High-temperature thermoelectric properties of nanostructured Ca3Co4O9 thin films

Abstract

We prepared nanostructured Ca3Co4O9 (CCO) thin films by promoting localized epitaxial growth on polycrystalline Al2O3 substrates. The thermoelectric properties of the CCO films were studied in the temperature range 300 to 1023 K. We confirmed that localized epitaxial growth occurred on the seed grains that dominate the (006) plane. The nanostructured CCO thin films were found to have a maximum Seebeck coefficient of 206 μV/K and a power factor (at 920 K) of 0.514 mW/mK2. Moreover, the presence of nanostructure was found to reduce the thermal conductivity, and thus, should enhance the overall performance of CCO films in thermoelectric devices.

High-throughput synthesis of thermoelectric Ca3Co4O9 films

“…As seen in the bulk, the positive value indicates hole-like carriers, and the shape of S(T) is typical of Ca 3 Co 4 O 9 , with a plateau observed for T > 100-200 K, which follows a steep increase of S at low temperature (which cannot be observed here due to the large value of resistivity). However, the value of S (close to 170 lV/K at 300 K) is slightly larger than for the bulk crystals (120 lV/K), 4 15 These power factors are smaller than the highest power factor reported for bulk samples, which are 0:3 mWm À1 K À2 . 30 In bulk misfits, an increase of Seebeck coefficient, associated to a more localized behaviour, has also been observed.…”

“…The grain size (mean area equivalent diameter) was about 50 lm, but exhibited a clear bimodal grain size distribution, with anisometric grains on the order of 100 lm and more isometric grains on the order of 10 lm. The misorientation distribution values across the grain boundaries ranged from 15 to 120 , and the relative fraction of boundaries in different ranges of misorientation angle are given in Figure 1(c). The majority of the grain boundaries are high angle boundaries, within 30 to 60 .…”

“…It should be clear that the Ca 3 Co 4 O 9 epitaxial film growth was achieved without the classical substrate-induced epitaxy arising from interactions with low-index surfaces of single-crystal substrates, which is very different from the previous reports. 15,23 The temperature-dependence of the Seebeck coefficient and the resistivity are shown in Figure 5. Contrary to the single crystals or polycrystals of Ca 3 Co 4 O 9 , 4,27 the resistivity presents here a localized behaviour with values of resistivity close to 100 lX cm at 300 K, and a local activation energy d(ln q/d(1/T)) increasing from 50 meV at low temperature to 300 meV at 300 K. The film resistivity increases dramatically below 150 K (see inset of Figure 5), and the activation energy is calculated to be 25.2 meV.…”

“…13,14 Deposition of Ca 3 Co 4 O 9 on polycrystalline Al 2 O 3 substrates has also recently been reported. 15 In this article, we explore more closely the local epitaxy of Ca 3 Co 4 O 9 films (deposited by the Pulsed Laser Deposition (PLD) technique) on polycrystalline Al 2 O 3 substrates, textured using SPS. When the grains of polycrystalline Al 2 O 3 , whose size is carefully controlled, act as seeds for localized epitaxial growth, one expects a large range of orientations, and these must be characterized with local structural probes.…”

Thermoelectric Properties of Polycrystalline Thin Films Under an External Magnetic Field

“…14 Thin films are traditionally used as low-dimensional materials with good TE properties. [15][16][17][18][19][20][21] However, to the best of our knowledge, there are no reported theoretical studies on the influences of applied magnetic fields on the TE properties of polycrystalline TE thin films. In this study, we focused on the effects of applied magnetic fields on the TE properties of polycrystalline thin film.…”