Revealing Anisotropic Dielectric and Thermal Behavior in Layered Ca3Co4O9 Thin Films

Abstract

Ca3Co4O9 (CCO) is a misfit-layered oxide with anisotropic thermal and dielectric properties. In this work, we compare bulk CCO, CCO thin films grown on LaAlO3 (LAO) and MgO, and an Au/CCO/SrRuO3/SrTiO3 parallel-plate heterostructure to investigate through-thickness heat transport and dielectric response. X-ray diffraction (XRD), cross-sectional scanning transmission electron microscopy (STEM), modified transient plane source, and temperature-dependent capacitance measurements were used. The CCO/LAO film was predominantly (00l)-textured, whereas CCO/MgO also exhibited domains with an additional crystallographic orientation. XRD also indicated a predominantly (00l)-textured heterostructure, while STEM revealed local tilting of the CCO layers. Bulk CCO had a thermal conductivity of 0.62 W m–1 K–1, while the evaluated effective through-thickness thermal conductivities of CCO/LAO and CCO/MgO were 0.91 and 1.46 W m–1 K–1, respectively. The higher CCO/MgO value is consistent with its mixed crystallographic orientation, which may allow a partial contribution from the higher in-plane thermal conductivity to the nominal through-thickness measurement. At 100 Hz, ε′ of bulk CCO increased from 5.31 × 104 at room temperature to 8.09 × 105 at 450 K. CCO/LAO showed a broad maximum of approximately 1.95 × 106 near 428 K, whereas CCO/MgO showed a substantially smaller effective response of approximately 3.7 × 102 to 2.5 × 103. In the parallel-plate heterostructure, ε′ increased from about 1.7 × 105 at room temperature to a broad maximum of 9.8 × 105 near 390 K. Because the CCO layers are locally tilted, this response should be interpreted as an effective through-thickness permittivity potentially containing a layer-parallel dielectric contribution, rather than as a pure intrinsic c-axis permittivity. Together, these comparisons associate the effective thermal and dielectric responses with texture, local layer orientation, and probing geometry, rather than with strain alone.