Thermo-mechanical stresses

In addition to the electrochemical reactions, reactants and biphasic water transport, other mechanisms limiting optimal catalyst utilization are charge transfer, thermo-mechanical stresses and irreversible materials degradation. Typical experiments show that the state-of-the-art DMFC in a stack environment loses 10-20 pV per hour of operation at a current density in the order of 100 mA.cm [24]. This limits DMEC lifetime by several thousand hours, which still is not sufficient for widespread commercialization. 

In order to improve durability and cost-efficiency of the cells, the stacks, and the system, much of the development has in the past focused on lower operating temperature, increased power density, and material savings based on reduced cell and stack component thickness. Current SOFC technology is relying upon cells made of ceramics with their inherent, somewhat unrehable properties in addition to their intolerance toward thermo-mechanical stresses [28-32]. Therefore, industrialization of SOFC concepts based on the current materials requires implication of large safety factors, which leaves further cost reduction as well as reliability improvements challenging. 

A 248 V33 - glass fibre reinforced (for parts which are subjected to a high thermo-mechanical stresses, such as under-bonnet automobile parts, housings for electro-mechanical hand tools, sports and leisure equipments)... 

Matrix The matrix having micropores is sandwiched between the anode and cathode (Fig. 10) electrodes with larger pores. The electrolyte matrix provides ionic transport, reactant gas separation and perimeter seal. It is a layer of tightly packed ceramic powder bed impregnated by alkali carbonate electrolyte to form a composite paste-like structure at the operating temperature. The stability of the matrix support materials and matrix robustness to withstand thermo-mechanical stress, are important considerations that impact the fuel cell performance and endurance. A comprehensive review of the matrix considerations, issues, and status is provided by Yuh, Farooque and Maru (Yuh 1999). 

The conductor layout has to be designed to avoid possible problems. For example, transitions should always be rounded and neither conductors nor pads should have sharp corners or right angles. Conductor tracks should not taper. Thermo-mechanical stresses and strains can cause cracking, which would damage the film. This can be avoided by rounding the features of the circuit layout. Excess film should be easy to remove after embossing, so the structures should not form enclosures around excess film. 

A solder joint is characterized by two diffusion zones or intermetallic phases (one on the circuit board, one on the component) and the intermediate solder material (Fig. 5.6). The diffusion zones account for a volume of between 5 and 15% of the solder joint, with the formation of the zones depending on solder composition and also heavily on the level and duration of heat input during the solder process and subsequent use. In principle, the intermetallic phases are brittle, and these areas, therefore, are often responsible for the formation of cracks due to thermo-mechanical stresses. The standard Cu-Ni-Au plating system of MID technology, however, is comparatively uncritical with regard to the formation of intermetallic phases [153,178]. 

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Previous studies of the interphase/interlayer have mainly focused on the coefficient of thermal expansion (CTE) and residual thermal stresses. The importance of residual thermal stresses cannot be overemphasized in composites technology because the combination of dissimilar materials in a composite creates inevitably an interphase across which residual stresses are generated during fabrication and in service due to the difference in thermo-mechanical characteristics. The importance of an interlayer is clearly realized through its effects in altering the residual stress fields within the composite constituents. 

For analytical purposes, the fiber composites are conveniently modeled using axisymmetric three-phase (i.e. fiber-interlayer-matrix), four-phase (i.e. fiber-interlayer-matrix-composite medium) cylindrical composites, or in rare cases multi-layer composites (Zhang, 1993). These models are schematically presented in Fig. 7.9. The three-phase uniform interphase model is typified by the work of Nairn (1985) and Beneveniste et al. (1989), while Mitaka and Taya (1985a, b, 1986) were the pioneers in developing four-phase models with interlayer/interphase of varying stiffness and CTE values to characterize the stress fields due to thermo-mechanical loading. The four phase composite models contain another cylinder at the outermost surface as an equivalent composite (Christensen, 1979 Theocaris and Demakos, 1992 Lhotellier and Brinson, 1988). 

A candidate interlayer consisting of dual coatings of Cu and Nb has been identified successfully for the SiC-Ti3Al-I-Nb composite system. The predicted residual thermal stresses resulting from a stress free temperature to room temperature (with AT = —774°C) for the composites with and without the interlayers are illustrated in Fig. 7.23. The thermo-mechanical properties of the composite constituents used for the calculation are given in Table 7.5. A number of observations can be made about the benefits gained due to the presence of the interlayer. Reductions in both the radial, and circumferential, o-p, stress components within the fiber and matrix are significant, whereas a moderate increase in the axial stress component, chemical compatibility of Cu with the fiber and matrix materials has been closely examined by Misra (1991). 

Attempts to give a quantitative analysis of plastisol extrusion were undertaken only in a few published papers. They were based on the analysis of plastisol viscosity as a function of temperature and time. If in the processing of thermosetting plastics their viscosity is assumed as practically independent of time (except of materials sensitive to structural and chemical transformation in temperature and stress fields which are accompanied by thermo-mechanical decomposition and cross-linking of macromolecular chains, the extent of the larter being influenced by the time of exposure to thermal and mechanical loads 18-21)), then at extrusion of plastisols, in view of their gelatination, the additional condition should be satisfield ... 

Thermal residual stresses are inherent to fibre reinforced composites due to the heterogeneity of the thermo-mechanical properties of their two constituents. Such stresses build up when composite structures are cooled down from the processing temperature to the test temperature. Residual stresses will be present on both a fibre-matrix scale (micro-scale), and on a ply-to-ply scale (macro-scale) in laminates built up from layers with different orientations. It is recognised that these stresses should be taken into account in any stress analysis. 

Good thermo-mechanical properties. Glass transition temperatures range from 300 to 340 °C. Thermal stability is good up to 400 °C. Elongation to break is typically 20%. On wafer stress is 18 MPa, less than half that found for typical polyimides. 

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