Enhanced Thermoelectric Performance in the SrTi0.85Nb0.15O3 Oxide Nanocomposite with Fe2O3-Functionalized Graphene.

Doped SrTiO3 is considered one of the potential thermoelectric (TE) candidates but its TE figure of merit, ZT needs to be improved for practical application of electricity generation from high-grade waste-heat. In the present work, enhanced TE performance has been realized for SrTi0.85Nb0.15O3 (STN) perovskite adopting the strategy of composite formation with Fe2O3-functionalized graphene (FGR). We have achieved a maximum electrical conductivity of 1.4 × 105 S m-1 for 1 wt % FGR added to STN, which is around 1185% larger than that of pristine STN. The presence of FGR in the STN matrix acts as a mobility booster of electrons, overcoming the effect of Anderson localization of electrons, which impedes the electron transport in STN. This is evident from the order of magnitude increase in weighted mobility of STN after FGR addition. Furthermore, the incorporation of FGR causes about a 34% decrease in the lattice thermal conductivity. The Debye-Callaway model demonstrates that the phonon-phonon Umklapp scattering is primarily responsible for reduced thermal conductivity. The presence of FGR sheets along the grain boundaries of STN, Fe2O3 nanoparticles, and lattice imperfections gives rise to the glass-like temperature-independent phonon mean-free-path, especially above Debye temperature. The maximum ZT ∼ 0.57 has been obtained at 947 K for the 1 wt % FGR sample, which is around 420% higher than that of pristine STN. Furthermore, we have fabricated a prototype of a four-legged n-type TE module, demonstrating one of the highest power outputs of 18 mW among reported oxide thermoelectrics.

Designing rare earth-free high entropy oxides with a tungsten bronze structure for thermoelectric applications.

In recent years, forming high entropy oxides has emerged as one of the promising approaches to designing oxide thermoelectrics. Entropy engineering is an excellent strategy to improve thermoelectric performance by minimizing the thermal conductivity arising from enhanced multi-phonon scattering. In the present work, we have successfully synthesized a rare-earth-free single phase solid solution of novel high entropy niobate (Sr0.2Ba0.2Li0.2K0.2Na0.2)Nb2O6, with a tungsten bronze structure. This is the first report on the thermoelectric properties of high entropy tungsten bronze-type structures. We have obtained a maximum Seebeck coefficient of -370 µV K-1 at 1150 K, which is the highest among tungsten bronze-type oxide thermoelectrics. The minimum thermal conductivity of 0.8 W m-1 K-1 is obtained at 330 K, which is so far the lowest reported value among rare-earth-free high entropy oxide thermoelectrics. This synergistic combination of large Seebeck and record low thermal conductivity gives rise to a maximum ZT of 0.23 which is so far the highest among rare-earth free high entropy oxide-based thermoelectrics.

Enhanced Thermoelectric Performance of Rare-Earth-Free n-Type Oxide Perovskite Composite with Graphene Analogous 2D MXene.

Here, the first experimental demonstration on the effect of incorporating new generation 2D material, MXene, on the thermoelectric performance of rare-earth-free oxide perovskite is reported. The charge localization phenomenon is predominant in the electron transport of doped SrTiO3 perovskites, which deters from achieving a higher thermoelectric power factor in these oxides. In this work, it is shown that incorporating Ti3 C2 Tx MXene in a matrix of SrTi0.85 Nb0.15 O3 (STN) facilitates the delocalization of electrons resulting in better than single-crystal-like electron mobility in polycrystalline composites. A 1851% increase in electrical conductivity and a 1000% enhancement in power factor are attained. Besides, anharmonicity caused by MXene in the STN matrix has led to enhanced Umklapp scattering giving rise to lower lattice thermal conductivity. Hence, 700% ZT enhancement is achieved in this composite. Further, a prototype of thermoelectric generator (TEG) using only n-type STN + MXene is fabricated and a power output of 38 mW is obtained, which is higher than the reported values for oxide TEG.

Enhanced Thermoelectric Performance in Oxide Composites of La and Nb Codoped SrTiO3 by Using Graphite as the Electron Mobility Booster.

Inherent insulating nature of oxides makes it challenging for use in thermoelectric applications that warrant reasonable electrical conductivity. In the present work, we have used graphite (G) to improve the electron transport in La0.07Sr0.93Ti0.93Nb0.07O3 (LSTN) by making composites. Graphite acts as the electron momentum booster in the LSTN matrix, which otherwise suffers from Anderson localization of electrons, causing an order of magnitude increase in weighted mobility and electrical conductivity. As a result, the thermoelectric power factor increases more than 6 times due to graphite incorporation in LSTN. Furthermore, the lattice thermal conductivity is suppressed due to enhanced Umklapp scattering, as derived from the Debye-Callaway model. Hence, we have recorded ∼423% increment in the figure of merit (ZT) in LSTN + G composites. The maximum ZT obtained is 0.68 at 980 K for the LSTN with 1 wt % graphite composite. Furthermore, we have fabricated a four-legged n-type thermoelectric power generator demonstrating a milliwatt level power output, which hitherto remained unattainable for oxide thermoelectrics.