TEOX · New Thermoelectric Oxides for Energy Harvesting

New Thermoelectric Oxides for Energy Harvesting

New Thermoelectric Oxides for Energy Harvesting (TEOX, FP7-MC-IIF, 624474) Ruizhi Zhang and Michael J. Reece School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS,UK E-mail addresses: nwuzhangrz@gmail.com (R. Zhang), m.j.reece@qmul.ac.uk (M.J. Reece). Thermoelectric (TE) materials can convert waste heat to electrical power, so they have received increasing attention due to the world’s demands for green energy generation and global environmental protection. There is a need for new high-performance thermoelectric materials that are inexpensive, non-toxic and earth abundant. Oxides ceramics are low density, inexpensive, contain non-toxic elements, and are potentially low cost and environmentally friendly thermoelectric material. They are particularly suitable for high temperature usage due to their excellent oxidation resistance. The main objective in this research is to identify and fabricate high performance n-type TE oxides, which can be divided into 4 steps: 1) screening for AE-Nb-O(AE=alkali earth metals Ca, Sr or Ba) based layered perovskite-related oxides with good TE properties; 2)development of spark plasma sintering processing routes to produce highly textured, fine grained and dense ceramics of identified compounds in step 1; 3) modeling of thermoelectric properties of the nanosheets containing ceramic composites to utilize energy filtering effect; and 4) spark plasma sintering and characterisation of the composites with optimal parameters for good TE properties. Following the original plan described in the proposal, we identified potential TE materials by screening materials from the Inorganic Crystal Structure Database (ICSD) as the first step. By developing a program written by Python, compounds with a Nb-O octahedral network in the crystal structure were identified. There are 922 entries containing Ca/Sr/Ba, Nb and O in the ICSD 2016 database. After deleting duplicate entries and entries with doping/vacancies, 453 phases remained. Among them there are 107 compounds containing a Nb-O octahedral network. Based on first principles band structure and phonon dispersion calculations, as well as literature review, two compounds, Sr1.6La0.4Nb2O7 and BaCa0.7La0.3Nb2O7, were identified as potential TE materials. The second step was to make single phase texturedSr1.6La0.4Nb2O7 and BaCa0.7La0.3Nb2O7 ceramics. The former is a perovskite-related layered structure and the latter is a Dion-Jacobson phase. They both have a high in-plane electrical conductivity, but in the out-of-plane direction they are insulators. Spark plasma sintering (SPS) processing with two-steps was developed to produce dense and textured ceramics, and the layered structure with in-plane direction perpendicular to the pressing direction during SPS was successfully obtained. The electrical conductivity of these compounds can be tuned by changing the doping level of La. As the third step, nanosheet containing ceramics were modeled in the framework of Boltzmann transport equations and energy filtering effect. A novel technique was developed to incorporate energy filtering into the first principles calculation based method by modifying the source code of BoltzTraP package. Using a simple composite model, where energy filtering occurs at the interface between the nanosheets and matrix, the optimal values for the maximum power factor enhancement of interfacial potential barrier height and carrier scattering probability were calculated. The calculations show that the power factor can be greatly enhanced at high carrier concentrations by energy filtering. This enhancement is very sensitive to potential barrier height and carrier scattering probability. The optimal potential barrier height also shows a weak dependence on carrier scattering probability. To verify our modeling results, nanosheet containing composite ceramics were fabricated in the final step. Graphene nanosheets were chosen as the fillers in the oxide matrix. By ball milling of graphene and oxide powders together, and then densifying by SPS, composite ceramics with well dispersed graphene nanosheets were successfully obtained. The volume ratio of graphene was 1%, 3% and 5%. Both electrical conductivity and Seebeck coefficient of the composites increased with graphene volume ratio, indicating the effectiveness of the energy filtering effect. Overall, the work done in the reported period progressed according to plan and all objectives were achieved.

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