Design of Transition-Metal Nitride Thin Films for Thermoelectrics

Abstract

Thermoelectric devices are one of the promising energy harvesting technologies, because of their ability to convert heat (temperature gradient) to electricity by the Seebeck effect. Furthermore, thermoelectric devices can be used for cooling or heating by the inverse effect (Peltier effect). Since this conversion process is clean, with no emission of greenhouse gases during the process, this technology is attractive for recovering waste heat in automobiles or industries into usable electricity. However, the conversion efficiency of such devices is rather low due to fundamental materials limitations manifested through the thermoelectric figure of merit (ZT ). Thus, there is high demand on finding materials with high ZT or strategies to improve ZT of materials.In this thesis, I discuss the basics of thermoelectrics and how to improve ZT of materials, including present-day strategies. Based on these ideas, I propose a new class of materials for thermoelectric applications: transition-metal nitrides, mainly ScN, CrN and their solid solutions. Here, I employed both experimental and theoretical methods to synthesize and study their thermoelectric properties. My study envisages ways for improving the thermoelectric figure of merit of ScN and possible new materials for thermoelectric applications.The results of my studies show that ScN is a promising thermoelectric material since it exhibits high thermoelectric power factor 2.5×10 −3 Wm −1 K −2 at 800 K, due to low metallic-like electrical resistivity while retained relatively large Seebeck coefficient. My studies on thermal conductivity of ScN also suggest a possibility to control thermal conductivity by tailoring the microstructure of ScN thin films. Furthermore, my theoretical studies on effects of impurities and stoichiometry on the electronic structure of ScN suggest the possibly to improve ScN ZT by stoichiometry tuning and doping. For CrN and Cr1−xScxN solid solution thin films, the results show that the power factor of CrN (8×10 −4Wm −1 K −2 at 770 K) can be retained for the solid solution Cr0.92Sc0.08N. Finally, density functional theory was used to enable a systematic predictionbased strategy for optimizing ScN thermoelectric properties via phase stability of solid solutions. Sc1−xGdxN and Sc1−xLuxN are stabilized as disordered solid solutions, while in the Sc-Nb-N and Sc-Ta-N systems, the inherently layered ternary structures ScNbN2 and ScTaN2 are stable. Svensk sammanfattningSedan den industriella revolutionen har fossila bränslen varit vår huvudkälla till energi i motorer för transport, elproduktion och uppvärmning av byggnader. Eftersom mänskligheten och vår teknik växer för varje år som går, fortsätter efterfrågan på fossila bränslen att öka. Med tanke på att fossila bränslen inte är förnybara, riskerar vi att de tar slut. Dessutom är resultatet av denna ständiga förbränning av fossila bränslen generering av växthusgaser, t.ex. kolmonoxid och koldioxid, som orsakar klimatförändringar, som ett ytterligare problem. Således finns det ett ökande behov...

Transition-metal-nitride-based thin films as novel energy harvesting materials

“…This can be addressed in a Cr 1– x Sc x N solid solution, which is thermodynamically stable at high temperature in cubic NaCl-structured form. 107 The fact that the 3d orbitals in Sc are empty can be exploited as a means of delocalizing the electrons in 3d orbitals, resulting in electrical conductivity reduction 107 and possibly also thermal-conductivity reduction due to alloy scattering. We have recently shown that the Seebeck coefficient of Sc-rich Cr 1– x Sc x N solid solution epitaxial thin films does indeed increase compared to pure ScN and that the thermoelectric properties of CrN are largely retained in Cr-rich Cr 1– x Sc x N solid solutions.…”

“…We have recently shown that the Seebeck coefficient of Sc-rich Cr 1– x Sc x N solid solution epitaxial thin films does indeed increase compared to pure ScN and that the thermoelectric properties of CrN are largely retained in Cr-rich Cr 1– x Sc x N solid solutions. 107 …”