Reduced temperature-dependent thermal conductivity of magnetite thin films by controlling film thickness
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NANO EXPRESS
Open Access
Reduced temperature-dependent thermal conductivity of magnetite thin films by controlling film thickness No-Won Park1, Won-Yong Lee1, Jin-A Kim2, Kyungjun Song3, Hyuneui Lim3, Wan-Doo Kim3, Soon-Gil Yoon2* and Sang-Kwon Lee1*
Abstract We report on the out-of-plane thermal conductivities of epitaxial Fe3O4 thin films with thicknesses of 100, 300, and 400 nm, prepared using pulsed laser deposition (PLD) on SiO2/Si substrates. The four-point probe three-omega (3-ω) method was used for thermal conductivity measurements of the Fe3O4 thin films in the temperature range of 20 to 300 K. By measuring the temperature-dependent thermal characteristics of the Fe3O4 thin films, we realized that their thermal conductivities significantly decreased with decreasing grain size and thickness of the films. The out-of-plane thermal conductivities of the Fe3O4 films were found to be in the range of 0.52 to 3.51 W/m · K at 300 K. For 100-nm film, we found that the thermal conductivity was as low as approximately 0.52 W/m · K, which was 1.7 to 11.5 order of magnitude lower than the thermal conductivity of bulk material at 300 K. Furthermore, we calculated the temperature dependence of the thermal conductivity of these Fe3O4 films using a simple theoretical Callaway model for comparison with the experimental data. We found that the Callaway model predictions agree reasonably with the experimental data. We then noticed that the thin film-based oxide materials could be efficient thermoelectric materials to achieve high performance in thermoelectric devices. Keywords: Iron oxide (Fe3O4); Thermal conductivity; 2D thin films; 3-ω technique; Callaway model; In-plane and out-of-plane
Background In recent decades, there has been a great interest in the application of thermoelectric (TE) effects in alternative clean energy sources [1-6]. For the evaluation of the thermoelectric performances of TE devices, their efficiencies can usually be quantified by a dimensionless figure of merit (ZT), S2σT/κ or a power factor S2σ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. High-performance thermoelectric materials with high ZT values should have a large Seebeck coefficient, high electrical conductivity, and low thermal conductivity [2,7,8]. To obtain an efficiently comparable to a * Correspondence: [email protected]; [email protected] 2 Department of Materials Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea 1 Department of Physics, Chung-Ang University, Seoul, 156–756, Republic of Korea Full list of author information is available at the end of the article
household refrigerator, a ZT value at least 3 is desired for more widespread applications [6]. Recently, several researchers have alternatively studied two-dimensional (2D) thin films [9,10] to overcome the limitations of 1D nanostructured materials whose thermal properties are highly dependent on their dimensionality and morphology [3,11-13]. In 2010, T
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