Thermal and Chemical Expansion

It follows from the work of Darling and others that formation of the beta phase palladium hydride must be avoided to prevent embrittlement, cracking and delamination of thin catalytic films of unalloyed palladium used on composite membranes of Nb, Ta, Ti, V, and Zr. This also holds true for alloys of palladium susceptible to hydrogen embrittlement. Likewise, transformation of Nb, Ta, Ti, V and Zr substrates into brittle hydrides at low temperatures or at high hydrogen partial pressures must also be avoided. 

Already by 1963, for a patent granted in 1966, Straschil and Lopez realized that the match of coefficient of thermal expansion between palladium membranes and (porous) substrates was critical, and stated that it would be virtually impossible to compensate for differences in dilation due to absorption of hydrogen [38]. They patented the use of dimpled or corrugated foils to accommodate differential thermal and chemical expansion [38]. Buxbaum and Hsu, in a 1992 patent, maintained that a rough substrate surface produced by abrasion with steel wool was critical for adherence of palladium on surfaces of Nb, Ta, V and Zr [39]. Other patents recommend corrugated or undulating configurations to allow for both thermal and chemical expansion [24, 26, 27, 29]. 

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Because of the existence of SrMo04 impurities in air-fired SMMO, it is likely that the same impurities are formed upon treatment of H2-synthesized SMMO in high temperature air. Depending on the thermal and chemical expansion properties of this phase, these impurities could result in a decrease in performance with redox... 

Chen, X., Yu, X Adler, S.B. Thermal and chemical expansion of Sr-doped lanthanum cobalt oxide (Lai xSrxCo03 j). Chemistry of Materials 17 (2005), pp. 4537-4546. 

Fig. 1 Combined thermal and chemical expansion of Lao.3Sro.7Feo.6Gao.403 in pure O2 (1 atm) and in inert atmosphere with p02 = 10 atm (redrawn from [80])...

Thermochemical stresses in SOFC and other energy conversion systems are also due to differences between thermal and chemical expansion of component materials, as well as differences between processing, prospective operation conditions... 

Challenges Imposed by Thermochemical Expansion of Solid State Electrochemical Materials covers the topic of chemical expansion, which is due to the creation of point defects in a material due to changes in oxygen activity/ stoichiometry. The chapter covers the measurement of chemical expansion, the analysis of various phases, and finally the stress induced in materials from both thermal and chemical expansions. 

The origin of chemically induced stresses is a change of the lattice distance of materials due to changes in the physical state. For example, it is known that the lattice constant of LaCrCL increases under a reducing atmosphere [3], On the basis of the analogy between the thermal expansion and chemical expansion, the chemical expansion behavior can be simulated along with the thermal expansion behavior. 

Many methods have been proposed to address this issue (see Chapter 9). Beside thermal and chemical resistances of the sealing materials other issues need to be considered as well. One such important issue is the mismatch of the thermal expansion coefficients between the membrane element and the sealing material or joining material. While similar material design and engineering problems exist in ceramic, metal and ceramic-metal joining developmental work in this area is much needed to scale up gas separation units ot membrane reactors for production. The efforts are primarily p ormed by the industry and some national laboratories. 

Electrodes The anodes of SOFC consist of Ni cermet, a composite of metallic Ni and YSZ, Ni provides the high electrical conductivity and catalytic activity, zirconia provides the mechanical, thermal, and chemical stability. In addition, it confers to the anode the same expansion coefficient of the electrolyte and renders compatible anode and electrolyte. The electrical conductivity of such anodes is predominantly electronic. Figure 14 shows the three-phase boundary at the interface porous anode YSZ and the reactions which take place. The cathode of the SOFC consists of mixed conductive oxides with perovskite crystalline structure. Sr doped lanthanum manganite is mostly used, it is a good /7-type conductor and can contain noble metals. 

S.4. Thermal and Chemical Ring Expansion of Mono- and Dihalobicyclo 4.1.0]heptanes, and Related Compounds... 

The structures with D = 3 (frameworks) are usually most stable mechanically, thermally, and chemically. The layered (D = 2) and chain (D = 1) structures, and especially structures with finite anions (D = 0) with similar bond strength and polarity, are more labile many of these may absorb water into the interlayer or interchain space, or hydrolyze to produce amorphous products or even be water-soluble. In addition, structures with D = 1 or 2 usually display strong anisotropy of the conductivity and/or thermal expansion (although some exceptions are possible). Multiple examples with D = 0, 2, and 3 are listed in Sections 7.4—7.6. 

Industrial applications of zeolite membranes can be considered only for separations where they offer some unique advantage in terms of flux, selectivity, or thermal and chemical stability. The very high fluxes obtained with LTA membranes (typically 100 times higher than those obtained with a polyacrylonitrile membrane at the same FLO/alcohol selectivity) explain the rapid expansion of this type of application at the end of the 90s [8J. 

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