The thermal conductivity of coatings 1=Chy is primarily deter-mined by the mean free path of phonons(l), where C, and v represent the volume specific heat and elastic wave velocity, respectively. As shown in Eq.
l is related to the phonon mean free paths corresponding to intrinsic lattice phonon scattering (lp), defect scattering (ld), grain boundary scattering (lb), and other mechanisms (li). Therefore, the increase in defects and boundary density will enhance phonon scattering and reduce the value of I, resulting in a decrease in the thermal conductivity. As mentioned above, the number of defects and the boundary density increase gradually with the addition amount of Al2O3 into t-YSZ, as indicated in Table 1 and Fig.1, which explains why Al2O3 addition can cause a steady reduction in the thermal conductivity of the Al2O3-doped t-YSZ coatings in Fig.2.
Table 1 Cohesive energy(Ecohesive) and formation enthalpy (Hfomation) of t-YSZ.
Fig.1.Mean value graph of grain boundary density and porosity of coatings under different addition amounts of Al2O3.
The thermal expansion properties of the coatings are visibly different from their thermal conductivities. Thermal conductivity is closely associated to microstructure properties, such as defects and grain sizes.However, thermal expansion is not significantly affected by the micro-structures but is significantly associated with electronic structures such as bonds. It is clear that thermal expansion is more difficult to achieve owing to the stronger bonding between ions, which is the reason that the change in the bonding strength of Al2Og-doped t-YSZ with the addition of Al2O3 illustrated by a total DOS analysis is consistent with that of the CTE shown in Fig.2.
Fig.2.Mean value graph of CTE and thermal conductivity of coatings under different addition amounts of Al2O3.
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