact that Tc is a sensitive function of composition (the maximum Tc for BaPb1 xBixOs is observed at 13K (13) for x = 0.25, and a Tc of 34K is found for x =. 6 in Ba K BiOg (11)) means that the width of the resistive transition can be used to determine the uniformity of the product. Use of the resistive Tc onset to characterize these solid solutions, especially in bulk form can lead to errors due to stoichiometric nonuniformity. The results of Tomeno and Ando (50) who have reported that Tc remains constant at 29K for x values from 0.28 to 0.44 in Ba RbjjBiOg, are... 

Equation (83) may also be considered as applicable for a reaction on a nonuniform surface with different stoichiometric numbers of nonequilibrium stages in this case, however, the average stoichiometric number in (83) is determined not by (80), but in a more complicated manner the definition must involve averaging with respect not only to stages, but also to the kinds of the surface sites. 

A further problem in DMFC operation is due to the evolution of gaseous CO2 at the anode (Ye et al 2005b). Then gas bubbles that can locally interfere with the flow of the aqueous methanol solntion may form in the flow field on the anodic side of the bipolar plates. This leads to a nonuniform distribntion of the reaction (and thus current) across the MEA snrface. This effect is particularly noticeable when the solution is snpplied passively (e.g., by free flow from a tank above). To overcome it, one shonld nse an active reactant supply at flow rates several times in excess of the stoichiometric requirements (Cowart, 2005). This raises the question of how to dimension the means of pumping the solution through (and what energy they wonld consnme). The dimensions would depend on the pressure drop within the flow field channels between solution input and outlet (Yang et al., 2005). 

---