The specific grain interior and grain boundary conductivities, obtained from impedance spectroscopy and the brick layer model, are reported for BaZr 0.9 Y 0.1 O 3 − δ as a function of p O 2 and temperature. p O 2-dependencies were indicative of dominating ionic and p-type electronic conduction for the grain interior under reducing and oxidizing conditions, respectively, while the grain boundaries showed an additional n-type electronic contribution under reducing conditions. Transmission electron microscopy revealed enrichment of Y in the grain boundary region. These findings indicate the existence of space-charge layers in the grain boundaries. A grain boundary core-space-charge layer model is therefore applied to interpret the data. Using a Mott-Schottky approximation, a Schottky barrier height of 0.5-0.6 V and an effective grain boundary width of 8-10 nm (= 2× space-charge layer thickness) is obtained at 250°C in wet oxygen. Finite-element modelling of the complex impedance over a grain boundary with a space-charge layer depletion of protons yields a distorted semicircle as observed in the impedance spectra.
The gradual change in the crystal structure of the high temperature proton conductor LaNbO(4) through a second order phase transition and its relation to the activation enthalpy of mobility of protons have been studied by means of first principles calculations and conductivity measurements. The computations have revealed that protons diffuse by an inter-tetrahedral mechanism where the activation enthalpies of mobility are 39 and 60 kJ mol(-1) in tetragonal and monoclinic LaNbO(4), respectively. The activation enthalpy of mobility of protons for tetragonal LaNbO(4), determined from the conductivity curve, is 35 kJ mol(-1). Below the transition temperature the conductivity curve bends; initially dropping off steeply, followed by a less steep decrease towards lower temperatures. The bend in the conductivity curve at the onset of the phase transition in LaNbO(4) should not be given the traditional interpretation as an abrupt change in the activation enthalpy of mobility. After application of the proper analysis of the conductivity data, which takes the second order transition into account, the activation enthalpy of mobility of protons is found to continuously increase with increasing monoclinic angle at decreasing temperature, reaching approximately 57 kJ mol(-1) at 205 degrees C for the end monoclinic phase.
This work was carried out within the project "Thermoelectric Oxides for High Temperature Waste Heat Recovery" (THERMEL), funded by the RENERGI programme of the Norwegian Research Council. Firstly, I would like to thank my supervisors, Terje Finstad and Truls Norby, for allowing me to work independently and develop my own ideas, but providing interest, advice, and guidance, whenever I got lost on the way. I am also grateful for the privilege to work in the social, helpful, and inspiring environment of the FASE group. Thanks to all previous and present members for making this a nice place to work and-of course-for uncounted coffee breaks, foosball matches, and social gatherings. Especially, I thank Harald Fjeld, for being a great office mate, patient teacher of (defect) chemistry, and omniscient lab-encyclopedia. It is safe to say that this thesis would have looked different without you! A big thank you also to our cohabitants from the structural physics group, especially Ole-Bjørn Karlsen and Kjetil Valset, for providing practical assistance and help. Simone Casolo deserves my deep respect for patiently enduring my endless questions around his DFT results. My stay in the group of Sean Li in Sydney was pleasant, interesting, and scientifically productive. I'd like to thank all the people I met there, who helped making me feel at home at the other side of the world within only three months. I am also happy to have met Leyre Sagarna and Christophe Heinrich on conferences, sharing the experience and struggles of being a PhD-student and resulting in minicollaborations. Further, I am deeply indebted to my parents and family for almost three decades of unconditional support and advice. Finally, I thank my friends and especially Tina for simply being there. Oslo, March 2014 Matthias Schrade iii Here, α is the Seebeck coefficient, ρ the electrical resistivity, and κ the thermal conductivity. The index n (or p) indicates the n-type (or p-type) leg. The effective figure of merit Z is usually close to the average of the individual figures of merit z of the p-and n-materials. Thus, the by far most reported number in thermoelectric
The ac conductivities of Er 6 WO 12 and 1% Ca-doped Er 6 WO 12 have been characterized as a function of oxygen and water vapor partial pressures in the temperature range 300-1200°C. Partial conductivities have been determined from the open-circuit voltage of p͑O 2 ͒, p͑H 2 ͒, and p͑H 2 O͒ concentration cells. The materials exhibit n-and p-type electronic conductivity and oxygen ion and proton conductivity, depending on the conditions. Protons are the major ionic charge carriers in wet atmospheres below 1150°C. Hydration thermodynamics and transport parameters have been determined under conditions where the acceptor doping is predominantly compensated by oxygen vacancies and protons.
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