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A3: High temperature materials
We report on high thermoelectric performance of 2% Al-doped ZnO (AZO) thin films fabricated by Pulsed Laser Deposition on cheap amorphous silica substrates. Dense AZO target was irradiated by Nd:YAG laser (266 nm, 10 Hz) which energy density is about 4.2 J/cm2 for deposition period of 30 min at Tdep = 300°C, 400°C, 500°C and 600°C keeping an oxygen pressure of 27 Pa. All the films are fully c-axis oriented, independently of Tdep. TEM analysis reveals columnar growth and very sharp interface between substrate and films. Electric conductivity (σ) and Seebeck coefficient (S) were evaluated in the interval T = 300 - 600 K. σ presents a clear semiconducting behaviour. S sign is always negative, confirming the n-type conduction, and his absolute value increases with Tdep. Best performance was obtained on the film deposited at 300 °C, with σ = 923 S/cm, S = -111 μV/K and very large power factor S2σ = 1.2 W/m×K2at 600K, twofold than for AZO film deposited on SrTiO3 single crystal under same experimental conditions [1]. The reason can be given by the lowest stress of thin films deposited on fused silica [2]. Thermal conductivity is κ = 4.89W/m×K at 300 K. Dimensionless figure of merit ZT = (S2σ )T/ κ is calculated in the range 300-600 K using κ at 300 K [3]: ZT for thin films is always higher than for corresponding bulk material. This result is quite encouraging for the practical applications of thermoelectric oxide thin films at high temperatures.
[1] P. Mele et al, Appl. Phys. Lett. 102 (2013) 253903
[2] M Novotny et al, J. Phys. D: Appl. Phys. 45 (2012) 225101
[3] Because κ is expected to decrease with T: see for example G. S. Nolas and H. J. Goldsmid, Thermal Conductivity: Theory, Properties and Applications, Kluwer Academic/Plenum Publishers, 2004, p. 114