Mismatch in thermal expansion

It has been observed that solid oxide fuel cell voltage losses are dominated by ohmic polarization and that the most significant contribution to the ohmic polarization is the interfacial resistance between the anode and the electrolyte (23). This interfacial resistance is dependent on nickel distribution in the anode. A process has been developed, PMSS (pyrolysis of metallic soap slurry), where NiO particles are surrounded by thin films or fine precipitates of yttria stabilized zirconia (YSZ) to improve nickel dispersion to strengthen adhesion of the anode to the YSZ electrolyte. This may help relieve the mismatch in thermal expansion between the anode and the electrolyte. 

Also, heating might cause decomposition of sensitive components of the device. In addition, any mismatch in thermal expansion between the NLO film and its package can lead to strain, cracking, and delamination during cooling from high temperature. 

The presence of mullite and hence compressive surface stresses appears to improve the hardness and fracture toughness (see Table 5.2). These values are at least two to three times higher than those reported for the mullite/ alumina system described above. Clearly, the presence of mullite is desirable for inducing compressive stresses in the vicinity of the surface region by virtue of the mismatch in thermal expansion between ZTA and mullite. This significant improvement in the observed fracture toughness was attributed to... 

The high elevated-temperature cures are damaging to adhesive systems due to a mismatch in thermal expansion coefficient that can occur between the epoxy and the substrate. The difference in rate of expansion when returning to room temperature from the cure temperature can lead to significant internal stress within the adhesive joint, which results in poor adhesion. 

Stresses caused by items 1 and 2 above are magnified by the mismatch in thermal expansion coefficients between the adhesive and the substrate. Incorporating fillers into the adhesive formulation can often reduce these stresses. Fillers also reduce the thermal shrinkage during aging by bulk displacement of the polymeric resin. 

The mechanical interaction between the different epitaxial layers may result in the formation of misfit dislocations. Nucleation and propagation of cracks can ensue if the mismatch in thermal expansion coefficient is relatively large. The defects significantly influence the physical properties of the thin films. Examples from different material combinations and models of how to predict the numbers for critical thicknesses are provided in Section 14.4. 

Similar to the case of semiconductor thin films and quantum well structures, there is a need to deposit buffer layers prior to deposition of the superconducting thin-film, see Fig. 14.6. The role of a buffer layer is to prevent detrimental interactions between the film and substrate and to diminish the effect of surface defects on the film growth. In addition, there are examples of film/substrate combinations where the mismatch in thermal expansion coefficients is severe enough to cause cracking in the thin films [14.16, 14.17]. A suitably chosen intermediate buffer layer can reduce the stress caused by such a mismatch. 

Cracks are frequently observed in YBCO films deposited on Si substrates owing to the relatively large mismatch in thermal expansion coefficient... 

The large mismatch in thermal expansion coefficient limits the film thickness of (110) oriented YBCO films [14.17]. Cracks form during cooling from the deposition temperature for a film thickness greater than a critical value, as described in Section 14.4. The phenomenon is similar to the case of c-axis-oriented films on Si substrates [14.16, 14.19]. 

Heating or cooling of multiphase ceramics in which the various constituents have differing thermal expansion coefficients. The stresses generated in this case will depend on the mismatch in thermal expansion coefficients of the various phases. These stresses cannot be avoided by slow heating or cooling. 

It is important to determine stresses in thin films in relation to mechanical stability as a result of the deposition process and the temperatures involved. The stress developed in a film is made up of three components. The first component is intrinsic, which is the result of factors such as deposition, structure, and mode of growth the second is the result of the mismatch in thermal expansion between film and substrate and the third is related to externally applied stresses (Lepienski et al., 2004). Once the values of intrinsic stress ( thermal stress ( (thermal)), and externally applied stress ((T(extemal)) are determined, the stress developed within a film can be calculated using the following equation ... 

Thermal Properties. Since polymers generally have a much larger thermal expansion coefficient than most rigid fillers, there is a significant mismatch in thermal expansion in a filled polymer. This mismatch could lead to generation of thermal stresses aroimd filler particles during fabrication and, most severely, induce microcracks at the filler interface that could lead to prematiu-e failure of the filled polymer. As for the thermal expansion coefficient of a filled polsrmer, it generally falls below the value calculated from the simple rule of mixtiu-es but follows the Kemer equation (40) for nearly spherical particles. 

The reader is directed to Subsection 3.5.1 of this book for a more detailed discussion of thermal stress caused by mismatches in thermal expansions. 

The electrowetting cell was set between two plastic injection molded lenses. A flat glass plate closed the cylinder on the oil side and a truncated glass sphere mounted on a thin metal diaphragm closed the side of the salt solution. The outer diameter of the cylinder was 4 mm, the inner diameter 3 mm, and the height was 2.2 mm. The flexible metal diaphragm compensated part of the mismatch in thermal expansion between the liquids and the cylinder. The achromatized lens stack had a high optical quality. The camera was able to focus faster than the refresh rate of the CMOS sensor. 

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