Critical durability

Whenever a(a a ) the second term of the sum from the equations (3.152) and (3.153) plays the essential role and the critical durability becomes ... 

The platinum eleetroehemical dissolution rate in fuel cell operation is a critical durability issue due to loss of catalyst surface area vs. time. The dissolution behavior and solubility of platinum are governed by the chemical state of the platinum surfaee and the equilibrium platinum species in the solution. Temperature, pH, eleetrolyte composition, potential, and particle size are all major factors influencing the solubility and the dissolution rate. Potential cycling of Pt electrodes has been extensively investigated, and the platinum electrode dissolution rates from the literature are summarized in Table 23.5 [9, 33, 70-74], Both Pt(II) and Pt(IV) speeies were detected in sulfuric solution after potential cycling from... 

Electrode Walls. Development of durable electrode wads, one of the most critical issues for MHD generators, has proceeded in two basic directions ceramic electrodes operating at very high surface temperatures (>2000 K) for use in channels operating with clean fuels such as natural gas, and cooled metal electrodes with surface temperatures in the range 500—800 K for channels operating with slag or ash-laden flows. 

Total interpenetration of chains (X approaches Rg) is not necessary to achieve complete strength when M > M and x < 7). However, a word of caution while complete strength may be obtained in terms of critical fracture measures such as G c and A lc, the durability, measured in sub-critical fracture terms, such as the fatigue crack propagation rate da/AN, may be very far from its fully healed state at r. We have shown that while the weld toughness A lc increases linearly with interdiffusion depth X as K f. X, the fatigue crack propagation behavior of partially healed welds behaves as [1]... 

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

Optimized grit blast/silane treatments can provide wedge test durability as good as PAA with failure entirely cohesive within the adhesive (Fig. 21) [89]. Maintaining the process parameters within acceptable tolerances is critical with the heat drying of the silane on the treated surface being the most sensitive process parameter. 

Surface cleaning/etches. As with aluminum and titanium, the most critical test for bonded steel joints is durability in hostile (i.e., humid) environments. The fact that the problem is a serious one for steel was illustrated in a study [117] that compared solvent cleaned (smooth) 1010 cold-rolled steel surfaces with FPL aluminum (microrough) substrates. Although the dry lap-shear strengths were not markedly different, stressed lap-shear joints of steel adherends that were exposed to a humid environment failed in less than 30 days, whereas the aluminum joints lasted for more than 3000 days. 

Engraved mold Unit cost low Labor cost low Investment moderate Limited Unrestricted Not critical Good durability No extra operations Best for simple lettering and texture. 

Inserted nameplates Unit cost high Labor cost high Investment moderate Partially limited Restricted Not critical Good durability Longer molding cycles Allows three-dimensional as well as special effects. 

Electrostate Unit cost low to moderate Labor cost low Investment moderate to high Limited Somewhat restricted Critical Moderate to good durability Dry process, no tool contact with product. 

Hot stamping Unit cost low Labor cost low to moderate Investment low to moderate Limited Somewhat restricted Critical Good durability Requires little floor space Dry process, tool contacts product. Produces bright metalics. 

Nameplates Unit cost high Labor cost moderate to high Investment low to moderate Unlimited Somewhat restricted Less critical Good durability Adaptable to many situations Dry process, tool contacts product. Multicolor graphics. 

For application of these fluorochemical finishes to textile fabrics, an extremely important factor is their formulation into suitable aqueous emulsions or dispersions. The quality of the formulation has a critical influence on stability during storage and application, as well as the efficacy of treatment and durability [501,502]. In particular, the choice of surfactant(s) for emulsifying or dispersing must ensure good stability with freedom from deposition on rollers, yet must not impair the water and oil repellency of the finished fabric. No individual product fulfils all requirements hence specifically formulated products are available for certain fibre types. 

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