Towards efficient RuO2 thermoelectric thin films through utilizing a quantum design approach
Final Report Abstract
The major objective of this project was to correlate composition, structure, and transport properties for RuO2 alloyed with various metals. Based on the predicted Seebeck coefficients for RuO2 alloyed with lanthanides, we have selected La for experimental validations. Our experiments on La-alloyed RuO2 films exhibiting a fourfold higher Seebeck coefficient validate the quantum mechanical prediction. Additionally, La dilute alloying also results in a twofold electrical conductivity decrease due to La-dissolution-induced local distortions to the RuO2 structure. Nevertheless, the combination of these effects yield a large power factor, exceeding the previous literature reports on RuO2 doped with 2 mol% BaO by an order of magnitude. This large power factor enhancement was further maximized by applying this research strategy to RuO2 alloying with 3d transition metals. We have obtained the power factor of 274 μWK^−2m^−1 which is not only the largest reported value for RuO2 based compounds but it also occurs at ∼600 °C thus increasing the Carnot efficiency significantly. We have also explored the thermal conductivity of alloyed RuO2 to enable the complete description of its transport properties. Again a correlative theoretical and experimental approach was used. Both the calculated and measured data consistently display an order of magnitude drop in the thermal conductivity upon alloying of RuO 2 with La. Furthermore, we predicted the thermal conductivity of Fe-alloyed RuO2 to be 9.3 W m^-1 K^-1 and validated this by measuring the thermal conductivity of magnetron sputtered thin films. Disregarding electronic contribution to the thermal conductivity, the Slack model together with grain boundary scattering effects can be used to explain the modulation of the thermal conductivity of RuO2 alloyed with La and Fe by a factor of 20. Even though there were no new questions raised from the synthesis perspective during this project, not requiring any further plasma analysis, there were still open issues with respect to unusual morphologies of these thin films. We have investigate the growth of two rutile oxides, namely RuO2 and MnO2, to allow for a direct comparison of drastically different morphologies, nanorods and faceted surface, respectively, obtained under equivalent experimental conditions. To identify the underlying mechanisms, we have carried out density functional theory based molecular dynamics simulations of the growth of one monolayer. Ru and O2 molecules are preferentially adsorbed at their respective RuO2 ideal surface sites. This is consistent with the close to defect free growth observed experimentally. In contrast, Mn penetrates the MnO2 surface reaching the third subsurface layer, resulting in atomic scale decomposition of MnO2. Due to this atomic scale decomposition, MnO2 may have to be renucleated during growth, which is consistent with experiments. Furthermore, we have demonstrated that the research methodology aiming at identifying and subsequent utilization of physical mechanisms to improve the transport properties of RuO2 is transferable to other materials systems, such as MnO2. Hence, all goals raised in this project have been reached and the obtained data were communicated in high-profile international journals.
Publications
- Modulation of transport properties of RuO2 with 3d transition metals, Mater. Res. Express 1 (2014) 045034
D. Music, Y.-T. Chen, R.W. Geyer, P. Bliem, J.M. Schneider
(See online at https://doi.org/10.1088/2053-1591/1/4/045034) - Multifold Seebeck increase in RuO2 films by quantum-guided lanthanide dilute alloying, Appl. Phys. Lett. 104 (2014) 053903
D. Music, F.H.-U. Basse, L. Han, Devender, T. Borca-Tasciuc, J.J. Gengler, A.A. Voevodin, G. Ramanath, J.M. Schneider
(See online at https://doi.org/10.1063/1.4864078) - Atomistic growth phenomena of reactively sputtered RuO2 and MnO2 thin films, J. Appl. Phys. 118 (2015) 015302
D. Music, P. Bliem, R.W. Geyer, J.M. Schneider
(See online at https://doi.org/10.1063/1.4926414) - Critical evaluation of the colossal Seebeck coefficient of nanostructured rutile MnO2, J. Phys.: Condens. Matter 27 (2015) 115302
D. Music, J.M. Schneider
(See online at https://doi.org/10.1088/0953-8984/27/11/115302) - Designing low thermal conductivity of RuO2 for thermoelectric applications, Appl. Phys. Lett. 106 (2015) 063906
D. Music, O. Kremer, G. Pernot, J.M. Schneider
(See online at https://doi.org/10.1063/1.4909513)
