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:
ε here is phenomenological adjustable parameter which represents the anharmonicity of lattice. Comparing with 8 YSZ, Γ is larger for 18YSH due to its much larger (M(Y3+)-M(Hf4+))2 and (δ(Y3+)-δ(Hf4+))2. Hereby, phonon scattering should be greatly improved around defect sites in HfO2 lattice. For charge balance, large amount of doping Y3+ions would lead 18YSH to produce higher concentration of oxygen va-
cancies than 8 YSZ. As an additional scattering mechanism, oxygen vacancies can improve phonon scattering to a higher extent. The two combined scattering mechanisms even reduce the phonon mean free path to the minimum, resulting in 18YSH coating with the observed temperature-independent thermal conductivity. We also note that 18YSH coating only shows a lower thermal conductivity from room temperature to 400 ◦C, when compared to 8 YSZ coating. At temperatures higher than 600 ◦C, the thermal conductivity is similar to 8 YSZ coating. It is well known that point defects can decrease
thermal conductivity at low temperatures, but the high temperature limit of the thermal conductivity does not depend on the constitutional point defect content. The minimum value of mean free path is related to the interatomic distance at high temperatures. In
the case of 8 YSZ, less Y3+ions are introduced into ZrO2, then the ZrO2 lattice expand less. As a result, 8 YSZ might have the smaller minimum phonon mean free path, so the conductivity of 8 YSZ is approaching that of 18YSH at high temperatures.
Fig. 1. Thermal diffusivity, heat capacity and thermal conductivity of 18 YSH coating dependence of temperature from room temperature to 800 ◦C.
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