Thermal diffusivity of oxide perovskite compounds at elevated temperature

The phonon component of thermal diffusivity ( D ) for eleven compounds (synthetic SrTiO 3 , SrTiO 3 : Fe 3 + , BaTiO 3 , KTaO 3 , KNbO 3 , NdGaO 3 , YAlO 3 , YAlO 3 : Tm , LaAlO 3 , La 0.29 Sr 0.66 Al 0.65 Ta 0.35 O 3 , 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 LaAlO 3 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 ∝ T 3 , so k lat = k 0 + k 1 T . Defects being present preclude scattering at sample walls, adding a small constant D 0 − 1 ∼ 0.0001 mm − 2 s as T → 0 , and an additional contribution of k dfct T 3 . 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.