The effect of annealing in carbon powders on the anisotropy of the thermoelectric properties of SrTiO3−δ single crystals on the (001) crystal plane
SrTiO3 has high thermoelectric properties and excellent high-temperature stability, which makes it a promising thermoelectric material for use in harsh environments. Hence, it has potential applications in power generation and sensors through direct conversion between heat and electricity. In this study, we investigated the crystal orientations on the crystal growth plane to seek the crystal orientation that is suitable for higher thermoelectric (TE) properties. The [510], [12 5 0] and [320] directions were selected which are on the (001) crystal plane and quarterly divide the angle between the [100] and [110] directions. Two common directions [100] and [110] were selected for comparison purposes. The SrTiO3 single crystals are annealed in carbon powders at 1573 K and 1673 K to form SrTiO3−δ. With the increase in annealing temperature, the electrical conductivity of SrTiO3−δ in the [510], [12 5 0] and [320] directions decreases, which is attributed to the decreasing carrier concentrations. This agrees well with the results calculated using the first-principles calculation. After annealing at 1573 K, the SrTiO3−δ single crystal exhibits the highest ZT value of 0.19 at 873 K in the [510] direction. However, the two common directions, [100] and [110], are not suitable for high thermoelectric properties. In particular, the SrTiO3−δ single crystal in the [12 5 0] direction can always exhibit higher PF and ZT values, regardless of the variation of the annealing temperature in carbon powders. The results of this study are imperative for the design and development of perovskite TE materials.
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