The delafossite structure (ABO 2 ) accommodates a wide range of transition and rare-earth cations on the B-site in combination with a short list of A-site cations. This paper reports the effects on defect chemistry and transport as a function of the B-site cation in copperbased delafossite materials, with CuScO 2 and CuYO 2 as examples. Large B-site cation delafossites exhibit small polaron conduction, a diffusion-limited conduction mechanism, with an activation energy approximately twice that of CuAlO 2 (0.22 eV vs 0.14 eV, respectively). The mobility for these materials is found to be <1 cm 2 V -1 s -1 , further evidence of small polaron conduction. The majority defect species is highly dependent on the B-site cation, such that CuScO 2 and CuYO 2 are dominated by extrinsic defects (acceptor doping) and free oxygen interstitials, whereas off-stoichiometry dominates CuAlO 2 . Dopant solubility is shown to be 1% for bulk CuScO 2 samples and lower in CuYO 2 .
Oxidation of SiC can occur in a passive mode where a protective film is generated or in an active mode where a volatile suboxide is generated and can lead to rapid material consumption. The transition between these two modes of oxidation is a critical issue. Evidence indicates that this transition occurs via a different mechanism for the active‐to‐passive transition as compared with that of the passive‐to‐active transition. In Part I of this article, the former (active‐to‐passive mode) is explored. Three different types of SiC are examined: Si‐rich SiC, stoichiometric SiC, and C‐rich SiC. Evidence suggests that the SiO2/SiC equilibrium requirements as well as formation of SiO(g) at the SiC surface and subsequent oxidation to SiO2(s) are critical issues in the active‐to‐passive transition.
The thermodynamic stability of ceramic coatings with respect to their reaction products is crucial to develop more durable coating materials for gas-turbine engines. Here, we report direct measurements using high-temperature solution calorimetry of the enthalpies of reaction between some relevant ceramic coatings and a corrosive molten silicate. We also report the enthalpy of mixing between the coatings and molten silicate after combining the results measured by high-temperature solution calorimetry with enthalpies of fusion measured by drop-andcatch calorimetry and differential thermal analysis. The enthalpies of solution of selected silicate and zirconia-based coatings and apatite reaction products are moderately positive except for 7YSZ, yttria-stabilized zirconia. Apatite formation is only favorable over coating dissolution in terms of enthalpy for 7YSZ. The enthalpies of mixing between the coatings and the molten silicate are less exothermic for Yb 2 Si 2 O 7 and CaYb 4 Si 3 O 13 than for 7YSZ, indicating lower energetic stability of the latter against molten silicate corrosion. The thermochemical results explain and support the very corrosive nature of CMAS melts in contact with ceramic coatings.
K E Y W O R D Scorrosion/corrosion resistance, environmental barrier coatings, thermal barrier coatings
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