Thermal diffusivity of oxide perovskite compounds at elevated temperature

The phonon component of thermal diffusivity (D) for eleven compounds (synthetic SrTiO₃, SrTiO₃: Fe³⁺, BaTiO ₃, KTaO₃, KNbO₃, NdGaO₃, YAlO ₃, YAlO₃:Tm, LaAlO₃, La_0.29 Sr _0.66 Al_0.65 Ta_0.35 O₃, and natural Ca_1.01 Mn_0.001 Fe_0.007 Ti_0.99 O ₃) with various perovskite structures was measured from ambient temperature (T) up to ∼2000 K using contact-free, laser-flash analysis, from which effects of ballistic radiative transfer were removed. Structural transitions (e.g., orthorhombic to tetragonal) below 800 K were manifest as sharp steps in 1/D. Above 800 K, structural transitions occur over intervals of ∼150 K. Similarly broad peaks accompany changes from colorless to black, attributable to partial reduction in Ti, Nb, or Ta from contact with graphite coatings. Otherwise, D decreases with increasing T and, if substitutional disorder exists, approaches a constant (D_sat) near 1600 K. Our data are best described as D^-1 following a low order polynomial in T. Ordered, cubic perovskites occupy a single trend for D (T) ^-1, defining the contribution of the ideal lattice. Distortion, disorder, and polymorphism affect D^-1 in a manner that is consistent with the damped harmonic oscillator-phonon gas model which relates phonon lifetimes to infrared peak widths. Calculated D -values at ambient and high T agree with measurements. The behavior of D is simple compared to that of thermal conductivity, k=ρ C_P D, where ρ is density and C_P is heat capacity. Combining our data with cryogenic measurements of YAlO ₃ and LaAlO₃ shows that D^-1 depends on T similarly to C_P, consistent with phonon lifetime depending on the density of states but, the best description for D^-1 (T) is a proportionality to α_T from ∼0 K up to the limit of measurements, where α is thermal expansivity, a strongly anharmonic property. At low T, D^-1 due to phonon scattering follows that of C_P, generally T³, so k_lat = k₀ + k₁ T. Defects being present preclude scattering at sample walls, adding a small constant D₀^-1 ∼0.0001 mm^-2 s as T→0, and an additional contribution of k_dfct T³. Forms previously inferred for thermal insulators include systematic errors stemming from ballistic radiative transfer and/or interface resistance. Our results show that optical phonons largely govern heat transport of complex insulators.