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Thermoelectric Properties of SrTiO3 Based Oxides through A-site Modifications

Ekren, Dursun

[Thesis]. Manchester, UK: The University of Manchester; 2019.

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Abstract

Donor doped SrTiO3 is one of the most promising thermoelectric oxides, and further improvement in the properties could provide a viable and efficient thermoelectric material that can be utilised to generate electricity from waste heat at elevated temperatures. In this study, the effects of additives, cation vacancies, and processing routes on the thermoelectric properties of ceramics based on SrTiO3 were investigated. High density (typically 95% theoretical) ceramics were prepared by initially calcining the powders with desired compositions for 8 hours in air at 1373-1523 K and then sintering the green bodies for 4-24 hours in either H2-Ar atmosphere or graphite at 1673-1723 K. Minor additives (B2O3 and ZrO2) were utilised for the modification of the structure of Sr0.9Ln0.1TiO3-d (Ln=Nd and Pr) ceramics at both the atomic and micro scales. B2O3 reduced the sintering temperature required to obtain dense ceramics while ZrO2 improved chemical homogeneity at the atomic scale and homogenised the microstructure. The electrical conductivity and power factor were significantly improved by the substantial increase in charge carrier mobility, while the Seebeck coefficient was maintained. Addition of only 0.5 wt% B2O3 and 0.3 wt% ZrO2 enabled 30-50% improvement in the maximum power factor for both Sr0.9Nd0.1TiO3-d (~2000x10-6 W/m.K2) and Sr0.9Pr0.1TiO3-d (~2200x10-6 W/m.K2). Moreover, values of the dimensionless figure of merit, ZT (0.37 at 1015 K for Sr0.9Nd0.1TiO3-d and 0.30 at 800 K for Sr0.9Pr0.1TiO3-d) were at least comparable to earlier work on these systems. The effect of lanthanide/vacancy doping on the structural and thermoelectric properties of Sr1-xLn2x/3Vx/3TiO3-d was studied for Ln=Pr and Nd, and x=0.1-1.0. Increasing cation/vacancy concentration changed the crystal structure from cubic (space group Pm-3m) to a layered orthorhombic (space group Cmmm) structure. A glass-like thermal conductivity (as low as 2 W/m.K) was achieved with a high level of doping, while low doping resulted in a high thermoelectric power factor (~1800x10-6 W/m.K2). The maximum ZT was 0.26 at 865 K for Sr0.60Pr0.267TiO3-d and 0.28 at 870 K for Sr0.75Nd0.167TiO3-d samples. Sintering sample by encasing in graphite was an effective processing route. The effect of the lanthanide cation on the thermoelectric properties of Ln0.6Sr0.1TiO3-d was investigated for Ln=La, Pr and Nd compounds (where RLa>RPr>RNd). The use of La resulted in short-range cation/vacancy ordering (space group Pban) while Pr and Nd containing compositions exhibited long-range ordering (space group Cmmm). Changes of the lanthanide cation changed the unit cell volume and distortion of the TiO6 octahedra. This led to a high thermoelectric power factor (~600x10-6 W/m.K2) with the use of larger ionic radius lanthanide (La), while a lower thermal conductivity (~1.88 W/m.K) was achieved with the smaller ionic radius lanthanide (Nd). The maximum ZT was 0.18 at 865 K for compositions with Pr or La regardless of the processing route.