Thermophysical properties such as thermal diffusivity and thermal conductivity have been measured to determine the thermal insulative potential of 18YSH TBC. The temperature dependence of thermal diffusivities for as-sprayed 18YSH coating are plotted in Fig. 1(a). It can be observed that thermal diffusivities monotonically decrease with temperatures consistent with a dominant phonon conduction mechanism. The heat capacity, derived from the Neumann–Kopp law, clearly increases with increasing temperature. Using the derived heat capacity and measured thermal diffusivity, thermal conductivity has been calculated and plotted as a function of temperature in Fig. 1(b). Thermal conductivity of 8 YSZ are presented for comparison. As shown, 8YSZ TBC shows a typical temperature-dependence thermal conductivity following the 1/T law. 18YSH TBC however, exhibits thermal conductivity that is almost independent of temperature, from 0.857 at room temperature to 0.832 W/m⋅K at 800 ◦C. This extremely low thermal conductivity means 18YSH may be well-suited for thermal barrier coatings.
In addition to the porous structure, the low and temperature independent thermal conductivities of 18YSH coating can be primarily attributed to improved phonon scattering upon introduction of Y3+into the HfO2 lattice. Taking into consideration mass fluctuation and elastic lattice strain, the phonon scattering coefficient Γ expresses this enhancement of phonon scattering. Γ is given below in terms of the mass, M, and the atomic size, δ, as well as dopant concentration, x:
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