Ceramic separators contact resistances

An experiment to determine contact resistance between the gas diffusion layer (GDL) and the separators was designed using the following materials SUS304, carbon A, carbon B, and alumina ceramics. The results are shown in Figure 6-3. 

Figure 6-3 shows the actual resistance measurements under increasing load conditions. The data reveal that contact resistance of the ceramic separator plate is smaller than carbon B and equivalent to gold-plated... 

The results for contact resistance were measured comparing the glass epoxy fabricated separators to the ceramic fabricated separators. These materials were fastened with 6 MIO screws under increasing torque conditions. Figure 6-6 compares the results between each material under the increasing torque and plots the resistances observed. The contact resistances measured are for one cell under torque and include the GDL resistance. This test shows that the ceramic separator resistance is much lower than that of glass epoxy. The measured resistance for the ceramic was 77% to that of the glass epoxy separator. 

Figure 6-6. Comparison results of contact resistance between a ceramic separator and a glass epoxy separator...

The results for using ceramic materials as an integral component for micro fuel cells have conclusive evidence that ceramics make an excellent separator plate for micro fuel cells. The thin structure of ceramics allows for rigidity without sacrificing increased resistance. A lower contact resistance when compared to other common separator materials increases power output. Ceramic s inert composition also provides excellent reliability in acidic conditions of the fuel cell. Ceramic separator plates achieved the highest power density of comparable materials and subsequently produced less than 10% voltage variation over 1,000 h of testing. 

There are also techniques involving the use of nonporous, solid or liquid membranes that separate the donor phase from the receiving phase by an evident phase boundary. Most often used are three-phase systems (donor phase, membrane, and acceptor phase) or two-phase systems, in which one of the surrounding phases is the same as the membrane. Solid membranes are made of chemically resistant, hydrophobic polymers (PTFE, PVDF, PS, PP, silicates), metals (Pd alloys), or ceramic materials. Channels of membrane modules have a volume ranging from 10 to 1000 pL and, according to their geometry, can be classified as planar or fibrous. For setting up a membrane system, two modes can be used the membrane can be immersed in a sample (membrane in sample, MIS) or the sample can be introduced into a membrane (sample in membrane, SIM). In both systems, only a small amount of sample is in direct contact with membrane, because ratio of the membrane surface area to the sample volume is small. 

The discovery of a solid conductor of sodium ions by Kummer and Weber made possible the construction of sodium-sulfur cells which utilize molten or dissolved reactants separated by the ceramic electrolyte j3-(cf. Fig. 12), or, usually, j3"-alumina. The latter ceramic has a three Al-0 spinel block structure, a molar ratio of Al203-Na20 = 5, and contains 1-4% of MgO or Li20. The resistivity of the polycrystalline material at 350°C is about 5 H cm, -4 times lower than that of alumina. Other recently reported solid Na ion conductors containing phosphorus oxides do not seem to be stable in contact with sodium at elevated temperatures. ... 

The direct measuremoit of the sq>aration between two bearing surfeces of metal-on-metal hip implants tested in a hip simulator under normal walking conditions was carried out by Dowson et al (2000) using the above electrical resistance technique. The mode of lubrication in flieir experimental study was mixed, with some periods of very effective surface separatioi and others of metal-on-metal contact in each simulated walking cycle. Using the same technique, Smith et al (2001a) have demonstrated surface separation of the femoral head and acetabular cup by a lubricating film in ceramic-on-ceramic hip implants. However, it was not clear whether protein films enhanced the sur ce separation in both of these studies. 

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