Electrical stability contact loss

A new area of concern for electrical stability arises because of the increasing use of conductive adhesives as replacements for solder. Some conductive adhesives show unstable electrical-contact resistance when used on non-noble metal surfaces such as copper or tin-lead solder. Although stable on gold, palladium, platinum, and silver surfaces, the same adhesives were found to be unstable on tin, tin-lead, copper, and nickel surfaces.The unstable resistance and increase in resistance in temperature-humidity exposures have been attributed to the growth of an oxide layer separating the filler particles from the substrate at the interface, a mechanism similar to that for the loss of backside contact in die-attach materials. 

The heart of a fuel cell is the membrane electrode assembly (MEA). In the simplest form, the electrode component of the MEA would consist of a thin film containing a highly dispersed nanoparticle platinum catalyst. This catalyst layer is in good contact with the ionomeric membrane, which serves as the reactant gas separator and electrolyte in this cell. The membrane is about 25-100 p,m thick. The MEA then consists of an ionomeric membrane with thin catalyst layers bonded on each side. Porous and electrically conducting carbon paper/cloth current collectors act as gas distributors (Figure 27.1). Since ohmic losses occur within the ionomeric membrane, it is important to maximize the proton conductivity of the membrane, without sacrificing the mechanical and chemical stability. 

An important function of the GDL is to even out the compression forces in the stack. These compression forces are also a source for degradation. Lee and Merida concluded from ex situ experiments that the compressive strain increased with the applied pressure but even more strongly with temperature, and the GDL strain was influenced by the PTFE stability [158]. Properties such as in-plane electrical resistivity, surface contact angle, bending stiffness, and porosity were not affected. However, it was found that convective airflow through the GDL under strain can lead to loss of material. The GDL degradation can also contribute to sudden failure as GDL fibers can puncture the membrane, either when as a result of degradation they do not distribute the compression forces well any more or when they are even broken themselves. 

Electric Fuel Ltd. which was modified by the inclusion of an anionic polymeric membrane. The polymeric membrane was composed of interpenetrated network of two polymers. A polycationic cross-linked polyepichlorohydrin was used as the ionic network and poly(hydroxyl ethyl methacrylate) was used as the structural polymer to provide mechanical stability and reduced swelling. The cyclic performance of the cell using a saturated aqueous solution of LiOH and untreated ambient air is shown in Fig. 13b. Relatively high capacities were obtained however, on cycling the lithium metal formed a porous or columnar deposit that increased in volume and caused a loss of contact between the hthium metal and O-LATP [50]. The hfetime of this composite air electrode when used with untreated air in 5 M or saturated LiOH aqueous solution was increased firom 10 h without the anion exchange membrane to 1000 h. 

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