Matching of thermal expansion

A reasonably close match of thermal expansion of the coating and substrate over a wide temperature range to limit failure caused by residual stresses is desired for coatings. Because temperature gradients cause stress even in a weU-matched system, the mechanical properties, strength, and ductUity of the coating as well as the interfacial strength must be considered. 

Mechanical stability against thermal cycling between ambient temperature and Top requires matching of thermal-expansion coefficients of the electrolyte and electrodes, the interconnects, and the seals. Note Ceramic strength is improved where cell design retains the ceramic membrane under a compressive stress. 

Matching of thermal expansion coefficients. In addition to possible phase transition or structural change problems described above, there is also the issue of matching the thermal expansion coefficients of the components in a composite membrane which is composed of layers of different materials. If the thermal expansion coefficients of two adjacent layers of materials as part of the membrane element differ appreciably, any thermal cycles in normal operation or erratic temperature excursions can induce significant stresses on the components and jeopardize the physical integrity of the membrane. 

Match of thermal expansion coefficient with other components with minimal defect-induced chemical expansion... 

Despite control, few glass systems allow a match of thermal expansion coefficient to other important cell materials (typically alkaline earth-alumina-silica glasses). In any case, the cell materials don t match each other close enough to allow a rigid seal in larger cells... 

Hence, the anode must be catalytically active to oxidize the fuel and electronically conductive to transport electrons. The other requirements include matching of thermal expansion coefficients of the electrolyte and the intercoimect integrity of porosity for gas permeation chemical stability with the electrolyte and the intercoimect and applicability to use with versatile fuels and impurities (such as sulfur). In addition, cost-effectiveness is always a factor in commercialization processes. 

This chapter first considers the complex mix of attributes required of SOFC anodes, including matching of thermal expansion coefficients, chemical compatibility with the electrolyte and the interconnect, porous structure to allow gas permeation, and corrosion resistance to the fuel and impurities therein. Then the nickel cermet anode is described in detail, especially its fabrication processes. Steady-state anode reactions of hydrogen and carbon monoxide are analysed, followed by a description of transient effects. Finally, behaviour under current load and operation on different fuels are discussed. The details of the anode reactions and polarisations are described in Chapter 9. 

In spite of having many favorable characteristics, the metallic interconnects also suffer fi-om certain drawbacks. Some of the pertinent issues are electrical contacts between metallic interconnect and ceramic electrodes (cathode and anode), matching of thermal expansion between the metallic interconnect and adjacent components, oxide scale formation on the metallic surface as well as cathode poisoning. All these issues need drastic improvement. For more than a decade, a number of alloys have been attempted, but the major interest for development of such metallic interconnect started only when SOFC developers started using metallic interconnects for SOFC operation, preferably < 750°C. 

The thermal expansivity of Ni—Fe alloys vary from ca 0 at ca 36 wt % Ni (Invar [12683-18-OJ) to ca 13 x 10 / C for Ni. Hence, a number of compositions, which are available commercially, match the thermal expansivities of glasses and ceramics for sealing electron tubes, lamps, and bushings. In addition, the thermal expansion characteristic is utilized ia temperature controls, thermostats, measuriag iastmments, and condensers. 

Cases can be classified as either hermetic or nonhermetic, based on their permeabiUty to moisture. Ceramics and metals are usually used for hermetic cases, whereas plastic materials are used for nonhermetic appHcations. Cases should have good electrical insulation properties. The coefficient of thermal expansion of a particular case should closely match those of the substrate, die, and sealing materials to avoid excessive residual stresses and fatigue damage under thermal cycling loads. Moreover, since cases must provide a path for heat dissipation, high thermal conductivity is also desirable. 

Film stress arises owing to the manner of growth and the coefficient of expansion mismatch between the substrate and film material (4). In many CVD processes, high temperatures are used. This restricts the substrate-coating material combinations to ones where the coefficient of thermal expansions can be matched. High temperatures often lead to significant reaction between the deposited material and the substrate, which can also introduce stresses. 

Where a chrect fusion of glass-to-metal or metal oxide is obtained (sometimes termed the wetting of metal by glass) two types of seal can result—one matched, the other mismatched, depending on how the relative coefficients of thermal expansion of the glass and the metal compare. 

Any one of the three components in SOFC, the cathode, anode, or electrolyte, can provide the structural support for the cells. Traditionally, the electrolyte has been used as the support however, this approach requires the use of thick electrolytes, which in turn requires high operating temperatures. Electrode-supported cells allow the use of thin electrolytes. The Siemens—Westinghouse Corporation has developed a cathode-supported design,although this has required electrochemical vapor deposition of the YSZ electrolyte. Most other groups have focused on anode-supported cells. In all cases, it is important to maintain chemical compatibility of those parts that come in contact and to match the thermal expansion coefficients of the various components. A large amount of research has been devoted to these important issues, and we refer the interested reader to other reviews. 

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