[21a-P08-1~2] 22.1 Joint Session M "Phonon Engineering"

English Presentation

[21a-P08-2] Direct Measurement of In-Plane Thermal Conductivity for Freestanding Oxide Films

〇(D)Kungwan Kang¹, Fumiaki Kato², Akitoshi Nakano², Ichiro Terasaki², Hyoungjeen Jeen³, Hiromichi Ohta⁴ (1.IST-Hokkaido Univ., 2.Physics-Nagoya Univ., 3.Physics-Pusan Nat'I Univ., 4.RIES-Hokkaido Univ.)

Keywords:

Thermal conductivity,Freestanding film,Ac-calorimetric method

Ba_1/3CoO₂ belongs to the layered crystal structure composed of rigid CoO²⁻ layer and Ba²⁺ layer alternately stacked along the c-axis. Recently, we have reported Ba_1/3CoO₂ epitaxial films ^{[1, 2]} exhibit rather high ZT ~0.55 at 600 °C in air along the layered structure. This ZT value is the highest among reliable values of oxides and comparable to that of p-type PbTe and p-type SiGe at 600 °C. We fabricated the Ba_1/3CoO₂ epitaxial films by the reactive solid-phase epitaxy (Na_3/4CoO₂) followed by the ion exchange treatment. We measured the electrical conductivity and thermopower along the layered structure directly using the dc four-probe method and steady-state method, respectively. Further, we measured the thermal conductivity values of the Ba_1/3CoO₂ epitaxial films with two different crystallographic orientations perpendicular to the film surface by the time-domain thermoreflectance (TDTR) method. Then, we extracted the thermal conductivity along the layered structure (κ_//). Here we show that the κ_// values were correctly estimated. We fabricated 1 μm thick freestanding films of Sr_1/3CoO₂ and Ba_1/3CoO₂ by peeling off the epitaxial films grown on sapphire substrates using hot water.^[3] Subsequently, we directly measured the κ_// by the ac-calorimetric method (Figs. a and b).^[4] The obtained κ_// of Sr_1/3CoO₂ and Ba_1/3CoO₂ were 4.1 and 3.4 W m⁻¹ K⁻¹ (Fig. c), which were similar to those of the TDTR results, clearly revealing that the TDTR method can be used to extract κ_// correctly.