Electrical contact resistances

Rhodium. Rhodium is the most commonly plated platinum-group metal. In addition to its decorative uses, rhodium has useful properties for engineering applications. It has good corrosion resistance, stable electrical contact resistance, wear resistance, heat resistance, and good reflectivity. The use of rhodium for engineering purposes is covered by an ASTM specification (128). Typical formulas are shown in Table 15. The metal content is obtained from prepared solutions available from proprietary plating supply companies. Replenishment is required because anodes are not soluble. Rhodium for decorative use may be 0.05—0.13 Jm thick for industrial use, it may be 0.50—5.0 Jm thick. 

Tin—Zinc, Baths for tin—zinc alloys stem from work done after World War II in efforts to find a substitute for cadmium. Although alloys of all concentrations are possible, 80% tin—20% zinc gives the best combination of properties. This alloy has a low coefficient of friction, low electrical contact resistance, is solderable, slightly anodic to steel, and does not form voluminous corrosion products. In addition, the tin—zinc alloy has good paint adhesion qualities, good ductility, and is easily spotwelded. 

ECR Both sulfonates and ZDDPs form electrically resistive film. Electrical contact resistance measurements may help to guide the work of synthetic and mineral oil formulation chemists (Komvopoulos, et al., 2002 Yamaguchi et al., 1997). 

XRF = X-ray fluorescence spectroscopy, XPS = X-ray photoelectron spectroscopy, AES = Auger electron spectroscopy, XANES = X-ray absorption near edge spectroscopy, RAIR = Reflectance-absorbance infrared spectroscopy, EXAFS = X-ray absorption fine-structure spectroscopy, ECR = Electric contact resistance, NMR = Nuclear magnetic resonance spectroscopy, IPS = Imaging photoelectron spectromicroscopy. 

Two compression bars are shown across the top of each module in Figure 12. These bars are used to maintain compression on all of the stacks during operation in order to minimise electrical contact resistance between the cells, flow fields and interconnects. The bars are held in compression via spring-loaded tie-downs located outside of the hot zone under the base plate. 

The internal resistance of a fuel cell includes the electric contact resistance among the fuel cell components, and the proton resistance of the proton-conducting membrane. In PEMFCs, the proton resistance of the polymer electrolyte membrane contributes the most to the total ohmic resistance. 

Additional parameters specified in the numerical model include the electrode exchange current densities and several gap electrical contact resistances. These quantities were determined empirically by comparing FLUENT predictions with stack performance data. The FLUENT model uses the electrode exchange current densities to quantify the magnitude of the activation overpotentials via a Butler-Volmer equation [1], A radiation heat transfer boundary condition was applied around the periphery of the model to simulate the thermal conditions of our experimental stack, situated in a high-temperature electrically heated radiant furnace. The edges ofthe numerical model are treated as a small surface in a large enclosure with an effective emissivity of 1.0, subjected to a radiant temperature of 1 103 K, equal to the gas-inlet temperatures. 

O2 (Air) fuel cell is ca. 0.5-0.6 V [28, 29]. This deviation is primarily caused by the large overpotential associated with the reduction of O, but the decrease also can be attributed to other resistances in the fuel cell itself, including solution resistance and electrical contact resistance. 

The electrical connection between bipolar plates, GDL and MEAs is achieved by applying force between the endplates of the stack. Since there are variations in the electrical contact resistance as a fimction of mechanical pressure, with possible swelling of the MEAs, an analogous thermal contact resistance between the cells was introduced. 

Electrical contact resistance is distinct from the intrinsic coating resistivity, which is quite high. A1 oxide films thickened by contact with chromate solutions [162], and CCCs themselves [160], demonstrate high electrical resistance. DC resistances of CCCs measured with a small (4 x 10 cm ) Hg droplet range from 10 to 1012 2 [163]. 

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. 

Thermal and electric contact resistances are taken into account as a function of local temperature and are referred to a specific contact surface (horizontal and vertical) between the different elements of equipment, i.e., die/compact, punch/ compact, punch/die, and graphite-spacer/punch, as shown in Fig. 6.7 [31]. As a result, the obtained contact conductances and thermal conductivities are in the range of 5 X 10 -7 X 10 S m and 3 x 10 -2 x 10 W m K respectively. 

Fig. 6.16 Drops of temperature or voltage at an imperfect interface with apparent contact area (SA), due to the thermal contact resistance or electrical contact resistance as heat (qc), or electric current flux (j), which flows across the interface, respectively. Joule heat (q ) is also generated and evenly distributed into two parts. qi and 52 sr heat flux in parts 1 and 2, respectively. Reproduced with permission from [4] by Zavaliangos et al. Copyright 2004, Elsevier...

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