This article discusses the changes in the behavior of strontium titanate (SrTiO3) ceramics, specifically in their thermoelectric properties and crystal structure, after being doped with different elements such as niobium (Nb), tantalum (Ta), and lanthanum (La) in the context of thermoelectric power generation.
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Limitations of Thermoelectric Material
Thermoelectric materials have gained considerable attention for harvesting energy from the waste heat liberated from electronic devices, industries, automobiles, and power plants.
The thermoelectric material efficiency is measured by the dimensionless thermoelectric figure of merit ZT = S2 σTқ-1, where σ refers to electrical conductivity/ S cm-1, S is the Seebeck coefficient/ µVK-1, қ is the thermal conductivity/ Wm-1K-1, T is the absolute temperature/K, and Z is the figure of merit.
However, achieving a high ZT is challenging owing to the interdependence of σ, қ, and S. To attain a high ZT, the thermoelectric material must possess a high power factor/S2σ value and a low thermal conductivity.
Typically, the Seebeck coefficient or electrical conductivity is enhanced by nano/micro structuring or doping approaches to increase the power factor of the thermoelectric material.
Significance of Perovskite-type SrTiO3
A range of compounds of potential thermoelectric properties in a large temperature range can be synthesized by doping perovskite-type cubic SrTiO3. The SrTiO3-based oxide thermoelectric materials have gained prominence owing to their low cost, non-toxicity, and excellent redox flexibility and thermal stability.
Undoped SrTiO3 acts as an insulator, which can be converted into a semiconductor by doping pentavalent elements in the B-site and trivalent elements in the A-site. The donor dopant with a higher cationic charge, such as Ta5+ and Nb5+, replaces the host cation titanium (Ti4+) with a lower charge in the doping process, which brings either more electrons or ions into the system compared to the host oxide and improves the electrical properties of SrTiO3 ceramics.
Change in the Behavior of Doped SrTiO3 Ceramics
A paper recently published in the journal Materials Today: Proceedings demonstrated changes in the thermoelectric properties and the structure of SrTiO3 ceramics after the ceramics were doped with Nb/Ta or co-doped with Nb and Ta.
The lattice constant was increased in the Ta/Nb-doped SrTiO3 compared to undoped SrTiO3 due to the higher ionic radii of Ta and Nb dopants compared to the replaced host Ti atom.
However, the lattice constant remained the same in the Nb-Ta co-doped and Nb/Ta doped SrTiO3 owing to the similar ionic radii of both Nb and Ta. The tolerance factor and unit cell volume of Nb/Ta substituted and co-doped SrTiO3 also displayed no difference due to the same ionic radii of dopants.
The thermoelectric properties, such as the Seebeck coefficient and electrical conductivity, were evaluated in an inert atmosphere to prevent the reoxidation of the doped SrTiO3 samples at extremely high temperatures.
In all doped samples, the electrical conductivity decreased with an increasing temperature, indicating the general metallic behavior of the Nb/Ta substituted and co-doped SrTiO3 ceramics.
However, the Nb-doped SrTiO3 samples displayed a typical semiconductor behavior as the electrical conductivity increased in Nb-doped SrTiO3 samples till 570 K. In the Ta-doped SrTiO3 samples, the electrical conductivity decreased consistently with the increasing temperature, which improved the metallicity of the samples.
Ta-Nb co-doped samples with an equal concentration of both dopants displayed the highest electrical conductivity of 235 S cm-1 at 470 K due to the creation of more charged carriers compared to other doped samples.
Unlike the electrical conductivity, the Seebeck coefficient was increased in correspondence with the increasing temperature. The inverse relationship between the Seebeck coefficient and electrical conductivity resulted in a higher electrical conductivity when the Seebeck coefficient value was low. The Ta doping decreased the Seebeck coefficient value, while Nb doping increased the value in the samples.
In Ta doped samples, the Seebeck coefficient decreased considerably while the electrical conductivity was enhanced, indicating that the increase in the number of charge carriers in Ta doping was due to the increased oxygen vacancies. The highest Seebeck coefficient value of 190 µVK-1 was observed at 1080 K for Nb-doped SrTiO3 samples.
The power factor values increased with the rising temperature in all samples. Although the Ta and Nb co-doped samples demonstrated the highest electrical conductivity, their power factor values decreased with the rising concentration of Ta, owing to the substantial decrease in the Seebeck coefficient. The maximum power factor value of 250 µWm-1K-2 was observed at 900 K in the Nb-doped SrTiO3 samples.
In another study published in the journal Chemistry of Materials, researchers investigated the thermoelectric properties and structure of lanthanum (La)-doped A-site-deficient SrTiO3 (Sr1−3x/2LaxTiO3) ceramics sintered in nitrogen/5%hydrogen and air.
The optimized ZT values of 0.41 obtained for n-type Sr1 −3x/2LaxTiO3‑δ ceramics at 973 K with 0.125 ≤ x ≤ 0.175 was the highest reported thermoelectric ZT values for any perovskite-based SrTiO3 ceramics.
The high ZT values were achieved by the close control of ceramic processing conditions and careful selection of starting compositions, which created defective perovskite lattices consisting of O- and A-site vacancies with mixed valent Ti4+ and Ti3+ on the B-site.
Although the optimized materials featured highly defective nonstoichiometric perovskites, they were resistant to rapid reoxidation in the air at temperatures up to 1273 K, which showed their potential in thermoelectric application such as n-type legs in thermoelectric generators.
To summarize, the doping approach has been found suitable for effectively improving the thermoelectric properties of perovskite-type cubic SrTiO3. Thus, the approach can be potentially implemented for the synthesis of efficient thermoelectric materials with a high figure of merit.
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References and Further Reading
Charan Prasanth S., Jose, R., Vijay, A., Vineetha P., Venkata Saravanan K. Tuning thermoelectric properties of Nb and Ta co-doped SrTiO3 ceramics. Materials Today: Proceedings. ISSN 2214-7853. 2022. https://doi.org/10.1016/j.matpr.2022.04.908
Lu, Z., Zhang, H., Lei, W., Sinclair, D.C., and Reaney, I. M. High-Figure-of-Merit Thermoelectric La-Doped A-Site-Deficient SrTiO3 Ceramics. Chemistry of Materials. 28, 3, 925–935. 2016. https://doi.org/10.1021/acs.chemmater.5b0461
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