The experimental procedures including thin films deposition, characterizations and thermoelectric
properties measurement, and fabrication and testing a thermoelectric module are described as follows.
2.1. Thin films deposition
Fig 1 shows the schematic diagram of a bipolar pulsed-dc magnetron sputtering system used for thin
film preparations. Details of a system have been given elsewhere [8]. The depositions were carried out
under an argon atmosphere (Ar 99.999%, at the gas flow rate of 23.3±0.1 sccm and total pressure of 70
mTorr) using the p-Ca3Co4O9 and n-ZnAlO targets of 60 mm diameter and 3.0 mm thickness. The
substrates were Al2O3:clay ceramics (25.0×25.0×1.0 mm3
) placed at the distance of 5.0 cm above the
target without additional heating facility. The pulsed-dc power applied to the targets was operated at
approximately 17 kHz, with the sputtering on-time (−ton) of 20 μs, the reverse sputtering on-time (+ton) of
10 μs, and the pulse off-time (toff) of 14 μs. The negative voltage pulse/current at the target were 244±15
V/120 mA, and 600±15 V/120 mA for p-Ca3Co4O9 and n-ZnAlO targets, respectively. The positive
voltage pulse (+V) was set at 100±10 V for both targets. The deposition time was 60 min for each sample.
The optical emissions from plasma during sputter deposition of films were observed in the wavelength
range of 360-800 nm using a high resolution spectrometer (the getSpec-2048 spectrometer, Sentronic
GmbH). The spectral lines were indexed to the Atomic Spectra Database (ASD) of the National Institute
of Standards and Technology (NIST) [9].