Determination of modified figure of merit validity for thermoelectric thin films with heat transfer model: Case of CuCrO2:Mg deposited on fused silica

Abstract

Thermoelectric performance of a material is determined using a figure of merit (FOM) determined as ZT (ZT = σS2T/κ where σ is the electrical conductivity, S is the Seebeck coefficient, κ is the thermal conductivity, and T is the temperature). In the case of a thin film, it is normal in the first approach to consider calculating the FOM by using the thermal conductivity of the film. However, both the thermal influence of the substrate and the emissivity of the film must also be taken into account. In the present work, the heat transfer model is used in order to study the influence of the thermal conductivity, the thickness, and the emissivity of the film on the thermal gradient of the stack (substrate + thin film). The limits of these three parameters are determined in order to have the temperature variation due to the presence of the film compared to the substrate alone that remains less than 1%. Under these limits, the thermal conductivity of the substrate can be taken into account instead of the thermal conductivity of the thin film, and a modified FOM (Z’T) can be calculated. The present study leads to the determination of the validity of modified ZT. In the case of CuCrO2:Mg thin films, the model shows that the use of Z’T is valid. The calculated value of Z’T with the measured Seebeck coefficient and the electrical conductivity as a function of the temperature for 100 nm thick films and the temperature dependent thermal conductivity taken from the literature reached 0.02 at 210 °C. A thermoelectric module made with this material showed 10.6 nW when 220 °C is applied at the hot side.

Insights into stability, transport, and thermoelectric properties of transparent p-type copper iodide thin films

“…Sinnarasa et al 55 , an effective can be calculated for the stabilized -CuI: Z'T = 0.02 at 100 °C. This value is 2.6 Z'T =  S² s T time larger than previously reported p-type Mg doped CuCrO 2 thin film at the same temperature 55 .…”

“…Sinnarasa et al 55 , an effective can be calculated for the stabilized -CuI: Z'T = 0.02 at 100 °C. This value is 2.6 Z'T =  S² s T time larger than previously reported p-type Mg doped CuCrO 2 thin film at the same temperature 55 . The carrier density (10 19 cm -3 ), the large Seebeck coefficient and the Z'T from this work are clearly adapted for thermoelectric applications 56 .…”

The effect of post-deposition annealing conditions on structural and thermoelectric properties of sputtered copper oxide films

“…In general, the thin lms maintained the high Seebeck coefficient values reported for copper oxides 3,35,45,[61][62][63][64][65] (see Table 1), comparing favourably with other common thermoelectric materials such as Bi 2 Te 3 , 66 PbTe, 67 and CuAlO 2 , 68 and advanced materials such as Ca 3Àx Bi x Co 4 O 9+d , 69 oxides, 62,63 and borides, 64,65 which makes them ideal for sensing applications. Interestingly, a correlation can be observed in Table 1 between the Seebeck coefficient of the lms and their electrical resistance: the lm with the highest conductivity (CuO:O) also possesses the highest Seebeck coefficient.…”

Microstructural and transport properties of Mg doped CuFeO2 thin films: A promising material for high accuracy miniaturized temperature sensors based on the Seebeck effect

“…It can be explained by the effects of micro and nano structuration which cause a decrease of the thermal conductivity compared to the same bulk materials. Moreover, as described by Sinnarasa et al [41], the impact of ther mal conductivity of the film can be neglected in comparison with the thermal conductivity of the substrate. In our case, the fused silica has a very low thermal conductivity (1.38 W m À1 K À1 at 300K) [56].…”

“…Temperature sensors based on Seebeck effect require also a low thermal conductivity and low material quantity to avoid thermal pumping. In previous work [41], we have demonstrated that in the case of thin films, the thermoelectric material can be considered as a composite made with the substrate and the thin film. The sub strate does not participate to the electrical conduction and the Seebeck effect but it definitely influences the thermal conductivity.…”