Ceramics contact resistance

The investigations of SOEs with oxygen ion conductivity (mainly based on zirconia, ceria and gallates) and with protonic conductivity (mainly cerates and zirconates) included detailed studies of their ceramic and transport properties, contact resistance of grains in SOE, ageing processes depending on the mode of fabrication, temperature, composition of material, impurities, gas atmosphere, and other factors. 

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. 

The main difficulty facing powder process engineers and powder plant designers is the need for information on two powder phenomena firstly, the resistance to product flow generated by particles within the product and secondly, how easily a powdered product can flow over the metal, plastic or ceramic contact area or internal surfaces of plant equipment. 

Fig. 37.13 Contact resistance (left axis) and sample resistance (right axis) as a function of temperature recorded for two sample-contact element combinations (A) YBa2Cu307-6 ceramic pellet sample with gold contacts (B) YBa2Cu307-6 ceramic pellet with copper contacts. (Adapted from Ref. 4.)...

This systematic study led to an optimised steel composition - at laboratory scale - with small additions of Mn, La, and Ti but without any Al and Si. This steel forms the desired thin arid electrically conductive oxide scales [55, 56], good contact resistances with ceramic coatings [57] (Figure 7.7), and reduced permeability for volatile chromium species [58]. 

Lanthanum chromite has provided long lifetimes, as long as 69,000 h in Siemens Westinghouse tubular cells, at 900-1000°C. However, metallic interconnects have not yet shown equivalent lifetime performance. Improvements in metallic interconnect compositions and contact layers between cells/interconnects are still issues for materials development. In particular, the metal/ceramic interface in cells should have low corrosion, low contact resistance and low permeability of chromium species. Recent results have shown that optimised steels for SOFC applications are available and alkaline earth-free and cobalt-containing perovskites are the most suitable materials for contact layers however, their long-term performance under fuel cell operation conditions needs to be proven. 

Cadmium Sulfide Photoconductor. CdS photoconductive films are prepared by both evaporation of bulk CdS and settHng of fine CdS powder from aqueous or organic suspension foUowed by sintering (60,61). The evaporated CdS is deposited to a thickness from 100 to 600 nm on ceramic substates. The evaporated films are polycrystaUine and are heated to 250°C in oxygen at low pressure to increase photosensitivity. Copper or silver may be diffused into the films to lower the resistivity and reduce contact rectification and noise. The copper acceptor energy level is within 0.1 eV of the valence band edge. Sulfide vacancies produce donor levels and cadmium vacancies produce deep acceptor levels. 

Materials that come in contact with wet halogens must be corrosion-resistant. Glass, ceramics, tantalum, and fiuoropolymers are suitable materials. Granite has been used in steaming-out towers. 

Phenolic-neoprene contact cements are used for structural metal-metal bonding. especially where fatigue resistance and low temperature performance are important [209]. They are also used for bonding textiles, wood, rubbers, plastics, ceramics, and glass to metal and to one another. Solvent toxicity and flammability has greatly reduced the use of contact cements in the wood products industry. Water-based contact cements persist, but generally do not perform as well as the solvent systems, thus allowing market erosion by alternative binders. 

Aluminum Foil. Studies of various foods wrapped in aluminum foil show that food products to which aluminum offers only fair resistance cause little or no corrosion when the foil is in contact with a nonmetallic object (glass, plastic, ceramic, etc.) The reactions, when found, are essentially chemical, and the effect on the foil is insignificant. However, when the same foods are wrapped or covered with foil that is in contact with another metallic object (steel, tinplate, silver, etc.), an electrochemical or galvanic reaction occurs with aluminum acting as the sacrificial anode. In such cases, there is pitting corrosion of the foil, and the severity of the attack depends primarily on the food composition and the exposure time and temperature. Results obtained with various foods cov-... 

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