Abstract
The layered cobaltates AxCoO2 (A: alkali metals and alkaline earth metals) are of interest in the area of energy harvesting and electronic applications, due to their good electronic and thermoelectric properties. However, their future widespread applicability depends on the simplicity and cost of the growth technique. Here, we have investigated the sputtering/annealing technique for the growth of CaxCoO2 (x = 0.33) thin films. In this approach, CaO–CoO film is first deposited by rf-magnetron reactive cosputtering from metallic targets of Ca and Co. Second, the as-deposited film is reactively annealed under O2 gas flow to form the final phase of CaxCoO2. The advantage of the present technique is that, unlike conventional sputtering from oxide targets, the sputtering is done from the metallic targets of Ca and Co; thus, the deposition rate is high. Furthermore, the composition of the film is controllable by controlling the power at the targets.
Highlights
Thermoelectricity, through its ability to harvest energy from waste heat, can contribute to environmentally friendly energy systems
The thermoelectric efficiency of a material system is determined by a dimensionless parameter, the thermoelectric figure of merit, ZT = S2/ρκ, where S is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the thermal conductivity
CaxCoO2 thin films can be promising for near room temperature thermoelectric applications, due to its higher power factor (S2/ρ) as compared to Ca3Co4O9 [33]
Summary
Thermoelectricity, through its ability to harvest energy from waste heat, can contribute to environmentally friendly energy systems. The interfacial phonon scattering in these materials systems is reported to drastically reduce their thermal conductivity, leading to the multifold enhancement of ZT [12,13] The disadvantage of such artificially grown superlattice materials is that they are not thermodynamically stable structures, reducing their stability at high temperature [15]. NaxCoO2 (x~0.7) is reported to have the highest power factor, as high as the standard thermoelectric material Bi2Te3, due to its low electrical resistivity (ρ = 0.2 mΩ·cm at 300 K) and high Seebeck coefficient (100 μV K−1 at 300 K) [23] Even with this high power factor, NaxCoO2 cannot offer reliable performance, due to its poor chemical stability, because the mobile Na+ ions tend to be ejected from the material at high temperatures. CaxCoO2 thin films can be promising for near room temperature thermoelectric applications, due to its higher power factor (S2/ρ) as compared to Ca3Co4O9 [33]
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