Growth and thermoelectric properties of CaMnO3-based thin films

Abstract

The field of thermoelectrics started in early 19 th century. Since the discovery of the Seebeck effect and the Peltier effect, thermoelectric modules have found their way into, mostly, niche applications such as radioisotope thermoelectric generators on space missions. Thermoelectric modules can also be used for cooling, utilizing the Peltier effect.Thermoelectrics are promising materials due to the operation nature of the modules. That is, they have no moving parts, no exhaust, long lifetime without maintenance, features that make them attractive for many applications. Despite these promising properties, thermoelectric modules are mostly used in niche applications. The main reason for this is conventional modules with the highest efficiency are commonly made of expensive and/or rare elements which prevents mass production.To tackle this problem, new materials are investigated to find a module that can be made widely available. Oxides are one possibility, where an added benefit is that they are chemically stable even at elevated temperature. The perovskite CaMnO3 is one of the more promising oxides, with elements that are abundant on earth and cheap. The material does suffer from low electrical conductivity which results in a low electrical conductivity and efficiency. A substantial effort has been put in to increase the efficiency of CaMnO3, but it still needs improvement.In my thesis, I have investigated the CaMnO3 system. CaMnO3 was synthesized using co-reactive RF-magnetron sputtering and post annealing. The synthesis method is already known but has not been used for deposition of perovskites. I have also demonstrated that this synthesis method can be used to dope CaMnO3 with niobium at appropriate levels for enhancing the efficiency. During the course of research underlying this thesis, I was enrolled in Agora Materiae, a multidisciplinary doctoral program at Linköping University, Sweden. VI VII Populärvetenskaplig SammanfattningMed dagens utveckling av teknologi ökar behovet av elektricitet. Många teknologier är små saker som behöver egna energikällor t.ex. sensorer. Dessa kan sitta på människokroppen, i datorer, i olika industrier och andra miljöer. Det de har gemensamt är att alla behöver en tillförlitlig kraftkälla som i många fall behöver fungera utan något underhåll samt ta lite plats.Ett annat problem i samhället är det stora slöseriet med energi i form av spillvärme, det vill säga en oönskad restprodukt, från i stort sett alla processer. Denna värme är i många fall helt outnyttjad och om man kunde ta till vara på denna värme skulle effektiviteten av många processer kunna ökas.Jag har undersökt material som skulle kunna lösa dessa problem, nämligen termoelektriska material. Sådana material har förmågan att omvandla värme till elektricitet och det som driver denna förmåga är Seebeck-effekten, uppkallad efter Thomas Seebeck som upptäckte den i början av 1820-talet. De fungerar genom att materialet har en varm och en kall sida. På den varma sidan rör sig elektronerna snabbare än på den kalla sid...